Rheological properties at small (dynamic) and large (yield) deformations of acid gels made from heated milk

Abstract

The effect of a range of milk heat treatments on the rheological properties, at small and large deformations, of acid skim milk gels was investigated. Gels were made from reconstituted skim milk heated at 75, 80, 85 and 90°C for 15 or 30 min by acidification with glucono-δ-lactone at 30°C. Gels were also made from skim milk powder (SMP) samples that had been given a range of preheat treatments during powder manufacture. Heating milks at temperatures [ges ]80°C for 15 min increased the storage moduli (G′) compared with unheated milk and produced gels with G′ in the range 300–450 Pa. Acid gels made from high-heat or medium-heat SMP had higher G′ than gels made from low-heat or ultra-low-heat SMP. Cooling gels to low temperatures resulted in an increase in G′. The yield stress of gels slightly decreased with mild heat treatments of milk, and then increased again to a maximum, finally decreasing slightly with very high heat treatments of milk. The strain at yielding decreased markedly with increasing heat treatment of milk, making these gels brittle and easier to fracture. We propose that denatured whey proteins aggregated with casein particles during the acidification of heated milk and were responsible for most of the effects observed in this study.

Article

“…Values of the minimum in tan d appeared rather constant at 0.33 (d = 18.3°) at both temperatures, while the "maximum in tan d" appeared higher at 42°C than at 30°C. These results have been previously reported on heat-treated milk acidified by GDL [22,25,26] and with lactic bacteria [2,27,36,40].…”

“…With heat treatment, tan d first decreases then increases to a local maximum value and finally decreases. According to Lucey et al [25], this behaviour of tan d was observed for a heat treatment at 85°C for 30 min. A great increase in the final value of G' was observed for a heat treatment ³ 80°C for 30 min, at more than 70% denaturation of whey proteins.…”

“…Increasing the temperature leads to a decrease of 20% in G' at pH 4.6 [27], but we could not detect any G' difference. This rheological method is powerful, but even if mechanical spectra are performed as done by Lucey et al [25,29], it does not alone allow complete comprehension of acid gelation.…”

A rheological study of acid-set "simulated yogurt milk" gels prepared from heat- or pressure-treated milk proteins

“…Denatured whey proteins could act as bridging material by interacting with the whey proteins which are associated with the micelles, which would increase the number and strength of bonds between protein particles. The concentration of potential gelling protein would also be increased due to active participation of denatured whey protein in the gel structure [22]. While denatured whey proteins are known to affect the formation of acid milk gels [22], the mechanism by which they affect the rheological properties is not adequately explained.…”

Post-processing of concentrated fermented milk: influence of temperature and holding time on the formation of particle clusters

“…In the milk gel, the clusters fuse to roughly homogeneous strands and cannot be (well) distinguished anymore (van Vliet et al 2004). According to the rheological properties, fermented milks can be regarded as weak gel networks (Lucey et al 1997a;van Vliet et al 1991). By stirring, this gel is broken down into more or less stable microgel particles suspended in the whey that conserve the properties of the initial gel (Renan et al 2009) and that have sizes ranging from 1 to 100 μm( C i r o ne ta l .2010; Weidendorfer and Hinrichs 2010).…”