Spore populations among bulk tank raw milk and dairy powders are significantly different

To accommodate stringent spore limits mandated for the export of dairy powders, a more thorough understanding of the spore species present will be necessary to develop prospective strategies to identify and reduce sources (i.e., raw materials or in-plant) of contamination. We characterized 1,523 spore isolates obtained from bulk tank raw milk (n = 33 farms) and samples collected from 4 different dairy powder-processing plants producing acid whey, nonfat dry milk, sweet whey, or whey protein concentrate 80. The spores isolated comprised 12 genera, at least 44 species, and 216 rpoB allelic types. Bacillus and Geobacillus represented the most commonly isolated spore genera (approximately 68.9 and 12.1%, respectively, of all spore isolates). Whereas Bacillus licheniformis was isolated from samples collected from all plants and farms, Geobacillus spp. were isolated from samples from 3 out of 4 plants and just 1 out of 33 farms. We found significant differences between the spore population isolated from bulk tank raw milk and those isolated from dairy powder plant samples, except samples from the plant producing acid whey. 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 spp. and Anoxybacillus spp., were more frequently isolated from finished powders. Importantly, we found that 8 out of 12 genera were isolated from at least 2 different spore count methods, suggesting that some spore count methods may provide redundant information if used in parallel. Together, our results suggest that (1) Bacillus and Geobacillus are the predominant spore contaminants in a variety of dairy powders, implying that future research efforts targeted at elucidating approaches 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 are significantly different, suggesting that targeting in-plant sources of contamination may be important for achieving low spore counts in the finished product. These data provide important insight regarding the diversity of spore populations isolated from dairy powders and bulk tank raw milk, and demonstrate that several spore genera are detected by multiple spore count methods.