Effects of Freezing Milk Samples on the Recovery of Alimentary Pathogens and Indicator Microorganisms

Abstract

The objective of the present study was to determine the effects of freezing and subsequent storage on quantitative results of bacteriologic culturing of selected alimentary 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 °C for 72 hours, 7 days or 21 days. After thawing, the following counts of indicator microorganisms were assessed: total bacteria count and counts of coliform and psychrotrophic microorganisms. The counts of these microorganisms determined before freezing served as control. In the second experiment, milk samples were inoculated with strains of shigatoxigenic Escherichia coli, enterotoxigenic Staphylococcus aureus and bacteria Listeria monocytogenes and frozen. After storage for the above times, recovery was performed and colony-forming unit counts per millilitre were assessed. It was documented that freezing had a considerable adverse effect on the recovery of shigatoxigenic E. coli (P < 0.01) after 7 days of storage. A decline in counts of coliform bacteria (P < 0.01) and psychrotrophic microorganisms (P < 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 < 0.05); 72 hours and 21 days (P < 0.01). In contrast, no effect was recorded for L. monocytogenes and total bacterial count. Freezing caused a slight increase (P < 0.05) in S. aureus counts in milk samples 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 potential risk of misdiagnosis may be high.