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We assessed the effects of freezing and refrigeration over long periods on the microbiological quality of sheep milk. The raw milk was frozen in 1-L plastic bags or 5-L milk buckets and, after 1 mo, thawed at 7 or 25°C. We evaluated these samples immediately after thawing (d 0) and after 1 d of storage at 7°C. Furthermore, we stored fresh raw milk at 7°C for 10 d in the same packages and in a bulk milk cooler at 4°C (adding 10% of fresh raw milk daily). The total bacterial, total psychrotolerant, and proteolytic psychrotolerant counts were evaluated before and after thawing (for previously frozen milk) and daily (for refrigerated milk). The frozen-thawed milks showed no significant increase in bacterial counts immediately after thawing for all samples (<0.7 log cfu/mL), but only the samples packaged in 1-L bags and thawed at 7°C remained microbiologically adequate after 1 d of storage. Findings of the refrigerated samples were modeled using a modified Gompertz equation, obtaining a lag phase of around 0.5 (5-L bucket), 2.6 (1-L bag), and 7.0 (bulk milk cooler) d for total bacterial and total psychrotolerant counts. Maximum growth rates (µ max ) were 1.0 and 1.0 (5-L bucket), 1.2 and 1.3 (1-L bag) and 3.0 and 1.5 (bulk milk cooler) ln(cfu/mL) per day for total bacteria and total psychrotolerant counts, respectively. Compared with total bacteria and total psychrotolerant bacteria, psychrotolerant proteolytic bacteria grew slowly, reaching unacceptable counts only after 9 to 10 d of storage. The studied methods are interesting alternatives for preserving raw sheep milk on smallholdings.

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The impact of microbial growth on the physicochemical parameters of sheep milk was evaluated. The total bacterial counts (TBC) showed a lag phase of 4 h and µ-max of 0.4 log/h. After 8.4 h, the TBC reached the limit established by European Community, pH had a reduction of 0.04, acidity had an increase in 0.04%, and no changes were observed in the ethanol stability. The milk became thermally unstable after 17 h (10 9 cfu/mL), when the pH was 5.71, acidity was 0.36%, and ethanol stability was 21.3%. These results highlighted the extreme stability of sheep milk protein and that acidity may be the best physicochemical parameters to predict the quality of sheep milk.

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