Functional milk proteins

The term Functional Properties of Proteins in relation to foods refers to those physicochemical properties of a protein which affect the functionality of the food, i.e. its texture (rheology), colour, flavour, water sorption/binding and stability. Probably the most important physicochemical properties are solubility, hydration, rheology, surface activity and gelation, the relative importance of which depends on the food in question these properties are, at least to some extent, interdependent. 

Probably because of the ease with which casein can be produced from skim milk, essentially free of lipids, lactose and salts, by rennet or isoelectric coagulation and washing of the curd, acid and rennet caseins have been produced commercially since the beginning of this century. However, until relatively recently, they were used for industrial applications, e.g. in glues. 

Although heat-denatured whey protein, referred to as lactalbumin, has been available for many years for food applications, it was of little significance, mainly because the product is insoluble and therefore has limited functionality. The commercial production of functional whey protein became possible with the development of ultrafiltration in the 1960s. Whey protein concentrates (WPCs) produced by ultrafiltration are now of major commercial importance, with many specific food applications. Superior whey protein products (whey protein isolates, WPI) are being produced on a limited scale by chromatography, although their substantially higher cost has limited their production. 

As discussed in section 4.16, many of the whey proteins have interesting biological and physical properties. It is now possible to isolate individual whey proteins on a commercial scale in a relatively pure form it is likely that in the immediate future such purified whey proteins will be readily available for specific applications. 

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Mulvihill, D.M. 1994. Functional milk protein products. In Biochemistry of Milk Products (A.T. Andrews and J. Varley, eds.) Royal Society of Chemistry, London. 

PF Fox. The milk protein system. In PF Fox, ed. Developments in Dairy Chemistry —Functional Milk Proteins. London Elsevier Science, 1989, pp 1-54. 

Mulvihill, D.M., and Fox, P.F. (1989). Physico-chemical and functional properties of milk proteins. In "Developments in Dairy Chemistry—4—Functional milk proteins" (P.F. Fox, ed.), pp. 131-172. Elsevier Applied Science, London. 

Morr, C.V. (1989) Whey proteins manufacture, in Developments in Dairy Chemistry, Vol. 4 Functional Milk Proteins, (ed. P.F. Fox), Elsevier Applied Science, London, pp. 245-84. 

The mmen is not functional at birth and milk is shunted to the abomasum. One to two weeks after birth, the neonate consumes soHd food if offered. A calf or lamb that is nursing tends to nibble the mother s feed. An alternative method of raising the neonate is to remove it from its mother at a very young age, <1 week. A common example of an early weaning situation is the dairy calf that is removed from the cow soon after birth so that the cow s milk supply might be devoted entirely to production. In this instance, the neonate requires complete dietary supplementation with milk replacer. Sources of milk replacer protein have traditionally included milk protein but may also include soybean proteins, fish protein concentrates, field bean proteins, pea protein concentrates, and yeast protein (4). Information on the digestibiUty of some of these protein sources is available (4). 

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). 

The increasing interest in nutritional and functional properties of soybean protein has promoted their use in the manufacturing of foods for human consumption. Soybean products (particularly infant formulas and soybean dairy-like) may also represent an interesting substitute for infants and people allergic to milk proteins. On the other hand, due to their technological properties and low cost, soybean proteins are increasingly employed as ingredients in milk, bakery, and meat products, in which their addition is forbidden or allowed up to a certain limit. 

Milk protein system. The nomenclature and physico-chemical properties of the major milk proteins and their subunits have been provided by Whitney et al. (13) and Brunner (1 ). The conformation and related properties of the individual proteins and their subunits and aggregates have been reviewed by Morr (15) with special reference to their functional properties in food systems, and drawing heavily upon previous considerations by Bloomfield and Mead (16) and Slatterly (17). 

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). 

Co-preclpltate is an insoluble milk protein product that is produced by heating skinimllk to high temperatures ( > 90 C) to denature the whey proteins and complex them with the casein micelles. The heated system is subsequently adjusted to isoelectric point conditions of pH 4.5-5 to precipitate the complexed whey protein-casein micelles, centrifuged or filtered to recover the precipitate, washed and dryed. The resulting product, which is virtually insoluble, exhibits only minor functionality in most typical emulsification applications. 

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). 

One of the important developments in dairy technology in recent years has been the fractionation of milk into its principal constituents, e.g. lactose, milk fat fractions and milk protein products (caseins, caseinates, whey protein concentrates, whey protein isolates, mainly for use as functional proteins but more recently as nutraceuticals , i.e. proteins for specific physiological and/or nutritional functions, e.g. lactotransferrin, immunoglobulins). 

The proteins can participate in sulphydryl-disulphide interchange reactions at temperatures above about 75°C at the pH of milk, but more rapidly at or above pH 7.5. Such interactions lead to the formation of disulphide-linked complexes of / -lg with K-casein, and probably as2-casein and a-la, with profound effects on the functionality of the milk protein system, such as rennet coagulation and heat stability. 

Kinsella, J. E. 1984. Milk proteins Physicochemical and functional properties. CRC Crit. Rev. Food Sci. Nutr. 21, 197-262. 

Morr, C. V. 1982. Functional properties of milk proteins and their use as food ingredients. In Developments in Dairy Chemistry, Vol. 1 Proteins. P. F. Fox (Editor). Applied Science Publishers, New York, p. 375-397. 

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