Silanikove, N., Biology of Lactation Laboratory, Institute of Animal Science, A.R.O., The Volcani Center, P.O. Box 6, Bet Dagan, Israel
Merin, U., Department of Food Science, Institute of Technology and Storage of Agricultural Products, A.R.O., The Volcani Center, P.O. Box 6, Bet Dagan, Israel
Leitner, G., National Mastitis Reference Center, Kimron Veterinary Institute, P.O. Box 12, Bet Dagan, Israel

Bulk milk is the mixture of all the milked udders in a given herd. Nowadays, about 15-40% of the udders in most herds are intramammary infected by different bacteria species, mainly coagulase negative staphylococci. The presences of bacteria in the lumen of the mammary gland induce impairment of milk quality and increase the number of somatic cells. A positive relationship between % casein (casein/total protein) and curd firmness (CF) and negative relationship between lactose, or somatic cell count (SCC) and CF are associated with bacterial infection and with late lactation milk, and therefore with reduction in cheese yield and quality. On the other hand, in milk of goats with intramammary infection, the correlation between the levels of fat, protein, casein and curd yield is minor compared to milk of uninfected animals. Thus, gross milk composition is an insufficient predictor of milk quality for cheese production, since a high percent of the bulk milk originates from subclinically infected glands. Research carried out in the past few years highlighted the effectiveness of lactose as a predictor of milk quality. The correlation between lactose and CF was higher than that for % casein and SCC. Lactose concentration of ≤4% is associated with non-coagulating milk and therefore, such milk is unsuitable for making cheese, but still meets the criterion for consumption as pasteurized milk. A model that describes the simultaneous and close association between reduction in lactose concentration and milk yield on the one hand and reductions in lactose concentration and milk quality on the other hand is presented. The physiological and biochemical basis for deterioration of milk quality in subclinically infected and in late lactation animals is reviewed and suggestions to improve the quality of milk produced by farmers and acquired by dairies are presented. © 2014 Elsevier B.V.