Study Of Casein-dextran Conjugate Modification

Casein was a full value protein due to its ideal essential amino acids composition. Howerer, solubility and emulsifying properties of casein were bad in low acid solution. The use of casein was limited while food was in low acid conditions,. In order to improve the functional properties, we should get it modified. In this paper, the technology,molecular structure and functional properties of conjugates were studied.Firstly, the prepration of casein-dextran conjugates by wet-heating and its molecular structure were studied. The effects of temperature, heating time, the concentration of casein , weight ratio of casein to dextran ,pH value and buffers on the degree of graft and browning were investigated. Based on the single factor test, the orthogonal test was used to optimize the process. The most optimum conditions were as follows: temperature 110℃, heating time 60min, the concentration of casein 2mg/mL, the weight ratio 0.1, pH 7.8. The degree of graft and browning reached 35.10% and 0.198 respectively. According to the analysis of SDS-PAGE, the appearance of a band at the top of the separating gel was observed in the conjugates. HPLC suggested the formation of higher molecular weight materials. Analysis of the components of amino acid showed that the content of Lys and Arg decreased. The conjugates by wet heating were confirmed by fourier transform IR spectroscopy and Fluorescence spectra.Secondly, the prepration of casein-dextran conjugates by dry-heating was studied. The effects of temperature, heating time, relative humidity, weight ratio of casein to dextran ,pH on the degree of graft were investigated . The conditions were temperature 60℃, heating time 24 h, the weight ratio 1/3, pH 7.4. The degree of graft and browning reached 20.61% and 0.096 respectively. SDS-PAGE, HPLC, analysis of amino acid and Ultraviolet spectrum suggested the structure of conjugates by dry-heating were similar to the conjugates by wet-heating. All of them confirmed the formation of conjugates by dry-heating.At last, Functional properties of casein-dextran conjugates were studied. In the range of acidity, with the increasing of the degree of graft, the solubility increased continuously. The solubility of the casein-dextran conjugates by wet-heating in the isoelectric point increased from 4% to 40%, while the casein-dextran conjugates by dry-heating increased to 30% and the results were not affected by the concentration of the salt. Meanwhile, the emulsifying activity of the casein-dextran conjugates improved and the conjugates, which was made by the dry-heating, had high level of emulsifying activity(1.5 times of casein) and emulsification stability(3 times of casein). The temperature had little effect of the result. With the pH from 3 to 6, the foaming ability of the conjugates increased with the increasing of the degree of graft. In the isoelectric point, the foaming ability of casin increased from 12% to 70% by wet-heating graft reaction and to 50% after dry-heating graft reaction. The results of the antioxidant activity showed that casein-dextran conjugate had strong ability of reducing power and antioxidant activity and it increased with the increasing of the degree of graft.