Effects of oregano essential oil and nisin on the shelf life of modified atmosphere packed grass carp (Ctenopharyngodon idellus)

Effects of oregano essential oil (EO) and nisin on the shelf life and microbial community of grass carp fillets packed under modified atmosphere were investigated. The shelf life of fish samples was evaluated based on sensory attributes and spoilage indicators, including color, odor, texture, overall acceptability, total volatile basic nitrogen, and biogenic amines. The addition of oregano EO, nisin, and oregano EO combined with nisin significantly extended the shelf life of fish fillets from 16 d to 20, 20, and 28 d, respectively. In addition, changes in microbial composition of fish fillets were measured by using Illumina-MiSeq high-throughput sequencing and classical microbiological analysis. Adding oregano EO and nisin significantly inhibited the microbial growth of fish fillets, especially lactic acid bacteria which were the dominant spoilage bacteria in fish fillets. While the use of oregano EO and/or nisin did not change the dominant bacteria of fish samples, they significantly lowered total viable counts. Treating fish fillets with nisin lowered the tyramine level and increased the pH value, due to the inhibitory effect on Gram-positive bacteria. The results suggest that oregano EO combined with nisin have great potential to be used as natural antimicrobials to extend the shelf life of grass carp.