41-6 |
Sulfate transport and its fixation into volatile sulfur compounds by Lactococci |
S. GHOSH, Department of Nutrition & Food Sciences, Utah State University, UMC 8700, Logan, UT 84322 and B. C. Weimer, Dept. of Nutrition & Food Sciences, Utah State Univ., 8700 Old Main Hill, Logan, UT 84322-8700. A particular balance of specific volatile sulfur compounds (VSC) contribute positively to the characteristic flavor of cheese. These are generated during ripening of cheese, via metabolic activities of lactococci. The content of methionine, the precursor of VSCs, increases beyond the amount that can be derived by hydrolysis of cheese proteins. This led us to hypothesize that these additional sulfur atoms are coming from the sulfate present natively in milk. The objective of this work was: to determine the effect of environmental pH on transport of sulfate by lactococci, measure assimilation of this transported sulfate into cellular protein, and verify conversion of sulfate into VSCs. Sulfate transport was determined by measuring the residual sulfate in spent media. The assimilation into cellular proteins was studied using two-dimensional PAGE followed by auto-radiography. The VSCs produced were determined by gas-chromatography of headspace. The fixation of sulfate into VSCs was determined by scintillation counting of headspace. Lactococcal strains, transported more sulfate in acidic conditions and during logarithmic growth. The assimilation of sulfate into cellular protein was not observed using two-dimensional gel electrophoresis, despite measuring radioactivity in the cell pellet. However, gas chromatographic analysis of the headspace with L. lactis ML3 determined VSCs were present with sulfate as the sole sulfur source. Consequently, sulfate fixation was confirmed using radioactive sulfate and finding radio active VSC in the headspace. Addition of methionine to the medium during growth inhibits fixation. A portion of the genes associated with fixation in other organisms exists in the genome of L. lactis IL1403. This work generated new information about inorganic sulfur metabolism by lactococci. Since these lactococcal strains transported more sulfate than is natively present in milk, it may also be exploited in cheese industry by extraneous addition of sulfate into the milk, thus increasing VSC production during cheese ripening.
Session 41, Dairy Foods: Dairy microbiology
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