A. Vegetative Growth
The presence and growth of bacteria in milk affects milk quality. Chemical components of milk can be degraded by bacterial metabolism and various enzymes secreted by bacteria. Products of these degradation reactions can have undesirable effects on milk structure, smell, and taste. Lactose present in milk is readily fermented by lactic acid bacteria, resulting in sour flavor notes and, if the pH of milk drops below 4.6, precipitation of casein proteins (Bylund, 1995; Jay, 2000). Fermentative metabolism of lactose by a variety of bacteria can also produce numerous volatile compounds, including acetic and butyric acids, carbon dioxide and hydrogen gas, and various alcohols that can adversely affect milk odor and flavor. Proteins are also subject to degradation by bacteria and their secreted enzymes. Digestion of proteins by extracellular proteases can create bitter-tasting peptides; cause curdling and clotting of the milk; result in production of ammonia and hydrogen sulfide; and ultimately cause gelation of the milk. Lecithinases hydrolyze lecithin molecules present in fat globule membranes, causing globule aggregation that results in flocking and lumping. Lipase, which breaks down triglycerides, creates short chain fatty acids that give milk a rancid smell and taste. Phospholipases hydrolyze phospholipids present in fat globule membranes making interior lipids more susceptible to lipase attack (Bylund 1995; Cousin, 1982). Growth of molds, yeasts, coliforms, Pseudomonas spp., Actinomyces spp., and Lactococcus lactis ssp. lactis biovar. maltigenes can give milk musty, fruity, cowlike, fishy, earthy, or malty odors, respectively.
Most microorganisms present in raw milk are destroyed by exposure to time and temperature combinations currently in use for milk pasteurization. Minimizing the time between production and pasteurization and maintaining low storage temperatures will help control enzymatic degradation of raw milk through growth of heat-sensitive organisms. However, some spores and thermoduric organisms can survive pasteurization and affect the quality of fluid milk and other processed dairy products. Thermoduric organisms, such as some species of Streptococcus and Lactobacillus, and spore-forming organisms, such as Bacillus, can multiply within pasteurized milk products resulting in off-flavors and protein and lipid degradation. Psychrotrophic spore formers present a particularly difficult challenge, as they can survive pasteurization, germinate, and multiply in refrigerated conditions under which milk is stored (Boor et al., 1998; Douglas, 2000; Ralyea, 1998).
Numerous organisms commonly found in raw milk produce degradative enzymes that remain functional following heat treatment. Once these enzymes have been secreted, they have the potential to degrade both raw and processed milk components. Furthermore, refrigeration conditions under which raw milk is stored selects for growth of psychrotrophs, many of which produce heat-stable enzymes. These psychrotrophs can grow and secrete heat-stable enzymes while milk awaits processing. Following heat treatment, these enzymes can continue to degrade milk in the absence of viable bacterial cells. A variety of psychrotrophic organisms, including P. fluorescens, P. putida, P. fragi, P. putrefaciens, Acinetobacter spp., Achromobacter spp., Flavobacterium spp., Aeromonas spp., and Serratia marcescens produce heat-stable extracellular proteases (Mottar, 1989). Many psychrotrophs, including P. fluorescens, P. fragi, P. putrefaciens, Achromobacter spp., Alcaligenes viscolactis, Acinetobacter spp., and Serratia marcescens, produce heat-stable extracellular lipases (Mottar, 1989). Among these organisms, Pseudomonas spp. are commonly isolated from raw milk, frequently comprising 50% of the psychrotrophic flora (Suhren, 1989).
Mastitis directly impacts milk quality by raising the total bacterial number of raw milk through shedding from the infected udder. An indirect effect of mastitis can also have significant implications for milk quality. Whereas healthy udders typically shed low numbers of somatic cells, mastitic udders frequently shed 106
somatic cells/mL. This increased somatic cell count (SCC) can impact the quality of fluid milk and other dairy products. Ma et al. (2000) found that high SCC pasteurized milk (849,000 cells/mL) experienced rates of lipolysis and casein hydrolysis three and two times faster than those of low SCC pasteurized milk (45,000 cells/mL), respectively. Sensory defects, such as rancid, oxidized, and fruity aroma; salty, rancid, bitter and astringent taste; and bitter and lingering aftertaste, were detected in high SCC pasteurized milk after 21 days at 5°C. Standard plate counts, coliform counts, and psychrotrophic bacterial counts remained below 100,000 cfu/mL for both high and low SCC milk, suggesting that these effects were likely to be independent of contaminating bacteria. The SCC also affects cheese making with high SCC milk resulting in reduced curd firmness, decreased cheese yield, increased fat and casein loss in the whey, and sensory defects (Munro et al., 1984; Politis and Ng-Kwai-Hang, 1988a, 1988b).
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