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Tuesday, March 20, 2018 at 4:00pm to 5:00pm
Microfiltration and centrifugation as physical, nonthermal methods for spore removal from milk
Presented by Emil Griep, PhD candidate, Field of Food Science and Technology, Cornell University
The presence of bacterial spores in raw milk can cause safety, quality, and shelf life issues for dairy products. While vegetative cells can be inactivated by HTST pasteurization, bacterial spores are not destroyed by this process. In this work, microfiltration (MF) and centrifugation were explored as nonthermal methods to physically remove spores from milk. MF is a pressure-driven filtration process that can effectively remove bacteria from skim milk based on size exclusion. Milk was inoculated with 106 CFU/mL spores of Bacillus licheniformis or Geobacillus spp. and processed by cold MF using 1.4 µm (current industry standard) and 1.2 µm pore size membranes. Removal of B. licheniformis spores was significantly increased by decreasing the MF pore size, with spore reduction averaging 2.17 and 4.57 log for 1.4 µm and 1.2 µm pore sizes, respectively. Geobacillus spp. spores were fully removed by both membrane pore sizes, due to the spore size and hydrophobic interactions between spores. Microbial shelf life for milk processed with both pore size membranes, followed by HTST pasteurization (72°C for 15s), reached 90 days under refrigeration conditions. Milk produced using 1.2 µm MF & HTST had a longer enzymatic shelf-life (49 days) compared to milk produced using the 1.4 µm MF & HTST process (38 days). Although MF can successfully increase the quality and shelf-life of milk, this process can only be applied to skim milk. To remove bacteria from whole milk, bactofugation, a process that allows separation of bacteria in a centrifugal field because they have a higher density than milk, can be used. The effect of centrifugal force and temperature on bacterial removal from whole milk inoculated with 106-107 CFU/mL spores of Bacillus licheniformis or Geobacillus spp. was investigated. Centrifugation lowered the concentration of spores in the milk by 0.16 – 2.16 log, and was also very effective in removing somatic cells. A concentration of bacteria in the cream portion was observed at 4°C, but not at 50°C. Bacterial removal via bactofugation was also verified in a dairy processing facility. In the plant study, whole milk bactofuged at 62 ± 2°C had significantly lower aerobic plate count and somatic cell counts than the raw milk, and spore counts were reduced from 0.92 log to below detection level. The ability to physically remove bacterial spores from milk by nonthermal methods can have a great impact on the safety and shelf life of dairy products, with minimal effects on their quality.