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Inhibitors in milk

Growth and acid production by starter cultures may be inhibited by bacterial viruses, bacteriophages,  or added substances including antibiotics, sterilant and detergent residues, or  free fatty acids produced by or as a result of the growth of microorganisms, and natural often called indigenous antimicrobial proteins.

Antibiotics

Milk should not contain antibiotic residues.  Milk production in the UK is regulated by the Dairy Products (Hygiene) Regulations 1995.  These regulations include the standards for raw milk.  Prior to 1990 milk was deemed to be contaminated if an antibiotic concentration of > 0.01 international units (iu) /ml was present, the standard has now been increased to 0.006 iu/ml.  Manufacturers buying milk from producers impose stringent financial penalties on farmers producing contaminated milk and have procedures to exclude this from the food chain.  Despite legislation and financial penalties, there is evidence to suggest that residues occasionally still cause problems.  In a survey of the causes of slow acid production by cheese starters in the UK (Boyle and Mullan, 2000, unpublished results) found that, some 28 % of respondents attributed slow acid problems to antibiotics. 

Antibiotics gain entry to milk because of mastitis treatment; mastitis means inflammation of the udder.  Although this term includes all inflammatory conditions of the udder, it isdefined here as a bacterial infection of the udder.  The common causative organisms of mastitis in the UK are Str. agalactiae, Str. dysgalactiae, coagulase-negative staphylococci and Staphylococcus aureus. 

The antibiotics used in veterinary medicine belong to six major groups:-

Aminoglycosides e.g. gentamicin
Penicillins and cephalosporins (ß-Lactams) e.g. cloxacillin
Macrolides-e.g. erythromycin
Quinolones and fluroquinolones see World Health Organisation
Sulphonamides e.g. trimethoprim
Tetracyclines e.g. tetracycline

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