Microfiltration and centrifugation as physical, nonthermal methods to remove bacterial spores from milk

The presence of bacterial endospores in raw milk can have negative effects on the safety, quality, and shelf-life of dairy products. While vegetative bacteria can be inactivated by traditional high temperature, short time (HTST) pasteurization, spores are not destroyed by this process. Instead, dairy producers can utilize nonthermal methods such as microfiltration and centrifugation to physically remove spores from raw milk. In this work, various factors impacting the efficacy of spore removal by these processing methods were investigated. Microfiltration (MF) is a membrane separation process that can remove spores from skim milk by size exclusion. In Chapter One, different pore size MF membranes were evaluated for their effectiveness in removing Bacillus licheniformis or Geobacillus sp. spores from milk. These spores have been frequently isolated from the farm, dairy processing environments, and in finished products. By decreasing membrane pore size from 1.4 µm to 1.2 µm, removal of B. licheniformis spores significantly increased, while near full removal of Geobacillus sp. spores was achieved with both membrane pore sizes. Investigation of cell size and surface properties determined that this difference in effectiveness was a function of both the spore size relative to membrane pore size, as well as differing hydrophobicity of the spores. In Chapter Two, a follow-up study was completed to determine the shelf-life of refrigerated milk produced using a combination 1.2 µm MF and HTST pasteurization process compared to HTST pasteurization or 1.2 µm MF alone. The MF-HTST process achieved the longest microbial shelf-life of at least 98 days. However, the findings demonstrated that while increased bacterial removal can result in a significantly increased microbial shelf-life, this may not translate in an equally long proteolytic shelf-life, which is limited by off-flavors due to casein degradation. MF processing is limited to skim milk due to the overlap of milk fat globule sizes with bacterial cell size, and the potential for membrane fouling by fat globules. Therefore, research on processing methods to remove spores from whole milk are of additional importance to the dairy industry. One such example is bactofugation, a specialized centrifugation process designed to decrease the microbial load of milk that is suitable for skim or whole milk. Chapter Three describes a study that was completed in a NY State dairy plant to evaluate the effectiveness of bactofugation on microbial removal from milk, and to provide a direct comparison of a fluid milk process with and without a bactofuge. In considering the growing interest of bactofugation in the dairy industry, yet the little available data on the impact of certain processing parameters, the effect of centrifugal force and temperature on spore removal was studied using a laboratory centrifuge, as described in Chapter Four. Centrifugation again decreased the concentration of bacteria in the skim portion. Bacterial concentration was observed in the cream fraction likely due to cold agglutination of fat globules by immunoglobulins present in the raw milk. Overall, the effectiveness of bacterial removal from skim milk was primarily affected by centrifugation temperature, rather than centrifugation speed. Together, this work provides useful information about the effectiveness of nonthermal processing alternatives in removing spores from milk. This in turn can lead to the production of higher quality milk, increasing the shelf-life and marketability of US dairy products