Levels of phenolic compounds, antioxidant capacity, and microbial counts of fresh-cut onions after treatment with a combination of nisin and citric acid

Abstract

Nisin and citric acid are generally recognized as safe (GRAS) for using as a food ingredient and often used to control the microbial contamination of fresh-cut fruits and vegetables. However, their effects on phenolics and antioxidants remain unclear. Here we used 50 μg·mL-1 nisin combination with 1% (w/v) citric acid (N+C) to wash fresh-cut onions and the phenolic compounds, antioxidant capacity, microbial load, slice color were evaluated during the storage period. N+C treatment decreased the total polyphenols (TP) and quercetin contents (p < 0.05), as compared to the control at day 0, but did not affect phenolic acids and antioxidant capacity (ABTS, DPPH, and FRAP assays). During storage, antioxidant capacity, TP, and flavonoid contents increased. Storage had mixed effects on the phenolic acids (caffeic acid, vanillic acid, and ferulic acid increased, chlorogenic acid remained fairly stable). N+C treatment reduced the microbial counts and total viable counts (TVC) of fresh-cut onions below the detection limit until 5 days of storage. After 15 days storage, TVC of N+C treated samples remained significantly (p < 0.05) lower than controls. In addition, N+C treatment led to better color retention during storage. Therefore, N+C might be used as a safe preservative for fresh-cut onions.