Characterization of casein micelle precipitation by chitosans

We have found that the addition of chitosan, a cationic polymer, on whole o r skim milk produces destabilization and coagulation of casein micelles tha t takes place without changes in the milk pH or the stability of most whey proteins. The amount of lipids recovered in the chitosan-casein aggregates was similar or higher than that obtained with rennet or acid precipitation. Approximately 70% of milk Ca2+ (similar to 750 mg/L) was found in the chit osan-induced aggregates, which is 10 and 50% higher than the amounts observ ed with acid or rennet coagulations, respectively. Purified alpha, beta-, a nd kappa -caseins were extensively precipitated by different molecular weig ht chitosans at pH 6.8. The phosphate groups of caseins seem not to be rele vant in this interaction because dephosphorylated alpha- and beta -caseins were equally precipitated with chitosans. Analysis by optical microscopy of the chitosan-casein complex reveals that the size of the aggregates increa se as the molecular weight of chitosans increase. Hydrophobic and electrost atic interactions particpate in the association and coagulation of casein m icelles with chitosans of different molecular weights. The phenomenon is ob served over a broad range of temperature (4 to 70 degreesC) with a reductio n in the concentration of chitosan needed to precipitate the caseins that p arallels a reduction in the viscosity of the chitosan solutions. Taken toge ther, the results indicate that the electrostatic interactions may contribu te energetically to the association between the two biopolymers, but the hy drophobicity of the complex would be the key determinant in the overall ene rgetics of the reaction.