Biochemistry, physicochemical properties, and processing of human milk and comparison to bovine milk

Human milk is a valuable biofluid that provides essential nutrients and immunological protection to newborns. In the thesis, levels and properties of macronutrients in human milk, their influencing factors, and physico-chemical properties were studied, and in some cases compared to those of bovine milk. A longitudinal analysis of human milk composition showed a decrease in protein level and plasmin activities in both term and preterm human milk over six months of lactation after the due date. Infant gender and pre-pregnancy maternal body mass index (BMI) influenced total fat, energy density, and fatty acid profiles in human milk. Concerning the physical stability of human milk, this is the first study to show that creaming rates of human milk increased with creaming temperatures in the range 5 to 40°C. There was no significant impact of freezing at -20°C for 1 week on creaming rates, while pre-heating at 70°C for 10 min slowed the creaming of human milk; the results may help to improve human milk handling methods. Compared to human milk creaming, bovine milk exhibited a lag phase at the beginning of creaming, followed by a much higher creaming rate at 5°C, suggesting the clustering of bovine milk fat globules through cold agglutination, which could occur in a greater extent than in human milk. Casein stability in human milk, in relation to ethanol stability, heat stability and heat-induced changes, chymosin-/pepsin-induced coagulation, chaotropic agent induced-dissociation, and cooling-induced dissociation were also examined in this thesis. Human milk showed higher resistance to heat-induced coagulation compared with bovine milk. Proteolysis occurred during incubation at 37°C; in human milk with low plasmin activity, proteolysis still occurred due to the action of aspartyl peptidases such as cathepsin D. In separate studies, novel processing methods that could be applied to human milk as alternatives to holder pasteurization were evaluated. High-pressure treatment at 400 MPa for 10 min was shown to be a promising preservation method to compromise between maintaining nutritional quality and ensuring microbial safety. Other non-thermal treatments showed better performance in preserving lysozyme and lactoperoxidase activities compared to holder pasteurization. However, addition of nisin A to human milk could not prevent the proteolysis by bacterial proteases and addition of lactose oxidase decreased the pH of human milk during storage at 4°C for 1 week. These preserving methods required further investigation. Overall, this thesis provides valuable data on nutrients in human milk provided by Irish mothers and expands the knowledge of stability of milk from different species. This study will help improve the handling and storage methods in homes, hospitals, or human milk banks, therefore, helping infants to obtain better human milk and benefit their growth in the future