Date of Award

2015

Document Type

Doctoral Thesis

Degree Name

Doctor of Philosophy

Department

Biological Sciences

First Advisor

Dr. Aidan Coffey

Abstract

The work presented in this thesis characterises natural cereal-derived lactic acid bacteria and examines their potential application in the dairy fermentation area, with special emphasis on the sourdough isolate, Weissella cibaria MGl and its associated glucan homoexopolysaccharide. This strain is notable for its high-level production of dextran, being termed a “hyper-producef’. Genome sequencing and analysis of a number of IV. cibaria strains was performed in the context of available genomic data for other Weissella species, with special focus on the metabolic traits of W. cibaria MGl, and the results represent the first study to explore members of the Weissella genus at a genomic level. The genetic determinant of exopolysaccharide production in W. cibaria, a single dextransucrase gene, was confirmed, as well as identification of a large proteolytic system in strain MGl. Subsequently, this strain was employed as an adjunct culture in a Cheddar cheese model system. In addition, its associated exopolysaccharide was purified and used as bioingredient in cheeses. Both use of the W. cibaria MGl strain alone, and its exopolysaccharide as ingredient, resulted in increased moisture levels in cheeses when com^pared to controls. The survival of this strain in the cheese environment and the negligible impact on cheese proteolysis suggests its suitability as a potential adjunct culture in dairy products. To further exploit the exopolysaccharide producing potential of W. cibaria MGl, the dextransucrase gene was heterologously cloned and expressed in Lactococcus lactis NZ9000. Transfer of the exopolysaccharide-positive trait to this strain was successful, yielding dextran levels comparable to the native strain. This ''proof-of-concept’ approach demonstrates the potential for the transfer of the exopolysaccharide “hyper-producer” trait to other starter or adjunct lactic acid bacteria. This thesis also presents research on the use of another cereal-derived lactic acid bacterium, a strain of Lactobacillus with demonstrated antifungal properties, in the dairy background. Cheddar cheeses prepared with this strain as adjunct culture exhibited delayed fungal spoilage when compared to cheeses prepared with an adjunct strain not producing antifungal compounds. Ill

Access Level

info:eu-repo/semantics/openAccess

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