Effect of processing on the composition, structure and digestibility of cow’s milk protein

Milk provides essential nutrients and serves as an energy source in our diet. The quality of protein in milk is closely linked to its nutritive value, which reflects its ability to support key metabolic functions essential for optimal health. Important aspects of dietary protein quality include amino acid composition, protein digestibility, and amino acid bioavailability. Before consumption, commercial milk undergoes various processing methods for safety and improving shelf-life, which can significantly alter its nutritional protein value. Processing techniques may impact the nutritional quality of milk proteins, either positively or negatively. Given the necessity of processing, it is essential to manage these methods carefully to preserve the protein functionality and nutritional value of milk, especially as it is a core component of many healthy and functional foods and beverages. Microfiltration of fresh milk is a relatively new commercial process designed to extend its shelf life in combination with pasteurisation. The aim of this study is to expand our understanding of the effects of microfiltration on milk proteins. This work could facilitate research and development of food products containing milk protein as a crucial nutritional component, as well as improve processing methods to optimize milk protein attributes. This thesis specifically addresses changes in protein structure, digestibility, and peptide profiles in commercially available filtered milk in the UK market, in comparison to pasteurized milk. By 2024, the production of filtered semi-skimmed milk had increased by about 25 %, representing 83 % of the brands available in the UK market, according to the data collected in this study. Meanwhile, filtered whole milk saw a 58 % increase, now comprising approximately 66 % of brands that sell fresh milk, as observed in this research. The combination of particle size measurements, thiol content analysis, and Confocal Laser Scanning Microscopy (CLSM) provided a comprehensive understanding of how microfiltration influences milk's structural properties in comparison to pasteurisation. A significant increase in Z-average particle size (average ~+12 %) and reduction in thiol content (average ~-24 %) indicate that filtration promotes the formation of larger aggregates, potentially through thiol-disulphide exchange interactions. CLSM imaging further revealed enhanced protein-fat interactions in filtered milk, suggesting a strengthened association between milk proteins and fat globule membrane proteins. Protein digestibility and peptides released after in vitro digestion were analysed by examining the static in vitro gastrointestinal digested milk samples using high-resolution quadrupole time of flight instruments (TOF LC/MS). Although no significant differences were observed in the in vitro gastrointestinal digestion between filtered and pasteurised milk, notable changes in peptide distribution were identified. These variations suggest that the filtration process may influence the types or quantities of peptides released. First, β-casomorphin 7 (BCM7), is an opioid peptide released during the digestion of β-casein, which has been associated with various health concerns, was measured in filtered and pasteurised samples following the in vitro digestion. Interestingly, the presence of fat appears to limit BCM7 release and has a greater impact on BCM7 release than microfiltration alone, suggesting that fat content may play a more prominent role in moderating bioactive peptide release. Microfiltration appears to influence BCM7 release more significantly when fat is present. In semi-skimmed filtered milk, quantification of BCM7 revealed no significant difference in levels compared to pasteurised milk, though correlations between fat content, processing methods, and protein digestion percentages highlighted distinct impacts of microfiltration. Comparing the percentage increase in BCM7 release between filtered and pasteurized milk of the same fat content (indicating the effect of processing) and between semi-skimmed and whole milk under the same process (indicating the effect of fat content) provides clearer insight into how these factors individually affect BCM7 production. This finding warrants further investigation to understand the mechanisms behind these differences in protein structure and peptide distribution and their potential implications for milk's bioactivity and nutritional properties.