Factors Affecting Growth and Survival of Salmonella in Onion Extracts and Onion Bulbs

This study investigated the survival and growth of Salmonella in onion extracts and bulbs. Extracts of an array of onion varieties and germplasms were used to evaluate their ability to retard or inhibit Salmonella growth and the antimicrobial activity of extracts prepared from onions that were subjected to different types of light during curing. Separately, the survival of internalized and non-internalized Salmonella enterica ser. Newport was tested in red, white, and yellow onions by inoculating onion bulbs on the external and internal onion layers and by internalizing Salmonella using a syringe and needle to inject the inoculum inside the bulb. The inoculated onions were stored at room temperature and collecting samples at intervals during storage. Results showed that the varied lighting used for postharvest stimulation resulted in extracts with different antimicrobial effects (P < 0.05). Extracts from red and white onions treated with blue light inhibited Salmonella growth. Extracts from onions treated with white light revealed slow and fast growth of Salmonella in red and yellow, respectively. Of the extracts from the various germplasms, only one corresponding to red onion, inhibited Salmonella growth, whereas the rest showed varying levels of growth retardation (P 2.7, and > 2.4 log cycles on red, white, and yellow onions, respectively within 3 days of storage. In the inner layers, Salmonella grew by 2.4, 2.6, and 2.8 log cycles of red, white, and yellow onions re-spectively, over 18-d storage. In separate trials, the outer layer again did not support the survival of Salmonella Newport. While the inner scales on red onion did not allow growth of Salmonella Newport, those of white and yellow onions allowed growth during the first 3 days of storage, followed by reduction to nondetectable levels. Although the antimicrobial effect of onions is attributed mainly to the polyphenols, factors such as tissue integrity or water content in internal tissues may hinder the deleterious effects of poly-phenols against Salmonella.