Effect of High Temperature Treatments on Buffalo Milk and Buffalo:Bovine Milk Blends

Milk has been a big part of the human diet for centuries, conferring essential nutrients such as protein, fat, lactose, vitamins, and minerals. The concentration of these nutrients varies with milk source, and these components are further modified when milk is subjected to various processes to extend its shelf-life. These process-induced changes in milk had been widely studied in bovine milk being the most consumed milk. In Asian countries, however, buffalo milk is sometimes more abundant than bovine milk hence it is common practice to market milk as mixture of buffalo and bovine milk. Blending milk from different species could improve the quality of dairy products derived from them, yet there have been very few studies conducted on milk mixtures. Thus, this study aimed to investigate heat-induced changes in the physicochemical, structural, and functional properties of milk blends consisting various ratios of buffalo skim milk to bovine skim milk. In the first stage of this research, the impact of heat treatment (80, 85, 90 or 95°C for 5 min) on the physicochemical characteristics and protein structural properties in buffalo:bovine milk blends (0:100, 25:75, 50:50, 75:25, and 100:0) was studied. Results showed that heat treatment at ≥85°C induced significant changes in pH and viscosity, and modified the levels of non-sedimentable caseins and whey proteins in milk. The extent of these changes increased with increasing proportion of buffalo skim milk, mainly due to differences in protein and calcium concentrations. Variation in protein and calcium contents led to differences in the level of heat-induced casein dissociation, alternation in salt balance, denaturation of whey proteins and subsequent association with caseins. Based on the physicochemical and structural properties of milk blends, heat treatment temperatures of 85 and 95°C were selected for the second stage of the study. In this stage, calcium equilibria in buffalo:bovine milk blends was modified through the addition of the calcium sequestering salts (CSS) trisodium citrate (TSC) and disodium hydrogen phosphate (DSHP) before the heat treatment. Results indicated that the physicochemical properties of milk blends were markedly changed by the addition of CSS mainly as a result of dissolution of colloidal calcium phosphate due to calcium sequestration. This subsequently affected the structural and physicochemical characteristics of proteins in the milk blends mostly during heat treatment at 95°C. The extent of these heat-induced changes increased proportionally to the concentration of buffalo skim milk in the mixture. Overall, DSHP addition impacted milk with higher proportion of bovine skim milk while the addition of TSC mostly affected milk with higher proportion of buffalo skim milk. This is mainly due to the presence of high amounts of micellar calcium in buffalo milk which subsequently formed calcium-citrate complexes that are not capable of associating with dispersed caseins. The process parameters used in the second phase of the research were also adopted for the third phase to assess the impact of CSS addition and heat treatment on the rheological and textural properties of buffalo:bovine milk blends. In this stage, buffalo:bovine milk blends were acidified through the addition of 2.5% glucono-δ-lactone (GDL). Gelation time decreased with heat treatment and with increasing proportion of buffalo skim milk in the samples. This is parallel to an increasing gel strength, gel firmness, and water-holding capacity (WHC). However, when milk is subjected to heat treatment at 95°C, gelation time of milk mixtures increased but the resulting acid gels exhibited higher gel strength and gel firmness than acid gels prepared from either buffalo skim milk or bovine skim milk alone. This underscores the influence afforded by the presence of micellar whey protein-casein complex on the rheological and textural properties of acid gels. The addition of CSS increased gelation time and WHC but decreased gel strength and gel firmness, irrespective of milk ratio and heat treatment. While TSC and DSHP had similar effects on gel WHC, gelation time was lower in DSHP-added milk but the resulting acid gels exhibited higher gel strength and gel firmness than TSC-added milk. This confirms the fact that CSS-induced casein dissociation and the formation of calcium-citrate complex greatly affect physicochemical and structural properties of buffalo:bovine milk blends and this subsequently influence the rheological and textural properties of acid gels prepared therefrom. The in vitro gastrointestinal protein digestibility of buffalo:bovine milk blends was assessed in the fourth stage of the study where milk was added with TSC or DSHP before heating at 85 or 95°C for 5 min. Results showed that protein digestibility decreased with increasing proportion of buffalo skim milk. Heat treatment of milk did not have a significant effect on the digestibility of proteins but the digestibility of κ-casein was slightly reduced for CSS added samples before heat treatment. When milk was heated at 85°C, the impact of TSC and DSHP addition on κ-casein digestibility was comparable but when milk was heated at 95°C, the addition of TSC prior to heating resulted to higher κ-casein digestibility than DSHP addition did. In all milk samples, α-lactalbumin and κ-casein were the most resistant proteins against degradation by digestive enzymes. Overall, mixtures of buffalo and bovine skim milk can be prepared and subjected to heat treatment with careful control of process parameters depending on the milk ratio. This study provides important insights that can be used to obtain desirable quality characteristics in heat-treated dairy products made from buffalo:bovine milk blends.