Interactions between cheese matrix physico-chemistry, microstructure and microbial metabolic activity for cheese diversification and quality

This research focuses on exploring the relationships and interactions between cheese manufacture parameters and subsequent variations in cheese chemical composition, starter and non-starter bacteria viability, enzyme activity, microstructure and sensory quality ripening indices over time. Knowledge gaps were identified relating to the movement and role of salt and its interaction with cook temperature on the microbial population and ripening cheese matrix; further development of analytical techniques such as flow cytometry and microscopy to increase our understanding of the various interactions which occur during the manufacture and ripening of cheese; the role of fat globules and fat globular membrane material on cheese ripening indices. Cook temperatures (40 or 50 °C) and salting methods (dry or brine) were varied to mimick conditions similar to Cheddar and Swiss-style cheese manufacture. It was observed that dry salting had a significant influence on starter viability, levels of proteolysis and chemical composition compared to brine salting, irrespective of cook temperatures of 40 or 50 °C. Brine salting creates salt gradients within the ripening cheeses. Bacteria subjected to high salt conditions in the outer layers of brine salted cheeses experience significantly higher levels of stress, resulting in altered membrane integrity, bacterial morphology and levels of reactive oxygen species and lower levels of intracellular enzyme release compared to those in inside layers. These results substantially alter our understanding of the relationship between salt concentration, cell lysis and subsequent intracellular enzyme release during cheese ripening. Addition of buttermilk powder at salting of Cheddar curds to increase phospholipid content of cheese compared more favourably to the addition of buttermilk to the cheese milk. Primary and secondary proteolysis was significantly lowered due to buttermilk powder addition, while volatile composition was influenced by the addition of 10 % buttermilk powder (w/w) and hedonic sensory characteristics were mostly comparable to the control cheese. Use of buttermilk in milk standardisation resulted in significantly elevated free fatty acid levels and the development of off- flavours in the resulting cheese. Addition of dairy powder to cheese curd has not previously been reported and this work indicates a possibility to create cheese varieties with increased phospholipid contents, potentially conferring added health benefits through buttermilk powder addition. This research has provided a new and deeper understanding of the developing cheese matrix and how entrapped bacteria interact with this matrix. It provides a greater understanding of the relationships between manufacture parameter variation and subsequent effects on cheese ripening, made possible through the application of new and advanced analytical techniques including confocal microscopy and multi- parameter flow cytometry. This research will help in the innovation of diverse cheeses for new market opportunities to achieve greater consistency in cheese quality.