Effect of somatic cell count on proteolysis and lipolysis in pasteurized fluid milk during shelf-life storage

Abstract

The general goal of this research was to provide fluid milk processors with data to enable them to estimate the economic benefits they might derive from longer fluid milk shelf-life or new marketing opportunities due to a reduction in raw milk SCC. The study objectives were: 1) to measure the time in days for pasteurized homogenized 2% milk to achieve a level of lipolysis and proteolysis caused by native milk enzymes present in milks of different somatic cell count (SCC) at 0.5 and 6°C that would be sufficient to produce an off-flavor, 2) to determine whether milk fat content (i.e., 1, 2, and 3.25%) influences the level of proteolysis or lipolysis caused by native milk enzymes at 6°C, and 3) to determine the time in days for milks containing 2% fat with different SCC to undergo sufficient lipolysis or proteolysis to produce an off-flavor due to the combination of the action of native milk enzymes and microbial growth at 0.5 and 6°C. In experiment 1, pasteurized, homogenized milks, containing 2% fat were prepared from raw milk containing four different SCC levels from <100,000 to >1,000,000 cells/ml. Each of the four milks was stored at 0.5 and 6°C for 61 d. In experiment 2, pasteurized, homogenized milks containing 1, 2, and 3.25% fat were prepared starting from two raw milks containing two different SCC levels, one <100,000 and the other >1,000,000 cells/ml. In experiment 3, pasteurized, homogenized 2% fat milks were prepared starting from raw milks containing two different SCC levels, one <100,000 and the other >1,000,000 cells/ml. For experiments 1 and 2, all milks were preserved with potassium dichromate to prevent microbial growth but to allow the activity of native milk proteases and lipases during storage. For experiment 3, one set of milk was preserved with potassium dichromate to prevent microbial growth but to allow the activity of native milk proteases and lipases, and a second set of milk was unpreserved during storage at 0.5 and 6°C for 29 d. Based on previous work, an off-flavor due to proteolysis was detected by 50% of panelists when the decrease in casein as a percentage of true protein (CN/TP) was > 4.76%. Our data indicated (assuming 50% of consumers would detect an off-flavor when CN/TP decreases 5%) that pasteurized milk containing 2% fat would develop an off-flavor at a time long after 61 and at 54 d for the low SCC milk, and at about 54 and 19 d for the high SCC milk, at 0.5 and 6°C, respectively. Previous research reported that 34% of panelists could detect an off-flavor in milk containing 2% fat due to lipolysis at a (free fatty acid) FFA concentration of 0.25 meq/kg of milk. Based on these results, it was estimated in the present study that 34% of panelists would detect an off-flavor in a 2% fat pasteurized milk with low SCC at a time long after 61 and just after 61 d at 0.5 and 6°C, respectively, while for milk with high SCC, an off-flavor would be detected by 34% of panelists at slightly longer than 61 and 35 d at 0.5 and 6°C, respectively. The combination of low SCC milk and low storage temperature when coupled with processing technology to produce very low initial bacteria count in fluid milk could produce fluid milk that will maintain flavor quality for more than 61 d of storage at temperatures < 6°C.