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.
Beta A1‐casein was treated with TPCK‐trypsin to give 3.2, 5.0, 5.8 and 7.4% hydrolysis of the peptide bonds. By sodium dodecyl sulfate‐polyacrylamide gel electrophoresis the resulting peptides had apparent molecular weights in the range 8,000–4,000. Size‐exclusion chromatography of hydrolyzed samples showed four major peaks near 15,000, 5,500, 3,500 and 2,500 molecular weights, representing 17, 15, 7 and 14% of the material, respectively, after 3.2% hydrolysis and 9, 6, 14 and 52% of the material, respectively, after 7.4% hydrolysis. Between the extremes 3.2% and 7.4% hydrolysis, a peak near 8,500 molecular weight was present until 5.8% hydrolysis then disappeared after 7.4% hydrolysis to be replaced by a peak near 12,000 molecular weight. Peptides recovered from reversed‐phase high performance liquid chromatography were analyzed by determination of their amino acid composition and identified in the sequence of β‐casein. After trypsin treatment, the solubility of β‐casein hydrolysates was largely increased at pH 4.0–7.5. The emulsifying activity of the hydrolysates was higher than that of β‐casein in the range of pH 1.5–3.5 and 6.5–10.0, but all the emulsions obtained with trypsin‐treated β‐casein were less stable than those obtained with original β‐casein.
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