Milk sulfhydryl groups

Bovine IgM as identified by immunoelectrophoresis is a pentamer of four-chain units linked together by disulfide bonds between the constant C-terminal regions of the heavy chains of the monomers and has a carbohydrate content of 12.3% (Kumar and Mikalajcik 1973). Each mole of the pentamer contains one mole of covalently bound J-chain and a maximum of 1.2 moles of noncovalently bound J-chain. The J-chain has a molecular weight of 16,500 and contains 9.7 sulfhydryl groups per mole (Komar and Mukkur 1974). The IgM output in bovine milk in mid-lactation is approximately 1 g/day (Guidry et al. 1980). 

Lactoglobulin has been shown by Larsson and Jenness (1950) to be the major source of sulfhydryl groups in milk, while the fat globule membrane material contributes a minor portion of these reducing compounds. This finding was confirmed by Hutton and Patton (1952). 

Hutton, J. T. and Patton, S. 1952. The origin of sulfhydryl groups in milk proteins and their contributions to cooked flavor. J. Dairy Sci. 35, 699-705. 

Taylor, M. J. and Richardson, T. 1980A. Antioxidant activity of skim milk Effect of heat and resultant sulfhydryl groups. J. Dairy Sci. 63, 1783-1795. 

The effects of heat treatment of milk on the oxidation-reduction potential have been studied to a considerable extent (Eilers et al 1947 Gould and Sommer 1939 Harland et al 1952 Josephson and Doan 1939). A sharp decrease in the potential coincides with the liberation of sulfhydryl groups by denaturation of the protein, primarily /3-lactoglobulin. Minimum potentials are attainable by deaeration and high-temperature-short-time heat treatments (Higginbottom and Taylor 1960). Such treatments also produce dried milks of superior stability against oxidative flavor deterioration (Harland et al 1952). 

Serum albumin has 35 cystein residues which are found as 17 intrachain disulfide linkages and one free sulfhydryl group. Except for the immunoglobulins, serum albumin is the largest milk protein (Walstra and Jenness 1984). 

Although batch and HTST pasteurization produces a cooked flavor in milk by activation of whey protein sulfhydryl groups, it is not severe... 

Effect of heat treatment on the sulfhydryl groups in skim milk and non-fat dry milk. J. Dairy Sci., 36, 427 (1953). With G. Zweig. 

A mixture of emulsifiers can improve the stability of colloid food systems by strengthening the film membrane and by interaction between emulsifiers at the interface. Combinations of oil-soluble and water-soluble surfactants (Spans and Tweens) are often used in food formulations to produce more stable oil-inwater emulsions. Mixtures of phospholipids and proteins in foods undergo complex interactions during various thermal processing that can either promote or retard oxidation. At elevated temperatures phospholipids can produce antioxidant substances by the browning reaction, and proteins can denature to release reducing sulfhydryl groups that increase the oxidative stability of heated foods (e.g. evaporated milk). 

There is another phenomenon, regarded as a deteriorative change in the protein of soy milk, caused also by the evaporation of water. This is a film formation on the surface of soy milk, which occurs when heated soy milk is kept open to the air. This phenomenon is observed not only in heated soy milk but also in heated cow s milk. Film formation of soy milk occurs only when the soy milk is heated above 60°C and there is evaporation of water from the surface of the soy milk. The mechanism of protein insolubilization is basically the same as that of soy milk powder produced from heated soy milk (10. When water is removed from the surface of heated soy milk by evaporation, the molecular concentration of protein near the surface increases locally and the exposed reactive groups of the denatured molecules come close enough to interact intermolecularly both by hydrophobic interactions and through the sulfhydryl/disulfide interchange reaction to form a polymerization (film) on the surface. The upper side of the film contains more hydrophobic amino acids because of orientation of the hydrophobic portions of the unfolded molecules to the atmosphere rather than into the aqueous solution. 

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