لخّصلي

Aerobic endospore-forming bacteria of the Bacillaceae family have been recognized as major contributors to dairy product quality issues over the past 2 decades . In spore form, these organisms are capable of surviving environmental stresses including low pH, high temperature, exposure to sanitizers, high pressure. These qualities, combined with sporeformers’ ubiquitous presence in natural environments, have led to interest in controlling their entry into the dairy product continuum, on the farm, in the transportation chain , and in the processing environment . In recent years, the presence of mesophilic and thermophilic spores in dairy powders has gained increasing attention, as specifications for these microorganisms in powders have become progressively more stringent.

Mesophilic spores have been shown to be the most prevalent sporeformer found in bulk tank raw milk . Organisms such as Bacillus licheniformis and Bacillus pumilus predominate in raw bulk tank milk and appear to originate primarily from the dairy farm environment . In contrast, thermophilic spores are more prevalent in dairy powders . Studies across the globe have consistently identified Bacillus licheniformis, Anoxybacillus sp., and Geobacillus sp. as the 3 primary sporeformers present in dairy powders . While Anoxybacillus sp. and Geobacillus sp. are considered obligate thermophiles (i.e., optimum growth temperatures of 50 to 62°C and 55 to 65°C, respectively; Pikuta, 2009; Logan et al., 2009) and are generally associated with the dairy processing environment Bacillus licheniformis is capable of growing at mesophilic temperatures as well as thermophilic temperatures and is found throughout the dairy production and processing continuum.

Strategies to reduce the prevalence and levels of spores in dairy powders include reducing their entry into raw milk and controlling their presence and growth in processing environments . The success of these approaches is evaluated based on results of spore testing in the final product (i.e., reducing spore counts). Unfortunately, this seemingly straightforward process is more complicated than it would seem due to the lack of standardization in spore testing methodologies. Although a host of testing methodologies have been devised to enumerate spores in dairy products there is little in the way of standardization when it comes to spore tests. Initial heat treatments to eliminate vegetative cells and select for spores range from 80°C to 125°C for 10 to 30 min; combined with incubation temperatures to select for mesophilic (i.e., 30–32°C) or thermophilic spores and various plating media, this leads to the potential for hundreds of unique spore test combinations. This makes national and global benchmarking and comparison nearly impossible. Additionally, although some spore tests are designed to target specific groups of sporeforming bacteria, in general, little is known regarding the effects of different spore treatments on the population of spores that will be detected.

To this end, the objective of this study was to utilize various commonly used spore enumeration methodologies to compare baseline mesophilic and thermophilic spore levels and populations in raw milk and dairy powders sourced from across the United States, and to test the specific hypotheses that (1) increasing spore counts throughout a processing run would indicate the presence of in-plant associated sporeforming bacteria (i.e., Anoxybacillus); and (2) spore testing parameters affect both the level and types of spores recovered from dairy powders. Results of this study will enable the US and global dairy industries to compare and reference spore levels in both raw milk and dairy powders, and define standard methods for enumeration of spores in dairy powder products, thereby allowing for targeted efforts to reduce spore levels in these products.