100A-17 |
Development of a new rapid method for monitoring coliforms in foods |
P. M. Gray and D. H. KANG. Department of Food Science and Human Nutrition, Washington State University, PO Box 646376, Pullman, WA 99164-6376 The conventional plating method on VRBA is the most popular method for monitoring coliforms in foods. However, this method requires about 24 h incubation. Therefore, a more rapid, simple, economical method is needed to monitor coliforms in foods. The purpose of this study was to develop a simplified rapid cultivation method utilizing the gelling reaction of alginate with calcium chloride to produce a semi-solid medium, and to compare this new procedure with the conventional plating method. Modified selective medium (3.33 ml) containing 2.0 % low viscosity alginate and 1.0 ml of sample (including 90 mM CaCl2) were mixed in 16 mm test tubes yielding a semi-solid mixture. The tubes were incubated at 37C and checked for positive reaction (gas production by lactose fermentation and color changes by acid production) at regular intervals. The positive reaction times were compared with VRBA plate counts. The first experiment was performed to validate the alginate method using pure coliform cultures (Escherichia coli group, Enterobacter cloacae group, and Klebsiella pneumoniae group). The results indicated that the alginate method saved about 10-14 h incubation time compared to conventional plating methods. In the case of low coliforms numbers, such as 1.0-1.5 log10 CFU/ml, the alginate method detected their presence within 10 h. When highly contaminated samples were assayed (6.0 log10 CFU/ml), the alginate method detected the presence of coliforms within 4-5 h. In a food system experiment, pasteurized skim milk inoculated with a coliforms mixture was assayed using the alginate method, which quickly detected their presence. It was also found that inoculated skim milk stored at 4C for 2 and 7 days (to produce cold-injured cells) had reaction times similar to milk containing actively growing cultures. According to these data, the alginate method is a rapid, simple, accurate, and economical way to monitor and estimate total coliforms in milk.
Session 100A, Food Microbiology: General II
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