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

Abstract

Raw skim milk was diluted 1000-fold using distilled water or various salt solutions as specified. Smooth, hyperbolic profiles of coagulation rate v. pH for casein were calculated from recordings of turbidity (400 nm) with time. The effects of pH, cation type, anion type and cleavage of/c-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), CaCl 2 (50 mM) or MgCl 2 (50 mM). The maximum of the pH-coagulation rate profile was 15 times greater when the Ca salt was changed from CaCl 2 to Ca(SCN) 2 (50 mM). The highest pH at which casein coagulation occurred increased from 4 -45 to > 6*0 when Cu 2+ (1 mM) was included with casein micelles dispersed in CaCl 2 solution (50 mM). The addition of chymosin to casein micelles suspended in CaCl 2 solution (70 mM) eliminated the inhibition of casein coagulation by Ca 2+ at pH 4 -5. It is proposed that ions such as Mg 2+ , Ca 2+ , 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 Ca 2+ , Mg 2+ 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~, N0 3~ and SCN~ bind to lysine, arginine and histidine groups and thereby decrease repulsive hydration forces between cationic casein micelles.

Rheology of Stirred Yogurt as Affected by Added Milk Fat, Protein and Hydrocolloids

“…This would will lead to different ratios of casein-bound inorganic ions. Bringe and Kinsella (1991) suggested that inorganic ions in general can affect the protein-protein interactions between casein micelles during acid gel formation by altering the repulsive hydration forces between protein surfaces. They also showed that Ca 2+ ions had a greater tendency to inhibit coagulation of casein than Na + ions.…”

Interfacial properties of acidified skim milk

“…The effect of pH on micelle structure was well known and acidification mechanisms appears to be more complex than a simple discharge of casein micelles. Nevertheless, one of the main effects of lowering the pH of milk is diminution of the micellar net negative charge [57][58][59]. By applying the mass action law (pH = pK + log[R-COO − ]/[R-COOH]), it would be possible to evaluate the R-COO − number due to carboxyl groups (acidic sugar N -acetylneuraminic acid, aspartic acid, glutamic acid) as related to pH value (Table 3).…”

Physicochemical characterization of calcium-supplemented skim milk

“…To describe the effects of calcium addition to milk, numerous physicochemical studies have been carried out [3,4,7,8,10,11,14,15,21,23,24,26,27,31,34,35,37,39]. These studies principally showed that addition of calcium to milk resulted in a new distribution of ions between milk serum and casein micelles, leading to physicochemical changes in casein micelles.…”