Factors Affecting the Survival of Salmonella in Onion Extracts and Crude Onion Juices

Abstract

Characterization of the effect of onion components effect on Salmonella and the factors which influence their antibacterial properties can be a valuable asset to the produce industry. Fruits and vegetables, often consumed raw or minimally processed, come with risk of contamination by pathogenic bacteria. Some natural compounds, which are present in large concentrations within onions, have been shown to inhibit bacterial growth and in some instances reduce their concentration. Despite the understanding of what these compounds are and how they act on bacteria, much is not understood in the way of how pre/ post harvest conditions may affect their bactericidal effects and concentrations. Here we observed the effect of extracts of Red Label (red), Dulciana (yellow), and Carta Blanca (white) onions grown within combinations of optimal/ nominal water and nitrogen conditions (high/low water and nitrogen) on the inhibition of Salmonella, and the survival of Salmonella in crude juices of these onion varieties. Only red onions showed inhibitory and bactericidal effects on Salmonella. Extracts of red onions grown under high water and high nitrogen soil conditions showed the lowest minimum inhibitory concentration (0.0808 mg/g w/w), whereas the lowest minimum bactericidal concentration was found in extracts of onions grown in high water, low nitrogen soil, 0.1170 mg/g w/w. Concentrations of phenolic compounds increased with the reduction of water and nitrogen content their interaction and their interaction played a significant role in the inhibitory effect of Red Label onion extracts. In crude onion juices all varieties managed to reduce bacterial concentrations. White onion juice had the greatest death rate, 1.70 log cycles/h, followed by red onion juice, 1.38 log cycles/h, and yellow, 0.68 log cycles/h. Despite its rapid reduction in Salmonella populations, red onion juice had a more efficient reduction over the 12 hour time period. These results suggest onions can reduce and limit Salmonella growth and these affects can be altered by their pre- harvest soil conditions.