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In this study, the interaction between lotus rhizome polyphenol oxidase (PPO) and ascorbic acid (AA) was discussed from the aspects of inhibitory activity, thermodynamics, and conformation. Results showed that PPO was purified from lotus rhizome by DEAE‐52 anion exchange chromatography and Sephadex G‐100 gel filtration chromatography, with its optimum substrate being determined as pyrogallic acid. Spectrophotometric and polarographic assays demonstrated that AA exhibited strong inhibitory activity against PPO. Thermodynamics, fluorescence, and circular dichroism spectral analysis showed that hydrophobic interactions caused the formation of AA–PPO complex, leading to the remarkable fluorescence quenching and conformational change of PPO. Atomic force microscopic analysis revealed that binding to AA induced significant changes in the surface morphology and molecular aggregation of PPO molecules. In this study, the interaction mechanism between PPO and AA was proposed for the first time, which provided a theoretical basis for AA to inhibit lotus rhizome browning. Practical applications: Lotus rhizome, an aquatic vegetable, is prone to enzymatic browning in processing operations, which leads to a decrease in market value and economic loss. At present, ascorbic acid (AA) is widely used in industries as an excellent antioxidant because of its good antibrowning effect and relatively low cost. However, the interaction between the enzymatic browning‐related polyphenol oxidase (PPO) from lotus rhizome and ascorbic acid has not been clearly studied. Understanding the mechanism of inhibiting PPO will help to prevent vegetable browning, especially fresh‐cut products. The inhibitory effect of AA on PPO in lotus rhizome favors simultaneous use with other types of PPO inhibitors because of their likely synergistic effects. [ABSTRACT FROM AUTHOR]

Tyrosinase (polyphenol oxidase) is the key enzyme of enzymatic browning in fruits and vegetables. In this research, the impact of ascorbic acid on tyrosinase and its anti‐browning effect on fresh‐cut Fuji apple were investigated. Ascorbic acid had a dual effect on tyrosinase with a half inhibitory concentration (IC50) of 13.40 ± 0.05 µM. Fluorescence assay demonstrated that ascorbic acid interacted with tyrosinase in a dynamic contaction caused by Förster's resonance energy transfer (FRET) and induced a conformational change of the enzyme. Thermodynamic analysis, copper interaction, and molecular docking further confirmed that ascorbic acid could chelate the copper ions located in active center and interact with amino acid residues of tyrosinase via hydrophobic interaction. In addition, ascorbic acid prevented the browning of fresh‐cut apples by increasing APX activity and inhibiting PPO and POD activities which reduce the oxidation of total phenolics and flavonoids. Practical applications: The present study demonstrated that ascorbic acid had a strong inhibitory activity against tyrosinase (IC50 = 13.40 ± 0.05 µM) and anti‐browning activity against fresh‐cut Fuji apple. It could delay the browning degree of apple juice, increase APX activity, inhibit PPO and POD activities, and reduce the oxidation of total phenolics and flavonoids. These findings provided a basis for the feasible application of ascorbic acid on the preservation of fruits. [ABSTRACT FROM AUTHOR]

The aim of this study was to compare the effects of sodium metabisulphite and the thiol compounds, glutathione (GSH), L‐cysteine (CYS), and N‐acetylcysteine (NAC), on the enzymatic browning, antioxidant activities, total phenolic, and ascorbic acid content of potatoes after 1, 24, and 48 hr. Three different concentrations (0.5%, 1.0%, and 2.0%) of each thiol compound were tested. While sulphite solution inhibited polyphenol oxidase as expected, NAC and CYS also decreased its activity. CYS‐treated samples exhibited the highest residual thiol content, while the amount of residual thiol in GSH‐treated samples was the lowest. The 2.0% NAC and 2.0% CYS solutions were the most effective at increasing antioxidant activity and ascorbic acid content; however, the results of total phenolic content assays were complicated. In summary, solutions containing 2.0% NAC, 1.0% CYS, and 2.0% CYS prevented enzymatic browning and increased the residual thiol content, ascorbic acid, and antioxidant activities of fresh‐cut potatoes significantly, but GSH did not significantly inhibit browning. Practical applications: Fresh‐cut potatoes are susceptible to enzymatic browning, which significantly reduces their commercial value. In literature, there have been several methods to protect the enzymatic browning of fruits and vegetables. Among these methods, thiols are good inhibitors of enzymatic browning. So, GSH, CYS, and NAC were used in this study. The outcomes of current work may help to inhibit polyphenol oxidase activity and increase the ascorbic acid content, residual thiol content, and antioxidant activity of fresh‐cut potatoes. Both CYS and NAC may be useful alternatives to sulphite anti‐browning agents, which may have adverse health effects. [ABSTRACT FROM AUTHOR]

The effect of ascorbic acid [AA (1%)] and Aloe vera gel [AVG (50%)] coating alone and in combination was investigated on enzymatic browning and quality of lotus root slices during storage at 20 ± 1°C. The combined application of AA and AVG coating delayed surface browning, reduced increase in relative electrolyte leakage (REL) and showed higher overall visual quality (OVQ). Similarly, AA and AVG combined treatment reduced superoxide anion (O2-·) and hydrogen peroxide (H2O2) production and malondialdehyde (MDA) content, and suppressed peroxidase (POD) and polyphenol oxidase (PPO) activities. In addition, AA and AVG treatment conserved higher AA content, ascorbate peroxidase (APX), superoxide dismutase (SOD) and catalase (CAT) enzymes activities along with higher total phenolics and radical scavenging activity. In conclusion, the combined application of AA and AVG coating could be an appropriate treatment to delay surface browning and quality loss of lotus root slices. Practical applications: Lotus root is an aquatic rhizome vegetable. The fresh‐cut slices of lotus roots are prone to post‐cut enzymatic browning and quality deterioration during postharvest storage. Browning induced loss of visual quality and microbial infestations are the leading constraints in extending storage and/or shelf life of lotus root slices. Surface browning results in loss of characteristic color eventually leading to significant reduction in market potential and visual quality. However, quality deterioration and development of browning could be delayed with some suitable postharvest treatments. So, the effect of AA and Aloe vera gel based coating was investigated for quality conservation of lotus root slices. The findings of the current work are of global importance in reducing browning and conserving visual quality of lotus root slices in particular and fresh‐cut produce in general. [ABSTRACT FROM AUTHOR]

The pineapples were dipped in salicylic acid (SA) (2.5 or 5.0 mM) or methyl jasmonate (MeJA) (0.01 or 0.1 mM) for 1, 2, or 3 hr and then held at 13 ± 1°C for 10 days. The parameters were determined after left the fruits at 25°C for 2 days. It was found that, in the tissue adjacent to the core, 5.0 mM SA for 2 hr and 0.01 mM MeJA for 3 hr alleviated chilling injury (CI) and maintained color than other treatments. The MeJA immersions lowered CI score, electrolyte leakage, and malondialdehyde content greater than other treatments. SA and MeJA immersions retarded polyphenol oxidase activity, total phenols, and induced antioxidant activity during storage. In the pulp tissue, SA treatment increased ascorbic acid and total sugar contents, and MeJA immersions enhanced bioactive compound and antioxidant enzyme activities than SA immersion. Therefore, MeJA immersions could alleviate CI and enhance antioxidant better than other treatments of pineapples during cold storage. Practical applications: Postharvest immersion at the optimum concentration of SA or MeJA can be applied for commercial scale in order to alleviate chilling of "Sawi" pineapple fruit during storage. MeJA treatment enhances antioxidant activity and reduces internal browning better than SA treatment in pineapple. [ABSTRACT FROM AUTHOR]