Ultrasonic Auxiliary Sterilization And Modification Technology For Dairy Products

With the improvement of living standard, people also have more demanding for the quality of dairy products. For its unique nutritional advantages, Pasteurized milk becomes more and more popular. However, for its short quality guarantee period and difficult storage method, which make it hold low market share. Therefore, it is urgent to adopt an effective method to get better sterilization and preservation, which can reduce the loss of nutrients effectively as well as improve storage performance. This thesis adopts ultrasonic technology to process micro-organisms in fluid milk and measure their quality. In addition, this thesis carrys on study on the heterogeneity of ultrasound and enzymatic modification. The results are as follows:(1)With the increase of time, temperature and power, the retention rate of Bacillus subtilis and Colibacillus in liquid milk has reduced. Under the ultrasound processing condition by the power of 1400 W, the processing temperature 60℃, the processing time 60 s to liquid milk, Bacillus subtilis bactericidal rate could reach 96.77 %, and Colibacillus bactericidal rate could reach 100 %, which could achieve the effect of pasteurization. Under the above conditions and by the combination of ultrasonic pasteurization, sterilization of Bacillus subtilis rate could reach 98.71 %. Compared with pasteurization, the number of bacteria colony reduces 60.24 %.(2)Select the optimal sterilization conditions to determine the quality of raw milk. According to the preservation experiment, ultrasound pasteurized milk has the longest preservation time for about 8 days, which increases 4 days than that of pasteurized milk. Moreover, the fat globule's average volume particle size of both Ultrasound and Ultrasound pasteurized milk decrease significantly, which reduce by 3.70μm to 1.68μm, and the average volume particle size decrease 54.59 %. So it has a good ultrasound homogeneity effect with equal fat size. Ultrasonic milk and ultrasound pasteurized milk have been stored under the condition of 4℃for 12 days, after the alcohol test, it presents negativity with no rancidity and fat stratification, there is still a good processing quality after 12 days'storage. Ultrasound has a certain effect on whey protein in raw milk, thermal denaturation rate is slightly higher than that of pasteurized milk, but far less than the impact of UHT milk on whey protein. After the ultrasonic treatment, casein diameter decrease slightly and the size is more equal. According to mid-infrared spectroscopy, the effect of pasteurization and ultrasonic(1400 W, 60℃, 60 s)on secondary structure of protein in raw milk is not obvious. Theα-helix,β-fold,β-corner in ultrasound pasteurized milk decrease to a large extent, and random coil increases, which shows that the hydrogen bonds in the internal structure of protein in ultrasound pasteurized milk have been destroyed, which result in the changes in protein structure and protein denaturation.(3)By orthogonal tests and with the consideration of cost, optimum conditions for ultrasonic homogenization are: 60℃, 60 s, 1400 W, intermittent rate 5:2. Under these conditions, the average size of ultrasonic fat globule ball reduces 54.49%, average particle size of fat globule is 1.67μm.(4)Under the ultrasonic power condition of 1400 W, and take temperature and time as factors to carry on laboratory analysis, the result shows that the best parameters for milk is: initial temperature 70℃, ultrasonic treatment 420 s. Under this condition, the prepared yogurt has better water holding capacity, gel strength, and viscosity.(5)Through Uniform mixing test to optimize the process of ultrasound on milk protein enzymatic modification, and the best process parameter obtained is: time 200 min, enzyme dosage 5000 U/g protein, ultrasonic power 1400 W, ultrasonic temperature 40℃. It is proven that under this condition, the ultrasonic enzyme DH is 18.39 %, which enhances 20.27 % compared with water bath enzymolysis DH of 15.29 %. It shows that ultrasound can significantly improve the proteinase degree of hydrolysis and hydrolisis efficiency.