Physical Factors Affecting Oxidative Changes

Physical Factors Affecting Oxidative Changes 1. Oxygen Tension 

Niell and Hastings (1925) demonstrated more rapid conversion of hemoglobin to methemoglobin at intermediate rather than at very high or very low oxygen tensions. This was true not only for the spontaneous oxidation of laked blood corpuscles but also for the oxidation of hemoglobin catalyzed by unsaturated fats. 

Light accelerates all oxidative changes in meats, provided, of course, that oxygen is available. Discolorations caused by exposure to light have become a particularly serious problem because of modern methods of merchandising which require exposure of retail outs in lighted display oases. Cured meats are much more susceptible to light discoloration than fresh (Ramsbottom et al., 1951 Urbain and Ramsbottom, 1948). Appar- 

As might be expected, the oxidation rate of both heme pigments and fat of meat is accelerated with increasing temperature. Lea (1939) has reviewed the earlier literature on temperature coefficients of fat oxidation. 

In general, the oxidation rate of pure dry oils was approximately doubled by a 10 C. (18° F.) rise in temperature in the absence of catalysts. In the presence of light or metal catalysts the coefficient was much smaller. There is no published information on the temperature coefficient of the coupled reaction between hemoglobin and unsaturated fat. Numerous more recent studies on frozen meats and poultry have emphasized the importance of low storage temperatures in retarding rancidity (Cook and White, 1939, 1941 Ramsbottom, 1947 Atkinson et al., 1947 Hall et al., 1949 Klose et al., 1950 Palmer et al., 1953). 

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With rubber base adhesives, it is necessary to prevent their properties from changing during service life. Oxidative changes induced by thermal, ozone exposure and UV light can dramatically affect service life of rubber base adhesives. More precisely, the rubber and the resin are quite susceptible to oxidative degradation. Environmental and physical factors exert detrimental effects on rubber base adhesive performance. These effects can be mitigated by the incorporation of low levels of stabilizers during the fabrication process of the adhesive. 

The physical and chemical characteristics of zinc oxide powders are known to affect cement formation (Smith, 1958 Norman et al., 1964 Crisp, Ambersley Wilson, 1980 Prosser Wilson, 1982). The rate of reaction depends on the source, preparation, particle size and surface moisture of the powder. Crystallinity and lattice strain have also been suggested as factors that may change the reactivity of zinc oxide powders towards eugenol (Smith, 1958). 

Not only can absorption alter the aroma and flavor of a product, it can also change the mechanical properties of the polymer. Swelling and gas permeability are factors that effect the physical properties of a polymer (Robertson, 2006 Sadler and Braddock, 1991 Safa and Bourelle, 1999). Swelling occurs when compounds are absorbed into the polymer and distort the shape of the package. As absorption increases there is also a subsequent increase in gas permeability. This increase in gas permeability can affect the shelf-life and sensory quality of a food by, for example, increasing oxidation. In very severe cases, absorption can affect package integrity. 

The hydroxyl radicals react with the SAMs and locally change the chemical and physical properties of the monolayer surface. Diaphorase patterns are then formed on the substrates by physical adsorption onto the hydrophobic area or by chemical linkage to the hydroxyl radical-attacked area (Figure 12.30). Diaphorase activity can be visualized using SECM by detecting the diaphorase-catalyzed current of feirocenylmethanol coupled with the oxidation of reduced nicotinamide adenine dinucleotide (135). The size of the hydroxyl-radical-attacked area, which determines the resolution of the enzyme patterns, is affected by different factors, including the tip size, the concentration of Fe +, and the potential pulse period for generation of the hydroxyl radical. 

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