Browning inhibition mechanisms by cysteine, ascorbic acid and citric acid, and identifying PPO-catechol-cysteine reaction products

Secondary product from strawberry (Fragaria ananassa) fruit for extended preservation and value addition

Strawberry is highly delicate and perishable fruit prone to microbial spoilage. To address these issues, secondary product(s) using strawberry pulp was prepared by adding sweetener (jaggery or sucrose) and anti-browning agent (citrate) to reduce the extent of drying induced sourness and discoloration, respectively. Subsequently, these products were LDPE packed and radiation processed (≤ 5 kGy) to ensure microbiological safety during ambient temperature storage. Physical, biochemical, functional (antioxidant and antimutagenic) and organoleptic properties (including flavor compounds) were found to be either retained or enhanced compared to dehydrated strawberry pulp. The product termed as dehydrated strawberry roll having jaggery and citrate (DSRJ) was found to be more acceptable than that having sucrose and citrate (DSRS) till 5 months. Scanning electron microscopy indicated better surface quality of DSRJ than DSRS. Electrochemical property when analyzed using cyclic voltammetry displayed strong correlation to radical scavenging antioxidant assays with marginal storage associated shift in the peak potential. GC-MS analysis indicated retention of most of the flavor compounds in DSRJ during storage. Thus, DSRJ, a value-added product could provide a solution to control post-harvest losses of the fruit by processing to a more stable form and ensuring availability beyond the season.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13197-021-05171-9.

The Effect of p-Coumaric Acid on Browning Inhibition in Potato Polyphenol Oxidase-Catalyzed Reaction Mixtures

There has been considerable interest in using natural polyphenol oxidase (PPO) inhibitors to control browning in fruit and vegetable products. p-Coumaric acid (pCA), a common secondary metabolite of plants, has been studied as an inhibitor of PPOs/tyrosinases from several foods (e.g., mushroom, apple, and potato). However, studies on the use of pCA for the inhibition of PPO-initiated browning in actual food systems are limited. Therefore, a study was carried out to ascertain the efficacy of using pCA to limit PPO-initiated browning in fresh potato juice. The extent of browning inhibition by pCA was shown to be reaction system-dependent. Browning in potato juice was unexpectedly enhanced by the addition of pCA. This was interpreted as pCA acting as an alternative substrate with significantly higher browning efficiency; extent of browning under this condition was higher than that observed in the native potato juice. The addition of pCA to any of the model reaction mixtures (i.e., those containing semi-purified enzymes and substrates) significantly inhibited browning. The discrepancy in pCA effects on browning inhibition in different reaction systems is postulated to be mainly due to non-enzyme and non-substrate components in potato juice that participate in the post-PPO reaction sequences, which ultimately lead to brown color formation.

Inhibitory effect of Malaysian coastal plants on banana (Musa acuminata colla "Lakatan"), ginger (Zingiber officinale Roscoe) and sweet potato (Ipomoea batatas) polyphenol oxidase

Undesired browning reaction catalyzed by polyphenol oxidase (PPO) has reduced the nutritional quality and customer acceptance of the products. The inhibitory effects of six coastal plants including Sonneratia alba, Rhizophora apiculata, Syzygium grande, Rhizophora mucronata, Hibiscus tiliaceus and Bruguiera gymnorhiza on PPO in banana, sweet potato and ginger were studied based on oxidation of pyrocatechol. Banana exhibited the highest PPO activity (141,600 U), followed by sweet potato (43,440 U) and ginger (26,880 U). Banana PPO was strongly inhibited by R. apiculata (70.87%) and sweet potato PPO was strongly inhibited by S. alba (82.00%). In general, most banana PPO was the most susceptible to inhibition with all inhibitors having inhibition higher than 60.00% at 0.5 mg/ml and ginger PPO was the least susceptible with all inhibitors showing less than 50.00% inhibition at 0.5 mg/ml. Coastal plant extracts are potentially to be developed as natural inhibitors for preventing enzymatic browning of fruits and vegetables.

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.

Spicy Herb Extracts as a Potential Improver of the Antioxidant Properties and Inhibitor of Enzymatic Browning and Endogenous Microbiota Growth in Stored Mung Bean Sprouts

The quality and shelf life of sprouts can be improved by postharvest application of water herb extracts. The effect of water infusions of marjoram, oregano, basil, and thyme on the phenolic content, antioxidant potential, and the microbiological and consumer quality of stored mung bean sprouts was studied. Compared to the control, the treatments increased total phenolic content. The highest amounts were determined in sprouts soaked in the thyme extract (6.8 mg/g d.m.). The infusions also inhibited the activity of enzymes utilizing phenolics, and marjoram and oregano were found to be the most effective. The increase in the level of phenolics was reflected in enhanced antioxidant properties (ability to quench cation radical ABTS^•+, reducing and chelating power). Both total phenolics and flavonoids, as well as antioxidant capacities, were highly bioaccessible in vitro. All the natural extracts effectively reduced the growth of total mesophilic bacteria, coliforms, and molds (they were more effective than ascorbic and kojic acids). The treatments did not exert a negative influence on the sensory properties or nutritional value of the sprouts, and even improved starch and protein digestibility. These results are very promising and may suggest a wider used of natural extracts as preservatives of minimally processed food.

Effect of ascorbic acid, oxygen and storage duration on patulin in cloudy apple juice produced on a semi-industrial scale

Patulin (PAT), a mycotoxin mainly produced by Penicillium expansum, is of high concern with regard to human food safety. This study examined the stability of PAT in artificially contaminated cloudy apple juice (CAJ) produced on a semi-industrial scale using an innovative technology allowing degassing and pressing under low-oxygen conditions (VaculIQ 1000). The effects of adding ascorbic acid (AA), degassing during production and storing in the dark at 20 °C on the PAT concentration were studied, as well as possible degradation and reaction products formed. The highest PAT degradation (50%) was observed for flash-pasteurised juice with AA added, produced under low-oxygen conditions and degassed and stored for 14 days at 20 °C in the dark in aluminium laminate aseptic bags. Juices produced showed no significant differences in the quality parameters measured and did not show significant formation of reaction products. Further research needs to be focused on the fate of PAT in CAJ produced on an industrial level with and without addition of AA.

The effect of physico-chemical treatment in reducing Listeria monocytogenes biofilms on lettuce leaf surfaces

The purpose of this research was to characterize Listeria monocytogenes from several environmental and clinical sources and assess the efficacy of single and combined physico-chemical treatments in reducing biofilm on lettuce leaves. PCR analysis of L. monocytogenes isolates collected from different clinical (10 strains) and environmental sources (12 strains) was used to look for the presence of one Listeria-specific gene and five virulence genes. Biofilms of L. monocytogenes were developed on lettuce leaves over 24 h. A 5-min ultrasound and a 300-ppm sodium hypochlorite (NaOCl) wash resulted in similar reductions in cell numbers of 0.82 log CFU cm-2. For chlorine dioxide (ClO2) at 60 ppm, the cell numbers were reduced by ∼5.45 log CFU cm-2. A combined treatment of 5 min of ultrasound plus 300 ppm NaOCl or 40 ppm ClO2, provided maximal efficacy, reducing the number of L. monocytogenes on the lettuce surface to non-detectable levels. Therefore, ClO2 has the potential to replace NaOCl for the disinfection of food products in the food industry.

Insight of Polyphenol Oxidase Enzyme Inhibition and Total Polyphenol Recovery from Cocoa Beans

A full factorial design (ascorbic acid/l-cysteine inhibitors, temperature, and time as factors) study was conducted to enhance inhibition of polyphenol oxidase (PPO) activity without decreasing cocoa polyphenol concentrations. The data obtained were modelled through a new equation, represented by Γ, which correlates both high polyphenol content with reduced specific PPO activity. At optimized values (70 mM inhibitory solution at 96 °C for 6.4 min, Γ = 11.6), 93.3% PPO inhibition and total polyphenol of 94.9 mg GAE/g were obtained. In addition, microscopy images confirmed the cell morphological changes measured as the fractal dimension and explained the possible cell lysis and denaturation as a result of heat treatment and chemical inhibitors. Results also showed that PPO enzyme was most suitable (higher v_max/K_m ratio) for catechol, with a reduction in its affinity of 13.7-fold after the inhibition heat treatment. Overall, this work proposed a suitable and food-safe procedure for obtaining enriched polyphenol extract with low enzyme activity.

Bioavailability and antioxidant potentials of fresh and pasteurized kiwi juice before and after in vitro gastrointestinal digestion

The aim of the present work was to find the changes in the chemical composition, chemical structure and antioxidant activity of bioactive nutrients of kiwi juices upon pasteurization and in vitro gastrointestinal digestion. Results showed that fresh kiwi juice (FKJ) contains total phenols, 162 mg GAE/mL, total flavonoids 1.44 mg QE/mL, ascorbic acid 4.5 mg/mL and fiber 60%; these contents were marginally affected by pasteurization (PKJ) to be 122, 1.02, 2.6 and 47 respectively. On the other hand, gastrointestinal digestion severely lowered total phenols, total flavonoids and ascorbic acid in digested fresh kiwi juice (DFKJ) to 49 mg GAE/mL,0.16 mg GE/mL and 2.2 mg/mL, and in digested pasteurized kiwi juice (DPKJ) to 42, 0.07 and 1.2 respectively; however fiber contents increased upon digestion as a result of decreasing other nutrients. The antioxidant and reducing power activities followed the same order of decreasing total phenols, total flavonoids and ascorbic acid contents i.e. FKJ > PKJ > DFKJ > DPKJ. The major flavonoid found was apigenin in the form of various glycosides differ in the number (1-3) and position of the sugar units. Pasteurization showed minor effects on the chemical composition of fresh juice while digestion, as a result of gastrointestinal enzymes had more effect but mainly on the number and position of the sugar units rather than affecting the flavonoid moiety which preserve the main nutritive values.

Effect of Chitosan-Ascorbic Acid Coatings on the Refrigerated Storage Stability of Fresh-Cut Apples

Using natural antimicrobial substances in edible films becomes crucial to extend the shelf-life of fresh-cut fruits due to the consumers’ preferences. In this study chitosan and ascorbic acid based film was used to improve the shelf-life of fresh-cut apples. Fresh-cut apple cubes were dipped in water (control), ascorbic acid (1%) or mixtures of chitosan–ascorbic acid in different ratios (1%:1%, 2%:2% or 1%:5%) for 5 min. After draining, fresh-cut apples were packed in sterile polypropylene jars and stored at 5 °C for 14 days. The treatment with chitosan and ascorbic acid suppressed browning, retained flesh firmness and maintained phenolic compounds throughout the storage period. Moreover, the treatment with chitosan–ascorbic acid significantly retarded the microbial growth during storage. Those findings suggested that the best performance was acquired in 1% chitosan and 5% ascorbic acid coating. That coating could be practical and useful to prolonging the chemical and microbial shelf lives of fresh-cut apples during refrigerated storage.

Biochemical Properties of Polyphenol Oxidases from Ready-to-Eat Lentil (Lens culinaris Medik.) Sprouts and Factors Affecting Their Activities: A Search for Potent Tools Limiting Enzymatic Browning

Enzymatic browning of sprouts during storage is a serious problem negatively influencing their consumer quality. Identifying and understanding the mechanism of inhibition of polyphenol oxidases (PPOs) in lentil sprouts may offer inexpensive alternatives to prevent browning. This study focused on the biochemical characteristics of PPOs from stored lentil sprouts, providing data that may be directly implemented in improving the consumer quality of sprouts. The purification resulted in approximately 25-fold enrichment of two PPO isoenzymes (PPO I and PPO II). The optimum pH for total PPOs, as well as for PPO I and PPO II isoenzymes, was 4.5–5.5, 4.5–5.0, and 5.5, respectively. The optimal temperature for PPOs was 35 °C. Total PPOs and the PPO I and PPO II isoenzymes had the greatest affinity for catechol (K_m = 1.32, 1.76, and 0.94 mM, respectively). Ascorbic acid was the most effective in the inhibition of dark color formation by total PPOs, and showed ca. 62%, 43%, and 24% inhibition at 20-, 2-, and 0.2-mM concentrations. Ascorbic acid, l-cysteine, and sodium metabisulfite (20 mM) significantly inhibited color development in the reactions catalyzed by both isoenzymes of PPO. Ba²⁺, Fe³⁺, and Mn²⁺ (10 mM) completely inhibited PPO activity. This study of the effect of antibrowning compounds and cations on PPO activity provides data that can be used to protect lentil sprouts against enzymatic browning during storage and processing.

Influence of Nucleophilic Amino Acids on Enzymatic Browning Systems

In the present study the enzymatic oxidation of gallic acid and catechin catalyzed by nashi pear polyphenol oxidase (PPO) in the presence of the amino acids lysine, arginine, or cysteine was investigated for polyphenol-amino acid adducts. HPLC analyses revealed the formation of two novel dihydrobenzothiazine carboxylic acid derivatives (8-(3',4'-dihydro-2 H-chromene-3',5',7'-triol)-3,4-dihydro-5-hydroxy-2 H-benzothiazine-3-carboxylic acid and 7-(3',4'-dihydro-2 H-chromene-3',5',7'-triol)-3,4-dihydro-5-hydroxy-2 H-benzothiazine-3-carboxylic acid) from 2'-cysteinyl catechin and 5'-cysteinyl catechin in cysteine incubations, respectively. In contrast, arginine and lysine did not lead to any amino acid adducts. Target compounds were separated by high-performance countercurrent chromatography and preparative HPLC and unequivocally characterized by mass spectrometry and nuclear magnetic resonance spectroscopy. Mechanistic incubations starting from the catechin-cysteine adducts showed that both catechin and PPO are crucial components in the formation of the dihydrobenzothiazines. The cysteine incubations showed a red-brown coloration, which coincided with formation and degradation of the dihydrobenzothiazines finally leading to the formation of high-polymeric melanins. Therefore, these compounds might be the key intermediates to understand development of color during cysteine-driven enzymatic browning reactions.

Isolation and Identification of Tyrosinase-Inhibitory and Copper-Chelating Peptides from Hydrolyzed Rice-Bran-Derived Albumin

Rice-bran albumin (RBAlb), which shows higher tyrosinase-inhibitory activity than other protein fractions, was hydrolyzed with papain to improve the bioactivity. The obtained RBAlb hydrolysate (RBAlbH) was separated into 11 peptide fractions by RP-HPLC. Tyrosinase inhibition and copper chelation activities decreased with increasing retention times of the peptide fractions. RBAlbH fraction 1, which exhibited the greatest activity, contained 13 peptides whose sequences were determined by using LC-MS/MS. Most of the peptide sequences contained features of previously reported tyrosinase-inhibitory and metal-chelating peptides, especially peptide SSEYYGGEGSSSEQGYYGEG. RBAlbH fraction 1 showed more effective tyrosinase inhibition (IC50 = 1.31 mg/mL) than citric acid (IC50 = 9.38 mg/mL), but it was less effective than ascorbic acid (IC50 = 0.03 mg/mL, P ≤ 0.05). It showed copper-chelating activity (IC50 = 0.62 mg/mL) stronger than that of EDTA (IC50 = 1.06 mg/mL, P ≤ 0.05). These results suggest that RBAlbH has potential as a natural tyrosinase inhibitor and copper chelator for application in the food and cosmetic industries.

Buffalo cheese whey hydrolyzed with Alcalase as an antibrowning agent in minimally processed apple

Buffalo whey was hydrolyzed with Alcalase for different times t_i (i = 0, 0.5, 1, 2, 3, 4 or 6 h) and the browning inhibition of minimally processed apples was investigated. The hydrolysis process was followed by determination of the degree of hydrolysis. In order to understand possible modes of action on the enzymatic browning, whey was submitted to the analysis of antioxidant activity (ABTS^·+ radical sequestration, Fe²⁺ chelating activity and reducing power), reactivity with quinones and inhibitory activity on polyphenol oxidases (PPO) extracted from Red Delicious apples. Buffalo whey showed significant increase in degree of hydrolysis, antioxidant activity, reactivity with quinones and PPO-inhibitory activity as a function of the hydrolysis time. Maximum PPO-inhibitory activity was observed from 4 h hydrolysis (t_4h hydrolysate), reaching about 50% inhibition. Then, slices of minimally processed apples were immersed in a buffered solution of the t_4h hydrolysate, packed and subjected to instrumental color evaluation during storage for up to 6 days. As for the ability to inhibit the browning of the minimally processed apples, the hydrolyzate kept the L∗ parameter of the apples during 6 days of storage, not statistically differing from the metabisulfite. In addition to the luminosity, the hydrolyzed whey was able to maintain the browning index of the apples at lower values during this storage time compared to the non-hydrolyzed whey. These results evidence possible applications of buffalo whey hydrolyzed with Alcalase as a natural substitute for additives conventionally used in the control of enzymatic browning in foods.

Rapid screening and quantification of major organic acids in citrus fruits and their bioactivity studies

Organic acids (OAs) are small non-volatile molecules with widespread usage in processed foods, feeds and instant beverages. The prime aim of this study was to explore major OAs in local citrus fruits (Citrus limetta, Citrus aurantifolia, Citrus nobilis, Citrus karna, Citrus medica, Citrus ichangensis and Citrus aurantium) and assessment of their bioactivities. A RP-HPLC-DAD method was developed using buffer free solvent system for rapid detection and quantification of major OAs from citrus fruits and derived products. Method validation studies showed good linear calibration curve (0.985-0.998) for all OAs. The values of %RSD ranged between 0.0001-1.129 and 0.142-1.941 for interday and intraday variability respectively. The limit of detection and limit of quantification values for different OAs were ranged between 1.5-12 and 5-40 µg mL^-1. The juice of above mentioned citrus fruit cultivars were assessed for OAs, total phenolics, free radical scavenging antioxidants and their antimicrobial potential against selected bacterial and fungal strains. The results showed variable contents of phenolics [0.28 ± 0.001-1.17 ± 0.014 mg (GAE) mL^-1] and antioxidant compounds (1.26 ± 0.009-2.84 ± 0.006 mg of trolox equivalents mL^-1) in all juice samples besides significant antifungal activity against C. albicans and A. niger strains. However, in case of antibacterial activity, only C. aurantifolia showed inhibitory effects against selected strains. It was found that citrus fruits have immense potential for their utilization as economic source of natural OAs and development of value added products, beverages and bio-preservatives.