Daniloski, Davor ORCID: 0000-0001-6219-0565 (2024) Understanding the effects of β-casein phenotype on the composition and quality of milk and dairy products. PhD thesis, Victoria University.

Abstract

Bovine milk is composed of milk proteins, with two major groups, caseins and whey proteins. One type of casein, β-casein, comes predominately in two genetic forms, known as β-caseins A2 and A1. The difference between these two variants appeared due to a single nucleotide polymorphism on the sixth chromosome of the CSN2 gene (essential for coding of β-casein) with the inclusion of either proline in β-casein A2 or histidine in β-casein A1 at position 67 in the peptide chain. There has been significant attention on the health implications of consuming milk containing β-casein A1, everything from links to detrimental health issues, none of which have been fully substantiated, but nevertheless, it has created a drive in some markets towards promoting A2/A2 milk. However, the fact that milk is now segregated for certain markets based on β-casein phenotype means that from a milk functionality perspective, there may be implications on product functionality. While the aim of the study is not focused on answering the health question, it is intended to address the issue of diversifying of β-casein as a function of cow genotypes and various processing conditions commonly applied in the dairy industry. The outcomes shed new light on discovering the effects of protein phenotype and gene polymorphism on protein structure and chemical composition, and consequently on the conformational, functional, and in vitro digestion properties of milk and dairy products with known β-casein phenotype. Therefore, the aim of this study is to assist and provide knowledge to the dairy industry on the possible impacts of changing national milk pools to the β-casein A2 variant. To improve our understanding of the structural and functional properties of casein, milk, and dairy products and how they are influenced by various processing parameters, A1/A1, A1/A2 and A2/A2 samples were studied simultaneously by utilising in-situ spectroscopic techniques, such as Fourier Transform Infrared, Nuclear Magnetic Resonance and Raman spectroscopy, supported by numerous physicochemical, imaging, chromatographic techniques and analysis and in vitro digestion patterns. The developed method successfully distinguished between temperature and pH in unheated and heat-treated β-casein A2 and A1 milk groups, revealing that a minimum of 50 % of all structural variation between the milk samples could be attributed to the β-casein variant. From a technological perspective, it was found that A1/A1 and A1/A2 milks had significantly different heat coagulation properties to A2/A2 milk, which was less heat stable. Differences were also observed between β-casein variants in acid- and rennet-induced gels, as well as, their counterpart products, i.e., yoghurt and cheese, respectively. It was found that the onset of gelation was faster in A1/A1 and A1/A2 milks compared to those from A2/A2 milk. Several reasons may account for the differences in these milks processability, including greater level of polyproline II helixes, larger casein micelle size, and lower total calcium, κ-casein amounts in A2/A2 samples compared to A1/A1 and A1/A2 samples that were comprised mainly of α-helical motifs. Although the greater gel strength observed in A1/A1 and A1/A2 milks may positively affect the techno-functional characteristics of yoghurt or cheese, it may also alter the curd formation properties during the gastric phase of digestion. Interestingly, the gastric digestion of these milks and dairy products showed significant differences with faster gastric digestion occurring in A1/A1 and A1/A2 samples compared to A2/A2 milk. This work suggests that the gastric transit of dairy products carrying β-casein A1 is more rapid, compared to A2/A2 samples and may have impacts in terms of product digestibility. This study has clearly identified that milk and dairy products with β-casein A2 and A1 variants are quite different, based on their structure, functionality, and behaviour to environmental factors. Overall, the findings from this project to date, have revealed significant implications if there was an initiative to change the national dairy herds to the A2/A2 phenotype, specifically impacting dairy processors and their products. However, there are both advantages and disadvantages to using A2/A2 milk only and must be assessed on an individual basis by the dairy companies.

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