Alcoholic Off-Flavor Disorders in Fresh Fruits

School of Agricultural Engineering and Food Science, Shandong University of Technology, Zibo 255049, Shandong Province, China School of Food and Agricultural Sciences, University of Management and Technology, Lahore 54000, Punjab Province, Pakistan Department of Botany, University of Agriculture, Faisalabad 38000, Punjab Province, Pakistan Guangdong Provincial Key Laboratory of Intelligent Food Manufacturing, Foshan University, Foshan 528225, China School of Food Science and Engineering, South China University of Technology, Guangzhou, China National Institute of Food Science and Technology, University of Agriculture, Faisalabad 38000, Pakistan


Introduction
Fruits are one of the most desired horticulture commodities among consumers due to their high nutritional content, appealing color, and favor [1][2][3][4][5]. However, due to a fuctuating ripening trend, most fresh fruits ripen rapidly after harvest, even at ambient and low temperatures. In turn, substantial-quality losses might occur in fruits, such as early fruit softening, decay incidence, weight loss, color degradation, fesh browning, skin pitting, nutritional breakdown, chilling injury, and alcoholic of-favor development [6][7][8]. Such quality losses arise due to the ongoing metabolic adaptations that alter the intended qualities of fruits until they are unmarketable.
Te perception of of-favor sensations characterizes alcoholic of-favor disorder as a result of an excess of fermentation metabolites in ethanol metabolism, such as acetaldehyde and ethanol [6,[9][10][11]. Acetaldehyde and ethanol concentrations in fruits are often present in trace amounts throughout normal ripening or postharvest storage but begin accumulating as fruits tend to ripen sharply under high respiration rates, elevated temperatures, and anaerobic respiration [12]. Such metabolites are involved in maintaining the postharvest quality of the fruit under aerobic respiration, notably during the volatile aroma production stage [13]. Also, they can improve fruit favor, suppressing ethylene production and reducing fruit frmness [14]. Conversely, increased levels of these metabolites develop physiological disorders in fruit during postharvest storage as their accumulation may impart the formation of of-favors in various horticultural crops such as mandarins, pear, dragon fruit, grapes, and kiwifruit.
Several studies have shown that anaerobic respiration [15], low O 2 stress [16], high CO 2 stress [17], and hightemperature stress [18] have a detrimental impact on ethanol metabolism, which leads inevitably to over biosynthesis or accumulation of acetaldehyde and ethanol, resulting in alcoholic of-favor disorder in fruits. Subsequently, numerous metabolic physiological mechanisms, including the c-aminobutyric acid (GABA) shunt pathway [6], mitochondrial energy metabolism [19], glycolysis and Krebs (TCA) cycle [12], cytosolic malate metabolism, and starch and sugar metabolism [20,21], tend to alter or regulate the ethanol metabolism under the consequences of such stresses. Te detrimental impact of undesirable modifcations in such metabolic physiological pathways in response to stress stimuli, specifcally accelerated ethanol metabolism, has been demonstrated by various studies.
To date, as per our knowledge, no review article on the alcoholic of-favor disorder in fresh fruits has been published. Hence, this review article aims to discuss the comprehensive literature regarding alcoholic of-favor disorders in fresh fruits. Tis article particularly addressed the primary causing factors involving physiological and metabolic mechanisms and postharvest strategies to overcome alcoholic of-favor disorders at an agroindustrial level. Furthermore, a research gap that needs to be investigated at a molecular level in future research has also been pointed out.

Ethanol Biosynthesis and Metabolism
Ethanol is an essential aroma-active volatile compound naturally synthesized in fruits and vegetables during the maturing and ripening processes via ethanol metabolism [22]. Ethanol metabolism is a two-step pathway involving the decarboxylation of pyruvate into acetaldehyde and ethanol via the activities of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH), as depicted schematically in Figure 1. Furthermore, acetaldehyde could be further reduced to acetic acid via aldehyde dehydrogenase (ALDH) activity, and ethanol could be converted to ethyl acetate via an esterifcation reaction catalyzed by alcohol acetyltransferase (AAT) activity using acetyl-CoA as a key substrate [6,23,24]. Ethanol metabolism is concomitant with the continuous recycling and reoxidation of nicotinamide adenine dinucleotide + hydrogen (NADH) to nicotinamide adenine dinucleotide (NAD + ) to generate adenosine triphosphate (ATP) [25][26][27]. Studies reported that ethanol metabolism is the only alternate pathway for ATP biosynthesis to fulfll the plant's metabolic energy demand under hypoxic or anoxic stresses [12,19].
Additionally, accelerated ethanol metabolism leads to over-accumulation and the induction of alcoholic of-favor, which deteriorate the fruit favor quality [19]. It has been reported that the high stimulation of ethanol metabolism is anticipated by cellular pH-stat. Pyruvate is catalyzed by lactate dehydrogenase (LDH) to lactic acid, and lactic acid production acidifed cytoplasmic pH and consequently activates the frst ethanol metabolism enzyme PDC [25,27,28]. Te intensity of ethanol metabolism can vary based on PDC and ADH enzyme activities, resulting in desired or immature favor development in various horticultural crops. For example, Botondi et al. [29] and Huan et al. [19] reported the correlation between excess ethanol and acetaldehyde production or accumulation due to enhanced activities of PDC and ADH, resulting in alcoholic of-favor disorder in "Hayward" and "Bruno" kiwifruit.
Similarly, Zhang and Watkins [30] reported the occurrence of of-favors compounds such as acetaldehyde and ethanol and enhanced PDC and ADH activity in strawberry fruit. Furthermore, Shi et al. [31] noticed enhanced acetaldehyde and ethanol contents due to upregulated PDC and ADH transcriptional levels in mandarin fruit. Our previous work also demonstrated that higher PDC and ADH activities result in higher acetaldehyde and ethanol accumulation, which leads to alcoholic of-favor disorder in "Bruno" kiwifruit at room temperature storage [6,12,32].
In contrast, some previous studies negatively correlated PDC and ADH activities with acetaldehyde and ethanol biosynthesis. For example, Ponce-Valadez and Watkins [33] reported that the accumulation of acetaldehyde and ethanol negatively correlated with changes in PDC and ADH activities in "Jewel" and "Cavendish" strawberry fruit. Imahori et al. [34] reported that increased activity of PDC and ADH in bell pepper fruit was not correlated with acetaldehyde and ethanol accumulation. Our previous study concluded a negative correlation between kiwifruit fermentation metabolites and enzymatic activities [32]. Such a correlation could be based on the sensitivity range of each crop's PDC and ADH expression under various stresses [35,36].

Factors Leading to Upregulated Ethanol Metabolism in
Fresh Fruits. Te development of alcoholic of-favor is limited during normal fruit growth or ripening, even at preand postharvest stages. Meanwhile, due to the upregulation of ethanol metabolism, fruits may be susceptible to the occurrence of alcoholic of-favor, which may result in the deterioration of fruit favor quality initiated by the following inducing factors:

Anaerobic Respiration.
Anaerobic respiration is a process that initiates under the efect of reduced aerobic mitochondrial activity and lower ATP biosynthesis as a result of undesired stresses, activating ethanol metabolism to regenerate ATP by using NADH [37,38]. Plenty of ethanol contents are produced and accumulated, resulting in alcoholic of-favor development in various fruits [12,19,29,39].
Moreover, a relationship between anaerobic respiration and alcoholic of-favor disorder is described in detail by Saltveit [15] as follows: "Anaerobic respiration is associated with the regeneration of NAD + and ATP to maintain the plant cell energy demand by moving the pyruvate fux to the ethanol fermentation/metabolism instead of the TCA cycle" (Figure 2). Tis process further leads to the accumulation of lactic acid in the lactate fermentation pathway, which results in the acidifcation of cells and activates the PDC in ethanol metabolism. PDC converts the pyruvate to acetaldehyde by removing the CO 2 and then to ethanol through the activity of the ADH to generate more NAD + content. Te anaerobic pathway accounted for only 20% of the energy synthesis ability, which is insufcient for plant survival. Tis fact upregulated ethanol metabolism for more ATP synthesis, resulting in an over-accumulation of ethanol and alcoholic of-favor.

Low O 2 Stress.
Oxygen concentration in fruit tissues is primarily based on each cultivar's respiration rate, as the ratio of O 2 and CO 2 fuctuates continuously due to organic compounds' consistent biochemical reactions and degradation [40]. Te correlation between various physiological disorders in many horticultural crops was investigated by applying diferent modifed gas conditions during storage. However, exposure of each cultivar to diferent concentrations of O 2 atmosphere can be benefcial or harmful depending on various factors such as storage temperature and cultivar type. Oxygen acts as an electron acceptor in mitochondrial cytochrome oxidase during aerobic pathways. However, in a scenario of a limited O 2 atmosphere, plant cells are exposed to fermentation pathways, energy defcits, and storage disorders [17,41]. Reduced O 2 concentration appeared to have a benefcial relationship with decreased respiration rate, senescence, ethylene synthesis, and enzymatic oxidation at a certain level. In contrast, lowered O 2 induced physiological disorders such as alcoholic of-favor, gummosis, fruit rots, fruit discoloration, superfcial scald, split and shattered pits, gel breakdown, greasiness, and pit burning in many fruits, including pear, apple, mango, and stone fruits (nectarines, plum, cherry, apricot, and peach) during storage [16,[42][43][44].
Te oxygen limit at which ethanol metabolism initiated and accumulation occurred was named the pasture point, or lower oxygen limit, and later referred to as fermentation induction point [16]. Recently, Park et al. [45] reported that the upregulated expression of ethanol metabolism-related genes MdPDC2 and MdADH1 led to higher production of acetaldehyde and ethanol in "Empire" apples during storage in a 0.5 kPa O 2 atmosphere compared to a 1.5 kPa O 2 atmosphere. Burdon et al. [46] reported increased PDC and ADH activities, resulting in a sharp accumulation of fermentation metabolites in the "Hass" avocado under the storage atmosphere with less than 0.5% O 2 concentration. Each crop has a specifc tolerance limit for low O 2 levels, and below that, plants adopt a fermentation pathway [47,48]. Previously, many researchers exposed various fruits to low O 2 atmosphere for a short period and concluded benefcial efects regarding quality. Meanwhile, susceptibility to alcoholic of-favor disorder has been observed with long-term exposure [16,19,49,50].
Regarding alcoholic of-favor disorder in fruits under low O 2 stress, Wood et al. [51] reported that exposing fruits to anoxic conditions (20.9 kPa O 2 ) results in a higher concentration of acetaldehyde and ethanol in apple fruit. Ntsoane et al. [43] concluded that acetaldehyde and ethanol accumulation were signifcantly higher in "Shelly" mango stored at 5% O 2 than at 10% O 2 in controlled atmosphere storage. Moreover, Pintó et al. [52] concluded that the change in pomegranate favor quality is caused by the accumulation of ethanol and acetaldehyde due to a low O 2 storage atmosphere. Under low O 2 stress, acetaldehyde is produced by the decarboxylation of pyruvate by PDC and then converted to ethanol by ADH by consuming NADH. Imahori et al. [53] reported that elevated PDC and ADH transcription and translation resulted in the synthesis of new mRNA and PDC and ADH protein in a low O 2 atmosphere. Tus, under low O 2 stress, activation of PDC and ADH is a critical response. Another efect of low O 2 stress is the accumulation of lactic acid that lowers cytoplasmic pH and inhibits LDH activity. Tis fact further activates PDC and ADH, resulting in excessive ethanol biosynthesis, as discussed above. Boeckx et al. [23] and Botondi et al. [29] reported the accumulation of acetaldehyde and ethanol and the occurrence of alcoholic offavor in "Jonagold" apple and "Hayward" kiwifruit under the storage of ultra-low oxygen atmosphere. Ethanol accumulation has been reported as a low-O 2 stress biomarker during horticultural crop storage [40].

High CO 2 Stress.
Optimal ranges of CO 2 levels in fruit tissue or the storage environment may regulate fruit quality [53]. However, each crop's sensitivity to elevated CO 2 injury varied according to genetic variation, resulting in diferent responses [54,55]. For example, the storage atmosphere containing 20-30% CO 2 concentration resulted in maintaining the fruit frmness, ascorbic acid regulation, and titratable acidity level without deteriorating the favor quality in several sweet cherry cultivars such as "Lapis," "Van," "Kristin," "Stella," "Sam," and "Huldra" [56,57]. On the other hand, Cozzolino et al. [58] described the occurrence of of-favor in sweet cherry cv. "Ferrovia" during storage based on the accumulation of fermentation metabolites under a 20% CO 2 storage atmosphere. Te benefcial efects of increased CO 2 may inhibit respiration rate, ethylene biosynthesis, and N-assimilation in crop tissues, even while promoting carbohydrate synthesis [59][60][61]. In contrast, Lu et al. [62] reported the occurrence of an alcoholic of-favor in satsuma mandarin fruit during storage in a modifed atmosphere accompanied by a 15% CO 2 concentration. Harman and McDonald [63] also reported the efect of a storage atmosphere consisting of 14% CO 2 concentration on the "Hayward" kiwifruit for 16 weeks and noticed the development of the alcoholic of-favor disorder. Surprisingly, the optimum favor was retained when fruit was stored at a lower CO 2 concentration during storage. Te of-favor sensation was identifed due to ethanol over-accumulation in "Bartlett" pears, tomatoes, and lettuce stored in an elevated CO 2 storage atmosphere [64][65][66][67][68]. Te mango cultivar "R2E2" was similarly reported to have an alcoholic of-favor disorder after being stored under 8% CO 2 rather than 6% CO 2 during the controlled storage atmosphere [69]. Similarly, the storage atmosphere of 10 kPa CO 2 produced less ethanol and acetaldehyde in mango fruit than the storage atmosphere of 25 kPa CO 2 [70]. Wang et al. [71] evaluated the efect of modifed atmosphere packing on the ethanol content of two sweet cherry varieties, "Lapins" and "Skeena," resulting in of-favor disorder due to ethanol buildup in the 10% CO 2 storage atmosphere rather than the 8% CO 2 storage atmosphere. Forney et al. [72] reported the accumulation of ethanol in several blueberry cultivars (such as "Duke," "Aurora," "Brigitta," "Jersey," and "Liberty") under the storage condition of 25 kPa CO 2 . Te same fndings were reported in the "Ottomanit" fg subjected to an elevated CO 2 atmosphere [73]. Alcoholic of-favor was observed in the "Italia" table grapes stored in a high CO 2 (>20%) atmosphere [9]. Furthermore, increased expression of the ethanol metabolism-related gene VvADH was associated with increased accumulation of of-favor aroma volatiles, including ethanol, in table grapes during storage under higher CO 2 conditions [74]. Overall, elevated CO 2 levels have mostly been associated with the deterioration of crop quality attributes, particularly favor balance and sensory quality [75][76][77][78][79]. Te overall impacts of O 2 and CO 2 stress conditions on ethanol metabolism-related metabolites, enzymes, and genes transcriptions in fresh fruits are summarized in Table 1.    Journal of Food Biochemistry 2.5. Storage Temperature Stress. An optimal storage temperature range is critical to maintaining fresh fruit quality and extending shelf life by reducing pathological and physiological deterioration [80]. Te ideal temperature for each crop's storage varied depending on its origin or ripening behavior. Kader and Yahia [37] recognized the optimal temperature range of 20-24°C for the anticipated ripening of most harvested fruits. However, some fruits experienced alcoholic of-favor disorder even at such temperature range (Table 2) along with others disorders such as freezing injury, heat injury, chilling injury, skin/fesh discoloration, softening, and AsA degradation [84,85].
Studies were conducted on the efect of varied temperature environments on the alteration of volatile aromatic compounds in several fresh fruits. For example, Saberi et al. [81] noticed the alcoholic of-favor in "Valencia" oranges at 20°C than 5°C during storage. Ali et al. [6] reported the alcoholic of-favor occurrence in "Bruno" kiwifruit during storage at 24°C. Obenland et al. [82] reported a rapid decline in the sensory quality of "W. Murcott Afourer's" mandarins due to an excess of alcohols and ethyl esters, resulting in alcoholic of-favor disorder at 20°C compared to 5°C and 10°C. Similarly, Yang et al. [18] also concluded that high temperatures have a deteriorative efect, particularly due to the enhanced biosynthesis of volatile aromatic compounds (primarily in ethanol) in banana fruit during storage at 30°C than 20°C. Furthermore, Petracek et al. [83] likewise reported an increase in ethanol content in sweet cherry (Prunus avium L., cv. "Sams") during storage at 20°C compared to storage at 5°C, 10°C, and 15°C, resulting in alcoholic of-favor disorder. On the other hand, Zou et al. [86] demonstrated that lower storage temperatures ranging from 4°C to 10°C signifcantly impacted the transcript expression of genes ADH2, PDC1-like1, and PDC1-like2 in tomato fruit. In addition, the efects of storage temperature either on the accumulation or reduction of of-favor compounds in a variety of fruits have been reported, including "Hort16A" kiwifruit [87], cantaloupe [88], strawberry [89], tomatoes [90], oranges [91], and nectarine [92].

GABA Shunt Pathway.
Te GABA shunt pathway regulates many physiological mechanisms such as C : N balance, pH-stat, plant development, plant defense against insects, signal transduction, and osmoregulation [6,93]. A short pathway generally metabolizes GABA via the TCA cycle that includes glutamate decarboxylase (GAD), GABAtransaminase (GABA-T), and succinate semialdehyde dehydrogenase (SSADH), called the GABA shunt pathway [94,95], as depicted in Figure 3(a). So apart from GABA benefcial roles, its accumulation under stress is linked to various postharvest physiological disorders, including the incidence of pear fruit core breakdown [96], pear fesh and spathe browning [97,98], reduced chilling tolerance in Natura zucchini fruit [99], and controlled atmosphere-related injury in "Honeycrisp" apples [100]. Enhanced GABA levels have also been linked to tomato surface pitting and soggy breakdown in "Honeycrisp" apples, respectively [101,102]. Regarding alcoholic of-favor disorder, Deewatthanawong et al. [103] found that lower GABA-T activity under high CO 2 storage conditions induced the synthesis and accumulation of alcoholic of-favor compound ethanol along with GABA accumulation as a stress marker in response to a high CO 2 environment in the "jewel" strawberry cultivar. Moreover, Mae et al. [104] also reported an increase in GABA levels due to upregulated GAD and limited GABA-T gene expressions in tomatoes stored in a hypoxic atmosphere. According to the preceding discussion, the relationship between GABA accumulation is mostly similar to the results of low O 2 /high CO 2 or anaerobic respiration, which resulted in an ATP defcit and reduced aerobic metabolic pathways. Interestingly, Deewatthanawong and Watkins [101] justifed the interlinking of GABA enhancement with the accumulation of fermentation metabolites. Our previous research concluded that the GABA shunt pathway might initiate ethanol metabolism, thus possibly causing the alcoholic of-favor disorder in kiwifruit cv. "Bruno" [6]. In Figures 3(b)-3(d), it has been shown that inhibited activity of the GABA-T enzyme from 12 days (d) to onward in control kiwifruit cv. "Bruno" appears to be directly linked to limited decarboxylation of GABA and thus shows a sign of suppressed GABA shunt pathway that impacts the functionality of the TCA cycle and thus leads to activation of PDC and ADH, resulting in oversynthesis or accumulation of acetaldehyde and ethanol contents. Our previous study noticed this relationship by using 1-methylcyclopropene (1-MCP) to reduce ethanol metabolism by regulating the GABA shunt pathway under reduced climacteric respiration, avoiding the occurrence of alcoholic of-favor disorder in kiwifruit during storage [6]. However, limited research has been conducted to investigate the GABA shunt pathway concerning ethanol fermentation, suggesting the need for an integrated approach to analyzing GABA shunt pathway-related gene expression, protein level, and involved metabolites at transcriptomic, proteomics, and metabolomics levels further to reveal the GABA shunt infuence on ethanol fermentation.

Mitochondrial Energy Metabolism.
Te cell mitochondria are a primary site for maintaining the ATP or energy status pool required for regulating the plant's physiological or biochemical reactions [105,106]. Te mitochondrial energy status is closely related to the activities of energy metabolism-related enzymes, including succinic dehydrogenase (SDH), cytochrome C oxidase (CCO), H +adenosine triphosphatase (H + -ATPase), and Ca 2+ -adenosine triphosphatase (Ca 2+ -ATPase) [12]. Previously, decreased ATP and energy charge (EC) in various fruits were adversely associated with several physiological disorders i.e., chilling injury [107], tissue, pericarp, or peel browning [108], skin pitting [109], and ethanol accumulation or alcoholic offavor disorder [12,19,95]. An energy defcit occurs due to dysfunctional mitochondrial energy metabolism at the cellular level [19,77].
In agreement, our previous study concluded a possible correlation between dysfunctional mitochondrial energy metabolism with reduced SDH, CCO, H + -ATPase, and Ca 2+ -ATPase activities with upregulated ethanol metabolism [12]. Moreover, Cukrov et al. [39] also reported the occurrence of alcoholic of-favor in kiwifruit due to downregulated mitochondrial energy metabolism during storage. Consequently, Zhang et al. [110]; Huan et al. [19]; and Blanch et al. [77] further coincided with the susceptibility of alcoholic of-favor disorder incidence under the same circumstances in strawberry and kiwifruit during storage.

Glycolysis and TCA Cycle.
Glycolysis and the TCA cycle are the central respiratory pathways in horticultural crops. Tese pathways are involved in maintaining the ripening or senescence processes by regulating the energy status, carbon fux, NAD(P), and NAD(P)H levels through sequential cell reactions [111]. However, research has revealed that such respiratory pathways exhibit considerable variation under diferent conditions. Te glycolysis ended up with pyruvate formation that acts as a primary substrate for the frst enzyme of the TCA cycle, pyruvate dehydrogenase (PDH), and ethanol fermentation (PDC) [11,12]. Meanwhile, the concentration and fate of pyruvate either in the TCA cycle or ethanol fermentation are profoundly infuenced by a high CO 2 /low O 2 atmosphere or anaerobic metabolism. For example, Ummarat et al. [11] concluded that the of-favor sensation caused by the over-accumulation of ethanol correlated with enhanced pyruvate content in "Pixie" mandarin during an anaerobic atmosphere generated by waxing. Interestingly, it has been reported that enhanced pyruvate is preferred for alcoholic fermentation over the TCA cycle in anaerobic respiration [112]. Mannucci et al. [113] reported that anaerobic metabolism eventually  converts pyruvate to acetaldehyde and ethanol, suggesting that pyruvate and ethanol contents are critically interlinked under stress conditions. Te increase or accumulation of glycolytic fux is interlinked with the enhanced activity of pyruvate kinases (PKs) or suppressed activities of TCA cycle enzymes that are eventually indicated by the accumulation of TCA cycle organic acid [12,21,114]. Te functional TCA cycle is vital in generating a vast pool of ATP for regulating the cell metabolic mechanisms via oxidative phosphorylation or electron transport chains to maintain the desired ripening processes of horticultural crops [115][116][117]. Meanwhile, the TCA cycle has been reported to be replaced by glycolysis, which directs pyruvate to ethanol metabolism to generate sufcient ATP to keep cell functions alive under hypoxic conditions [12,118,119]. A number of previous research reported that ethanol metabolism occurs under stressed conditions as a result of a malfunctioned TCA cycle, which is associated with suppressed PDH, SDH, and GABA-T enzyme activities as well as increased levels of organic acids such as citric acid, succinic acid, and GABA content [12,26,[120][121][122].

Cytosolic Malate Metabolism.
In plants, malate is important in regulating the cytoplasmic pH, cell acidity, and carbon metabolism. Malate is synthesized by converting the phosphoenol-pyruvate (PEP) into oxaloacetate (OAA) by the activities of cytosolic enzymes, including phosphoenolpyruvate carboxylase (PEPC) and NAD-linked malate dehydrogenase (NAD-MDH) [33]. It has been demonstrated that both OAA and malate can enter the mitochondrial TCA cycle to generate numerous organic acids. Malate can also be converted to pyruvate by the cytosolic NADP-linked malic enzyme (NADP-ME) via a dicarboxylate carrier [123].
Little research has been done into the possible link between cytosolicmalate metabolism and ethanol metabolism. In a recent study, Huan et al. [21] reported the promising involvement of higher expression of NADPdependent malic enzymes (NADP-MEs) in increased ethanol production that eventually develops an alcoholic offavor in kiwifruit cv. "Bruno" during ambient storage conditions. Malate metabolism has previously been shown to induce ethanol metabolism by converting stored malate into pyruvate under various atmospheric conditions in harvested fruit such as grapes, berries, and strawberries [20,123]. In contrast, Ponce-Valadez and Watkins [33] found a discrepancy between ethanol metabolism-related metabolites and enzymes involved in cytosolic malate metabolism. However, more omics-based research is needed to validate the role of cytosolic-malate metabolism in causing alcoholic of-favor. A proposed schematic diagram of the relationship between cytosolic malate and ethanol metabolism is depicted in Figure 4.

Starch and Sugar
Metabolism. Fruits primarily convert their starch stores into sugars as their primary energy source [124]. Te favor of ripe fruit is largely determined by the number of soluble sugars that accumulate during ripening [125]. Cultivars difer signifcantly in starch degradation and sugar composition at various stages of fruit development and ripening [126]. Te starch in starch-based crops reportedly needs to be converted into simple sugars before it can be fermented into ethanol [127]. Previous research has indicated that sugars might be the most important substrates utilized by ethanol fermentation during fruit ripening [128]. β-amylase plays a crucial role in carbohydrate metabolism by depolymerizing α-glucan chains, thereby facilitating starch degradation during fruit ripening [129,130], as evidenced in blueberries, as starch content drops at the same time as the rise in β-amylase gene expression [131]. It has been shown that the conversion of mango fruit starch to soluble sugars occurs simultaneously when the anaerobic respiration pathway is initiated [132]. Terefore, elevated starch degradation to sugars phenomenon may afect the biosynthesis of ethanol metabolism metabolites, which could initiate an of-favor due to the over production and accumulation of ethanol content under anaerobic respiration.
Regarding this, Huan et al. [21] revealed that starch-tosucrose metabolism (starch degradation) might induce alcoholic of-favor disorder in "Bruno" kiwifruit during postharvest storage based on the fndings associated with a dramatic decline in starch contents coincided with increased sucrose, fructose, and glucose levels accompanied with enhanced acetaldehyde and ethanol contents along with higher expressions of key genes such as starch phosphorylases (SPs), beta-amylases, UDP-glucose pyrophosphorylases (UGPases), sucrose synthases (SSs), and invertases (INVs) which are responsible for accelerating starch conversion to soluble sugars. Tis study provided transcriptome evidence that increased starch catabolism during fruit ripening may function as a substrate to promote ethanol fermentation, which induces an alcoholic of-favor.

Chemical Treatments.
Numerous chemical treatments have been studied and applied to various fresh fruits to eradicate the susceptibility of alcoholic of-favor disorder via suppressing the ethanol metabolism in both pre-and postharvest stages. Our recent study indicates that 1-MCP (1 μL·L −1 ) treatment signifcantly reduced the occurrence of alcoholic of-favor disorder in kiwifruit during storage at ambient conditions via inhibiting the ethanol metabolismrelated enzymatic activities of PDC and ADH [12]. Subsequently, Huan et al. [133] also reported that 1 μL·L −1 1-MCP treatment could efectively reduce the alcoholic of-favor disorder by suppressing ethanol metabolism in kiwifruit during ambient storage. Tewes et al. [134] concluded that applying 1-MCP together with dynamic controlled atmosphere (DCA) was particularly efective in lowering acetaldehyde and ethanol biosynthesis in apple fruit. Such fndings were correlated with reduced PDC and ADH activity in apples, except during the mature stage, regardless of the cultivar. Te lower levels of acetaldehyde and ethanol contents were also noticed in "Galaxy" apples under the efect of 1-MCP treatment and a dynamically controlled atmosphere based on respiratory quotient [135]. It has also been observed that 1-MCP could substantially lower the expression of PDC2 and ADH, reducing acetaldehyde and ethanol synthesis in apple fruit [136].
Recently, Lv et al. [22] demonstrated that exogenous application of 100 μmol·L −1 nordihydroguaiaretic acid (NDGA) on "Nanguo" pear fruit resulted in lower acetaldehyde and ethanol contents concomitant with downregulated expressions of PDC1, ADH1, and ADH2 genes. Our previous work also suppressed the ethanol metabolism via preharvest spraying of 5 mmol·L −1 oxalic acid (OA), which resulted in the absence of alcoholic of-favor disorder in kiwifruit "Bruno" during storage under ambient conditions [32]. In another study, Zhang et al. [137] demonstrated that fumigation with 10 μmol·L −1 carbon monoxide (CO) on the winter jujube might restrain the ethanol metabolismrelated metabolites (acetaldehyde and ethanol) under reduced PDC and ADH activities. Li et al. [17] reported that treating strawberries with 1 mM ATP in a 20% CO 2 storage atmosphere reduced acetaldehyde and ethanol overaccumulation (72% and 75% lower in ATP + CO 2 -treated fruit, respectively) that was linked to lower PDC and ADH activities during fruit storage. In a recent study, the efects of melatonin on ethanol metabolism in kiwifruit were also investigated, resulting in reduced acetaldehyde and ethanol contents under suppressed PDC and ADH activities and downregulated expressions of potential genes such as AdADH1, AdPDC1, and AdPDC2 during storage [138]. Tese changes may eventually result in decreased synthesis of aromatic compounds such as acetaldehyde and ethanol, thus controlling the alcoholic of-favor disorder in fresh fruits during storage.

Hypobaric and Other
Treatments. Several studies have found that hypobaric treatments can help suppress or delay various physiological disorders in fresh fruits. To suppress the alcoholic of-favor disorder, Huan et al. [19] reported the signifcant efects of hypobaric treatment (25 ± 5 kPa for 30 min/twice treatment) in alleviating the alcoholic offavor disorder by inhibiting the activities of PDC and ADH in kiwifruit during storage. Previously, it had been demonstrated that fresh fruits could reduce the occurrence of alcoholic of-favor by reducing the ethanol metabolism metabolites during storage under modifed superatmospheric O 2 exposure [44,139]. Similarly, Wood et al. [51] observed a decrease in acetaldehyde and ethanol levels under controlled storage conditions (O 2 kPa −1 CO 2 kPa −2.5 ) for "Golden Delicious" and "Jonagold" apple fruit. Furthermore, Chen et al. [140] reported the decrease in ethanol and acetaldehyde production in "Benihoppe" strawberries under low oxygen application (2 kPa O 2 ) during storage. Zuo et al. [141] investigated the efects of high relative humidity (RH: 98 ± 2%) on ethanol metabolism in zucchini fruit, identifying that it inhibited the activities of PDC and ADH, as well as CpPDC1 and CpADH1 gene transcripts, resulting in decreased acetaldehyde and ethanol levels during cold storage. Previously, certain coating formulations were reported to initiate ethanol metabolism, resulting in of-favor disorder in various fruits due to a change in the internal gas atmosphere of the fruit.
Meanwhile, a recent study showed that Carnauba wax nano-emulsion (attributed to being an efective moisture barrier and relatively permeable to gases) as a coating material had the least impact on the occurrence of alcoholic of-favor by avoiding the production of ethanol above the threshold level in citrus fruit during storage [142]. Moreover, Velazco et al. [143] also concluded that Brillaqua F6 (18% solids (9.35% oxidized polyethylene wax and 5.7% shellac)), Citrosol AK (18% solids (Carnauba E903 and shellac)), and Teycer GLK (18% solids (Carnauba and shellac)) coatings on citrus fruit had promising efects about reduced acetaldehyde and ethanol contents, thus decreasing the susceptibility to alcoholic of-favor disorder. Te overall impacts of chemical, hypobaric, and other treatments on ethanol metabolism to suppress the alcoholic of-favor disorder in fresh fruits are summarized in Table 3.  Li et al. [17] Melatonin (100 μmol·L −1 ) "Bruno" kiwifruit Reduced acetaldehyde and ethanol contents and downregulated expressions of potential genes including AdADH1, AdPDC1, and

Citrus
Prevented the above threshold level biosynthesis of ethanol Miranda et al. [142] Coatings (Brillaqua

Conclusion and Future Work
Anaerobic respiration, low O 2 stress, high CO 2 stress, and storage temperature stress are the most important factors for inducing alcoholic of-favor disorder by upregulating ethanol metabolism in fresh fruits. Tese factors further cause undesirable metabolic changes at cellular levels by altering physiological mechanisms such as the GABA shunt pathway, mitochondrial energy metabolism, glycolysis and TCA cycles, cytosolic-malate metabolism, and starch and sugar metabolism. Tese physiological metabolic mechanisms might further activate ethanol metabolism, resulting in overbiosynthesis and the accumulation of alcoholic of-favorrelated metabolites such as acetaldehyde and ethanol in fresh fruits. Several chemicals, hypobaric, and coatings treatments have been practiced to overcome alcoholic of-favor disorders in fresh fruits during storage. Despite this, a wide research gap still prevails that needs further investigation at the omics level to evaluate or validate the impact of numerous physiological mechanisms on ethanol metabolism and reveal the detailed mechanistic relation. In addition, other mechanisms and factors, including respiratory mechanisms, fruit genetic makeup, degree of maturity, and postharvest handling and storage conditions, are also recommended for future work concerning the alcoholic offavor disorder in fresh fruits during storage. Te overall thematic scheme representing the current review is shown in Figure 5.

Data Availability
Te dataset supporting the conclusions of this article is included within the article.

Conflicts of Interest
Te authors declare that there are no conficts of interest.

Authors' Contributions
Maratab Ali conceptualized the study and wrote the original draft. Sara Batool, Muhammad Faisal Manzoor, Rana Muhammad Aadil, and Xiuming Zhao wrote, reviewed, and edited the article was responsible for writing and review and editing. Nauman Khalid and Fujun Li performed visualization and wrote, reviewed, and edited the paper. Xiaoan Li was responsible for validation, writing, reviewing, and editing. Xinhua Zhang was responsible for conceptualization, writing and review, editing, funding, and supervision.