In the basic steps in milk processing, second step is separation. The purpose of the separation step is separate raw milk into cream and skim milk. Raw milk is transferred to separator, which is a high-speed centrifuge machine. Lighter part of raw milk is separated from heavier skim milk fraction and stored as cream. Skim milk and cream are stored separately for further processing. At the next step, standardization, these two fractions of raw milk may be mixed by blending to produce products that require milk fat content [1].
Skim milk is the raw material of ice cream, frozen yogurt, and frozen desserts. And also it is commonly used as a source of milk solids in dairy applications [2]. Since fat content is separated from skim milk, some vitamins such as A and D, dissolving in fat, must be added.
Skim milk can given as an example to colloidal suspension of caseins, formed by four casein proteins and calcium phosphate, which are dispersed in an aqueous solution of salts, lactose and whey proteins [3]. Casein protein makes milk supersaturated with calcium phosphate. Packaging of the calcium phosphate is one of the biological functions of the micelle [4]. As a result solute dispersed in liquid phase is these micelles and liquid phase is water that is one of the major components with 86% weight fraction.
A casein micelle [5]
At the figure, representation of casein micelle is shown where A molecules are sub-micelle, B molecules are protruding chains, C molecules are calcium phosphate, D molecules are casein and E molecules are phosphate groups.
Homogenization is an important step for increasing stability of both milk and skim milk. After homogenization, the size of the fat globules and casein micelles is reduced to 400-600 nm and 300 nm [6]. Homogenization reduces size of particles by pumping milk at high pressure through a small orifice. Homogenizer utilizes particles turbulence and cavitation. That causes forming of particles that is in very large number and small size. This process is called single-stage homogenizer as a result of usage of a single valve. It is preferable for low-fat products. On the other hand, there is another process that particle sizes are reduced in two steps. This process is called dual-stage homogenizer. At the first step of two steps, milk is subjected to high pressure. At the second step, milk is subjected to relatively lover pressure. This process is suitable for milk that has high fat or not fat content [7].
References:
1 Chandan, R. (2011). Dairy ingradients for food processing: An owerview. In R. Chandan & A. Kilara (Eds.),Dairy ingradients for food processing (1 ed., Vol. 1, p. 5). Iowa: Blackwell Publishing Ltd.
2 Chandan, R. (2011). Dairy ingradients for food processing: An owerview. In R. Chandan & A. Kilara (Eds.),Dairy ingradients for food processing (1 ed., Vol. 1, p. 18). Iowa: Blackwell Publishing Ltd.
3-6 Dahbi, L., Alexander, M., Trappe, V., Dhont, J. K. G., & Schurtenberger, P. (2010). Rheology and structural arrest of casein suspensions. (2 ed., Vol. 342, pp. 564-570).
4 Thompson, Abby; Boland, Mike; Singh, Harjinder (2009). Milk Proteins - from Expression to Food. (pp: 143). Elsevier. Online version available at:
http://www.knovel.com/web/portal/browse/display_EXT_KNOVEL_DISPLAY_bookid=3003&VerticalID=0
5 http://www.food-info.net/uk/protein/milk.htm
7 Chandan, R. (2011). Dairy ingradients for food processing: An owerview. In R. Chandan & A. Kilara (Eds.),Dairy ingradients for food processing (1 ed., Vol. 1, p. 12). Iowa: Blackwell Publishing Ltd.
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