Purification, characterization, and milk coagulating properties of ginger proteases

Ginger proteases are used as milk coagulants in making a Chinese traditional milk product (Jiangzhinai or Jiangzhuangnai), suggesting their potential as a source of rennet substitute that might be applicable in the modern dairy industry. In this study, ginger proteases were extracted from fresh ginger rhizome by using phosphate buffer and subsequently purified by ion exchange chromatography. Ginger proteases, all with a molecular weight around 31 kDa, were found to exist in 3 forms with isoelectric point values around 5.58, 5.40, and 5.22, respectively. These enzymes had very similar biochemical behavior, exhibiting optimal proteolytic activity from 40 to 60°C and maximum milk clotting activity at 70°C. They were capable of hydrolyzing isolated αS1-, β-, and κ-casein, of which αS1-casein was most susceptible to the enzyme; κ-casein was hydrolyzed with a higher specificity than αS1- and β-casein. In addition, the ginger proteases exhibited a similar affinity for κ-casein and higher specificity with increasing temperature. Gel electrophoresis and mass spectra indicated that Ala90-Glu91 and His102-Leu103 of κ-casein were the preferred target bonds of ginger proteases. The milk clotting activity, affinity, and specificity toward κ-casein showed that ginger protease is a promising rennet-like protease that could be used in manufacturing cheese and oriental-style dairy foods.