Modification of Milk Proteins

Primary structure of proteins can be chemically modified in order to improve their functional properties. This approach has been used with success to study the structure-function relationships (enzymatic function, biological function, physico-chemical and functional properties). Deliberate chemical modification of food proteins can result in alteration of the nutritive value, formation of potentially toxic amino acid derivatives, and contamination by toxic chemicals. 

Phosphorylation is an effective way to increase negative charges in a protein molecule and thereby to improve functionality, particularly solubility. Either O- or N-esterification reactions can transfer inorganic phosphate (P ) 

Enzymatic phosphorylation by phosphorylases and phosphatases produces phosphoesters such as phosphoserine and phosphothreonine. Chemical phosphorylation of proteins changes their functional properties, improving them sometimes (Yoshikawa et al., 1981 Hirotsuka et al., 1984 Huang and Kinsella, 1986 Chobert etal, 1989 Matheis, 1991). However, the properties of the phosphorylated proteins depend entirely on the degree of denaturation and substitution defined by the reaction conditions and the protein (Medina etal, 1992 Sitohy etal, 1994). Casein was phosphorylated by the commonly used methods, characterized by use of excessive amounts of phosphorus oxychloride and with important additions of concentrated inorganic bases (Matheis et al, 1983 Medina et al, 1992). Thus, obtained phosphorylated caseins were highly cross-linked and partially insoluble and difficult to characterize. Hence, there arose a need to produce monomeric over-phosphorylated caseins more suitable for use and for study of their 

The SDS-PAGE patterns of the same samples showed small intermole-cular associations in the phosphorylated caseins especially when compared with the results obtained by Matheis et al. (1983) and Medina et al. (1992) whose phosphorylated samples were entirely unable to enter the SDS-PAGE gel due to high cross-linking. 

The possibility of using basic amino acids in the form of free bases as the only base of the reaction was studied in order to eliminate the use of tertiary amines, which are nutritionally unacceptable (Sitohy et al., 1995b,c Haertle and Chobert, 1999). 

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Modification of Milk Protein during Lactic Acid Fermentation..209... 

Most chemical agents used for studying the chemical modification of proteins are not suitable for food applications. These studies nevertheless demonstrate how changes in amino acid side chains, and both the structure and conformation of proteins can impact on functionality. A comprehensive review of chemical modification of milk proteins has been carried out (Chobert, 2003). Only some highlights and more recent work on modification with chemical agents are covered here. 

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. 

The properties of many dairy products, in fact their very existence, depend on the properties of milk proteins, although the fat, lactose and especially the salts, exert very significant modifying influences. Casein products are almost exclusively milk protein while the production of most cheese varieties is initiated through the specific modification of proteins by proteolytic enzymes or isoelectric precipitation. The high heat treatments to which many milk products are subjected are possible only because of the exceptionally high heat stability of the principal milk proteins, the caseins. 

Iametti, S. and Bonomi, F. 1993. Monitoring the surface hydrophobicity of milk proteins A realtime study on heat-induced modifications. Int. Diary Fed. Spec. Issue 9303 111-116. 

Elimination of Immunoreactive Properties of Milk Proteins in Chemical Modifications (1 g of Modifying Agent/g Protein)... 

Both the protein and fat components in milk influence the properties of food, but the ability of the milk to impart desirable properties to food is mostly influenced by the physical functional properties of the milk protein components (Kinsella, 1984 Mulvihill and Fox, 1989). The inherent functionality of milk proteins is related to the structural/ conformational properties of protein, which is influenced by both the intrinsic properties of the protein and extrinsic factors. Modification of the protein composition or structure and the organization of the proteins within the dairy ingredient through the application of physical, chemical, or enzymatic processes, alone or in combination, enable the differentiation of the functionality of the ingredient and designing the required functionality for specific applications (Chobert, 2003 Foegeding et al., 2002). 

Most of the current interest in hydrolysis of milk proteins is directed at the production of bioactive peptides. This aspect is not covered here but reviews provide an update of these interests (Korhonen and Pihlanto, 2006). Hydrolysis of proteins for modification of functionality has been also covered by reviews (Chobert, 2003 Foegeding et al., 2002 Kilara and Panyam, 2003). 

Some authors have focused on opportunities for genetic manipulation of dairy cattle beyond BST (79). Opportunities include production of heterologous proteins in milk (20,27), modification of milk and milk protein composition (22,25), and genetic modification of the rumen or intestinal microflora of cows to improve their digestive capacity, feed efficiency, or ability to degrade plant toxicants (24,25). 

Allergenicity of proteins of milk and various (heat-treated, fermented, and enzymatically modified) milk products was determined in vitro by ELISA [203]. Food processing like heat treatment or fermentation does not reduce the allergenic character of the proteins. The enzymatic modification, however, showed a significant reduction in the allergenicity of milk proteins (Fig. 7). 

Thus, the synthesis and secretion of milk proteins involves eight steps transcription, translation, segregation, modification, concentration, packaging, storage and exocytosis, as summarized schematically in Figure 4.35. 

Mating of C females with B6 males caused pronounced modification of milk composition milk from C females that had given birth to hybrids contained a higher percentage of proteins and lipids compared to the control C females that mated with a C male (Figure 2). There were no effects of gestation type on milk composition in B6 females. 

Farm animals produce recombinant proteins less expensively than bacteria or cells in culture because the farm animals produce large volumes of milk containing up to 5 g/L of recombinant protein. In addition, modifications to the proteins that can be performed only by mammalian cells are made by the cells of the mammary gland. Therefore, numerous pharmaceuticals that previously could only be made by cells in culture or extracted from human tissue or blood are being produced by lactating farm animals. 

Fat Content of Milk. Raw milk as well as many dairy products are routinely analyzed for their fat content. The Babcock test, or one of its modifications, has been a standard direct measure for many years and is being replaced by indirect means, particularly for production operations. The Babcock test employs a bottle with an extended and caHbrated neck, milk plus sulfuric acid [7664-93-9] to digest the protein, and a centrifuge to concentrate the fat into the caHbrated neck. The percentage of fat in the milk is read direcflv from the neck of the bottle with a divider or caHper, rea ding to... 

Two other practical appHcations of en2yme technology used in dairy industry are the modification of proteins with proteases to reduce possible allergens in cow milk products fed to infants, and the hydrolysis of milk with Hpases for the development of Hpolytic flavors in speciaHty cheeses. 

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