Milk proteins processing

Phosphates, which react with calcium to reduce the calcium ion activity, assist in stabilizing calcium-sensitive proteins, eg caseinate and soy proteinate, during processing. Phosphates also react with milk proteins. The extent of the reaction depends upon chain length. Casein precipitates upon addition of pyrophosphates, whereas whey proteins do not. Longer-chain polyphosphates cause the precipitation of both casein and whey proteins. These reactions are complex and not fully understood. Functions of phosphates in different types of dairy substitutes are summarized in Table 9 (see also Food additives). 

Ultrafiltration (UF) is used for the separation and concentration of macromolecules and colloidal particles. Ultrafiltration membranes usually have larger pore sizes than RO membranes, typically 1 to 100 nanometer (nm). Operating pressures are generally low (30-100 psig). Applications include electropaints, gray water, emulsions, oily wastes, and milk, cheese, and protein processing. 

Singh, R. K., Nielsen, S. S., Chambers, J. V., Martinez-Serna, M., and Villota, R. (1991). Selected characteristics of extruded blends of milk protein raffinate or nonfat dry milk with com flour. /. Food Process. Preserv. 15, 285-302. 

The protein content of milk is primarily influenced by the breed of cow, the stage of lactation, type of diet being fed and the health status of the cow, and is important in processing because the protein (and specifically casein) content of milk determines its cheese yield. Milk provides a highly digestible source of protein for a large proportion of the world s population, either as raw milk or processed into dairy products. In addition to this basic nutrition, milk... 

Milk is biochemically well characterized, and the physicochemical properties of the major native milk proteins of various species are well known. This helps rational development of appropriate downstream processing protocols (Table 5.8). 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

Gobbetti, M., Stepaniak, L., Angelis, M. D., Corsetti, A., and Cagno, R. D. (2002). Latent bioactive peptides in milk proteins Proteolytic activation and significance in dairy processing. Crit. Rev. Food Sci. Nutr. 42,223-239. 

Milk protein products. As indicated in Table 1, the food industry is placing major emphasis on the production and utilization of milk protein products in a wide variety of formulated food products (20,21,22). Although nonfat dry milk (NFDM) and whey powder are major milk protein ingredients in formulated foods, casein and whey protein concentrates, which contain their proteins in a more highly concentrated and functional form, are essential for certain food product applications, such as those products that require the proteins as an emulsifier agent. Additional details on the processing methods and conditions used to produce the various milk protein products are available (23). 

Whey protein concentrates (WPC), which are relatively new forms of milk protein products available for emulsification uses, have also been studied (4,28,29). WPC products prepared by gel filtration, ultrafiltration, metaphosphate precipitation and carboxymethyl cellulose precipitation all exhibited inferior emulsification properties compared to caseinate, both in model systems and in a simulated whipped topping formulation (2. However, additional work is proceeding on this topic and it is expected that WPC will be found to be capable of providing reasonable functionality in the emulsification area, especially if proper processing conditions are followed to minimize protein denaturation during their production. Such adverse effects on the functionality of WPC are undoubtedly due to their Irreversible interaction during heating processes which impair their ability to dissociate and unfold at the emulsion interface in order to function as an emulsifier (22). 

The following factors appear to control the emulsification properties of milk proteins in food product applications 1) the physico-chemical state of the proteins as influenced by pH, Ca and other polyvalent ions, denaturation, aggregation, enzyme modification, and conditions used to produce the emulsion 2) composition and processing conditions with respect to lipid-protein ratio, chemical emulsifiers, physical state of the fat phase, ionic activities, pH, and viscosity of the dispersion phase surrounding the fat globules and 3) the sequence and process for incorporating the respective components of the emulsion and for forming the emulsion. 

In the fractionation of the milk proteins, usually the first step in the process is to separate the so-called whole casein from the whey in a skim milk. A number of procedures are available (McKenzie 1971C), but the most commonly used method is based upon classical acid precipitation at the pH of minimum solubility. Several different temperatures have been employed 2, 20, and 30°C. Except for precipitation at 2°C, where minimum solubility occurs at pH 4.3, the skim milk is adjusted to pH 4.5-4.6 with hydrochloric acid (1 M). A more recent investigation of the relationship of temperature and pH to the completeness of casein precipitation indicated that optimum yield was obtained at pH 4.3 and 35°C (Helesicova and Podrazky 1980). 

Parker, T. G. and Dalgleish, D. G. 1977B. The potential application of the theory of branching processes to the association of milk proteins. J. Dairy Res. 44, 79-84. 

The mix is then homogenized at 105 to 210 kg/cm2 (1500 to 3000 lb/in2) to subdivide milk fat globules to sizes ranging from 0.5 to 2 m in diameter. This process is essential to produce a mix with adequate aeration properties so that the final product will contain < 175- m-diameter air cells to contribute a smooth texture. The homogenized mix is cooled and aged to fully hydrate the hydrocolloids, e.g., milk proteins, stabilizers and corn sweetners, and to provide adequate viscosity to the mix. 

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