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The cross-linking of casein was demonstrated by capillary zone electrophoresis analysis. The central composite design using response surface methodology was used to optimize cross-linking conditions
of casein. The optimal cross-linking conditions of casein were as follows: the addition level of HRP was 4.73 mkat·g-1 proteins, temperature was 37°C and reaction time was 2.9 h when casein concentration
and pH of reaction medium were fixed at 5% (w/w) and 9.5, respectively. Cross-linked casein was prepared with these optimal conditions and used to analyze its emulsifying activity index, emulsifying stability index and microstructure of acidified gel. The emulsifying activity index and emulsifying stability index of the cross-linked casein were enhanced about 10 and 6% compared to that of casein. The microstructure of acid-induced gel of the cross-linked casein observed by scanning electron
microscopy was more compact and uniform than that of casein without cross-linking. Cross-linking of food proteins induced by horseradish peroxidase might serve as an alternative approach to modify
functional property of the proteins.