Essential Oil Emulsions as Postharvest Sanitizers To Mitigate Salmonella Cross-Contamination on Peppers.

Alternative postharvest sanitizers to chlorine are of increasing interest for many organic growers and consumers. An emulsion of clove bud oil (CBO; 0.2 and 0.5%) or thyme oil (0.2 and 0.5%) was evaluated as a sanitizer for produce washing against a five-serovar cocktail of Salmonella on snacking peppers and compared for antimicrobial efficacy with sodium hypochlorite (200 ppm). To further evaluate these compounds, the sanitation efficacy of an emulsion was examined after the addition of 1% organic load (OL). Emulsion treatments at 0.2 and 0.5% thyme oil and 0.5% CBO were the least effected by OL and effectively reduced cross-contamination of Salmonella on clean peppers, in many cases to below the limit of detection (1 CFU/10 g; P < 0.05). Chlorine and 0.2% CBO were rendered ineffective by the addition of OL in preventing cross-contamination and performed similarly to the water control. For surface-inoculated peppers, none of the evaluated treatments performed better than a water-only wash. The antimicrobial efficacy of the essential oil emulsions in the presence of OL indicates these emulsions may be suitable replacements for chlorine in postharvest produce wash systems.

Salmonella survival and differential expression of fatty acid biosynthesis-associated genes in a low-water-activity food.

UNLABELLED: The purpose of this study was to investigate the difference in expression of fatty acid biosynthesis genes and survival of different serotypes of Salmonella when incubated in a low-water-activity (aw ) food over a 14-day period. Stationary cells of five strains of Salmonella enterica belonging to 3 different serovars (Typhimurium ATCC 2486, Enteritidis H4267, Tennessee ARI-33, Tennessee S13952 and Tennessee K4643) were inoculated into granular sugar (aW   = 0·50) and held aerobically over a 14-day period at 25°C. Survival was determined by enumerating colonies on TSA and XLT-4 plates at 0, 1, 3, 5, 7 and 14 days. Correspondingly, gene expression was evaluated for three selected genes involved in fatty acid biosynthesis and modification (fabA, fabD and cfa). After 14 days of incubation, the population was reduced from 2·29 to 3·36 log for all five strains. Salmonella Tennessee ARI-33 and Salm. Tennessee K4643 displayed greater survival than Salm. Typhimurium and Salm. Enteritidis. The increased expression of the cfa gene (involved in cyclopropane fatty acid biosynthesis) over 14 days was found associated with strains with a lower survival rate. The fabA gene (involved in unsaturated fatty acid biosynthesis) was observed up-regulated for all strains for at least one sampling time and for Salm. Tennessee ARI-33 for all time points tested, suggesting its potential role in enhancing Salmonella survival in low aw foods. SIGNIFICANCE AND IMPACT OF THE STUDY: Numerous outbreaks of salmonellosis associated with low-water-activity foods have been reported. Therefore, the adaptive mechanisms utilized by Salmonella to survive in low-water-activity foods for prolonged periods of time need to be better understood. The results in this study showed that low-water-activity environments increase expression of gene fabA, which is involved in unsaturated fatty acid biosynthesis of Salmonella, while the increased expression of cfa, associated with cyclopropane fatty acid synthesis, was associated with decreased survival over 14 days.