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Membrane Separation in the Dairy Industry Time:2018.03.21
     Hair, grass and flies. Raising hair, grass and flies here seems ridiculous, but it is a very real problem in the dairy industry. Flies are easy to remove and hair is hard to find and remove. When dealing with diluted raw milk, you can find a surprising amount of cow hair and brush hair, often with some green grass. To solve these problems, bag filters that can be cleaned and disinfected are usually used, and the effect is quite good.
    Casein. In whey contains a lot of casein fine particles. Larger particles are generally removed using a sieve that can be self-cleaned, while the remaining casein particles are generally removed using centrifugation. If the operating efficiency of the centrifuge is low, particulate matter is effectively reduced when the whey is processed through the membrane system. Experience has shown that a RO system with a 47-mil flow path can handle a certain amount of suspended solids that pass through a casein filter so that a centrifuge is not needed.
Milk, fat and phospholipids. The proportion of natural fat contained in milk ranges from 3.0% to 4.5%. Membrane systems can handle fats in milk, but they work better when the fat content is less than 0.05%. In dairy processing plants, the most commonly used equipment is the use of centrifuges to remove fat from milk. Phospholipids form on the skin of the fat globules and are therefore very annoying and they need to be cleaned with high pH and high temperatures. There is currently no way to remove Phospholipids from milk and whey.
 
    Pasteurization. It is no exaggeration to say that pasteurization before membrane filtration has become an industry standard. Pasteurization requires heating to 72°C for 15 to 20 seconds and cooling to operating temperature later. There are few exceptions to this principle. The purpose of pasteurization is to reduce the motility of bacteria while forming calcium phosphate crystals (see section on calcium phosphate). If pasteurization is not performed, the performance of the system will be reduced and at the same time the pH of the product will drop after several hours of operation due to the formation of lactic acid. The decrease in PH causes the product to become a waste product. Although pasteurization has been applied for 25 years, in order to reduce investment and operating costs, the use of this method has gradually begun to be avoided. If successful membrane filtration without pasteurization is to be successful, very detailed investigation of the operating equipment must be carried out.
 
    Calcium phosphate. Calcium phosphate or calcium hydrogen phosphate is present in all dairy products. The higher the temperature, the lower the solubility of calcium phosphate. The trouble is that many dairy products must be processed at elevated temperatures, and it is difficult to remove calcium phosphate from milk and whey before membrane filtration. Pretreatment can make the amount of precipitation within the acceptable range, but it can not completely avoid this phenomenon.
 
    The general rule of thumb is to pasteurize the dairy product prior to membrane filtration, ie heating to 72°C for 15-20 seconds and then cooling to operating temperature. If not immediately cooled, and according to the so-called “heat-keeping” method, the temperature of the product (mostly sweet cheeses) is kept at 55-60°C for one hour, a surprising amount of calcium phosphate microcrystals will form and sink precipitate. A portion of the crystals can be discharged as sludge and they will not affect the membrane process. On the contrary helps prevent further precipitation on the membrane. When the membrane system is concentrated, the concentration of calcium phosphate exceeds its solubility, and the already formed crystals will continue to grow, so that excess calcium phosphate can be removed from a large amount of liquid.
    This method can be compared to crystal seeding techniques that are well known in many other industries. Although not completely effective, it can also be operated for 20 hours in a cleaning cycle.
 
    Deformation protein. It is also a trouble in the membrane system feed solution. Minor deformations usually reduce solubility, and thorough deformations produce a substance that is similar to hard-boiled proteins. Soluble slightly denatured proteins cannot be removed by pretreatment, thus resulting in decreased flux and increased membrane fouling. The precipitated protein can be easily removed using conventional filtration methods.
Oxidation environment. The resistance of the membrane to the oxidant varies greatly. The film cannot tolerate any oxidizing environment other than hypochlorous acid. All polysulfone membranes (Desal EF and P series), PVDF membranes (Desal J series), PTFE membranes (Desal K series), and polyacrylonitrile membranes (Desal Q series) are well tolerant to various oxidants. force.
 
    Therefore, in principle, all oxidizing substances must be removed before the feed liquid enters the membrane system. What will be discussed later is only the oxidant related to the roll membrane.
² chlorine, sodium hypochlorite
 
    If chlorine or sodium hypochlorite is present in the feed solution, dechlorination pretreatment must be performed. The following method can be used:
n  The commonly used method is to add a chlorine-reducing agent such as sodium sulfite. As a result, sulfate will form in the system.
n  Activated carbon filters are also commonly used. However, it should be noted that it may be a potentially terrible source of microbial growth and biological obstruction. There is also the problem of carbon reprocessing.
n  It may be best to use UV light, but it is more expensive.
² Hypochlorous acid
 Hypochlorous acid is reported to be harmless to the film but lacks genuine convincing evidence. Hypochlorous acid is difficult to handle and it is difficult to form free chlorine, so it is rarely used in membrane filtration plants.
² Hydrogen peroxide/(H2O2), Peracetic acid
 
    Both are very weak oxidants and are used for weekly disinfection of the membrane. However, if they are in the feed, they must be removed by adding chemicals.
² Bromine, iodine
 
Similar to chlorine, the same method can be used for pretreatment.
² Nitric acid
 
    At room temperature, normal concentrations («0.5%) of nitric acid can be used for daily cleaning. However, it cannot be used at higher temperatures or at higher concentrations. We strongly recommend not using nitric acid but replacing it with other non-oxidizing acids. Better alternatives are phosphoric acid, hydrochloric acid and citric acid.
² chromate
 
    Hexavalent chromium in the feed solution is a problem because it is a fairly strong oxidant. The hazard of chromate is related to pH. The most harmful at neutral pH. The service life of the film in Ph 7 and high chromate containing systems is in weeks instead of months. Most membranes work well at pH below 2. Other pretreatment methods include: reducing hexavalent chromium, which is harmful to the film, to make it trivalent chromium. This method makes it possible to recycle chromate.
The suspended solids in the feed solution are very difficult to handle. So far, the commonly used pretreatment method is conventional filtration. Centrifugal methods are also used in the initial dairy industry.
 
    Microbes represent a special problem. Whether they are dead or alive, they can block the diaphragm. Live microorganisms may cause obstruction by reacting with a spontaneous metabolite or with bacteria in the feed solution. The cells of dead microorganisms are mutilated, mainly cell membranes, which can cause blockages. Removal of cell debris and metabolites from the material is very difficult because the various substances produced by the cells generally have the ability to resist oxidation and withstand high pH environments.
 
    Table 28 and Table 29 list the conventional methods used in the pretreatment. Although there are many pretreatment methods that can be listed, there are few methods that are commonly used. Therefore, if you need to deal with a product that is not listed here, you can use the same pretreatment method for the same products listed in this table. Although this part of the information is organized by industry, it can also be classified by the type of product as listed below.
 
E.g:
Protein: cheese, milk (female milk, goat milk, horse milk, buffalo milk), soy protein, pea protein, enzyme protein, plasma, fish bone water, and fish jelly.
Polysaccharides: Carrageenan, pectin, agar, resin, xanthan gum.
Juice: apple juice, pear juice, pineapple juice.
The pre-filtering requirements vary greatly. Tubular membranes are resistant to particles in the feed solution. All other systems require the feed solution to be filtered. Table 26 provides some guidance.

 
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