Oxidative Deterioration Processes

Although the above mentioned add catalyzed hydrolysis is the main cause for the deterioration of cellulose and the connected loss of permanence, the influence of oxidation cannot be neglected. The oxidative damage is much more complex in its 

Cellulose can be oxidized not only by various oxidants Hke pollution gases from the air (ozone, NOx, etc.) and bleaching agents in the wood pulp production, but also by atmospheric oxygen. In the oxidation by atmospheric oxygen an additional activation or catalysis is needed. 

Acids as well as bases have a catalyzing effect. The catalysis by heavy metal ions is really effective. A well-known example of this mechanism is the ink corrosion which leads to a total destruction of paper in the area of ink due to the excess of iron ions in the ferro-gaUic ink and the acid. Some of the most valuable manuscripts are affected (Fig.13.3 and 13.4). Temperature and electromagnetic radiation, especially UV rays, have an activating effect. 

The observed reactions are often summarized under the term autoxidation as reaction products of such oxidation processes organic adds Kke ethanoic acid, formic add, oxahc add etc., cause a further addification of the paper. Due to these oxidation processes the acid catalyzed hydrolytic deterioration of cellulose is accelerated. 

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It is now widely appreciated that polyunsaturated fatty acids (PUFAs) are highly susceptible to oxidative damage. Indeed, the process of lipid peroxidation was broadly defined as the oxidative deterioration of polyunsaturated lipids by Tappel (1979). The presence of a double... 

Flavor is one of the major characteristics that restricts the use of legume flours and proteins in foods. Processing of soybeans, peas and other legumes often results in a wide variety of volatile compounds that contribute flavor notes, such as grassy, beany and rancid flavors. Many of the objectionable flavors come from oxidative deterioration of the unsaturated lipids. The lipoxygenase-catalyzed conversion of unsaturated fatty acids to hydroperoxides, followed by their degradation to volatile and non-volatile compounds, has been identified as one of the important sources of flavor and aroma components of fruits and vegetables. An enzyme-active system, such as raw pea flour, may have most of the necessary enzymes to produce short chain carbonyl compounds. 

Lipid peroxidation is a free radical-mediated, chain reaction resulting in the oxidative deterioration of polyunsaturated fatty acids (PUFAs) defined for this purpose as fatty acids that contain more than two double covalent carbon-carbon bonds. Singlet oxygen can produce lipid hydroperoxides in unsaturated lipids by non-radical processes (Pryor and Castle, 1984), but the reaction usually requires a radical mechanism (Porter, 1984). Polyunsaturated fatty acids are particularly susceptible to attack by free radicals. Lipid peroxidation is a complex process, and three distinct phases are recognized (a) initiation, (b) propagation and (c) termination (see Fig. 2.10). 

Autoxidation is the most common process leading to oxidative deterioration and is defined as the spontaneous reaction of atmospheric oxygen with lipids (3). The process can be accelerated at higher temperatures, such as those experienced during deep-fat frying, which is called thermal oxidation, with increases in free fatty acid and polar matter contents, foaming, color, and viscosity (4). Lfnsaturated fatty acids... 

The thiobarbituric acid (TBA) test was proposed over 40 years ago and is now one of the most extensively used methods to detect oxidative deterioration of fat-containing foods (41). During lipid oxidation, malonaldehyde (MA), a minor component of fatty acids with 3 or more double bonds, is formed as a result of the degradation of polyunsaturated fatty acids. It is usually used as an indicator of the lipid oxidation process, both for the early appearance as oxidation occurs and for the sensitivity of the analytical method (42). In this assay, the MA is reacted with thiobarbituric acid (TBA) to form a pink MA-TBA complex that is measured spectrophotometrically at its absorption maximum at 530-535 nm (Figure 2) (9,43,44). The extent of oxidation is reported as the TBA value and is expressed as milligrams... 

The oxidative deterioration of most commercial polymers when exposed to sunlight has restricted their use in outdoor applications. A novel approach to the problem of predicting 20-year performance for such materials in solar photovoltaic devices has been developed in our laboratories. The process of photooxidation has been described by a qualitative model, in terms of elementary reactions with corresponding rates. A numerical integration procedure on the computer provides the predicted values of all species concentration terms over time, without any further assumptions. In principle, once the model has been verified with experimental data from accelerated and/or outdoor exposures of appropriate materials, we can have some confidence in the necessary numerical extrapolation of the solutions to very extended time periods. Moreover, manipulation of this computer model affords a novel and relatively simple means of testing common theories related to photooxidation and stabilization. The computations are derived from a chosen input block based on the literature where data are available and on experience gained from other studies of polymer photochemical reactions. Despite the problems associated with a somewhat arbitrary choice of rate constants for certain reactions, it is hoped that the study can unravel some of the complexity of the process, resolve some of the contentious issues and point the way for further experimentation. 

The deterioration of metals caused by an oxidation-reduction process is termed corrosion. Metal atoms are converted to metal ions the structure, hence the properties, changes dramatically, and usually for the worse (Figure 9.8). 

Autoxidation is the process in foods and bulk lipids, which leads to rancidity. Rancidity is the spoiled off-flavor obtained by subjective organoleptic appraisal of food. Autoxidation is the oxidative deterioration of unsaturated fatty acids via an autocatalytic process consisting of a free radical mechanism. This indicates that the intermediates are radicals (odd electron species) and that the reaction involves an initiation step and a propagation sequence, which continues until the operation of one or more termination steps. Autoxidation of lipid molecules is briefly described by reactions 1-3. ... 

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