Oxidation, radical species present during

The changes in the protein composition and flavor of "cooked" and "cooked- -stored" beef seemed to be related to degradation by free radical species induced during lipid oxidation 1-10, 16, 22, 23), Based on the information presented in the aforementioned publication, it seemed reasonable to suggest that the appearance of "bitter" and "sour" tastes and the disappearance of "meaty" and "beefy" flavors were a result of the activity of the free radicals derived from lipid oxidation on flavor proteins (2, 24-28),... 

The pyridinyl radical dimer (2-2) is formed by brief reduction (0.5-8.0s) of l-methyl-2-carbomethoxypyridinium ion 2 ) around an Hg drop. A rapid jump (tp = 0.8-90 ms) to a potential suitable for oxidation of any new species present near the electrode 1 1 to an increase during irradiation (Fig. 8) in the amount of monomeric pyridinyl radical (2 ) detected. The detection technique is termed reverse pulse polarography, RPP (see below). Variation of the wavelength of irradiation yielded a variation in the excess current, corrresponding to a photodissociation spectrum similar to the alsorption spectrum of the dimers (Fig. 9). 

The lead oxide aerosol is finely divided, with a particle size <10 nm [44], but compared with gas phase substances it is much less available for reaction. Why then is it so effective compared with ashless materials Two possibilities are first, that it reacts with species which are present in larger amounts than OH, the radical which is mainly removed by the ashless materials and second, that it acts catalytically - the lead oxide is not destroyed during the removal of radicals and goes on to react with more. 

When the polypropylene sample is irradiated in air, as is the case during a sterilization cycle, the oxidative process is initiated at the onset of irradiation. As a consequence of oxygen diffusion during irradiation a spectrum intermediate between the alkyl and peroxy radicals is generally observed (Figure 3). Whether oxygen is present during irradiation or added later, the peroxy radical becomes the dominant radical species with time. 

An outline of the polymerization scheme for polypyrrole is given in Fig. 1, The mechanism for polymerization can be deduced from several observations. First, a radical cation produced by the initial oxidation of pyrrole can either react with another radical cation to produce a pyrrole dimer or make an electrophilic attack on a neutral molecule. The coupling of two radicals is favored over electrophilic attack, since the electrochemical polymerization reaction occurs only when a sufficient potential is applied to oxidize the polymer, and at the potentials normally applied the amount of neutral species present at the electrode surface is essentially zero. Second, the amount of charge consumed during polymer formation has an initial linear time dependence and is independent of pyrrole concentration during constant-potential electrolysis [20,40]. This evidence indicates that the entire electrochemical reaction occurs at the electrode surface. Third, the dimer (bipyrrole). 

Studies on the photochemical stability achieved of polymeric materials upon addition of mineral fillers yielded in some unexpected results, since the decrease in properties is, in some cases, more intense after the UV exposure, in comparison with unfilled materials [80-82]. Thus, for PP and PE filled with clays (montmo-rillonite, MMT) [9] such behavior was explained considering the very small amounts of metal ions (i.e., Fe " ) present as impurities in clays and which can act as catalysts in UV-initiated oxidation reactions that entails accelerated degradation. At the same time, hydroperoxides formed during photo-oxidation are a source of free radical species able to further promote the degradation. Even more, light stabilizers effectiveness was found to be much reduced in polymer-MMT composites, maybe because these molecules were adsorbed on the surface of clay platelets so that their action was diminished or even blocked. Such results were reported for PC/MMT composites [5], as well as for PA6/MMT [83]. 

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