Secondary oxidation products acid value

Secondary oxidation products oxidation indices, 656, 665-72 acid value, 672 anisidine value, 656, 666 carbonyl compounds, 656, 669-71 conjugated dienes value, 671-2 thiobarbituric acid reactive substances, 656, 666-9... 

It should be noted that both linoleic and a-linolenic acids form hydroperoxides that absorb UV radiation at 233 nm (i.e., the same wavelength as that of CDs). Furthermore, CDs are formed upon decomposition of hydroperoxides from a-linolenic acid, absorbing at 233 nm, whereas secondary oxidation products, particularly ethylenic diketones and a-unsatu-rated ketones, show a maximum absorbance at -268 nm. Carotenoid-containing oils may interfere in the assay by giving higher than expected absorbance values at 233 nm, due to the presence of double bonds in the conjugated structures of carotenoids. 

Although the work just described used canola oil, similar increases in free fatty acid levels as a consequence of higher bleach temperatures have been reported for soya (118) and pahn (86, 119) oils. The latter workers also reported decreased peroxide values but increased anisidine values as bleach temperature was increased. This conforms to expectation because the anisidine test is diagnostic for shorter chain aldehydes (including alkenals and dienals), which are secondary oxidation products of peroxide decomposition. 

AOCS has a recommended practice (Cg 3-91) for assessing oil quality and stability (AOCS, 2005) for measuring primary and secondary oxidation products either directly or indirectly. For example, peroxide value analysis (AOCS method Cd 8-53) (AOCS, 2005) determines the hydroperoxide content and is a good analysis of primary oxidation products. To determine secondary oxidation products, the procedure recommends p-anisidine value (AOCS Method Cd 18-90, 2005) volatile comlb by gas chromatography (AOCS Method Cg 4-94, 2005) and flavor evaluation. (AOCS Method Cg 2-83, 2005). The anisidine value method determines the amounts of aldehydes, principally 2-alkenals and 2, 4-dienals, in oils. The volatile compound analysis method measures secondary oxidation products formed during the decomposition of fatty acids. These comlb can be primarily responsible for the flavors in oils. The... 

A variety of compounds such as hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds are formed as secondary oxidation products and are responsible for the undesirable flavors and odors associated with rancid fat." The off-flavor properties of these compounds depend on the structure, concentrations, threshold values, and food systems. Aliphatic aldehydes are the most important volatile breakdown products because they are major contributors to unpleasant odors and flavors in food products. 

A variety of compounds such as hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds are formed as secondary oxidation products and are responsible for the undesirable flavors and odors associated with rancid fat. The off-flavor properties of these compounds depend on the structure, concentration, threshold values, and the tested system. Aliphatic aldehydes are the most important volatile breakdown products because they are major contributors to unpleasant odors and flavors in food products. The peroxidation pathway from linoleic acid to various volatiles is determined in several researchs, - by using various techniques (Gas chromatography mass spectrometry, GC-MS, and electron spin resonance spectroscopy, ESR), identified the volatile aldehydes that are produced during the oxidation of sunflower oil. In both cases, hexanal was the major aldehyde product of hydroperoxide decomposition, whereas pentanal, 2-heptenal, 2-octenal, 2-nonenal, 2,4-nonadienal, and 2,4-decadienal were also identified. 

It can be seen from Equation (5.1) that the volume of steam required for deodorization is directly proportional to the system pressure and inversely proportional to the vapour pressure of the free fatty acid. Thus, a reduction in the former and an increase in the latter, brought about by increasing temperature, result in a reduction of time on temperature for a set steaming rate. This is correct for the simple reduction of fatty acid levels. However, oils vary in their content of pigments and oxidation products. Practical experience has shown that, whereas these products can be removed in the time required to reduce free fatty acid to the desired level from a good-quality feed oil, this is not so with oxidized oils. For such oils, an extended time at the selected temperature is required to allow thermal reactions to take place in which some of the oxidation products are further decomposed and the derivatives removed from the oil (Andersen, 1962 Brekke, 1980). If such oxidation products are not removed, the deodorized product will have a poorer taste and reduced oxidative stability. The limitations of this aspect of the deodorization process can be noted in the fact that to date the anisidine value, which is a measure of secondary oxidation products in the oil, is not reduced to zero. Commercial plants are currently designed for holding time on temperature of 30-120 min, but all are capable of extension. 

More recently, a series of sol-gel hydrophobized nanostructured silica matrices doped with the organocatalyst TEMPO (SiliaCat TEMPO) entered the market as suitable oxidation catalysts for the rapid and selective production of carbonyls and carboxylic acids. In the former case, SiliaCat TEMPO selectively mediates the oxidation of delicate primary and secondary alcohol substrates into valued carbonyl derivatives (Scheme 5.2), retaining its potent activity throughout several reaction cycles (Table 5.2).33 Using this catalyst, for example, enables the synthesis of extremely valuable a-hydroxy acids with relevant selectivity enhancement by coupling of SiliaCat TEMPO with rapid Ru04-mediated olefin dihydroxylation (Scheme 5.3).34... 

Recovery of vanadium with peroxygens involves both oxidation and com-plexation. In solution, conversion of lower oxidation states into vanadium(V) allows separation by solvent extraction (Figure 6.18).269 This chemistry can be used for vanadium by-products in uranium extractions. With hydrogen peroxide, vanadium(IV) is not oxidized in acidic solution, but rather in alkaline conditions, e.g. 60 °C at pH 9 (Figure 6.19).270 Use of excess hydrogen peroxide readily forms peroxo complexes and this is of value in selective dissolution of vanadium from secondary sources. 

Shelf Storage Test The test material is stored under similar conditions as in retail and is evaluated for the effectiveness of antioxidants in prolonging the premium quality of the product. Periodic evaluation of the hpid oxidation products (primary or secondary) by chemical tests (e.g., peroxide value, conjugated diene value, 2-thiobarbituric acid reactive substances, hexanal content) or sensory evaluation will be used to find out the onset of oxidation. The main drawback of this kind of evaluation is the time taken therefore, rapid evaluation or accelerated methods are often preferred (19, 51). 

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