Physicochemical Properties of Skimmed Milks, Milk Gels, and Sodium Caseinates from Bovine A1A2 and A2 Milks

Abstract

Globally, milk and the Dairy Industry is an important part of the food sector. Many studies have been conducted to investigate the behaviour of dairy products and their ingredients. The appearance of commercial A2 milk in the recent years has sparked a huge number of debates on the health benefits of A2 β-casein (A2 ꞵ-CN) compared to the regular cow’s milk containing both A1 and A2 β-CN variants. As the consumption of the A2 milk is increased due to its claimed health benefits, more research on the physicochemical properties of this milk is needed. The main aim of this thesis is to compare some physicochemical properties of A2 and A1A2 milks to determine if they are different when utilised as milk products or as dairy ingredients such as sodium caseinate. Additionally, the structural alterations of A2 and A1A2 milks under processing should be also compared in order to reveal their effects on the properties of these two milks.

In this thesis, sodium caseinates (SC) were extracted from regular A1A2 and A2 milks, and compared in terms of their physicochemical properties using different methods. Rheological analysis revealed that the viscosity of SC increased with increasing total solid concentration, especially with the concentrations > 1wt%. Moreover, an increase in the particle size of SC was observed with increasing their concentrations, likely due to the presence of large aggregate, as measured by dynamic light scattering (DLS). The analysis of SC adsorption behaviour onto the latex particles showed that the adsorbed layer of SC was around 10 nm and the protein full protein surface coverage attained at around 16.5 mg/m2. The internal structure of SC by small angle X-ray scattering (SAXS) revealed that the radius of gyration (Rg) of SC particles is around 7 nm and consist of protein inhomogeneities (~ 3 nm) within the particles. The results of SAXS and DSL are not in agreement due likely to the sensitivity of DLS to large aggregates. This study showed that there is no difference in the investigated physicochemical properties of the SCs extracted from regular A1A2 and A2 milks.

Milk gels were prepared by acidifying heated and non-heated A1A2 and A2 milks using glucono-δ-lactone (GDL) with the milk concentration ranging from 5 to 20 wt%. Both these acid milk gels and selected commercial stirred yoghurts were examined by diffusing-wave apectroscopy (DWS) coupled with multiple speckle diffusing-wave spectroscopy (MSDWS). DWS and MSDWS were both designed and developed in-house. It is found that the measurement of acid milk gels samples was extremely difficult due to the nature of the samples and the structural characteristics of the gels. It is found that the structure of the milk gels continues to evolve, which leads to the importance of time when the measurement was taken. Two dynamics were detected for the milk gels; a first fast dynamic with a relaxation time at around 2 ms due to the diffusion of the individual protein aggregates, and a second slow dynamic (>10 s) associated with the dynamic of the whole system. DWS-MSDWS was found to be very adequate to measure the stirred commercial yoghurts. Only a slow relaxation time which seems to be dependent of the product of the viscosity by the protein aggregate size is observed. Based on the experimental results, it was difficult to differentiate the dynamic behaviour of acid milk gels prepared from A1A2 and A2 milks.

In-situ study on the impact of high hydrostatic pressure (HHP) on milk was presented in the last part of this thesis. The first experiment was conducted using SAXS coupled a diamond anvil cell (DAC) to elucidate the effect of the pressure on the substructures of the casein micelles. The dissociation of casein micelles was monitored with the pressure increasing up to approximately 1 GPa. The results indicate that casein micelles, under high pressure, dissocia