Effects of freezing milk samples on the recovery of alimentary pathogens and indicator microorganisms
2007
Hubackova, M.,Vyzkumny Ustav Veterinarniho Lekarstvi, Brno (Czech Republic) | Rysanek, D.,Vyzkumny Ustav Veterinarniho Lekarstvi, Brno (Czech Republic)
The objective of the present study was to determine the effects of freezing and subsequent storage on quantitative results of bacteriologic culturing of selected food pathogens and indicator microorganisms in milk. Two model experiments were carried out. In the first experiment, raw milk samples were frozen and stored at -20 deg C for 72 hours, 7 days or 21 days. After thawing, total bacterial count and counts of coliform and psychrotrophic microorganisms were assessed. In the second experiment, milk samples were inoculated with strains of shigatoxigenic Escherichia coli, enterotoxigenic Staphylococcus aureus and Listeria monocytogenes and frozen. Freezing had a considerable adverse effect on the recovery of shigatoxigenic E. coli (P less than 0.01) after 7 days of storage. A decline in counts of coliform bacteria (P less than 0.01) and psychrotrophic microorganisms (P less than 0.01) was detected as soon as after 72 hours storage. The decline in counts of E. coli and psychrotrophic microorganisms continued with the time of storage - 72 hours and 7 days (P less than 0.05); 72 hours and 21 days (P less than 0.01). In contrast, no effect was recorded for L. monocytogenes and total bacterial count. Freezing caused a slight increase (P less than 0.05) in S. aureus counts after 72 hours and 7 days of storage. The results of this study indicate that the freezing of milk samples is unsuitable for sample storage before the assessment of hygienic quality because the potential risk of misdiagnosis may be high.
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