Linolenic acid, autoxidation

The volatiles produced by the LOX pathway and autoxidation are typically volatile aldehydes and alcohols responsible for fresh and green sensorial notes. In the LOX pathway these volatile compounds are produced in response to stress, during ripening or after damage of the plant tissue. The pathway is illustrated in Scheme 7.2. Precursors of the LOX (EC 1.13.11.12) catalysed reactions are Cis-polyunsaturated fatty acids with a (Z,Z)-l,4-pentadiene moiety such as linoleic and a-linolenic acids that are typically oxidised into 9-, 10- or 13-hydro-peroxides depending on the specificity of the LOX catalyst. These compounds are then cleaved by hydroperoxide lyase (HPL) into mainly C, C9 and Cio aldehydes, which can then be reduced into the corresponding alcohols by alcohol dehydrogenase (ADH EC 1.1.1.1) (Scheme 7.2) [21, 22]. The production of volatile compounds by the LOX pathway depends, however, on the plants as they have different sets of enzymes, pH in the cells, fatty acid composition of cell walls, etc. 

Litwinienko, G, Kasprzyska-Guttman, T., and Studzinski, M. 1997. Effects of Selected Phenol Derivatives on the Autoxidation of Linolenic Acid Investigated by DSC Non-Isothermal Methods. Thermochim. Acta., 307, 97-106. 

Hydrogen abstraction also increases at elevated temperature as thermal energy decreases bond dissociation energy. Typical H abstraction rates for ROO at room temperature are < 1 M s, but this increases to 10 -10" L M s at 65°C (223). For example, in linolenic acid autoxidized neat at room temperature to PV 1113, products were not quantified, but estimates from intensities of HPLC peaks gave about 40% LnOOH, 12% dihydroperoxides, 12% hydroperoxy epidioxides, and 4% epoxides (228). At 40°C, H abstraction occurred more as a secondary process. Hydroperoxides per se were still the main products, but fewer were present as mono- and dihydroperoxides (36% total) and more had formed after cyclization or addition (31%). Data are not available to distinguish whether this... 

The development of a characteristic, objectionable, beany, grassy, and hay-like flavor in soybean oil, commonly known as reversion flavor, is a classic problem of the food industry. Soybean oil tends to develop this objectionable flavor when its peroxide value is still as low as a few meq/kg, whereas other vegetable oils, such as cottonseed, com, and sunflower, do not (15, 51). Smouse and Chang (52) identified 71 compounds in the volatiles of a typical reverted-but-not-rancid soybean oil. They reported that 2-pentylfuran formed from the autoxidation of linoleic acid, which is the major fatty acid of soybean oil, and contributes significantly to the beany and grassy flavor of soybean oil. Other compounds identified in the reverted soybean oil also have fatty acids as their precursors. For example, the green bean flavor is caused by c/i-3-hexenal, which is formed by the autoxidation of linolenic acid that usually constitutes 2-11% in soybean oil. Linoleic acid oxidized to l-octen-3-ol, which is characterized by its mushroom-like flavor (53). 

Because linolenic acid is much more subject to autoxidation than is linoleic acid, nutritionists and industrial laboratories attempted to eliminate linolenic acid from food formulations to minimize rancidity during storage. Autoxidation was the great enemy for designers of stable foods, and a lifetime of effort was required to breed linolenic acid out of soybeans sufficiently to make soybean oil a more stable and convenient component of industrial products intended for human consumption. 

This aldehyde is a product of autoxidation of linolenic acid (Grosch, 1998a), not linoleic as quoted in the publication by Full et al., which is present in green coffee, representing 1.1 1.5% of the total acid content of immature or mature beans (Guyot et al., 1988a). 

During the refining, the sensory value of the oil is substantially improved but the nutritional value is impaired by partial removal of carotenoids, tocopherols, and phytosterols, and by moderate isomerization of polyunsaturated acyls into their cis,trans isomers linolenic acid is especially sensitive (Cmolik et al., 2000). The amount of trans isomers should not exceed 1%. Refined oils are nearly flavorless, tasteless, and colorless, and possess good stability against autoxidation. 

At least one vitamin E deficiency sign is known to occur only when a certain class of polyunsaturated fatty acids is furnished in the diet. For instance, encephalomalacia in chicks appears when the diet contains linoleic or arachidonic acid, but not when the diet is fat free or contains linolenic acid. It is possible that encephalomalacia is due to the lack of an antioxidant effect, but attempts to demonstrate autoxidation products in the affected tissue have not been successful. 

Enzymatic or autoxidative decomposition of imsaturated fatty acids, mainly linoleic and linolenic acids could lead to the formation of aldehydes in peas (Homostaj Robinson, 2000 Barra et al., 2007). Differences observed in the concentration of these carbonyl compounds could be due to differences in linoleate compositions in pea cultivars (Oomah ... 

Meijboom, P.W. and Stroink, J.B.A. (1972) 2-trans,4-cis,7-cis-decatrienal, the fishy off-flavour occurring in strongly autoxidized oils containing linolenic acid or o)3,6,9, etc., fatty acids. JAOCS 49, 555-558. 

Off-Flavor Occurring in Strongly Autoxidized Oils Containing Linolenic Acid or (0-3,6,9, etc. Fatty Acids. J. Amer. Oil Chem. Soc. 49, 555 (1972). 

The hypothesis presented in Fig. 3.19 is valid only for the initiation phase of autoxidation. The process becomes less and less clear with increasing reaction time since, in addition to hydroperoxides, secondary products appear that partially au-toxidize into tertiary products. The stage at which the process starts to become difficult to survey depends on the stability of the primary products. It is instructive here to compare the difference in the stractures of monohydroperoxides derived from linoleic and linolenic acids. 

Autoxidation of linolenic acid yields four monohydroperoxides (Table 3.28). Formation of the monohydroperoxides is easily achieved by H-abstraction from the bis-allylic groups in positions 11 and 14. The resultant two pentadiene radicals then stabilize analogously to linoleic... 

Among the peroxy radicals of linolenic acid which are formed by autoxidation, the isolated P,y double bond system exists only for the... 

Odor-Active Monocarbonyl Compounds. Model expriments showed that the volatile fractions formed during the autoxidation of oleic, linoleic and linolenic acid contain mainly aldehydes and ketones (Table 3.31). Linoleic acid, a component of all lipids sensitive to autoxidation, is a precursor of hexanal that is predominant in... 

The rapid deterioration of food containing linolenic acid should not be ascribed solely to the preferential oxidation of this acid but also to the low odor threshold values of the carbonyl compounds formed, such as (Z)-3-hexenal, (E,Z)-2,6-nonadienal and (Z)-l,5-octadien-3-one (Table 3.32). Aldehydes with exceptionally strong aromas can be released in food by the autoxidation of some fatty acids, even if they are present in low amounts. An example is octadeca-(Z,Z)-11, 15-dienoic acid (the precursor for... 

The occurrence of 2,4-heptadienal (from the 12-hydroperoxide isomer) and of 2,4,7-decatrienal (from the 9-hydroperoxide isomer) as oxidation products is, thereby, readily explained by accepting the fragmentation mechanism outlined above (option B in Fig. 3.26) for the autoxidation of a-linolenic acid. The formation of other volatile carbonyls can then follow by autoxidation of these two aldehydes or from the further oxidation of labile monohydroperoxides. 

Malonic Aldehyde, This dialdehyde is preferentially formed by autoxidation of fatty acids with three or more double bonds. The compound is odorless. In food it may be bound to proteins by a double condensation, crosslinking the proteins (cf. 3.7.2.4.3). Malonic aldehyde is formed from a-linolenic acid by a modified reaction pathway, as outlined under the formation of hydroperoxide-epidioxide (cf. 3.7.2.1.3). However, a bicyclic compound is formed here as an intermediary product that readily fragments to malonic aldehyde ... 

Rapeseed Canola) oil is used as an edible oil. It is susceptible to autoxidation because of its relatively high content of linolenic acid. It is saturated by hyrogenation to a melting point of 32-34 °C and, with its stability and melting properties, resembles coconut oil. 

Linseed Oil. Flax, used for fiber and seed production and the subsequent processing of the seed into linseed oil, is grown mainly in Canada, China and India (cf. Table 14.0). Due to its high content of linolenic acid (cf. Table 14.11), the oil readily autoxidizes, one of the processes by which some bitter substances are created. Since autoxidation involving polymerization reactions proceeds rapidly, the oil solidifies Jast drying oir). Therefore, it is used as a base for oil paints, varnishes and linoleum manufacturing, etc. A comparatively negligible amount, particularly of the coldpressed oil, is utilized as an edible oil. 

The observed oxidation of linolenic acid to (Z)-3-hexenal, which then partly isomerizes to (E)-2-hexenal, is catalyzed by a lipoxygenase and a hydroperoxide lyase (cf. 3.7.2.3) and also occurs by autoxidation. (Z)-3-Hexenal contributes to the aroma of green tea. 

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