Microbial populations and foodborne pathogens control of mung bean sprouts

The two main objectives in this study were investigating the microbial quality and microbial communities of 'use-by date' Mung bean sprouts by using conventional culture and 16S/18S rDNA PCR-DGGE methods, and evaluating the efficacy of natural antimicrobial substances, chemical disinfectants, and thermal treatments in reducing and inhibiting the growth of the pathogens on mung bean seeds. Retail samples of pre-packed mung bean sprouts were obtained from three retailers in the local area. The microbial quality and communities were evaluated on the 'use-by date'. The highest counts of total aerobic counts (7.86 log10 CFU/g), yeasts and moulds (7.0 log10 CFU/g), total lactic acid bacteria (6.24 log10 CFU/g) and total coliforms (6.63 log10 CFU/g) were found in samples from one shop and the DGGE band sequences also identified major populations of LAB from the same samples, These indicated poor quality and spoilage of the samples from this location and could be related to improper storage at temperatures above 5°C. The combination of conventional culture methods with the PCR-DGGE technique revealed a larger diversity of bacterial communities than eukaryotic ones based on the relative number of amplimers present on most of the OGGE gels. Identification based on band analysis revealed that the Enterobacteriaceae (29.6%), soil bacteria (20.4%), lactic acid bacteria (18.5%). yeast (14.8%). Pseudomonas spp. (13%), and Flavobacterium (3.7%) constituted the major populations in bean sprout samples. Cluster analysis of the OGGE patterns of both 16S and 18S rDNA amplimers found no strong relationship between sample sources and batches indicating the variability of natural populations. The use of natural antimicrobial products, such as a mixture of lime juice and vinegar (1: 1, pH 2.83) and bacteriocin-like substances produced by Pediococcus acidilactici, failed to reduce and inhibit the growth of Listeria monocytogenes on mung bean seeds. The former solution had higher antimicrobial efficiency in reducing the pathogen on seeds (1.93 log10 CFU/g) compared to the Pediococcus broth culture (1.22 log10 CFU/g), but both solutions failed to inhibit the re-growth of the pathogen during the sprouting process and also reduced seed germination percentage by 13-18%. The evaluation of efficacy of sequential washing using a combination of chemical treatments (two-step dipping) against the pathogens on seeds showed that a two-step dipping treatment in a solution containing 2% sodium hypochlorite for to min followed by 5% lactic acid solution for 5 min was the most effective treatment. This treatment achieved the highest reductions of L. monocytogenes (2.91 log10 CFU/g) and Salmonella Typhimurium (3.20 l0g10 CFU/g) after treatment and continued to reduce the pathogen during the sprouting process. This may be due to the chemical residues on treated seeds which lowered both pathogens on sprouted seeds to below the limit of detection <50 CFU/g) by direct plating without significantly affecting seed viability. The use of thermal treatments based on a hot and cold water dipping was found to be more effective in reducing the normal flora on seeds and less affecting of seed germination compared to microwave heating. The use of a hot and cold water dipping treatment at 92°c for 1 min followed by ice-cold water at 5°C for 30 sec achieved the highest reduction of L. monocytogenes on seeds (>5 log10 CFU/g) but had the lowest germination percentage (89%) compared to other hot and cold water dipping treatments. Microwave heating at 1-4 kW showed a poorer efficiency in reducing nonnal flora on seeds and severely affected seed viability. Overall, a two-step washing with 2% sodium hypochlorite followed by 5% lactic acid seems to be the most successful treatment in reducing and inhibiting the recovery of the pathogen during the sprouting process. However, the chemical residues on treated seed may become a negative image to apply this treatment in the sprout industry.