Fractionation of cheese nitrogen using chitosan

The use of chitosan (0.02%) to fractionate the water-soluble (WSE) and water-insoluble (WISF) fractions of cheese was studied. Chitosan, a polymer of N-acetylglucosamine, is polycationic at acidic pH values and can form complexes with negatively charged molecules present either in solution or in the colloidal state, inducing their flocculation. Experiments in the pH range 2.0–7.0 showed that chitosan did not selectively precipitate peptides in the solubilized WISF, but fractionated WSE quite effectively, especially at pH 4.0. Analysis of the soluble and insoluble material obtained from WSE at pH 4.0 showed that the concentration of peptides, especially low molecular weight peptides, was much higher in the supernatant than in the precipitate, and there were large qualitative and quantitative differences between the peptide profiles of the two fractions. These results indicate that chitosan may offer an alternative to other methods (e.g. ultrafiltration or fractionation with ethanol) for the selective precipitation of water-soluble peptides in cheese prior to further analysis.