Background Casein is the main proteinaceous component of milk and has made us interest due to its wide applications in the food, drug, and cosmetic industries as well as to its importance as an investigation material for elucidating essential questions regarding the protein chemistry. Enzymatic hydrolysis is an important method commonly used in the modification of protein structure in order to enhance the functional properties of proteins. The relationship between enzymatic hydrolysis and structure change of casein need to make more study. Results During hydrolysis, degree of hydrolysis in the casein hydrolysates increased rapidly in the initial 20 minutes, reached a plateau after 45 minutes, and then kept relative constant for the rest of the hydrolysis. The relative percentage of the released peptides with molecular weight of over 50 kD significantly decreased with hydrolyzation, while those with MW of 30–50 kD and below 20 kD increased significantly. The contents of a-helix and β-turn in the hydrolysates increased compared to the original casein. Moreover, the molecular flexibilities of the casein hydrolysates, estimated by the ratio of α-helix to β-structure, were lower than that of original casein protein. Conclusions The significant changes in molecular weight distribution and structure characteristics of casein hydrolysates were found compared to the control sample. This change should be the basis of enhancement of functional properties.
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