Valorisation of Selected Fruit Pomaces in Animal Nutrition

Abstract

Rising feed costs and the shift towards circular bioeconomy systems have generated interest in agro-industrial by-products as livestock feeds, but the evidence on fruit pomaces is fragmented across single pomaces, single species and single processing methods. This review compares apple, citrus and grape pomaces against a common set of nutritional, processing, functional and environmental criteria, and grades the strength of the evidence supporting each comparison. Under particular species-, processing- and dose-specific conditions, properly prepared pomaces have been reported to improve ruminal fermentation, nutrient digestibility, milk composition and oxidative status in ruminants, and gut health, antioxidant status and product quality in non-ruminants. Responses are heterogeneous, and null or adverse effects on intake, digestibility and performance also occur, particularly at high inclusion levels and with lignin- and tannin-rich material. Reductions in enteric methane have been measured in vivo for grape by-products in dairy cattle, whereas the evidence for citrus flavonoids remains in vitro; whole-system environmental and economic benefits are unquantified. Research priorities include standardised processing and compositional reporting, routine control of mycotoxins, pesticide residues and heavy metals, species-specific dose–response trials, and whole-chain techno-economic and life-cycle assessment.

1. Introduction

Rising costs and the limited supply of conventional feed ingredients, particularly cereal grains and protein meals that compete directly with human food and biofuel markets, pose a growing constraint on the economic and environmental sustainability of livestock production worldwide. This challenge has driven increasing scientific interest in alternative and underutilised feed resources capable of reducing reliance on conventional feedstuffs, such as microalgae [1], insects [2] and underutilised fodder resources like Azolla pinnata, bitter vetch (Vicia ervilia (L.) Willd), Sulla flexuosa (L.) Medik. and sorghum [3,4,5,6]. Within this broader landscape, fruit agro-industrial by-products, and pomaces in particular, occupy a distinctive position; unlike microalgae, insects, or purpose-grown fodder crops, they are generated as an unavoidable co-product of already-established juice, cider, and wine industries, requiring no additional land, water, or cultivation input beyond that of the primary fruit crop. This positions fruit pomace as a readily available, low-opportunity-cost feed resource warranting focused evaluation. Global fruit production reached approximately 933 million tonnes in 2022 [7]. The expansion of fruit-processing industries has generated large quantities of fruit agro-industrial by-products. If not properly managed, these by-products create significant environmental and economic challenges. Apple processing illustrates the scale involved: global apple production reached 95.84 million tonnes in 2022 [7], and annual apple pomace generation has been estimated at approximately 4 million tonnes [8]. Fruit processing sectors, particularly juice, beverage, and wine industries, are the major contributors to this waste stream. It is estimated that 20–22% of the mass of processed fruit remains as pomace following juice extraction [9], contributing to greenhouse gas emissions, unpleasant odours, and soil and water contamination, and it is this fraction, generated at a limited number of industrial sites and therefore geographically concentrated, that constitutes the practically recoverable feed resource. In recent years, increasing emphasis on sustainable livestock production has stimulated interest in valorising agro-industrial by-products as alternative feed resources. Several studies have described the positive impacts of such pomaces on animal performance, rumen fermentation, gut health, antioxidant status, product quality, and the environmental sustainability of both ruminants and non-ruminants. However, this evidence remains fragmented. Previous reviews have generally focused on an individual fruit pomace [10,11], on particular processing or extraction approaches [11,12,13], or on food rather than feed applications [8], whereas the present review provides a comparative livestock-oriented evaluation of fruit pomaces.

In the global fruit processing industry, the processing of apples, citrus and grapes is of particular importance. For example, China alone generates more than one million tonnes of apple pomace annually [14], while global grape production exceeds 75 million tonnes, most of which is processed into wine [13,15]. In terms of processing volume and availability, these crops are among the largest fruit categories processed industrially, so that the resulting pomaces arise in concentrated and predictable volumes at identifiable industrial sites rather than being dispersed across many small producers. On the other hand, in terms of the evidence base, controlled livestock-feeding studies are available for all three pomaces in both ruminant [16,17,18] and non-ruminant species [19,20,21], which is a precondition for the comparative evaluation attempted here. Mango, pineapple, banana, berry and other fruit by-products fall outside this predefined scope due to its small quantity and lack of research evidence, although they are also applied to feed production in some regions [22,23]. This review mainly aims to provide a comprehensive evaluation of selected fruit pomaces from apple, citrus, and grape by comparing their nutritional compositions, processing methods, functional characteristics for ruminant and non-ruminant feeding, bioactive compounds, and environmental benefits. It will also explore the roles of fruit pomaces as animal feeds in enhancing animal performance, product quality, and environmental sustainability.

2. Methodology

This narrative review synthesises the literature on citrus, apple, and grape pomaces regarding nutritional composition, bioactive compounds, preservation, and livestock utilisation.

2.1. Literature Search Strategy

A structured search of Scopus, Web of Science, PubMed, and Google Scholar covered publications from 1984 to 2026. The search combined terms for pomace type (e.g., “apple pomace”, “grape marc”), livestock species (e.g., “ruminant”, “monogastric”), and outcomes (e.g., “digestibility”, “methane emission”). Searches were adapted per database syntax. Google Scholar results were limited to the first 200 records. References of retrieved articles were hand-searched. Only records with an English full text were retained.

2.2. Inclusion and Exclusion Criteria

Primary studies were included if they reported original quantitative data on the target pomaces in livestock feeding and were peer-reviewed. Studies focusing solely on human food or nutraceuticals, or lacking quantitative data, were excluded. Background sources (reviews, statistical and institutional reports, regulatory documents and methodological references) provided context but were not part of the quantitative evidence base. Incidental studies on non-target by-products [23,24] were cited only for comparison.

2.3. Study Selection

Records were screened by title/abstract and full text against eligibility criteria. Exclusions were primarily due to irrelevant applications, out-of-scope species, or a lack of quantitative data. After duplicates were removed, the synthesis was based on 112 primary studies, supplemented by 39 contextual sources. As a narrative review, no formal risk-of-bias assessment or meta-analysis was conducted.

2.4. Evidence Appraisal and Reporting of Consistency

Findings are characterised by reproducibility: consistently supported (replicated across species/settings), conditionally supported (dependent on factors like dosage or processing), or limited (few studies or conflicting results). Mechanistic claims distinguish between evidence from target livestock and inferences from in vitro or lab animal models. We acknowledge the publication bias favouring positive results [25]; consequently, null or adverse effects are explicitly reported where available rather than omitted.

3. Processing and Preservation of Fruit Pomaces for Animal Nutrition

China is the world’s leading producer of both citrus and apples, generating more than one million tonnes of apple pomace annually [14] and accounting for 28.80% of global citrus output [26], while Asia as a whole contributes 52.8% of world citrus and 65.4% of world apple production, with the United States, Turkey, and Poland as other major apple-producing countries [10]. Grape production, concentrated primarily in Italy, France, and Spain, exceeds 75 million tonnes annually, most of which is processed into wine [13,15], generating substantial volumes of grape pomace. This geographical concentration has direct practical implications for livestock feeding systems, since pomace generation is tied to discrete regional harvest seasons rather than occurring continuously. Apple and grape pomace arise mainly from the autumn harvest and pressing, while citrus pomace follows the winter harvest typical of Mediterranean and subtropical growing regions, meaning fresh pomace is available in large, low-cost volumes only during and shortly after each crop’s processing season, after which preserved forms are needed to maintain year-round supply. Fruit pomace primarily consists of peels, seeds, and residual pulp [8] and is characterised by high moisture content and susceptibility to rapid microbial spoilage [27], which makes its storage, transportation, and disposal difficult. Moreover, the high levels of fibre and lignin in the pomace can lead to poor nutrient digestibility, while variations in composition and anti-nutritional components make it difficult to utilise the product as animal feed. For this reason, it is important to process the material to improve stability, maintain its nutritional value, enhance digestibility, and increase utility. Processing methods can also reduce or concentrate anti-nutritional compounds, depending on the technique applied.

3.1. Hot-Air Drying

Dry preservation of fruit pomace is considered the most common method because it reduces moisture content, extends shelf life, improves handling, and lowers transportation costs. The drying process affects nutrient preservation, and its practical significance depends on the intended end use. For food and nutraceutical applications, antioxidant and phenolic retention is often the primary quality criterion. For livestock feed, the retention of fibre, protein, and digestible energy generally matters more than antioxidant content alone, though some antioxidant retention remains beneficial for animal health and product quality.

Hot-air drying is the most widely used method of drying because of its simplicity and applicability in industrial processes. The use of moderate temperatures during drying enhances storage stability and maintains acceptable levels of fibre and bioactive components, whereas higher temperatures lead to nutrient losses. Hot-air drying of citrus pomace at 60 °C preserved fibre and antioxidants but caused partial losses of vitamin C and some phenolic compounds [28]. In addition, moderate drying increased the stability of flavonoids and carotenoids in citrus waste material. Drying temperature also significantly affected the polyphenol content of grape pomace: high drying temperatures resulted in a considerable loss of phenolics, whereas lower drying temperatures, around 45 °C, favoured the preservation of the antioxidants. Higher temperatures were associated with an apparent increase in protein concentration, which reflects a concentration effect arising from greater moisture loss rather than any true increase in protein deposition [29,30]. References [29,30] report grape pomace specifically; equivalent temperature-response data for apple pomace obtained under comparable analytical conditions were not identified, so this temperature retention relationship is regarded here as conditionally supported and should not be assumed to transfer quantitatively between pomace types. In a feed context, this trade-off is notable: producers seeking to maximise antioxidant content for functional feed applications may prefer lower drying temperatures, whereas those prioritising protein concentration for basic nutritional value may find moderate-to-higher temperatures acceptable, depending on the specific production goal.

3.2. Other Drying Technologies

3.2.1. Microwave Drying

The process of microwave drying helps increase the rate of moisture evaporation, thereby reducing the total drying time compared to hot-air drying. Nevertheless, drying efficiency is highly dependent on microwave power and the pretreatment method used. For instance, microwave drying of orange pomace increased the rate of drying, but excessive microwave power degraded the colour quality [31]. This finding indicates that microwave-assisted drying may improve processing efficiency and product quality in high-end product applications, where colour and appearance are also commercially important attributes.

3.2.2. Infrared Drying

Infrared drying allows for faster heat transfer and a shorter drying period than traditional drying methods. Infrared drying of apple pomace, alongside hot-air pre-drying, helped improve the water removal rate [32]. In grape pomace, infrared drying improved drying rate, energy efficiency, and microbial stability while maintaining polyphenol and proanthocyanidin contents [33]. Natural convection infrared drying generally provides higher drying efficiency than forced-convection systems [34].

3.2.3. Vacuum Drying

Vacuum drying helps reduce heat degradation by lowering the boiling point of water under reduced pressure. Optimal vacuum-drying conditions for apple pomace (68.33 °C, 46.33 kPa, 4.27 h) enhanced antioxidant content while reducing moisture and energy consumption [35]. Similarly, vacuum drying at 70 °C effectively preserved polyphenols and organic acids in grape pomace [36]. Therefore, vacuum drying offers a balance between drying efficiency and preservation of bioactive compounds.

3.2.4. Freeze-Drying

Freeze-drying is considered one of the most effective methods for preserving heat-sensitive compounds because water is removed via sublimation at low temperatures. Freeze-drying preserved higher concentrations of polyphenols and antioxidants in lemon, apple, and grape pomaces than conventional hot-air drying [30,37,38]. In apple pomace, freeze-dried material also showed high suitability for the recovery of phenolic compounds through supercritical extraction techniques [39]. Despite its superior preservation efficiency, freeze-drying remains expensive due to high energy and equipment requirements, which limit its large-scale application in livestock feed systems.

3.2.5. Emerging Drying Technologies

A study reported a biodrying process that reduced the weight and volume of apple pomace by over 70 percent in just five days, producing a stable material with minimal energy input [40]. Drum drying preserved antioxidant activity in apple pomace while reducing processing time and energy consumption compared with freeze-drying [41]. Heat-pump and electrohydrodynamic drying enhanced thermal efficiency and the preservation of heat-sensitive compounds in grape pomace [29,42]. These technologies have been reported to show potential for sustainable large-scale pomace processing, although the supporting evidence remains limited to laboratory and pilot scales.

Taken together, the drying technologies reviewed above are best interpreted not as a ranking from inferior to superior, but as a trade-off between nutrient and bioactive retention on one axis and capital cost, energy demand and throughput on the other, with the appropriate choice determined by the intended end use. For bulk livestock feed, where fibre, protein and digestible energy retention are the primary value drivers, the technologies that perform best on bioactive retention are generally the least economically realistic. The economic realism of the drying methods described above for livestock feed therefore remains limited, despite their individual technical advantages. The specialised equipment and higher operating costs make these technologies more justifiable for high-value nutraceutical applications, bioactive extracts, functional additives, or research-scale use than for bulk feed, where fibre and protein retention outweigh colour and antioxidant content as the primary value drivers. It follows that the description of freeze-drying as the most effective preservation method refers strictly to retention of heat-sensitive compounds; technical superiority in nutrient preservation does not imply economic or industrial suitability for processing large tonnages of livestock feed, and freeze-drying should not be presented as the preferred option for this application. On the evidence currently available, hot-air drying remains the only thermal method demonstrably applied at the industrial scale to pomace destined for feed, with infrared and heat-pump systems most plausibly deployed as complementary or pre-drying steps rather than as standalone solutions. These emerging technologies remain supported mainly by laboratory- or pilot-scale evidence, with equipment availability, capital cost, standardisation, and batch-to-batch performance consistency still largely unaddressed in the literature; until such data become available, they should be regarded as promising research directions rather than feed-ready processing solutions.

3.3. Ensiling

Among the preservation methods discussed in this review, ensiling deserves particular emphasis as one of the most practically important and widely applicable approaches for fruit pomace, especially for high-moisture pomaces. Fruit pomace can be preserved by ensiling to make it more stable during storage and less susceptible to spoilage and of higher nutritional quality. Ensiling is a useful process for conserving fruit pomace through anaerobic fermentation and microbial breakdown. It helps maintain nutrient value, prolong shelf life, and reduce spoilage losses [43]. While there was some reduction in energy content, ensiled orange pulp fed to finishing pigs at a 500 g/kg substitution level of the basal diet retained satisfactory fibre digestibility, and this conservation method was associated with reduced ammonia and methane emissions from slurry [20]. This finding is therefore most directly relevant to non-ruminant manure management. The potential for ensiling apple pomace is also promising, particularly for ruminants. In a digestibility trial using wether sheep, apple pomace ensiled with wheat straw and urea (1 tonne apple pomace mixed with 100 kg wheat straw and 5 kg urea, fresh-weight basis) was compared with maize silage and supported acceptable digestible organic matter and metabolisable energy values, indicating suitability as a maize silage substitute [44]. In finishing lambs, ensiled apple pomace enhanced feed intake and growth performance compared with the dried form [16], supporting its practical value as a ruminant feed ingredient at the inclusion levels tested in these species-specific trials. Ensiling of grape pomace increased in vitro digestibility of organic matter and fibre while maintaining microbial and antioxidant stability [12,45,46]. The addition of microbial inoculants and additives can further improve silage fermentation quality and nutrient preservation. The fermentation and preservation of citrus pomace silage were facilitated by lactic acid bacteria [43]. The digestibility of grape pomace was increased by the use of Lactiplantibacillus plantarum [12]. Additives such as zeolite and enzymatic-bacterial preparations improved silage by lowering the pH, ammonia-N levels, and the extent of partial tannin degradation [47]. A mixture of apple and pomegranate pomaces resulted in lower dry matter loss and higher silage density, while essential oils improved the quality of the fermentation process [24]. Fermented apple, citrus and grape pomaces have been reported to improve antioxidant status, feed utilisation efficiency and meat quality in growing animals [48,49,50]. Citrus by-products are very important in ruminant diets due to their high energy content and capacity to stimulate growth and milk production without negatively affecting rumen fermentation, as opposed to feeds rich in starch [51]. Moreover, citrus pomaces can also be used to produce enzymes, organic acids, ethanol, and microbial proteins using fermentation processes [52]. While these represent valorisation pathways rather than direct animal feed applications, some of the fermentation-derived products, particularly microbial protein and specific enzymes, can themselves be recovered and incorporated into animal feed formulations as protein or enzyme supplements, and industrial fermentation for non-feed products competes for the same pomace feedstock as direct animal feed use, which has practical implications for the availability, allocation, and economic viability of pomace as a livestock feed resource within a circular economy framework. Nonetheless, the effect of fermentation depends on the type of pomace and fermentation conditions.

3.4. Chemical Preservation

Chemical preservation methods are mainly used to control microbial growth and extend the storage stability of fruit pomace. In grape pomace, sodium benzoate combined with γ-irradiation effectively controlled microbial deterioration and preserved anthocyanin content during refrigerated storage [53]. The antimicrobial activity of acetic acid, natamycin, iodine, and chitosan was observed to reduce the microbial population and increase the shelf life of apple pomace [54]. The use of chemicals and biochemicals to enhance the properties of pomace fibre has also been considered. This can be seen in the biochemical processing of citrus residues to produce ethanol and organic acids [52], as well as in the enhancement of fibre functionality through alkaline and enzymatic processes [55]. Similar valorisation approaches have been applied to apple pomace for the recovery of bioactive compounds [56].

Despite their demonstrated effectiveness in controlling spoilage, the practical adoption of chemical preservatives for pomace intended as livestock feed is subject to several important considerations beyond preservation efficacy. Safety for the target animal species must be established, since preservative compounds and their residues may accumulate in animal tissues or milk, with downstream implications for food safety in animal-derived products intended for human consumption. Regulatory status varies considerably by preservative, jurisdiction, and animal species: compounds such as sodium benzoate and organic acids are approved feed additives in some regions but subject to maximum permitted inclusion levels, while others (e.g., irradiation-based treatments) face more restrictive or inconsistent regulatory acceptance across different countries. Residue monitoring and maximum residue limits for preservative compounds in animal feed and derived products are therefore important, though often under-addressed, aspects of adopting these methods at a commercial scale. Consumer acceptance represents an additional practical constraint, as the use of chemical additives in livestock feed, particularly synthetic preservatives or irradiation, can raise concerns among consumers regarding food naturalness and safety perceptions, potentially affecting market acceptance of the resulting animal products, even where regulatory approval has been granted. Collectively, these safety, regulatory, and consumer-acceptance factors should be weighed alongside preservation efficacy when evaluating the practical suitability of chemical preservation methods for pomace-based livestock feed.

3.5. Pelleting

Pelleting enhances the handling, transportation, storage, and physicochemical properties of feeds that contain fruit pomace. The addition of dried apple pomace improved pellet durability and hardness and reduced fines during production, but also increased energy consumption [57]. Grape pomace has been included in pelleted feed for ruminants without negative effects on feed intake or rumen fermentation [58]. The use of binding agents such as molasses has enhanced pellet quality by increasing density and durability [59]. However, excessive processing intensity may alter nutrient availability and digestion dynamics, particularly in ruminants, where an excessive proportion of fines may reduce effective fibre length and rumen buffering capacity, which, depending on overall diet fermentability, particle size distribution and feeding management, can contribute to an increased risk of ruminal acidosis [60]. The major processing methods are compared in

Table 1. Comparison of fruit pomace processing methods.

Processing Method	Advantages	Disadvantages	Effect on Nutrient Value	Cost/Energy Demand	Farm Scale vs. Industrial Suitability	Supporting Evidence
Hot-air drying	Simple, widely applicable	Nutrient losses high at high temperature	Moderate temperatures (45–60 °C) preserve fibre and antioxidants	Low–moderate equipment cost and energy	Generally regarded as the most cost-effective option at industrial scale and adaptable to farm scale with basic equipment; no comparative techno-economic study specific to the pomace was identified	[28,29,30,32]
Microwave drying	Fast moisture removal shortens drying time	Excessive power reduces colour quality and energy efficiency; risk of uneven heating	Can preserve nutrients if power is optimised	Moderate–high equipment cost, can lower overall energy consumption	Reported as economically unrealistic for bulk feed use and better suited to higher-value end product applications where colour and appearance justify the cost; based on energy and exergy analysis rather than full costing	[31]
Infrared drying	Rapid heat transfer shortens processing time, preserves polyphenols and proanthocyanidins, improves microbial stability	Evidence mainly at lab/pilot scale, equipment cost and throughput at industrial volumes unproven	Good retention of polyphenols and bioactive compounds	Moderate cost and energy demand	Evidence at commercial feed scale is lacking; currently more promising as a complementary or pre-drying step	[32,33,34]
Vacuum drying	Preserves heat-sensitive compounds	Equipment complexity and cost, slower throughput	High retention of antioxidants, phenolics, organic acids	Moderate–high cost; reduced energy use via lower boiling point under vacuum	Industrial-scale, especially for high-value products	[35,36]
Freeze-drying	Best preservation of heat-sensitive antioxidants and polyphenols	Very expensive, high energy and equipment requirements, slow processing	Highest retention of polyphenols and antioxidants among all drying methods	Highest cost and energy demand of all methods evaluated	Generally considered impractical for bulk feed and realistically limited to high-value extracts, functional additives or research-scale use; ranking is qualitative, as comparative cost data for pomace are not available	[30,37,38,39]
Emerging drying technologies	Improved energy efficiency, comparable to freeze-drying at lower cost	Mostly lab/pilot-scale evidence, not yet validated or adopted in commercial feed systems, cost/standardisation data lacking	Preserves antioxidant activity	Lower energy demand than freeze-drying, solar drying is low-cost	Evidence remains limited to laboratory and pilot scales; currently best regarded as promising research directions rather than feed-ready options	[29,40,41,42]
Conventional ensiling	Low cost, simple, extends shelf life, reduces spoilage, may reduce methane emissions, exploits (rather than requires removing) high moisture content pomaces	Some energy value reduction; excessive inclusion may impair digestibility	Maintains nutrient quality and digestibility; improves fibre/organic matter digestibility	Low cost and energy demand, minimal equipment needed	Well suited to farm-scale use for high moisture pomaces, also applied industrially	[16,20,43,44,51]
Ensiling with additives/inoculants	Improves fermentation quality, reduces spoilage, pH, and ammonia-N, partial tannin degradation	Added cost of inoculants/additives, requires precise application	Improved digestibility and nutrient preservation compared with conventional ensiling	Low–moderate cost (additive-dependent)	Feasible at farm-scale, also used in industrial silage production	[12,24,45,46,47]
Fermentation/biotransformation	Improves antioxidant status, feed efficiency, and meat quality, can yield feed-relevant outputs (microbial protein, enzymes) alongside industrial products (ethanol, organic acids)	Effects vary with substrate and conditions, may reduce non-fibre carbohydrates, industrial-scale biotransformation competes with feed use for the same pomace feedstock	Improved bioactive compound release, but some carbohydrate losses during fermentation	Low–moderate for basic fermentation; higher for industrial biotransformation	Basic fermentation suitable at farm-scale, biotransformation/ enzyme production is mainly industrial	[19,49,50,52,55]
Chemical preservation	Effective microbial control, extends storage stability	Requires additional safety, residue, and regulatory consideration; consumer acceptance concerns with synthetic preservatives/irradiation	Preserves anthocyanins, bioactive compounds and antioxidants, especially combined with irradiation	Low–moderate chemical cost; low energy demand	Mainly industrial, where regulatory approval for feed-grade preservatives is in place	[53,54,61]
Pelleting	Improves handling, transport, storage, and physical feed quality/durability	Increased processing energy, excessive fines may reduce effective fibre length and contribute to ruminal acidosis risk, depending on diet fermentability and management	Generally neutral to positive if not over-processed; binders (e.g., molasses) improve pellet quality	Moderate–high cost and energy demand	Mainly industrial-scale feed manufacturing	[57,58,59,60]

Note: Entries under ‘Advantages’, ‘Disadvantages’ and ‘Effect on nutrient value’ are drawn from the cited studies. Entries under ‘Cost/energy demand’ and ‘Farm scale vs. industrial suitability’ are qualitative appraisals by the authors, derived from the equipment, throughput and energy characteristics reported in those studies; no comparative techno-economic analysis of these processes applied specifically to fruit pomace intended for livestock feed was identified in the searched literature, and these columns should be read as reasoned judgment rather than as quantitative evidence.

based on their advantages, disadvantages, effects on nutrient value, cost and energy demand, and suitability for farm-scale and industrial applications.

3.6. Whole-Chain Economic Feasibility

The comparison above addresses the technical performance of individual preservation processes. Whether fruit pomace actually reduces feeding cost, however, is determined at the level of the whole supply chain rather than at the level of the processing step, and the two questions should not be conflated. Fresh pomace leaves the processing plant at approximately 70–85% moisture, so a large fraction of the mass transported is water; the cost per unit of dry matter delivered therefore rises steeply with distance, and beyond a threshold haulage distance the ingredient ceases to be competitive with conventional fibre and energy sources irrespective of its nutritional merit. Dewatering or drying before transport reduces this burden but transfers the cost to the energy account, and drying has been identified in life-cycle analyses of agro-industrial residue valorisation as the dominant contributor to overall environmental burden [62]. Storage introduces a third cost centre: because pomace generation is tied to discrete harvest and pressing seasons, year-round use requires either preserved stock, with its associated dry-matter and quality losses, or a feeding strategy that accepts seasonal inclusion. Spoilage losses between generation and use are a direct deduction from the nutritional value ultimately delivered and are greatest where handling and storage capacity are weakest.

Two further factors are frequently omitted from cost comparisons. The first is the spatial relationship between processing plants and livestock operations. Apple, citrus and grape processing is geographically concentrated, and the economics of pomace feeding are correspondingly favourable close to processing regions and unfavourable at a distance so that a single national or global statement about feasibility is not meaningful. The second is competition for the same feedstock: pomace is also a substrate for anaerobic digestion, composting, insect rearing, pectin and polyphenol extraction, and industrial fermentation for enzymes, organic acids and ethanol [52,63]. Where these alternative uses command a higher price, pomace is not a free or low-cost residue but an ingredient carrying an opportunity cost determined outside the feed market.

The evidence base for these considerations is weak, and this should be stated plainly rather than assumed away. No study identified in the searched literature reports a complete techno-economic assessment of apple, citrus or grape pomace as a livestock feed ingredient that jointly accounts for collection, transport, dewatering or drying, storage, spoilage and opportunity cost. Statements in the literature that fruit pomace reduces feeding costs rest principally on the ingredient’s low or zero purchase price at the factory gate, which is not equivalent to a delivered cost advantage. Accordingly, the economic case for fruit pomace feeding is presented throughout this review as plausible and locally demonstrable rather than as established, and the development of transparent, region-specific techno-economic models is identified in Section 8 as a research priority.

4. Nutritional Composition of Fruit Pomaces

4.1. Nutritional Variability of Apple, Citrus and Grape Pomaces

A major practical challenge in utilising apple, citrus, and grape pomace as feed ingredients is the substantial compositional variability reported across studies, which limits the precision of any single “recommended” inclusion level. This variability arises from several interacting sources specific to each pomace type. For citrus pomace, physicochemical composition depends on cultivar, cultivation method, harvesting time, ripening stage, and the juice-extraction technique used, with dried citrus pomace composition reported to range from 30 to 40% for sugars and 14–25% for pectin depending on these factors [11,64]. For grape pomace, seed and skin fractions differ substantially in polyphenol content: seed extracts consistently show higher total polyphenols and condensed tannins than skin extracts (144–298 mg GAE/g d.w. for skins versus 327–540 mg GAE/g for seeds), so the seed-to-skin ratio remaining after pressing directly shifts pomace-level polyphenol values, independent of cultivar [65]. This composition also varies with cultivar, geographic origin, climatic conditions (“vintage effect”), and winemaking technique (white versus red vinification, maceration duration) [65]. For apple pomace, processing technology has a particularly large effect on fibre values: comparing unwashed versus washed apple pomace from the same source, acid detergent fibre (ADF) increased from 30.5% to 43.4% and neutral detergent fibre (NDF) from 40.5% to 54.8% because washing removes residual soluble sugars and pulp, concentrating the structural fibre fraction that remains [66]. Because of this variability, single reference values for NDF, ADF, lignin, sugars, pectin, or polyphenol content in apple, citrus, or grape pomace should be interpreted as indicative rather than fixed, and inclusion-level recommendations derived from one study’s material may not transfer reliably to pomace from a different cultivar, extraction process, or storage history. This has direct practical implications: feed formulation based on these pomaces ideally requires batch-level compositional analysis, particularly of fibre fractions, moisture and polyphenol content, rather than reliance on the literature averages, and the inclusion thresholds discussed throughout this review should be understood as ranges contingent on the specific pomace source and processing history used in each cited study, rather than universal constants.

4.2. Nutritional Profile of Selected Fruit Pomaces

The nutritional value of fruit pomaces varies considerably depending on their nutrient profiles, dietary fibre properties, and the bioactivity of their compounds. Among commonly used fruit pomaces, citrus, apple, and grape pomaces play a vital role, as they contain dietary fibre, fermentable carbohydrates, minerals, and antioxidants that may positively affect gut health, digestion, and antioxidant status in farm animals. Protein content is a key limiting factor across all three pomace types, though to differing degrees: apple pomace has the lowest and most consistently limiting crude protein content (1.1–6.2% DM), grape pomace has the highest and most variable range (3.6–14.2% DM), and citrus pomace falls in between (5.1–6.6% DM) (

Table 2. Nutritional composition of selected fruit pomaces.

Nutritional Components	Basis	Citrus Pomace (%)	Apple Pomace (%)	Grape Pomace (%)	Analytical Basis of Reported Values
Dry matter	% fresh matter	87.1–94.5	16.5–26.4	60.3–90.2	Oven drying, 105 °C (AOAC 934.01)
Crude protein	% dry matter	5.1–6.6	1.1–6.2	3.6–14.2	Kjeldahl N × 6.25 (AOAC 984.13)
Ether extract	% dry matter	0.9–4.7	0.9–3.7	1.1–13.9	Soxhlet extraction (AOAC 920.39)
Crude fibre	% dry matter	10.3–15.7	4.2–33.5	5.3–20.6	Weende crude fibre (AOAC 962.09)
Ash	% dry matter	2.9–7.5	0.4–4.3	1.7–9.1	Incineration, 550 °C (AOAC 942.05)
Soluble fibre (pectin)	% dry matter	15.8–19.6	1.5–19.8	0.7–12.8	Enzymatic–gravimetric soluble dietary fibre (AOAC 991.43); pectin by galacturonic acid assay in some sources
Total insoluble fibre	% dry matter	14.7–41.7	77.8–89.2	16.4–63.7	Enzymatic–gravimetric insoluble dietary fibre (AOAC 991.43) and Van Soest NDF-derived estimates
Cellulose	% dry matter	12.4–17.3	3.6–42.4	15.2–22.3	Van Soest, calculated as ADF–ADL
Hemicellulose	% dry matter	6.3–15.2	4.3–24.4	10.2–24.6	Van Soest, calculated as NDF–ADF
Lignin	% dry matter	2.2–3.3	3.37–6.2	36.4–53.6	Mixed basis: Van Soest acid-detergent lignin (ADL) and Klason/acid-insoluble lignin; Klason values are systematically higher
Total fibre	% dry matter	33.8–46.9	82.2–90.4	17.28–88.7	Enzymatic–gravimetric total dietary fibre (AOAC 991.43); some sources report NDF as a proxy

Compiled from [75,76,77,78,79,80,81,82,83,84,85,86,87,88,89]. Note: Values are the minimum and maximum reported across the cited sources and are presented as literature ranges, not as measurements obtained under a common protocol. The analytical basis of each row is stated in the final column, because the fibre fractions were determined by three non-equivalent systems: Weende crude fibre, Van Soest detergent fractionation (NDF, ADF, ADL) and AOAC enzymatic–gravimetric dietary fibre. Values obtained by different systems are not interconvertible and should not be compared across rows or summed. Direct quantitative comparison between pomaces is valid only within a row whose analytical basis is singular and stated; the two rows flagged as mixed basis (total insoluble fibre and lignin) should be read as indicative of order of magnitude only. Central values (means or medians) are deliberately not reported: the underlying studies differ in cultivar, geographic origin, extraction technology, washing and drying regime, and analytical method, and an arithmetic mean computed across such heterogeneous material would convey a precision the literature does not support. Where a representative value is required for diet formulation, batch-level analysis of the specific material is recommended in preference to any of the literature’s central values.

). This means apple pomace in particular cannot be used as a sole feed source and requires protein supplementation, whereas grape pomace, depending on the specific source and processing method, may approach levels adequate to partially offset this limitation. Fibre fraction composition also differs substantially, although this comparison must be made with care because the reported values derive from different analytical systems, as indicated in the analytical-method column of Table 2. When the comparison is restricted to studies reporting Van Soest detergent fractions, apple pomace combines a high proportion of soluble pectin with moderate acid-detergent lignin, and citrus pomace combines high soluble pectin with very low acid-detergent lignin. Grape pomace is consistently reported as the most lignified of the three, but the very high values quoted in the literature (36.4–53.6% DM) derive predominantly from Klason or acid-insoluble lignin determinations, which include condensed tannins and other acid-insoluble polyphenols and therefore return substantially higher figures than acid-detergent lignin measured on the same material; the numerical gap between grape pomace and the other two pomaces in Table 2 consequently overstates the true difference in lignin content, and the ranges should not be subtracted or otherwise compared arithmetically. The direction of the difference is nonetheless consistently supported across analytically matched studies: grape pomace fibre is the least digestible of the three, and its greater lignification, together with its condensed tannin content, is the principal reason, notwithstanding its higher total fibre content. Apple pomace is distinguished by its high level of total dietary fibre and fermentable carbohydrates. This type of pomace contains considerable amounts of pectins and structural polysaccharides, including galacturonic acid, arabinose, galactose, cellulose, and hemicellulose [67,68]. A high amount of pectin provides good water-holding and fermentability properties, making this kind of pomace a valuable source of dietary fibre for both ruminants and non-ruminants. Furthermore, apple pomace contains compounds that can be classified as antioxidants [69,70]. Citrus pomace contains a moderate amount of crude protein and ether extract; however, this feed ingredient is most valued for its high levels of soluble fibre and pectins. Citrus fibres have high water- and oil-binding capacities, which may be useful for enhancing gut performance and digesta traits [71,72]. Pomace from citrus fruits is rich in fermentable carbohydrates and bioactive compounds such as the flavonoids hesperidin and naringin, along with essential oils dominated by d-limonene, conferring antioxidant and antimicrobial activities. However, the presence of components such as d-limonene and moisture in citrus pomace necessitates processing before it is incorporated into animal feeds. The effect of d-limonene, however, is dose-dependent and should not be characterised simply as a processing obstacle. At low concentrations, d-limonene contributes antimicrobial and antioxidant properties that may support gut health and help control pathogenic microflora. At higher concentrations, however, d-limonene can inhibit rumen microbial activity and fermentation, and its strong aroma may reduce feed palatability and voluntary intake, particularly when fresh, unprocessed citrus pomace is included at high dietary inclusion levels. This dose-dependent duality means that d-limonene content, which varies with citrus cultivar, processing method, and the proportion of peel versus pulp in the pomace, is an important factor to monitor and manage rather than a uniformly negative attribute. Processing methods such as drying or ensiling reduce d-limonene concentration through volatilisation, which helps mitigate its inhibitory effects at high inclusion levels while still preserving lower, potentially beneficial concentrations. The pomace from grapes differs from that of apples and citrus fruits due to higher levels of lignin, lipids, and polyphenols. The pomace of grapes has considerable amounts of insoluble fibre consisting mainly of lignin and cellulose as well as phenolic compounds such as anthocyanins (e.g., malvidin-3-glucoside), catechins, and quercetin [73,74]. These compounds provide potent antioxidant action and may have beneficial effects on rumen fermentation, oxidative stability, and meat quality. Excessive levels of lignin and tannin may decrease the digestibility of nutrients in feed when pomace from grapes is used in high concentrations. Grape pomace tends to have higher crude protein and ether extract content than apple or citrus pomace, but its substantially higher lignin content means its fibre is less digestible than that of apple or citrus pomace. The appropriate comparison depends on which nutritional attribute is under consideration—protein and fat supply versus fibre digestibility—and on the animal species involved, since ruminants tolerate lignin-rich fibre through fermentative digestion far better than non-ruminants, which are more limited by high-lignin fibre.

These substantial differences in chemical composition among fruit pomaces directly affect their suitability as livestock feed and support the case for pomace-specific, rather than generalised, feeding recommendations. Table 2 compares the nutritional composition of citrus, apple, and grape pomaces.

4.3. Bioactive Compounds and Anti-Nutritional Factors of Selected Fruit Pomaces

The use of fruit pomace as a feed ingredient has become common due to its abundance of bioactive substances, which can have various physiological effects on animals, including improving antioxidant status, immunity, gut health, and the quality of animal products. Nevertheless, anti-nutritional factors contained in fruit pomace can limit its nutritional value. Condensed tannins can bind dietary protein and reduce protein and amino acid digestibility, whereas lignin, as an indigestible structural polymer, physically limits access of digestive enzymes to encapsulated nutrients and fermentable fibre. Pectin, though generally beneficial as a soluble fibre source that supports gut fermentation, can increase digesta viscosity at high inclusion levels, potentially impairing nutrient absorption in monogastrics [90,91]. Essential oils, present notably in citrus pomace, contribute antimicrobial and antioxidant properties but may also affect palatability and feed intake if included at excessive concentrations. These structural and phytochemical antinutrients are intrinsic properties of fruit pomace. Fruit pomace is additionally subject to safety-related risks common to agricultural by-products, including mycotoxins, pesticide residues, heavy metals and microbiological spoilage; because these determine whether the material is fit to enter a diet at all, rather than how well it performs once included, they are treated separately in Section 4.4. Grape pomace is particularly rich in polyphenolic compounds such as resveratrol, catechins, and anthocyanins [92]. Citrus pomace contains substantial amounts of carotenoids, terpenes, limonoids and flavonoids, particularly hesperidin and naringin, together with essential oils and pectin [93]. Apple pomace contains hydroxycinnamic acids, flavan-3-ols, dihydrochalcones, dehydroascorbic acid, carotenoids, chlorogenic acid, epicatechin, phloridzin, quercetin and catechins [69,70,84], which have been associated with antioxidant and immunomodulatory effects in animals [94].

Table 3. Major bioactive compound classes in fruit pomaces by type.

Pomace Type	Compound Class	Representative Compounds	Reported Physiological Effects
Grape	Non-flavonoid polyphenols	Resveratrol	Antioxidant; may partially replace synthetic antioxidants in diets
	Flavan-3-ols/condensed tannins	Catechins, proanthocyanidins	Antioxidant; at high concentrations (especially seed fraction) may reduce protein/fibre digestibility
	Anthocyanins	Malvidin, cyanidin derivatives (concentrated in skin)	Antioxidant; contribute to meat oxidative stability and pigmentation
Citrus	Flavonoids	Hesperidin, naringin	Antioxidant, anti-inflammatory; supports immune responsiveness
	Carotenoids	β-cryptoxanthin, β-carotene	Antioxidant; contribute to yolk/meat pigmentation
	Limonoids/terpenes	Limonin, nomilin	Antioxidant; bitter compounds affecting palatability at high levels
	Essential oils	D-limonene	Antimicrobial, antioxidant; may reduce palatability/intake if excessive
	Structural polysaccharide	Pectin	Soluble fibre; supports beneficial gut fermentation; may increase digesta viscosity at high inclusion
Apple	Hydroxycinnamic acids	Chlorogenic acid	Antioxidant, immunomodulatory
	Flavan-3-ols	Epicatechin, catechins	Antioxidant
	Flavonols	Quercetin	Antioxidant, anti-inflammatory
	Dihydrochalcones	Phloridzin	Antioxidant; reported to modulate glucose metabolism
	Vitamins	Dehydroascorbic acid	Antioxidant precursor
	Carotenoids	Lutein, β-carotene (minor fraction)	Antioxidant; pigmentation

Compiled from [21,69,70,84,90,91,92,93,102].

summarises these major bioactive compound classes by pomace type. A clear distinction should be maintained between the presence of a bioactive compound in the pomace and the effective biological exposure of the animal to that compound. The analytical polyphenol concentration of an ingredient does not by itself establish physiological efficacy: the compound must first be released from the feed matrix, then survive or be transformed during gastrointestinal transit, undergo ruminal or hindgut microbial metabolism, and be absorbed either intact or as a metabolite before any systemic effect is possible. Bioavailability along this chain is highly variable and depends on the degree of polymerisation, the feed matrix, the processing method applied and interactions with dietary fibre and protein; condensed tannins in particular are poorly absorbed and act predominantly within the digestive tract rather than systemically. The mechanistic pathways most often invoked for pomace polyphenols, including direct radical scavenging, antimicrobial action, modulation of gut microbial composition, immune regulation and signalling through the Nrf2 and NF-kB axes, are supported to markedly different degrees. Antimicrobial and microbiota-modulating effects have been observed directly in target livestock species [50,81,95], and changes in systemic antioxidant markers have likewise been measured in vivo [48,96,97]. Transcription-factor-level mechanisms such as Nrf2 or NF-kB modulation are, by contrast, largely inferred from in vitro systems and laboratory-animal models rather than demonstrated in cattle, sheep, pigs or poultry fed pomace, and are therefore presented here as plausible hypotheses consistent with the observed phenotypes rather than as established mechanisms in livestock species. Therefore, the nutritional effects of fruit pomaces depend heavily on the type of pomace, processing method, and dietary inclusion level. Several processing strategies have been developed to improve the feeding value of fruit pomaces and reduce the impact of anti-nutritional compounds. Enzymatic treatments using cellulases, hemicellulases, ligninolytic enzymes, pectinases, and related enzymes can improve fibre degradation and nutrient availability [98], and multi-enzyme supplementation of apple-pomace diets has been associated with improved feed conversion ratio and nutrient digestibility in broilers [99]. Fermentation techniques, particularly solid-state fermentation, are also effective in reducing tannin and lignin contents while increasing the bioavailability of beneficial compounds and improving digestibility. These compositional improvements have translated into measurable performance responses in several feeding studies, although the species and pomace type differ between them and the findings should not be merged into a single statement about broilers. In broilers, fermented citrus pomace improved growth performance, antioxidant capacity, fatty acid profile and meat quality [49], and fermented sweet orange pulp fed with or without a multi-enzyme preparation maintained normal haematological status while increasing white blood cell counts [100]; in broiler chicks, fermented grape pomace improved antioxidant status and intestinal morphology relative to the raw material [50]. In ruminants, fermented apple pomace increased plasma antioxidant activity and promoted rumen papillae development in lambs [48]. In pigs, fermented apple pomace improved plasma biochemical and antioxidant indicators and modified faecal microbiota in weaned animals [101]. The consistent element across these studies is that fermentation improves the response relative to the unfermented material of the same type; the magnitude and the specific endpoints affected are species- and substrate-dependent. In addition, maintaining appropriate dietary inclusion levels is essential for optimising animal performance and minimising adverse effects.

4.4. Feed Safety, Contaminants and Quality Control

Fruit pomaces are agricultural by-products, and their use in livestock diets raises safety questions that are distinct from their nutritional evaluation. These questions should be settled before, rather than after, a recommendation for widespread use, but the evidence available to settle them is considerably less than that found in the nutritional literature. Mycotoxins are the most immediate concern, because pomace is a moist, sugar-rich substrate that supports fungal growth from the moment of pressing. Apple pomace derives from a crop susceptible to Penicillium expansum and to contamination from associated mycotoxin patulin, while grape pomace derives from a crop susceptible to Aspergillus and Penicillium species producing ochratoxin A. Direct measurement in the pomace itself, as distinct from the parent fruit or the finished beverage, is nonetheless scarce. A systematic review of mycotoxin occurrence in food-industry by-products identified grape pomace as the only fruit-derived by-product for which occurrence data were available, reporting ochratoxin A in 12 of 13 samples but at trace concentrations (mean 0.07 µg/kg), and identified no eligible occurrence data for apple pomace or citrus by-products [103]. The appropriate conclusion is not that fruit pomaces are safe with respect to mycotoxins, but that the occurrence data required to support such a statement have not yet been generated. Risk is in any case governed less by the raw material than by handling: delay between pressing and preservation, incomplete drying, and aerobic exposure during storage or feed-out are the conditions under which fungal proliferation and toxin formation occur, and these are precisely the conditions most likely to arise in the on-farm use of a seasonal, high-moisture by-product. Pesticide residues represent a second route of carry-over. Residues applied to the parent crop concentrate preferentially in the peel and seed, which are the fractions that dominate pomace, so residue concentrations in pomace may exceed those in the juice or in the whole fruit. In the European Union, maximum levels for undesirable substances in feed materials, including heavy metals and certain pesticide-related compounds, are established by Directive 2002/32/EC and its subsequent amendments [61], and comparable frameworks operate in other jurisdictions; compliance rests with the feed business operator placing the material on the market, which is a material consideration where pomace moves informally from a processing plant to a neighbouring livestock enterprise. Heavy metals, principally cadmium and lead, may accumulate from soil, irrigation water or atmospheric deposition and likewise concentrate in the peel fraction. Grape pomace has been investigated as a biosorbent for metal ions, which indicates a binding affinity that is advantageous in effluent treatment but that also raises the question of accumulation within the material when the parent crop is grown or the pomace stored in contaminated environments.

Microbiological quality constitutes a third component. Inadequately preserved pomace supports yeasts, moulds and spoilage bacteria; aerobic instability in silage and mould growth in under-dried material reduce hygienic quality, cause dry-matter loss and may generate mycotoxins in situ. Preservation practice therefore serves a safety function as well as a nutritional one, and the choice between drying and ensiling should be evaluated against both criteria. Processing modifies these risks in both directions: drying arrests fungal growth but does not destroy mycotoxins already formed, most of which are heat-stable, whereas well-managed anaerobic ensiling with a rapid pH decline suppresses mould development, though poorly sealed or slowly acidifying silage may aggravate it. No process should be assumed to act as a decontamination step in the absence of direct evidence that it does so. The practical implication is that fruit pomace should be treated as a feed material requiring routine quality control rather than as an inherently benign residue. A proportionate control programme would comprise supplier-level knowledge of the parent crop’s plant-protection programme; batch-level verification of dry matter and preservation quality; periodic analysis for the mycotoxins relevant to the specific fruit, namely patulin for apple and ochratoxin A for grape, and for cadmium and lead; rejection of visibly mouldy, overheated or malodorous material; and documented storage conditions and shelf life. Generating occurrence data for apple and citrus pomaces, and carry-over data for milk, meat and eggs, is identified in Section 8 as a research priority. Until such data exist, the statement that fruit pomaces are safe feed ingredients is not supported by direct evidence and should be replaced by the more defensible statement that no substantial contamination has been demonstrated in the limited material examined to date.

5. Fruit Pomaces in Ruminant Nutrition

5.1. Rumen Fermentation and Microbial Modulation

Fruit pomaces can greatly affect rumen fermentation because they contain fermentable carbohydrates, fibre, and bioactive substances that affect microflora growth and the fermentation process. However, the effects of pomaces depend on the differences in carbohydrate content, lignification, and polyphenolic content. Citrus pomaces affect rumen fermentation more consistently than other types of pomaces owing to high amounts of pectin and fermentable carbohydrates in this type of pomace. These effects are strongly dependent on the inclusion rate and the ingredient that is replaced. In Barki goats, replacing wheat straw with orange waste silage at increasing levels (0, 500, and 1000 g/kg DM) produced a linear increase in ruminal acetate concentration (p = 0.012), along with linear changes in propionate and butyrate proportions (p < 0.01) and a linear decrease in blood urea nitrogen, indicating improved nitrogen utilisation as inclusion level increased [17]. In feedlot cattle, ruminal fistulation studies confirm this dose-dependency for grape pomace: increasing dietary inclusion from 0% to 15% to 30% DM altered total-tract nutrient digestibility and the nitrogen route of excretion without affecting total ruminal short-chain fatty acid concentration [18]. In vitro, grape marc inclusion at 10% and 20% DM reduced ammonia-N (NH₃-N) concentration in a dose-related manner [104], further supporting the conclusion that fermentation and nitrogen metabolism outcomes scale with pomace inclusion level rather than occurring as a fixed effect. In a direct comparison in Angus steers, dried citrus pulp and dried grape pomace were each included at 150 g/kg DM (15% of diet DM) as alternative fibre sources. The dried citrus pulp diet was characterised by higher pectin, sugar, and non-fibre carbohydrate content and greater 24 h and 48 h in vitro NDF digestibility, while the dried grape pomace diet had substantially higher proanthocyanidin and total tannin content [105], directly illustrating the compositional trade-off between the two pomace types at a matched, moderate inclusion level. In addition to stimulating fibre digestion, apple pomace increases rumen fermentation owing to the significant amount of fermentable carbohydrates and pectin contained in this ingredient. Variations in fermentation responses are primarily associated with different processing techniques, fibre composition, and inclusion rate. The effects of condensed tannins on rumen fermentation are similarly dose-dependent, and moderate and excessive levels should be distinguished. At the moderate inclusion levels used in the reported cattle and dairy studies, condensed tannins have been associated with beneficial effects, including a shift in nitrogen excretion from urine to faeces (a desirable outcome, since urinary nitrogen contributes more to reactive nitrogen emissions than faecal nitrogen) and reductions in enteric methane emissions, attributed to tannin-driven changes in the ruminal microbiome [106,107]. In contrast, at higher condensed tannin concentrations, these compounds inhibit the activity of cellulolytic and proteolytic microorganisms, reducing fibre and protein degradation and potentially depressing digestibility and intake. This distinction indicates that moderate tannin inclusion can be nutritionally advantageous, whereas excessive levels are inhibitory, underscoring the importance of inclusion rate rather than tannin presence alone in determining the net effect on rumen fermentation. These effects are not uniform across pomace types and depend on their distinct compositions. Citrus pomace is a fermentable, pectin-rich energy source; apple pomace is a variable fibre or pectin source with low protein; and grape pomace is polyphenol-rich but higher in lignin and tannins. High tannin, lignin, moisture, essential oil, or fibre content can reduce intake, digestibility, or performance, so these benefits should not be generalised across pomace types. Applying the appraisal framework set out in Section 2.4, the pectin-driven fermentative response to citrus pomace is consistently supported; the dose-dependence of the grape pomace tannin response, beneficial at moderate and inhibitory at high inclusion, is conditionally supported and reproducible once inclusion level and basal diet are specified; and the fermentative contribution of apple pomace rests on a smaller number of studies with more variable material and is therefore treated here as limited evidence rather than an established effect.

5.2. Nutrient Digestibility and Feed Utilisation

The digestibility and nutritive value of fruit pomaces are significantly affected by fibre constituents, lignification, and the processing/preservation method applied; dried, ensiled and fermented forms are not nutritionally equivalent, even when derived from the same raw pomace. Among the different types of pomaces, citrus pomace has been found to be the most digestible owing to its higher proportion of pectin and lower quantity of lignin. The addition of citrus pomace enhances the digestibility of dry matter, organic matter and fibre, making it possible for citrus pomace to partly replace cereals or concentrates; this has been demonstrated directly in steers [105] and in grazing Jersey cows in which dried citrus pulp replaced maize grain in the concentrate [108], and is consistent with the earlier synthesis of citrus by-product feeding [51]. Comparisons of dried versus ensiled citrus pomace indicate that ensiling generally preserves acceptable digestibility relative to the dried form, though some reduction in energy value may occur with ensiling, depending on the preservation conditions. The fermentable nature of the carbohydrates in apple pomace is another factor that leads to more efficient energy utilisation. Apple pomace has moderate but variable digestibility [91]. The processing technique will also affect the feeding value. Dried apple pomace has higher digestibility and metabolisable energy than ensiled apple pomace [44], as ensiled apple pomace may contain ethanol depending on fermentation quality, since apple pomace’s high sugar content can support yeast activity alongside the intended lactic acid bacterial fermentation when ensiling conditions are suboptimal; this ethanol formation reduces the energy available to the animal [109]. Overall, these comparisons indicate that the relative digestibility ranking of dried, ensiled, and fermented pomace forms is not consistent across pomace types: dried forms are generally favoured for apple pomace, ensiling is broadly effective with modest trade-offs for citrus pomace, and microbially inoculated ensiling appears specifically advantageous for grape pomace due to its distinct fibre-tannin composition.

5.3. Milk Production, Composition, and Fatty Acid Profile

Fruit pomaces can affect milk production, milk composition and milk fatty acid profile by acting on rumen fermentation, nutrient availability and antioxidant status. These are distinct outcomes with distinct determinants, and an improvement in one does not imply an improvement in the others; they are therefore reported separately below rather than aggregated under the heading of improved production. Citrus pomace mainly supports milk production by improving rumen fermentation and energy availability. In a trial using forty lactating Holstein cows (550 ± 50 kg body weight, ~20 kg daily milk yield at baseline), dried orange pulp fed at 0, 25, 50, and 75% substitution of yellow corn grain improved nutrient digestibility, blood metabolic parameters, and plasma antioxidant capacity, and reduced pathogenic faecal bacteria counts without adverse effects on production [75]. Replacing cereal grains with citrus pomace can also improve the fatty acid composition of milk and its antioxidant capacity, as reported in lactating dairy goats supplemented with dried orange pulp throughout a full lactation period [110]. In addition, citrus by-products such as lemon by-product silage, evaluated in vitro as a partial replacement for lucerne, can improve silage fermentation and digestibility characteristics relevant to feed preservation [111]. In early-lactation dairy cows, ensiled apple pomace included at 15% or 30% of dietary dry matter intake, in a 21-day-period crossover trial, increased standardised (4% fat) milk yield by approximately 9% and elevated milk fat and protein content at the higher inclusion level, compared with a pasture silage and concentrate control diet [112]. Moreover, the polyphenolic compounds in apple pomace may improve the antioxidant properties of milk [113]. Apple pomace has also been used successfully in silage systems, where it improves fermentation characteristics and digestibility [114]. Unlike citrus and apple pomaces, grape pomace has a greater effect on the milk fat content and oxidative stability of milk than on milk production. Feed supplementation with grape pomace has been associated with increased concentrations of polyunsaturated fatty acids (PUFAs), monounsaturated fatty acids (MUFAs), and conjugated linoleic acid (CLA) in milk, together with enhanced oxidative stability of dairy products [74,115,116]. These effects are mainly attributed to grape polyphenols, which modify ruminal lipid biohydrogenation pathways. However, these benefits should be interpreted alongside the condensed tannin content of grape pomace, which at higher dietary inclusion levels can bind dietary and microbial protein, reduce feed intake, and depress fibre digestibility; the net effect on milk yield and composition therefore depends on the balance between the beneficial modulation of ruminal biohydrogenation and any tannin-associated reduction in intake or digestibility at the specific inclusion level used, underscoring that grape pomace’s milk-quality benefits are not necessarily accompanied by production benefits. When these studies are synthesised, the conditions under which milk responses differ can be stated more precisely than a single summary effect would allow. Milk yield responses are observed principally where the pomace substitutes for a lower-energy component of the diet and thereby raises fermentable energy supply, as with dried orange pulp replacing part of the cereal fraction [75,108] or ensiled apple pomace replacing pasture silage and concentrate in early lactation [112]; where the pomace substitutes for an ingredient of comparable energy density, yield responses are small or absent. Milk composition responses, principally fat and protein percentage, track the acetate-to-propionate balance in the rumen and are therefore most evident with the pectin-rich pomaces at moderate inclusion. Milk fatty acid and antioxidant responses follow a different mechanism entirely, arising from polyphenol-mediated modification of ruminal biohydrogenation and from the transfer of antioxidant capacity to the milk, and are consequently most pronounced with grape pomace, which improves milk quality attributes without a corresponding yield benefit [74,115,116]. Lactation stage, basal diet forage-to-concentrate ratio, pomace processing form and trial duration further modify these responses: most published trials are short, frequently of crossover design over 21-day periods, and cannot establish whether responses persist across a full lactation. Within the framework of Section 2.4, the milk fatty acid and oxidative-stability responses to grape pomace are consistently supported, the yield responses to citrus and apple pomace are conditionally supported and contingent on what the pomace replaces, and evidence for persistence over a full lactation remains limited.

5.4. Meat Quality, Fatty Acid Composition, and Oxidative Stability

Fruit pomaces could enhance meat quality and oxidative stability through effects on antioxidant status and lipid metabolism. Apple pomace, in particular, has been shown to reduce lipid oxidation and enhance the antioxidant profile of meat without deteriorating its physical and chemical characteristics [117]. In lambs, citrus pulp combined with wheat straw silage improved growth performance and carcass and meat quality traits [118]. In finishing cattle, grape pomace inclusion improved fatty acid composition, increasing concentrations of PUFA and CLA in meat, alongside favourable effects on carcass traits [119]. In lambs, dietary grape pomace supplementation affected growth performance, carcass traits and meat quality [96]. In Dorper × Hu hybrid sheep, grape pomace supplementation improved the serum biochemical and antioxidant profile and the amino acid composition of muscle, and altered rumen microbial diversity, without negatively affecting growth performance [120]. Grape pomace is considered the most effective for modifying the fatty acid content and oxidation of meat due to its high levels of polyphenols and tannins. The use of grape pomace helps to boost the concentration of PUFA, CLA, and n-3 fatty acids and prevent lipid oxidation [96,119]. The effects of grape pomace on the sensory qualities of meat products have been proposed, but they have not been consistently confirmed through validated sensory-panel studies; therefore, this potential benefit should be regarded as preliminary pending further investigation.

Excessive feeding levels might adversely affect feed intake and performance due to the high levels of tannins and lignin in grape pomace.

Table 4. Comparative functional characteristics of selected fruit pomaces in ruminant feeding.

Pomace Type	Key Nutritional Components	Effects on Rumen & Animal Performance	Inclusion Levels Tested (Species, Production Stage, Processing Form)
Apple pomace	High pectin, fermentable carbohydrates, dietary fibre, polyphenols; low crude protein; variable lignin	Enhances rumen fermentation and microbial modulation; increases PUFAs and n-3 fatty acids; improves milk yield, fat, protein, and antioxidant capacity; promotes cellulolytic bacteria and rumen health; improves meat oxidative stability; however, digestibility may decrease at higher inclusion due to lignin content and fermentation losses	Dried pomace, dairy cows: 10–25% of diet DM tested, with responses in milk yield and composition reported within this range. Ensiled pomace, beef cows and finishing lambs: 15–30% of DM tested. Lambs (meat quality endpoints): 10–11% of DM tested.
Citrus pomace	High pectin (22–40%), soluble carbohydrates, low lignin, flavonoids, moderate protein, organic acids	Highly digestible energy source; improves rumen fermentation (increased VFAs and acetate, reduced ammonia-N and protozoa) and fibre digestibility; enhances intake, milk yield, and feed efficiency; supports microbial activity and rumen health; improves milk fatty acid profile and antioxidant status; enhances meat quality and oxidative stability	Dried citrus pulp, dairy cows and growing cattle: 15–30% of diet DM tested. Beef and growing cattle, dried pulp replacing cereal grain: up to 20–40% of DM tested. Ensiled orange waste, goats: up to 1000 g/kg of the forage fraction tested.
Grape pomace	High fibre, lignin, polyphenols (tannins, flavonoids), residual lipids, essential fatty acids	Modulates rumen fermentation through tannin–microbe interactions. At the moderate levels tested in cattle and lamb studies, improves nitrogen efficiency by shifting excretion from urine to faeces, reduces enteric methane emissions, and improves milk and meat fatty acid profile (PUFA, CLA, MUFA) and oxidative stability; growth performance was not impaired in these trials. At higher condensed tannin and lignin loads, and with seed-rich or unprocessed material, fibre and protein degradation are inhibited and feed intake and digestibility may decline. The direction of the response depends on inclusion level, the seed-to-skin ratio of the material, and the processing form. Dried, seed-rich pomace at high inclusion is the combination most associated with depressed intake and digestibility; ensiled or microbially inoculated pomace at moderate inclusion is the combination most associated with the beneficial responses above.	Dried pomace, feedlot cattle: 15% and 30% of diet DM tested. Ensiled grape marc, lambs: up to 30% of DM tested. Dairy cows and ewes: lower levels, typically 5–10% of DM, tested for milk quality endpoints. Values are the levels used in the cited experiments; the upper end of this range is the highest level tested.

PUFAs = polyunsaturated fatty acids; DM = dry matter; VFAs = volatile fatty acids; CLA = conjugated linoleic acid; MUFAs = monounsaturated fatty acids. Compiled from [17,48,51,74,75,91,96,105,108,109,110,112,113,115,117,120,121,122,123,124,125,126,127,128,129]. Note: The values in the final column are the dietary inclusion levels at which each reported outcome was actually tested in the cited experiments. They are not optimal levels, recommended levels or demonstrated safety thresholds, and no such thresholds can be derived from the available literature, which does not include dose–response studies designed for that purpose. Levels tested in one species, production stage or processing form should not be transferred to another; because pomace composition varies with cultivar, extraction technology and processing history (Section 4.1), levels should, in any case, be adjusted following batch-level compositional analysis.

provides a comparative summary of selected fruit pomaces, giving their key nutritional components, the reported effects on rumen fermentation and animal performance, and the inclusion levels at which those effects were actually tested in ruminant feeding systems. The table reports tested levels as well as the species, production stage and processing form in which they were tested; it does not identify optimal or maximum safe levels, which would require dose–response studies that have not been conducted for these ingredients.

6. Fruit Pomaces in Non-Ruminant Nutrition

Fruit pomaces have attracted considerable attention in non-ruminant nutrition for their ability to provide dietary fibre, phytochemicals, and antioxidants that enhance digestive health, immunity, meat quality, and sustainability. However, their nutrient composition and physiological activity vary with the type of pomace, preparation methods, and incorporation rate. Species-specific digestive anatomy is a key factor governing these responses: pigs possess a functional hindgut fermentation compartment that supports moderate fermentable-fibre tolerance, poultry have a short gastrointestinal tract with rapid transit and limited hindgut fermentation capacity, and rabbits, as strict hindgut fermenters, tolerate and require substantially higher dietary fibre levels than either pigs or poultry. Citrus pomace is an important fruit pomace commonly used in non-ruminant nutrition, primarily for its role in providing fermentable energy and promoting gut health, owing to its high levels of soluble fibres and pectins. On the other hand, apple pomace is an important fruit pomace for its fermentability and moderate antioxidant activity, while grape pomace stands out owing to its high levels of polyphenols. The benefits of feeding fruit pomaces are more constrained in non-ruminants than in ruminants, since pigs, broilers, and rabbits differ substantially in fibre tolerance and digestive physiology. High fibre, lignin or tannin content can reduce palatability, nutrient utilisation and performance in these species unless mitigated through enzyme supplementation or appropriate processing; therefore, positive effects on gut health, antioxidant status, immunity and meat quality should not be assumed uniformly across non-ruminant species or inclusion levels. The evidence is also unevenly distributed between species, and this is stated here rather than bridged by extrapolation. Broilers account for the majority of published non-ruminant pomace studies; pigs are represented by a smaller but substantive body of work concentrated on weaned and finishing animals; rabbit evidence is confined to a small number of grape pomace trials; and no controlled feeding studies of apple, citrus or grape pomace in aquaculture species were identified in the searched literature. Responses in the poorly represented species are therefore reported as knowledge gaps rather than inferred from poultry or ruminant data, which is particularly important because the physiological basis for fibre and polyphenol utilisation differs between hindgut fermenters, omnivorous monogastrics and rapid-transit avian species.

6.1. Citrus Pomace in Non-Ruminant Nutrition

Citrus pomace has been widely studied as a potential alternative energy source and functional feed component for chickens and pigs. In finishing pigs specifically, dietary inclusion of orange pulp (as dehydrated or ensiled sun-dried citrus by-product) has been associated with reduced ammonia and methane emissions from slurry, reflecting altered manure composition and fermentability rather than a change in the animal’s own enteric or metabolic emissions [20]; this finding should not be generalised to other non-ruminant species (e.g., poultry, rabbits), which have different manure/excreta handling systems and have not been evaluated in this respect. Due to the high levels of soluble carbohydrates and pectin in citrus pomace, it can be used as a partial replacement for cereal-based energy sources in pigs. Orange pulp in dried, ensiled, or sun-dried forms could be used as a replacement for the cereal fraction to reduce ammonia and methane emissions from manure [20]. In poultry, fermented citrus pomace has shown promising effects on growth performance, antioxidant status, and meat quality; feeding 10% fermented citrus pomace (vs. 10% unfermented pomace) to yellow-feathered broilers improved these outcomes relative to unfermented inclusion [49]. Supplementation with 5–10% fermented sweet orange peel, with or without multi-enzyme supplementation, maintained normal haematological parameters while increasing white blood cell counts, suggesting improved immune responsiveness [100]. Separately, incremental (non-fermented) sweet orange peel meal at 2.5–7.5% of the diet improved antioxidant status and humoral immunity without impairing growth or carcass traits [97]. Similarly, inclusion of 7% fermented citrus pomace in broiler diets enhanced antioxidant activity in serum and muscle tissues, improved polyunsaturated fatty acid content, and positively regulated genes associated with lipid metabolism and oxidative stress [130]. Increased serum catalase activity and antioxidant protection have also been reported in broilers fed fermented citrus pomace [49]. In addition, citrus by-products have been shown to affect intestinal morphology and micro-flora composition in non-ruminants: long-term dietary inclusion of citrus pomace increased villus height and the villus-to-crypt depth ratio and altered colonic microbiota in Tibetan pigs [81]. In broiler chicks, fermented pomace diets (e.g., fermented grape pomace) have been reported to influence gut morphology and bacterial populations relative to raw pomace and control diets [50]. Fermented diets also reduce intestinal pH and promote beneficial bacterial populations while suppressing pathogenic microorganisms. These effects indicate that citrus pomace may contribute to improved gut health and intestinal function in non-ruminants.

6.2. Apple Pomace in Non-Ruminant Nutrition

Apple pomace, especially in its dried and fermented forms, has been extensively studied in broilers and pigs for its high levels of soluble fibre and polyphenols. Incorporation of apple pomace into broiler rations at levels up to approximately 5% is commonly recommended as a practical guideline to support acceptable growth performance without detrimental effects [131], though optimal thresholds reported in the literature vary (generally 3–10%) depending on formulation and enzyme supplementation. Its soluble fibre fraction promotes beneficial intestinal fermentation and increases short-chain fatty acid production, which may improve intestinal health and digestive activity [132]. Moderate inclusion levels have also been associated with improved antioxidant status, immune response, and intestinal morphology [133]. However, at higher inclusion levels (typically >12–15%), the high fibre and lignin content of apple pomace can impair nutrient digestibility and depress growth performance. Villus height and crypt depth may also be negatively affected, and these effects are only partially mitigated by exogenous fibre-degrading enzymes. Various approaches, such as fermentation, could improve the nutrient composition of apple pomace and reduce its fibre-related anti-nutritional burden. Solid-state fermentation has been used to increase apparent crude protein content and feed efficiency [19]. However, this increase largely reflects the accumulation of microbial (single-cell) proteins derived from the fermenting organism’s biomass rather than an intrinsic increase in the substrate’s own protein content. Accordingly, the source of nitrogen and the resulting amino acid profile and protein digestibility should be considered when evaluating the nutritional value of fermented apple pomace, rather than relying on crude protein content alone. In addition, apple pomace contains polyphenolic compounds that contribute antioxidant and immunomodulatory effects [94]. In pigs, dietary apple pomace may also reduce environmental pollution by decreasing urinary nitrogen excretion and ammonia emissions from pig housing systems [134].

6.3. Grape Pomace in Non-Ruminant Nutrition

Grape pomace and its related products, including grape skin residue, grape seed and fermented grape residues, contain high levels of polyphenolic antioxidants and fibre, though these fractions differ chemically: grape seed is generally richer in total polyphenols and condensed tannins, whereas anthocyanins are concentrated mainly in the skin of red cultivars, and whole pomace (a mixed fraction of seed, skin, and stem) has an intermediate, more variable profile that is further altered by fermentation. These fractions have been investigated for their roles in chickens, pigs, and rabbits. For instance, the inclusion of 2–6% grape pomace in broiler diets improved antioxidant status, increased the polyunsaturated fatty acid content of meat and improved oxidative stability without impairing growth [135]. Furthermore, grape pomace can modulate gut microbial populations and suppress pathogenic bacteria, including Clostridium perfringens [50]. Moderate dietary supplementation with grape pomace has been reported to improve gastrointestinal morphology, enhance immune responses, reduce pro-inflammatory cytokine expression and promote beneficial intestinal microbiota in pigs [95,136]. Comparable benefits have been reported in rabbits, although the evidence rests on only two studies. Dietary grape pomace improved antioxidant status, fatty acid composition and meat shelf life in fattening rabbits without impairing growth performance [137]. Separately, dehydrated grape pomace improved growth and reproductive performance, including semen traits, in male rabbits [138]. Because these are the only two rabbit trials identified, the rabbit response is treated here as limited evidence rather than an established effect. The beneficial effects of grape pomace are mainly attributed to its high concentrations of flavonoids, stilbenes, and phenolic acids, which act as natural antioxidants and may partially replace synthetic antioxidant additives in animal diets [102]. However, the high levels of lignin and tannins in grape pomace can hinder nutrient digestibility and feed efficiency when used in excess. Consequently, processes such as fermentation and enzyme addition have been used to improve its feed value [139].

Table 5. Comparative functional characteristics of selected fruit pomaces in non-ruminant feeding.

Parameters	Citrus (Orange, Lemon, Tangerine) Fermented	Apple (Dried or Fermented)	Grape (Pomace, Seed, Peel, Fermented)
Inclusion Levels Tested (processing form and species)	Fermented citrus pomace, broilers: 7–10% of diet. Fermented sweet orange pulp with or without multi-enzyme, broilers: 5–10%. Non-fermented sweet orange peel meal, broilers: 2.5–7.5%. Dehydrated or ensiled sun-dried orange pulp, finishing pigs: up to 500 g/kg of the basal diet. Dried citrus pulp as a fibre source, weaned piglets: partial replacement of the fibre fraction.	Dried apple pomace, broilers: approximately 3–5%, with levels up to 10% reported in enzyme-supplemented diets. Dried apple peel waste, heat-stressed broilers: tested as a dietary antioxidant. Solid-state fermented apple pomace, growing and weaned pigs: 4–10%.	Grape pomace, broilers: 2–6%. Grape pomace, extract or fermented forms, finishing pigs: 5–10%; weaned piglets, lower levels. Dehydrated or dried grape pomace, fattening and male rabbits: up to 20%.
Target Species	Broilers	Broilers, weaned pigs	Broilers, finishing pigs, rabbits
Growth Performance	Improved ADG, feed efficiency	Maintains growth, Improved feed efficiency	Improved ADG, feed intake (moderate levels)
Gut Morphology & Microbiota	Improved villus height, V/C ratio; reduced coliforms	Improved villus height, crypt depth, beneficial gut bacteria	Improved villus height (dose-dependent), beneficial bacteria (Lactobacillus, Firmicutes), reduced pathogenic bacteria
Antioxidant/Immune Effects	Improved serum antioxidant activity, catalase	Improved total antioxidant status, immune response, reduced oxidative stress	Improved antioxidant enzyme activity (CAT, SOD, GPx), reduced pro-inflammatory markers (IL-1β, TNF-α)
Meat Quality/Fatty Acids	Improved PUFA content, reduced abdominal fat	Reduced drip loss in meat, Improved protein content	Improved PUFA content, improved oxidative stability, intramuscular fat
Remarks	Fermentation enhances the response relative to unfermented material of the same type; excessive inclusion may reduce growth or feed efficiency. Evidence is concentrated in broilers; the pig data derive mainly from manure-emission and dietary fibre studies and no aquaculture data were identified.	High inclusion may require exogenous enzyme supplementation. Reduces urinary nitrogen excretion and ammonia emission in pigs. Increases in crude protein following solid-state fermentation reflect accumulation of microbial biomass rather than an increase in the substrate’s own protein, so amino acid profile and protein digestibility should be assessed directly.	High lignin and tannin content may reduce digestibility; fermentation or enzymatic treatment is commonly applied. Broiler and pig evidence is reasonably consistent; the rabbit evidence is limited. No aquaculture studies were identified.

ADG = average daily gain; V/C ratio = villus height to crypt depth ratio; CAT = catalase; SOD = superoxide dismutase; GPx = glutathione peroxidase; IL-1β = interleukin-1 beta; TNF-α = tumour necrosis factor alpha; PUFAs = polyunsaturated fatty acids. Compiled from [19,20,21,49,50,81,94,99,101,102,131,132,134,136,137,138,140,141,142,143,144,145]. Note: The values in the first row are the dietary inclusion levels at which each reported outcome was tested, together with the processing form (raw, dried, ensiled, fermented or extracted) and the species in which it was tested. They are not optimal levels, recommended levels or demonstrated safety thresholds. Where evidence for a species is drawn from a small number of trials, this is stated in the Remarks row rather than concealed within a summary range.

presents a comparative summary of selected fruit pomaces used in non-ruminant feeding, giving the processing form and species in which each inclusion level was tested together with the reported effects on animal health and productive performance.

7. Greenhouse Gas Emissions and Environmental Sustainability

Methane production in the rumen is a natural anaerobic process carried out by methanogenic archaea such as Methanobrevibacter, Methanospirillum, and Methanomicrobia, which utilise hydrogen to reduce carbon dioxide to methane. Although methanogenesis helps maintain rumen fermentation balance, it results in a loss of dietary gross energy and contributes significantly to greenhouse gas emissions from livestock systems [146,147]. Consequently, various dietary methane mitigation strategies have been investigated; however, excessive suppression of methanogenesis may negatively affect rumen fermentation and animal productivity [148]. Fruit pomaces have gained attention as sustainable feed resources because of their polyphenol, tannin, flavonoid and fermentable fibre content, which can modify rumen fermentation and nitrogen utilisation and reduce methane and ammonia emissions. The supporting evidence is pomace-specific and should be attributed accordingly: for grape pomace and grape marc, reductions in enteric methane have been measured in lactating dairy cows [107,121] and in dairy ewes [116]; for apple pomace, reductions in ruminal ammonia and in methane emission have been measured in cannulated dairy cows [149]; and for citrus, the evidence is at present mechanistic rather than whole-animal. In vitro, flavonoids extracted from citrus peel showed synergistic inhibitory effects on rumen methanogenesis and ammoniagenesis, associated with shifts in the rumen microbiome [150], and ruminal microbial colonisation of citrus pomace during incubation has been characterised in the same experimental system [129]. Within the framework of Section 2.4, methane mitigation by grape by-products in dairy cattle is consistently supported by in vivo measurement; the antimethanogenic effect of citrus flavonoids is supported only in vitro and should be regarded as an emerging mechanism awaiting confirmation in the intact animal. In vivo evidence corroborates these effects across several ruminant systems. Feeding dried apple pomace (150 g/kg DM) to cannulated dairy cows for 64 days decreased ruminal ammonia concentration and methane emissions while altering biohydrogenation of unsaturated fatty acids [149]. Fruit rinds other than those considered here have shown broadly favourable nutritional characteristics in small ruminant diets [23]; this observation lies outside the defined scope of the present review and is noted only for context. In beef cattle, inclusion of ensiled crimped grape marc affected energy intake and gas emissions, illustrating that whole-animal responses to grape by-products depend on the processing method (ensiling vs. drying) and inclusion level rather than being uniformly antimethanogenic [151]. A measured reduction in enteric methane at the level of the individual animal does not by itself demonstrate a lower whole-system environmental footprint, and the two should not be equated. Drying energy, transport, storage losses, the emissions associated with the conventional ingredient that the pomace displaces, and the alternative uses forgone all affect the net outcome, and several of these act in the opposite direction to the animal-level effect. Life-cycle or farm-system-level evidence specifically quantifying the greenhouse gas impact of feeding fruit pomace, as distinct from laboratory- or animal-level methane measurements, remains limited in the current literature and represents a clear gap for future research linking pomace valorisation to farm-level carbon footprint reduction. Nevertheless, the effectiveness of fruit pomaces as methane-mitigating feed ingredients depends on the inclusion level, processing method, and chemical composition, as excessive supplementation may reduce feed efficiency and energy availability despite lowering methane emissions [148].

The overall environmental sustainability of using fruit pomace as feed also depends on handling between its generation and use. Fresh pomace is highly perishable and prone to microbial spoilage, so it is typically dried or ensiled shortly after generation to prevent nutrient loss; drying is usually the most energy-intensive step, with life-cycle assessments of agro-industrial residue valorisation identifying electrical energy consumption during drying as the dominant contributor to overall environmental burden [62]. Since processing facilities and livestock operations are not always co-located, transport distance and mode further influence the net environmental balance, favouring processing close to the point of generation or pre-transport dehydration to reduce bulk. Comparative life-cycle assessments of alternative valorisation pathways for fruit pomace, including animal feed, anaerobic digestion and insect-rearing substrates, show that the relative environmental performance of the feed-use pathway is highly sensitive to these drying, storage, and transport choices [63], indicating that the nutritional and methane-mitigation benefits described above must be weighed against these logistical energy and emissions costs to determine net environmental benefit.

8. Conclusions and Future Perspectives

Apple, citrus and grape pomaces show potential as partial replacements for conventional fibre and energy ingredients in livestock feeds, yet the available evidence remains fragmented and the responses observed are heterogeneous. Null or adverse effects on intake, digestibility and performance occur, particularly at high inclusion levels and with lignin- and tannin-rich material. Among the three, citrus pomace is consistently the most fermentable and most digestible, although the antimethanogenic potential of its flavonoids has so far been demonstrated only in vitro. Grape pomace carries the highest polyphenol and condensed tannin load, produces the most consistent effects on the fatty acid profile and oxidative stability of milk and meat, and offers the strongest in vivo evidence for enteric methane mitigation in dairy cattle. Apple pomace occupies an intermediate position and is the most compositionally variable of the three.

Four gaps prevent firmer recommendations. First, no dose–response study has established an optimal or maximal inclusion level in any species. Second, most trials are too short to establish whether short-term responses persist over a full production cycle. Third, data on mycotoxins, pesticide residues and heavy metals in pomaces are scarce, and carry-over into milk, meat and eggs remains uncharacterised. Fourth, neither the delivered cost relative to conventional ingredients nor the net whole-system environmental balance has been quantified.

These gaps define the research agenda: standardised processing and compositional reporting to enable quantitative comparison across studies; species-specific, adequately powered dose–response trials of sufficient duration; routine monitoring of mycotoxins, pesticide residues and heavy metals, together with characterisation of carry-over into animal products; and whole-chain techno-economic and life-cycle assessment benchmarked against competing valorisation routes.