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

Growth and location of bacterial colonies within dairy foods using microscopy techniques: a review

The growth, location, and distribution of bacterial colonies in dairy products are important factors for the ripening and flavor development of cheeses, yogurts, and soured creams. Starter, non-starter, spoilage, and pathogenic bacteria all become entrapped in the developing casein matrix of dairy foods. In order to visualize these bacterial colonies and the environments surrounding them, microscopy techniques are used. The use of various microscopy methods allow for the rapid detection, enumeration, and distribution of starter, non-starter and pathogenic bacteria in dairy foods. Confocal laser scanning microscopy is extensively utilized to identify bacteria location via the use of fluorescent dyes. Further study is needed in relation to the development of micro- gradients and localized ripening parameters in dairy products due to the location of bacteria at the protein-fat interface. Development in the area of bacterial discrimination using microscopy techniques and fluorescent dyes/tags is needed as the benefits of rapidly identifying spoilage/pathogenic bacteria early in product manufacture would be of huge benefit in relation to both safety and financial concerns

The effect of buttermilk or buttermilk powder addition on functionality, textural, sensory and volatile characteristics of Cheddar-style cheese

The influence of buttermilk or buttermilk powder addition to cheese milk or cheese curds respectively on cheese functional properties, free fatty acid profiles and subsequent volatile and sensory characteristics was investigated. Buttermilk addition to cheese milk resulted in a softer cheese compared to other cheeses, with a significantly reduced flowability, while buttermilk powder addition had no influence on cheese firmness but cheese flowability was also reduced compared to the control cheese. Larger pools of free fat, higher levels of free fatty acids, volatile compounds and significant differences in sensory profiles associated with off-flavour were also observed with the addition of buttermilk to cheese milk. Application of light microscopy, using toluidine blue stain, facilitated the visualisation of fat globule structure and distribution within the protein matrix. Addition of 10% buttermilk powder resulted in significant increases in volatile compounds originating from proteolysis pathways associated with roasted, green aromas. Descriptive sensory evaluation indicated few differences between the 10% buttermilk powder and the control cheese, while buttermilk cheeses scored negatively for sweaty, barnyard aromas, oxidized and off flavors, correlating with associated volatile aromas. Addition of 10% buttermilk powder to cheese curds results in cheese comparable to the control Cheddar with some variations in volatile compounds resulting in a cheese with similar structural and sensory characteristics albeit with subtle differences in overall cheese flavor. This could be manipulated to produce cheeses of desirable quality, with potential health benefits due to increased phospholipid levels in chees

Measurement of pH micro-heterogeneity in natural cheese matrices by flourescence lifetime imaging

Cheese, a product of microbial fermentation may be defined as a protein matrix entrapping fat, moisture, minerals and solutes as well as dispersed bacterial colonies. The growth and physiology of bacterial cells in these colonies may be influenced by the microenvironment around the colony, or alternatively the cells within the colony may modify the microenvironment (e.g., pH, redox potential) due to their metabolic activity. While cheese pH may be measured at macro level there remains a significant knowledge gap relating to the degree of micro-heterogeneity of pH within the cheese matrix and its relationship with microbial, enzymatic and physiochemical parameters and ultimately with cheese quality, consistency and ripening patterns. The pH of cheese samples was monitored both at macroscopic scale and at microscopic scale, using a non-destructive microscopic technique employing C-SNARF-4 and Oregon Green 488 fluorescent probes. The objectives of this work were to evaluate the suitability of these dyes for microscale pH measurements in natural cheese matrices and to enhance the sensitivity and extend the useful pH range of these probes using fluorescence lifetime imaging (FLIM). In particular, fluorescence lifetime of Oregon Green 488 proved to be sensitive probe to map pH micro heterogeneity within cheese matrices. Good agreement was observed between macroscopic scale pH measurement by FLIM and by traditional pH methods, but in addition considerable localized microheterogeneity in pH was evident within the curd matrix with pH range between 4.0 and 5.5. This technique provides significant potential to further investigate the relationship between cheese matrix physico-chemistry and bacterial metabolism during cheese manufacture and ripening