Evaluation of indirect impedance for measuring microbial growth in complex food matrices.

The suitability of indirect impedance to accurately measure microbial growth in real food matrices was investigated. A variety of semi-solid and liquid food products were inoculated with Bacillus cereus, Listeria monocytogenes, Staphylococcus aureus, Lactobacillus plantarum, Pseudomonas aeruginosa, Escherichia coli, Salmonella enteriditis, Candida tropicalis or Zygosaccharomyces rouxii and CO2 production was monitored using a conductimetric (Don Whitely R.A.B.I.T.) system. The majority (80%) of food and microbe combinations produced a detectable growth signal. The linearity of conductance responses in selected food products was investigated and a good correlation (R(2) ≥ 0.84) was observed between inoculum levels and times to detection. Specific growth rate estimations from the data were sufficiently accurate for predictive modeling in some cases. This initial evaluation of the suitability of indirect impedance to generate microbial growth data in complex food matrices indicates significant potential for the technology as an alternative to plating methods.

Behavior of Enterobacter pulveris in amorphous and crystalline powder matrices treated with supercritical carbon dioxide.

The resistance of an Enterobacter pulveris strain to combined heat and supercritical carbon dioxide (scCO(2)) treatments in different powder matrices was examined. The strain proved resistant to scCO(2) treatment up to 50 MPa pressure at temperatures >73 °C for at least 20 min in a commercial infant formula. Water availability was shown to be important for the observed thermotolerance, because introduction of water in the scCO(2) gas flow during treatment resulted in a 1 log(10) cfu/g reduction of the initial inoculum. Interestingly, similar tolerance to heat and scCO(2) treatment was observed in a less complex matrix, a maltodextrin powder. In contrast, the bacterial strain proved sensitive to lower temperatures (55-65 °C) over shorter times (≤10 min) in a dextrose powder composed of crystalline particles. Therefore, the microorganism demonstrates heat sensitivity in the crystalline powder matrix closer to that of nonpowder liquid matrices. These data demonstrate the increased heat tolerance of the bacterium specifically in amorphous powders and indicate that this characteristic is not dependent on fat and other components commonly found in infant formula. The information is important in designing strategies to deal with contamination of powders with Enterobacteriacae, including pathogenic Cronobacter spp.