Influence of Pasteurisation (Conventional vs. Radiofrequency) and Chill Storage on Retention of Ascorbic Acid, Tocopherol and Carotenoids in Salmorejo

Salmorejo, a Mediterranean tomato-oil puree, is considered a dietary source of antioxidant vitamins C and E and carotenoids lycopene and β-carotene, the latter endowed with provitamin A activity. However, these antioxidants can be degraded in oxidation reactions catalysed or not by enzymes during pasteurisation and storage treatments used to stabilise the salmorejo before consumption. Due to its better penetration, the use of dielectric heating by radiofrequency (RF) may improve results of pasteurisation in this product. The objective was to assess the effects of pasteurisation temperature (70-100 °C, at 5 °C intervals) and storage time (0-5 months, at one-month intervals) on levels of ascorbic acid, α-tocopherol and carotenoids and antioxidant capacity (AC) in salmorejo pasteurised (over 10 s) by conventional (CH) or RF continuous heating. Two successive experiments were conducted to select an adequate pasteurisation temperature for use in the shelf-life study. Pasteurisation upon tested conditions allows a good retention of salmorejo antioxidants. Either CH or RF pasteurisers can be used with similar results. Vitamin C (L-ascorbic + dehydroascorbic acids) was more abundant (15-19 mg 100 g-1) than carotenoids (0.9-2.6 mg 100 g-1) (all-trans + cis lycopene and β-carotene) and α-tocopherol (0.8-1.2 mg 100 g-1) in the pasteurised product. Using excessively low temperatures (70 °C) resulted in partial losses of the three antioxidants, possibly due to oxidase residual activities. Intensifying thermal treatment improved this issue with minor losses of the thermolabile vitamin C and increased carotenoid content. Using a suitable temperature (80 °C) did not prevent most vitamin C from being degraded by the first month, while α-tocopherol, and, to a lesser extent, carotenoids, showed good retention levels during shelf life under refrigeration. Vitamins C and E and carotenoids, either by degradation, regeneration or releasing, likely contribute to the AC in salmorejo. Phenolic antioxidant response, radical-scavenging activities and redox potential values confirmed this finding. The pasteurised-chilled salmorejo shows good antioxidant properties with potential health implications, a positive nutritional aspect for consumers of this tomato-oil homogenate. The losses of antioxidants and AC due to pasteurization would be of little relevance compared to the losses accumulated during shelf life.

Inhibition Mechanism of Chitooligosaccharide-Polyphenol Conjugates toward Polyphenoloxidase from Shrimp Cephalothorax

Crustaceans are perishable with a short shelf-life. They are prone to deterioration after capture, particularly during handling, processing, and storage due to melanosis caused by polyphenoloxidase (PPO). Therefore, inhibitory effects of chitooligosaccharide (CHOS) in comparison with CHOS-catechin (CHOS-CAT), CHOS-epigallocatechin gallate (CHOS-EGCG), and CHOS-gallic acid (CHOS-GAL) conjugates on Pacific white shrimp cephalothorax PPO were studied. IC₅₀ of CHOS-CAT (0.32 mg/mL) toward PPO was less than those of all conjugates tested (p < 0.05). CHOS-CAT exhibited the mixed-type inhibition. K_ic (0.58 mg/mL) and K_iu (0.02 mg/mL) of CHOS-CAT were lower than those of other conjugates (p < 0.05). CHOS-CAT showed static fluorescence-quenching, suggesting a change in micro-environment around the active site of PPO. Moreover, CHOS-CAT was linked with various amino acid residues, including Tyr208 or Tyr209 of proPPO via van der Waals, hydrophobic interaction, and hydrogen bonding as elucidated by the molecular docking of proPPO. Although CHOS-CAT had the highest PPO inhibitory activity, it showed a lower binding energy (-8.5 kcal/mol) than other samples, except for CHOS-EGCG (-10.2 kcal/mol). Therefore, CHOS-CAT could act as an anti-melanosis agent in shrimp and other crustaceans to prevent undesirable discoloration associated with quality losses.

Chemical inhibition of polyphenol oxidase and cut surface browning of fresh-cut apples

Fresh-cut apples, which offer consumers health benefits and convenience, have become popular in recent years. One of the main challenges for processing fresh-cut apples is rapid development of cut surface browning, immediately after fruits are cut. Browning, a physiological response that impacts organoleptic properties and deters consumer purchase of fresh-cut fresh produce, is mainly a result of enzymatic reaction of phenolic compounds with oxygen catalyzed by polyphenol oxidase (PPO), a decapper enzyme. Many antibrowning agents have been developed and evaluated to inhibit PPO activities by using reducing agents (antioxidants), chelating agents, acidulants, etc. The present manuscript reviews the diverse characteristics of PPO (such as optimum pH and temperature, and molecular weight) in apples reported in the literature and the enzyme's latency, multiplicity and copper states in the active site. It also summarizes the latest development in the investigation and formulations of antibrowning compounds, and discusses future research needs. This review should stimulate further research to discover more effective, low cost, and natural antibrowning compounds to meet the demand of consumers as well as the food industry for clean label and long shelf-life of fresh-cut apples.

A Preliminary Color Study of Different Basil-Based Semi-Finished Products during Their Storage

Basil-based semi-finished products, which are mainly used as an intermediate to produce the typical pesto sauce, are prepared and exported all over the world. Color is a fundamental organoleptic requirement for the acceptability of these semi-finished products by the manufacturers of the pesto sauce. Some alternative formulations, which adjust the typical industrial recipe by both changing the preservative agent (ascorbic acid, citric acid, or a mixture of both) and introducing a preliminary thermic treatment (blast chilling), were evaluated. In this work, a fast and non-destructive spectrophotometric analysis, to monitor the color variations in these food products during their shelf-life, was proposed. The raw diffuse reflectance spectra (380-900 nm) obtained by a UV-visible spectrophotometer, endowed with an integrating sphere, together with the CIELab parameters (L*, a*, b*) automatically obtained from these, were considered, and elaborated using multivariate statistical analysis (principal component analysis). From this preliminary study, blast chilling, together with the use of ascorbic acid, proved to be the best solution to better preserve the color of these products during their shelf-life.

Increasing the Storability of Fresh-Cut Green Beans by Using Chitosan as a Carrier for Tea Tree and Peppermint Essential Oils and Ascorbic Acid

The quality of fresh-cut green beans deteriorates rapidly in storage, which contributes to increased food waste and lower perceived customer value. However, chitosan (Cs) and certain plant essential oils show promise in reducing postharvest quality loss during storage. Here, the effect of Cs and the combinations of Cs + tea tree oil (TTO), Cs +x peppermint oil (PMO), and Cs + ascorbic acid (AsA) on the quality of fresh-cut green bean pods (FC-GB) is studied over a 15-d storage period at 5 °C. All four FC-GB treatments reduced weight loss and maintained firmness during storage when compared to uncoated FC-GB. Furthermore, all treatments showed higher total chlorophyll content, AsA, total phenolic compounds, and total sugars compared to the control. The best treatment for reducing microbial growth was a combination of Cs + AsA. Additionally, the combination of Cs with TTO, PMO, or AsA showed a significant reduction in the browning index and increased the antioxidant capacity of FC-GB up to 15 d postharvest.

Role of Ascorbic Acid in the Extraction and Quantification of Potato Polyphenol Oxidase Activity

The ability to accurately measure the activity of polyphenol oxidase (PPO) in complex matrices is essential. A problem encountered when using spectrophotometric methods is interference due to ascorbic acid (AA), often used as an enzyme “protecting agent” during PPO extraction. This study focuses on the nature of AA’s effect on spectrophotometric determinations of PPO activity as well as enzyme extraction. Potato extracts and semi-purified PPO were used as enzyme sources. The inactivation of PPO attributed to AA is substrate-mediated. The extent of AA-dependent inactivation of PPO in model systems varied between substrates. AA only slows mechanism-based inactivation of PPO induced by catechol, possibly owing to the prevention of quinone formation. AA minimally protects PPO activity during enzyme extraction. The problem associated with AA in PPO assay could be circumvented by using ascorbate oxidase to remove AA when catechol is the primary substrate or by using chlorogenic acid as the primary substrate.

A Transcriptional Analysis of the Genes Involved in the Ascorbic Acid Pathways Based on a Comparison of the Juice and Leaves of Navel and Anthocyanin-Rich Sweet Orange Varieties

Sweet oranges are an important source of ascorbic acid (AsA). In this study, the content of AsA in the juice and leaves of four orange clonal selections, different in terms of maturity time and the presence/absence of anthocyanins, was correlated with the transcription levels of the main genes involved in the biosynthesis, recycling, and degradation pathways. Within each variety, differences in the above pathways and the AsA amount were found between the analysed tissues. Variations were also observed at different stages of fruit development and maturation. At the beginning of fruit development, AsA accumulation was attributable to the synergic action of l-galactose and Myo-inositol, while the l-gulose pathway was predominant between the end of fruit development and the beginning of ripening. In leaves, the l-galactose pathway appeared to play a major role in AsA accumulation, even though higher GalUr isoform expression suggests a synergistic contribution of both pathways in this tissue. In juice, the trend of the AsA content may be related to the decrease in the transcription levels of the GME, GDH, MyoOx, and GalUr12 genes. Newhall was the genotype that accumulated the most AsA. The difference between Newhall and the other varieties seems to be attributable to the GLDH, GalUr12, APX2, and DHAR3 genes.

Effects of melatonin treatment on browning alleviation of fresh-cut foods

Enzymatic browning is the main quality issue of fresh-cut foods. This study investigated the effects of different concentrations (0.05, 0.1, and 0.2 mM) of melatonin (MT) treatment on the enzymatic browning of four fresh-cut foods, including apples (Malus domestica), pears (Pyrus spp.), potatoes (Solanum tuberosum), and taros (Colocasia esculenta), and found that only 0.05 mM MT was most significant at enhancing the L* values and reducing the browning index (BI) in all four foods. This suggests that 0.05 mM MT might be a universal concentration for the browning alleviation of fresh-cut foods. MT treatment increased the total phenolic contents and PAL activities but reduced the activities of POD, PPO, and LOX in fresh-cut taros. Furthermore, MT significantly affected the expression of sixteen browning-related genes in fresh-cut taros during storage. These results suggest that MT reduced fresh-cut food browning by regulating the activities of browning-related enzymes. However, MT did not significantly influence the browning degree or PPO activity of the taro mash in vitro, suggesting that MT might regulate enzyme activity in an indirect manner. Overall, these results indicate that MT might be a promising anti-browning agent to alleviate the browning of fresh-cut foods. PRACTICAL APPLICATIONS: Melatonin (MT) is an endogenously produced indoleamine. Previous studies have reported that MT displays protective activities in many agricultural products. However, the reported protective MT concentrations vary between different products. To extend this agent to the fresh-cut industry, it is necessary to determine the universal dosage of MT needed for application efficiency. In this study, the effects of three concentrations (0.05, 0.1, and 0.2 mM) of MT on four fresh-cut foods, including apples, pears, potatoes, and taros, were investigated. The results showed that MT treatment alleviated browning development in the slices of these four foods and that 0.05 mM MT was the most effective treatment. Moreover, 0.05 mM MT significantly affected the activities of browning-related enzymes. These results suggest that 0.05 mM MT is a universal dosage for reducing surface browning in fresh-cut foods. This study provides a foundation for the application of MT in the processing of fresh-cut foods, especially fresh-cut taros.

The Combined and Single Effect of Marjoram Essential Oil, Ascorbic Acid, and Chitosan on Fresh-Cut Lettuce Preservation

Increasing demands by consumers for fresh, nutritional, and convenient food has led to the increase of fresh-cut produce market. Nowadays, there is a turn towards the investigation of natural products (i.e., essential oils, organic acids, and edible coatings) in an effort to lower the usage of chemical synthetic compounds (i.e., chlorine) as postharvest sanitizers. The aim of the present study was to assess the effectiveness of Origanum majorana essential oil (EO), ascorbic acid (AA), chitosan, and their combinations on quality attributes of fresh-cut lettuce stored for six days at 7 °C. When applied, Chitosan+AA resulted to a less acceptable product (visual quality and aroma), while the application of marjoram EO was able to preserve the visual quality of fresh-cut lettuce and at the same time resulted in a pleasant aroma. The application of EO+AA and Chitosan+AA increased total phenolics and antioxidant levels of fresh-cut lettuce on the fourth and sixth day of storage. The EO and EO+AA increased damage index (hydrogen peroxide and lipid peroxidation) of fresh-cut lettuce, while at the same time these treatments decreased the activity of enzymes related with plant tissue browning (i.e., peroxidase activity and polyphenol oxidase). Chitosan decreased total valuable counts and yeasts and molds counts on the sixth day of storage, while EO, AA, EO+Chitosan, and Chitosan+AA decreased yeasts and molds after four days of application. The findings of the present work indicating that the combination of marjoram EO, AA, and chitosan could be considered further as alternative means for fresh-cut produce preservation.

Application of sodium chloride and quince seed gum pretreatments to prevent enzymatic browning, loss of texture and antioxidant activity of freeze dried pear slices

ABSTRACT

In the study the effect of sodium chloride and quince seed gum solutions as immersion pretreatment to prevent enzymatic browning reaction in pear slices before freeze drying was evaluated. Four levels of gum concentrations (0%, 0.25%, 0.5% and 1%) and three concentrations of salt (0%, 0.25% and 0.5%) were used. Subsequently, freeze drying method was used to dry the pear slices and the qualitative, structural and sensory characteristics of dried pear slices were investigated. The moisture content of the dried pear slices decreased significantly with increasing sodium chloride concentration, while increasing the concentration of the gum significantly increased the moisture content (P < 0.05). Use of both immersion pretreatments were effective in maintaining the antioxidant activity and reducing browning index of the slides and with increasing concentration of the immersion solutions, the antioxidant activity loss decreased in the slices (P < 0.05). Electron microscopy study revealed that the microstructural changes of the drying process on the slices are very slight, although gum pre-treatment at the high levels made a layer on the surface of the pear slices which slowed down the transfer of water vapor molecules. By considering the results of qualitative, structural and sensory evaluations, using of immersion pretreatment with 0.25% gum and 0.25% salt solution to prevent enzymatic browning along with preserving the quality properties of pear slices prior to the drying process is recommended.

GRAPHIC ABSTRACT

SEM of cross section of dried pear slices after processing.

Production and characterization of freeze-dried banana slices pretreated with ascorbic acid and quince seed mucilage: Physical and functional properties

The objective of this investigation was to illustrate the effects of quince seed mucilage (QSM) and ascorbic acid pretreatments to prevent the quality of freeze-dried banana slices. The studied parameters were moisture content, antioxidant activity, total phenol, color properties, structural properties, and sensory evaluation. Both treatments were effective in protecting total phenolic content and antioxidant activity in dried banana slices (P ˂ .05). The control slices showed greater increase in browning index (BI) and greater decrease in lightness (L*) than pretreated dried samples. Ascorbic acid and QSM treatments can be effective in the control of the enzymatic browning along with maintaining the quality properties of banana chips. Therefore, using of immersion pretreatment with 0.25% QSM and 0.05% ascorbic acid is recommended to prevent enzymatic browning as well as maintain the quality of banana chips before the drying process.

Recent Trends in Controlling the Enzymatic Browning of Fruit and Vegetable Products

Enzymatic browning because of polyphenol oxidases (PPOs) contributes to the color quality of fruit and vegetable (FV) products. Physical and chemical methods have been developed to inhibit the activity of PPOs, and several synthetic chemical compounds are commonly being used as PPO inhibitors in FV products. Recently, there has been an emphasis on consumer-oriented innovations in the food industry. Consumers tend to urge the use of natural and environment-friendly PPO inhibitors. The purpose of this review is to summarize the mechanisms underlying the anti-browning action of chemical PPO inhibitors and current trends in the research on these inhibitors. Based on their mechanisms of action, chemical inhibitors can be categorized as antioxidants, reducing agents, chelating agents, acidulants, and/or mixed-type PPO inhibitors. Here, we focused on the food ingredients, dietary components, food by-products, and waste associated with anti-browning activity.

Inhibitory effects of organic acids on polyphenol oxidase: From model systems to food systems

Organic acids are widely utilized in the food industry for inhibiting the activity of polyphenol oxidase (PPO) and enzymatic browning. This review discusses the mechanisms of inhibition of PPO and enzymatic browning by various organic acids based on studies in model systems, critically evaluates the relevance of such studies to real food systems and assesses the implication of the synergistic inhibitory effects of organic acids with other physicochemical processing techniques on product quality and safety. Organic acids inhibit the activity of PPO and enzymatic browning via different mechanisms and therefore the suitability of a particular organic acid depends on the structure and the catalytic properties of PPO and the physicochemical properties of the food matrix. Studies in model systems provide an invaluable insight into the inhibitory mechanisms of various organics acids. However, the difference in the effectiveness of PPO inhibitors between model systems and food systems and the lack of correlation between the degree of PPO inhibition based on in vitro assays and enzymatic browning imply that the effectiveness of organic acids can be accurately evaluated only via direct assessment of browning inhibition in a particular food system. Combination of organic acids with physical processing techniques is one of the most viable approaches for PPO inhibition since the observed synergistic effect helps to reduce the undesirable organoleptic quality changes from the use of excessive concentration of organic acids or intense physical processing.

Evaluation of Pseudomonas graminis CPA-7 as a biopreservation method for fresh-cut pear: Physicochemical, enzymatic, and nutritional quality

Biological preservation methods with bacterial antagonists have emerged as alternatives to chemical sanitizers for extending shelf-life and reducing the population of pathogenic microorganisms. In addition, calcium plays an important role in maintaining the quality of fruit, and postharvest calcium treatments might determine the potential of fruit for processing. The objective of this work was to evaluate the effect of the postharvest application of calcium and biopreservation with the CPA-7 strain of Pseudomonas graminis on the quality parameters of fresh-cut pears. After harvest, whole pears were dipped in calcium chloride solution (1%, w/v) or water (control) for 10 min at 25 ℃ and stored for five months at temperatures ranging from 0 to -0.5 ℃. Both batches of fruit were minimally processed and dipped in a solution containing CPA-7 and an antioxidant solution or kept untreated, and both groups were stored at 4 ℃ for six days. The postharvest calcium treatment had no remarkable effect on the quality of the whole and fresh-cut pears. The enzymatic activities (PPO, PME and PG) related to browning and softening were constant in fresh-cut pears after storage, and the application of P. graminis CPA-7 had a positive effect on the activity of PPO. Finally, a combined effect of the biocontrol agent and calcium treatment was not demonstrated.

Effect of high-hydrostatic pressure and moderate-intensity pulsed electric field on plum

Moderate intensity pulse electric fields were applied in plum with the aim to increase bioactive compounds content of the fruit, while high-hydrostatic pressure was applied to preserve the purées. High-hydrostatic pressure treatment was compared with an equivalent thermal treatment. The addition of ascorbic acid during purée manufacture was also evaluated. The main objective of this study was to assess the effects on microorganisms, polyphenoloxidase, color and bioactive compounds of high-hydrostatic pressure, or thermal-processed plum purées made of moderate intensity pulse electric field-treated or no-moderate intensity pulse electric field-treated plums, after processing during storage. The application of moderate intensity pulse electric field to plums slightly increased the levels of anthocyanins and the antioxidant activity of purées. The application of Hydrostatic-high pressure (HHP) increased the levels of bioactive compounds in purées, while the thermal treatment preserved better the color during storage. The addition of ascorbic acid during the manufacture of plum purée was an important factor for the final quality of purées. The color and the bioactive compounds content were better preserved in purées with ascorbic acid. The no inactivation of polyphenoloxidase enzyme with treatments applied in this study affected the stability purées. Probably more intense treatments conditions (high-hydrostatic pressure and thermal treatment) would be necessary to reach better quality and shelf life during storage.

Enzymatic browning inhibition and antioxidant activity of pear juice from a new cultivar of asian pear (Pyrus pyrifolia Nakai cv. Sinhwa) with different concentrations of ascorbic acid

Different ascorbic acid (AA) concentrations of 0.16, 0.20, and 0.24% (w/v) were added to pear juice from the new cultivar Pyrus pyrifolia Nakai cv. "Sinhwa". Enzymatic browning reduction and antioxidant activity were analyzed after 24 h at 37°C. Juices with 0.20% added AA showed the highest inhibition of 78.8% of polyphenol oxidase (PPO) activity. L* values of juices a with 0.20 and 0.24% added AA decreased more slowly than controls lacking AA addition and juice with 0.16% added AA after storage for 24 h. Browning indices of juices with added AA were lower than for controls. However, indices increased after storage for 24 h. The DPPH radical-scavenging activity, reducing power, and nitrite scavenging activity of all juices with added AA were higher than for controls and decreased after storage for 24 h. Addition of 0.20% AA to pear juice from the new "Sinhwa" cultivar showed the highest browning activity reduction.

Tyrosinase-Catalyzed Hydroxylation of 4-Hexylresorcinol, an Antibrowning and Depigmenting Agent: A Kinetic Study

4-Hexylresorcinol (HR) is a compound used in the food and cosmetic industries as an antibrowning and lightening agent. Its use is mainly attributed to its inhibitory effect on the enzyme tyrosinase. However, the enzyme hydroxylates HR to an o-diphenol, which it then oxidizes to an o-quinone, which rapidly isomerizes to p-quinone. For tyrosinase to act in this way, the Eox form (oxy-tyrosinase) must be present in the reaction medium, which can be brought about by (a) hydrogen peroxide, (b) ascorbic acid, or (c) catalytic concentrations of o-diphenol and a reductant (NADH) to maintain it constant. This work demonstrates that HR is a substrate of tyrosinase and proposes a mechanism for its action. Its kinetic characterization provides a catalytic constant of 0.85 ± 0.04 s(-1) and a Michaelis constant of 60.31 ± 6.73 μM.

Intrinsic polyphenol oxidase-like activity of gold@platinum nanoparticles

Au@Pt NPs showed PPO mimetic activity over a wider range of pH and temperatures compared to PPO. In the oxidation of all substrates, Au@Pt NPs exhibited higher affinity to the substrates, especially to catechol and pyrogallol, compared with PPO.

Genetic, metabolite and developmental determinism of fruit friction discolouration in pear

BACKGROUND

The unattractive appearance of the surface of pear fruit caused by the postharvest disorder friction discolouration (FD) is responsible for significant consumer dissatisfaction in markets, leading to lower returns to growers. Developing an understanding of the genetic control of FD is essential to enable the full application of genomics-informed breeding for the development of new pear cultivars. Biochemical constituents [phenolic compounds and ascorbic acid (AsA)], polyphenol oxidase (PPO) activity, as well as skin anatomy, have been proposed to play important roles in FD susceptibility in studies on a limited number of cultivars. However, to date there has been no investigation on the biochemical and genetic control of FD, employing segregating populations. In this study, we used 250 seedlings from two segregating populations (POP369 and POP356) derived from interspecific crosses between Asian (Pyrus pyrifolia Nakai and P. bretschneideri Rehd.) and European (P. communis) pears to identify genetic factors associated with susceptibility to FD.

RESULTS

Single nucleotide polymorphism (SNP)-based linkage maps suitable for QTL analysis were developed for the parents of both populations. The maps for population POP369 comprised 174 and 265 SNP markers for the male and female parent, respectively, while POP356 maps comprised 353 and 398 SNP markers for the male and female parent, respectively. Phenotypic data for 22 variables were measured over two successive years (2011 and 2012) for POP369 and one year (2011) only for POP356. A total of 221 QTLs were identified that were linked to 22 phenotyped variables, including QTLs associated with FD for both populations that were stable over the successive years. In addition, clear evidence of the influence of developmental factors (fruit maturity) on FD and other variables was also recorded.

CONCLUSIONS

The QTLs associated with fruit firmness, PPO activity, AsA concentration and concentration of polyphenol compounds as well as FD are the first reported for pear. We conclude that the postharvest disorder FD is controlled by multiple small effect QTLs and that it will be very challenging to apply marker-assisted selection based on these QTLs. However, genomic selection could be employed to select elite genotypes with lower or no susceptibility to FD early in the breeding cycle.

Role of ascorbic acid in the inhibition of polyphenol oxidase and the prevention of browning in different browning-sensitive Lactuca sativa var. capitata (L.) and Eruca sativa (Mill.) stored as fresh-cut produce

BACKGROUND

Polyphenol oxidase (PPO) and, to a minor extent, peroxidase (POD) represent the key enzymes involved in enzymatic browning, a negative process induced by cutting fresh-cut produce such as lettuce (Lactuca sativa) and rocket salad (Eruca sativa). Although ascorbic acid is frequently utilised as an anti-browning agent, its mechanism in the prevention of the browning phenomenon is not clearly understood.

RESULTS

The activity of PPO and POD and their isoforms in lettuce (a high-browning and low-ascorbic acid species) and rocket salad (a low-browning and high-ascorbic species) was characterised. The kinetic parameters of PPO and in vitro ascorbic acid-PPO inhibition were also investigated. In rocket salad, PPO activity was much lower than that in lettuce and cutting induced an increase in PPO activity only in lettuce. Exogenous ascorbic acid (5 mmol L(-1)) reduced PPO activity by about 90% in lettuce. POD did not appear to be closely related to browning in lettuce.

CONCLUSION

PPO is the main enzyme involved in the browning phenomenon; POD appears to play a minor role. The concentration of endogenous ascorbic acid in rocket salad was related to its low-browning sensitivity after cutting. In lettuce, the addition of ascorbic acid directly inhibited PPO activity. The results suggest that the high ascorbic acid content found in rocket salad plays an effective role in reducing PPO activity.

Synthesis of 4'-thiosemicarbazonegriseofulvin and its effects on the control of enzymatic browning and postharvest disease of fruits

4'-Thiosemicarbazonegriseofulvin, a new thiosemicarbazide derivative of griseofulvin, was synthesized and evaluated for its potential in the control of enzymatic browning and postharvest disease of fruits. Browning on fruits is mainly due to the enzymatic oxidation of phenolic compounds catalyzed by tyrosinase. 4'-Thiosemicarbazonegriseofulvin could effectively inhibit the activity of tyrosinase, and its 50% inhibitory concentration (IC(50)) against tyrosinase was determined to be 37.8 μM. It was a reversible and noncompetitive inhibitor of tyrosinase, and its inhibition constant (K(I)) was determined to be 38.42 μM. The antifungal activity of 4'-thiosemicarbazonegriseofulvin was studied against four fungi (Fusarium oxysporum, Fusarium moniliforme, Fusarium solani, and Colletotrichum truncatum) that often cause postharvest diseases of fruits. The results showed that 4'-thiosemicarbazonegriseofulvin could also strongly inhibit the mycelial growth of the four target fungi; the 50% lethal concentration (LC(50)) values were 5.4, 7.0, 15.3, and 1.5 mM, respectively.

Effect of bruising on respiration, superficial color, and phenolic changes in fresh Manzanilla olives (Olea europaea pomiformis): development of treatments to mitigate browning

The aim of the work was to study the postharvest changes in Manzanilla olives and to find treatments to mitigate damages because of bruises. The phenolic content in bruised and unbruised fruits exposed to air always decreased, but the loss in phenols and the respiratory activity were greater in bruised olives; these changes were related to the appearance of brown spots. Immersion of the picked fruits in a cold (8 °C) acidic solution (pH 3), ascorbic acid solution (100 mM), or sodium metabisulfite solution (100 mM) significantly reduced the loss in phenols in olives and led to lighter brown bruised areas. This immersion did not affect the behavior of the fruits during the lye treatment and the subsequent fermentation. In the final product, no influence on the surface color of unbruised olives was observed and there was a significant color improvement in the bruised areas of damaged olives.

Bioactive compounds during storage of fresh-cut spinach: the role of endogenous ascorbic acid in the improvement of product quality

Spinach is rich in bioactive constituents such as vitamin C, flavonoids and phenolic acids. In this work, the biochemical modifications occurring during one week of storage at 4 degrees C were evaluated both in intact and in fresh-cut spinach. Results showed that vitamin C concentration is less affected by storage in fresh-cut spinach with respect to intact spinach. MS/MS analysis showed that the main flavonoids are not modified during storage in intact leaves, while some of them increased significantly during storage in the fresh-cut samples. Fresh-cut spinach did not show color alteration even if PPO activity increased significantly during storage. This finding was related to the high ascorbic acid content, which delays the subsequent polymerization events. This finding was confirmed by the unaltered concentration of phenolic compounds in fresh-cut spinach during storage. In conclusion, data about nutritional content and visual performance concurrently suggest that spinach is a suitable species for marketing as a fresh-cut product.

Tissue printing to visualize polyphenol oxidase and peroxidase in vegetables, fruits, and mushrooms

A simple tissue-printing procedure to determine the tissue location of the endogenous enzymes polyphenol oxidase and peroxidase in a variety of vegetables, fruits, and mushrooms is described. In tissue printing, cell contents from the surface of a cut section of the tissue are transferred to an adsorptive surface, commonly a nitrocellulose membrane. Because of the considerable expense of nitrocellulose, our procedure utilizes artists' hot-press watercolor paper as a novel and more economical alternative. Tissue prints are then exposed to an enzyme substrate from which an insoluble, colored product is produced. The appearance of color in specific areas of the print is an indication of the presence of the enzyme in those tissue locations. The experiment is designed to enable students to learn some fundamental concepts about enzymes. It has been used in an introductory-level organic and biochemistry course for nonscience majors, but would also be appropriate for advanced high school students or could be adapted for an upper-level undergraduate biochemistry course.