COMPARATIVE IMPEDANCE EVALUATION OF THE MICROBIAL LOAD OF DIFFERENT FOODSTUFFS

The traditional microbial colony counts (total viable counts = TVC, En terobacteriaceae count) are compared with the results obtained with tw o commercially available impedance systems (Bactometer 128 and Malthus 2000). Samples, in part artificially contaminated, were a selection o f dehydrated raw materials and finished products, including leeks, mea ts, mushrooms, onions and spices as ra w materials, and fresh salads, dry soups and bouillons as finished products. The investigated impedan ce systems revealed detection limits at a cell concentration of 3 x 10 (7) CFU/mL. A microbial load of 10(6) colony forming units in leeks fo r example, is detected within 5 h in the Bactometer, and about 6 h in the Malthus 2000 system, respectively. At low microbial loads of 10(2) colony forming counts per g, the corresponding detection times were b etween 12 and 14 hours. Compared to the traditional colony count, this provides a time gain of al least 2 days, if the analysis is done over night. Inhibitory effects observed with spices, onions and leeks could partially be eliminated by adding 1 g/L lecithin and 3 g/L Tween 80 t o the diluting solvent. The selectivity of the Enterobacteriaceae medi um used in the Bactometer could be improved by using a double concentr ated medium with an addition of 50 mg/L monensin.