The Study On Immobilized Lactase And Browning-inhibition Technology For Low-lactose Milk

In the paper,the immobilized method of lactase and hydrolysis characteristics of several commercial lactases were comparedly studied.The immobilized parameters and an optimum lactase used in the study ware determined,and the dosage,hydrolysis condition and process to product low lactose milk were determined.5-HMF is one of the indictors for Maillard reaction.It was also discussed the effects of the sterilized conditions and browning-inhibitors'anti-browning on the low lactose milk.The technical system of the low lactose milk was established(lactose hydrolysis rate 60%),providing the technical support for the low lactose milk.The results indicated that:(1)Sodium alginate-gelatin were used to entrap lactase.The optimums as follows: 1.78%sodium alginate,1.0%gelatin,0.48%lactase,under which condition the gel got the activity yield was 81.74%,but the mechanical strength of the immobilized enzyme was not ideal.(2)Sodium alginate--κ-carrageenan were used to entrap lactase.The optimums as follows:2.0%sodium alginate,0.81%κ-carrageenan,0.23%lactase,under which condition the gel could be used 8 times on end and the activity yield was 89.19%.Then immobilized enzyme entrapped in gel was characterized.The immobilized enzyme entrapped with sodium alginate--κ-carrageenan was showed that its optimal temperature was 40℃,which did not change,optimal pH was 7.0 raised 0.2,endurance to temperature and pH were both better than soluble enzyme which showed that this kind of immobilized enzyme could be used directly and conveniently.(3)The activity of the lactase decreased significantly when heat treatment temperature was above 45℃.The sterilized conditions of milk production can inactive the lactase.To lactase,the optimum pH value was 6.5～7.5.When milk was treated with I enzyme at 4℃for 20 hours,with dosage of 0.10%,or 38 to 40℃for 3 hours,with dosage of 0.08%,the degree of hydrolysis for lactose could reached 60%.(4)5-HMF concentration was measured by using HPLC with UV detection at 284nm.The 5-HMF concentration in low lactose milk that treated under 110℃,10min reached to 1.57μg/ml,which was 3.6times higher than that in fresh milk,2.5 times higher than that in low lactose milk of adding browning-inhibitors,respectively.(5)The 5-HMF concentration in high temperature low lactose milk was affected significantly by several factors,such as hydrolysis degree of lactose,the combination of temperature and time when sterilization,and stored conditions.If optimum hydrolysis dgree was used,cooling rapidly after sterilization,and stored at a lower temperature,5HMF concentration and browning can be controlled effectively.When using mixtured inhibitors BHA.Vc.NaHSO₃ and L-cys,5-HMF concentration was lower more than 50%than without inhibitors.(6)The residual content of SO₂ in low lactose milk was 21.7mg/kg＜25mg/kg, which accords with the level of national standard(GB2760-85).