Study On Enzymatic Hydrolysis Of Chicken Protein And Antioxidant Activities Of Its Hydrolysate

Chinese chicken production ranks second in the world. The sustainable development of chicken industry is seriously restricted by those outdated processing techniques in China. This thesis aimed at improving the technical level of further processing of chicken protein. Several proteases were chosen to hydrolyze chicken protein. The optimal conditions and some intrinsic mechanisms of enzymatic hydrolysis were studied. The nutrition, flavor, appearance and antioxidant activity were investigated for supporting the product development of nutritional stuff, flavouring stuff and functional stuff.The composition of chicken meat showed it had high content of protein, low content of fat and well-balanced amino acid combination. The ratio of constituent amino acid indicated that chicken protein had high content of necessary amino acids, umami amino acids, branched-chain amino acids and antioxidant amino acids. But the content of these amino acids showed significant differences in different chicken protein parts (sarcoplasmic, myofibrillar and stroma proteins). The hydrophobic values of chicken protein and its parts were low. Therefore, chicken meat was a good protein resource for preparing nutritional, flavouring and functional food stuff.The differential scanning calorimetry (DSC) was employed to analyse the thermal properties of chicken protein. How the content of sulfhydryl groups and disulfide bonds changed under conditions of different heating temperature was investigated. The changes of hydrolysate composition were determined to understand the effect of heat treatment prior to submitted to enzymatic hydrolysis on the hydrolysis process. The results showed heat denaturation decreased the release of free amino acids, low-molecular-weight peptides and the recovery of soluble nitrogen, but increased the release of large-molecular-weight peptides. Moderate heat treatment could improve the peptide content of chicken protein hydrolysates. Therefore, heat modification could be used to regulate the peptide molecular weight range and functional properties of chicken protein hydrolysates.Five proteases (alcalase, protamex, neutrase, papain and ptn 6.0s) were empolyed to hydrolyze chicken protein and the hydrolytic conditions were optimized. The content of soluble nitrogen, amino nitrogen and peptide-based nitrogen in chicken protein hydrolysates were determined and compared. The results indicated that alcalase was the proper enzyme for extensive hydrolysis of chicken protein, and the hydrolysates obtained by papain and ptn 6.0s had relatively large amounts of peptides. By comparison of the molecular weight distribution profiles of hydrolysates, it showed that significant differences existed in the molecular weight range of less than 10000 Da. This suggested that the molecular weight distribution profile and functional properties of hydrolysates could be controlled by using relevant protease. Combination of endoproteinase (alcalase, papain) and exoproteinase (flavourzyme) were not helpful to improve the content of soluble nitrogen and low-molecular-weight peptide. But it could improve the content of free amino acid significantly.The umami, bitterness and browning extent during the hydrolysis process of chicken protein were analysed. The results showed chicken protein hydrolysate was of good flavor.Addition of browning inhibitor prior to hydrolysis could prevent the browning progress significantly. The necessary amino acid indexes of hydrolysates after 24 hr of hydrolysis were lower than chicken protein, but higher than the index of ideal adult amino acid pattern recommended by FAO/WHO (1973). Except for neutrase hydrolysates, hydrolysates obtained by proteases had higher amino acid score than chicken protein. Therefore, selective enzymatic hydrolysis of chicken protein was useful for improving the nutritive value of its hydrolysate.Different chicken protein parts were hydrolyzed by alcalase and the hydrolysates were compared with chicken protein hydrolysate. The results indicated that all chicken protein parts could be degraded into soluble protein. Free amino acids in chicken protein hydrolysate were mainly from degradatin of sarcoplasmic proteins. The peptide fraction with molecular weight lower than 4500 Da was mainly from degradation of sarcoplasmic proteins. The peptide fraction with molecular weight in the range of 4500~10000 Da was mainly from degradation of stroma proteins. Myofibrillar proteins contributed relatively little to the composition of chicken protein hydrolysate. The reaction rate which slowed down during the late period of enzymatic hydrolysis was mainly because of decreasing substrates available for being hydrolyzed by protease. The necessary amino acid index and amino acid score of hydrolysates didn't increase linearly during the hydrolysis process. Therefore, controlled enzymatic hydrolysis of chicken protein could be used to improve the nutritive value of its hydrolysate.Analyses of antioxidant acitivities showed the water-soluble extract of chicken meat, hydrolysates of chicken protein had relatively high scavenging activities of DPPH radical and reducing power. No relativity between reducing power and DPPH radical scavenging capacity was detected in this research. The same conclusion was drawn between the degree of hydrolysis and antioxidant activity. Gel filtration chromatography was employed to separate the water-soluble extract of chicken meat and chicken protein hydrolysates by papain after 4 hr of hydrolysis. The peptide fraction with molecular weight lower than 2200 Da was found that had the highest antioxidant activity. It might be related to the existence of carnosine and anserine. The antioxidant activities of chicken protein hydrolysates had good thermal stability and preserving stability.