Characterization of 16S Ribosomal RNA Probe-Isolated Lactococci from nature for use in Cheddar cheese manufacture Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v979v5518

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  • Strains of Lactococcus lactis ssp. lactis and Lactococcus lactis ssp. cremoris with potential use in the food industry which had been previously isolated from nature using 16S rRNA probes were characterized for unique identifying traits and for attributes important for potential use in cheese manufacture. Strains of Lc. cremoris which showed desirable characteristics for cheesemaking were selected for Cheddar cheese making trials where they were used first as single strains then in combination. Biochemical testing included lithium chloride sensitivity using Alsan medium. Strains of Lc. cremoris showed greater sensitivity to lithium chloride than Lc. lactis. strains. however sensitivity was generally indicated by reduced colony size rather than absence of growth as predicted for Lc. cremoris. strains. Strains of both Lc. lactis and Lc. cremoris produced blue colonies on Alsan medium indicating citrate utilization. Citrate utilization could not be confirmed by other traditional methods. Carbohydrate utilization using API 50 test kits revealed no pattern which could definitively differentiate strains of Lc. lactis from Lc. cremoris. It was found that all strains fermented galactose, D-glucose. D-fructose. D-mannose. N-acetyl glucosamine and lactose. Regardless of genotype. strains which were shown to be phenotypically Lc. lactis generally fermented additional carbohydrates. whereas Lc. cremoris phenotype strains did not. Only one probe-isolated strain of Lc. lactis fermented D-xylose. Tests to identify potential Cheddar cheese starter strains were performed on all environmental isolates. All strains which were phenotypically identified as Lc. lactis were rejected as potential starters because they railed to coagulate milk after 15 to 18 hours or they developed undesirable flavors in milk. Nine of twenty strains that were both phenotypically and genotypically Lc. cremoris were identified as having good potential for use as Cheddar cheese starter cultures. Four strains (MS-9, MS-23, MS-24, MS-51) were selected for Cheddar cheesemaking trials. Twenty small vats of cheese were made using each of the four strains individually then in every possible combination. In each case the experimental strains showed good functional characteristics with no sensitivity to bacteriophage present in the manufacturing environment. Five commercial scale cheese trials (53,500 lbs of milk) with automated equipment were also undertaken. Each strain was used first individually then in combination. Starter activity in the automated system using the experimental strains was sufficient to allow a 14% reduction in starter usage and still obtain good acid development. Flavor development of the cheese was monitored for one year of aging. Single strains varied in flavor development, with bitter notes being the most prominent off flavor detected. For some strains the bitter flavor was transitory and varied in intensity. Cheese produced from the four combined strains never developed the bitter defect, indicating that proteolysis of the combined strains was sufficient to degrade the bitter peptides as they were produced.
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