Graduate Thesis Or Dissertation
 

Studies to improve lactic acid bacterial starter cultures

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  • Three commercial sausage starter cultures were grown in an internal-pH-controlled medium (PHASE 4) and in a soluble phosphate-buffered medium. The cultures were tested by comparing viable counts before and after freezing at -40 C or freeze-drying and subsequent storage for 3 months in the frozen or dried state. Strains varied in storage survival. PHASE 4 preserved activity and viability of cells much better than the other medium. Cultures were tested for ability to ferment a simulated sausage mixture in comparison to commercial cell concentrates. Penicillin-resistant mutants for six strains of Streptococcus cremoris currently used in commercial Cheddar cheese manufacture were isolated after treatment with N-methyl-N-nitro-N-nitrosoguanidine. Resistance was associated with beta-lactamase production. Both parents and mutants were characterized with regard to growth, acid production, optimum temperatures, phage sensitivity and adsorption, lysogeny, beta-lactamase production, penicillin degradation, flavor, rate of death at Cheddar cheese-cooking temperature, susceptibility to other antibiotics, arginine hydrolysis, slime production, phosphate tolerance and plasmid profile. Mutants were resistant to most penicillins. This resistance was retained after several passages in absence of the antibiotic. Mutants could be added to bulk storage raw milk not only to degrade residual penicillins but also to inhibit the growth of psychrotrophs. Nisin production by four strains of Streptococcus lactis was compared in four different media. The highest concentrations were obtained with cells grown in an internal-pH-controlled medium (PHASE 4). Mutants of Streptococcus lactis strains C10, 401 and 406, which lacked the ability to produce ammonia from arginine, were obtained by ultraviolet light irradiation and selection on arginine-tetrazolium chloride agar plates. Parent and mutant strains were compared for their respective characteristics and ability to grow at 40 C. No linkage was found between the arginine hydrolysis trait and any of the plasmids. Replacing Streptococcus cremoris strains by these mutants for Cheddar cheese-making is not recommended.
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