Study On Characteristics Of Goat Kid Rennet Extracted By Ultrasound

Rennet, a milk-clotting agent, plays a key role in cheese making. The characteristics of rennet would have an important influence on its application for the manufacture of cheese. The study investigated the processing parameters for the ultrasonic extraction of goat kid rennet, and the quality and milk-clotting properties of goat kid rennet extracted by ultrasound, and primarily investigated the salting-out condition of goat kid rennet extracted by ultrasound.The optimum processing parameters for the extraction of goat kid rennet by ultrasound were: ultrasound intensity, 44 W · cm^-2;extraction time, 25 min;NaCl concentration in extraction solution, 16 %;and the ratio, 1:30;pH of extraction solution, 7.0;temperature of extraction solution, 25 ℃;and extraction times, one time. On the optimal condition, the highest extracting activities (11941 SU · g^-1 ) of goat kid rennet could be obtained, which was insignificantly different from the predicted value (12154SU·g^-1(p>0.005)Quality characteristics of goat kid rennet extracted by ultrasound were investigated. Results showed that, compared with the traditional method, ultrasonic extraction could significantly increase the extracting activity of goat kid rennet (p<0.05), decrease extracting rate of protein (p<0.05), but had insignificant influence on proteolytic activity (p>0.05), thus increased the extraction efficiency of goat kid rennet (p<0.05). Furthermore, ultrasonic extraction could also remarkably decrease the curd tension and syneresis of goat kid rennet (p<0.05), while had insignificant influence on the storage stability of goat kid rennet (p>0.05).The milk-clotting properties of goat kid rennet extracted by ultrasound were investigated. The results indicated that the optimum milk-clotting temperature was 50 ℃. The milk-clotting activity of goat kid rennet was significantly decreased after treated at 35⁶0℃ for 25 min (p<0.05), but remained relatively stable when kept at pH 3.0-6.0 for 20 h (p>0.05). The milk-clotting activity of goat kid rennet significantly decreased with the increase of milk pH from 5.5 to 7.5 (p<0.05). Al³⁺ , Ca²⁺ , and Na⁺ in goat kid rennet solution could accelerate the curd formation (p<0.05), and K⁺, Mg²⁺ , Mn²⁺ andFe3+ had no distinctly positive or negative effects on the milk-clotting activity (p>0.05), but Cu2+ and Zn2+ obviously inhibited the milk-clotting activity of goat kid rennet (p>0.05). When the concentration of milk solution increased above 200 g ? L"1, the milk-clotting activity tended to be stable (p>0.05). With the increase of dilution time of goat kid rennet solution, the milk-clotting activity was decreased steadily (p<0.05). Compared with the traditional method, ultrasonic extraction decreased the thermal stability of goat kid rennet (p<0.05), but on the action of other influential factors, ultrasonic extraction had no significant influence on the change rule of the milk-clotting activity (p>0.05).The salting-out conditions of goat kid rennet extracted by ultrasound were primarily investigated. Results showed that no more than 56 % milk- clotting activity of goat kid rennet was recovered by salting-out with (NH^SC^. And the milk-clotting activity would lose during dialysis. So ammonium sulfate was proved not to be suitable to precipitate rennet. But NaCl could be present in the goat kid rennet preparations, so it could be an optimum salting-out agent in the salting-out of goat kid rennet. Adding Al2(SO4)3 sqlution to the activated goat kid rennet could remove partial protein and was contributed to precipitate rennet, but the additive amount of Ala(SO4)3 solution had insignificantly influences on the purification efficiency (p>0.05). The optimal condition for salting-out by NaCl was 65 % NaCl saturation, pH 4.60, 4 °C and 16 h. On the condition, the recovered milk-clotting activity of goat kid rennet could reach 86% and the factor purification approximately reached 1.6.