Ultra high temperature (UHT) stability of casein-whey protein mixtures at high protein content: Heat induced protein interactions

abhiDecember 28, 20186min1350
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Although whey proteins have high biological value, their application to UHT processed high protein neutral beverages is limited due to their low heat stability. This research aimed to exploit the chaperone activity of caseins to improve UHT stability of whey proteins. UHT stability of reconstituted milk protein concentrate (RMPC), reconstituted whey protein concentrate (RWPC) and samples with various casein to whey protein ratios (C:W) (80:20 to 40:60) was studied. A 2% protein RWPC caused severe fouling suggesting its poor UHT stability. However, 10% protein RMPC was very stable (UHT run-time > 120 min). Inclusion of caseins caused stabilization of whey proteins to UHT processing and 10% protein C:W-50:50 was successfully processed for >120 min. Further increase in whey proteins proportion in milk protein dispersions caused a drop in run-times (below 120 min) and overall heat transfer co-efficient (OHTC), corresponding with increase in particle size and apparent viscosity. Presence of higher amounts of casein in the serum phase of samples caused formation of smaller protein aggregates (D(4,3) was 0.23 and 0.16 μm for supernatants of C:W-40:60 and RMPC, respectively) after heating. These results can help to increase the whey protein content of neutral pH, UHT processed high protein ready to drink beverages.
Heat treatment is commonly applied to milk during industrial processing in order to ensure the microbial safety of dairy products as well as to extend shelf life (McKinnon and others 2009). Whey protein denaturation is one of the main effects of milk heating which causes modification of the chemical and nutritional properties of milk. Depending on the physicochemical conditions in milk, the denaturation process is either reversible, where partial unfolding of the whey proteins takes place with a loss of helical structure, or irreversible where an aggregation process occurs involving sulfhydryl (–SH)/disulfide (S–S) interchange reactions (Vasbinder and De Kruif 2003) and other intermolecular interactions, such as hydrophobic and electrostatic interactions (McMahon and others 1993; Hoffmann and vanMil 1997; Anema and Li 2000). These denaturation and aggregation reactions are of considerable interest to dairy scientists because knowledge of them is essential for devising ways to manipulate the chemical and nutritional properties of dairy products. Of practical interest is the production of heat‐stable beverages containing high levels of whey proteins for which there is an increasing demand.

In general, whey protein aggregation involves the interaction of a free –SH group with the S–S bond of cystine‐containing proteins such as β‐Lg, κ‐casein (κ‐Csn), α‐La, and BSA via –SH/S–S interchange reactions (Considine and others 2007). These protein–protein interactions lead to irreversible aggregation of proteins into protein complexes of varying molecular size depending on the heating conditions and protein composition. Knowledge of ways of inhibiting the formation of these protein complexes is needed in order to minimize the negative practical consequences that may arise. Over recent decades, studies on the mechanism of formation of these protein complexes have been conducted and various approaches have been followed in order to prevent it.

The effect of the addition of individual whey proteins to skim milk on the interaction between casein micelles and whey proteins was studied, during heat treatment at 75, 80, and 90 °C. Not only temperature and time but also concentration affected the extent of the heat-induced reations of the whey proteins with casein micelles. In general, higher concentration of α-lactalbumin (α-la) in skim milk caused an increase in the amount of this protein associated with the micelles. On the other hand, a further addition of β-lactoglobulin (β-lg) hardly affected the maximum incorporation of this protein with the casein micellar fraction. To determine the effect of a lower concentration of whey protein than that present in natural skim milk, purified α-la and β-lg were added to separated casein micelles suspended in milk permeate from ultrafiltration, and the reconstituted mixture was heated at 80 °C. In the absence of β-lg, very little α-la was found associated with the micellar pellet after heating. When comparable amounts of whey proteins were present, the interaction behaviors of α-la and β-lg with casein micelles were very similar, resulting in an α-la/β-lg ratio close to 1. In general, the ratios of α-la/β-lg associated with the heated micelles were significantly affected by the amount of protein present, either in skim milk or in the presence of resuspended micelles. In milk, at temperatures <90 °C, α-la and β-lg may interact together in the serum phase, with a ratio depending on the original protein concentration, before interacting with casein micelles; this hypothesis could explain most of the observations made in the study.

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