Impedance measurement of the microbial flora in dairy-based desserts

Abstract The microbial flora of refrigerated dairy-based desserts was assessed by impedance measurement with a Bactometer M123 and compared to that obtained by conventional microbiological plating analysis (aerobic mesophilic organisms, yeasts and moulds, and Enterobacteriaceae and other Gram negative bacteria). Both Bactometer and conventional analysis (semi-quantitative) were performed on the same dessert sample after a 36 h pre-incubation at 30°C, to reach detectable low microbial levels. The desserts were divided into three groups: (pH) neutral desserts ( n = 840), fresh cheeses ( n = 1114) and yoghurts ( n = 571). Results for aerobic mesophilic organisms in neutral desserts after 14–16 h incubation in the Bactometer were similar to those obtained by conventional plating analyses (incubation 3 days at 30°C). For yeasts, moulds and Enterobacteriaceae there was good agreement between Bactometer and conventional analyses. In yoghurts, more yeasts and moulds were detected with the Bactometer than by plating procedures, whereas in cheeses the conventional plating detected more yeasts and moulds. Cut-off times of 16 h are recommended for aerobic mesophilic organisms, 15 h for Enterobacteriaceae and 22 h for yeasts and moulds, in addition to the 36 h product pre-incubation.