IMPEDANCE MEASUREMENT OF THE MICROBIAL-FLORA IN DAIRY-BASED DESSERTS

The microbial flora of refrigerated dairy-based desserts was assessed by impedance measurement with a Bactometer M123 and compared to that o btained by conventional microbiological plating analysis (aerobic meso philic organisms, yeasts and moulds, and Enterobacteriaceae and other Gram negative bacteria). Both Bactometer and conventional analysis (se mi-quantitative) were performed on the same dessert sample after a 36 h pre-incubation at 30 degrees C, to reach detectable low microbial le vels. The desserts were divided into three groups: (pH) neutral desser ts (n = 840), fresh cheeses (n = 1114) and yoghurts (n = 571). Results for aerobic mesophilic organisms in neutral desserts after 14-16 h in cubation in the Bactometer were similar to those obtained by;conventio nal plating analyses (incubation 3 days at 30 degrees C). For yeasts, moulds and Enterobacteriaceae there was good agreement between Bactome ter and conventional analyses. In yoghurts, more yeasts and moulds wer e 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.