Identification and characterization of some psychrotrophic heat resistant/sporeforming bacteria in the Grade A raw milk supply of Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/j9602329q

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  • Heat resistant sporeforming psychrotrophic bacteria were isolated from raw milk samples from 59 Grade A farms in Oregon. Forty-nine of the 59 (83%) raw milk samples in this survey contained sporeforming psychrotrophic bacteria; isolates from twenty-four (40%) of the samples exhibited proteolytic properties. Populations of sporeforming psychrotrophic bacteria ranged from <10 to >10,000 CFU/mL for all samples. One hundred-two isolates were identified as Bacillus species. Twelve different Bacillus species were identified with B. licheniformis being the most predominate (18% of the samples) and B. laterosporus the least frequently isolated species, (2%). Fifty-eight percent of the bacilli isolates produced a bitter off-flavor and putrid odor, while 42% produced a fruity and/or rancid off-flavor when inoculated into sterile whole milk. Based on biochemical activity tests, 83% of the thermoduric isolates hydrolysed casein while 56% were proteolytic (in litmus milk), 57% demonstrated lipolytic activity and 31% produced acid in litmus milk. Forty-eight isolates that tested positive for proteolysis were evaluated quantitatively for activity, which ranged from 0.93 to 1.93 units (expressed as mM of alanine). Isolates of Bacillus cereus var. mycoides demonstrated significantly higher (p>0.05) proteolytic activity than other Bacillus species isolated.
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