The microbiological quality of hot water-washed broccoli florets and cut green beans

AIMS: To determine the effect of hot water washing on the microbiological quality of cut broccoli florets and trimmed green beans. METHODS AND RESULTS: Broccoli florets and trimmed beans were washed for 90 s in tap water at either 20 degrees C or 52 degrees C and stored at 7 and 10 degrees C. The numbers of naturally occurring aerobic mesophilic organisms, Pseudomonas spp., Enterobacteriaceae, yeast and moulds and lactobacilli or lactic acid bacteria were enumerated at intervals for up to 2 weeks. The ability of Listeria monocytogenes, Bacillus cereus and Escherichia coli O157:H7 inoculated onto the tissue post heat treatment to survive or grow was also measured to mimic the effect of postprocess contamination. Using a hot wash treatment improved the initial appearance of the vegetables and resulted in a small, but significant, reduction in populations of all groups of endogenous flora measured. The number of yeast and moulds on the vegetables washed at 52 degrees C remained below the levels observed on the 20 degrees C washed vegetables throughout the observation period, but Pseudomonas spp., lactobacilli and Enterobacteriaceae were better able to grow on the hot-washed vegetables such that the counts at the end of storage were greater on hot-washed than ambient-washed vegetables. All three of the pathogens tested were better able to grow on hot-washed broccoli and beans than on equivalent product washed at 20 degrees C. CONCLUSIONS: Hot water washing can be used to control enzymic browning or yeast and moulds growth but it can also allow more rapid and extensive growth by pathogens and spoilage organisms. SIGNIFICANCE AND IMPACT OF THE STUDY: Reduced protection against growth by pathogens means that the hot wash treatment of vegetables should be used with caution and requires careful assessment of risk.