2. Read and translate the text into your native language: Membrane Processes in the Food Industry Microfiltration

As a solid–liquid separation process, microfiltration is increasingly becoming a preferred alternative to the more expensive traditional processes of dead-end filtration and centrifugation. In the food industry, microfiltration is extensively used for the clarification of cloudy fluids. Membranes with nominal pore sizes in the order of 0.1 to 0.5 μm or less produce permeates that are practically free of microorganism cells.

Consequently, microfiltration is being increasingly used for the purification of drinking water and of water for the production of soft beverages. Food fluids clarified by MF include clear fruit juices, wine, vinegar and sugar syrups.

Microfiltration frequently serves for the pre-treatment of fluids, before ultra filtration or reverse osmosis. Removal of suspended particles and colloidal material by MF is essential for reducing the rate of fouling in subsequent UF or RO steps. Oil droplets and particles of fat are also removed by MF. Membranes made of hydrophobic polymers, such as polyvinylidene fluoride, are particularly suitable for that application.

Brines used in the manufacture of cheese and in fish processing contain suspended solids and fats, which must be removed before recycling or disposal of the fluid. Other wastewater streams generated in food processing often require pre-filtration before further treatment. MF is used with advantage in such applications.

Ultra filtration

Dairy applications: with the development of improved membranes, ultra filtration has become a major separation process in the dairy industry. The two largest areas of application are the pre-concentration of milk for cheese manufacture and the production of protein concentrates from whey. Both applications are related to cheese-making.

Whole cow’s milk contains typically 3.5% protein, 4% fat, 5% lactose and 0.7% inorganic salts (ash). The balance (about 87%) is water. In the traditional cheese-making process, milk (usually, but not always, after adjustment of fat content and pasteurization) is coagulated through a combination of lactic acid fermentation and enzymatic reactions. After cutting and temperature adjustment, the coagulated mass separates into solid particles (curd) suspended in liquid (whey). The curd contains the major milk protein – casein, and most of the fat. The whey is essentially a dilute aqueous solution of lactose, mineral salts and non-casein milk proteins (e.g. lactalbumins and lactoglobulins). The curd is subjected to various methods of treatment and curing, resulting in the vast variety of different cheeses. The whey is usually a problematic waste, due to its high volume (often 90% of the mass of milk used) and its high cost of disposal due to its extremely high biological oxygen demand (BOD).

Ultra filtration of milk retains the fat globules and the proteins. The inorganic salts and the lactose, along with some of the water are partially removed as permeate. The resulting retentate is partially concentrated milk with reduced lactose and mineral content. The concentration ratio is often 3- to 5-fold. If further reduction of lactose and mineral content is desired, the retentate is diluted with water and ultra filtered once again. This kind of ultra filtration is sometimes called “diafiltration”. The pre-concentrated milk is mainly used in the manufacture of many types of cheeses (most commonly soft cheeses). The advantages of using UF pre-concentrated milk for making cheese are (Cheryan, 1986):

• Increased yield, probably because of the inclusion of some of the non-casein proteins in the curd

• Lower energy consuption

• Reduced volume of whey.