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20 March 2026

Composition, Fatty Acids Profile, Antioxidant Capacity and Nutritional Indices of Saanen Goats Milk Fed on Dehydrated Grape Pomace

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1
Department of Animal Science, Federal University of Bahia, Salvador 40170-110, Brazil
2
Campus Agricultural Sciences, Federal University of São Francisco Valley, Petrolina 56300-990, Brazil
3
Department of Food Science, Federal University of Santa Maria, Santa Maria 97105-900, Brazil
4
Department of Food Technology, Federal Institute of Sertão Pernambucano, Petrolina 56314-522, Brazil
Ruminants2026, 6(1), 21;https://doi.org/10.3390/ruminants6010021
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Simple Summary

The use of agri-industrial waste in ruminant feed ensures food security for animals in arid and semi-arid regions, where seasonality in forage production is more pronounced. Another benefit is the reduced inclusion of high-quality foods in the diet, which also competes with human food, as well as the improvement in the quality of their products and derivatives through the inclusion of fatty acids beneficial to health. This study evaluated the effects of dehydrated grape pomace, partially replacing cactus pear, on the quality of milk from Saanen goats. We analyzed data from goats fed increasing levels of grape pomace in their diet and found that pomace inclusion did not affect dry matter intake or milk composition. However, improvements in the fatty acid profile were observed, including reductions in total saturated fatty acids and atherogenic and thrombogenic indices, and an increase in milk’s antioxidant capacity. Our results may reflect improvements in the dairy goat production chain, adding value to the product and its derivatives with a view to the sustainability of activity in partnership with viticulture.

Abstract

Grape pomace is an agri-industrial by-product rich in fatty acids with the potential to be used in diets for goats and increase the nutraceutical properties of milk. This study aimed to investigate the effect of incorporating dehydrated grape pomace (DGP) into the diets of Saanen goats on the composition, fatty acid profile, nutritional indices of fatty acids, and antioxidant capacity of their milk. Eight multiparous Saanen goats, averaging approximately four years of age and weighing 41.2 ± 15.7 kg, were used in a double Latin square (4 × 4) design. Diets were formulated with increasing levels of grape pomace (0, 90, 150, and 210 g/kg Dry Matter—DM), replacing cactus. The data underwent analysis of variance using GLM procedure and regression analysis (both linear and quadratic) using REG procedure at a significant level of 5%. The concentration of C18:2 n-6 cis and C18:3 n-3 increased (+20.1 and +15.5%, respectively) with the grape pomace inclusion. There was a reduction in the atherogenic (−24.0%) and thrombogenic (−9.9%) indices of goat milk with the increase in DGP levels. There was a reduction in de novo fatty acids (−10.5%), Δ−9 desaturase of C14 (−21.8%) and C18 (−9.5%) indexes with the highest level of DGP. There was a quadratic effect for Ferric Reducing Antioxidant Power (FRAP) and a linear effect for phenolic compounds (PC), where the highest values were observed at the estimated levels of 160 g/kg and 210 g/kg DM, respectively. Supplementing dairy goats’ diets with dehydrated grape pomace up to 210 g/kg dry matter enhances the fatty acid profile and nutritional indices of fatty acids of Saanen goat milk without altering its basic composition. As grape production is prevalent in low rainfall regions, pomace may provide an alternative feed in areas with forage production constraints. Additionally, grape residue could establish a link between the wine and dairy sectors for cheese production, expanding markets for farmers.

1. Introduction

Dairy goat farming is commonly practiced in arid and semi-arid regions worldwide, where goat milk is considered a functional food. Some studies point out a lower cholesterol content, better digestibility, and presence of fatty acids in goat milk that can be exploited by the nutraceutical industry because they are easily metabolized by the body compared to raw milk [1]. Cactus pear is commonly used in semi-arid regions in feed ruminants, as it is a food rich in water and energy value, with an average value of 64.47% DM of non-fibrous carbohydrates [2]. However, due to its higher concentration of soluble carbohydrates, it must be associated with a source of fiber in the diet. For this, agri-industrial by-products obtained from processing oilseeds or the fruit industry present potential alternatives for animal feed that do not compete with human food and can help lower feeding costs [3]. The agri-industrial by-products, predominantly generated in substantial volumes by the pulp and juice sectors, presents a significant risk of environmental contamination when not disposed of appropriately [4]. The reintroduction of a potentially disposable component into the production chain add value, either as an alternative feed for animal production or as a fertilizer for crops [5]. Among the by-products used, grape pomace is notable as a source of fiber in dairy ruminants’ diets, primarily used as silage or for dehydration [5,6].
This product, derived from grape processing, consists of skins, seeds, and stems, and is highly regarded by researchers due to its polyunsaturated fatty acids and phenolic compounds. When incorporated into the diets of ruminants, these fatty acids modulate the biohydrogenation process in the rumen, influencing the fatty acid profile of milk and dairy products [7]. Most studies on the effects of grape pomace on lipid composition and antioxidant potential in milk focus on sheep and dairy cattle, with limited information available on dairy goats. The fatty acid profile of grape pomace, can positively influence the composition of milk fatty acids and their antioxidant capacity [5,6,7], thereby contributing to improved functional properties [7].
Ref. [8] observed an increase in monounsaturated fatty acids without altering the overall composition of milk in dairy ewes. Phenolic compounds can influence the biohydrogenation process of fatty acids by modulating the microbial profile, which in turn affects the profile of these acids in milk [9]. In the rumen, a specific group of Gram-negative bacteria (Butyrivibrio fibrisolvens) is responsible for converting linoleic acid into rumenic acid and subsequently into vaccenic acid. With the addition of tannins to the diet, ref. [10] observed that phenolic compounds inhibit the first stage of the biohydrogenation process in Butyrivibrio fibrisolvens, thereby favoring an increase in linoleic acid in ruminants. Ref. [9] noted an increase in the proportions of C18:2 n-6, C18:3 n-6, C18:3 n-3, and long-chain fatty acids, along with a decrease in the amounts of C11:0, C12:0, C14:0, the n-6/n-3 ratio, and the atherogenicity index in goat milk fat.
In dairy sheep, ref. [11] observed a decrease of 38.2% in medium-chain saturated fatty acids and an increase of 27.1% in polyunsaturated fatty acids. Ref. [12] found that incorporating 2% grape flour into sheep feed enhanced the milk’s antioxidant capacity, health, and production efficiency. Research on the use of grape pomace in dairy goat feed and its effects on goat milk quality is crucial for implementing alternative feeds in dairy goat nutrition, eliciting adaptive responses from the animals, and improving milk quality in arid and semi-arid regions, where seasonality in forage production is more pronounced, with a strong dependence on high-cost foods that compete with human food, and goat milk serves as the main food source for the population.
Understanding the changes brought about by grape pomace on the composition and fatty acid profile of goat milk enables better guidance on utilizing this by-product in animal feed. This aims to increase the quantity of health-beneficial fatty acids for consumers while providing oxidative stability to the product and its derivatives. Given the above, this study aimed to evaluate the effects of replacing cactus pear with grape pomace on qualitative milk traits of lactating Saanen goats. We hypothesize that dehydrated grape pomace as a substitute for cactus pear in the diet of lactating goats will improve the fatty acid profile of milk, enhance the antioxidant potential and consequently improve goat milk quality.

2. Material and Methods

2.1. Animals, Experimental Design and Treatments

The research and procedures involving animals were performed following the Ethical Principles of Animal Experimentation established by the Brazilian College of Animal Experimentation (COBEA). The study received approval from the Ethics Committee on the Use of Animals (CEUA) at the Federal University of São Francisco Valley (UNIVASF) under protocol number 0004/241121. The experiment occurred at the Sheep and Goat Sector of UNIVASF, located on the Agricultural Sciences Campus in Petrolina, Pernambuco, Brazil. The climate in this region is classified as semi-arid (BSh) according to the Köppen climate classification, with an average temperature of 31.6 °C and a relative humidity of 46.1% during the study.
Eight healthy, multiparous Saanen goats, with an average 4 years in age, weighing 41.2 ± 15.7 kg (mean ± standard deviation), average of 30 days of lactation, and an average production of 1.4 kg of milk/day were used in a double Latin square design (4 × 4). The treatments consisted of four diets in which cactus pear, serving as a source of roughage, was partially substituted with dehydrated grape pomace at 0, 90, 150, and 210 g/kg based on dry matter (DM). The goats were housed in individual pens measuring 4.50 m2, each equipped with a feeder, drinker, artificial shade to reduce the incidence of sunlight on the sides in the pens, and access to mineral salt ad libitum. The experiment lasted 80 days and was divided into four experimental periods of 20 days each, with the first 15 days allotted for adaptation to the diet, followed by five days for collection. After milking, the animals were fed twice daily with a total mixed ration, at 9:00 am and 5:00 pm, with 50% of the daily ration in the morning and 50% in the afternoon. The diet was adjusted daily based on the amount of leftovers. Leftovers were collected and weighed in the morning, with the adjustment maintaining at 10% of the food offered. The observed daily dry matter intake was calculated by the difference between the amount of food provided and the amount of leftovers.
Samples of the ingredients were collected and submitted for physicochemical analysis in triplicate (Table 1). Physicochemical analyses were executed within the Nutrition Laboratory for Animals (LANA) at the Federal University of Paraná (UFPR—Palotina Campus) to ascertain the concentrations of dry matter (DM; method 967.03), crude protein (CP; method 984.13), lignin (lignin; method 973.18), and ash (ash; method 942.05), adhering to the methodologies established by the Association of Official Analytical Chemists [13]. The content of ether extract (EE; Am 5-04 method—[14] was quantified via extraction using ethyl ether, employing the ANKOM XT10 Extractor (Ankom Technology Corp., Macedon, NY, USA) and utilizing fat filter bags (XT4 bags with a porosity of 3 µm; Ankom Technology Corp.). Analyses of neutral detergent fiber (NDF) and acid detergent fiber (ADF) were performed in accordance with the protocol recommended by [15], using heat-stable α-amylase and without making adjustments for the exclusion of residual ash. The quantification of non-fibrous carbohydrate (NFC) content was performed utilizing the equation NFC = 100 − ((CP + EE + ash + NDF). The digestible components of crude protein (CP), ether extract (EE), non-fibrous carbohydrates (NFC), and neutral detergent fiber (NDF) were ascertained in accordance with the methodologies delineated by [16,17,18,19]. Total digestible nutrients (TDN) were estimated using the equations proposed by [20]. The diet on dry matter (DM) basis was composed of 600 g/kg of roughage based on Elephant grass (Cenchrus purpureus Schum Syn. Pennisetum purpureus Schum), cactus and grape pomace, and 400 g/kg of concentrate: soybean meal, ground grain corn, cottonseed cake, soybean oil, mineral, and limestone salt (Table 2). The diet met protein requirements (133.3 g/kg CP) for goats producing 0.89 to 2.08 kg milk/day according to [21] adjusted to the maintenance and milk production requirements of lactating goats.
Table 1. Chemical composition of the ingredients.
Table 2. Proportion of ingredients, chemical composition and fatty acid profile (g/100 g FAME) of diets fed to lactating goats in the experimental period.
The Tropical Winery in Lagoa Grande, Pernambuco, Brazil, supplied grape pomace for this study. The material was removed from the press after seven days of fermentation, yielding approximately 350 g/kg DM, primarily consisting of grape skins and seeds (Vitis vinifera cv. Carménère, BRS Magna, BRS Violeta, Itália, Benitaka, and BRS Vitória). Grape pomace underwent solar dehydration on a masonry floor, being turned every two hours and collected at the end of the day until the pomace attained approximately 850 g/kg DM. It was then grinded into 8 mm particles and stored in 60 × 115 cm polypropylene bags.
The cactus used in the experiment was the Opuntia stricta Haw. Cactus pear was harvested by hand at the start of each experimental period, with an estimated dry matter content of 120 g/kg DM. Following the harvest, it was stored in shaded conditions and sliced daily into 20 mm pieces using a motorized slicer (FP3001n, Laboremus, Campina Grande, Paraíba, Brazil).

2.2. Milk Collection and Physicochemical Analysis

Daily milk production (kg milk/day) was quantified during the five collection days of each experimental period. Milking was conducted manually twice a day, at 0800 and 1600 h, where samples were collected. Aliquots of 30 mL were taken during the morning and afternoon milking sessions and stored in sterile 300 mL bottles pooled by period. These samples were maintained at −20 °C for later physicochemical analysis and lipid extraction. The milk was thawed gradually, and a 25 mL aliquot underwent physicochemical analysis in triplicate to assess fat, defatted dry extract, density, protein, lactose, and solids using the Master Mini Milk Analyzer (AKSO®, São Leopoldo, RS, Brazil), which is equipped with an ultrasonic sensor. The fat-corrected milk yield for 3.5% fat (FCMY) was calculated using the equation suggested by [22]: FCMY 3.5% = (0.4255 × kg milk) + [16.425 × (% fat/100) × kg milk].
Lipids from milk were extracted according to [23] with modifications using a mixture of chloroform, methanol, and water [24]. Subsequently, 25 mg of lipids were subjected to a transesterification procedure described in method 5509 of the International Organization for Standardization [25]. This procedure produced fatty acid methyl esters (FAME) through alkaline catalysis (KOH 2 M in methanol; 200 µL) and then partitioned into 1 mL of hexane. The FAME extract was subjected to automatic injection and subsequent analysis using a gas chromatograph equipped with a flame ionization detector (GC/FID, Star CX 3400, Varian, Palo Alto, CA, USA) following the methodologies established by [26].
One microliter of the extract was introduced in split mode (1:20), with the injector temperature set to 250 °C. Hydrogen gas with a purity level of 99.999% was used as the carrier gas, due to its efficiency and compatibility with common detectors and kept at a steady pressure of 25 psi. The separation of analytes took place in an HP-88 capillary column (100 m × 0.25 mm i.d.; 0.20 µm stationary phase thickness; Bellefonte, PA, USA). Initially, the column oven’s temperature was set to start at 100 °C for 1 min. After that, it increased to 180 °C at a rate of 15 °C/min, followed by a gradual rise to 195 °C at 0.5 °C/min. Finally, it reached 230 °C at 10 °C/min, sustaining isothermal conditions for 5 min.
The elucidation of fatty acids was carried out through a comparative analysis of the experimental retention times against those of reference compounds, which included FAME Mix 37 (P/N 47885-U), linoleic acid conjugated methyl ester isomers (P/N O5632), cis/trans isomers of linoleic acid methyl ester (P/N 47791), a mix of linolenic acid methyl ester isomers (P/N 47792), trans-vaccenic acid methyl ester (P/N 46905-U), and docosapentaenoic methyl ester (P/N 47563-U) (Sigma-Aldrich, St. Louis, MO, USA). The findings were expressed as a percentage of the total chromatographic area, considering the correction factors relevant to the Flame Ionization Detector (FID) and the conversion from ester to acid [27].
The atherogenic index (AI) and thrombogenic index (TI) were calculated using the equations proposed by [28], with modifications by [29] AI = [C12:0 + (4 × C14:0) + C16:0]/[(PUFA) + (MUFA)] and TI = (C14:0 + C16:0)/[(0.5 × MUFA) + (0.5 × n-6) + (3 × n-3) + (n-3/n-6)]. The desirable fatty acids (DFA), EPA + DHA and LA/ALA were determined according to [30], calculated as the sum of C18:0 + PUFA + MUFA, the sum of C22:6n-3 + 20:5n-3 and relation of the linoleic/alpha-linolenic acid, respectively. The de novo fatty acids were calculated as the sum of C4:0 to C14:0 and 50% of C16:0 [31]. The Δ-9 Desaturase ratios were determined according to [32] as follows: C14 index = [C14:1/(C14:0 + C14:1)] × 100, C16 index = [C16:1/(C16:0 + C16:1)] × 100, and C18 index = [C18:1 n-9cis/(C18:0 + C18:1 n-9 cis)] × 100.
The Ferric Reducing Antioxidant Power (FRAP) method was conducted according to [33] and described by [34]. The FRAP reagent was synthesized by combining 25 mL of acetate buffer solution (300 mM; pH 3.6), 2.5 mL of a 2,4,6-Tri(2-pyridyl)-s-triazine (TPTZ) solution (10 mM TPTZ in 40 mM HCl), and 2.5 mL of FeCl3 (20 mM) in an aqueous medium. A 90 µL aliquot of the previously diluted milk sample was added to 2.7 mL of the FRAP reagent and kept at 37 °C in a water bath for 30 min. The absorbance was measured at a wavelength of 595 nm using a spectrophotometer, with calibration against the FRAP solution. The obtained results were subsequently analyzed against a standard curve of ferrous sulfate at concentrations ranging from 100 to 2000 µmol/kg and expressed as mmol of Fe2+ per kg of the analyzed sample.
Phenolic compounds (PC) were determined using the spectrophotometric method with the Folin–Ciocalteu test [35]. In a test tube, 100 µL of the milk sample, 7.90 mL of distilled water, and 0.50 mL of Folin–Ciocalteu reagent were added. After 3 to 8 min, 1.50 mL of a saturated Na2CO3 solution (20%) was added, and the mixture was allowed to rest for 2 h. The absorbance was subsequently measured at 765 nm in a 10 mm optical path glass cuvette using a UV-visible spectrophotometer model UV 2000A (Instrutherm, São Paulo, Brazil), which was zeroed with the reagent blank. Based on a calibration curve, the results were expressed in milligrams of gallic acid equivalent per 100 g of milk (mg GAE/100 g). A calibration curve was created using various concentrations of gallic acid (Sigma-Aldrich®) ranging from 1 to 85 μg/mL.

2.3. Statistical Analysis

The data underwent analysis of variance using GLM procedure and regression analysis (both linear and quadratic) using REG procedure at a significance level of 5%, utilizing the Statistical Analysis System 9.1 software (SAS Institute, Cary, NC, USA, 2003). The statistical model used for regression analysis was Yijk = µ + LSi + αi + βj + γk(ij) + eijk, where Yijk is the value observed in the experimental unit that received the fixed effect of treatment k (in line i and column j), µ is the overall mean, LSi is the fixed effect of the Latin Square (1 and 2), αi is the random effect of line i (animal), βj is the random effect of column j (period), γk(ij) is the treatment effect k applied to line i and column j (inclusion levels of the residue, 0, 90, 150, and 210 g/kg DM), and eijk represents random error.

3. Results

Table 1 and Table 2 present the chemical composition of the experimental ingredients and the proportions of these ingredients in the diets, respectively.
The DGP, as a substitute for cactus pear, did not alter the production and composition of goat’s milk, averaging 1.55 kg; 1.57 kg; 22.4 g/kg; 85.1 g/kg; 1031.1; 31.3 g/kg; and 46.8 g/kg; for milk yield, FCMY, fat content, solids, density, protein, and lactose, respectively (Table 3). While the diets did not affect SFA C10:0 and C21:0, the fatty acids C4:0 (+47.6%), C6:0 (+29.4%), C8:0 (+14.9%), C18:0 (+54.1%), C20:0 (+29.4%), and C22:0 (+70.3%) increased linearly with the inclusion of DGP. In contrast, a linear decrease was noted for C11:0 (−57.7%), C12:0 (−23.7%), C13:0 (−48.2%), C14:0 (−13.5%), C15:0 (−24.1%), C16:0 (−16.5%), and C17:0 (−14.6%) with the inclusion of DGP, leading to an overall reduction in total SFA (−4.48%). A quadratic response was observed for C23:0 (p = 0.011) and C24:0 (p = 0.012), with the lowest values estimated at the inclusion of 100 g/kg and 125 g/kg of pomace, respectively (Table 4).
Table 3. Milk production and composition of goats fed with different levels of dehydrated grape pomace.
Table 4. Saturated, monounsaturated and polyunsaturated fatty acids profile in milk of goats fed with different levels of dehydrated grape pomace.
Total MUFA and C18:1 n-9 cis increased linearly (+14.9% and +19.5%, respectively) with the inclusion of DGP. However, the levels of C14:1 (−38.3%), C16:1 (−22.3%), and C18:1 n-7 cis (−22.7%) decreased as DGP was incorporated into the diets (Table 4). Increases were seen in the concentrations of C18:2 n-6 cis9 trans12 (+32.5%), C18:2 n-6 cis (+20.1%), and C18:3 n-3 (+15.5%), while decreases were observed for C18:3 n-6 (−8.9%) and C20:4 (−12.4%). A quadratic response was noted for C20:5 n-3 (p = 0.005) and C22:5 n-3 (p < 0.001), with the lowest values estimated for the inclusion of 71.4 g/kg DM and 116.7 g/kg DM of pomace, respectively. The total amounts of C18:2 cis9, trans11, and PUFA were unaffected (p > 0.05) by the levels of grape pomace, with average values recorded at 0.795 and 4.563 g/100 g, respectively (Table 4).
The omega-3 and omega-6 levels were unaffected by the diet, averaging 3.354 and 0.396 g/100 g, respectively (Table 4). The omega-6 to omega-3 ratio exhibited a quadratic response with the addition of grape pomace (p = 0.012), with the highest value estimated at an inclusion of 134.0 g/kg DM (Table 4). A decrease in AI (−24.0%; p < 0.001) and TI (−9.9%; p < 0.013) and an increase in DFA (+ 22.0%; p < 0.001) were observed in goat milk with the inclusion of DGP (Table 5). The DGP had a quadratic effect on EPA + DHA (p = 0.001) and FRAP (p <0.001), with the highest value estimated at 150 and 160 g/kg DM, respectively.
Table 5. Atherogenic index (AI), thrombogenic index (TI), antioxidant activity (FRAP), and total phenolic compounds (Folin) in the milk of goats fed with different levels of dehydrated grape pomace.
An increase (+18.4%; p < 0.001) in phenolic compounds (FOLIN) was observed in goat milk with the inclusion of grape pomace (Table 5). Decreases were observed in de novo fatty acids (−10.5%; p < 0.001), the Δ−9 desaturase index of C14 (−21.8%; p = 0.018), and C18 (−9.5%; p = 0.004) with the inclusion of DGP (Table 5). The LA/ALA and C16 index were not significantly affected (p > 0.05) by levels of DGP, which had an average value of 10.431 and 1.95, respectively (Table 5).

4. Discussion

When formulating diets for ruminants in semi-arid regions (Table 1), various protein and energy sources are utilized to minimize reliance on traditional foodstuffs that compete with human food. The grape pomace used in this study had a crude protein (CP) value of 153.4 g/kg dry matter (DM), enabling a reduction in the amount of soybean meal (Table 2), which is a costly protein source in the semi-arid region. Among the ingredients used (Table 1), the grape pomace exhibited a high level of ether extract (EE), leading to an increase in this nutrient’s presence in the experimental diets (Table 2). The absence of the fatty acid profile of the experimental diets in the manuscript can be attributed to methodological limitations associated with the analytical procedures used to recover these fatty acids and does not reflect what was fed to the animals.
The lack of effects on milk production and composition may be related to the composition of the diets (Table 2) and the absence of any effect on dry matter intake (3.80 kg/day—Table 3), factors considered to be determinants of animal performance [36]. No effects on milk production and composition were also reported by [5,37,38].
The discovery of health-promoting fatty acids in grape pomace has spurred research into enhancing the quality of milk and dairy products, thereby improving the quality of life for regular consumers. In this study, the change in fatty acid profile may be related to the source of roughage used, with grape pomace replacing cactus pear, resulting in higher NDF content with differences in the protein and energy content of the diets (Table 2). Other factors may include possible lipomobilization [39] and changes in de novo fatty acid synthesis [40].
The observed increases in short-chain fatty acids (C4:0, C6:0, and C8:0) in goat milk can be explained by lipomobilization, which occurs when there is a reduction in the intake of fermentable organic matter [39]. The reduction in medium-chain fatty acids suggests that DGP levels influenced rumen carbohydrate fermentation, likely due to the increased lignin content in the experimental diets (Table 2). Higher levels of grape pomace, as shown in Table 2, reduce non-fibrous carbohydrates and may decrease the synthesis of de novo fatty acids in the mammary gland [41]. Raising levels of short-chain fatty acids is highly desirable, as they improve the digestibility of goat milk compared to cow milk [6] and offer significant nutraceutical potential.
The decrease in C12:0, C14:0, C16:0, and C16:1 in milk may reflect a decrease in fermentable carbohydrates as the inclusion of grape pomace increases, since these carbohydrates serve as precursors for de novo fatty acid synthesis [40]. Odd and branched-chain fatty acids, such as heptadecanoic acid (C17:0), may partially stem from the elongation of pentadecanoic acid (C15:0) or from bacteria serving as indicators of ruminal function, reflecting changes in microbial populations due to dietary modifications [41], which may explain the reduction in C17:0 concentration observed in this study.
In this study, the increase in C18:0 can be attributed to the ruminal environment’s influence on the accumulation of biohydrogenation intermediates or its role in limiting the conversion of trans-11 C18:1 to C18:0, which is affected by changes in pH and the type of ingested lipids [11]. A significant reduction in the amount of C14:1 was also reported by [42] which is attributed to the presence of tannins in mango meal. This acid is produced by the desaturation of C14:0, which occurs entirely in the mammary gland [8].
The increase in the concentration of C18:1 n-9 cis can be attributed to its synthesis in the mammary gland via the enzyme Δ-9 desaturase, which utilizes stearic acid as a substrate, thereby making it the primary source of C18:1 n-9 cis in milk [41]. The decrease in cis-vaccenic acid (C18:1 n-7 cis) in milk can be explained by the biohydrogenation process, through the conversion of unsaturated fatty acids into saturated fatty acids (C18:0), the final product of this stage [43]. The decrease in cis-vaccenic acid and increase in stearic acid in milk (Table 4) suggest that even with the inclusion of grape residue in the diet, the microorganisms responsible for the conversion process were not affected. Otherwise, decreases in stearic acid would be observed, demonstrating the biohydrogenation process’s sensitivity to residue levels, similar to that reported by [43].
The increase in linoleic acid (Table 4) can be explained in two ways. First, this acid shows a dose-dependent relationship with dietary acid intake, since it originates in milk only from exogenous sources [7,41]. Another explanation is that inhibition of linoleic acid isomerization to conjugated linoleic acid in the rumen increases its concentration in milk [43].
The increase in the amount of C18:3 n-3 observed in this study can be explained by the amount of this acid that escapes biohydrogenation and accumulates in the milk [41]. Mammary glands synthesize CLA through Δ-9 desaturase of vaccenic acid (C18:1 trans11), an intermediate formed during the biohydrogenation of C18:3 n-3 in the rumen. The lack of effect on trans-vaccenic acid (C18:1 n-7 trans), may explain why there is no effect on conjugated linoleic acid. The lack of effect observed for C18:2 cis9, trans11 (Conjugated Linoleic Acid—CLA) is consistent with findings by [7,11].
The reduction in C20:4 can be attributed to the presence of phenolic compounds in grape pomace, which alter the biohydrogenation pattern in the rumen and consequently affect the availability of unsaturated fatty acids in milk fat [44]. The presence of arachidonic acid, along with other very long-chain fatty acids, plays a significant role in brain and retinal function [45]. The decrease in the amount of C20:4 differs from the findings reported by [38,41,46], who attribute the increase in C20:4 to higher levels of its precursor (C18:2 n-6 cis).
EPA, DPA, and DHA acids are precursors of eicosanoids, which play an important role in controlling allergic, inflammatory, and cardiovascular diseases. The ability to convert C18:3 n-3 into health-promoting PUFAs, such as EPA and DHA, is limited in human metabolism, underscoring the importance of these acids in the food supply [30]. The traces of EPA, DPA and DHA are consistent with those reported by [6,7,47]. The decrease in total SFA and an increase in MUFA (mainly oleic acid) lead to a lower thrombogenic index, reducing cardiovascular disease. This demonstrates that including grape pomace in lactating goats’ diet can enhance milk’s functional properties [11,38]. The ω-6 values found in goat milk are associated with a higher amount of C18:2 n-6 cis in the diet, as omega-6 cannot be synthesized by the animal [45].
An increased concentration of omega-6 in goat milk is not beneficial for human health, while higher levels of CLA and omega-3 are recommended due to their positive effects, including anticarcinogenic, antioxidative, antiatherogenic, and other benefits [48]. For beneficial effects on lipid metabolism, inflammation, and oxidative stress in humans, it is recommended to maintain the ω-6/ω-3 ratio below 4, as indicated by [41].
The reduction observed in this study for AI and TI, and the increase in the DFA value indicates that milk from goats fed DGP may provide relative benefits. This finding highlights the potential of DGP as a valuable dietary component for dairy goats. The quadratic behavior observed in this study for EPA + DHA can be explained by the inclusion of grape pomace in the diet, which must to overcome ruminal degradation to have effects on milk. In this study, up to 150 g/kg, the biohydrogenation process is efficient, as EPA and DHA are converted into saturated fatty acids before reaching the milk. Beyond this point, microorganisms are unable to biohydrogenate all the acid present in the rumen, causing it to ‘escape’ to the small intestine, where it is absorbed and directed to the mammary gland. This explanation is similar to that observed by [49].
The results suggest that the effect of grape pomace may not be uniform in terms of the decrease in the synthesis of new fatty acids in the mammary gland. Although the literature reports that almost all C6:0 to C14:0 production occurs almost exclusively in the mammary gland [40], our results suggest that not all fatty acids are equally affected by grape pomace. While C4:0 to C8:0 fatty acids increased with grapes, acids above eight carbons decreased. The effect of grapes appears to be greater in reducing than in increasing de novo synthesis. This impairment in de novo synthesis has been reported more frequently in the literature, most often attributed to reduced intake of fermentable organic matter due to higher concentrations of polyunsaturated fatty acids and/or lignin in grape pomace [40].
The decrease in the C14 index can be explained by the lower concentration of myristic acid in milk due to the increase in DGP levels (Table 5) since the only source of C14:1 in milk is the desaturation of C14:0 in the mammary gland [8]. Although the amount of C18:0 and C18:1 n-9 cis in this study increases with the substitution of cactus pear with grape pomace (Table 4) and stearic acid is the substrate most used by Δ-9 desaturase, a decrease in the C18 index cannot be easily explained [11]. Ruminants can convert saturated fatty acids into unsaturated fatty acids via the enzyme stearoyl CoA desaturase. This enzyme introduces double bonds at positions 9 and 10 of the fatty acid [32]. The results observed in this study regarding the C14 index support those reported by [8], who reported a decrease of −25.94% in the C14 index of sheep milk after 60 days of feeding on grape pomace. The decrease in the C18:0 index observed in this study contrasts with the result observed Table by [8] who observed an increase of 5.06% for this index. The presence of phenolic compounds (Folin) and capacity antioxidants (FRAP) in the diet can be transferred to the animal product [7]. Although phenolic compounds are often associated with reduced nutrient digestibility and, consequently, lower animal performance, their inclusion in the diet of dairy ruminants may help prevent the oxidation of fatty acids in milk [5].

5. Conclusions

Incorporating up to 210 g/kg of dry matter from dehydrated grape pomace as a replacement for cactus pear in the diet of lactating goats does not affect milk production or its physicochemical composition. However, it reduces medium-chain and total saturated fatty acids while enhancing goat milk’s antioxidant activity and monounsaturated fatty acid levels. Therefore, including 210 g/kg dry matter of grape pomace is recommended for its potential to improve the antioxidant capacity and increase the concentration of health-promoting fatty acids in milk. Although the results using grape pomace as a substitute for cactus pear have positively modified goat milk quality, it is suggested that further studies evaluate the use of grape pomace with a larger number of animals, reporting effects on the lactation curve, economic viability, feed efficiency measures, rumen parameters, greenhouse gas mitigation, protein and energy metabolism.

Author Contributions

E.M.d.N.: Conceptualization, Investigation, Data curation, Formal analysis, Validation, Methodology, Writing—original draft, Writing—review and editing; T.M.S.: Conceptualization, Investigation, Software, Project administration, Supervision, Writing—review and editing; A.F.G.N.: Conceptualization, Investigation, Formal analysis, Software, Validation, Methodology, Project administration, Supervision, Writing—review and editing; F.B.R.: Conceptualization, Investigation, Data curation, Validation; É.B.L.d.S.: Investigation, Data curation, Validation, Methodology, Writing—original draft; V.A.S.: Investigation, Data curation, Validation; A.G.V.d.O.L.: Formal analysis, Software, Validation, Methodology, Writing—review and editing; M.W.S.C.: Software, Validation, Methodology, Writing—original draft, Writing—review and editing; R.W.: Software, Validation, Methodology, Writing—original draft, Writing—review and editing; A.J.d.B.A.C.: Methodology, Writing—original draft; M.d.S.L.: Methodology, Writing—original draft; S.A.d.M.: Funding acquisition, Methodology, Supervision; T.V.V.: Funding acquisition, Methodology, Supervision Writing—review and editing; M.A.Á.Q.: Funding acquisition, Methodology, Supervision, Writing—review and editing:, S.A.F.M.: Investigation, Data curation, Validation; S.N.B.: Investigation, Data curation, Formal analysis, Software, Validation, Supervision; D.R.M.: Conceptualization, Investigation, Software, Funding acquisition, Validation, Methodology, Project administration, Supervision, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed in part by the National Council for Scientific and Technological Development—Brasil (CNPq)—Finance Code 408334/2021-5; and Science and Technology Support Foundation of the State of Pernambuco (FACEPE)—Finance code APQ-1493-5.04/22. This project was developed with support from the Bahia State Research Support Foundation (FAPESB) through the granting of a Doctoral scholarship.

Institutional Review Board Statement

The research and procedures involving animals were performed following the Ethical Principles of Animal Experimentation established by the Brazilian College of Animal Experimentation (COBEA). The study received approval from the Ethics Committee on the Use of Animals (CEUA) at the Federal University of São Francisco Valley (UNIVASF) under protocol number 0004/241121 on 24 November 2021.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

We would like to thank Tropical Winery for donating the grape pomace, Casa de Queijos da Nia for providing part of its animals for this study, the Animal Nutrition Laboratory (LANA) of the Federal University of Paraná—Campus Palotina, Embrapa—Semiarid, Federal University of Santa Maria and Federal Institute of Sertão Pernambucano for the analyzes carried out.

Conflicts of Interest

Dra. Salete Alves de Moraes and Dr. Tadeu Vinhas Voltolini are affiliated with Brazilian Agricultural Research Corporation, they declare no conflicts of interest. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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