Date of Award:

1998

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

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Department name when degree awarded

Nutrition and Food Sciences

Advisor/Chair:

Jeffery R. Broadbent

Abstract

This study investigated the effect of cold, heat, or osmotic shock treatment on the resistance of L. lactis subsp. cremoris MM160 and MM310 and Lactococcus lactis subsp. lactis MM210 and FG2 cheese starter bacteria to freezing and freeze-drying. The ability to withstand freezing at -60°C for 24 h was variable among lactococci, but resistance to this treatment was significantly improved (P < 0.05) in most strains by a 2-h cold shock at l0°C or a 25-min heat shock at 39°C (L. lactis subsp. cremoris) or 42°C (L. lactis subsp. lactis). Stress treatments that improved lactococcal freeze resistance were also found to significantly (P < 0.05) enhance the resistance of most strains to lyophilization. Increased resistance to freezing or lyophilization was not detected when stress treatments were performed in broth that contained erythromycin, which indicated stress-inducible proteins were involved in cell protection. Membrane fatty acid analysis of stress-treated cells suggested that enhanced resistance to freezing and lyophilization may be related to heat or cold shock-induced changes in cell membrane composition. Heat-shocked cells had a higher 19:0 cyclopropane fatty acid content than did control cells, and cold-shocked cells contained a lower ratio of saturated to unsaturated fatty acids. Other factors must also be involved in cell protection, however, because similar changes in membrane composition were also detected in strains whose resistance to freezing and lyophilization was not improved by heat or cold shock.

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