Polyphenol oxidase activity and browning of three avocado varieties

A comparison between the in vitro total and specific activity of polyphenol oxidase of three avocado varieties, Fuerte, Horeshim and Lerman, showed clear differences that were correlated with the in vivo rate of browning of the corresponding freshly cut mature fruit; Fuerte had the highest values, followed by Horeshim and then Lerman. A good correlation existed between the PPO activity in the crude homogenate of the fresh fruit and the crude enzyme extracted from the acetone powder prepared from each variety. The possibility that the relatively low PPO activity in the crude enzyme of the Lerman avocado was due to the presence of an inhibitor and to factor(s) degrading the enzyme, or that the relatively high activity of the Fuerte enzyme was due to an activator, was tested in various ways and ruled out. It was therefore concluded that the differences in the rate of browning of the three avocado varieties studied were directly related to the PPO activities as expressed in the crude enzyme fraction.     

Muscadine Grape Polyphenoloxidase: Partial Purification by High Pressure Liquid Chromatography and Some Properties

A rapid method of isolating grape polyphenoloxidase (PPO) was developed, using two muscadine cultivars, “Noble” and “Welder”. PPO extracts from acetone powders were partially purified followed by filtration through 0.2μ membrane filter by HPLC. Reactivity of the active fraction per unit protein, increased 10-fold for Noble and 33-fold for Welder cultivars over values for the corresponding crude extracts. Electrophoretic analysis and activity stain showed that the enzyme displayed a single band in Welder and two bands for Noble. Optimum pH for PPO activity in Welder was pH 4.5 while that for Noble was pH 4.5–5.5. The enzyme in the Noble cultivar was relatively more active at alkaline pH. Activites of the enzymes were stable at temperatures up to 30°C for both cultivars.     

Enzymatic browning in sliced and puréed avocado: A fractal kinetic study

Modelling of browning is essential to evaluate the effectiveness of the processes applied to obtain fresh-appearance foods and also to determine their shelf life. Computer vision and the fractal kinetic method were used to quantify browning in three avocado cultivars. Samples (avocado slices and purée) were stored at 4 °C and surface images were captured in the tiff format. Browning kinetic was derived from images using the L^∗ mean method and the fractal method. In another experiment, inhibitor kinetic was also quantified, using the polyphenol oxidase activity method (in purée samples) with 0.1%, 0.5% and 1% of sodium bisulfite. In the results for avocado slices, kinetic rate derived by the fractal kinetic method was higher than that obtained with the L^∗ mean method, but the opposite was observed when the avocado was pulping (avocado purée). Kinetic quantified by the fractal method was similarly inhibited by bisulfite, when compared with the L^∗ mean value or PPO activity value methods. In general, the fractal method can be used to record browning kinetic and to discriminate between avocado cultivars, initial structural state of the sample (slice or purée), or in experiments using browning inhibitors.     

Inhibition of loquat enzymatic browning by sulfhydryl compounds

The effectiveness of a series of sulfhydryl compounds in inhibiting polyphenoloxidase (PPO) activity in a model system (chlorogenic acid solution) and in loquat juices was evaluated. Application of different concentrations of sulfhydryl compounds to chlorogenic acid solution and fresh loquat juice showed that l-cysteine appeared to be an effective browning inhibitor. The required concentration of l-cysteine for 90% browning inhibition depended on loquat cultivars, and ranged from 0.6 mM for “Nagasaki” to 2.0 mM for “Yukawa” or “Toi”. The difference may be due to the concentration of endogenous phenolic compounds inherent in each cultivar. The results from oxidized product analysis by loquat PPO implied that the mechanism of browning inhibition by thiols involved the formation of a thiol-conjugated reaction product.     

Changes in Apple Polyphenoloxidase and Polyphenol Concentrations in Relation to Degree of Browning

Polyphenoloxidase (PPO) activity and polyphenol concentrations decreased during maturation and remained relatively constant during cold storage. However, there was no set pattern of changes in degree of browning during maturation and cold storage. Not all cultivars showed the same relationship between degree of browning and enzyme activity. In some cultivars (Classic Delicious, RI Greening, McIntosh and Cortland), PPO activity was directly related to degree of browning while in others (Empire, Rome and Golden Delicious), degree of browning was related more to phenolic concentration. High performance liquid chromatography analysis of the phenolics in apples showed that the types of phenolic compounds in all cultivars were similar and that no one particular compound could account for the differences observed in browning.     

板栗品种褐变度差异性及其多酚氧化酶活性的相关性研究

以37个板栗品种为试材,研究了板栗果实褐变差异性及其与多酚氧化酶（PPO）活性之间的关系,并根据褐变差异性对各品种进行聚类分析。结果表明:板栗不同品种间褐变差异显著（p<0.05）,根据褐变差异性聚类分析,可将37个品种可以分为5个等级;不同品种间PPO活性差异显著（p<0.05）;通过探讨不同品种间褐变度与PPO活性的关系,发现随着板栗品种间PPO活性的升高,褐变度也开始升高,当PPO活性达到90 U·min-1·g-1时,PPO活性对褐变影响开始降低;通过褐变聚类分析,将板栗PPO活性分为5类,在优选或改良品种时,可将品种的PPO活性在第一或第二类活性范围以下作为选择依据之一。      

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.     

Evaluation of browning effect on avocado purée preserved by combined methods

The inhibition of enzymatic browning catalyzed by polyphenol oxidase is still an important challenge in food processing. The aim of this work was to evaluate the feasibility of using combined methods in reducing browning of avocado purée through the evolution of PPO activity and CIELab color parameters. Higher reductions of activity were achieved when EDTA was added as an antioxidant and stored under N₂ conditions. Purées with greater activity decays were less sensitive to colour changes. These changes occurred in avocado, thus losing the original lightness and greenness of the pulp. The antioxidant utilised was the factor that most affected the initial color, EDTA being the more effective additive in order to avoid browning reactions. The combination of factors had to be taken into account to evaluate these changes. The influence of combined methods over PPO activity was also assessed. Greater activity inhibition was reflected in better color maintenance.     

Occurrence and distribution of ppo activity in floral organs of some standard and local cultivars of tea

Variation of the polyphenol oxidase (PPO) activity in tea flower organs was studied among established cultivars and plants grown locally. Intervarietal differences among the soluble and bound components of the enzyme in floral organs were observed. Quantitative traits were found to be highly conserved among plants of the same clones but differed widely among plants of different clones and of local varieties/cultivars. The latter originated from seeds and some heterogenity was to be expected.     

CHARACTERIZATION AND INHIBITION OF POLYPHENOL OXIDASE FROM PEARS (PYRUS COMMUNIS L. CV. BOSC AND RED)

Red pears had higher PPO activity, total phenolics and chlorogenic acid concentration than Bosc pears. PPO activity and phenolics both decreased in fruits held at room temperature. pH and temperature optima for Bosc and Red pears PPOs were 5.0 and 5.5, and 20 and 23C, respectively. 4-Methylcatechol, catechol and dopamine were good substrates for PPO from both pear cultivars; however, no activity was observed with any of the mono-hydroxy substrates studied. Higher K_m and lower V_max values were observed for Bosc pear PPO. Heating at 75C for 30 min completely inactivated the enzyme from both cultivars. Heating at 55 and 65C for the same duration resulted in partial inactivation (45–60%) of this enzyme. Ascorbic acid, L-cysteine, sodium metabisulfite and thiourea effectively inhibited browning due to pear PPOs.     

Phenolic Composition and Browning Susceptibility of Various Apple Cultivars at Maturity

Phenolic compounds and polyphenol oxidase (PPO) activity in the fruits of 11 apple cultivars before and after bruising were studied in relation to their susceptibility to browning. The degree of browning has been determined by the simultaneous measurements of soluble (absorbance at 400nm) and insoluble (lightness) brown pigments. The sum of these two expressing the extent of browning, closely correlated with the amount of phenolics (hydroxycinnamic derivatives and flavan-3-ols) degraded. Maturity did not appear to greatly influence the development of browning.     

Pre-drying treatment of plant related tissues using plasma processed air: Impact on enzyme activity and quality attributes of cut apple and potato

During post-harvest processing of fresh cut and dried fruits and vegetables, polyphenol oxidase (PPO) and peroxidase (POD) need to be inactivated or inhibited in order to avoid undesirable browning reactions and loss of sensorial or nutritional quality. To meet this goal, the application of plasma processed air (PPA) offers a promising “gentle” alternative to traditional methods, such as pasteurization or the addition of anti-browning compounds. Using ambient air as process gas instead of an expensive noble gas, such as argon, exhibits a substantial improvement for the development of large-scale plasmas at ambient pressure and allows the indirect treatment of larger goods within a remote exposure reactor. In this study the ability of PPA to inactivate PPO and POD in complex food matrices and its impact on quality parameters, such as color, texture and cell integrity directly after freshly cutting and during storage of warm air dried and freeze dried produce was evaluated.The study evidently shows that PPA processing is capable of reducing the activity of PPO and POD in the freshly cut tissue from both apple and potato. Following exposure to PPA for 10 min the PPO activity was reduced by about 62% and 77% in fresh cut apple and potato tissue, respectively. POD, as the more temperature-stable enzyme, was even less stable upon PPA treatment for 10 min and was reduced by about 65% and 89% in fresh cut apple and potato tissue, respectively. Blackening of the potato tissue could be completely prevented by plasma treatment while a browning different from the habitual nature of enzymatic browning occurred upon exposure of the apple tissue to PPA. In both cases, the pH value on the tissue surface dropped to 1.5 while cell integrity and dry matter content were not significantly affected.Industrial relevanceThe quality and shelf life of freshly cut and dried fruits and vegetables greatly depend on the activity of naturally occurring enzymes which catalyze browning reactions at cut surfaces. This study shows that the application of PPA, as a promising nonthermal “pasteurization” technology, enables the inactivation of PPO and POD in complex food matrices. It further describes the impact of the PPA treatment on quality parameters of the freshly cut tissue from apple and potato and goes beyond on evaluating color, texture and enzyme activity in warm air dried and freeze dried tissue over a storage time of three weeks. The results contribute to the understanding and product-specificity of PPA-induced effects on quality and shelf life of fresh cut and dried fruit and vegetable produce and could be a basis for a possible industrial implementation.     

Characterization of polyphenol oxidase and peroxidase from peach mesocarp (Prunus persica L. cv. Babygold)

The two enzymes involved in enzymatic browning reactions—polyphenol oxidase (PPO) and peroxidase (POD)—were extracted from peach fruit mesocarp. PPO was mainly located in the membrane fraction and was in its latent state. However, POD activity was found in the soluble fraction. The kinetic characterization of PPO and POD was carried out with a natural substrate (chlorogenic acid) and with a non‐physiological substrate. Under native isoelectric focusing (IEF), several PPO isoenzymes were present in the pH range 5.4–5.8. A partially denaturing sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS‐PAGE) showed the presence of two active bands with apparent molecular masses of 49 and 50 kDa. A totally denaturing SDS‐PAGE indicated the presence of a single polypeptide with a molecular mass of 60 kDa, as revealed by western blot. POD was also analyzed by IEF, showing the presence of two strongly basic isoenzymes, which were resolved by cationic native PAGE into two different bands. Copyright © 2007 Society of Chemical Industry     

Postharvest quality comparison of six blackberry cultivars under two storage conditions

The fruit of six blackberry cultivars “Ningzhi 3,” “Arapaho,” “Kiowa,” “Hull,” “Shuofeng,” and “Triple Crown” were stored under different storage conditions 25°C (RT) and 4°C (LT). Compared to RT treatment, LT treatment-restricted deterioration and nutritional loss, and maintained antioxidant capacity of samples. Among LT samples, color of “Ningzhi 3,” “Hull,” and “Shuofeng” changed fewer than other cultivars, and the decay rate of “Shuofeng” and weight loss of “Hull” were lower than others. At the end of LT, total soluble solid and soluble sugar content of “Arapaho,” soluble protein content of “Shuofeng” were the highest. During the storage period, “Arapaho,” “Hull”, and “Shuofeng” maintained higher antioxidant enzyme activity and lower hydrogen peroxide content. The storage stability of “Arapaho,” “Hull,” and “Shuofeng” was better than the other three blackberry cultivars because of their better postharvest quality. This study provides a comprehensive insight into the postharvest treatment to maintain the quality of blackberry.     

三种巨大口蘑菌株贮藏期间褐变及相关酶活性

以巨大口蘑SCAU1、新3和新4三种菌株子实体为试材,于12℃和相对湿度95%下贮藏,定期取样测定褐变度、多酚氧化酶(PPO)、酪氨酸酶(TYR)、漆酶和总酚等生理指标,研究各菌株的褐变度与酶活性及总酚的相关性。结果表明,贮藏期间三种巨大口蘑菌株的褐变度呈上升趋势,PPO、TYR及漆酶的活性均呈下降趋势,其中SCAU1菌株的褐变度和PPO、TYR及漆酶活性最低,而总酚含量与新3和新4差异不显著(p>0.05);相关性分析结果显示,新3、新4菌株的褐变度均与PPO活性呈极显著负相关(p<0.01),SCAU1菌株的褐变度与PPO活性呈显著负相关(p<0.05),相关性从高到低依次为新3菌株>新4菌株>SCAU1菌株,此外,新3菌株的褐变度与TYR活性呈显著负相关(p<0.05)。该结果可为巨大口蘑优良菌株的选育和褐变内在机理的揭示提供依据。     

Effects of dehydration temperatures on colour and polyphenoloxidase activity of Amasya and Golden Delicious apple cultivars

During dehydration at different temperatures (60, 70 and 80 °C) of Amasya and Golden Delicious apple cultivars, changes in colour, polyphenoloxidase activity (PPO), browning index and hydroxymethylfurfural (HMF) content were studied. Effects of dehydration time on the L* values of both cultivars were not significant. Only Amasya samples dehydrated at 80 °C were found to have significantly higher L* values than the remaining samples. In all cases, Golden Delicious samples had higher L* values than those of the Amasya cultivar. a* values of samples increased within the first hour of dehydration and then remained almost unchanged. The enzyme activity showed a rapid decrease in the first hour of dehydration and residual enzyme activities (%) reached 9.8%, 5.3% and 4.5% for Amasya, and 17.4%, 10.3% and 4.6% for the Golden Delicious cultivar at 60, 70 and 80 °C, respectively. In all final samples the residual enzyme activities were around 1%. The highest browning values were observed at the second hour of dehydration at 60 and 70 °C and the first hour of dehydration at 80 °C. In all conditions Amasya apples had a higher browning tendency. The presence of HMF was detected only at the 4th hour of dehydration at 80 °C for both cultivars. Effects of dehydration temperature on colour and PPO of the final product were insignificant. However, an important effect of temperature was determined on the browning index of Golden Delicious samples. The lowest browning tendency was measured on samples dehydrated at 80 °C. The results showed that cultivar and dehydration temperature had considerable effect on the browning of apple slices. Copyright © 2006 Society of Chemical Industry     

Suitability for minimal processing of non‐melting clingstone peaches

Clingstone non‐melting peach cultivars (Prunus persica L. Batsch) are used primarily for canning, but their processing as fresh‐cut products is also of interest. Four clingstone peach cultivars (‘Andross’, ‘Babygold 8′, ‘Calante’ and ‘Romea’) were evaluated for their suitability for minimal processing (washing, peeling and slicing) followed by storage for 9 days at 4 °C in a modified atmosphere using microperforated films. Romea cultivar, which has low polyphenol oxidase (PPO) enzymatic activity, proved to be the most suitable cultivar for minimal processing, given its lower degree of browning as well as the results from a sensory evaluation. Maturity indicators varied slightly during storage at a different degree depending on cultivars. We have found that PPO activity and browning potential are adequate indicators for surface browning in fresh‐cut non‐melting peaches.     

Carrot browning on simulated market shelf and during cold storage

Browning index, degree of browning, total phenolics content and polyphenoloxidase (PPO) activity of three carrot cultivars—Kendo, Ricardo and Stefano—were studied on a simulated market shelf (20 °C) and during cold storage (4 °C). There was a general increase in browning indices and degree of browning during simulated marketing storage for all three cultivars. The three cultivars had similar levels of browning after cold storage but Ricardo reached the highest level after 10 days of simulated shelf life. There appeared to be a rapid increase in total phenolics content during cold storage, which increased further during a simulated marketing trial. The PPO activities of Kendo and Stefano decreased during cold storage whereas that of Ricardo increased, reaching its highest level after 2 months in cold storage. A positive and significant correlation between total phenolics and degree of browning was found for all three cultivars stored for different lengths of time. This research suggests that shorter periods of cold storage reduce the potential for carrot browning and hence prolong the shelf life of carrots. Copyright © 2004 Society of Chemical Industry     

Browning prevention by ascorbic acid and 4-hexylresorcinol: different mechanisms of action on polyphenol oxidase in the presence and in the absence of substrates

ABSTRACT:  We have investigated the mechanism of action of 4-hexylresorcinol (4-HR) and ascorbic acid (AA) on the polyphenol oxidase (PPO) catalyzed oxidation of phenolic substrates. Incubation of PPO with 4-HR diminishes strongly PPO activity. This effect can be erroneously interpreted, due to the high affinity of 4-HR for PPO, as irreversible inactivation of PPO. However, PPO activity can be recovered by dialysis after incubation with 4-HR. 4-hexylresorcinol is a canonical enzyme inhibitor that binds preferentially to the oxy form of PPO. It is a mixed-type inhibitor, because it influences both apparent V _max (1.26 compared with 0.4 units in the absence and presence of 4-HR, respectively) and K _m values (0.28 mM compared with 0.97 mM in the absence and in the presence of 4-HR, respectively) of PPO. AA can prevent browning by 2 different mechanisms: In the absence of PPO substrates it inactivates PPO irreversibly, probably through binding to its active site, preferentially in its oxy form. In the presence of PPO substrates, AA reduces PPO oxidized reaction products, which results in a lag phase when measuring PPO activity by monitoring dark product formation but not when monitoring O₂ consumption. The simultaneous use of both 4-HR and AA on PPO results in additive prevention of browning.     

Polyphenoloxidase activity and total phenolic content as related to browning of minimally processed ‘Jonagored’ apple

Phenolic content, polyphenoloxidase (PPO) activity and browning have been reported by different authors to vary in different ways, depending on cultivar, picking maturity, storage conditions, etc. Numerous authors have attempted to correlate browning results with the phenolic content and/or enzymatic activity of apples, but data published in the literature are divergent as to which of the two factors, ie the enzyme or the substrate, plays the decisive role in the overall phenomenon of enzymatic browning. The aim of this work was to follow the PPO activity and phenolic content of minimally processed apple (cv ‘Jonagored’) during cold storage and to establish a potential relationship with enzymatic browning. Increasing storage time of minimally processed apple seemed to result in an increase in PPO activity. No changes (P > 0.05) in total phenolic content were observed during storage. Colour degradation of apple cubes, as indicated by changes in colour parameters, was found to be highly correlated with PPO activity. Browning of ‘Jonagored’ apple cubes during storage was found to be moderately correlated with PPO activity. No correlation was found between the browning index and the total phenolic content. © 2001 Society of Chemical Industry