| Summary: | 碩士 === 國立臺灣大學 === 畜產學研究所 === 92 === Proteolytic enzymes are used for meat tendering and milk clotting purposes. Ginger rhizome grows in tropical Asia and has often been used as drugs in Chinese medicine for a long times. The protease in ginger rhizome was reported and indicated that it belongs to the cysteine protease family such as papain and bromelin. The coagulation of milk is the basic step in the manufacture of all cheeses. Milk clotting enzymes suitable for cheese making must have a high cleavage capacity toward ��-casein. Chymosin is a milk clotting enzyme from the fourth stomach of calf and is recently scarce. This study with ginger rhizome investigated a new source of plant proteolytic enzymes that are important for milk processing.
Experiment I was conducted to assay protease activity of ginger. Protease activity was distinct to milk clotting activity and proteolytic activity. It was found that the optimal temperature of ginger protease on milk clotting and proteolytic activity was 60℃, and the pH optima was 5.0. It was heat stable but alkaline unstable. Adding some metal ions such as Ca2+, Mg2+ and Fe2+ could increase the milk clotting activity, but the proteolytic activity has no function significantly. Hg2+ could inhibit both the milk clotting and proteolytic activity of ginger protease.
Experiment II was compared ginger protease with other milk clotting enzymes such as rennet, papain and bromelain. ��-, ��-, ��- and whole caseins were used as substrates. SDS-PAGE of casein cleavage by ginger protease indicated the protease could digest casein. In general, milk clotting enzymes attack ��-casein first and the ��-casein plays an important role to stabilize the micelle structure in the milk. Rennet was the perfect milk clotting enzyme which can digest ��-casein but not digest ��- and ��-casein. It was found that ginger protease could digest ��-casein, ��-casein and ��-casein, such as papain and bromelain, and it would decrease the curd hardness and some bitter peptides were obtained. But it would not cause extremely proteolysis because the enzyme activity of ginger protease was lower than papain and bromelain. The MCA/PA of four enzymes were as follows: rennet＞ginger protease＞papain＞bromelain.
Experiment III was conducted by different treatments and measured the curd firmness, syneresis to find the optimal constitution. The factors that affected the curd quality when producing milk clotting product by ginger juice were as follows: holding time, ginger juice added, SNF content, CaSO4˙2H2O and fructose concentration. Increasing holding time, ginger juice amount, SNF content, and CaSO4˙2H2O concentration, the curd firmness increased. Increasing SNF content and fructose concentration, the syneresis decreased. Sensory evaluation showed good results of the new milk clotting products. Mixture of 2.5mL ginger juice/50mL milk with 30mM CaSO4˙2H2O and 9%（v/v）fructose added, then holding at 80℃ for 20 minutes showed the best result. The microstructure of ginger milk curd was similar to renneted milk curd. Freeze dried ginger juice could maintain the enzyme activity during 3 month storage at -20℃. According to above results, milk could be coagulated by ginger protease and form a curd. This may manufacture a delicious milk clotting product and increase the varieties for dairy products.
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