Abstract #T192

# T192
The effect of raw milk cooling on sensory perception and shelf life of pasteurized skim milk.
Andy Lee*1, D. M. Barbano2, M. A. Drake1, 1Southeast Dairy Foods Research Center, North Carolina State University, Raleigh, NC, 2Cornell University, Ithaca, NY.

The cooling rate of raw milk may influence sensory properties and pasteurized shelf life. Under the Pasteurized Milk Ordinance (PMO) for grade A milk, raw milk may be cooled instantaneously by on-farm heat exchangers but is also acceptable if “cooled to 10°C 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°C) was obtained and transported within 1 h of milking to North Carolina State University. After comingling, the batch was split and a plate heat exchanger was used to quickly cool one treatment to <6°C for all milkings. The second treatment was stored in a jacketed bulk tank and slowly cooled over 4 h to <10°C. Three consecutive milkings were collected every 12 h with subsequent milkings added to the previous collections. The bulk milk was kept below 10°C while adding milk for the slow cool milk treatment. After 72 h, each whole milk was separated, pasteurized at 73 or 78°C for 20 s, homogenized, and held at 4°C. Difference tests (n = 75) and consumer acceptance tests (n = 100) were conducted to determine if consumers could detect differences among milks. Descriptive analysis (DA) and microbial testing for aerobic, psychrotrophic, and spore counts were conducted through shelf life. The entire experiment was repeated in triplicate. Raw milks averaged 4.29 Log cfu/mL by aerobic plate count, 52 cfu/mL coliforms, 300,000 SCC and 3.15 ± 0.7% protein. Spores were < 20 cfu/mL in raw milk. After processing, consumers could not detect differences (P < 0.05) between cooling treatments of the same pasteurization temperature nor between different temperatures of the same cooling treatment. Milks reached sensory failure 35–42 d after processing, and aerobic counts were between 5 and 7 log cfu/mL. Higher pasteurization temperature decreased shelf life. 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 largest effect on milk quality.

Key Words: fluid milk, cooling, quality