From Biosynthesis to Regulation: Recent Advances in the Study of Fruit-Bound Aroma Compounds

Abstract

Aroma volatiles constitute the primary molecular basis of fruit flavor quality, governing sensory attributes and marketability. Based on their chemical states, aroma compounds are categorized into bound and free forms. Bound aroma compounds predominantly exist as non-volatile glycosides, which can be hydrolyzed enzymatically or through acid treatment to release volatile free aroma compounds, thereby enhancing fruit fragrance. Although the dynamic interconversion between free and bound aroma compounds is pivotal for fruit flavor development, the governing mechanisms, including the principal controlling factors, regulatory networks, and external influences, are still under investigation. This review primarily synthesizes recent advances regarding the structural diversity, analysis, biosynthesis, and regulation of bound aroma compounds. Additionally, it examines how key regulatory networks and environmental factors modulate the synthesis and transformation of these compounds. The integrated overview provides new insights for future regulation of aroma metabolism in fruits.

1. Introduction

Fruits are widely consumed due to their attractive coloration, high nutritional value, and intense flavors. Aroma, a key sensory attribute of fruits, is not only detectable by humans but also serves as a critical marker for assessing fruit quality and freshness [1]. More than 2000 flavor compounds have been identified in fruit aroma, primarily encompassing terpenes, esters, alcohols, aldehydes, and volatile phenols [2]. The aroma of fruits is primarily determined by volatile compounds in both free and bound forms [3]. Free aroma compounds are volatile, readily perceived, and rapidly detectable, directly influencing the flavor profile of fruits. Moreover, some volatile organic compounds (VOCs), including linalool, benzaldehyde, and eugenol, exhibit potent antibacterial properties [4]. Bound aroma compounds are non-volatile and formed by glycosidic bonds with sugar molecules, consisting of glycoside aglycones (terpenes, fatty alcohols, hydroxy esters, phenols, phenyl derivatives, etc.), sugar moieties (D-glucose, L-arabinose, etc.), and glycosidic linkages (β-glycosidic bonds) [5]. These conjugated forms constitute a major reservoir of latent aroma compounds in fruits.

In plant systems, the dynamic interconversion between free and bound aroma compounds represents a crucial biochemical process governing aroma production and release. Using free aroma compounds as substrates and uridine diphosphate glucose as the sugar donor, the glycosylation reaction is catalyzed by UDP-glycosyltransferases (UGTs) to generate non-volatile bound aroma compounds. These glycosylated derivatives are subsequently stored in vacuoles, serving as stable reservoirs of aroma precursors. Conversely, aroma precursors interact with specific hydrolases, whereby glycosidically bound aroma compounds are converted back into volatile forms through acidolysis or enzymatic hydrolysis [6]. The conversion of bound to free aroma compounds is particularly active during fruit development, ripening, or in response to external stimuli such as biotic stress. For instance, during winemaking, bound-aroma precursors in grapes are converted into volatile substances, which enhances the overall aroma intensity and contributes to the formation of a unique flavor profile of wines [7]. This biochemical transformation mechanism holds potential applications in the modulation of fruit flavor and its processing (Figure 1).

The initial identification of bound aroma glycosides was reported by Francis and Allcock in roses, with subsequent confirmation of their presence in fruits by Cordonnier and Bayonove [8,9]. Concomitant with the expansion of related research, studies on bound aroma compounds in fruits have proliferated. To date, aroma glycosides have been identified in various fruits, including grapes, mangoes, lychees, kiwis, oranges, strawberries, cherries, pineapples, and blackberries [10]. For instance, terpenes primarily exist in glycosidically bound forms, representing the predominant class of conjugated aroma compounds identified existing in both grapes and kiwifruits [11,12]. The principal bound aroma compounds in Okinawa pineapples include eugenol, geraniol, phenethyl alcohol, benzyl alcohol, 2-ethyl-1-hexanol, 1-hexanol, and 3-methyl-2-buten-1-ol [13]. In dates, the major bound aroma compounds, including 3-hydroxy-2-butanone, (E)-2-butenoic acid, and hexanoic acid, were detected. Moreover, enzymatic hydrolysis can enhance the total aroma intensity of dates [14]. Notably, Ren et al. [15] demonstrated in six varieties of Citrus fruits that the concentration of bound aroma compounds was higher than that of free aroma compounds. Several substances, including linalool oxide, benzoic acid, α-terpineol, and vanillin, were exclusively present in bound form in Eureka lemons [16]. Previous reviews have provided detailed analyses of bound aroma precursors; most focus on their biosynthetic pathways or examine aroma composition from the perspective of the whole fruit [10,17]. Furthermore, a comprehensive understanding of their interconversion mechanisms and the regulatory factors that mediate the accumulation of bound aromas is still lacking. Therefore, it is imperative to elucidate the regulatory network associated with the metabolism of bound aroma compounds, thereby providing a scientific basis for modulating aroma quality at the molecular level.

Glycosylation is a mechanism employed by plants to conjugate secondary metabolites, thereby facilitating the storage and transport of hydrophobic substances and diminishing their reactivity by masking active hydroxyl groups [10]. High concentrations of linalool, for instance, can be phytotoxic. Glycosylation converts this free compound into a bound form, thereby reducing its toxicity to fruit cells while retaining a high level of the aroma substance [18]. The levels of bound aroma compounds are influenced by numerous pre- and post-harvest factors. Previous studies indicated that temperature, light, fertilization, and ultraviolet irradiation can alter the composition and content of these compounds in postharvest fruits [19,20,2122]. A systematic analysis of these external factors is thus essential for controlling fruit aroma quality.

With the continuous improvement of living standards, consumers have placed greater emphasis on the aromatic quality of fruits. This review aims to provide a comprehensive overview of the structural composition, biosynthetic pathways, extraction and analytical techniques for bound aroma compounds in fruits, as well as the possible regulatory network and the effects of pre-harvest and post-harvest factors on the accumulation and transformation of fruit bound aroma. The objective is to provide a theoretical foundation for understanding the metabolic regulation mechanisms of bound aroma and a technical foundation for the development of aroma quality enhancement in fruits.

2. Structure and Composition of Bound Aroma Compounds

2.1. Glycosidic Ligands

Glycosidically bound aroma compounds represent sugar conjugates of various volatile compounds, arising from the glycosylation of a volatile with a sugar moiety. The aromatic constituents are termed glycoside aglycones, also known as genins [10]. Volatile aromatic aglycones primarily consist of volatile substances containing hydroxyl groups. They form glycosidic bonds with sugar molecules, encompassing for example, terpenes, phenols, C13-norisoprenoids, and certain phenyl derivatives [15]. Among the volatile aroma constituents in fruit-based matrices, terpenes hold a position of paramount significance. Terpenes predominantly consist of monoterpenols and sesquiterpenols, typically imparting fruity aromas to fruits. For instance, grape berries are characterized by a dominance of terpenes, with compounds such as geraniol, nerol, and citronellol contributing to their signature rich rose aroma [23]. D-limonene is extensively distributed throughout the pulp and peel tissues of citrus fruits. As a fundamental aromatic compound, it releases a sweet, floral-fruity scent upon hydrolysis, forming the chemical basis of sweet orange aroma profile [3]. Linalool, a key aroma compound in peach fruit, contributes floral and grassy aromas [24]. Moreover, the phytochemical profile of each citrus cultivar’s juice is distinguished by its unique terpenoid fingerprint, serving as a critical chemotaxonomic marker for aromatic differentiation among varieties.

Phenols and certain phenyl derivatives, such as benzyl alcohol, phenethyl alcohol, eugenol, and guaiacol, constitute another important class of volatile aromatic aglycones. Eugenol is the most prevalent of these substances in fruits and possesses a characteristic clove-like aroma. Benzyl alcohol and phenethyl alcohol are significant glycoside aglycones in sweet cherry and grape varieties, enhancing the floral and citrus aromas of cherries upon enzymatic hydrolysis of bound aroma compounds [17,25,26]. Furthermore, phenethyl alcohol exhibits a rose-like aroma and is commonly utilized in cosmetics and perfumes [27]. Raspberry ketone, an aromatic phenolic compound, is most commonly found in red raspberries and is also a significant aroma compound in berries such as sea buckthorn and cranberries [28].

Furthermore, C13-norisoprenoid volatiles, formed through the degradation of carotenoids, can be classified into mono-oxygenated, di-oxygenated, and highly oxygenated compounds. These compounds, based on varying structural frameworks, generate a diverse array of glycosidically bound aroma compounds, such as β-ionone and β-damascenone, which significantly contribute to the aroma of fruits like tomato and grape [29,30]. Common aglycone chemical structures found in fruits are presented in

Table 1. Common ligand chemical structures.

Substance	Chemical Structure					Typical Fruits
Terpenoid acyclic monoterpenes						Grape, Peach
Monocyclic monoterpenes						Orange, Mango
						Lemon, Grape
Isoprene compounds						Papaya, Grape, Raspberry, Passion
Phenolic compounds						Raspberry, Mango
Phenyl derivatives						Papaya, Grape

. Chemical structures were drawn using ChemDraw (version 22.0.0; PerkinElmer Informatics, Waltham, MA, USA).

Glycosylation modifications effectively mitigate oxidation and volatilization by attaching hydrophilic groups to terpenoid compounds, facilitating stable sequestration within vacuoles. For instance, the glycosylated form of linalool exhibits markedly increased resistance to photodegradation and thermal stress compared to its free form [18]. This enhanced stability presents challenges for aroma release. Many aromatic-rich cultivars display subdued flavor profiles when consumed fresh, with full aromatic expression heavily reliant on post-harvest processing methods such as juicing or fermentation, or enzymatic hydrolysis during mastication. Consequently, strategies aimed at augmenting fruit volatile aroma profiles must consider not only the precursor compounds but also glycoside structures and hydrolytic conditions to effectively enhance fruit aroma quality.

2.2. Sugar Ligands

The glycosyl unit is another essential component of glycosidically bound aroma compounds. During the biosynthesis of these bound forms, volatile aroma compounds can directly link with β-D-glucopyranose to form monoglycosides. These sugar conjugates can further incorporate additional glycosyl moieties, forming diglycosides or polyglycosides [31]. Previous research on glycosidically bound aroma compounds in plants and microorganisms have demonstrated that α-L-rhamnopyranosides, α-L-arabinofuranosides, α-L-arabinopyranosides, β-D-xylopyranosides, β-D-apiosylfuranosides, and β-D-glucopyranosides can serve as potential terminal glycosyl moieties in diglycosides [32].

In recent, advancements in analytical techniques have facilitated a more comprehensive characterization of the carbohydrate profile associated with bound aroma compounds in fruits. In addition to prevalent monosaccharides such as glucose and rhamnose, research suggests that the glycosidic moieties in citrus peel may comprise glucose, galactose, and arabinose [33]. Sun et al. [34] verified that orange juice contains a spectrum of saccharides, including mannose, glucose, sucrose, and galactose. These glycosyl moieties can be linked through the hydroxyl groups at different positions of the initial β-D-glucopyranose, thereby generating diverse diglycoside structures [31]. For example, the three substances 3-methylbutyric acid, 2-methylbutyric acid, and phenylethanol primarily form connections via rhamnosidic bonds; in contrast, 1-chlorooctane, 1-chlorononane, hexanoic acid, 3-hydroxyethyl ester, and undecenone bind through glycosidic bonds [35]. There is a significant difference in the types of glycosidic linkages between these two groups. Compared to monoglycosides and diglycosides, triglycosides are less prevalent in common fruits. For instance, geraniol, nerolidol, and linalool can be found in both diglycosidic and triglycosidic forms within grape pulp [36].

Glycosylation enables plants to convert volatile free-state aroma compounds into non-volatile bound precursors. This transformation permits the efficient and substantial accumulation of these aroma substances within vacuoles without loss. The sugar moiety significantly enhances the stability of the aroma compounds through several key mechanisms: (1) Enhanced oxidative stability: Many aroma molecules, particularly unsaturated terpenoids, are highly susceptible to oxidation, which leads to deterioration and off-flavor formation. The glycosidic group acts as a protective shield, substantially reducing the oxidation rate. (2) Improved tolerance to pH variations: The glycosidic bond is more stable than certain functional groups present in the free-state molecules against pH fluctuations that occur during different physiological stages or processing conditions. (3) Resistance to non-specific enzymatic degradation: The cellular environment contains various enzymes that can degrade free aroma compounds. The conjugated form alters the molecular structure, rendering it less recognizable and susceptible to degradation by non-specific enzymes. Moreover, these glycosides contribute to the enhancement and equilibrium of flavors in fruit-derived products. Therefore, a comprehensive understanding of the glycosidically bound fraction of volatile compounds in fruits is essential for applications in the food processing sector.

Table 2. Chemical structure of common sugar ligands.

Common Glycosides		Chemical Structure	Function	References
Monosaccharides	β-D-Glucopyranoside		Enhanced oxidation stability	[37]
	Malonyl-β-D-glucopyranoside		Enhanced the solubility of secondary metabolite glycosides	[38]
Disaccharides	α-L-Arabinofuranosyl-β-D-glucopyranoside		Exhibit strong stability	[39]
	β-D-Apiosyl-β-D-glucopyranoside		Bind to aroma moieties such as geraniol, benzyl alcohol, 2-phenylethanol, and various monoterpenols	[40]
	α-L-Arabinopyranosyl-β-D-glucopyranoside		Combinate with linalool, benzyl alcohol, and 3-methyl-2-buten-1-ol	[41]
	β-D-Xylopyranosyl-β-D-glucopyranoside		Disaccharides containing 2-phenylethanol, benzyl alcohol, linalool, 3-hexenol, and geraniol	[42]
	α-L-Rhamnopyranosyl-β-D-glucopyranoside		Storage forms or aroma precursors	[39]
	β-D-Glucopyranosyl-β-D-glucopyranoside		As an aroma precursor for substances such as citronellol	[43]
Trisaccharides	β-D-Glucopyranosyl-β-D-xylopyranosyl-β-D-glucopyranoside		As a derivative or complex of malic acid	[44]

illustrates the chemical structures of common sugar ligands found in fruits. Chemical structures were drawn using ChemDraw (version 22.0.0; PerkinElmer Informatics, Waltham, MA, USA).

3. Extraction and Detection Techniques for Bound Aroma Compounds

To elucidate the flavor of postharvest fruit and processed food, it is necessary to analyze the changes in composition and content of bound aroma compounds. The comprehensive characterization analysis of bound aroma profiles demands meticulous sample preparation protocols [45]. Inside, the selection of adsorbent materials and optimization of extraction methodologies represent the predominant factors governing the accurate identification and quantification of flavor constituents.

3.1. Adsorbents

Bound aroma compounds in fruits are typically isolated and extracted from fruit extracts, whole fruits, juices, or wines, primarily through the use of adsorbent materials. The extraction of bound aroma compounds usually begins with an appropriate solvent extraction, followed by the removal of low-molecular-weight compounds such as sugars and acids using deionized water. Free aroma compounds are typically separated using extraction solvents like dichloromethane, pentane, or diethyl ether-dichloromethane, and bound aroma compounds are subsequently eluted with organic solvents such as methanol. Common adsorbents used for extracting bound aroma compounds from fruits include C18 reversed-phase adsorbents, C18 reversed-phase silica gel adsorbents, Amberlite XAD-2 resin, and cleaner PEP Column. The Amberlite XAD-2 resin adsorption separation method exhibits strong adsorption capacity and relatively ideal results, making it the most commonly used method to date. However, for laboratory extraction of bound aroma compounds in strawberries, cleaner PEP adsorption columns are more appropriate. This technique markedly increases the variety and total yield of bound aroma compounds extracted while also enriching key constituents that define the characteristic strawberry aroma, such as esters, terpenes, and aldehydes. This is essential for comprehensive analysis of the strawberry aroma precursor profile [6].

Bound aroma compounds are complex, variable, and unstable, making them susceptible to interference during extraction and separation. They typically exist in glycosidic forms, which are undetectable by detectors. Thus, bound aroma precursors must first be enriched and then transformed into free aroma compounds prior to detection. Moreover, most impurities must be removed to achieve an appropriate concentration of the target aromatic glycoside before detection. Based on this, it is essential to select adsorbents with appropriate characteristics during the adsorption phase to minimize the loss of aromatic compounds and achieve efficient recovery of target aromatic glycosides. Future research should focus on developing milder, more universal adsorption materials with reduced bias, employing validated internal standards for accurate recovery correction to ensure data accuracy and scientific rigor.

3.2. Extraction and Purification Techniques

Current primary methodologies for the extraction of aroma compounds include solvent extraction, simultaneous distillation extraction (SDE), microwave extraction, solvent-assisted extraction, and solid-phase microextraction (SPME). Oussou et al. [46] evaluated the efficiency of cold-pressed extraction, solvent extraction, and traditional extraction methods for the extraction of volatile compounds from shea butter. The results demonstrated that the choice of extraction technique can significantly influence both qualitative and quantitative outcomes, thereby substantially affecting the quality and chemical composition of the extracted material.

Solvent extraction is used to extract volatile compounds without requiring specialized equipment. However, this method is inefficient and unsuitable for fruits with high oil content, such as avocados, for extracting aromatic compounds. It is time-consuming, requires large amounts of solvent, and may result in residue formation [47]. SDE is a classic and frequently used method, enabling extraction under relatively mild conditions, making it suitable for obtaining highly stable substances such as plant essential oils and aromatic compounds [48,49]. Nevertheless, it struggles to capture high-boiling-point components and exhibits low efficiency with samples containing high water content, potentially resulting in the loss of water-soluble components [50]. Microwave extraction is simpler and does not require complex pretreatment, but this method requires further purification of the extracted bound aroma compounds [51]. Solvent-assisted extraction utilizes liquid nitrogen freezing and high-vacuum, low-temperature separation of aroma compounds, offering gentle operation. For example, sensory analysis and solvent-assisted flavor evaporation can evaluate the flavors and volatile compounds of different grape varieties across multiple vintages, including α-pinene and β-pinene, thereby helping to confirm the flavor diversity of table grapes, but are cumbersome and time-intensive [52]. Notably, each of these methods presents distinct limitations.

In contrast, SPME represents a non-solvent-selective extraction method characterized by high sensitivity and excellent recovery rates, making it particularly suitable for the analysis and identification of fruit aroma components [53]. This method can be applied in various modes, including direct extraction, headspace extraction, and membrane-protected extraction. Moreover, compared to solvent-assisted extraction, SPME encompasses the dynamic release process of volatile compounds from the matrix. However, it should be noted that the extraction efficiency of SPME is critically dependent on several key parameters, such as headspace time, constant temperature, extraction duration, fiber coating material, and the extension depth of the extraction fiber [54]. These parameters are predominantly determined by the characteristics of the sample matrix. It is particularly noteworthy that the appropriate selection of the fiber coating can significantly enhance extraction efficiency, with a range of commonly used stationary phases available for this purpose. Nonpolar polydimethylsiloxane (PDMS) is optimal for capturing volatile, nonpolar analytes such as sesquiterpenes, while the typical polar stationary phase polyacrylate is better suited for separating polar, semi-volatile compounds including aldehydes, alcohols, ketones, and acids. Among composite phases, carbon wide range/PDMS exhibits bipolar characteristics, favoring the extraction of highly volatile analytes. Divinylbenzene-PDMS is also bipolar and suitable for aromatic compounds and semi-volatile analytes. In contrast, Divinylbenzene/carbon wide range/PDMS, which contains both polar and non-polar components, accommodates a wider polarity range of volatile and semi-volatile analytes, thereby offering broader applicability [55].

Currently, integrated extraction methods are increasingly being developed and applied to achieve comprehensive analysis of volatile compounds in samples. For instance, the combined application of headspace SPME and solvent-assisted flavor evaporation enables simultaneous extraction of aroma components from both fresh and vacuum freeze-dried mulberries, revealing the impact of different processing methods on the aromatic characteristics of mulberries. In the analysis of isolating aroma constituents associated with grape skins, a novel methodology known as SPME-Arrow has been developed. This technique combines the benefits of SPME and stir bar sorptive extraction. Unlike traditional SPME fibers, the SPME-Arrow is characterized by its coating, which contains a higher volume of sorbent material. This enhancement allows for fully automated sampling and improves the extraction efficiency and detection of a wider spectrum of volatile analytes [56,57].

3.3. Detection Techniques

After sample pretreatment, complex aroma mixtures need to be separated into individual components and identified. When combined with gas chromatography–mass spectrometry (GC-MS), this approach provides an efficient means for the quantitative and qualitative analysis of aroma compounds. In recent years, ultra-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS) has emerged as a novel analytical approach for glycoside aroma precursors due to its high separation efficiency, sensitivity, and specificity in multiple reaction monitoring mode. This technique has been successfully applied to simultaneously determine components such as geraniol β-primeveroside and linalool β-primeveroside, revealing their distribution differences across different plant organs [58]. For complex systems, a derivatization method based on ultra-performance HPLC-MS/MS enables precise quantification of thiols and TCEP-releasable sulfides, providing a basis for flavor regulation [59]. Another method, first applied to the analysis of glycoside precursors, utilizes nitrogen-atmosphere direct analysis ionization-high-resolution mass spectrometry. This technique eliminates the need for chromatographic separation, enabling real-time, high-throughput detection of glycosides via dip-it sampling, significantly enhancing analytical efficiency [60]. Research employing a combination of multidimensional analytical techniques, including GC×GC-ToFMS and UHPLC-DAD-MSn, has revealed how different bottle cap types influence the chemical evolution and sensory characteristics of wine by regulating the microenvironment within the bottle [61]. Additionally, online thermal desorption-GC-MS technology provides theoretical support for optimizing thermal processing parameters by simulating the thermal release behavior of precursors such as linalool β-primeveroside during tea processing [62]. These methods hold broad application prospects in the field of glycosidic aroma precursor research, serving as important tools for aroma fingerprint identification, freshness detection, and processing technology monitoring of fruits.

4. Biosynthesis of Bound Aroma Compounds in Fruits

The biosynthesis of bound aromatic compounds involves the conjugation of an aglycone with an activated sugar donor, catalyzed by glycosyltransferases (GTs) [63]. Among the GT families, UGTs could specifically catalyze the glycosylation of free aroma compounds, thereby converting them into bound aroma precursors. The major sugar donors involved in this biosynthesis process include UDP-glucose, UDP-galactose, UDP-arabinose, UDP-xylose, UDP-rhamnose, and UDP-glucuronic acid. To date, more than 100 UGTs have been identified in plants such as Arabidopsis, maize, soybean, and citrus fruits, which are classified into 14–17 groups based on the conservation of their amino acid sequences [64,65,66]. Notably, significantly elevated expression of UGT genes is observed during fruit growth and developmental stages. Based on the origin of aroma ligands, the biosynthetic pathways involving UGTs include fatty acid metabolic, phenylpropanoid pathway, and terpenoid metabolic. Currently, members of the UGT family implicated in the biosynthesis of bound aroma compounds have been identified across various fruit species (Table 3).

4.1. Fatty Acid Metabolic Pathway

The fatty acid metabolic pathway serves as a primary route for the formation of volatile aroma compounds in most plants. Inside, C6 and C9 aldehydes and alcohols are a particularly prevalent accumulation in foliar tissues and immature fruit organs. These constituents predominantly include cis-3-hexenol, trans-2-hexenol, hexanal, hexanol, cis-2-hexenol, and trans-2-hexenal [67]. Their biosynthesis predominantly proceeds via the lipoxygenase (LOX) pathway. In this process, LOX catalyzes the oxidation of linoleic acid and linolenic acid to generate 9- or 13-hydroperoxides (HPOs), which are subsequently cleaved by hydroperoxide lyase to yield C6/C9 aldehydes. These aldehydes are further metabolized into esterified compounds through the catalytic actions of alcohol dehydrogenase and alcohol acyltransferase [68,69]. Alternatively, 13-HPO can be enzymatically transformed into jasmonic acid through the sequential action of allene oxide synthase and allene oxide cyclase, ultimately resulting in the biosynthesis of methyl jasmonate, a bioactive signaling molecule with notable ecological roles [70].

Notably, aldehyde derivatives lack hydroxyl functionalities and thus are not directly glycosylated by most UGT enzymes. Sugimoto et al. [71] identified a UGT91R1 gene, which encodes a glycosyltransferase with specific catalytic activity toward (Z)-3-hexenol. This enzyme facilitates the β-D-glucopyranosyl arabinose conjugation of (Z)-3-hexenol, resulting in the biosynthesis of HexVic ((Z)-3-hexenyl β-vicianoside) in plant tissues. Conversely, Yauk et al. [72] characterized the glycosyltransferase AdGT4, a key enzyme implicated in the regulation of fruit ripening, through targeted biochemical screening in Actinidia deliciosa. In vitro enzymatic assays confirmed that recombinant AdGT4 exhibits broad substrate specificity, with pronounced glycosylation activity towards terpenoid compounds such as linalool and limonene, as well as C6 alcohols including hexanol, (Z)-3-hexenol, (E)-2-hexenol, and 3-hexanol. Research on strawberry UGT has also significantly advanced, particularly in characterizing their role in aroma biosynthesis via glycosylation. For example, Song et al. [73] demonstrated that FaUGT71W2 specifically catalyzes the glycosylation of key volatile compounds such as 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) and its analogs, thereby directly influencing the development of characteristic strawberry aroma profiles. Furthermore, Yamada et al. [74] reported FaUGT85K16, which exhibits substrate specificity for HDMF aromatic volatiles. Separately, FaFAGT2 glycosylates various aromatic acids such as cinnamic acid, exhibits broad substrate promiscuity, and contributes to aroma compound diversification [75].

4.2. Phenylpropanoid Pathway

The phenylpropanoid pathway initiates with the amino acid phenylalanine, leading to the production of various secondary metabolites, including flavonoids and anthocyanins, which are essential for plant defense against both biotic and abiotic stresses [76]. As the first enzyme in this pathway, phenylalanine ammonia-lyase (PAL) is a membrane-associated enzyme that orchestrates the biosynthesis of phenylpropanoid derivatives in vascular plants. PAL catalyzes the deamination of phenylalanine to generate trans-cinnamic acid, which subsequently undergoes methylation, hydroxylation, and redox modifications, ultimately contributing to lignin formation [77].

In this metabolic network, the aromatic amino acid L-phenylalanine functions as a carbon skeleton donor, facilitating the synthesis of high-molecular-weight polymers such as lignin through the phenylpyruvate pathway. Subsequently, these compounds are converted into glycosylated hydroxymethylfurans along with associated aldehydes, alcohols, and esters through UGT catalysis [78]. For instance, NSGT1 regulates the bioavailability of volatile phenylpropanoid pathway intermediates, including lignin monomer derivatives, via glycosylation modifications. The NSGT1 gene is upregulated during tomato fruit ripening, facilitating the glycosylation of phenylpropanoid volatiles linked to smoking, such as guaiacol and eugenol. This enzymatic modification transforms β-glucosidase-sensitive disaccharides into more stable trisaccharides, thereby modulating the timing of phenylpropanoid volatile release throughout fruit maturation and defense mechanisms [79]. However, some compounds in phenylpropanoid pathway, including catechins, isoflavones, flavones, flavonoids, and anthocyanins, do not possess the distinctive phenylpropane backbone. Consequently, they are categorized as secondary metabolites [80]. Louveau et al. [81] identified the UGT5 gene, highlighting its substrate specificity for a diverse array of compounds such as flavonoids, flavonols, hydroxybenzoquinones, biotin analogs, and chlorinated pollutants.

4.3. Terpenoid Metabolic Pathway

Terpenoid compounds are essential constituents of plant volatile organic compounds and are core components of fruit aroma profiles [82]. Their biosynthetic process predominantly proceeds mediated by the cytosolic mevalonate (MVA) and plastidial methylerythritol phosphate (MEP) pathways [83]. The MEP pathway employs pyruvate and glyceraldehyde-3-phosphate as substrates, with geranyl pyrophosphate synthase facilitating the biosynthesis of geranyl pyrophosphate, the monoterpene precursor. Conversely, the MVA pathway begins with acetyl-CoA and, via farnesyl pyrophosphate synthase, generates farnesyl pyrophosphate, the precursor for sesquiterpenes [84,85]. Metabolic crosstalk occurs through the transmembrane exchange of isopentenyl pyrophosphate and its isomer, dimethylallyl pyrophosphate, between those two pathways. Of these, terpene synthase is a key enzyme involved in the synthesis of monoterpene aroma compounds such as geraniol and linalool. This type of enzyme is encoded and regulated by a family of TPS genes. Currently, they have been identified in citrus, tomatoes, peaches, grapes, and strawberries fruits [86].

Terpenoid biosynthetic pathways generate aroma substances such as linalool, citral, α-terpineol, and nerolidol [87,88,89]. Throughout fruit maturation, the persistent expression of UGT facilitates the glycosylation of aromatic compounds generated by terpenoid biosynthetic pathways. Through integrated multi-omics correlation analysis, Wu et al. [87] systematically examined metabolomic and transcriptomic datasets across various tissues, developmental stages, treatment conditions, and peach cultivars, leading to the identification of a pivotal gene, PpUGT85A2. The glycosyltransferase encoded by this gene specifically mediates the glycosylation of linalool, with its expression markedly increasing during fruit ripening, aligning with the observed accumulation pattern of glycosylated derivatives. Cao et al. [88] further characterized two members of the UGT enzyme family, with UGT85A27 demonstrating high substrate specificity for R-linalool, whereas UGT85A26 preferentially glycosylates S-linalool. Moreover, the external application of free volatile compounds markedly enhanced the accumulation of glycosidically bound volatiles in tobacco leaves, suggesting that the biosynthesis of glycosylation products is likely regulated by substrate availability, specifically free volatile compounds. Similarly, Bönisch et al. [89] pinpointed VvGT7 gene via comparative analysis across five grapevine cultivars. The enzyme encoded by this gene, a monoterpene β-D-glucosyltransferase, specifically catalyzes the glycosylation of linalool and nerol during fruit maturation, thereby promoting the biosynthesis of their conjugated derivatives.

Table 3. Functional characterization of the UGT family involved in the biosynthesis of fruit aroma compounds in the conjugated state.

Species	UGT Members	Sugar Donor	Substrate	Function	References
Wild tomato Solanum pennellii	UGT91R1	UDP-arabinose UDP-xylose	(Z)-3-hexenyl-β-D-glucopyranoside	Conversion of exogenous (Z)-3-hexenol into its diglycoside for defense against herbivores	[71]
	UGT91R4	UDP-arabinose	(Z)-3-hexenyl-β-D-glucopyranoside		[71]
Strawberry Fragaria × ananassa	UGT73B23	UDP-glucose	4-Hydroxy-2,5-dimethyl-3(2H)-furanone	Enhance strawberry flavor	[73]
	UGT73B24	UDP-glucose	4-Hydroxy-2,5-dimethyl-3(2H)-furanone		[73]
	UGT71W2	UDP-glucose	1-Naphthol, Vanillin, Quercetin, Abscisic acid		[73]
	GT2	UDP-glucose	Cinnamic acid, O-aminobenzoic acid, Trans-2-hexenoic acid, Niacin, 2,5-Dimethyl-4-hydroxy-3(2H)-furanone, Gallic acid	Induction of oxidative stress	[75]
	UGT85K16	UDP-glucose	4-Hydroxy-2,5-dimethyl-3(2H)-furanone	Catalyzes the glycosylation of furaneol	[74]
Kiwifruit Actinidia chinensis	GT4	UDP-glucose	Phenylethanol, 2-Methylbutanol, Nerol, N-hexanol, Linalool, Benzyl alcohol, α-Terpineol, (Z)-2-hexenol, Furfural, 2-Butanol, 3-Methylbutanol, 2-Pentanol, (Z)-3-hexenol, Geraniol	Influence floral and fruity aromas	[72]
	GT4	UDP-glucose UDP-galactose	Linalool, 3-Hexanol		[72]
Peach Prunus persica	UGT85A2	UDP-glucose	Linalool, Citral, α-Terpineol	Conversion of linalool into its glucoside.	[87]
Grapes Vitis labrusca × Vitis vinifera	UGT85A26	UDP-glucose	Geraniol, Citronellol, S-linalool, α-Terpineol, Benzyl alcohol, Phenethyl alcohol	Elevate glycosylated volatiles; enhance the defensive response Participates in the biosynthesis of aroma compounds	[88]
	UGT85A27	UDP-glucose	Geraniol, Citronellol, R-linalool, α-Terpineol, Benzyl alcohol, Phenethyl alcohol		[88]
	GT7	UDP-glucose	Linalool, Benzyl alcohol, Geraniol, Phenylethyl alcohol, Citronellol, Eugenol	Synthesis of geraniol and pinene glycosides during ripening	[89]
Apple Malus domestica	UGT83L3	UDP-glucose	Cyanidin, Quercetin, Kaempferol	Promote flavonoid glycoside accumulation and regulate stress tolerance	[90]
Grapefruit Citrus maxima	2RhaT	UDP-rhamnose	Flavanone-7-O-neohesperidosides	Catalyze the biosynthesis of neohesperidin, create the distinctive bitterness	[91]
Orange Citrus sinensis	6RhaT	UDP-rhamnose	Anthocyanins, Flavonols, Flavones, Flavanones	Maintaining species in a selectively bred state	[92]
Tomato Solanum lycopersicum	NSGT1	UDP-glucose	Eugenol-GX, Guaiacol-GX, Methyl-GX	Response to wounding stress	[79]
	UGT5	UDP-glucose	Methyl salicylate, Cinnamaldehyde, Benzyl alcohol, Thymol	Involve in the production of phenolic volatiles and enhance antioxidant capacity	[81]
	UGT75C1	UDP-glucose	Abscisic acid	Accelerate fruit ripening and enhanced resistance to drought stress	[93]
	UGT73C1	UDP-glucose	Stevioside	Response salt tolerance and drought resistance	[94]

Table 3. Functional characterization of the UGT family involved in the biosynthesis of fruit aroma compounds in the conjugated state.

Species	UGT Members	Sugar Donor	Substrate	Function	References
Wild tomato Solanum pennellii	UGT91R1	UDP-arabinose UDP-xylose	(Z)-3-hexenyl-β-D-glucopyranoside	Conversion of exogenous (Z)-3-hexenol into its diglycoside for defense against herbivores	[71]
	UGT91R4	UDP-arabinose	(Z)-3-hexenyl-β-D-glucopyranoside		[71]
Strawberry Fragaria × ananassa	UGT73B23	UDP-glucose	4-Hydroxy-2,5-dimethyl-3(2H)-furanone	Enhance strawberry flavor	[73]
	UGT73B24	UDP-glucose	4-Hydroxy-2,5-dimethyl-3(2H)-furanone		[73]
	UGT71W2	UDP-glucose	1-Naphthol, Vanillin, Quercetin, Abscisic acid		[73]
	GT2	UDP-glucose	Cinnamic acid, O-aminobenzoic acid, Trans-2-hexenoic acid, Niacin, 2,5-Dimethyl-4-hydroxy-3(2H)-furanone, Gallic acid	Induction of oxidative stress	[75]
	UGT85K16	UDP-glucose	4-Hydroxy-2,5-dimethyl-3(2H)-furanone	Catalyzes the glycosylation of furaneol	[74]
Kiwifruit Actinidia chinensis	GT4	UDP-glucose	Phenylethanol, 2-Methylbutanol, Nerol, N-hexanol, Linalool, Benzyl alcohol, α-Terpineol, (Z)-2-hexenol, Furfural, 2-Butanol, 3-Methylbutanol, 2-Pentanol, (Z)-3-hexenol, Geraniol	Influence floral and fruity aromas	[72]
	GT4	UDP-glucose UDP-galactose	Linalool, 3-Hexanol		[72]
Peach Prunus persica	UGT85A2	UDP-glucose	Linalool, Citral, α-Terpineol	Conversion of linalool into its glucoside.	[87]
Grapes Vitis labrusca × Vitis vinifera	UGT85A26	UDP-glucose	Geraniol, Citronellol, S-linalool, α-Terpineol, Benzyl alcohol, Phenethyl alcohol	Elevate glycosylated volatiles; enhance the defensive response Participates in the biosynthesis of aroma compounds	[88]
	UGT85A27	UDP-glucose	Geraniol, Citronellol, R-linalool, α-Terpineol, Benzyl alcohol, Phenethyl alcohol		[88]
	GT7	UDP-glucose	Linalool, Benzyl alcohol, Geraniol, Phenylethyl alcohol, Citronellol, Eugenol	Synthesis of geraniol and pinene glycosides during ripening	[89]
Apple Malus domestica	UGT83L3	UDP-glucose	Cyanidin, Quercetin, Kaempferol	Promote flavonoid glycoside accumulation and regulate stress tolerance	[90]
Grapefruit Citrus maxima	2RhaT	UDP-rhamnose	Flavanone-7-O-neohesperidosides	Catalyze the biosynthesis of neohesperidin, create the distinctive bitterness	[91]
Orange Citrus sinensis	6RhaT	UDP-rhamnose	Anthocyanins, Flavonols, Flavones, Flavanones	Maintaining species in a selectively bred state	[92]
Tomato Solanum lycopersicum	NSGT1	UDP-glucose	Eugenol-GX, Guaiacol-GX, Methyl-GX	Response to wounding stress	[79]
	UGT5	UDP-glucose	Methyl salicylate, Cinnamaldehyde, Benzyl alcohol, Thymol	Involve in the production of phenolic volatiles and enhance antioxidant capacity	[81]
	UGT75C1	UDP-glucose	Abscisic acid	Accelerate fruit ripening and enhanced resistance to drought stress	[93]
	UGT73C1	UDP-glucose	Stevioside	Response salt tolerance and drought resistance	[94]

5. UGT Regulatory Network

5.1. UGT Gene Family Identification

UGTs, which serve as family 1 GTs, represent the most prevalent group in plants. A characteristic feature of UGT proteins is a highly conserved C-terminal region known as the plant secondary product glycosyltransferase (PSPG) box, which spans approximately 44 amino acids [95]. Advances in genomic sequencing have led to the identification of an expanding repertoire of putative UGT genes across diverse plant species. For instance, 120 UGTs have been reported in Arabidopsis thaliana, 276 in Nicotiana tabacum, 181 in grape, 10,769 in tomato, 295 in potato, and 241 in apple [96,97,98]. The multiplicity and diversity of UGT family members enable their coordinated action in regulating complex biochemical pathways, thereby influencing a wide range of plant physiological processes and biological functions.

To elucidate the transcription factors regulating UGT genes, cis-regulatory elements in their promoter regions were predicted and analyzed. In the model plants, a total of 4472 cis-elements, belonging to 63 distinct types, were identified (Figure 2). The most abundant elements included MYB-binding sites (MBSs, 20%), abscisic acid-responsive elements (ABREs, 12%), low-temperature-responsive elements (LTRs, 11%), methyl jasmonate-responsive elements (TGACG-motifs, 10%), MYC-binding sites (9%), G-boxes (5%), GT1-motifs (4%), and salicylic acid-responsive elements (W-boxes, 4%) (Figure 2). Functional annotation revealed that MBSs are associated with drought stress, light responsiveness, and flavonoid biosynthesis regulation; ABREs participate in abiotic stress and light responses; G-boxes and GT1-motifs are involved in light responsiveness; LTRs are linked to low-temperature responses; MYC play roles in light and temperature responsiveness; TGACG-motifs mediate methyl jasmonate responses; and W-boxes are associated with salicylic acid responsiveness. Additionally, other cis-elements related to gibberellin (e.g., TATC-box, GARE-motif, P-box), auxin (e.g., TGA-element), and light signaling (e.g., GATA-motif, MRE, TCT-motif) were detected. These findings suggest that UGT gene expression is potentially regulated by a diverse array of developmental processes and environmental factors. A compelling example is MdUGT83L3 in apple. This gene is transcriptionally regulated by MdMYB88 and fine-tunes flavonoid metabolism, thereby playing an important role in enhancing tolerance to salt and cold stress [90].

5.2. UGT Functional Analysis and Signal Transduction

UGT enzymes are integral to plant defense mechanisms, mediating interspecific chemical signaling and abiotic stress responses. In Solanum lycopersicum [94], the research team successfully cloned the SlUGT73C1 gene. Analysis of its promoter region revealed the presence of multiple stress-responsive cis-elements, such as antioxidant response elements and low-temperature response elements. Under saline stress conditions, the expression of the SlUGT73C1 gene is markedly upregulated. As a pivotal regulatory hub, SlUGT73C1 orchestrates multiple signal transduction pathways to enhance osmotic and drought resilience in plants. This involves upregulation of proline biosynthetic enzymes, increased activity of sodium-potassium ion exchangers, and heightened expression of critical enzymes in abscisic acid biosynthesis. Research conducted by Sugimoto et al. [71] has identified the UGT91R1 gene, which encodes an enzyme responsible for the biosynthesis of the volatile compound (Z)-3-hexenol during herbivore attack on tomato plants. Neighboring plants subsequently convert this volatile into (Z)-3-hexenyl β-glucoside through the action of specific glycosyltransferases. This glycosylated derivative acts as a chemical signaling molecule, initiating defense mechanisms in uninjured plants. In Arabidopsis thaliana, UGT76B1 regulates plant defense mechanisms via the glycosylation of N-hydroxypiperidine acid [99].

Furthermore, UGTs are vital in enhancing fruit quality by markedly regulating the biosynthesis and accumulation of volatile aromatic compounds in horticultural crops via glycosylation modifications. Research has shown that overexpressing the PbUGT72AJ2 gene in pear fruit effectively boosts the glycosylation efficiency of recombinant proteins involved in pinosylvin and mustard alcohol pathways, thereby refining the fruit’s flavor profile [100]. The glycosyltransferase encoded by the Cis1, 6RhaT gene in citrus specifically facilitates the branched rhamnosylation modification of diverse flavonoids, including flavanones, flavones, flavonols, and anthocyanins [92]. Exogenous application of methyl jasmonate (MeJA) markedly induces the expression of UGT85A26 and UGT85A27 genes in grapes, thereby enhancing the biosynthesis of key flavor compounds such as linalool-β-glucoside, citronellol-β-glucoside, and phenylethyl-β-glucoside [88].

6. Key Factors Influencing the Formation of Bound Aroma Compounds in Fruits

The formation of bound aroma compounds in fruits is influenced by a multitude of factors, encompassing fruit variety, cultivation and harvesting practices, and postharvest storage and processing methods, as shown in Figure 3. Significant variations in the types and concentrations of bound aroma compounds are observed among different fruit varieties, with varietal selection determining the fundamental composition of their characteristic aroma profiles. Cultivation management practices, such as light exposure, fruit maturity, and the application of growth regulators, significantly impact the formation and accumulation of bound aroma compounds. Furthermore, postharvest storage and processing treatments are crucial for regulating the fruit’s aroma quality by modulating biochemical reaction pathways and the release of volatile compounds.

6.1. Fruit Cultivar Variability

The bound aroma compounds in fruits exhibit significant cultivar-dependent variations, with notable differences in their compositional profiles and characteristic aroma constituents. The diversity in bound aroma profiles across species can be attributed in part to variations in the UGT gene family, including differences in gene number, expression levels, and substrate specificity. These factors directly influence the capacity to glycosylate aglycones. For instance, in grape berries, four members of the UGT85A subfamily (UGT85A24–UGT85A27) are localized in the cytoplasm. Their recombinant proteins exhibit activity toward various grape-derived volatiles, with a potential preference for catalyzing the glycosylation of geranio [88].

Bound aroma compounds have been characterized in several fruit species. 1-Hexanol and benzyl alcohol dominated the bound aroma substances in Shiikuwasha [101]. Kiwifruit is rich in bound aroma terpenes, presenting a fruity aroma [12]. In jujubes, the most significant bound aromatic substance is phenylmethanol, which imparts a floral aroma [14]. On one side, UGT enzymes synthesize the bound state; On the other side, the bound state’s aroma is being released by glycosidase hydrolysis. Some volatile compounds coexist in both forms; for example, eight compounds in nectarines—linalool, benzaldehyde, phenylacetaldehyde, decanal, nonanal, methyl benzoate, 2-ethylhexanol, and 3-octanone—exist simultaneously in free and bound states [102]. The seven aroma substances in turnjujube include 2-phenylethanol, α-terpinene, γ-terpinene, geraniol, 4-pineneol, 5-citronellol, and p-ethylguaiacol, in both bound and free forms. Camphenol, eugenol and isoeugenol were the main bound aroma substances [19]. Moreover, bound and free forms of γ-elemene, cyclocitral, and (E)-β-damascenone have been found to coexist in quince fruits [103].

The distinctive aromas of different strawberry varieties are closely associated with differential expression of specific members within the AP2/ERF transcription factor family, such as the FaAP2 gene. Research identified six FaAP2 paralogs (specifically four WRI and two AP2 homologs) as key candidate genes regulating distinctive fruit aromas. Their differential expression patterns across varieties ultimately drive aroma differences by modulating the synthesis of downstream aroma compounds [104]. For example, six widely grown strawberry varieties were selected; among them, Benihoppe had the highest number of bound state substances, with esters accounting for about 80% of its total. However, in Tokun, Kaorino, Snow White, Ssanta, and Seolhyang strawberries, terpenoids accounted for the highest number of bound state species, followed by esters, and the proportion of bound state substances varied among the varieties [6].

In summary, the type and concentration of bound aroma compounds in fruits are significantly influenced by cultivar, which in turn affects their sensory aroma profile. Consequently, the selection of appropriate fruit cultivars for cultivation can substantially enhance their economic value and commercial attributes (

Table 4. Aroma characteristics of different species of fruit combinations.

Fruits	Number of Volatile Aroma Compounds	Characteristic Compounds	References
Turnjujube	24	Camphene alcohol, Eugenol, Isoeugenol	[19]
Shiikuwasha	20	1-Hexanol, Benzyl alcohol	[101]
Kiwifruit	79	β-Damascenone, Acetyl geranylgeranyl alcohol	[12]
Jujube	17	3-Hydroxy-2-butanone, 5-Ethyloxolan-2-one, (E)-But-2-enoic acid, 4-Hexanoic acid, 3-Methyl-butanoic acid	[14]
Quince	87	Ethyl acetate, Hexyl pentafluoropropanoate, Ethyl hexanoate, γ-elemene, Cyclocitral, Megastigmatrienone-II, Dihydro-β-ionol	[103]
Nectarines	34	Linalool, Benzaldehyde, Phenylacetaldehyde, Decanal, Nonanal, Methyl benzoate, 2-Ethylhexanol, 3-Octanone.	[102]

).

6.2. Cultivation and Harvesting

6.2.1. Cultivation

Variations in light intensity, photoperiod, environmental climate, and application rates of growth regulators during cultivation can significantly affect the synthesis and accumulation of bound aroma compounds, leading to alterations in fruit flavor profiles. For example, the total amount of bound aroma terpenols (nerol, linalool, geraniol, and geranic acid) in shaded grape clusters is lower than in fruits exposed to sunlight [105]. Research indicates that light intensity or the composition of the cluster zone has a certain impact on bound aroma molecules and C13-norisoprenoids. Defoliation of grapevines, specifically the removal of 100% of leaves, increases the concentration of bound terpenes in grapes [106,107]. Additionally, the application of growth regulators also exerts a positive influence on the formation type and concentration of bound aroma compounds in fruits.

For example, glycosylated terpenoids are potential aroma enhancers in wine, and the application of exogenous ethylene to Cabernet Sauvignon grapes for one week prior to ripening resulted in a significant increase in eight glycosylated monoterpenoids, including linalool, citronellol, and nerolidol, after 12 h of ethylene treatment [108]. During grape cultivation, spraying with benzothiadiazole can lead to a significant increase in bound Z-3-hexenyl esters, hexyl acetate and free hexyl acetate levels, thereby augmenting the aromatic profile of the grapes and optimizing their organoleptic properties [109].

6.2.2. Fruit Ripening

Fruit ripening is characterized by dynamic alterations in the composition and relative concentrations of bound aroma compounds. Although unripe fruits exhibit superior storage and transportability, their aromatic profiles are typically less developed. This is primarily attributed to the synthesis and accumulation of volatile compounds being highly dependent on fruit ripeness. Multiple studies indicated that aromatic compounds in fruit increase as the fruit ripens.

In strawberries, the accumulation of aromatic compounds commences significantly in the early developmental phases, with a subsequent reduction occurring after overripening. During maturation, the levels of esters, alcohols, terpenes, and phenolic compounds exhibit a progressive rise. Moreover, the contents of acids and benzenes demonstrate an initial increase followed by a decrease at the red-ripe stage, which coincides with the period of maximum aromatic abundance [6]. Similarly, across the young fruit, expansion, transformation, and ripening stages, the concentration of bound aromatic substances consistently exceeds that of their free forms. Inside fruits, bound terpenes are the key precursors for floral and citrus aromas. Highly active bound terpenes during the young fruit stage impart an intense floral aroma to the fruit. By the ripening stage, metabolic shifts reduce their activity, and they synergize with free terpenes to form the characteristic mature aroma profile of rich fruitiness and pure green notes [110]. These findings demonstrate that fruit maturity not only influences aroma intensity but also determines the complexity and nuance of fruit flavor profiles. Consequently, the selection of an appropriate harvest period is a critical prerequisite for ensuring optimal fruit aroma quality.

6.3. Postharvest Storage Strategies

6.3.1. Low-Temperature Storage

Low-temperature storage is currently the most common postharvest technique used to delay fruit ripening and prolong shelf life. The stability, transformation, and release of bound aroma compounds can be significantly altered under low-temperature storage conditions. Taking nectarines as an example, their primary bound aroma compounds after harvest are aldehydes and linalool. These components release floral and herbal aromas upon hydrolysis. Research indicates that, under 8 °C cold storage conditions, the content of C6 aldehydes and alcohols in nectarines decreases sharply, while ester content increases significantly. This suggests that, at this temperature, aldehyde and alcohol aroma compounds are converted into esters via the lipoxygenase pathway. Furthermore, total bound aroma compounds exhibit an increasing trend during the early storage period. This may be related to the degradation of cell walls and membranes during storage. Such structural breakdown stimulates β-glucosidase activity, facilitating enzyme contact with bound volatiles and promoting their conversion from bound to free states [102]. Collectively, low temperatures suppress the expression of genes involved in the synthesis of bound-type aroma compounds within the fruit, thereby delaying the flavor development process.

6.3.2. Phytohormone Treatment

Ethylene, MeJA, and salicylic acid (SA), which serve as phytohormone, are often used to accelerate fruit ripening and mediate the composition and content of bound aroma substances in fruits. The mechanism of action of ethylene and MeJA involves activating or inhibiting gene expression and enzyme activity related to glycoside synthesis and hydrolysis, ultimately altering bound aromatic compounds.

Ethylene enhances enzymatic activity by upregulating key hydrolases in fruit, such as β-glucosidase and α-rhamnosidase, thereby accelerating hydrolysis. Following ethylene treatment, kiwifruit exhibited a 30–50% reduction in bound linalool glucoside and geraniol glucoside content during late ripening, accompanied by a concurrent increase in corresponding free aromatic compounds. This promotes the hydrolysis of bound alcohol compounds, resulting in more pronounced aroma during kiwifruit storage [111]. MeJA treatment significantly upregulated the expression of the glycosyltransferase genes UGT85A26 and UGT85A27, thereby promoting the biosynthesis of multiple bound-state aroma compounds, including the accumulation of linalool-β-D-glucoside, citronellol-β-D-glucoside, and phenethyl alcohol-β-D-glucoside [88]. Zhang et al. [112] treated refrigerated Nanguo pears with jasmonic acid, increasing the accumulation of ester compounds. PuCBF5, acting as a nuclear localization transcription factor, bound to the GCC-box motif within the PuCXE15 promoter and suppressed its transcription, thereby reducing CXE enzyme synthesis and inhibiting ester degradation. Concurrently, PuAAT1 expression remained unaffected, ultimately leading to ester accumulation and improved fruit aroma quality. Similarly, exogenous treatment with 50 mg/L of SA promoted the accumulation of bound terpene aroma substances in ripe grape berries, resulting in higher concentrations of bound geraniol, nerol, nerolidol, nerolidic acid, nerolidol oxide, and phellandrene [113].

6.4. Processing Procedure

Fruit processing methods include thermal processing, dehydration, fermentation, pickling, and non-thermal processing. Variations in temperature, pressure, and moisture content induced by these different processing techniques can accelerate the transformation of aroma compounds, leading to modifications in the flavor profile of the final products. For example, in strawberry jam production, pasteurization reduces the content of bound aroma compounds, such as 2,5-dimethyl-4-hydroxy-3(2H)-furanone [114]. Drying processes can alter the volatile characteristics of fruits. Compared to ambient storage, hot air drying of grapes increases the bound forms of phthalic acid dimethyl ether, linalool oxide, linalool, and terpinen-4-ol [115]. During the thermal treatment of hawthorn, increasing processing temperatures leads to a decrease in free phenolic compounds and an increase in bound phenolic compounds [116]. This phenomenon may arise partly due to the degradation of cell walls during processing—once the cell wall structure is disrupted, the free phenolic compounds originally enclosed within are released [117]. On the other hand, the released free phenolic compounds may undergo transformation through condensation reactions with proteins and sugars, further forming bound phenolic compounds mediated by hydrophobic interactions and hydrogen bonds [118]. High hydrostatic pressure processing (HPP), a novel technique, promotes the synthesis of bound phenolic compounds in carrots [119]. HPP, as a non-thermal processing technology, does not disrupt the molecular structures of vitamins, pigments, and other compounds. However, it alters the properties of non-covalent bonds within the system, such as hydrogen bonds and hydrophobic interactions. Following treatment, the hydrophobic associations and hydrogen bonding between sugars and aromatic compounds are significantly enhanced, enabling these aromatic compounds to be more firmly retained within the juice matrix [120]. Similarly, fermentation and juice processing can also influence the flavor of the products. These processes generate novel bound aroma compounds, including isoamyl acetate, ethyl hexanoate, ethyl benzoate, diethyl succinate, ethyl decanoate, and ethyl laurate, thereby enhancing the flavor complexity of the processed foods [118]. Therefore, understanding the effects of various processing methods on fruit-bound aroma compounds is crucial for optimizing postharvest procedures, improving fruit flavor quality, and formulating effective storage strategies.

7. Summary and Outlook

This review summarizes the two primary forms of fruit aroma compounds, the transformation of bound and free aroma compounds, and their regulatory mechanisms. Bound aroma compounds primarily exist as glycosides, which are non-volatile. Upon enzymatic or acid degradation, the glycosidic bond is cleaved, generating volatile free aroma compounds, a process critical for fruit flavor quality. The article details the types of bound aroma compounds, their biosynthetic pathways, and metabolic regulatory networks. It also analyzes key factors influencing the production and changes in bound aroma compounds, such as light exposure, fruit variety, maturity, and postharvest handling techniques. Furthermore, the extraction and detection methods for bound aroma compounds are introduced, and future research directions are proposed. This review provides new insights into the metabolic regulation technology of fruit aroma compounds.

Recent years have witnessed substantial progress in elucidating the chemical structures, formation mechanisms, and influencing factors of fruit aroma volatiles. Current research primarily emphasizes the analysis of free aroma compounds across diverse fruit cultivars; comparative studies between free and bound aroma compounds are relatively scarce. This study aims to investigate the influencing factors of bound aroma precursors and provide a basis for understanding their transformation patterns under environmental stresses. However, further investigation is warranted to elucidate the biosynthetic pathways of bound aroma precursors in fruits, the mechanisms of associated enzymatic reactions, and the alterations in aroma volatiles during postharvest storage and processing. A more comprehensive understanding of the transformation and regulatory mechanisms of fruit aroma components is critical for optimizing storage conditions, extending shelf life, and enhancing flavor quality. This knowledge will facilitate advancements in the fruit industry and satisfy consumer preferences for superior fruit products.