Full inhibition of Whangkeumbae pear polyphenol oxidase enzymatic browning reaction by l-cysteine

ANN-GA optimized composite color protectant combined with magnetic field assisted freezing: Effects on the quality of mushroom (Agaricus bisporus)

Due to their high water content, frozen mushrooms (Agaricus bisporus) were greatly affected by ice crystal formation, which can lead to the destruction of tissue structure, serious browning, high juice loss, and difficulty in maintaining good sensory characteristics. In order to improve the quality of frozen Agaricus bisporus, this study employed Artificial neural network and genetic algorithm (ANN-GA) to optimize the amount of composite color protectant, and identified the optimal freezing conditions for freezing Agaricus bisporus by determining the freezing curves under different magnetic field-assisted freezing conditions, the color variance, texture and structure, drip loss, and distribution of moisture. Furthering, using X-ray μCT three dimensional images were taken to characterize the microstructure of the samples. Among them, the 6 mT magnetic field-assisted freezing treatment group was significantly better than the control group, and the results showed that the magnetic field-assisted freezing combined with chemical color protectant as a composite processing technology improved the quality of frozen Agaricus bisporus. This provides a theoretical basis and technical support for enhanced processing of frozen Agaricus bisporus.

Affinity gel synthesis from the p-aminobenzoic acid derivative 4-amino-2-methylbenzoic acid and purification of polyphenol oxidase from various plant sources

The polyphenol oxidase (PPO) enzyme, which causes enzymatic browning, has been repeatedly purified from fruit and vegetables by affinity chromatography. In the present research, Sepharose 4B-l-tyrosine-4-amino-2-methylbenzoic acid, a novel affinity gel for the purification of the PPO enzyme with high efficiency, was synthesized. Additionally, Sepharose 4B-l-tyrosine-p-aminobenzoic acid affinity gel, known in the literature, was also synthesized, and 9.02, 16.57, and 28.13 purification folds were obtained for the PPO enzymes of potato, mushroom, and eggplant by the reference gel. The PPO enzymes of potato, mushroom, and eggplant were purified 41.17, 64.47, and 56.78-fold from the new 4-amino-2-methylbenzoic acid gel. Following their isolation from the new affinity column, the assessment of PPO enzyme purity involved the utilization of SDS-PAGE. According to the results from SDS-PAGE and native PAGE, the molecular weight of each enzyme was 50 kDa. Then, the inhibition effects of naringin, morin hydrate, esculin hydrate, homovanillic acid, vanillic acid, phloridzin dihydrate, and p-coumaric acid phenolic compounds on purified potato, mushroom, and eggplant PPO enzyme were investigated. Among the tested phenolic compounds, morin hydrate was determined to be the most potent inhibitor on the potato (Ki: 0.07 ± 0.03 μM), mushroom (Ki: 0.7 ± 0.3 μM), and eggplant (Ki: 4.8 ± 1.2 μM) PPO enzymes. The studies found that the weakest inhibitor was homovanillic acid for the potato (Ki: 1112 ± 324 μM), mushroom (Ki: 567 ± 81 μM), and eggplant (Ki: 2016.7 ± 805.6 μM) PPO enzymes. Kinetic assays indicated that morin hydrate was a remarkable inhibitor on PPO.

Phytochemical Profiling and Bioactive Potential of Grape Seed Extract in Enhancing Salinity Tolerance of Vicia faba

Salinity stress poses a significant threat to crop productivity worldwide, necessitating effective mitigation strategies. This study investigated the phytochemical composition and potential of grape seed extract (GSE) to mitigate salinity stress effects on faba bean plants. GC-MS analysis revealed several bioactive components in GSE, predominantly fatty acids. GSE was rich in essential nutrients and possessed a high antioxidant capacity. After 14 days of germination, GSE was applied as a foliar spray at different concentrations (0, 2, 4, 6, and 8 g/L) to mitigate the negative effects of salt stress (150 mM NaCl) on faba bean plants. Foliar application of 2-8 g/L GSE significantly enhanced growth parameters such as shoot length, root length, fresh weight, and dry weight of salt-stressed bean plants compared to the control. The Fv/Fm ratio, indicating photosynthetic activity, also improved with GSE treatment under salinity stress compared to the control. GSE effectively alleviated the oxidative stress induced by salinity, reducing malondialdehyde, hydrogen peroxide, praline, and glycine betaine levels. Total soluble proteins, amino acids, and sugars were enhanced in GSE-treated, salt-stressed plants. GSE treatment under salinity stress modulated the total antioxidant capacity, antioxidant responses, and enzyme activities such as peroxidase, ascorbate peroxidase, and polyphenol oxidase compared to salt-stressed plants. Gene expression analysis revealed GSE (6 g/L) upregulated photosynthesis (chlorophyll a/b-binding protein of LHCII type 1-like (Lhcb1) and ribulose bisphosphate carboxylase large chain-like (RbcL)) and carbohydrate metabolism (cell wall invertase I (CWINV1) genes) while downregulating stress response genes (ornithine aminotransferase (OAT) and ethylene-responsive transcription factor 1 (ERF1)) in salt-stressed bean plants. The study demonstrates GSE's usefulness in mitigating salinity stress effects on bean plants by modulating growth, physiology, and gene expression patterns, highlighting its potential as a natural approach to enhance salt tolerance.

Color, antioxidant and nutritional composition of dehydrated country bean (Lablab purpureus) seeds using solar drying techniques and pretreatments in Bangladesh

The country bean (Lablab purpureus), is a significant contributor of dietary protein and other essential components in human nutrition. Because of its elevated moisture content, it is susceptible to rapid decay, leading to losses after harvesting. The utilization of solar drying has attracted significant attention as a tactic to minimize nutrient depletion in dried goods and enhance their longevity. This study employed four solar drying techniques, namely long chimney, short chimney, box solar drying and open sun drying, along with pretreatments such as potassium metabisulfite, potassium-sodium tartrate, citric acid and ascorbic acid. The objective was to determine an effective solar drying method, combined with pretreatment, that can maintain the color and nutritional qualities of dried country bean seeds. The treatment combinations were organized in a factorial randomized complete block design (RCBD) with three replications. The data were subjected to a two-way analysis of variance (ANOVA) and a Duncan Multiple Range Test (DMRT) was conducted at a significance level of 5 % (p < 0.05). Results revealed that box solar dryer having the highest drying efficiency, retained the highest β-carotene (82.94 %), vitamin C (90.15 %), protein (96.48 %), fat (11.63 %), and ash (90.50 %) with maximum DPPH radical scavenging activity (lowest IC50 209.49 μg/ml) compared to other driers. Besides, country bean seeds have noteworthy proximate compositions, antioxidant activity, and bioactive components treated with 1 % potassium metabisulfite. Furthermore, the country bean seeds dehydrated in box solar dryer after 1 % potassium metabisulfite treatment received the highest acceptance score on the five-point Hedonic scale (4.83-4.89 out of 5.00) and color appearance and the similar trend was further supported by principal component analysis. Thus, it can be inferred that using a box solar dryer with a 1 % potassium metabisulfite pretreatment is a feasible method for preserving the color and nutritional value of country bean seeds and reducing postharvest losses.

Salicylic Acid Spray Delays Sand Pear Fruit Senescence during Room Temperature Shelf Life by Regulating Antioxidant Capacity and Senescence-Related Genes

'Whangkeumbae' (Pyrus pyrifolia) is a variety of sand pear fruit well-known for its smooth surface and good taste. However, the fruit quality is adversely affected by postharvest ethylene production. Therefore, improving postharvest shelf life by regulating fruit senescence is critical to promoting the 'Whangkeumbae' fruit industry. Here, we investigated the effect of salicylic acid (SA) spray on fruit senescence in sand pears during room temperature shelf life. Exogenous SA reduced polyphenol oxidase (PPO) activity and malondialdehyde (MDA) content during room temperature shelf life. Additionally, SA effectively maintained the fruit skin coloration and increased the activity of antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). SA treatment inhibited PpPPO1 expression and upregulated PpSOD1, PpAPX6, and PpGST2 expression. Furthermore, SA application downregulated the expression of PpACO2, PpEIN3a, PpNCED1, and PpAOC2, while upregulating PpNPR-1, PpTAR2, and PpCOMT1 during room temperature shelf life. SA treatment also influenced cell wall metabolism and modification genes by inhibiting PpPG1, PpPME2, and PpCEL3 and inducing PpPGIP1 expression. Additionally, SA treatment affected sugar and acid metabolism genes and increased the expression of PpSPS1, PpSUS1, PpSOT1, PpTMT4, PpSWEET15, and PpcyNAD-MDH, but suppressed the expression of PpcyNADP-ME. The Pearson correlation analysis indicated that PPO activity and MDA content were positively correlated with the expression of PpPPO1, PpACO2, PpEIN3a, PpNCED1, PpAOC2, PpPG1, PpPME2, PpCEL3, and PpcyNDA-MDH. Conversely, these factors were negatively associated with the activities of SOD, POD, CAT, and APX, as well as the expression levels of PpSOD1, PpPOD1, PpCAT1, PpAPX6, PpGST2, PpNPR-1, PpTAR2, PpCOMT1, PpPGIP1, PpSPS1, PpSUS1, PpSOT1, PpTMT4, PpSWEET15, and PpcyNAD-MDH. Our results reveal that exogenous SA could delay fruit senescence in sand pear fruit by regulating various biochemical and molecular mechanisms and can be used to effectively extend fruit shelf life during room temperature storage. However, further research is necessary to determine whether the fruits sprayed with SA are suitable for direct human consumption.

Colour, nutritional composition and antioxidant properties of dehydrated carrot (Daucus carota var. sativus) using solar drying techniques and pretreatments

Carrot is a seasonal perishable tuberous root vegetable which presents a preservation challenge owing to its elevated moisture content. Recently, carrot processing has received more attention because of its many health-promoting qualities and the reduction of postharvest losses in a cost-effective safe way. This study was designed to sort out the effective solar drying technique including pre-treatment that would retain the color and quality characteristics of dehydrated carrot. Carrot slices were subjected to dry using open sun drying (D1), solar drying long chimney (D2), solar drying short chimney (D3) and box solar drying (D4) techniques with the pretreatments of ascorbic acid 1 % (C3), citric acid 5 % (C4), potassium metabisulfite 1 % (C5) and potassium sodium tartrate 0.3 % (C6) before drying. Drying characteristics, nutritional attributes, phytochemicals and antioxidant of the dehydrated carrot samples were compared with the fresh sample and untreated (control) sample. Results showed that D4 was a good drying method to preserve nutritional quality with good appearance. Among the pretreatments, C5 and C4 resulted improved nutritional quality retention, enhanced visual acceptability and enriched antioxidant activities. PCA (Principal Component Analysis) and correlation matrix revealed that D4 with C5 retained the maximum amount of vitamin, minerals, total phenolic content, antioxidant and admirable dehydrated carrot color by inactivating enzymatic reaction. Therefore, box solar drying with potassium metabisulfite pretreatment would be very promising for functional carrot drying retaining acceptable color and nutrition composition.

Advances in the Preparation, Stability, Metabolism, and Physiological Roles of Anthocyanins: A Review

Anthocyanins are natural flavonoid polyphenolic compounds widely found in fruits and vegetables. They exhibit antioxidant properties and prophylactic effects in the immune and cardiovascular systems, confer protection against cancer, and contribute to the prevention of cardiovascular diseases. Thus, their incorporation into functional foods, pharmaceuticals, supplements, and cosmetic formulations aims at promoting human well-being. This review comprehensively outlined the structural attributes of anthocyanins, expanding upon diverse methodologies employed for their extraction and production. Additionally, the stability, metabolic pathways, and manifold physiological functions of anthocyanins were discussed. However, their constrained fat solubility, susceptibility to instability, and restricted bioavailability collectively curtail their applicability and therapeutic efficacy. Consequently, a multidimensional approach was imperative, necessitating the exploration of innovative pathways to surmount these limitations, thereby amplifying the utilitarian significance of anthocyanins and furnishing pivotal support for their continual advancement and broader application.

Inhibitory mechanism of sodium hexametaphosphate on enzymatic browning in yellow alkaline noodles

The effect and mechanism of sodium hexametaphosphate (SHMP) on polyphenol oxidase (PPO) enzymatic browning in yellow alkaline noodles (YAN) were investigated. The browning degree and PPO activity in YAN or PPO solutions decreased with the SHMP concentrations increased. Variations in pH values (pH 7-8.5) adjusted by HCl or acetic acid slightly affected the PPO activity, but SHMP inhibited PPO activity more pronounced. The inhibition of SHMP on PPO activity was irreversible. SHMP formed coordinate covalent bonds with Cu²⁺ to make PPO inactive. HPLC analysis revealed that SHMP reduced the browning products and led to the color of PPO-catechol systems being lightened. Furthermore, SHMP inhibited browning by hampering the auto-oxidization of intermediate products due to the change in pH value. Besides, the HPLC chromatogram, UV-vis spectrum, and mass spectrometry revealed that SHMP could convert melanin (m/z 248.97, 723.5, and 836.58) to light-colored substances (m/z 137.11).

Effects of L-Cysteine and γ-Aminobutyric Acid Treatment on Postharvest Quality and Antioxidant Activity of Loquat Fruit during Storage

Sichuan is the China's leading producer of loquat, with the largest cultivation area and yield ranked first in China. Loquat is a seasonal fruit highly appreciated by consumers; however, the fruit is prone to browning and lignification after harvest, affecting its storage quality. The effects of L-Cysteine (L-Cys, 0.01, 0.05, 0.1, 0.2%) and γ-aminobutyric acid (GABA, 0.025, 0.05, 0.075, 0.1%) on the sensory quality and antioxidant activity of loquat fruit during cold storage at 4 °C for 35 days and simulated shelf life for 5 days were investigated. The results showed that after 40 days of storage, compared with the control, 0.05% L-Cys and 0.05% GABA treatment of 'Zaozhong No. 6' loquat fruit effectively reduced the weight loss rate, browning index, decay index, respiratory rate, firmness, and lignin content and slowed the decreases in total soluble solids, soluble sugar, titratable acidityand vitamin C contents. The application of 0.05% L-Cys and 0.05% GABA significantly increased the contents of total phenols, total flavonoids, flavanols, and carotenoids; delayed the increase of relative electric conductivity, MDA, POD, and PPO activities; and significantly enhanced the activities of SOD and CAT, DPPH free radical scavenging ability, and FRAP, thereby improving antioxidant capacity. In summary, 0.05% L-Cys and 0.05% GABA treatment promotes the quality of loquat fruit after 40 days of storage, and significantly enhances antioxidant capacity, thus delaying senescence after harvest.

Class III Peroxidases (POD) in Pepper (Capsicum annuum L.): Genome-Wide Identification and Regulation during Nitric Oxide (NO)-Influenced Fruit Ripening

The class III peroxidases (PODs) catalyze the oxidation of several substrates coupled to the reduction of H₂O₂ to water, and play important roles in diverse plant processes. The POD family members have been well-studied in several plant species, but little information is available on sweet pepper fruit physiology. Based on the existing pepper genome, a total of 75 CaPOD genes have been identified, but only 10 genes were found in the fruit transcriptome (RNA-Seq). The time-course expression analysis of these genes showed that two were upregulated during fruit ripening, seven were downregulated, and one gene was unaffected. Furthermore, nitric oxide (NO) treatment triggered the upregulation of two CaPOD genes whereas the others were unaffected. Non-denaturing PAGE and in-gel activity staining allowed identifying four CaPOD isozymes (CaPOD I-CaPOD IV) which were differentially modulated during ripening and by NO. In vitro analyses of green fruit samples with peroxynitrite, NO donors, and reducing agents triggered about 100% inhibition of CaPOD IV. These data support the modulation of POD at gene and activity levels, which is in agreement with the nitro-oxidative metabolism of pepper fruit during ripening, and suggest that POD IV is a target for nitration and reducing events that lead to its inhibition.

Purification, characterization and inactivation kinetics of polyphenol oxidase extracted from Cistanche deserticola

PPO was purified from Cistanche deserticola, and its enzymatic characteristics were clarified. It was found that microwave treatment was an efficient way to inactivate PPO. Polyphenol oxidase (PPO) from Cistanche deserticola was obtained and purified through an acetone precipitation and anion exchange column, the enzymatic characteristics and inactivation kinetics of PPO were studied. The specific activity of PPO was 73135.15 ± 6625.7 U/mg after purification, the purification multiple was 48.91 ± 4.43 times, and the recovery was 30.96 ± 0.27%. The molecular weight of the PPO component is about 66 kDa by SDS-PAGE analysis. The optimum substrate of PPO was catechol (V_max = 0.048 U/mL, K_m = 21.70 mM) and the optimum temperature and pH were 30 °C and 7, respectively. When the temperature is above 50 °C, pH < 3 or pH > 10, the enzyme activity can be significantly inhibited. The first-order kinetic fitting shows that microwave inactivation has lesser k values, larger D values and shorter t_1/2. It was found that microwave treatment is considered as an efficient and feasible way to inactive PPO by comparing the Z values and E_a values of the two thermal treatments.

A "turn-on" inverse opal photonic crystal fluorescent sensing film for detection of cysteine and its bioimaging of living cells

A "turn-on" inverse opal photonic crystal fluorescent sensing film infiltrated with a coumarin derivative is reported for the reliable and accurate detection of cysteine in human serum and fluorescence imaging of living cells. The coumarin derivative containing allyl ester specifically reacts with cysteine by ammonolysis to generate a fluorescent product whose emission wavelength is at ~ 535 nm, providing a selective fluorescence detection for cysteine. The emitted fluorescence is significantly enhanced due to the slow photon effect derived from the photonic crystal film. This is because the emission wavelength is overlapped with the blue-band edge of the photonic stopband of the selected inverse opal film. The fluorescence enhancement effect endows the prepared inverse opal film with highly sensitive detection with a limit of detection of 3.23 × 10^-9 mol/L and a wide linear detection range of 1 × 10^-7 - 1 × 10^-3 mol/L. A fast response within 30 s toward cysteine is also achieved due to the three-dimensional interconnected macroporous structure with a high-specific surface area of the inverse opal film. The prepared inverse opal fluorescent sensing film has been successfully applied to the detection of cysteine in human serum and bioimaging of living cells. In the diluted human serum, the recoveries for the detection of cysteine were 97.92 - 107.20%, and the relative standard deviations were 2.61-9.04%, demonstrating the potential applicability of the inverse opal fluorescent sensing film to real sample analysis. The method may provide a universal strategy for constructing various photonic crystal fluorescent sensing films by using different fluorescent probes.

Comparison of Inhibitory Effects of Cinnamic Acid, β-Cyclodextrin, L-Cysteine, and Ascorbic Acid on Soluble and Membrane-Bound Polyphenol Oxidase in Peach Fruit

There has been considerable interest in controlling polyphenol oxidase (PPO) activity to prevent enzymatic browning in foods. However, studies on inhibitions of different forms of PPO are very limited. Thus, this study focuses on the effects of cinnamic acid, β-cyclodextrin, L-cysteine, and ascorbic acid on soluble PPO (sPPO) and membrane-bound PPO (mPPO) in peach fruit. The activity of partially purified sPPO was 3.17 times higher than that of mPPO. However, mPPO was shown to be more stable than sPPO in the presence of inhibitors with different concentrations (i.e., 1, 3, 5 mM); activation of mPPO was found by 5 mM L-cysteine. Both sPPO and mPPO inhibitions were PPO substrate-dependent. Ascorbic acid showed the highest inhibitory effect on both sPPO and mPPO with all studied inhibitors and substrates. The inhibition of 1 mM ascorbic acid on sPPO and mPPO reached 95.42 ± 0.07% and 65.60 ± 1.16%, respectively. β-Cyclodextrin had a direct inhibitory effect only on sPPO, while the other three inhibitors had direct effects on both sPPO and mPPO. Cinnamic acid exhibited a non-competitive inhibition on sPPO and mPPO, with L-cysteine showing the same, though on sPPO. The inhibition of studied inhibitors on sPPO and mPPO is highly related to the substrate environment, type, and concentration of inhibitors. This study provides a basis for the further prevention of peach fruit browning from the perspective of different enzyme forms.

Elucidating the Role of Santalol as a Potent Inhibitor of Tyrosinase: In Vitro and In Silico Approaches

This research work focuses on the potential application of an organic compound, santalol, obtained from santalum album, in the inhibition of the enzyme tyrosinase, which is actively involved in the biosynthesis of melanin pigment. Over-production of melanin causes undesirable pigmentation in humans as well as other organisms and significantly downgrades their aesthetic value. The study is designed to explain the purification of tyrosinase from the mushroom Agaricus bisporus, followed by activity assays and enzyme kinetics to give insight into the santalol-modulated tyrosinase inhibition in a dose-dependent manner. The multi-spectroscopic techniques such as UV-vis, fluorescence, and isothermal calorimetry are employed to deduce the efficiency of santalol as a potential candidate against tyrosinase enzyme activity. Experimental results are further verified by molecular docking. Santalol, derived from the essential oils of santalum album, has been widely used as a remedy for skin disorders and a potion for a fair complexion since ancient times. Based on enzyme kinetics and biophysical characterization, this is the first scientific evidence where santalol inhibits tyrosinase, and santalol may be employed in the agriculture, food, and cosmetic industries to prevent excess melanin formation or browning.

Efficient regeneration of mature castanopsis hystrix from in vitro stem explants

Castanopsis hystrix is one of the main timber trees grown in China. However, severe shortage of natural seeds and the difficulty of explant regeneration has limited seedling supply. As such, there is a need for research on asexual multiplication of C. hystrix. This study established a rapid propagation technology system for C. hystrix genotypes, including explant treatment, proliferation, and rooting. HZ (a modified MS medium) supplemented with 4.4 μM BA and 0.5 μM IBA was found to be the optimal medium for shoot sprouting. The maximum proliferation coefficient and the number of effective shoots was obtained on HZ medium supplemented with 2.6 μM BA and 1.0 μM IBA, were 3.00 and 5.63, respectively. A rooting rate of 83.33% was achieved using half-strength HZ medium supplemented with 3.2 μM NAA. Adding vitamin C (80 mg⋅l-1) for 7 days in a dark environment reduced the browning rate, while increasing the proliferation rate. Additionally, through cytological observation, we established how and where adventitious roots occur. The survival rate of transplanted plantlets was > 90%. This is the first report of an in vitro regeneration technique that uses stem segments of mature C. hystrix as explants.

Comparative Transcriptomic Analyses Provide Insights into the Enzymatic Browning Mechanism of Fresh-Cut Sand Pear Fruit

Pear (Pyrus spp.) is one of the most commonly consumed temperate fruits, having considerable economic and health importance. Fresh-cut or processed pear fruits are prone to browning because of the abundant phenolic compounds; however, little is known about the molecular mechanisms underlying enzymatic browning of fresh-cut sand pear fruit. In this study, fruits of two sand pear genotypes (low browning cultivar ‘Eli No.2′ and high browning cultivar ‘Weiningdahuangli’) were used to analyze the molecular mechanism of enzymatic browning by SMRT-seq and RNA-seq. The results generated 69,122 consensus isoforms, 21,336 new transcripts, 7105 alternative splicing events, and 254 long non-coding RNAs (lncRNAs). Furthermore, five genes related to enzymatic browning were predicted to be targets of six lncRNAs, and 9930 differentially expressed genes (DEGs) were identified between two different flesh browning cultivars. Meanwhile, most DEGs (e.g., PAL, 4CL, CAD, CCR, CHS, and LAR) involved in the phenylpropanoid biosynthesis pathway were up-regulated, and the expression of PPO and POD were highly expressed in the high-browning cultivar. Interestingly, the transcript level of PbrPPO4 (Pbr000321.4) was significantly higher than other PPO and POD genes, and a high level of total polyphenol and PPO activity were observed in the high browning cultivar. We found that the expression of lncRNA PB.156.1 was significantly positively correlated with the target gene PbrPPO4 (Pbr000321.4). The results suggest that PbrPPO4 might act as a major contributor and a key enzyme encoding gene in regulating fresh-cut sand pear fruit enzymatic browning; the expression of PbrPPO4 was probably regulated by lncRNA PB.156.1. Altogether, the transcriptomic and physiological analyses expand the knowledge of sand pear flesh enzymatic browning at the molecular level and provide a foundation for germplasm resources for molecular breeding of high polyphenol and low browning cultivars in sand pears.

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.

Cellular autofluorescence and browning in trichomes of Chinese cabbage (Brassica campestris) in response to mechanical stimulation and senescence

There is limited information concerning the formation of dot-like browning appearing at the base of trichomes on mature leaves on the Chinese cabbage (Brassica campestris L. ssp. pekinensis). This study confirmed for the first time that enhanced autofluorescence can be induced in the base of trichomes when pressure stimuli is applied to trichomes; the enhanced autofluorescence gradually moves to the top of trichomes and the neighbouring mesophyll tissue within 15min. The excitation of autofluorescence in trichomes was found to be more effective in mature leaves compared to newly emergent leaves. Increased polyphenol oxidase (PPO) activities and reactive oxygen species (ROS) accumulation were also detected in the basal region of trichomes that were subjected to mechanical stimuli. Enhanced fluorescence was observed at the top of the trichomes in senescencing leaves. A browning in the base of the trichomes during leaf senescence was observed. In contrast, no browning occurred at the base of the trichomes in leaves that were subject to pressure stimuli. The blue fluorescence in the trichomes in senescent leaves arises mainly from the condensed cytoplasm. No direct evidence was able to prove that the enhanced autofluorescent substances in the trichomes during leaf senescence are the cause of the browning at the early growth stages.

Terpenoids and Polyphenols as Natural Antioxidant Agents in Food Preservation

Synthetic antioxidant food additives, such as BHA, BHT and TBHQ, are going through a difficult time, since these products generate a negative perception in consumers. This has generated an increased pressure on food manufacturers to search for safer natural alternatives like phytochemicals (such as polyphenols, including flavonoids, and essential oils rich in terpenoids, including carotenoids). These plant bioactive compounds have antioxidant activities widely proven in in vitro tests and in diverse food matrices (meat, fish, oil and vegetables). As tons of food are wasted every year due to aesthetic reasons (lipid oxidation) and premature damage caused by inappropriate packaging, there is an urgent need for natural antioxidants capable of replacing the synthetic ones to meet consumer demands. This review summarizes industrially interesting antioxidant bioactivities associated with terpenoids and polyphenols with respect to the prevention of lipid oxidation in high fat containing foods, such as meat (rich in saturated fat), fish (rich in polyunsaturated fat), oil and vegetable products, while avoiding the generation of rancid flavors and negative visual deterioration (such as color changes due to oxidized lipids). Terpenoids (like monoterpenes and carotenoids) and polyphenols (like quercetin and other flavonoids) are important phytochemicals with a broad range of antioxidant effects. These phytochemicals are widely distributed in fruits and vegetables, including agricultural waste, and are remarkably useful in food preservation, as they show bioactivity as plant antioxidants, able to scavenge reactive oxygen and nitrogen species, such as superoxide, hydroxyl or peroxyl radicals in meat and other products, contributing to the prevention of lipid oxidation processes in food matrices.

Inhibition of L-Cysteine on the Browning of Fresh Wet Noodles

This research explored the effect of L-cysteine on the browning of fresh wet noodles (FWN). With the increasing addition of L-cysteine (0.02–0.1%), the ΔL* decreased and Δa*, Δb* increased. The L-cysteine could reduce the pH value and polyphenol oxidase (PPO) activity and increase the retention rate of polyphenol of FWN. It suggested that L-cysteine could inhibit the browning of FWN by decreasing pH value, PPO activity, and the oxidation of polyphenols. In the in vitro PPO solution, the inhibitory effect of L-cysteine on PPO activity was related to the decrease in pH and the ability of chelating Cu²⁺. According to UPLC-TOF-MS analysis, L-cysteine could reduce the generation of browning products, which suggested that L-cysteine could react with the browning intermediate product (quinone) and generate a light-colored substance (-C₉H₁₀NO₄S). L-cysteine effectively inhibited the browning of FWN and had the potential to be used in noodle industry.

A novel method for explaining the product inhibition mechanisms via molecular docking: inhibition studies for tyrosinase from Agaricus bisporus

The present study aims to investigate the substrate (4-methyl catechol and catechol) specificity and inhibition mechanisms (l-ascorbic acid, citric acid, and l-cysteine) of the tyrosinase enzyme (TYR), which is held responsible for browning in foods and hyperpigmentation in the human skin, through kinetic and molecular docking studies. During the experimental studies, the diphenolase activities of TYR were determined, following which the inhibitory effects of the inhibitors upon the diphenolase activities of TYR. The inhibition types were determined as competitively for l-ascorbic acid and citric acid and noncompetitive for l-cysteine. The kinetic results showed that the substrate specificity was better for catechol while l-cysteine showed the best inhibition profile. As for the in silico studies, they also showed that catechol had a better affinity in line with the experimental results of this study, considering the interactions of the substrates with TYR's active site residues and their distance to CuB metal ion, which is an indicator of diphenolase activity. Besides, the inhibitory mechanisms of the inhibitor molecules were explained by the molecular modeling studies, considering the binding number of the inhibitors with the active site amino acid residues of TYR, the number and length of H bonds, negative binding energy values, and their distance to CuB metal ion. Based on our results, we suggest that the novel method used in this study to explain the inhibitory mechanism of l-cysteine may provide an affordable alternative to the expensive methods available for explaining the inhibitory mechanism of TYR and those of other enzymes. HighlightsThe best affinity for the tyrosinase enzyme occurred with catechol.l-Ascorbic acid, citric acid, l-cysteine inhibited the diphenolic activity of tyrosinase.In silico studies confirmed the best affinity shown by catechol.Product inhibition mechanism of l-cysteine explained by in silico for the first time.Communicated by Ramaswamy H. Sarma.

Purification and biochemical characterization of polyphenol oxidase extracted from Kirmizi Kismis grape (Vitis vinifera L.)

The purification of the polyphenol oxidase (PPO) enzyme from Kirmizi Kismis grape (Vitis vinifera L.) was performed 61.23 times using affinity chromatography. The molecular weight of the enzyme was found to be about 38.1 kDa by SDS-PAGE as a single band. The optimum pH and temperature values were revealed to be 5.0 and 30°C, respectively, in the presence of 4-methyl catechol substrate. The thermal stability of PPO was examined and it was observed to maintain its activity at 20°C for 1 hr. K_m and V_max values were determined to be 4.8 mM and 2000,0 EU/ml for 4-methyl catechol as a substrate. IC₅₀ and K_i values and inhibition types were found for various browning agents and ascorbic acid had the strongest inhibitory impact on PPO. The inhibitory impact of Na⁺ , K⁺ , Mg²⁺ , Cu²⁺ , and Al³⁺ metal ions on the enzyme activity at final concentrations of 1 mM and 10 mM was examined. PRACTICAL APPLICATIONS: Grapes grown and processed take a significant place in our life. The grape has antioxidant, anticarcinogenic, antidiabetic and protective properties against bacteria and viruses. Furthermore, it takes an important position in the country's economy and social life due to providing raw materials to the food industry and having high export potential. Polyphenol oxidase, which is the leading actor of enzymatic browning reactions causing serious economic losses every year, was purified and characterized from Kirmizi Kismis grape (Vitis vinifera L.). This ancient grape variety has industrial processing and export potential due to its long storage life and resistance to oxidation. Therefore, the purification and biochemical characterization of polyphenol oxidase from Kirmizi Kismis grape are of great importance.

Effect of production system and inhibitory potential of aroma volatiles on polyphenol oxidase and peroxidase activity in tomatoes

BACKGROUND

Polyphenol oxidase (PPO) and peroxidase (POD) are key enzymes associated with shelf life and defense responses. Thus, the activity of PPO and POD enzymes is usually assessed to check the quality of food samples and to understand the physiological responses of plants to different stresses. However, the outcomes of PPO and POD activity assessment studies are highly dependent on assay conditions. Hence, in this study, we initially optimized PPO and POD extraction and high-throughput 96-well plates-based enzymatic activity assessment methods to evaluate the inhibitory potential of tomato volatile compounds. Later, we explored the effects of net-house and open-field growing on the PPO and POD activity in tomato fruits of eight cultivars.

RESULTS

We found 150 mM of catechol and pH 7.0 were the optimal conditions for the maximum activity for the PPO assay. Conversely, 24 mM guaiacol with 12 mM H₂ O₂ and pH 6.0 was the best condition for the POD assay. Thermal inactivation studies confirmed that tomato POD is more resistant to heat than PPO. We found that the production systems had a considerable genotype-specific impact on tomato PPO and POD activity. Moreover, amongst the volatiles that were studied, β-damascenone and d-limonene showed 50% PPO inhibition at 40 and 80 mM, respectively.

CONCLUSION

The results of this study can be used to improve the shelf-life of fresh tomato fruit and its products. The findings also underscore the significance of PPO and POD enzymes as physiological trait markers in the tomato crop and fruit quality improvement programs. © 2020 Society of Chemical Industry.

Polyphenol oxidase plays a critical role in melanin formation in the fruit skin of persimmon (Diospyros kaki cv. 'Heishi')

In this study, the melanin in persimmon and its formation were investigated. Melanin was found to be deposited on the cell walls of the upper epidermis and subepidermal cells in persimmon skin and the isolated pigment appears to have lamellar structures. Diagnostic analysis of the isolated pigment showed results that were similar to those of melanin from other sources. Ultraviolet-visible spectroscopy revealed that the extracted skin pigment displayed a broadband, structureless absorption profile that increased progressively towards shorter wavelengths. The Fourier transform infrared spectroscopy assay revealed that melanin in persimmon skin exhibits many characteristic absorption peaks. The phenolic profile analysis suggested that the precursors of this pigment may include gallic acid, procyanidin B1, procyanidin B2, ferulic acid and epigallocatechin gallate. The PPO activity and DkPPO expression significantly increased during melanin formation, and transient overexpression of DkPPO promoted melanin synthesis. These results indicate that the isolated pigment was a type of melanin and that PPO plays a critical role in its formation.

The effect of ultrasonic on reducing anti-browning minimum effective concentration of purslane extract on fresh-cut potato slices during storage

Enzymatic browning is one of the major difficulties for the preservation and commercial value of fresh-cut products. To research more healthy and inexpensive anti-browning methods, we investigated the effect of ultrasonic coupling purslane extract on the browning resistance of fresh-cut potato during 8d storage at 4 °C. Firstly, the optimal ultrasonic time (10 min) was obtained. Then, the results showed that the combined application with lower purslane extract concentration (0.02%, w/w) could achieve a better anti-browning effect than the optimal concentration of alone purslane extract (0.05%, w/w). The combined application not only significantly inhibited the key enzyme activities of polyphenol oxidase (PPO) and peroxidase (POD), but also effectively reduced the damage to cell membrane, maintained its integrity and permeability. Meanwhile, it also improved antioxidant capacity during storage. Overall, the ultrasonic cavitation combined with purslane extract would be a promising method for fresh-cut industry.

Purification, kinetic parameters, and isoforms of polyphenol oxidase from "Xushu 22" sweet potato skin

We purified and compared the polyphenol oxidase (PPO) isoenzymes present in "Xushu 22," a sweet potato. A membrane-bound form (mPPO) and two soluble forms (sPPO1 and sPPO2) were identified and purified using ammonium sulphate precipitation, ion exchange chromatography, gel filtration chromatography, and mass spectrometer. The three PPO isoforms were characterized enzymatically. The specific activity of mPPO was significantly higher than that of the two sPPO isoforms, being 24.55- and 13.89-fold higher than sPPO1 and sPPO2. The preferred substrates for mPPO and the two sPPOs were catechol and chlorogenic acid, respectively. They can be efficiently and safely applied to phenolic wastewater treatment after being immobilized. Both mPPO and the two sPPOs were rapidly inactivated under acid or base conditions and were unstable at 65°C. The most effective inhibitors of mPPO, sPPO1, and sPPO2 were glutathione, ascorbic acid, and L-cysteine, respectively. PRACTICAL APPLICATIONS: According to incomplete statistics, about 5% of sweet potatoes in china are wasted due to enzymatic browning every year. PPO was regarded as a key molecule contributing to enzymatic browning in fruits and vegetables during ripening, processing, and storage and responsible for economic and industrial loss. It's great importance to characterize the PPO from "Xushu 22" sweet potato and provide data on its inactivation. The three PPO isoforms were purified and identified by chromatography and mass spectrometer. This study will provide useful informations to have a better command of PPO from "Xushu 22" sweet potato and give ideals to solve the browning of sweet potato.

Kinetic, spectroscopic, and molecular docking studies on the inhibition of membrane-bound polyphenol oxidase from Granny Smith apples (Malus domestica Borkh.)

We investigated the inhibitory effect and binding mechanism of four selected compounds (ascorbic acid, l-cysteine, glutathione, and citric acid) on membrane-bound polyphenol oxidases (mPPO) using spectroscopic and molecular docking techniques. Kinetic analysis demonstrated that these inhibitors reversibly inhibited the mPPO activity. Fluorescence spectroscopy revealed that the intrinsic fluorescence intensity of mPPO was quenched by inhibitors with a single class of the inhibition site on mPPO. Amino acid residues His 180, His 201, His 366, Cys 184, Glu 328, and Asn 333 were the important binding sites in the active center. These sites were identified using molecular docking techniques. Our findings suggested that the inhibitors were allosterically bound to the active center of mPPO through hydrogen bonds and ion contacts. This study provides new insights into the active site residues responsible for catalyzing mPPO and provides applicable information about the design of mPPO inhibitors.

Transcriptome sequencing and identification of key callus browning-related genes from petiole callus of tree peony (Paeonia suffruticosa cv. Kao) cultured on media with three browning inhibitors

Tree peony (Paeonia suffruticosa Andrews) has ornamental, oil, and medicinal values, and demand in the markets for uniform tree peony seedlings is increasing. Micropropagation could quickly propagate uniform seedlings. However, the heavy browning phenomenon hinders large-scale development of uniform tree peony seedlings. In this paper, we measured the total phenolic compounds content, and sequenced the transcriptomes of tree peony 'Kao' petiole calluses cultured on media with three browning antagonist treatments and fresh petioles to identify the key genes involved in callus browning. Polyvinylpyrrolidone (PVP) treatment can reduce production of phenolic compounds and promote callus regeneration. A total of 218,957 unigenes were obtained from fresh petiole and three kinds of browning petiole calluses by transcriptome sequencing. The average sequence length of unigenes was 446 bp with an N50 of 493 bp. Functional annotation analysis revealed that 43,428, 45,357, 31,194, 30,019, and 21,357 unigenes were annotated using the NCBI-NR database, Swiss-Prot, KOG, GO, and KEGG, respectively. In total, 33 differentially expressed genes (DEGs) were identified as potentially associated with callus browning. Among these DEGs, 12 genes were predicted to participate in phenolic compounds biosynthesis, three genes were predicted to be involved in phenolic compounds oxidation, and six genes were predicted to participate in callus regeneration. Moreover, six transcription factors were observed to be differentially expressed in the fresh petiole and three treated petioles in tree peony. This study comprehensively identifies browning-related gene resources and will possibly help in deciphering the molecular mechanisms of callus browning of tree peony in the future.

Fluorometric and colorimetric analysis of alkaline phosphatase activity based on a nucleotide coordinated copper ion mimicking polyphenol oxidase

In this work, a fluorometric and colorimetric analysis of alkaline phosphatase (ALP) activity was developed based on nanozymes. The nanozymes were composed of nucleotides (ATP, ADP and AMP) coordinated with copper ions. All three kinds of nanozymes (ATP-Cu, ADP-Cu and AMP-Cu) exhibited polyphenol oxidase (PPO)-mimic activity by catalyzing a chromogenic reaction of 2,4-dichlorophenol (2,4-DP) and 4-aminoantipyrine (4-AP). However, there were obvious differences in the PPO-like activity and the fluorescence of the three nanozymes produced from the same concentration of nucleotides (keeping the concentration of Cu²⁺ unchanged at 5 mM). The catalytic activities of produced ADP-Cu and AMP-Cu were obviously higher than that of ATP-Cu at a certain nucleotide concentration of 3 mM. In addition, when ATP was hydrolyzed into ADP and AMP by ALP, more nanozymes were produced and the catalytic activity of the system was enhanced, which resulted in an obvious increase of the colorimetric signal. The signal intensity was proportional to ALP concentration in the range of 0-30 U L^-1, and the detection limit for ALP was 0.3 U L^-1 from the colorimetric detection. Moreover, the fluorescence intensity of the produced nanozymes was also proportional to the ALP concentration in the range of 1-30 U L^-1 and the detection limit was 0.45 U L^-1 from the fluorescence detection. A fluorometric and colorimetric sensing ALP method was thus established. The method showed a high selectivity for ALP activity compared with proteins, amino acids and other interference components. Furthermore, the proposed method was also used to detect ALP activity in human serum samples, which showed great potential for diagnostic and practical purposes.

Effect of electrolyzed water on enzyme activities of triticale malt during germination

Triticale malt can be used as a source of enzymes or as a raw material for the production of functional foods. In this study, triticale malt was produced by soaking triticale seeds either in tap water (TW) or slightly acidic electrolyzed water (SAEW) and then rinsing with TW, SAEW, or alkaline electrolyzed water (AEW). We determined the length of the hypocotyl of triticale malt and the activities of α-amylase, phytase, proteases, and lipase during 4 days of germination. The electrolyzed water (EW) treatments promoted the growth of triticale malt. On the 4th day of germination, the hypocotyl length of triticale malt soaked in TW and watered with SAEW was 24.57% longer than that of triticale malt soaked and watered with TW. The α-amylase, phytase, acidic protease, and lipase activities of triticale malt soaked in SAEW and watered with AEW were high on the 4th germination day (0.11, 1.24 × 10^-4, 0.62, and 0.51 units/mg protein, respectively). The main finding of this study is that the use of EW, especially during the soaking procedure, may be a promising way to obtain triticale malt with high enzyme activity for use in the production of functional foods.