| Cheddar cheese is the most commonly consumed bacterial-ripened cheese in the world. The microbiota of Cheddar cheese and the metabolites formed by this microbiota are determined, in part, by cheese composition. However, the interactions of cheese composition and microbiota remain poorly defined due to many of the variables (i.e. pH and lactate concentration) being inter-related and difficulties in reproducibly producing Cheddar cheeses with specific compositions. To overcome these difficulties, we utilized either Cheddar cheese extract or a defined media as model systems as they allow for reproducible alterations in single variables. This thesis consists of three studies related to the influence of Cheddar cheese composition on microbial growth/survival and metabolism. The first study examined the influence of variable components of cheese composition [NaCl in the moisture, lactate in the moisture, and pH] on the survival of microbial pathogens in Cheddar cheese. The results indicate that Shiga toxin-producing Escherichia coli is the pathogen of greatest concern in low-Na Cheddar varieties, and that pH is the primary microbial hurdle. The second study examined the influence of cheese composition on the growth and metabolites formed by Lactobacillus casei strains. Lb. casei strains were selected as this species is a common component of the non-starter microbiota of Cheddar cheese, and its enzymes and metabolites are known to influence Cheddar cheese flavor development. NaCl concentration was determined to influence volatile compound accumulation, while lactate and lactose concentration did not have a a significant effect. Volatile compounds associated with buttery, sulfury, nutty, and rosy flavor were produced in a strain specific manner. These results indicate that strain selection is a critical for adjuncts targeted for Cheddar varieties with alternative compositions and that the conditions employed to evaluate the strains is critical. The third study characterized growth enhancement of Lb. casei by citrate and the metabolic pathways involved. We identified that citrate utilization enhances growth of Lb. casei, but the dependence on Mg2+ and metabolites formed were strain specific. In addition, the oxaloacetate (OAA) decarboxylase was determined to be the enzyme responsible for the conversion of OAA to pyruvate by gene expression and inactivation studies. |
