Acidic coagulation of casein micelles: mechanisms inferred from spectrophotometric studies

Synergistic Effect of Heating pH and Transglutaminase on the Gelation Kinetics and Texture of Yak Skim Milk Gels

Textural defects (including syneresis and poor consistency) often occur in yogurt gels produced from yak milk. In this research, the synergistic effects of transglutaminase (TGase) and heating pH on the textural properties of acidified yak skim milk gels, as well as the related mechanism of action, were investigated. The pH values of yak skim milk were adjusted to 6.3, 6.7, and 7.1, respectively. The samples were heated at 80°C for 30 min and then cooled to 42°C. After treatment with different contents of TGase (0, 3, and 10 U TGase per gram proteins), the samples were acidified with glucono-delta-lactone. For a given TGase content, the final storage modulus (G′) of gels was positively related to the heating pH, whereas the opposite trend was observed for the gelation time. This effect was obvious between pH 6.3 and 6.7. At a definite heating pH value, the final G′ of the gels was highest at 3 U TGase per gram proteins. The highest water holding capacity and firmness value were observed in gels prepared using pH 7.1 and 3 U TGase per gram proteins. In the samples treated with 3 U TGase per gram proteins (preheating pH 7.1), more rigid network structures were seen in the gel than 0 or 10 U TGase per gram proteins. Therefore, adjusting the heating pH values and TGase contents is an effective way of improving the textural properties of yak milk gels.

Effects of cations and anions on the rate of the acidic coagulation of casein micelles: the possible roles of different forces

Raw skim milk was diluted 1000-fold using distilled water or various salt solutions as specified. Smooth, hyperbolic profiles of coagulation ratev.pH for casein were calculated from recordings of turbidity (400 nm) with time. The effects of pH, cation type, anion type and cleavage ofk−casein by chymosin (EC 3.4.23.4) were determined. The maximum of pH-coagulation rate profiles decreased by 63, 85 and 94% when the skim milk diluent was changed from water to salt solutions of NaCl (100 mM), CaCl2(50 mM) or MgCl2(50 mM). The maximum of the pH–coagulation rate profile was 15 times greater when the Ca salt was changed from CaCl2to Ca(SCN)2(50 mM). The highest pH at which casein coagulation occurred increased from 4·45 to > 6·0 when Cu2+(1 mM) was included with casein micelles dispersed in CaCl2solution (50 mM). The addition of chymosin to casein micelles suspended in CaCl2solution (70 mM) eliminated the inhibition of casein coagulation by Ca2+at pH 4·5. It is proposed that ions such as Mg2+, Ca2+, and Na+, which generally associate with casein phosphate and carboxylate groups, increased the H+concentration required to initiate the coagulation of casein, because H+must displace bound Ca2+, Mg2+or Na+to reduce repulsive hydration forces between casein micelles, allowing attractive hydration forces (e.g. hydrophobic phenomena) to cause casein coagulation. Furthermore, it is proposed that ions such as Cl−, Br−,and SCN−bind to lysine, arginine and histidine groups and thereby decrease repulsive hydration forces between cationic casein micelles.

Ultrafiltration with a microfiltration membrane of acid skimmed and fat-enriched milk coagula: hydrodynamic, microscopic and rheological approaches

The effect of acidification method (microbiological with or without renneting, HCl addition) on mass transfer, fouling structure and the rheology of the retentate was studied in the ultrafiltration of skim milk coagula using a mineral microfiltration membrane. The increase in fouling with time appeared to determine permeate flow rates, which were higher in biological coagula, and the protein retention rates which were higher in chemical coagula. Fouling was investigated using scanning electron microscopy. The rheological study showed that at the same total solids, biological coagula were more viscous than chemical coagula. The initial coagula (total solids 97 g/kg) all displayed pseudoplastic behaviour at low shear velocities and Newtonian behaviour at high velocities. Ultrafiltration of fat-enriched milk coagulum to a dry weight corresponding to a soft cheese (total solids 334 g/kg; fat in total solids 60%) gave satisfactory permeate flow rates and protein retention rates. Performance was related to the composition of the product, the hydrodynamic parameters used and the resulting fouling. The rheological study showed that the initial coagulum behaved as a pseudoplastic body at low shear rate and for higher velocities as a Newtonian liquid. The concentrated retenate behaved as an ideal viscoplastic body (Bingham body).

Electroacoustic determination of size and zeta potential of casein micelles in skim milk

Measurements of the zeta potential and particle size of casein micelles in skim milk suspensions at natural and lower pH have been made using the technique of electroacoustics. This technique requires no dilution or change of environment of the casein micelles. The zeta potential obtained at natural pH for a commercial skim milk suspension was −18 mV; it became less negative with decreasing pH. The median particle size observed at natural pH for a commercial skim milk suspension was 0·2 εm, in good agreement with previously reported values. The particle size increased as the pH was decreased.

Chemical species in cheese and their origin in milk components

Cheese making is the process of concentrating milk fat and protein by separation from water and soluble components. The objective of the cheese maker is to maximize yield efficiency by optimum utilization of each milk component while not compromising cheese quality. Cheese yielding potential of milk may be increased by selective breeding for specific protein genotypes, especially the BB variant of both kappa-casein and beta-lactoglobulin. Milk fat is included in cheese by occlusion into the protein coagulum. Participation of casein in both lactic and rennet coagulation is nearly complete so that casein losses to the whey occur mainly during cutting and the early stages of cooking. In lactic cheese, excepting cottage cheese, it is possible to eliminate losses of fines by centrifugal or membrane separation of curd. In heat-acid precipitated varieties protein recovery is increased by inclusion of whey proteins but fat recovery is very dependent on coagulation conditions. In ripened cheese obtaining the correct basic structure and composition is critical to texture and flavour development during curing.