BACKGROUND: Polyphenol oxidase (PPO) is considered to have a key role in the food industry because it initiates enzymatic browning in the processing and storage of fruit and vegetables. Increasing numbers of benzoic and cinnamic acid derivatives have been found to be efficient inhibitors of polyphenol oxidase, but a comparison study on activity and action mechanism is lacking. In this study, 18 benzoic acid and cinnamic acid hydroxy derivatives were selected and investigated. RESULTS: Three substrates, four activators and 11 inhibitors were identified from benzoic and cinnamic acid derivatives. 2,4-Dihydroxy-cinnamic acid and benzoic acid showed the strongest inhibitory effect on PPO, with IC 50 of 0.092 and1.425 mmol L −1 , respectively. Benzoic acid reversibly inhibited PPO in a competitive manner, while 2,4-dihydroxycinnamic acid showed a mixed-type inhibition. Both of them showed that static-type fluorescence quenching and electrostatic interaction were the main driving force in the bonding process. Compared with benzoic acid, 2,4-dihydroxycinnamic acid more easily formed hydrogen bonds in the active site of PPO, making the interaction more stable. CONCLUSION: Comparative analysis showed that the inhibition effect of cinnamic acid hydroxyl derivatives on PPO was stronger than that of benzoic acid derivatives. Benzoic acid and 2,4-dihydroxycinnamic acid were the strongest inhibitors. PPO inhibitors identified from benzoic and cinnamic acid derivatives are expected to be promising inhibitors for controlling fruit and vegetable browning.
Galangal essential oil is obtained from the rhizomes of galangal with proven anti-inflammatory, antioxidant, antiviral, and antimicrobial properties, which are valuable in the food industry. To explore the effect of galangal essential oil on the quality of pineapple juice, 0.05, 0.1, 0.2, and 0.4% galangal essential emulsion were added, and their influence on the physical stability, physicochemical properties, microbial quantity, and aroma profiles of cloudy pineapple juice were evaluated. The essential oil emulsion of galangal is a milky white liquid with a strong aroma of galangal. The pH values of emulsion increased from 4.35 to 5.05 with the increase in essential oil concentration, and there was no significant difference in the particle size of the pineapple juice. The results showed that the galangal essential oil emulsion was stable and the stability of the cloudy pineapple juice was significantly enhanced by the essential oil emulsion determined using LUMiSizer. The cloudy pineapple juice with a 0.2% essential oil emulsion showed the most stability during storage. The lightness of the cloudy pineapple juice increased instantly with the essential oil emulsion addition. In addition, the microbial quantity of the cloudy pineapple juice was decreased by the individual essential oil emulsion or combined with thermal treatment to hold a longer shelf life. The microbial counts in pineapple juice treated by 0.4% essential oil emulsion and thermal treatment only increased from 1.06 to 1.59 log CFU/ml after 4 days of storage at 25°C. Additionally, the pH and total soluble solids showed a slightly increasing trend; however, the value of titratable acidity, free radical scavenging capacity, and ascorbic acid content of the cloudy pineapple juice showed no significant change. Finally, the results of the electronic nose showed that the aroma components of the pineapple juice were changed by the essential oil emulsion and thermal treatment, and the difference was especially evident in the content of the sulfur, sulfur organic, and aromatics compounds. Consequently, the results indicated that galangal essential oil emulsion can be used as juice additives to improve the quality attributes and extend the shelf-life of cloudy pineapple juice.
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