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The secondary structure transformation of beta-lactoglobulin from a predominantly beta-structure into a predominantly alpha-helical one, under the influence of solvent polarity changes is reversible. Independent of the alcohol used--methanol, ethanol, or 2-propanol--the midpoints of the observed structural transformation occur around dielectric constant epsilon approximately 60. The structural change destroying the hydrophobic core formed by the beta-barrel structure leads, at room temperature, to the dissociation of the retinol/beta-lactoglobulin complex in the neighborhood of dielectric constant epsilon approximately 50. However, when the dielectric constant of the medium is raised back to epsilon approximately 70 by the decrease of the temperature, both the refolding of BLG into a beta-structure and the reassociation of the retinol/beta-lactoglobulin complex are observed. The esterification of beta-lactoglobulin carboxyl groups has two effects: on the one hand it accelerates the beta-strand<==>alpha-helix transition induced by alcohols. On the other hand, the esterification of beta-lactoglobulin strengthens its interaction with retinol as it may be deduced from the smaller apparent dissociation constant of retinol/methylated beta-lactoglobulin complex. The binding of retinol to modified or unmodified beta-lactoglobulin has no influence (stabilizing or destabilizing) on the folding changes induced by alcohol.

Study of heat denaturation of major whey proteins (beta-lactoglobulin or alpha-lactalbumin) either in separated purified forms, or in forms present in fresh industrial whey or in recomposed mixture respecting whey proportions, indicated significant differences in their denaturation depending on pH, temperature of heating, presence or absence of other codenaturation partner, and of existence of a previous thermal pretreatment (industrial whey). alpha-Lactalbumin, usually resistant to tryptic hydrolysis, aggregated after heating at > or = 85 degrees C. After its denaturation, alpha-lactalbumin was susceptible to tryptic hydrolysis probably because of exposure of its previously hidden tryptic cleavage sites (Lys-X and Arg-X bonds). Heating over 85 degrees C of beta-lactoglobulin increased its aggregation and exposure of its peptic cleavage sites. The co-denaturation of alpha-lactalbumin with beta-lactoglobulin increased their aggregation and resulted in complete exposure of beta-lactoglobulin peptic cleavage sites and partial unveiling of alpha-lactalbumin tryptic cleavage sites. The exposure of alpha-lactalbumin tryptic cleavage sites was slightly enhanced when the alpha-lactalbumin/beta-lactoglobulin mixture was heated at pH 7.5. Co-denaturation of fresh whey by heating at 95 degrees C and pH 4.5 and above produced aggregates stabilized mostly by covalent disulfide bonds easily reduced by beta-mercaptoethanol. The aggregates stabilized by covalent bonds other than disulfide arose from a same thermal treatment but performed at pH 3.5. Thermal treatment of whey at pH 7.5 considerably enhanced tryptic and peptic hydrolysis of both major proteins.

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

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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.