Spore Removal by Centrifugation and Its Effect on Ultra-High Temperature Commercial Sterilization of Milk

Abstract

The removal of spores of Bacillus subtilis, Bacillus cereus, and Bacillus stearothermophilus from milk by continuous centrifugation at 71, 82 C or both and flow rates of ∼1,800 or ∼5,400 kg per hour was studied. The effect of spore reduction on spoilage after various ultra-high temperature treatments also was investigated. With a centrifugal force at 9,000 × g, spore reduction was approximately 98 to >99% for one centrifugation at slow milk-flow rate, or two centrifugations at the faster rate. Milk solids (sludge) losses were approximately three times higher when centrifuging at 1,800 compared to 5,400 kg per hour. A third centrifugation at 1,800 or 5,400 kg per hour had only a small effect on further reduction of spores. Species of spores did not significantly affect the final count with two centrifugations. The initial spore content of the milk resulted in only a small difference on percentage removal with two centrifugations but the total number remaining was higher with the larger initial count. Comparative centrifugations at 82 C for removing spores showed no significant difference from 71 C. If the milk contained a high spore content of 10,000 per milliliter or more, centrifugation removed 98 to >99% of the spores allowing a small reduction in temperature, or ensuring a slightly reduced spoilage rate compared to the milk that was not centrifuged. © 1971, American Dairy Science Association. All rights reserved.