Effect of extended storage on microbiological quality, somatic cell count and composition of raw goat milk on farm

S.S. Zeng, S.S. Chen, B. Bah, and K. Tesfai

Dairy goat herds in the U.S. are small scale, scattered around and distant from processing facilities. It is not cost-effective to collect goat milk everyday or every other day as it is with cow milk. In some areas goat milk is collected only once a week, which is in violation of regulations specified in the Pasteurized Milk Ordinance (PMO) for Grade A milk. This study was conducted to determine the effect of extended storage time up to seven days over a lactation on composition, somatic cell count (SCC), pH and microbiological quality of goat milk in refrigerated storage tank on farm. Duplicate samples were taken daily, after the morning milking, for seven consecutive days each month of the lactation season. Samples were analyzed immediately for all variables except free fatty acids (FFA). There were no significant changes (P > 0.05) detected in milk fat, protein, lactose, solids-non-fat, SCC or pH during the extended storage period, although effects of stage of lactation (P < 0.05) were observed. Mean standard plate count (SPC) in goat milk increased to 1.8x105 CFU/ml on the 6th day of the extended storage, exceeding the Grade-A limit (i.e., 1.0x105 CFU/ml). Mean psychrotrophic bacteria count increased steadily to 1.5 x 104 CFU/ml at 6 days of storage. Mean coliform count was approximately 500 CFU/ml for the first 3 d and fewer than 2,500 CFU/ml throughout 7 days of storage. No significant changes (P > 0.05) in FFA concentrations except for butyric and caprylic acids were observed as storage of goat milk advanced. In conclusion, when stored under refrigerated and sanitary conditions, goat milk in bulk tank on the farm could meet the Grade-A limits of both SPC and SCC within 5 days of storage but would have low quality due to growth of psychrotrophic bacteria thereafter.