Abstract:

This study investigated the changes in lipase activity of box- and bag-packaged UHT milk stored under different environmental conditions like different temperatures and mechanical vibration, and the changes in lipase activity with different combinations of pasteurization time and temperature as well as the change characteristics of lipase activity during the enzymatic hydrolysis of lactose. The lipase activity in UHT milk was significantly inhibited within the range of 9－10.5 ℃, while the lipase activities in box- and bag-packaged UHT milk both were obviously higher than those at low temperature when the temperature reached 14－21 ℃, and the bag-packaged UHT milk presented higher lipase activity than that the box-packaged. However, at the constant temperature of 35 ℃, the lipase activity in the box-packaged UHT milk remained very low and stable. At the low temperature of 9－10.5 ℃, continuous mechanical shaking for 2 h resulted in the increase of 65.5% in lipase activity compared to standing for 5 h while this continuous shaking for a long period of time caused sharp reduction of lipase activity. A higher activity of lipase was observed in milk pasteurized at 70 ℃ for 20 min than those at 90 ℃ for 5 min and at 98 ℃ for 5 min while the milk pasteurized above 90 ℃ for 5 min exhibited very low lipase activity during 4 ℃ storage. During the lactose hydrolysis, the inhibition of lipase activity became stronger with the temperature ascending (25－41 ℃) while with the extension of lactose hydrolysis, its activity was enhanced.

Key words: UHT milk, pasteurized milk, lactose hydrolysis, lipase, activity