Treatment of mastitis in cows involves application of antibiotics. Milk from treated cows cannot be legally sold, but, occasionally, it becomes mixed with salable product. The resulting low-level antibiotic contamination may be sufficient to inhibit starter culture microorganisms. As antibiotic levels in milk increase, acid production decreases. Lactic acid bacteria are very sensitive to antibiotics commonly used for mastitis treatment. These include penicillin, cloxacillin, streptomycin, and tetracycline. Milk that tests negative for antibiotics, using Bacillus stearothermophilus as an indicator, can still have sufficient antibiotic to cause starter culture inhibition (Valladao and Sandine, 1994a). When antibiotics other than penicillin are present, available methods may not be sufficiently sensitive to detect residues that could cause a 20% reduction in lactic acid production (Schiffmann et al., 1992).
Sensitivity of starter cultures to antibiotics is highly strain and species dependent. S. thermophilus is more susceptible to penicillin and cloxacillin (P-lac-
tam antibiotics) than are the lactococci, but lactococci are more sensitive to streptomycin and tetracycline (Desmazeaud, 1996). Swiss (Emmenthal) cheese made with antibiotic-contaminated milk (0.005 IU/mL) exhibited abnormal eye formation, presumably from inhibition of propionibacteria (Mayra-Makinen and Migret, 1993).
Occasionally, chemical sanitizers may contaminate milk, usually as a result of human error. Chlorine- and iodine-based sanitizers lose their activity in milk and are, therefore, unlikely to cause starter culture inhibition. Quaternary ammonium compounds present more potential problems, because they maintain activity in milk, and lactic acid bacteria are sensitive to low concentrations. Valladao and Sandine (1994b) observed that all tested Lactococcus strains were inhibited by 20 |g/mL and some were inhibited by only 10|g/mL quaternary ammonium compound. Thermophilic starter cultures are inhibited at 0.5-2.0 |g/mL quaternary ammonium compound (Guirguis and Hickey, 1987a).
Peracetic acid and acid anionic sanitizers can also maintain some activity in milk (Dunsmore, 1985). Relatively high concentrations of hydrogen peroxide or quaternary ammonium compound are required to give positive results in antibiotic screening tests (Richard and Kerhave, 1975). The amount of chemical sani-tizer that might enter milk through lack of rinsing should not be sufficient to cause culture inhibition (Desmazeaud, 1996). However, problems can be encountered when sanitizer solution is not drained from tanks or trucks.
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