SPORES ISOLATED FROM MILK POWDER PRODUCTS MAY NOT BE THE SAME AS THOSE ISOLATED FROM RAW MILK

Discipline: contamination; Key words: spore populations, dairy powders, bulk tank raw milk

Milk powder is well traded internationally and usually also from and to South Africa. In terms of contamination, spore forming organisms are of concern. These should be identified if sources of contamination have to be reduced or stringent spore limits for export are to be mandated. A possible natural deduction would be that the same organisms found in fresh milk would be the culprits in milk powder. The question is whether this is indeed the case, which was the topic of investigation of Dr R.A. Miller and colleagues. They published their findings in the Journal of Dairy Science, Volume 98 of 2015, pages 8492 to 8504, using the title:Spore populations among bulk tank raw milk and dairy powders are significantly different.

The authors characterized 1523 spore isolates obtained from bulk tank raw milk from 33 farms and samples collected from four different dairy powder-processing plants. The spores isolated comprised 12 genera, at least 44 species and 216 types. Bacillus and Geobacillus represented the most commonly isolated spore genera. Whereas Bacillus licheniformis was isolated from samples collected from all plants and farms, Geobacillus species were isolated from samples from three out of four plants and just one out of 33 farms. Also, significant differences were found between the spore populations of bulk tank raw milk and those isolated from dairy powder plant samples. A comparison of spore species isolated from raw materials and finished powders showed that although certain species, such as B. licheniformis, were found in both raw and finished product samples, other species, such as Geobacillus and Anoxybacillus species, were more frequently isolated from finished powders.

In summary, the results suggest that (1) Bacillus and Geobacillus were the predominant spore contaminants in a variety of dairy powders, implying that future research efforts to reduce levels of spores in dairy powders should focus on controlling levels of spore isolates from these genera; and (2) the spore populations isolated from bulk tank raw milk and some dairy powder products were significantly different, suggesting that in-plant sources of contamination should be controlled if low spore counts in the finished product are to be achieved.