Quality, in reference to milk, includes its composition and aesthetic factors such as flavor, odor, and appearance. Approximately 60% of variation in milk composition is caused by genetic factors, with environmental factors such as feeding, nutrition, climate, disease, processing, etc. making up the remainder of the variation. Off-flavors and/or odors may be: 1) feed induced; 2) environmentally or chemically induced; 3) indigenous milk enzyme induced; or 4) bacteria induced. Off-flavors or odors described as cowy or barny may be a result of unsanitary housing or milking conditions. Chemical or chemical-induced off-flavors may result from improper use of sanitizers, old or poorly maintained equipment, or prolonged exposure to sunlight, resulting in light-induced oxidized off-flavors. Hydrolytic rancidity or rancid flavors arise through the action of lipases, which originate from indigenous milk lipoprotein lipase or from bacterial contamination.
Sources of bacterial contamination of raw milk include the cow's udder, the exterior of the animal (e.g., bedding, soil, manure, feed residues), and the environment (e.g., milk-handling equipment and personnel, water, air).[2-4] Raw milk is pasteurized to extend its shelf life, or quality, during refrigerated storage (by minimizing numbers of spoilage microorganisms), and to ensure its microbiological safety (by eliminating pathogenic microorganisms, which caused major health problems before pasteurization was implemented). The extent to which pasteurization reduces microbial levels depends on the initial number of contaminants and the types of microorganisms. The primary spoilage organisms of raw and pasteurized milk are psychrotrophic bacteria, including species of Pseudomonas, Alcaligenes, and FlavobacteriumP31 Most of these microorganisms are heat-labile, although some may survive at low levels. Additional microorganisms may be introduced as postpasteurization contaminants. During refrigerated storage, these bacteria proliferate and produce proteinases and lipases, which affect the quality of milk. Proteolysis results in bitter, putrid, fermented, unclean, and fruity off-flavors and coagulation, while lipolysis and lipid oxidation result in hydrolytic or oxidative rancidity. Organoleptic defects become evident when microbial levels reach 106 to 107 per ml. Pasteurized milk may also be spoiled by surviving sporeformers (e.g., Bacillus and Clostridium spp.) or thermo-duric species (microorganisms that survive pasteurization, but do not grow at high temperatures).[2,3] Lactic acid bacteria (e.g., Lactococcus, Leuconostoc, Enterococcus, Pediococcus, and Streptococcus spp.) become the dominant spoilage organisms when milk is stored at high enough temperatures that allow these organisms to outgrow the psychrotrophs. Lactic acid bacterial spoilage results in sour odors, malty flavors, and ropiness. Spoiled raw milk may also exhibit ropiness due to exopolysaccharide production by Alcaligenes viscolatis.[2,3]
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