As the old saying goes, there is no substitute for good-quality raw milk. This is also true for milk destined for fermented milk manufacture. Quality criteria for raw milk should be based on compositional quality of milk, microbial count, somatic cell count, freedom from inhibitory substances, and reception temperature (7). In the United States, grade A milk is required for fermented milk manufacture. Such milk has standard plate count of less than 100,000 cfu/mL and storage temperature not exceeding 7°C (within 2 hr of milking). Somatic cells are an indicator of udder health, and individual states have set limits for acceptable somatic cell count (8). Generally, a somatic cell count above 750,000/mL is not acceptable. High somatic cell count is indicative of mastitis conditions; milk may also be abnormal in composition. Leukocytes in milk may cause inhibition of lactic starters by phagocytosis, and abnormal composition can influence other technological parameters affecting quality of resultant fermented milks.
Good-quality raw milk is free from all kinds of inhibitory substances, including residual antibiotics and traces of detergents, sanitizers, or insecticides. Treatment of mastitis in cows involves application of various antibiotics, and milk from treated cows may occasionally contain residual antibiotics if the milk is not withheld for the recommended period. The resulting low-level contamination may be sufficient to inhibit starter microorganisms (9); for example, penicillin at a concentration of 0.01 IU/mL of milk will inhibit thermophilic starters. Apart from the possibility of partial or complete inhibition of starter bacteria due to antibiotic action, antibiotic testing is also mandatory for consumer safety reasons due to potential allergic reaction by sensitive individuals (10). High levels of residual detergents and sanitizers can inhibit starter bacteria. Under Good Sanitation Practices, the amount of residual sanitizers that might enter into milk from milk contact surfaces is not sufficient to cause culture inhibition except when sanitizer solution is not completely drained from tanks or trucks (11). Quarternary ammonium compounds (QAC) present a potential problem due to their residual activity, and lactic acid bacteria are sensitive to low concentrations of QAC. For example, Lactococcus strains are inhibited at 10 ag/mL level of QAC (12) and thermophilic cultures are inhibited at 0.5 to 2 ag/mL of QAC (13).
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