With the constant improvement of people’s living water, people want to drink and eat more and more high, dairy products have become a popular nutrition. At present, a new type of liquid milk is emerging abroad — micro-filtered milk. In the people’s advocacy of green food, micro-filtered milk has won the favor of the majority of consumers with its sudden advantages. Micro-filtration technology has low energy consumption, which not only avoids the damage of high temperature heating to the nutrients. It also avoids the disadvantage that the dead bacteria can still release heat-resistant enzymes after high temperature sterilization and affect the quality of milk. The health index is similar to that of ultra high temperature sterilized milk.
Meaning of milk microfiltration for bacteria removal.
The diameter of fat pellets in milk is 0.l ~ 22.0μm, which basically covers the size of all the bacteria in milk, and seriously affects the effect of bacteria removal and filtration. Therefore, milk fat should be separated before microfiltration, and skim milk should be filtered to remove bacteria. The separated thin cream can be treated differently according to the needs of different products, such as sterilization alone or after mixing with the trap solution, and then mixed with the sterilized skim milk, standardized into products or as the raw material of the next process.
Microfiltration technology can replace pasteurization and chemical preservatives. The effective range of microfiltration is 0.05 ~ 2.50μm, and most of the size of microorganisms that need to be killed in food processing is included in this range. Therefore, aseptic filtration using microfiltration membrane separation is the most effective. It can trap fine bacteria, yeast and mycobacteria completely, and the active ingredients in the food can pass through the membrane.
Improvement process of microfiltration technology for removing bacteria
The application of microfiltration in dairy industry is mainly degreasing, bacterialization and concentration of macromolecular casein materials, which is determined by the size of various components in milk. The size of bacteria in milk is generally greater than 0.2μm, and the size of fat granules is between 0.1 and 22.0μm, which are larger than other components. Therefore, the use of microfiltration membrane can remove the bacteria and most of the fat in milk. For skim milk, the quality changed little before and after microfiltration. For whole milk, the fat will be removed and its quality will change to some extent. In the pilot study of whole milk microfiltration, it was found that the number of bacteria was greatly reduced by using 1.8μm alumina membrane microfiltration, but at the same time 98% of the fat was removed. A 1.4μm alumina film was used for the microfiltration of skim milk under the condition of constant pressure difference. The membrane flux was over 700L/m 2·h, and the bacteria removal rate was 99.7%. The suitable filtration pressure difference was 0.03 ~ 0.08M Pa. Because of the serious membrane pollution, it is necessary to regenerate the membrane after 5 ~ 10h operation. In the process of microfiltration, the “dynamic membrane layer” formed by polarized components such as macromolecular proteins is easy to be compacted, so that the microfiltration membrane has the separation characteristics of ultrafiltration membrane, resulting in the interception of casein and whey protein. Increase the velocity of membrane surface (> 6m /s) and reducing the film pressure difference can reduce the degree of compaction of the polarized layer, and can reduce this dynamic film layer to a minimum. However, high flow rates often lead to high pressure drops along the axial direction of membrane components, especially for small caliber (< 3 m m) membrane tube, the high pressure operation at the inlet leads to serious membrane pollution, and the low pressure operation at the outlet leads to some membrane area cannot be effectively used, from which the permeability of the product decreases sharply, the protein interception rate increases, affecting the quality of milk.
The solution to the concentration polarization problem is to use cross flow filtration. Cross-flow filtration has two characteristics. First, there are two outflow liquid, namely osmotic liquid (or filtrate) and circulating fluid used to provide scour effect on the membrane surface. The other is the velocity in two directions, preferably the filtrate velocity perpendicular to the membrane surface and the cross-flow velocity parallel to the membrane surface. The obvious difference between the transmission cross-flow microfiltration and the B actocatch process is that, in the traditional cross-flow microfiltration, the pressure on the permeation side remains constant, while the pressure on the feed side is lost due to cross-flow filtration. Therefore, the operating pressure differential changes continuously along the length of the component. In the Bactocatch process, the osmotic side pressure changes along the length direction of the membrane component to maintain the constant operating pressure difference along the length square direction of the component. This operation mode can be achieved by adding a pump loop to the osmotic side. In traditional cross-flow filtration, higher operating pressure at component inlet leads to higher filtration flux of membrane. However, when operating at high membrane flow rates, the pressure drops faster along the length of the component, so the flux drops significantly. This kind of uneven flux makes the film resistance distribution uneven along the length direction of the component (large inlet, small outlet), which seriously affects the total permeability flux. The B actocatch process overcomes the disadvantages of traditional cross-flow microfiltration, and the cleaning period of the membrane increases significantly and the total permeability increases due to the decrease of concentration polarization.
The Bactocatch method reported the production of 12,000 L/h low fat (0.5% w t) and medium fat (1% w t) milk. It showed that under the condition of concentration factor of 10, the average osmotic flux was 500L/m 2h, and the bacteria removal rate was greater than 99.6%. For low fat milk, under the condition of concentration factor of 20, the maximum permeability of up to 750L/m 2·h can be obtained. If the film is regenerated every 5 ~ 10h, the cleaning of the contaminated film is much easier than that of the usual cross-flow filtration operation.
The most effective method to remove bacteria from milk microfiltration is to adopt the constant pressure operation mode of B actocatch method. The milk treated by this method almost maintains its original flavor, and the health index is close to that of ultra-high temperature sterilized milk. Compared with pasteurized fresh milk, the shelf life increased from 6 ~ 8 days to 16 ~ 21 days at the same 8℃ environment, greatly extending the shelf life of the product. Therefore, the microfiltration technology has a broad application prospect in dairy industry.
Post time: Jan-14-2023