Effect Of Heat Treatment And Storage Conditions On The Quality Of Cow Milk

Abstract

This study was conducted to evaluate the influence of elevated heat treatments and storage conditions on chemical and microbial quality of three cow milk samples, raw milk samples treated at 85ºC for 10-40 min., and at 98ºC for 0.5-1.87min., and commercial ultra high temperature (UHT) treated samples, produced by two dairy companies in Khartoum State, collected and examined at one month of expiry. All milk samples were examined at day 1, 10, 20, 30, of storage for determining the percentages of protein, fat, lactose, total solids, moisture content, ash content, and average values of acidity and pH. All milk samples were also evaluated for total bacterial count, lactic acid bacterial count and identification of microorganisms that survived the pasteurization process. The results suggest that pasteurized milk samples increase significantly (P<0.01) in acidity compared to UHT treated milk. No bacterial count was changed up to 10 days for all samples. The significant change in total bacterial count was obtained beyond day 20for 98ºC treated samples, while change was significant beyond day 10 for 85ºC treated samples. But no significant change was obtained in ultra high treated samples for the entire period of storage. Ash content and total solids were insignificantly affected by storage conditions, but slightly affected by heat treatments. Lactose content was significantly higher in ultra high treated milk samples than pasteurized ones. And showed significant decrease (P<0.05) in their lactose content during storage, compared to pasteurized samples. Pasteurization treatment accompanied by refrigeration storage significantly improved the keeping quality of milk and increased its shelf life. However, increased microbial load towards the end of storage period adversely affected the quality of pasteurized milk and even ultra high treated milk samples. Storage conditions significantly affected milk constituents such as protein, fat, lactose and moisture, but the least effect was obtained for 98ºC treated samples. Dominant bacteria that survived heat treatments and storage conditions were identified as: Staphylococci, Streptococci, Bacilli, Enterobacter, Micrococci, Pseudomonas, Lactobacilli and Pediococci