| Somatic cell counts (SCCs) of bovine milk is high-level in China and the shelf life of the pasteurized milk was only 3 days, much less to produce Low-Heat milk powder. Although UHT milk can kill microorganisms, but the heat-resistant enzyme from somatic cells and the microorganisms is can not be completely inactivated, the residual activities of the enzymes could hydrolysis the milk fats, proteins and other nutrients during the shelf life, which lead to some quality problems. Thereby improving the quality of milk and dairy products quality is one of the main tasks. In this study, ceramic membrane with 1.4μm pore size was used to microfiltraed skim milk. We investigated the changes of SCCs, microorganisms and their heat resistance enzymes. Meanwhile, in order to improve the quality of dairy products, use of microfiltraed (MF) skim milk to produce UHT milk and Low-Heat milk powder was researched. The main results are as follows:(1) By means of single factor experiment of the cross-velocity and different trans-membrane pressure (TMP), we determined the best operating parameters of skim milk microfiltration to removal bacteria and spores in 50℃. With cross-velocity of 2.75 m/s, the proper TMP is between 50-77.5 kPa and the average permeate flux is 11.92 L/ (m2·min). By the same time, the compositions of the milk are not rarely retained while the bacteria and spore count is reduced remarkblely, which is 3.471 Log and 2.079 Log respectively. If not operating at the best conditions, it would result in blockage of membrane, leading to a large amount of protein retention.(2) The influence of microfiltration, ultrasonic processing and heat treatment on the Plasmin system was determined. The urokinase-plasminogen activators(uPA) was retained with the somatic cells during the microfiltation, thus reducing the concentration of plasminogen activators (PA) of milk. Plasmin (PL) activity was reduced in both control milk and MF milk after heat treatment. However, the PL activity of MF milk decreased more than the control milk, the difference is significant. After UHT treatment, the PL activity of control milk is still a slight residual, but can not be detected in the MF milk, almost completely inactivated. Heat-resistant lipase activities of skim milk and cream were measured before and after microfiltration. The lipase from the psychrophilic bacteria is combined with the fat globules, which transfer into the cream during the process of cream separation, therefore microfiltration have little effect on the retaining of lipase.(3) The four batches UHT milk, preparing by the two groups bovine milk (with two different levels of SCCs) with or without MF, were stored for shelf life experiments at 37℃. The difference of NCN/TN showed significant difference in the later storage stage between the MF and non-MF UHT milk and NPN/TN showed no significant difference during the shelf time. Some indexes of MF UHT milk, including pH, titratable acidity and the upper globule particle size were better than the non-MF UHT milk, the difference is significant. The shelf life of MF UHT milk prepared with low SCCs increased 21 days than the non-MF UHT milk and the MF UHT milk prepared with High SCCs increased 63 days than non-MF.(4) Three preparation methods of Low-Heat milk powder were investigated. Method 1 is traditional preparation. As the high bacterial counts in raw milk, those heat-resistant bacteria and spores are left after pasteurizing and still grow in the concentrating tower, eventually leading to excessive number of spores in milk powder. In method 2, the raw milk was microfiltrated first and then pasteurized, almost an entirely removal of the bacteria and spores occurred, thus greatly reducing the micro-organism number in the milk powder. In method 3, MF milk was put into the concentration tower directly without the pasteurization. The bacteria and spore number of the final milk powder is also up to standard. The WPNI of there Low-Heat milk powders were both larger than 6 mgN g-1, and emulsification had no significant difference. As method three dispensed the heat treatment process, the nutrients and whey protein of the milk were reserved at the greatest level. Which means, low heat treatment (75℃, 15 s) could be displaced by the microfiltration in the production of Low-Heat milk powder...
|
PDF Full Text Request