Date of Award:

2007

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Nutrition, Dietetics, and Food Sciences

Department name when degree awarded

Nutrition and Food Sciences

Advisor/Chair:

Jeffery R. Broadbent

Abstract

This study investigated genetic predictions for amino acid biosynthesis and catabolism by Lactobacillus helveticus CNRZ 32, a commercial cheese flavor adjunct that reduces bitterness and intensifies flavor notes. Conversion of amino acids into volatile and nonvolatile flavor compounds by L. helveticus and other lactic acid bacteria in cheese is thought to represent the rate-limiting step in the development of mature cheese flavor and aroma. One of the primary mechanisms for amino acid breakdown by these microbes involves the reversible action of enzymes involved in biosynthetic pathways, so our group investigated the genetics of amino acid biosynthesis in L. helveticus CNRZ 32. Most lactic acid bacteria are auxotrophic for several amino acids, and phenotypic characterization of L. helveticus CNRZ 32 has shown this bacterium requires 14 amino acids. Reconstruction of amino acid biosynthetic pathways from a draft-quality (incomplete) genome sequence for L. helveticus CNRZ 32 showed generally good agreement between gene content and phenotypic amino acid requirements. One exception involved the requirement ofCNRZ 32 for Asp (or Asn) for growth, where predictions derived from the genome sequence suggested this strain may be able to synthesize Asp from citrate. This prediction was confirmed as Asp auxotrophy in L. helveticus CNRZ 32 could be alleviated by the addition of citrate to a chemically defined medium that lacked Asp and Asn. Genome analysis also predicted that L. helveticus CNRZ 32 possessed ornithine decarboxylase activity, and would therefore catalyze the conversion of ornithine to putrescine, a volatile biogenic amine. Putrescine production in cheese would be undesirable because this compound may impart a rotting flesh flavor and can also have adverse effects on human health. Experiments to confirm ornithine decarboxylase activity in L. helveticus CNRZ 32 using a special growth medium, thin layer chromatography, high performance liquid chromatograph, or 13C nuclear magnetic resonance were unsuccessful, however, which indicated this bacterium does not contribute to putrescine production in cheese.

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