Graduate Thesis Or Dissertation
 

Lactic streptococci : the use of defined strains and bacteriophage-insensitive mutants in commercial manufacture of cheddar and cottage cheeses

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  • Phage-insensitive Streptococcus cremoris starter strains were selected by assaying cheese whey against potential starter strains. Six strains were selected and characterized for continual use in cheesemaking. Upon phage-infection, strains were removed from the blend. Cheesemaking continued with remaining strains. A phage-insensitive, fast-acid-producing mutant of the infected strain was isolated and characterized. This mutant, similar to the parent, was returned to the strain mixture. Multiple-blend starters were also used in cottage cheese and cultured buttermilk manufacture. Individual strains were used as antigens for a rapid detection test for lactic-streptococcal agglutinins in cheese milk. When sedimentation was encountered, agglutinin-sensitive strains were identified and replaced instead of an entire culture blend. Phage-insensitive mutants were compared to their respective parent strains. Traits examined included acid-producing activity, optimum temperature, generation time, proteolysis, phosphate and NaCl tolerance, phage adsorption, agglutination, morphology, and induction. Mutant strains showed variations in individual characteristics, but no general pattern of variation was observed. Bulk starters, prepared by growing then freezing individual strains in a commercial internal-pH-control medium (PHASE 4), were stored for 3 mo with and without glycerol. Strains varied in storage survival at -20 C. Glycerol enhanced cell viability and activity at -20 C. Storage in PHASE 4 at -40 C and -80 C preserved activity and viability without glycerol. Unfrozen PHASE 4 cultures retained original activity and viability after 1 mo refrigerated storage. Frozen and refrigerated PHASE 4 starters have been used in Cheddar and cottage cheese manufacture for more than 1 yr. Exclusive use of defined-strain cultures resulted in significant manufacturing and economic improvements including elimination of culture rotations and starter failure from phage infection, no ripening period, greater cheese uniformity, predictable starter activity, standardized manufacture, and improved cheese quality. Grade-A cheese production was increased by almost 10%. This technology enabled some factories to increase cheese yields by adding whey cream to cheese milk. The combined improvements, based on defined-strain technology, have enabled factories to increase production—some by nearly 50%. To date, more than 150 million lb of Cheddar cheese have been manufactured with defined-strain cultures.
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