Short communication: The effect of raw milk cooling on sensory perception and shelf life of high-temperature, short-time (HTST)-pasteurized skim milk

摘要

The cooling rate of raw milk may influence sensory properties and pasteurized shelf life. Under the Pasteurized Milk Ordinance for grade A milk, raw milk may be cooled instantaneously by on-farm heat exchangers but is also acceptable if "cooled to 10℃ or less within four (4) hours of the commencement of the first milking." The objective of this study was to determine the effect of raw milk cooling on consumer perception and shelf life. Raw milk (18-21℃) was obtained and transported within 1 h of milking to North Carolina State University (Raleigh). The batch of raw milk was split in 2 portions, and a plate heat exchanger was used to quickly cool one portion to <6℃ within 1 min. The second portion was stored in a jacketed bulk tank and slowly cooled over 4 h to <10℃. Milk from 3 consecutive milkings was collected every 12 h, with subsequent milkings added to the previous collections. The bulk milk was kept below 10℃ while adding milk for the slow cool milk treatment. After 72 h, each whole milk was separated; the skim milk was pasteurized at 73 or 78℃ for 20 s, homogenized, and held at 4℃. Difference tests (n = 75) and consumer acceptance tests (n = 100) were conducted to determine if consumers could detect differences among milks. Descriptive analysis and mi-crobial testing for aerobic, psychrotrophic, and psychro-tolerant spore counts were conducted through shelf life. The entire experiment was repeated in triplicate. Raw milks averaged 3.3 log cfu/mL by aerobic plate count, <25 cfu/mL coliforms, somatic cell count of 300,000 cells/mL, and 3.15 ± 0.07% protein. Psychrotolerant spores were not found in the raw milk. Consumers could not detect differences between cooling treatments of the same pasteurization temperature or between different temperatures of the same cooling treatment. Milks reached sensory failure 49 ± 4 d on average after processing, and aerobic counts were between 5 to 7 log cfu/mL. Cooling treatment had no effect on shelf life. These results suggest that pasteurized milk quality is due to a combination of many factors. Raw milk cooling rate is not the most important factor affecting milk quality when raw milk quality is excellent.