Fruit Astringency: Mechanisms, Technologies, and Future Directions

Abstract

Fruit astringency, which primarily results from the interaction between polyphenolic compounds such as tannins and salivary proteins, is a critical sensory attribute that limits the commercial value and consumer acceptance of many fruits. A thorough understanding of the mechanisms underlying astringency formation and the development of efficient and safe de-astringency technologies are crucial for the fruit industry. This review systematically elucidates the molecular basis of fruit astringency, focusing on the biosynthesis pathways, accumulation dynamics, and transcriptional regulatory networks of key phenolic substances, such as tannins, as well as their modulation by environmental factors. It also evaluates the efficacy and current applications of existing de-astringency methods and discusses the potential impacts of different treatments on fruit quality attributes. This study thoroughly analyzes the major challenges faced by current technologies, including balancing de-astringency efficiency with quality preservation, ensuring environmental friendliness and food safety, reducing costs, and promoting wider application. Finally, future research directions are discussed, emphasizing the importance of precise genetic improvement using tools such as gene editing, developing green and efficient processes, achieving intelligent process control, and focusing on synergistic quality regulation and the exploration of functional value. This review aims to provide an integrated knowledge framework for developing innovative, efficient, safe, and sustainable fruit de-astringency solutions, offering a scientific reference to advance technological upgrades in the fruit industry.

1. Introduction

The sensory quality of fruits is pivotal in determining their market acceptance and economic value. Among these qualities, astringency, characterized by an unpleasant perception of dryness, puckering, and roughness in the oral cavity [1], constitutes a major barrier to consumption, significantly deterring consumer purchasing intent [2]. Particularly in fruits such as Asian persimmons (Diospyros kaki Thunb.), unripe grapes (Vitis vinifera L.), and pomegranates (Punica granatum L.), intense astringency renders them difficult to consume fresh, severely limiting their market circulation despite potentially being nutritious and visually appealing [3,4,5]. Individual physiological differences, such as salivary characteristics, also influence the perceived intensity of astringency, exacerbating the avoidance of astringent fruits by some consumers [6,7]. Therefore, the thorough understanding and effective management of fruit astringency are crucial for enhancing the competitiveness of the fruit industry.

The chemical basis of astringency primarily stems from polyphenolic compounds, a class of plant secondary metabolites including tannins, catechins, and certain phenolic acids [8,9]. These compounds interact non-specifically with salivary proteins, particularly proline-rich proteins (PRPs), leading to protein precipitation, disruption of oral lubrication, and, ultimately, the sensation of astringency [10,11,12]. Among the various polyphenols responsible for astringency, soluble tannins are widely considered the primary contributors [13]. Based on their chemical structure, tannins are mainly classified into two major categories, hydrolyzable tannins and condensed tannins (also known as proanthocyanidins) [14], which are widely distributed in various fruits and vegetables. Tannins bind with proteins, cell wall polysaccharides, or fibers in fruits, forming insoluble macromolecular complexes that lose the ability to bind with salivary proteins. This binding effect significantly enhances during fruit ripening. Studies have confirmed that the reduction in astringency in Carob (Ceratonia siliqua) during ripening is mainly due to the increase in cell wall polysaccharides rather than a decrease in the absolute concentration of tannins [15]. The ripening process of Kakadu plums (Terminalia ferdinandiana) also supports this mechanism—although the total hydrolyzable tannin content in the mature stage is indeed lower than that in the immature stage [16], the reduction in soluble tannins is relatively limited. The structural changes in the cell wall and the accumulation of polysaccharides accompanying fruit development likely play a more critical role in the reduction of astringency through a similar binding mechanism. Research by Catherine on the interactions between polyphenols and the cell wall indicates that during ripening, the binding of tannins (such as proanthocyanidins) to cell wall polysaccharides, especially pectin, increases significantly, which may be the main reason for the reduction in astringency [17]. These findings suggest that the decrease in astringency is not only related to the decline in tannin concentration but more importantly to the increase in cell wall polysaccharides and the structural changes in the cell wall. Notably, tannins exhibit dual effects on health: excessive intake may have anti-nutritional effects or potential toxicity [18,19], whereas moderate consumption demonstrates various beneficial biological activities, including antioxidant and anti-inflammatory properties [20,21]. This duality establishes a crucial objective for de-astringency treatments: to balance astringency removal with the preservation of functional value.

To overcome the limitations imposed by astringency on fruit utilization, researchers have developed various de-astringency techniques, encompassing traditional physical methods, chemical treatments, biological approaches, and emerging technologies (Figure 1). Traditional methods, while simple and inexpensive, often suffer from drawbacks, such as low efficiency and quality deterioration [22]. Chemical methods, particularly CO₂ treatment, are widely used due to their higher efficiency but still face challenges related to potential chemical residues, stress-induced damage (e.g., flesh browning), and safety concerns [23,24]. Biological techniques offer advantages in terms of environmental friendliness and specificity, yet cost, efficiency, and process stability remain challenges to their large-scale application [25]. In summary, existing de-astringency technologies generally face core challenges, including balancing efficiency with quality preservation, insufficient technological universality (due to variations among fruit species), and inadequate safety assessment systems [26,27]. Meanwhile, the emergence of cutting-edge research involving multi-technology synergy, molecular targeted regulation, and intelligent monitoring [28] has increased the complexity of the knowledge base in this field, necessitating systematic integration.

Given the diverse technologies, significant challenges, and rapid development within the field of fruit de-astringency research, this review aims to systematically summarize the biochemical mechanisms of astringency formation; comprehensively evaluate the principles, progress, and limitations of various de-astringency techniques; thoroughly analyze the key current challenges; and explore future research directions and technological trends. By integrating and refining the existing knowledge, this review aspires to provide theoretical references and practical guidance for developing more efficient, safe, economical, and quality-preserving innovative de-astringency strategies, thereby promoting the sustainable development and value enhancement of the fruit industry.

2. Formation of Fruit Astringency: Biosynthesis, Accumulation, and Environmental Regulation

2.1. Biosynthesis and Accumulation of Phenolic Compounds

Phenolic compounds are crucial components of plant secondary metabolism, significantly influencing the color, flavor (especially astringency), and nutritional value of fruits. Tannins, a major class within phenolic compounds, are the primary contributors to fruit astringency [29]. The biosynthesis of these compounds involves complex metabolic networks that primarily rely on the shikimate and phenylpropanoid pathways [30,31] (Figure 2).

Firstly, the phenylpropanoid pathway serves as the common upstream route for the synthesis of various phenolic compounds, including tannins. This pathway begins with phenylalanine, which is converted through the action of key enzymes, such as phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H), and 4-coumarate:CoA ligase (4CL) [32,33], into cinnamic acid and its derivatives, ultimately yielding p-coumaroyl-CoA [34]. p-Coumaroyl-CoA is not only a precursor for simple phenolic acids, such as caffeic acid and ferulic acid [35], which themselves can form more complex phenolic structures through oxidation and polymerization, but it also serves as a critical hub entering the downstream flavonoid/tannin biosynthesis pathway [36].

Secondly, within the flavonoid metabolic pathway, important phenolic compounds, such as tannins, are synthesized in a stepwise manner. p-Coumaroyl-CoA condenses with malonyl-CoA, catalyzed by chalcone synthase (CHS), to form chalcone, which is then isomerized to naringenin by chalcone isomerase (CHI) [37]. Naringenin, a key intermediate, undergoes a series of enzymatic reactions involving flavanone 3-hydroxylase (F3H) and dihydroflavonol reductase (DFR), among others, to produce leucoanthocyanidins [38]. Leucoanthocyanidins are direct precursors for tannin synthesis. On one hand, they can polymerize under the action of anthocyanidin synthase (ANS) and anthocyanidin reductase (ANR) to form condensed tannins (i.e., proanthocyanidins, PAs) [39]; on the other hand, they can also be hydrolyzed to generate hydrolyzable tannins [40]. In addition to tannins, different branches of this pathway also produce other phenolic substances, such as flavonoids and anthocyanins [30].

The synthesis of phenolic compounds, particularly tannins, is controlled by a stringent transcriptional regulatory network. Transcription factor families, such as MYB, bHLH, and WD40, interact with the promoter regions of key enzyme genes, such as CHS, CHI, and DFR, finely regulating their expression and thus influencing the synthesis rate and final accumulation levels of phenolic compounds, especially tannins [41,42]. For instance, in persimmon, DkMYB4 and DkMYB6 have been confirmed as the key transcription factors regulating tannin synthesis [43].

In recent years, significant research progress has been made concerning the function, modification, transport, and regulatory mechanisms of key enzymes in the tannin biosynthesis pathway. Regarding key enzymes, Bogs et al. [44] pioneered the elucidation of the core roles of LAR and ANR in tannin precursor synthesis in grapes and their spatiotemporal expression specificity, laying a molecular foundation for understanding the tissue-specific accumulation of tannins. Subsequently, research by Yu et al. [45] further revealed subfunctionalization within the LAR gene family, notably identifying LAR2 as a key “molecular switch” regulating tannin content in the fruit peel, providing a precise target for directed improvement of peel astringency. Concurrently, research has delved into tannin modification and transport mechanisms. Bontpart et al. [46] identified the SCPL-AT enzyme responsible for tannin galloylation and clarified how this modification enhances astringency and antioxidant properties by increasing phenolic hydroxyl groups. Further studies have suggested that SCPL-AT might cooperate with chaperone proteins and that modified tannins are transported to the vacuole via MATE transporters. Efficiency is influenced by the degree of acylation, which offers a molecular explanation for the persistent astringency of high-tannin varieties [47]. Additionally, UGTs discovered by Khater et al. [48] provide clues about tannin glycosylation and potential targeted transport. The identification of the MATE-type tannin transporter DkDTX1 in persimmon and its grape homolog VvMATE41-2 by Liu et al. [49] highlights the value of cross-species comparative studies in dissecting tannin metabolic networks, offering potential new targets for astringency regulation. At the regulatory level, Deluc et al. [50] identified the first R2R3-MYB transcription factor, VvMYB5a/b, which directly regulates the phenylpropanoid pathway and key downstream enzymes for tannin synthesis. Later, Huang et al. [51] discovered MYBC2-L type inhibitors and constructed an “activation–inhibition” dynamic balance model, providing a detailed explanation for the regulation of tannin accumulation during fruit development. Cutting-edge techniques, such as single-cell sequencing, were employed by Shi et al. [52] to reveal the co-expression and synergistic regulation mechanisms of transcription factors (MYBPA1) and modification enzymes (SCPL-AT) in specific cell subpopulations spatially. Research trends also underscore the importance of multi-omics integration analysis; for example, Wang et al. [53] identified multiple loci associated with tannin synthesis via GWAS, providing targets and early selection markers for molecular breeding. Furthermore, the successful application of gene editing technologies (e.g., CRISPR/LbCas12a) in grapes [54,55] offers revolutionary new strategies for the targeted, efficient improvement of tannin content, overcoming challenges in traditional breeding.

Finally, the accumulation of phenolic compounds, including tannins, is a dynamic process influenced by a combination of internal and external factors. The developmental stage of the fruit is a key factor. For instance, during the early development of persimmon fruit, large amounts of soluble tannins (mainly PAs) accumulate in the flesh and peel, causing strong astringency (e.g., PAs can account for 25% of dry weight); as the fruit matures, the soluble tannin content significantly decreases, partially converting into insoluble tannins, thereby collectively reducing perceptible astringency [56]. Moreover, the fruit cultivar (genotype), cultivation environment conditions, and postharvest handling methods also significantly affect the final content and composition of phenolic compounds [30,57].

2.2. Influence of Environmental Factors on Fruit Astringency Formation

As illustrated in Figure 3, the formation of fruit astringency is significantly regulated by various environmental factors, with light, temperature, water availability, and soil conditions being key influencers [58]. Light, particularly its intensity and spectral composition, has a substantial impact on fruit astringency. Generally, sufficient light exposure can activate key enzymes in the phenylpropanoid pathway, such as PAL and CHS, thereby promoting the synthesis and accumulation of tannins and other phenolic compounds [59]. For instance, a study by Xu et al. [60] demonstrated that spraying “Cabernet Sauvignon” grapes with 24-epibrassinolide significantly upregulated the expression of key genes in the flavonoid synthesis pathway (e.g., DFR, ANR), leading to a 28% increase in the proanthocyanidin (condensed tannin) content in the peel and consequently enhancing fruit astringency. Furthermore, ultraviolet (UV) radiation, especially the UV-B spectrum, has been proven effective in inducing phenolic compound synthesis in fruits [61]. The mechanism involves inducing the expression of MYB transcription factors, which, in turn, upregulate downstream genes, such as DFR and ANR, accelerating the conversion of leucoanthocyanidins into condensed tannins. Specific studies have shown that UV-B-treated persimmon fruits had a 30% higher condensed tannin content compared to controls, which was attributed to UV-B-induced enhanced gene expression and metabolic flux remodeling [59].

Temperature also plays a complex role in phenolic metabolism and astringency regulation. Generally, higher growing temperatures may promote tannin synthesis and accumulation in some fruits, leading to increased astringency [62]. Moreover, temperature is critical during postharvest treatments. Ding et al. [63] found that the hot air drying (65 °C) of persimmons caused a dramatic increase in the soluble tannin content (up to six times that of fresh samples) and total tannin content, along with significant increases in the mean degree of polymerization (mDP) and degree of galloylation (DG) of tannins. This could be due to high temperatures destabilizing insoluble tannin–protein complexes, releasing soluble tannins, while enzymatic oxidation during drying might further promote tannin polymerization and modification, thus intensifying astringency. Additionally, in the CO₂ de-astringency treatment of persimmons, there is a significant interaction between temperature and treatment duration. Research indicates that at a specific temperature (e.g., 22 °C), the optimal CO₂ treatment time varies for persimmons of different maturity levels, directly affecting de-astringency efficacy, fruit firmness retention, and shelf life, highlighting the importance of precise temperature control in managing fruit astringency and quality [64].

Water stress is known to activate plant stress responses, including the upregulation of phenylalanine ammonia-lyase (PAL) activity within the phenylpropanoid pathway, leading to an increased in the synthesis of phenolic compounds that enhance fruit astringency [65]. In a comparative study on Suncrest peach (Prunus persica (L.) Batsch), Tavarini found that the effect of water stress on phenolic profiles strongly depended on the rootstock genotype: hydroxycinnamic acids significantly increased in fruit grafted onto GF677 under water deficit, while the total anthocyanin content rose in fruit from both GF677 and Penta rootstocks. In contrast, fruit from the more drought-tolerant Montclar rootstock exhibited minimal change, indicating genotype-dependent modulation of phenolic accumulation [66]. Conversely, excessive water input can disturb the metabolic equilibrium of the fruit, possibly impairing phenolic biosynthesis and altering astringency-related traits, although the underlying mechanisms remain less understood than those under deficit conditions [67].

The physicochemical properties of soil, including nutrient content and pH, are also important factors influencing tannin and phenolic synthesis in fruits. Regarding nutrient management, excessive nitrogen fertilization may inhibit tannin synthesis, while an adequate supply of phosphorus and potassium might be involved in regulating tannin metabolism or degradation [68]. Soil pH indirectly regulates plant secondary metabolism by affecting nutrient availability. Typically, acidic or alkaline soil conditions deviating from the plant’s optimal growth pH can cause stress, consequently stimulating phenolic accumulation and increasing astringency [69]. Although the study by Xia et al. [70] involved multiple crops, the principle revealed is generally applicable: plants grow best under their respective optimal soil pH conditions. Phenolic and tannin contents are often higher under these conditions. This underscores the profound impact of the soil environment on the synthesis of secondary metabolites (including those affecting astringency) via the regulation of the plant’s physiological state.

3. Mechanisms of Fruit De-Astringency (Astringency Removal)

The core principle of fruit de-astringency involves reducing the content or altering the structure of soluble tannins and other phenolic compounds, thereby lessening their ability to bind with oral proteins. This is primarily achieved through enzymatic degradation, non-enzymatic degradation, and transformation reactions of the phenolic compounds themselves (such as oxidation, polymerization, and condensation).

3.1. Degradation Mechanisms of Tannins

Tannin degradation is a key pathway for de-astringency and can be categorized into enzymatic and non-enzymatic processes (Figure 4). Enzymatic degradation mainly relies on the catalytic action of specific enzymes [71]. Among these enzymes, tannase (tannin acyl hydrolase) is an acyl hydrolase that specifically hydrolyzes ester bonds within tannin molecules, for example, breaking down gallotannins into gallic acid and glucose [72]. It plays an important role in the natural ripening or artificial de-astringency of fruits such as persimmons and olives (Olea europaea L.) [73]. Studies have shown that treating persimmon fruit with tannase significantly reduces its tannin content, effectively diminishing astringency [63]. Research by Natalia Jiménez et al. [74] further elucidated the complex mechanism of tannin degradation using microorganisms (e.g., Lactobacillus plantarum), revealing the synergistic action of two tannases (TanA and TanB): the intracellularly induced TanB efficiently hydrolyzes small tannin molecules, while the extracellularly secreted and constitutively expressed novel TanA acts on more complex gallotannins. This dual-enzyme system is adapted to plant environments rich in structurally diverse tannins.

Another critical class of enzymes includes polyphenol oxidase (PPO) and peroxidase (POD). These enzymes catalyze the oxidation of tannins and other phenolic compounds, generating quinones. These intermediates can subsequently undergo polymerization reactions to form larger, insoluble precipitates, thus reducing perceptible astringency [25,28,75]. For instance, when fruits sustain mechanical damage, PPO/POD activity typically increases significantly, accelerating the oxidative polymerization of tannins [24,76]. Research by Novillo et al. [77] linked the acetaldehyde (AcH)-induced insolubilization of tannins following CO₂ de-astringency treatment with subsequent enzymatic browning. They found that mechanical damage or stress could trigger reactive oxygen species (ROS) accumulation, activating enzymes such as POD, which further oxidize the already formed insoluble tannins, leading to flesh browning that is particularly evident in certain “pinkish-bruising” disorders.

Non-enzymatic degradation utilizes physical or chemical means to directly disrupt the structure of tannin or alter its solubility [78]. High-temperature treatments (e.g., warm water de-astringency) are thought to promote tannin precipitation by disrupting non-covalent bonds between tannins and proteins [25]. Alkaline environments (e.g., lime water de-astringency) facilitate the oxidation and polymerization reactions of tannins [28]. Furthermore, a core mechanism attributed to the widely used CO₂ de-astringency method involves inducing anaerobic respiration in the fruit to produce acetaldehyde (AcH). Research by Najafabadi [24] emphasized that AcH can undergo non-enzymatic, irreversible cross-linking reactions with soluble tannins to form insoluble complexes, which is key to astringency removal. Their study also found that the rate of this non-enzymatic reaction is significantly influenced by temperature (much faster at 20 °C than at 12 °C) and that excessive AcH accumulation might non-enzymatically induce ROS generation, subsequently accelerating the oxidative polymerization of insoluble tannins and increasing the risk of later flesh browning.

3.2. Transformation and Complexation of Phenolic Compounds

In addition to direct degradation, the transformation of phenolic compounds (including tannins) is another important aspect of de-astringency; this primarily involves oxidation, polymerization, and condensation/complexation reactions (

Table 1. Comparison of phenolic compound transformation mechanisms.

Transformation Mechanism	Oxidation Reaction	Polymerization Reaction	Condensation/Complexation Reaction
Key Conditions/Triggers	O2, Polyphenol Oxidase (PPO)	Alkaline pH > 9, Ca(OH)2	Proteins (e.g., ovalbumin)
Mode of Action	Phenols → Quinones → Insoluble polymers (browning)	Tannin cross-linking into high-MW polymers	Tannins bind proteins via hydrophobic interactions
Target Compounds	Soluble tannins (e.g., catechins)	Condensed tannins	Proanthocyanidins (PAs) and proteins
Example Result/Effect	↓ Phenols 30–50%, ↓ Astringency 60%	↓ SCT from 15.76 → 9.04 mg CE/g (↓ 42.6%)	↓ TPC 35%, ↓ TPAC 66.7%, ↓ Astringency score 7.75 → 4.25
Applicable De-astringency Methods	Mechanical damage	Lime-water treatment	Warm-water de-astringency/protein-addition de-astringency
References	[24,76]	[28]	[28,77,81]

References in the table are inferred examples based on the text description for illustrative purposes. The original table lacked specific references per column. SCT: soluble catechin tannins; CE: catechin equivalents; TPC: total phenolic content; TPAC: total proanthocyanidin content.

). As previously mentioned, the oxidation and polymerization reactions of phenolic compounds, mediated by PPO/POD or chemical factors (such as alkaline environments), generate insoluble macromolecules, serving as a common mechanism for reducing astringency [77,79,80]. Another significant transformation mechanism is condensation or complexation, where phenolic compounds (especially tannins) bind with endogenous or exogenous macromolecules (such as proteins) to form insoluble complexes, thereby losing their ability to interact with oral proteins [77]. Warm-water de-astringency might also involve the co-precipitation of endogenous proteins with tannins [28]. A more direct application involves adding exogenous proteins to actively complex with tannins. For example, Huang et al. [81] found that adding egg white powder to chokeberry (Aronia melanocarpa) juice, where ovalbumin (the main component of egg white powder) binds with proanthocyanidins (PAs) via hydrophobic interactions to form aggregates, significantly reduced the juice’s astringency score (from 7.75 to below 4.25). Concurrently, the soluble total phenolic and proanthocyanidin contents decreased substantially (by 35% and 66.7%, respectively), demonstrating the feasibility and effectiveness of de-astringency by promoting phenolic–protein complexation and precipitation.

4. Quality Effects of De-Astringency

While effectively reducing astringency, de-astringency treatments often exert collateral effects on other key fruit quality attributes. These impacts are critical factors that must be considered when evaluating and optimizing de-astringency technologies.

4.1. Color Changes

The visual appeal of fruit is paramount, and de-astringency processes can lead to significant alterations in color. These may manifest as desirable changes associated with ripening, such as the development of characteristic mature coloration due to chlorophyll degradation and the synthesis of pigments such as carotenoids [82]. However, undesirable browning, resulting from enzymatic or non-enzymatic oxidation and the polymerization of phenolic compounds (including residual tannins), is also a common concern that varies in severity among different treatments [83]. Different methods pose varying risks of browning; for example, comparative studies by Chung et al. found that warm-water treatment relatively effectively inhibited browning in fresh-cut persimmons [84].

4.2. Texture Changes

Fruit texture, particularly firmness, is a key indicator of freshness and consumer acceptance that often undergoes modification during de-astringency. Many treatments, especially those involving heat or certain chemical agents, can impact cell wall structural integrity (e.g., through pectin degradation) or alter cell turgor, frequently leading to a reduction in firmness and a softer texture. The extent of these changes is highly dependent on the treatment conditions [85,86,87].

4.3. Flavor Changes

The impact on flavor is complex. Reducing astringency enhances palatability, but the process can also alter the balance of sugars, acids, and volatile compounds. Residual by-products, such as the acetaldehyde from CO₂ or ethanol treatments, may cause off-flavors if not properly managed [88,89]. Buttery noted that low-threshold compounds (e.g., β-damascenone, threshold of 0.002 μg/kg) strongly affect aroma, while high-threshold substances, such as ethanol, require high concentrations (>100 mg/kg) to be perceived [90]. Young and Paterson found that while ethanol is a major volatile compound in kiwifruit, its aroma contribution is weak due to its high threshold, and excess ethanol can cause undesirable flavors [91]. Zhang’s CO₂ method optimized aroma and astringency tolerance, maintaining the activity of low-threshold compounds and avoiding ethanol buildup, raising consumer acceptance to 86.8 points [92].

4.4. Nutritional Component Changes

The nutritional value of fruit can also be affected, primarily through changes to the phenolic compounds themselves and to other labile nutrients. Since the core of de-astringency involves reducing the soluble tannin content, this is often accompanied by a decrease in the total phenolic content and associated antioxidant activity [93]. Furthermore, heat-sensitive or oxidation-prone components, such as certain vitamins (e.g., vitamin C), may be degraded depending on the harshness and duration of the treatment [89].

Consequently, selecting and optimizing de-astringency techniques requires a comprehensive assessment that balances their efficiency against potential impacts on overall fruit quality (color, texture, flavor, and nutrition). The changes in various quality attributes during fruit de-astringency are summarized in

Table 2. Changes in key fruit quality attributes during de-astringency.

Quality Attribute	Change Trend/Manifestation	Mechanism/Cause	Examples and Methods	Key References
Color	Peel turns orange-red; ↓ L*, b*, C*; warm water reduces browning	Phenolic oxidation/polymerization; PPO/enzymatic browning	Warm water: Best for color CO2: Maintains pigments	[82,83,84]
Texture	↓ Firmness; softening especially with heat; enzyme/microbial milder	Cell wall breakdown; pectin loss; acetaldehyde disrupts structure	Warm water: High softening Enzyme: Minor effect	[22,85,86]
Flavor	↓ Astringency improves taste; ethanol may add off-flavor; CO2 retains original aroma	Tannin reduction; aroma compound shift; ethanol threshold effects	Alcohol: Risk of off-flavor CO2: Preserves aroma Ethephon: Less effective	[27,88,89]
Nutritional Components	↓ Total phenolic content, antioxidants; some methods retain Vit C, acids	Tannin degradation/insolubilization; vitamin loss from heat or oxidation	CO2: Keeps Vit C, loses polyphenols Enzyme: Gentle on nutrients	[87,89,93]

.

5. Fruit De-Astringency Technologies: Principles and Applications

Fruit de-astringency is a critical technological step for improving edibility and enhancing market value and primarily focuses on reducing the content or altering the molecular structure of soluble tannins and other astringent compounds. Based on their operating principles, the existing de-astringency techniques can be broadly classified into physical methods, chemical methods, biological methods, and strategies integrating these principles (

Table 3. Comparison of various de-astringency methods.

Method Type	Specific Method	Principle	Tannin Reduction	Treatment Time	Cost Level	Advantages	Disadvantages	Applicable Fruits
Physical	Warm Water	Disrupts cells, promotes tannin precipitation	~96.5%	5 h	Low	Simple, low-cost	Slow, softens texture	Persimmon, Olive
	Mechanical Damage	Stimulates tannin oxidation via injury	Not significant (CO2-assisted)	Minutes	High	Simple	Affects appearance/storage	Persimmon, Apple
	Irradiation	Breaks tannin molecules via radiation	~20%	Minutes	High	High efficiency	Expensive, may affect quality	Persimmon, Grape
	Ultrasound	Cavitation enhances tannin degradation	10–30%	10–30 min	High	Efficient	High equipment cost	Persimmon, Olive
Chemical	Alcohol	Ethanol induces tannin polymerization	85–90%	24–36 h	Low	Efficient	Alters flavor/aroma	Persimmon, Pear
	Ethephon	Ethylene accelerates ripening and metabolism	80–90%	24–48 h	Very Low	Efficient, effective	Residue and safety concerns	Persimmon, Pear
	Lime Water	Alkalinity + Ca2+ promote oxidation/polymerization	70–80%	24–48 h	Very Low	Extremely cheap	May affect taste/nutrition	Persimmon, Olive
	Carbon Dioxide	CO2 induces acetaldehyde → tannin polymerization	~91.6%	24–36 h	Medium	Efficient, green	High equipment cost	Persimmon, Apple
	Nitrogen	Anaerobiosis triggers acetaldehyde-mediated polymerization	50–55%	2 h	Low–Medium	Efficient, mild	Requires inert atmosphere	Persimmon, Grape
Biological	Microbial	Microbial enzymes metabolize/degrade tannins	35–40%	~3.5 months	Medium	Eco-friendly, preserves quality	Slow, requires fermentation control	Persimmon, Tea leaves
	Enzymatic	Exogenous enzymes (e.g., tannase) degrade tannins	50–80%	4–12 h	Low–Medium	High specificity, mild conditions	Expensive enzymes	Persimmon, Juice
	Genetic Engineering	Gene edits regulate tannin synthesis/metabolism	80–90%	3+ years (breeding)	Very High	Long-lasting, no postharvest treatment	High cost, technical/regulatory barriers	Persimmon, Apple

).

Physical de-astringency techniques primarily utilize physical factors to alter the fruit’s internal environment, thereby promoting tannin transformation (Figure 5). Warm-water treatment is a traditional, long-established, and simple to operate method. It involves immersing fruits in water at a specific temperature (typically 35–50 °C). The heat affects cell structure and molecular interactions, promoting the binding and precipitation of tannins with endogenous macromolecules or directly reducing their solubility [25,83]. For example, astringent persimmons can be effectively de-astringed by soaking in suitable warm water for 12–48 h. Some studies have suggested this method can also inhibit browning during subsequent processing, possibly due to the heat inactivation of oxidative enzymes [94]. However, the main drawbacks of warm-water treatment are its lengthy duration, frequent causation of fruit softening, and the loss of some nutritional components [94,95]. Another simple physical method is mechanical damage induction, which is achieved by puncturing or cutting, which disrupts tissue integrity. This allows contact between previously compartmentalized tannins, enzymes (such as PPO and POD), and oxygen, accelerating the enzymatic oxidative polymerization of tannins [77]. While easy to perform [96], this method readily damages the fruit’s appearance, increases its susceptibility to spoilage, and shortens its shelf life [87]. Recently, emerging physical technologies, such as irradiation and ultrasound, have been explored for de-astringency. Irradiation treatment uses ionizing radiation, which may directly damage tannin structure or trigger free radical reactions [97]; however, its effects can be inconsistent and may even increase astringency under certain conditions [98]. Ultrasound utilizes cavitation effects to induce physicochemical changes that promote tannin degradation or structural alteration [99]. It shows potential in treating extracts [100]. However, for fruit de-astringency, these emerging physical methods generally face challenges of high equipment costs, complex mechanisms, and the need for further validation of efficacy [101].

Chemical de-astringency techniques employ chemical agents or modified atmospheres to induce the chemical transformation of tannins. Among these, gas treatment methods, particularly high-concentration carbon dioxide (CO₂) treatments, are currently widely used and relatively safe and efficient. The core mechanism of these treatments involves high CO₂ (or nitrogen, N₂) atmospheres that inhibit aerobic respiration, forcing the fruit into anaerobic respiration, which leads to the accumulation of endogenous acetaldehyde. Acetaldehyde rapidly undergoes irreversible cross-linking polymerization reactions with soluble tannins, forming insoluble macromolecules and thus eliminating astringency [87] (Figure 6). Maldonado et al. confirmed changes in the chemical structure of tannins in cherimoya (Annona L.) after CO₂ treatment using spectroscopic analysis [102]. Nitrogen treatment operates on a similar principle [103] and is sometimes considered gentler [104]. These methods yield good de-astringency results but require specialized controlled atmosphere equipment, entailing a certain amount of capital investment [105]. Alcohol (ethanol) treatment also utilizes a similar principle: ethanol penetrates the fruit and induces acetaldehyde production, which then polymerizes tannins [106]. Treating fruits with appropriate concentrations of alcohol solution or vapor (e.g., treating persimmons for 24 h) can significantly reduce soluble tannins [27] and may improve some quality attributes [107]. However, the main disadvantage of this method is the potential alteration of the fruit’s original flavor by introducing alcohol or acetaldehyde off-odors [22]. Lime-water (calcium hydroxide) treatment is a low-cost traditional chemical method. It primarily utilizes the strong alkaline environment it provides to promote tannin oxidation and polymerization, along with potential complexation and precipitation involving calcium ions [28,108]. Soaking fruits in dilute lime water for a period of time achieves de-astringency [109]. This method is also applied to de-astringe processed products, such as banana flour [110]. However, lime-water treatment is often crude and can significantly negatively impact fruit taste, texture, and nutritional content [111]. Additionally, the plant growth regulator ethephon can be used in de-astringency treatments. It releases ethylene within the plant tissues, initiating and accelerating the fruit ripening process, which indirectly regulates tannin-metabolism-related enzymes and gene expression, promoting the reduction or transformation of the tannin content [112,113,114]. Ethephon treatment is relatively efficient but poses risks of chemical residues and potentially causing over-ripening [115].

Biological de-astringency techniques utilize organisms or their products to achieve tannin transformation and are generally considered more environmentally friendly and specific (Figure 7). Microbial fermentation relies on specific microorganisms (e.g., lactic acid bacteria, yeasts) that secrete enzymes, such as tannase, to hydrolyze tannins [116]. By screening superior strains for fermentation treatment, the fruit tannin content can be significantly reduced within a certain timeframe [117]. This method is crucial in processing some traditional fermented foods (such as fermented persimmons) and can be enhanced through techniques such as immobilization [118]. Microbial de-astringency typically has a lesser impact on fruit quality but often requires longer treatment times and stringent control of fermentation conditions [119]. Enzymatic treatment involves the direct application of purified or crude enzyme preparations (mainly tannase) to the fruit or its products. The high catalytic efficiency of the enzyme specifically breaks down tannin molecules [120]. Research has also found that some non-tannase enzymes (such as pectinase) can indirectly promote de-astringency by altering the fruit’s tissue structure [63,121]. Enzymatic treatment is efficient, highly specific, and operates under mild conditions, but the high cost of enzyme preparations is a major factor limiting its widespread application [122]. The most disruptive biological strategies are genetic engineering and molecular breeding. These strategies aim to inhibit key tannin synthesis genes or regulatory factors or overexpress genes promoting tannin transformation at the genetic level using techniques such as CRISPR or RNAi, thereby breeding low-astringency or non-astringent cultivars. Zhang et al. successfully obtained low-astringency transgenic persimmon lines by inhibiting the key proanthocyanidin synthesis gene DkANR or overexpressing DkADH1 [123,124,125]. This approach can fundamentally solve the astringency problem, with lasting effects, but faces challenges, including technical complexity, long development cycles, high costs, and issues related to transgenic regulations and public acceptance [126]. Turnbull et al.’s global policy comparative study shows that the core regulatory challenges facing CRISPR crops stem from regional policy fragmentation: countries such as the U.S. and Argentina have implemented product-oriented regulation, exempting SDN1/SDN2-edited crops (such as the non-browning mushroom) from GMO review [127]. Meanwhile, the EU, based on process-oriented regulation (Directive 2001/18/EC), classifies gene-edited crops as GMOs and mandates labeling, leading to differentiated management of innovative products, such as high-GABA tomatoes, in the EU and U.S. [128]. Kato et al. further revealed in their consumer survey that public acceptance is hampered by “technological stigmatization”—72% of EU consumers confuse gene editing with traditional genetic modification [129], while U.S. consumers are more concerned with end-product characteristics [130]. Zhang et al. proposed non-transgenic editing pathways through RNP complex/mRNA delivery to achieve zero foreign DNA integration (e.g., Wx1 gene-edited corn), which has been exempted from regulation in the U.S. and Japan [131]. Smyth emphasized that establishing a Canadian-style “trait risk classification framework” (PNTs system) can synergize technological innovation with public trust, overcoming the current regulatory dilemma [132].

Considering the limitations of single methods, integrated de-astringency technologies have emerged. These aim to combine the advantages of two or more methods to achieve synergistic effects, resulting in better de-astringency outcomes, shorter processing times, or improved preservation of fruit quality [133]. For instance, studies have shown that combining CO₂ treatment with a small amount of ethanol pretreatment can significantly shorten the de-astringency time for persimmons. Combined ethanol and CO₂ treatment might also improve fruit color and soluble solids content [27]. The potential of integrated methods lies in optimizing their overall effectiveness, although they may also increase operational complexity and cost [134].

6. Current Applications and Challenges of Fruit De-Astringency Technologies

The development and application of fruit de-astringency technologies aim to address the limitations imposed by astringency on consumer acceptance and enhance fruit value. However, practical applications face numerous challenges stemming from the diverse characteristics of different fruits and the limitations of existing technologies. This section outlines the practical application of de-astringency techniques in several representative fruits and delves into the current major issues and challenges.

6.1. Application Practices in Different Fruits

The choice and effectiveness of de-astringency technology largely depend on the fruit species and the characteristics of its phenolic compounds (Figure 8). Persimmon is the most extensively studied and mature example of de-astringency research and application. Although traditional methods, such as warm-water soaking and lime-water treatment, are still used in some regions or specific processing scenarios due to their simplicity and low cost, their low efficiency, long treatment times, and tendency to damage fruit texture limit their large-scale application [135]. Currently, controlled CO₂ atmosphere de-astringency has become the mainstream technology in commercial persimmon production and is widely adopted, particularly in major producing countries such as Japan and China. This method typically reduces the de-astringency time to 24–48 h while maintaining fruit color and flavor reasonably well [136,137]. Ethephon-induced de-astringency is also used due to its high efficiency, but concerns about its ripening effects and residue levels need attention. Furthermore, non-destructive testing technologies, such as hyperspectral imaging (HSI), are being introduced for precise control and quality assessment, enabling the real-time monitoring of soluble tannin content changes and distribution during de-astringency, thus providing powerful tools for process optimization [138].

Unlike persimmons, the core of olive processing is the removal of characteristic bitter compounds, mainly oleuropein and its derivatives. This process is typically termed “debittering,” but its principles share common ground with de-astringency, as they both involve the transformation or removal of phenolic compounds. Widely adopted commercial chemical methods include the Spanish style (alkali treatment combined with brine fermentation), the Greek style (prolonged natural fermentation in brine), and the Californian style (multiple alkali treatments plus oxidative color fixation) [139]. These methods have respective advantages and disadvantages but commonly face issues related to efficiency, wastewater treatment, flavor impact, and potential chemical risks (e.g., acrylamide formation). Consequently, research is actively exploring more environmentally friendly and milder alternative technologies, such as using specific microorganisms (e.g., lactic acid bacteria) for fermentation debittering [140], applying enzymes, such as β-glucosidase, to hydrolyze bitter compounds, and employing physical aids, such as ultrasound and vacuum impregnation [141].

For pears (Pyrus communis L.) and apples (Malus domestica (Suckow) Borkh.), astringency is generally less problematic than in persimmons and olives. However, specific cultivars (especially pears used for Perry production) or unripe fruits can still exhibit significant astringency. Therefore, dedicated commercial de-astringency treatments for pears and apples are relatively uncommon. Common practices include using ethephon to accelerate the natural transformation of tannins during ripening [114] or employing cold storage to inhibit relevant enzyme activities and promote slow tannin conversion [24]. A more fundamental solution lies in breeding improvement. Research has found that perceived astringency in pears is related not only to the proanthocyanidin (PA) content but also, critically, to PA’s mean degree of polymerization (DPn) and possibly its binding state with cell wall polysaccharides. For instance, some highly astringent Perry pears have PAs with moderate polymerization, whereas certain Tunisian dessert pears (Pyrus L.), despite containing PAs with extremely high polymerization degrees, are not perceived as astringent, possibly because the PAs form complexes that are not readily perceived [26]. These findings provide important theoretical bases and molecular targets for breeding low- or non-astringent pear and apple cultivars.

6.2. Challenges in De-Astringency Technologies

Despite the development of diverse de-astringency techniques tailored for different fruits, the field still faces several common challenges that hinder the achievement of ideal outcomes.

The primary challenge lies in balancing de-astringency efficiency with the preservation of fruit quality. The pursuit of rapid and complete astringency removal often comes at the risk of sacrificing the fruit’s inherent quality attributes. Traditional methods, such as warm-water and lime-water treatments, although inexpensive, frequently lead to excessive softening and flavor deterioration [25]. Even the widely used CO₂ treatment, if the parameters are improperly controlled (e.g., excessively high concentration or prolonged duration), can induce fruit softening, off-flavors, or other physiological disorders. These negative effects may be exacerbated when combined with other treatments, such as ethanol [142]. Research indicates that the impact of CO₂ on cell membrane structure and function is a key factor causing changes in firmness, and this effect is closely related to the fruit’s maturity stage [77]. Chen’s experiments showed that acetaldehyde, as a reactive carbonyl compound, reacts with phospholipids in the cell membrane, damaging the tonoplast, inhibiting pectin methylesterase (PME), and leading to cell wall disassembly and tissue softening [143]. Boeckx found that high-CO₂-induced hypoxia suppresses cytochrome c oxidase, blocking aerobic respiration and shifting pyruvate to the PDC pathway, forming acetaldehyde and ethanol that are typical of anaerobic metabolism [144]. Benkeblia confirmed that acetaldehyde not only forms off-flavors, such as acetic acid via the ADH pathway, but also disrupts membrane systems, increasing permeability, reducing turgor, and accelerating softening while inhibiting PME and aggravating tissue degradation [145]. Therefore, maximizing the retention of firmness, color, flavor, and nutritional value while ensuring effective de-astringency remains a core difficulty in the optimization and innovation of current technologies [28].

Secondly, environmental and safety concerns are receiving increasing attention. Some traditional chemical de-astringency methods, such as the use of ethephon or strong alkaline solutions, raise concerns about environmental pollution (e.g., wastewater disposal) and food safety (e.g., chemical residues) [108]. Ethephon, being a pesticide-like plant growth regulator, has strict regulatory limits on its residues in food. Consequently, developing and promoting environmentally friendly, safe, and reliable green de-astringency technologies has become an inevitable trend for industry development. This includes utilizing naturally sourced enzyme preparations, screening beneficial microorganisms for controlled fermentation, and optimizing the application of safer physical methods. For instance, studies have explored using food-grade collagen peptides as additives to inhibit astringency by binding with tannins, offering a new safety-conscious approach for processing astringent fruits [146]. For emerging technologies, such as irradiation and genetic engineering, despite their significant potential, their long-term ecological and food safety require continuous, cautious, and comprehensive assessment [147].

Finally, technological costs and barriers to adoption limit the widespread use of advanced de-astringency techniques. Some highly efficient and precise methods, such as the application of specific enzyme preparations, sophisticated controlled atmosphere equipment (for CO₂ and N₂ treatments), and long-term, high-investment genetic engineering breeding programs, often entail substantial equipment investment, technical expertise, or R&D costs [148]. This makes their broad application difficult for many small- and medium-sized enterprises or those in regions with limited resources. To facilitate the adoption of advanced de-astringency technologies by a wider range of producers, future research must focus on reducing costs, simplifying operational procedures, and enhancing the universality and user-friendliness of the technologies. Potential strategies include developing low-cost, highly activity enzyme or microbial agents; designing more economical, intelligent, and automated controlled atmosphere equipment and control systems, for instance, by using artificial intelligence to dynamically optimize treatment parameters based on real-time monitoring data [149]; and developing rapid, low-cost, reliable non-destructive testing techniques for process monitoring and final product quality assessment, thereby improving overall production efficiency and standardization [150].

7. Conclusions and Perspectives

Fruit astringency remains a key factor limiting the commercial value and widespread consumer acceptance of many fruits. The future research and application of de-astringency technologies are poised for significant advancements toward greater precision, sustainability, intelligence, and functionalization. Key breakthrough areas include the following:

Precision Regulation and Genetic Improvement: Leveraging a deeper understanding of tannin metabolism and advanced gene editing (e.g., CRISPR/Cas9) to develop fruit cultivars with inherently low astringency alongside enhanced yield and quality.

Green and Efficient Process Innovation: Advancing sustainable de-astringency through novel natural agents, optimized enzymatic/microbial processes, innovative non-thermal physical technologies, and their integration with functional packaging to preserve quality.

Intelligent Monitoring and Process Control: Improving treatment efficiency and consistency by integrating advanced sensors, IoT, and AI for real-time monitoring, precise control, and optimized de-astringency protocols tailored to fruit characteristics.

Synergistic Quality Regulation and Functional Value Exploration: Shifting focus from mere astringency removal to the holistic improvement of fruit quality (flavor, texture, and nutrition) and the exploration of the health-promoting properties of de-astringed fruits, paving the way for value-added products.

In-depth investigation into how different de-astringency treatments affect the bioactivity and bioavailability of phenolic compounds in fruits will provide a scientific basis for developing high-value-added de-astringed fruit products with specific health claims, meeting the growing consumer demand for healthy and delicious fruit options.