Identification of (−)-epicatechin as the direct substrate for polyphenol oxidase isolated from litchi pericarp

Postharvest browning of litchi fruit results in a short life and a reduced commercial value. The experiments were conducted to separate, purify and identify the polyphenol oxidase (PPO) substrates that cause litchi fruit to brown. PPO and its substrates were, respectively, extracted from fruit pericarp tissues. The substrates for litchi PPO were separated and purified using polyamide column chromatography, silica gel column chromatography and preparative thin layer chromatography. The substrate was further identified by 0.5% FeCl₃ solution and enzymatic reaction with litchi PPO. On the basis of UV, ¹H NMR, ¹³C NMR, and ESI-MS data, the direct substrate for the PPO from litchi fruit pericarp tissues was identified to be (−)-epicatechin.     

Role of endogenous and exogenous phenolics in litchi anthocyanin degradation caused by polyphenol oxidase

The degradation of anthocyanins and/or the oxidation of phenolics caused by polyphenol oxidase (PPO) results in an enzymatic browning reaction of fruits and vegetables. This work was conducted with a view to explaining the unexpected observation that litchi (Litchi chinensis Sonn.) PPO did not directly oxidise litchi anthocyanins. PPO and anthocyanin from litchi fruit pericarp were extracted and purified, respectively, and then the anthocyanin degradation by PPO in the presence of (−)-epicatechin (endogenous PPO substrate), and catechol and gallic acid (exogenous PPO substrates) were analysed comparatively. The results showed that catechol was the most effective in litchi anthocyanin degradation, followed by (−)-epicatechin and gallic acid, but no significant differences existed between catechol and (−)-epicatechin. The study suggested that litchi PPO directly oxidised (−)-epicatechin; then oxidative products of (−)-epicatechin in turn catalysed litchi anthocyanin degradation, and finally resulted in the browning reaction, which can account for pericarp browning of postharvest litchi fruit.     

Identification of (−)-epicatechin as the direct substrate for polyphenol oxidase from longan fruit pericarp

Postharvest browning of longan fruit results in a short life and a reduced commercial value. The experiments were conducted to separate, then purify and finally identify the polyphenol oxidase (PPO) substrates that cause longan fruit to brown. PPO and its substrates were, respectively, extracted from longan fruit pericarp tissues. The substrate for longan PPO was separated and purified using polyamide column chromatography, Sephadex LH-20 column chromatography and silica gel column chromatography, respectively. The substrate was further identified by 0.5% FeCl₃ solution and enzymatic reaction with longan PPO. On the bases of UV, ¹H NMR, ¹³C NMR, and ESI-MS data, the direct substrate for the PPO from pericarp tissues of longan fruit was identified to be (−)-epicatechin. Furthermore, the contents of (−)-epicatechin of pericarp tissues of longan fruit of two major cultivars were determined by high performance liquid chromatography (HPLC). The HPLC analysis exhibited that the contents of (−)-epicatechin of fruit pericarp of ‘Shixia’ and ‘Chuliang’ were 0.26 and 0.56 mg/g on fresh weight (FW) basis at harvest and 0.15 and 0.09 mg/g FW after 3 days of storage. The more rapid decrease in the (−)-epicatechin content of ‘Chuliang’ was due to the oxidization catalyzed by PPO, which was in agreement with the higher browning index.     

Oxidation of (−)-epicatechin is a precursor of litchi pericarp enzymatic browning

The degradation of flavanols had been studied both in the litchi pericarp during the storage and in the model system containing PPO and flavanols of litchi pericarp. The results showed that (−)-epicatechin was the optimal endogenous substrate of litchi pericarp PPO, and the procyanidins of litchi pericarp were oxidised very slowly when incubated alone with PPO. However, (−)-epicatechin could accelerate the oxidation of the other flavanols in litchi pericarp through a coupled oxidation pathway. The results obtained allowed us to draw a conclusion that the oxidation of (−)-epicatechin was a precursor of litchi pericarp browning. A pathway of enzymatic browning of litchi pericarp was proposed as follows: with the loss of cellular compartmentation, the litchi pericarp PPO and flavanols mixed and, then, (−)-epicatechin was oxidised by the PPO and o-quinones formed. The o-quinones reacted with other flavanols and anthocyanins, accelerating the oxidation of other polyphenols. Finally, the oxidation of (−)-epicatechin and other polyphenols led to the formation of the brown-coloured compounds, resulting in the enzymatic browning of litchi pericarp.     

Antioxidant activities and contents of polyphenol oxidase substrates from pericarp tissues of litchi fruit

The experiments were performed to extract and purify substrates for polyphenol oxidase (PPO) from pericarp tissue of postharvest litchi fruit. Two purified PPO substrates were identified as (−)-epicatechin and procyanidin A2. The antioxidant properties of two PPO substrates were further evaluated in the present study. Variation in the content of the major substrate (−)-epicatechin of litchi fruit during storage at 25 °C was analysed using the HPLC-UV method. The results showed that (−)-epicatechin exhibited stronger antioxidant capability than procyanidin A2, in terms of reducing power and scavenging activities of DPPH radical, hydroxyl radical and superoxide radical. Furthermore, (−)-epicatechin content in pericarp tissue tended to decrease with increasing skin browning index of litchi fruit during storage at 25 °C. Thus, these two compounds can be used as potential antioxidants in litchi waste and the fresh pericarp tissue of litchi fruit exhibited a better utilisation value.     

EFFECT OF OXYGEN CONCENTRATION ON THE BIOCHEMICAL AND CHEMICAL CHANGES OF STORED LONGAN FRUIT

ABSTRACT

Longan fruits were stored for 6 days in atmosphere of 5, 21 (air) or 60% O₂ (balance N₂) at 28C and 90–95% relative humidity to examine effects of low and high O₂ concentration on enzymatic browning and quality attributes of the fruit. Changes in pericarp browning, pulp breakdown, disease development, total phenol content, activities of phenol metabolism‐associated enzymes, relative leakage rate, α,α‐diphenyl‐β‐picrylhydrazy (DPPH) radical scavenging activity, and contents of total soluble solids, titratable acidity and ascorbic acid were evaluated. Storage of fruit in a 5% O₂ atmosphere markedly delayed pericarp browning in association with maintenance of high total phenolic content and reduced activities of polyphenol oxidase (PPO), peroxidase (POD) and phenylalanine ammonia lyase. Moreover, the fruit stored in a 5% O₂ atmosphere exhibited a lower relative leakage rate and higher DPPH radical scavenging activity than fruit stored in air. This presumably was beneficial in maintaining compartmentation of enzymes and substrates, and thus, reducing pericarp browning. Pulp breakdown and disease development were also reduced by exposure to a 5% oxygenatmosphere. On the contrary, exposure of longan fruit to a 60% O₂ atmosphere accelerated pericarp browning, pulp breakdown and decay development. PPO and POD activities and relative leakage rate were similar for control and 60% O₂‐treated fruit after 4 and 6 days of storage. Furthermore, treatment with 60% O₂ significantly decreased the phenolic content and DPPH scavenging activity of fruit. In addition, exposure to 5 or 60% O₂ resulted in a higher level of total soluble solids, but a lower level of ascorbic acid of longan fruit flesh. In conclusion, exposure to a 5% O₂ atmosphere showed great potential to reduce pericarp browning and extend shelf life of longan fruit.

PRACTICAL APPLICATIONS

Pericarp browning and pulp breakdown are the major causes of deterioration in postharvest longan. Conventional controlled atmosphere with low O₂ and high CO₂ is effective in maintaining quality and extending shelf life of fruits and vegetables, including inhibition of tissue browning. In this study, 5%‐controlled atmosphere reduced significantly pericarp browning, pulp breakdown and rot development. It could potentially be useful as a postharvest technology of longan fruit for reducing or replacing the use of chemicals such as SO₂ and fungicides, but it requires further investigation.     

Two Endogenous Substrates for Polyphenoloxidase in Pericarp Tissues of Postharvest Rambutan Fruit

Abstract: The catalytic oxidation of phenolic substrates by polyphenoloxidase (PPO) causes pericarp browning of postharvest rambutan fruit. In the present study, PPO and its endogenous substrates were extracted from rambutan pericarp tissues (RPT). The substrate extracts were sequentially partitioned with ethyl acetate and n-butanol. The analysis of total phenolic content showed that the most phenolic compounds were distributed in ethyl acetate fraction. By high-performance liquid chromatography (HPLC), (−)-epicatechin (EC) and proanthocyanidin A2 (PA2) were identified from this fraction. After reacting with rambutan PPO, EC turned brown rapidly within 10 min, indicating that it was a significant endogenous substrate. Although PA2 could also be oxidized by the PPO, it turned brown very slowly. In addition, because EC and PA2 were continually catalyzed into browning products by PPO during storage of the fruit at 4 and 25 °C, their contents in RPT gradually declined with the extended storage time. It was further observed that both substrate contents in rambutan fruit storing at 25 °C decreased more rapidly than that storing at 4 °C, suggesting that low temperature inhibited the catalytic oxidation of substrates so as to slow down pericarp browning. Practical Application: Pericarp browning is a serious problem to storage and transport of harvested rambutan fruit. A generally accepted opinion on the browning mechanism is the oxidation of phenolic substrates by PPO. Ascertaining PPO substrates will effectively help us to control enzymatic reaction by chemical methods so as to delay or even prevent pericarp browning of harvested rambutan fruit.     

Vital Characteristics of Litchi (Litchi chinensis Sonn.) Pericarp that Define Postharvest Concepts for Thai Cultivars

To assess the fruit-specific determinants of pericarp browning, litchi pericarp was characterized in terms of appearance, the polyphenol pattern as specified by HPLC-DAD-MS ⁿ without and after thiolysis, and the activities of polyphenol oxidase (PPO) and peroxidase (POD) by exploring “Kwang Jao,” “O-Hia,” “Kim Cheng,” and “Chacapat” fruit on the respective harvest day, “Hong Huey” fruit also throughout 52 days of cold storage (5 °C, 95% relative humidity). At harvest, PPO activity was maximum for “Kim Cheng” pericarp (126 μkat/hg), whereas POD activity was striking for that of “O-Hia” (512 μkat/hg, including membrane-bound isoforms). Flavan-3-ol and proanthocyanidin patterns were consistent for all cultivars. However, cultivars with sharp-pointed and round–obtuse protuberances differed in pericarp anthocyanin and flavonol glycosylation patterns. The molar ratio of cyanidin 3-O-rutinoside to its glucoside was ≤6:1 for “Hong Huey” and “Kwang Jao,” but ≥43:1 for “Kim Cheng” and “Chacapat” pericarp. Long-term storage gave evidence of two key processes involved in pericarp browning: (1) PPO-mediated oxidation of abundant (?)-epicatechin (1.4–2.0 g/hg), resulting in dark brown pigments, and (2) microcrack-induced formation of light brown surface scurf, supposably with involvement of POD. Accordingly, an improved scheme for litchi pericarp browning was proposed. As regards recommendable postharvest concepts for each cultivar, “Chacapat” suited most for long-distance transports due to its overall low susceptibility to pericarp browning. Properties of “O-Hia” litchi, being prone to surface scurf formation, suggested preferred distribution via domestic markets. High contents of flavonols (e.g., quercetin glycosides, 166 mg/hg) and A-type-linked procyanidins (e.g., procyanidin A2, 1,092 mg/hg) qualified pericarp of “Hong Huey” litchi as raw material for polyphenol extracts exerting antioxidant properties.     

Short‐term anoxia treatment maintains tissue energy levels and membrane integrity and inhibits browning of harvested litchi fruit

Pericarp browning is the main limitation to the postharvest storage, handling and marketing life of litchi fruit. Pre‐storage treatment with pure N₂ gas is potentially effective in reducing skin browning and maintaining eating quality of litchis. To better understand inhibition of pericarp browning by a short period of anoxia, adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) levels, adenylate energy charge and membrane permeability were investigated. Litchi fruit were exposed to pure N₂ gas for 6 h and then kept in closed but vented containers for 6 days in the dark at 25 °C and 95–100% relative humidity. Changes in the mentioned fruit physiology and biochemistry parameters and in browning index were measured. ATP concentration and adenylate energy charge decreased rapidly and membrane permeability (relative leakage) increased gradually during storage. Fruit exposed to N₂ gas exhibited higher concentrations of ATP, ADP and AMP and adenylate energy charge levels, and lower levels of browning index and membrane permeability, compared to control (non‐N₂‐treated) fruit. Greater differences in ATP and ADP concentrations and adenylate energy charge levels of pericarp tissues between N₂‐treated and control fruit were more manifest after 4 and 6 days of storage, in association with significant differences at the 5% level in the pericarp browning index. It is suggested that pre‐storage anoxia treatment maintains membrane integrity of pericarp tissues, with high ATP and ADP concentrations and high adenylate energy charge levels. Thus, the loss of cellular compartmentalization (mixing of enzymes and substrates) that leads to enzymatic browning of litchi fruit pericarp is delayed. Copyright © 2007 Society of Chemical Industry     

Inhibition of polyphenol oxidase and the browning control of litchi fruit by glutathione and citric acid

Polyphenol oxidase (PPO, EC 1.10.3.2) from litchi peel was partially purified by ammonium sulfate fractionation and gel filtration, and a 16-fold purification of PPO achieved. The use of 10 mmol litre⁻¹ glutathione and 100 mmol litre⁻¹ citric acid was found to give good control of the browning of litchi fruit and 80–85% inhibition of PPO observed. Application of glutathione in combination with citric acid is recommended as a way of slowing the browning of litchi fruit.     

PARTIAL PURIFICATION AND CHARACTERIZATION OF POLYPHENOL OXIDASE FROM FRESH-CUT CHINESE WATER CHESTNUT

The characteristics of polyphenol oxidase (PPO) from Chinese water chestnut (CWC) and its potential inhibitors for browning reactions were investigated. PPO was isolated from fresh‐cut CWC and was purified on a Sephadex G‐100 column, with a yield of total activity close to 10%. The molecular weight, Michaelis constant (K_m), substrate specificity, optimal pH and temperature of CWC PPO were examined. Kinetic studies indicated that the K_m and V_max values of CWC PPO for catechol were 10.32 mmol/L and 6.452 × 10⁴ U/min, respectively. The optimal pH and temperature for CWC PPO was 6.5 and 40C, respectively. Among the browning inhibitors tested, 4‐hexylresorcinol, at a concentration of 0.3 mmol/L, showed the strongest inhibition (70%) against the PPO activity of CWC, followed by 3.0 mmol/L N‐acetyl‐L‐cysteine with an inhibition of 53%.     

INHIBITORY EFFECT OF ANTHOCYANIN EXTRACT FROM SEED COAT OF BLACK BEAN ON PERICARP BROWNING AND LIPID PEROXIDATION OF LITCHI FRUIT DURING STORAGE

ABSTRACT

Anthocyanins were extracted from seed coats of black beans (Glycine max[L.]) and the inhibitory effects of anthocyanin extract on pericarp browning and lipid peroxidation of litchi fruit were investigated. Litchi fruit were infiltrated for 3 min with 0 (control) or 50 mg/L of anthocyanin extract at a reduced pressure of 53 kPa, then packed in 0.03 mm thick polyethylene bags, and finally stored at 28C for 6 days. Changes in browning index, contents of anthocyanins and total phenol, peroxidase (POD) activity, levels of relative leakage rate and lipid peroxidation, α,α‐diphenyl‐β‐picrylhydrazy (DPPH) radical scavenging activity and reducing power were evaluated. Application of anthocyanin extract from black bean delayed pericarp browning of litchi fruit during storage, which was associated with reduced POD activity and higher contents of anthocyanins and total phenol. Moreover, the anthocyanin extract was found to have a direct inhibition on the POD activity in vitro. Furthermore, application of the anthocyanin extract apparently reduced lipid peroxidation and relatively maintained membrane integrity, which may account for browning inhibition to an extent. Finally, higher DPPH radical scavenging activity and reducing power of the fruits treated with the anthocyanin extract than control fruits possibly benefited in scavenging free radicals and reducing lipid peroxidation. It is, therefore, suggested that inhibited POD activity and reduced lipid peroxidation by the anthocyanin extract from seed coats of black beans were responsible for the inhibition of pericarp browning of litchi fruits.

PRACTICAL APPLICATIONS

Lipid peroxidation is a major cause of quality deterioration of postharvest fruits and vegetables. Some synthetic antioxidants are beneficial in inhibiting lipid peroxidation. However, considering that synthetic antioxidants, such as butylhydroxyanisole and butylated hydroxytoluene (BHT), have potential toxicity, the use of natural extracts to extend the shelf life of postharvest fruits and vegetables is a tendency. In our previous study, it has been found that anthocyanin extract from litchi pericarp has stronger antioxidant activities or free radical scavenging activities than BHT and ascorbic acid ( Duan et al. 2007 ). In this study, the application of anthocyanin extract from seed coat of black soybean showed reduced lipid peroxidation and pericarp browning. It could be used potentially as a postharvest technology for reducing or replacing the use of other chemicals, but it requires further investigation.     

Antioxidant properties of anthocyanins extracted from litchi (Litchi chinenesis Sonn.) fruit pericarp tissues in relation to their role in the pericarp browning

Anthocyanins were extracted and purified from litchi fruit pericarp and their antioxidant properties were investigated. Effects of exogenous anthocyanin treatments on pericarp browning and membrane permeability of harvested litchi fruit were also evaluated. Anthocyanins from litchi fruit pericarp strongly inhibited linoleic acid oxidation and exhibited a dose-dependent free-radical-scavenging activity against DPPH radical, superoxide anions and hydroxyl radical. The degradation of deoxyribose by hydroxyl radicals was shown to be inhibited by anthocyanins acting mainly as chelators of iron ions rather than directly scavenging hydroxyl radicals. Anthocyanins were also found to have excellent reducing power. The reducing power of anthocyanins, ascorbic acid and butylated hydroxytoluene all at 100 μg/ml were 3.70, 0.427 and 0.148, respectively, indicating that anthocyanins from litchi pericarp had a strong electron-donating capacity. Furthermore, application of anthocyanins to harvested litchi fruit significantly prevented pericarp browning and delayed the increase in membrane permeability. It was therefore suggested that anthocyanins could be beneficial in scavenging free radicals and reducing lipid peroxidation of litchi fruit pericarp.     

Effect of pure oxygen atmosphere on antioxidant enzyme and antioxidant activity of harvested litchi fruit during storage

The effects of pure oxygen on pericarp browning, reactive oxygen species (ROS) metabolism, antioxidant enzyme and antioxidant activity of harvested litchi fruit were investigated. Application of pure oxygen significantly prevented pericarp browning and delayed the increase in membrane permeability of litchi fruit during storage. Litchi fruit exposed to pure oxygen showed a lower level of lipid peroxides, compared to control fruit, with the delay in the increases of both H₂O₂ content and superoxide production rate. Furthermore, it was found that the treatment with pure oxygen induced the activities of superoxide dismutase (SOD), ascorbated peroxidase (APX) and catalase (CAT), which could be beneficial in scavenging of H₂O₂ and superoxide and alleviating lipid peroxidation. In addition, antioxidant ability (reducing power and free-radical scavenging activity against DPPH radical, superoxide anions and hydroxyl radical) of methanol extracts from litchi fruit pericarp declined gradually, with decreasing contents of anthocyanins and phenolic compounds, as storage time of the fruit progressed. There was a linear relationship between the contents of either anthocyanins or phenolic compounds and antioxidant ability or free radical scavenging activity. Treatment with pure oxygen markedly increased antioxidant ability, which was related to higher levels of anthocyanins and phenolic compounds, compared with those of control fruit. It is suggested that enhanced antioxidant activity and antioxidant enzyme induced by pure oxygen may contribute to alleviating lipid peroxidation and maintenance of membrane integrity, which reduced decompartmentation of enzymes and substrates, resulting in enzymatic browning.     

Role of anthocyanin degradation in litchi pericarp browning

Pericarp browning is the main problem of post-harvest litchi fruit, resulting in an accelerated shelf life and reduced commercial value of the fruit. Underhill and Critchley (1994). Anthocyanin decolorisation and its role in lychee pericarp browning. Australian Journal of Experimental Agriculture, 34, 115-122 found that there was not an obvious change in the content of anthocyanins when the fruit browned. This work was conducted with a view to explaining this unexpected observation. Litchi pericarp browning index increased while the content of anthocyanins decreased with storage time when 0.1 M HCl was used as the extract solution instead of acidic methanol. The visible spectum of the anthocyanin extract, at a range of 400–600 nm and pH values of 1.0, 3.0 and 5.0, were recorded, with an absorbance peak of about 510 nm. The colour of the extract depended on the pH values and the half-degradation constants for anthocyanins at pHs 1.0, 3.0 and 5.0 were, respectively, 29, 15.3 and 10.5 days, as calculated from the kinetics of the degradation. Compared with the anthocyanin extract, anthocyanidin is more vulnerable, with a half-degradation of about 5.3 min at pH 5.0. Furthermore, the product from the anthocyanidin degradation had a similar structure to catechol (a good substrate for polyphenol oxidase), which, in turn, could accelerate enzymatic browning reaction by the enzyme polyphenol oxidase. In addition, an anthocyanase, catalyzing anthocyanin hydrolysis and producing anthocyanidin was extracted from litchi fruit pericarp. High activity of the enzyme was observed in the pericarp. Thus, it is suggested that anthocyanase might contribute to the browning of litchi pericarp involved in the anthocyanase-anthocyanin-PPO reaction.     

Effects of O2 and CO2 concentrations on physiology and quality of litchi fruit in storage

Litchi (Litchi chinensis Sonn. cv Heiye) fruit were stored in air, modified atmosphere packaging (MAP) and controlled atmospheres (CA) at 3 °C to determine the effects of different O₂ and CO₂ atmospheres on physiology, quality and decay during the storage periods. The results indicated that CA conditions were more effective in reducing total phenol content, delaying anthocyanidin decomposition, preventing pericarp browning, and decreasing fruit decay in comparison with MAP treatment. Polyphenol oxidase (PPO), peroxidase (POD), anthocyanin and total phenols were involved in cellular browning. High O₂ treatment significantly limited ethanol production of litchi flesh in the early period of storage. The fruit stored in CA conditions for 42 days maintained good quality without any off-flavour.     

Polyphenol Oxidase Inhibitor from Blue Mussel (Mytilus edulis) Extract

Enzymatic browning remains a problem for the fruit and vegetable industry, especially new emerging markets like pre‐cuts. A crude inhibitor from blue mussel (Mytilus edulis) showed broad inhibition for apple (58%), mushroom (32%), and potato (44%) polyphenol oxidase (PPO) and was further characterized. Inhibition increased as the concentration of inhibitor increased in the reaction mixture eventually leveling off at a maximum inhibition of 92% for apple PPO. The inhibitor was capable of bleaching the brown color formed in the reaction mixture with apple PPO. Identification of the inhibitor by mass spectrometry and high‐performance liquid chromatography revealed it to be hypotaurine (C₂H₇NO₂S). Hypotaurine and other sulfinic acid analogs (methane and benzene sulfinic acids) showed very good inhibition for apple PPO at various concentrations with the highest inhibition occurring at 500 μM for hypotaurine (89%), methane sulfinic acid (100%), and benzene sulfinic acid (100%). Practical Application: An inhibitor found in the expressed liquid from blue mussel shows very good inhibition on enzymatic browning. Since this enzyme is responsible for losses to the fruit and vegetable industry, natural inhibitors that prevent browning would be valuable. Finding alternative chemistries that inhibit browning and understanding their mode of action would be beneficial to the fruit and vegetable industries and their segments such as pre‐cuts, juices, and so on. Inhibitors from products ingested by consumers are more acceptable as natural ingredients.     

Effect of hexanal vapour on longan fruit decay,quality and phenolic metabolism during cold storage

The effects of postharvest treatment with hexanal vapour on longan fruit decay, quality, hexanal residue, phenolic compound content, and polyphenoloxidase (PPO) and peroxidase (POD) activities were studied during storage at 5 °C for 30 days. Hexanal exposure for 2 h at 900 μL L^?1 before cold storage reduced the percentage of fruit with decay and was deemed the optimum treatment. Hexanal exposure resulted in a pericarp that was more reddish brown and less intense in colour. Hexanal residue in the pericarp and aril of fumigated fruit was several fold higher than that of nonfumigated fruit, although levels were low at the end of cold storage. Electrolyte leakage of pericarp increased during 5 °C storage and was further increased by hexanal exposure. Hexanal reduced pericarp phenolic content, and increased PPO and POD activities. Overall, use of hexanal vapour reduced postharvest disease of longan fruit but increased the likelihood of pericarp browning.     

荔枝与荔枝酒褐变控制

以荔枝及荔枝酒为试料,研究荔枝多酚氧化酶(PPO)的酶学特性及荔枝酒发酵过程中的褐变机理及控制措施。结果表明：荔枝PPO存在同功酶,荔枝PPO最适底物为没食子酸;以没食子酸为底物,荔枝PPO最适反应pH值为8.0,最适反应温度50℃,荔枝PPO的动力学参数为：米氏参数(Km)为26.033mmol/L,最大反应速度(Vmax)为34.816g/(L·min);荔枝PPO对酸碱、对热稳定性较差;荔枝酒发酵过程中褐变的主要原因是其PPO引起的酶促褐变。因此,低温贮藏和高温热处理可有效抑制荔枝及荔枝酒在贮藏和加工中酶促褐变反应。