Factors affecting the ethanol stability of bovine milk: IV. Effect of forewarming

Effects of milk heat treatment and solvent composition on physicochemical and selected functional characteristics of milk protein concentrate

Milk protein concentrate (MPC) powders (∼81% protein) were made from skim milk that was heat treated at 72°C for 15 s (LHMPC) or 85°C for 30 s (MHMPC). The MPC powder was manufactured by ultrafiltration and diafiltration of skim milk at 50°C followed by spray drying. The MPC dispersions (4.02% true protein) were prepared by reconstituting the LHMPC and MHMPC powders in distilled water (LHMPC_w and MHMPC_w, respectively) or milk permeate (LHMPC_p and MHMPC_p, respectively). Increasing milk heat treatment increased the level of whey protein denaturation (from ∼5 to 47% of total whey protein) and reduced the concentrations of serum protein, serum calcium, and ionic calcium. These changes were paralleled by impaired rennet-induced coagulability of the MHMPC_w and MHMPC_p dispersions and a reduction in the pH of maximum heat stability of MHMPC_p from pH 6.9 to 6.8. For both the LHMPC and MHMPC dispersions, the use of permeate instead of water enhanced ethanol stability at pH 6.6 to 7.0, impaired rennet gelation, and changed the heat coagulation time and pH profile from type A to type B. Increasing the severity of milk heat treatment during MPC manufacture and the use of permeate instead of water led to significant reductions in the viscosity of stirred yogurt prepared by starter-induced acidification of the MPC dispersions. The current study clearly highlights how the functionality of protein dispersions prepared by reconstitution of high-protein MPC powders may be modulated by the heat treatment of the skim milk during manufacture of the MPC and the composition of the solvent used for reconstitution.

Casein maps: effect of ethanol, pH, temperature, and CaCl2 on the particle size of reconstituted casein micelles

Although conditions favoring casein micelle aggregation are well known, factors promoting the dissociation of the casein micelle are not fully understood. It was our objective to investigate the ethanol-induced dissociation of micellar casein as affected by temperature and a wide range of pH, along with the concentrations of calcium and casein. Two different concentrations of casein micelles were dispersed in imidazole buffer with 0 to 80% ethanol (vol/vol) and 2 and 10mM calcium. Apparent micelle size was determined by dynamic light scattering at 5, 30, and 60°C. In the absence of ethanol, casein precipitation occurred at pH 4.6 in imidazole buffer. Ten to forty percent ethanol promoted casein aggregation (>1,000 nm) and higher temperature (30 and 60°C) enhanced this effect. Higher ethanol concentrations at 50 to 80% induced the dissociation (<40 nm) of the casein micelle upon acidification (pH <5) and alkalization (pH>8) in imidazole buffer. In addition, higher concentrations of casein (0.25mg/mL) and calcium (20mM) caused the formation of larger aggregates (>1,000 nm) in the presence of ethanol when comparing with the initial lower concentrations of casein (0.1mg/mL) and calcium (2mM). Casein micelle dissociation can be achieved near the isoelectric pH by modifying the solvent composition and temperature.

Relationship between ethanol stability of bovine milk and natural variations in milk composition

Several ionic components of ultrafiltrate were measured in bulk and individual cow milks and an assessment was made of their relationship with the parameters of the corresponding ethanol (EtOH) stability/pH profiles. From linear regression analysis the strongest relationships (P < 0·001) were between soluble salt balance [expressed as (Ca+Mg) minus (Pi+Cit) or as the ratio to (Pi+Cit)] and pK (correlation coefficient, γ ∼ 0·82) or Smax, the maximum stability at high pH (γ ∼–0·72), and between Pi and pK(γ = –0·84)or Smax (γ = –0·61). These relationships agree with the view that the parameters of the EtOH stability/pH profile are determined by pH-induced changes in concentration of divalent cations. Natural variations in these parameters may be attributed to variations in relative concentrations of divalent cations and their chelators. EtOH stabilities at the natural pH of bulk milks from winter/spring- and autumn-calving animals were lowest in early and late lactation. The most important contributory factors appeared to be a high salt balance ratio in late lactation and a low natural milk pH in early lactation. The main component responsible for variable salt balance ratio was usually soluble Pi. Decrease in EtOH stability at the natural pH of late lactation milks reflected a more general change in the characteristics of the EtOH stability/pH profile, seen as an increase in pK and, in extreme cases, a decrease in Smax and profile gradient.

Estimation of casein micelles' surface energy by means of contact angle measurements

The surface energies of highly hydrated casein micelle layers isolated from variously pretreated skim milks have been determined by means of contact angle measurements. The long range Lifshitz-Van der Waals (LW) and the short range hydrogen bonding (SR) components of surface energy were determined using α-bromonaphthalene and water for contact angle measurements. Casein micelles isolated from untreated and heat treated milks showed similar surface energy values of about 63·5 mJ.m-2 with an LW component of 19·2 mJ.m-2 and an SR component of 44·3 mJ.m-2. The calculated attraction potential energy was − 0·7 mJ.m-2. Casein micelles isolated from renneted milk showed a surface energy of 33·0 mJ.m-2 with an LW component of 30·7 mJ.m-2 and an SR component of 2·3 mJ.m-2. The attraction potential energy of renneted micelles was nearly two orders of magnitude higher than those of micelles from other milks ( − 63·3 mJ.m-2). The SR component of interfacial energy accounted for 98% of this attraction potential. The importance of attractive forces in relation to casein micelle stability is discussed.

Ethanol stability of casein micelles – a hypothesis concerning the role of calcium phosphate

The ethanol (EtOH) stability of skim milk and the stability towards aggregation of casein micelles diluted into ethanolic buffer solutions were compared using data obtained from previously published experiments. Differences in absolute stability and in relative response were observed when Ca2+ level and pH were adjusted, the buffer system results lying below those from skim milk in both cases. Increasing the ionic strength of skim milk adjusted to pH 7·0 lowered its EtOH stability whereas increasing the ionic strength of the diluting buffer increased the stability of the casein micelles. The hypothesis is put forward that the differences are due to the simultaneous precipitation of Ca phosphate when EtOH is added to skim milk. This draws calcium from the caseinate sites of the micelle, counteracting the destabilizing effects of the EtOH towards the micelle. Such removal and the consequent restructuring are kinetically controlled and micellar precipitation in skim milk finally occurs when the micellar coagulation time falls within the time scale of the restructuring reactions.