Rheological and syneretic properties of acid gels fortified with whey proteins

Abstract

Acid gels form the basis of most fermented products such as yoghurts, quarg. Yet the production of consistently high quality acid gel based dairy products that are free from defects (i.e. variation in viscosityhexture and in level of wheying-off) continues to be a problem for the dairy industry.On heating milk, whey protein (P-lactoglobulin, p-lg) undergoes interaction with the casein via disulphide bonding. This results in an increase in effective concentration of gel forming protein and finer acid gel network, which in turn markedly influence the rheological and syneretic properties of acid gel based dairy products. However little information exists on the influence of individual whey protein fractions on the quality of acid gel based products. The objective of this study was to compare the rheological and syneretic properties of glucono-delta-lactone induced acid milk gels stabilised using whey protein concentrate and individual whey protein fractions (a-lactalbumin, a-la and p-lg).Dried dairy ingredients: (1) skim milk powder (SMP), (2) 75% whey protein concentrate (WPC). (3) p-lg and (4) a-la were added to skim milk in sufficient quantities to yield 5% protein. Fortified milks were heated and glucono-delta-lactone was added at 4OoC and incubated until the pH decreased to 4.6. The product was then agitated at 70 rpm while cooling to 10°C, and stored at 4OC for 48 hours prior to testing. The susceptibility of the acid gels to syneresis was assessed by the level of whey expressed on cenmfugation at various g-forces. The rheological characteristics of acid gels were quantified by measuring shear stress (0) and viscosity as functions of shear rate (y) which was increased linearily from 18.2 to 116.2s-1 over 25 minutes.For all ingredients. the susceptibility to syneresis at all g forces decreased with increasing milk heat treatment prior to acidification. The susceptibility to syneresis at both heat treatments generally decreased in the following order SMP c a-la -c W C c p-lg ( Figure 1).In all gels, ts increased less than proportionately with y ( Figure 2). Thus the power law dependence of ts on y was found ts = Ky, where n c 1.0. The power law relationship was = linearised to the form log ts = log K + log ts. where K = consistency index (Pas n) and n = flow behaviour index (dimensionless). K is a measure of viscosity at shear rates approximating 1.01-1 and n is a measure of the degree of shear thinning. Increasing the milk heat treatment prior to acidification resulted in increases in K and viscosity at 116.2 sec-1 and decreases n (Figure 3).This indicates that increasing heat treatment resulted in acid gels which had higher viscosities which underwent more shear thinning. The consistency index and viscosity generally decreased in the following order S M P , WFC 75 > p-lg > a-la ( Figure 3).(1) Increasing the milk heat treatment from 72OC to 95OC reduced susceptibility to syneresis. increased in the consistency (K) and final viscosities (i.e. g = 116.2s-1); and the susceptibility to shear thinning (2) Fortifi...

Whey drainage during soft cheese manufacture and properties of drained curd as modified by casein concentration, whey protein to casein ratio, and pasteurisation of milk

“…VII). A negative effect of heat treatment on drainage has already been reported [3,6,11]. The late effect we observed was never reported in the literature, presumably because the studies of draining times generally did not exceed 200 min.…”