Casein amino groups were modified with aldehydes and dialdehydes via reductive alkylation at pH 8-0. The degree of alkylation was controlled by the amount of the alkylating reagent applied. The initial rates of a-chymotrypsincatalysed hydrolysis of alkylated /?-casein were inversely related to the size of the modifying group. Proteolysis of modified /?-casein with trypsin (18 h) or with a-chymotrypsin (48 h) depended on the nature and size of the substituent applied. The measurements of tryptophan fluorescence indicate that the modifications also induced conformation change. Solubilities of methyl-, ethyl-or benzyl-/?-casein slightly increased; solubilities of /?-casein dialdehyde derivatives were significantly lower than that of native /?-casein. Emulsion stability of methyl-or ethyl-/?-casein was higher than that of native /?-casein in the acidic pH range. After modification with glyoxal, the emulsifying activity and the emulsion stability of /?-casein decreased. The emulsifying activity of benzyl-/?-casein was lower than that of native /?-casein. Phthalylated /?-casein displayed the poorest emulsion stability.For a long time, caseins have been available to the food industry in large quantities because of the ease and the low cost of their preparation at a considerably high level of purity. Their use as food ingredients relies upon their specific functional characteristics such as water retention, thickening and emulsifying properties. The amphipathic structure of caseins causes them to concentrate at polar-non-polar interfaces.The properties of proteins can be significantly altered by physical, chemical and enzymic treatment. There is an extensive literature on this

Kappa‐casein A was treated with chymosin in order to isolate the caseino‐macropeptide corresponding to the C‐terminal 106–169 residues of K‐casein. Whole casein, K‐casein and the caseinomacropeptide (CMP) were studied for their water solubility and emulsifying activity. The CMP was soluble over the range of pH from 1 to 10, with a “minimum” solubility (88%) in the range of pH 1–5 and a “maximum” solubility (98%) in the range of pH 5–10. For whole casein and K‐casein, at pH values above 5.5, the emulsifying activity increased when pH increased and the maximum value was obtained for very alkaline solutions; for pH values below 4.5, the increase in emulsifying activity was much more pronounced at pH 2.5; below pH 2.5, emulsifying activity decreased. For CMP, the increase in emulsifying activity was much more pronounced in the acidic range than in the alkaline range. After 24 h storage and heating of the emulsion, a large pH‐dependant decrease of emulsifying activity (22–60%) was observed for CMP for pH values below 4.0; under the same conditions, the emulsifying activity of whole casein and K‐casein showed a 5–19% and a 1–21% decrease, respectively. For pH values above 6.0, a 22–59% decrease was observed for CMP as compared to a 1–12% and a 4–17% decrease with whole casein and K‐casein, respectively.

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The study of the solubility of unstructured proteins (alpha s1-, beta-, and kappa-casein) and well-structured globulin (beta-lactoglobulin) in low water binary solvent systems demonstrated the crucial importance of solvent polarity and neutralization of protein polar functions on the final outcome of solubility experiments. The solubilities up to 38, 56, and 96% in CHCl3/CH3OH (1/1, v/v) acidified with HCl and up to 5, 10, and 25% in CHCl3/CH3OH (1/1, v/v) in the presence of triethylamine (TEA) were obtained for kappa-, alpha s1-, and beta-casein, respectively. The importance of protein charge neutralization was apparent when the solubilization was performed in basified CHCl3/CH3OH media, giving the optimal results when the studied proteins were brought before to their isoionic point. The maximum solubility of beta-casein at its pI in 30-70% methanol in CHCl3 was reaching 50-60% with triethylamine (TEA) added. beta-lactoglobulin could be solubilized up to 70% in CHCl3/CH3OH (7/3, v/v) acidified with HCl and up to 40% in CHCl3/CH3OH (3/7, v/v) in the presence of TEA. The observed yield of reductive alkylation of beta-lactoglobulin was much higher (98%) when performed in studied solvent system than in aqueous conditions (75%). Apparently, steric hindrance of the well-folded beta-barrel (in aqueous conditions) structure masks the portion of epsilon-NH2 groups. In the case of unstructured aqueous media beta-casein, 90% alkylation yields were obtained in organic and aqueous conditions.