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The effect of interactions of denatured whey proteins with casein micelles on the rheological properties of acid milk gels was investigated. Gels were made by acidification of skim milk with glucono-δ-lactone at 30°C using reconstituted skim milk powders (SMP; both low- and ultra-low-heat) and fresh skim milk (FSM). The final pH of the gels was ∼4·6. Milks containing associated or ‘bound’ denatured whey proteins (BDWP) with casein micelles were made by resuspending the ultracentrifugal pellet of heated milk in ultrafiltration permeate. Milks containing ‘soluble’ denatured whey protein (SDWP) aggregates were formed by heat treatment of an ultracentrifugal supernatant which was then resuspended with the pellet. Acid gels made from unheated milks had low storage moduli, G′, of <20 Pa. Heating milks at 80°C for 30 min resulted in acid gels with G′ in the range 390–430 Pa. The loss tangent (tan δ) of gels made from heated milk increased after gelation to attain a maximum at pH ∼5·1, but no maximum was observed in gels made from unheated milk. Acid gels made from milks containing BDWP that were made from low-heat SMP, ultra-low-heat SMP and FSM had G′ of about 250, 270 and 310 Pa respectively. Acid gels made from milks containing SDWP that were made from ultra-low-heat SMP or FSM had G′ values in the range 17–30 Pa, but gels made from low-heat SMP had G′ of ∼140 Pa. It was concluded that BDWP were important for the increased G′ of acid gels made from heated milk. Addition of N-ethylmaleimide (NEM) to low-heat reconstituted milk, to block the —SH groups, resulted in a reduction of the G′ of gels formed from heated milk but did not reduce G′ to the value of unheated milk. Addition of 20 mm-NEM to FSM, prior to heat treatment, resulted in gels with a lower G′ value than gels made from reconstituted low-heat SMP. It was suggested that small amounts of denatured whey proteins associated with casein micelles during low-heat SMP manufacture were probably responsible for the higher G′ of gels made from milk containing SDWP and from milk heated in the presence of 20 mm-NEM, compared with gels made from FSM.

S. The effects of heat treatment of milk, and a range of rennet and gluconoδ-lactone (GDL) concentrations on the rheological properties, at small and large deformation, of milk gels were investigated. Gels were made from reconstituted skim milk at 30 mC, with two levels each of rennet and GDL. Together with controls this gave a total of sixteen gelation conditions, eight for unheated and eight for heated milk. Acid gels made from unheated milks had low storage moduli (Gh) of 20 Pa. Heating milks at 80 mC for 30 min resulted in a large increase in the Gh value of acid gels. Rennet-induced gels made from unheated milk had Gh values in the range " 80-190 Pa. However, heat treatment severely impaired rennet coagulation : no gel was formed at low rennet levels and only a very weak gel was formed at high levels. In gels made with a combination of rennet and GDL unusual rheological behaviour was observed. After gelation, Gh initially increased rapidly but then remained steady or even decreased, and at long ageing times Gh values increased moderately or remained low. The loss tangent (tan δ) of acid gels made from heated milk increased after gelation to attain a maximum at pH " 5n1 but no maximum was observed in gels made from unheated milk. Gels made by a combination of rennet and GDL also exhibited a maximum in tan δ, indicating increased relaxation behaviour of the protein-protein bonds. We suggest that this maximum in tan δ was caused by a loosening of the intermolecular forces in casein particles caused by solubilization of colloidal calcium phosphate. We also suggest that in combination gels made from unheated milk a low value for the fracture stress and a high tan δ during gelation indicated an increased susceptibility of the network to excessive large scale rearrangements. In contrast, combination gels made from heated milk formed firmer gels crosslinked by denatured whey proteins and underwent fewer large scale rearrangements.Most natural cheeses are made by using rennet to coagulate milk and adding starter culture to produce acid. In these cheeses (e.g. Cheddar) rennet coagulation occurs at pH values close to the natural pH of milk since little acidification has ‡ For correspondence.

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