Peter A. Munro

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

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.

Effect of interactions between denatured whey proteins and casein micelles on the formation and rheological properties of acid skim milk gels

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 m m -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 m m -NEM, compared with gels made from FSM.

A comparison of the formation, rheological properties and microstructure of acid skim milk gels made with a bacterial culture or glucono-δ-lactone

Abstract Gels were made by acidification of milk with glucono-δ-lactone (GDL) or a starter culture, at two gelation temperatures. Rheological properties and microstructure of these gels were determined using oscillatory rheometry, permeability and confocal laser scanning microscopy. On addition of GDL to milk the pH decreased rapidly and it stabilized at pH ∼4.6. After the addition of starter culture to milk initially the pH decreased slowly and then decreased steadily but stabilized at pH ∼4.0. GDL-induced gels had much shorter gelation times but higher storage moduli (G′), yield stresses and strains, permeability and whey separation than gels formed by a bacterial culture. Gels formed at 42°C had shorter gelation times but higher pH at gelation, G′, permeability and whey separation. Loss tangent of all gels increased to a maximum shortly after gelation. Microstructure of gels formed with a bacterial culture was not greatly affected by gelation temperature in contrast to GDL-induced gels.

Viscosity, microstructure and phase behavior of aqueous mixtures of commercial milk protein products and xanthan gum

Abstract The behavior of commercial milk protein/xanthan mixtures was studied at neutral pH. Four milk protein ingredients; skim milk powder, milk protein concentrate, sodium caseinate and whey protein isolate were considered. For the xanthan concentrations used, up to 1wt%, the viscosity of the mixtures was dominated by the viscosity of xanthan. Mixtures of xanthan with skim milk powder or milk protein concentrate showed phase separation, as seen by confocal micrographs, and phase diagrams have been established for these two systems. No visible phase separation was observed in the case of mixtures of sodium caseinate or whey protein isolate systems. However, mixtures of sodium caseinate and xanthan, under certain conditions, showed formation of ‘thread-like’ xanthan-rich regions by confocal microscopy. We believe that the phase separation occurring in milk protein concentrate/xanthan or skim milk powder/xanthan mixtures was a result of depletion flocculation of casein micelles by the xanthan macromolecules, but thermodynamic incompatibility was likely to occur in sodium caseinate/xanthan mixtures.

Rheological properties of milk gels formed by a combination of rennet and glucono-δ-lactone

The effects of heat treatment of milk, and a range of rennet and glucono-δ-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 °C, 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 ( G ′) of < 20 Pa. Heating milks at 80 °C for 30 min resulted in a large increase in the G ′ value of acid gels. Rennet-induced gels made from unheated milk had G ′ 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, G ′ initially increased rapidly but then remained steady or even decreased, and at long ageing times G ′ 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 ∼ 5·1 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.

Microstructure, permeability and appearance of acid gels made from heated skim milk

Abstract The effects of a range of heat treatments on the microstructure and appearance of acid skim milk gels were investigated using confocal scanning laser microscopy, permeability measurements and photography. 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, with a few samples at 40 °C. Heating milks, at temperatures ≤80 °C, resulted in a gel microstructure that appeared to have thinner but more numerous branches, and had a higher ‘apparent interconnectivity’ (in the thin optical section of the x - y plane) of aggregates compared with unheated or mildly heated milks that had tortuous, bent or irregular clusters and strands making up the gel network and much less ‘apparent interConnectivity’ of strands and clusters. There were no major differences in the microstructure of acid milk gels formed from milk heated in the range 80–90 °C. Permeability measurements, which gave information on the size and number of the largest pores in the gel matrix, indicated that heat treatment had little effect on the overall porosity of the gels. It was proposed that the aggregation of denatured whey proteins during the acidification of heated milk altered gel formation and were responsible for the modified microstructure. Heating probably reduced the thickness and altered the orientation of the strands in the network. High treatment of milk also resulted in gels having large visible cracks and a rough surface appearance, but gels made from unheated milk had a smooth, unblemished appearance. It was observed that these structural rearrangements of the network, which were responsible for the large cracks and rough appearance of gels made from heated milk, occurred just after gel formation. It was proposed that a reduction in the shear deformation at fracture of gels made from heated milk may have contributed to the greater susceptibility of these gels to localized fracture

Process-induced changes in whey proteins during the manufacture of whey protein concentrates

Abstract Samples of cheese whey or acid casein whey obtained from commercial plants during the different stages of whey protein concentrate (WPC) manufacture were analyzed for protein composition and the extent of protein aggregation. Protein composition was determined by size exclusion chromatography (SEC) and electrophoresis, while the degree of protein aggregation was determined through the changes in molecular weight distributions monitored by SEC combined with multi-angle laser light scattering. The results show that there were some differences between the compositions of protein between the two types of wheys with the acid casein whey also containing lower proportions of “soluble” aggregates than the cheese whey. Ultrafiltration/diafiltration processes caused a removal of the non-protein, low molecular weight components, but had no significant effect on protein composition and aggregation. Evaporation and subsequent spray drying of the ultrafiltered retentates caused no significant changes in any of the whey protein components. Differences in functionality of WPC products are probably more related to modifications of proteins and other components caused by the processes that are used in cheese or casein manufacture rather than to the WPC manufacturing process.

Effect of heat treatment on the physical properties of milk gels made with both rennet and acid

The effects of heat treatment of milk, and a range of rennet and glucono-δ-lactone (GDL) concentrations on the physical properties of combined rennet/acid gels, made from reconstituted skim milk at 30°C, were investigated. Microstructure was determined using confocal scanning laser microscopy and permeability measurement. Whey separation was determined using two differently shaped containers. Confocal scanning laser micrographs of combined gels made from unheated milk showed that these gels had much larger pores compared with combined gels made from heated milk. This trend was confirmed by permeability measurements. Combination gels made from unheated milk were very prone to spontaneous whey separation, possibly due to considerable rearrangements of aggregated particles at an early stage of the gelation process. Gels made with both rennet and acid are a useful model system for studying the behaviour of natural and fresh cheese products.

Formation and stability of sodium caseinate emulsions: influence of retorting (121°C for 15 min) before or after emulsification

Oil-in-water emulsions containing 30% soya oil and various concentrations of sodium caseinate were prepared in a two-stage valve homogenizer. The emulsions were sealed in glass bottles and then heated at 121°C for 15 min in an autoclave. In some experiments, the caseinate solutions were heated at 121°C for 15 min first, mixed with soya oil (to give 30% oil in the final emulsion) and then homogenized. Heat treatment (121°C for 15 min) of either sodium caseinate emulsions or sodium caseinate solutions prior to emulsion formation, at all caseinate concentrations used, resulted in an increase in surface coverage, an increase in creaming stability and a change in the proportions of individual caseins at the droplet surface. Heat treatment of sodium caseinate solutions resulted in the formation of several new peptides, due to protein degradation, as well as polymerization of casein molecules, as revealed by SDS–PAGE. Both the polymerized caseinate material and degradation products were adsorbed efficiently during emulsification; the degradation products were more readily adsorbed than the parent protein. Experiments on heated emulsions indicated that the adsorbed caseinate molecules were more susceptible to degradation during heating than those in solution.

Adsorption behaviour of sodium and calcium caseinates in oil-in-water emulsions

Abstract Adsorption behaviour of commercial sodium and calcium caseinates at the oil droplet surface in 30% (w/w) soya oil-in-water emulsions has been studied. The concentration of caseinate in the aqueous phase was varied from 0.5 to 5.0% (w/w). Size distribution of sodium caseinate emulsions appeared monomodal and showed no change with protein concentration. In contrast, a bimodal size distribution was obtained for calcium caseinate emulsions at low-caseinate concentrations, indicating flocculation of oil droplets. In sodium caseinate emulsions, the surface protein load increased as caseinate concentration in the aqueous phase was increased from 0.5 to 4.0%, but it levelled off at higher concentrations. However, surface protein loads in calcium caseinate emulsion increased gradually with caseinate concentration in the aqueous phase. In general, surface protein loads were higher in calcium caseinate emulsions than in sodium caseinate emulsions. In sodium caseinate emulsions, β-casein was adsorbed preferentially at caseinate concentrations below 1% but no distinct preference of any caseins was observed at higher concentrations. By contrast, in calcium caseinate emulsions αs-caseins appeared to be adsorbed preferentially at all caseinate concentrations used. These results are discussed in relation to the adsorption behaviour observed in emulsion systems.