Fatty acids from hexane oxidation

After acidification, the lipid fraction is extracted into hexane and the solvent subsequently removed by evaporation. This gives a material consisting of 60-75% (w/w) CLA (depending on the starting materials), with the remainder consisting of unchanged saturated and monounsaturated fatty acids from the starting material. Deodorization of CLA removes volatile compounds such as hexane that may be residual solvent or secondary autoxidation products from CLA. Fractional distillation and crystallization remove metal catalysts (which may stimulate oxidation), as well as undesirable components such as dimers and polymers. The reactor type will affect the amounts of metals found. 

Another source of rubredoxins was found in an aerobic bacterium, Pseudomonas oleovorans, utilizing n-hexane as a carbon source (10). This particular rubredoxin differs from those commonly found in anaerobic bacteria in some of its properties it has a molecular weight of 19,000, and one iron form of the protein is readily converted to a two-iron form (11). The rubredoxin of P. oleovorans functions as a terminal electron transfer component in an enzyme system which participates in the ( -hydroxylation of fatty acids and hydrocarbons. The hydrocarbon-oxidizing... 

Products of the LOX pathway or compounds formed by autoxidation of fatty acids (Scheme 7.2) are also important for leek aroma [31, 163]. Volatile compounds of the LOX pathway are not pronounced in the aroma profile of freshly cut leeks owing to a high content of thiosulfinates and thiopropanal-S-oxide [30]. In processed leeks that have been stored for a long time (frozen storage), however, these aliphatic aldehydes and alcohols have a greater impact on the aroma profile owing to volatilisation and transformations of sulfur compounds [31, 165]. The most important volatiles produced from fatty acids and perceived by GC-O of raw or cooked leeks are pentanal, hexanal, decanal and l-octen-3-ol (Table 7.5) [31, 35, 148, 163, 164]. 

Raw potato possesses little aroma. Approximately 50 compounds have been reported to contribute to raw potato aroma. Raw potatoes have a high content of LOX, which catalyses the oxidation of unsaturated fatty acids into volatile degradation products (Scheme 7.2) [187]. These reactions occur as the cells are disrupted, e.g. during peeling or cutting. Freshly cut, raw potatoes contain ( ,Z)-2,4-decadienal, ( ,Z)-2,6-nonadienal, ( )-2-octenal and hexanal, which are all products of LOX-initiated reactions of unsaturated fatty acids [188,189]. It is reported that two compounds represent typical potato aroma in raw potato methional and ( ,Z)-2,6-nonadienal [189]. Other important volatiles in raw potatoes produced via the LOX pathway are l-penten-3-one, heptanal, 2-pen-tyl furan, 1-pentanol and ( , )-2,4-heptadienal [189]. Pyrazines such as 3-iso-propyl-2-methoxypyrazine could be responsible for the earthy aroma of potato [35]. Some of the most important character-impact compounds of raw potatoes are summarised in Table 7.8. Aroma compounds from cooked, fried and baked potatoes have previously been reviewed [35]. 

Rancidity measurements are taken by determining the concentration of either the intermediate compounds, or the more stable end products. Peroxide values (PV), thiobarbituric acid (TBA) test, fatty acid analysis, GC volatile analysis, active oxygen method (AOM), and sensory analysis are just some of the methods currently used for this purpose. Peroxide values and TBA tests are two very common rancidity tests however, the actual point of rancidity is discretionary. Determinations based on intermediate compounds (PV) are limited because the same value can represent two different points on the rancidity curve, thus making interpretations difficult. For example, a low PV can represent a sample just starting to become rancid, as well as a sample that has developed an extreme rancid characteristic. The TBA test has similar limitations, in that TBA values are typically quadratic with increasing oxidation. Due to the stability of some of the end-products, headspace GC is a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic and solid-phase microextraction (SPME) methods. Hexanal, which is the end-product formed from the oxidation of Q-6 unsaturated fatty acids (linoleate), is often found to be a major compound in the volatile profile of food products, and is often chosen as an indicator of oxidation in meals, especially during the early oxidative changes (Shahidi, 1994). 

Cobalt(II) salts are effective catalysts for the oxidation of 1,2-glycols with molecular oxygen in aprotic polar solvents such as pyridine, 4-cyanopyridine, benzonitrile, DMF, anisole, chlorobenzene and sulfolane. Water, primary alcohols, fatty acids and nitrobenzene are not suitable as solvents. Aldehydic products are further oxidized under the reaction conditions. Thus, the oxidative fission of rram-cyclo-hexane-l,2-diol gives a mixture of aldehydes and acids. However, the method is of value in the preparation of carboxylic acids from vicinal diols on an industrial scale for example, decane-1,2-diol is cleaved by oxygen, catalyzed by cobalt(II) laurate, to produce nonanoic acid in 70% yield. ... 

The ability of flavonoids to enhance the resistance to oxidation and to terminate free-radical chain reactions in lipophilic systems can be monitored using low-density lipoproteins (LDL) as a model (Rice-Evans et al., 1996). The LDL oxidation is initiated either by copper or by a peroxyl radical [2,2 azobis(2-amidinopropane hydrochloride) (AAPH)] (Abuja et al., 1998). Hexanal liberated from the decomposition of oxidized n-6 polysaturated fatty acids in LDL may be determined by static headspace gas chromatography (Frankel and Meyer, 1998). Also, bleaching of P-carotene (Velioglu et al., 1998 Fukumoto and Mazza, 2000) and the tracing by HPLC (Fukumoto and Mazza, 2000) of malonaldehyde formed in lipid emulsion systems in the presence of iron (Tsuda et al., 1994) have been used to measure antioxidants in lipophilic systems. 

Methods of analysis (ASTM D-128, IP 37) are available for the measurement of excessive acidity derived from oxidation. These methods cover conventional grease that consists essentially of petroleum oil and soap. Thus these test methods are applicable to many types but not all grease. The constituents covered by the test series are soap, unsaponifiable matter (base oil), water, free alkalinity, free fatty acid, fat, glycerin, and insoluble. A supplementary test method is also provided and is intended for application to grease that contains thickeners that are essentially insoluble in n-hexane and to grease that cannot be analyzed by conventional methods because of the presence of such constituents as nonpetroleum fluids or nonsoap-type thickeners, or both. These methods may not be applicable to grease analysis when lead, zinc, or aluminum soaps are present or in the presence of some additives such as sodium nitrite. 

Phospholipids contribute specific aroma to heated milk, meat and other cooked foods through lipid oxidation derived volatile compounds and interaction with Maillard reaction products. Most of the aroma significant volatiles from soybean lecithin are derived from lipid decomposition and Maillard reaction products including short-chain fatty acids, 2-heptanone, hexanal, and short-chain branched aldehydes formed by Strecker degradation (reactions of a-dicarbonyl compounds with amino acids). The most odor-active volatiles identified from aqueous dispersions of phosphatidylcholine and phos-phatidylethanolamine include fra 5 -4,5-epoxy-c/5-2-decenal, fran5,fran5-2,4-decadienal, hexanal, fra 5, d5, d5 -2,4,7-tridecatrienal (Table 11.9). Upon heating, these phospholipids produced cis- and franj-2-decenal and fra 5-2-undecenal. Besides fatty acid composition, other unknown factors apparently affect the formation of carbonyl compounds from heated phospholipids. 

Oxidized fish oils, rich in n-3 polyunsaturated fatty acids, produced volatile compounds more readily than oxidized vegetable oils, rich in linoleic acid. Activation energy for the formation of propanal from fish oils was lower than for the formation of hexanal from vegetable oils. A mixture of aldehydes contributed to the characteristic odors and flavors of oxidized fish, described as rancid, painty, fishy and cod liver oil-like (Table 11.21). Oxidation of unsaturated fatty acids in fish was related to the formation of 2-pentenal, 2-hexenal, 4-heptenal, 2,4-heptadienal and 2,4,7-decatrienal. The fishy or trainy characteristic of fish oil was attributed to 2,4,7-decatrienal. Studies of volatiles from boiled trout after storage showed significant increases in potent volatiles by aroma extraction dilution analysis (Table 11.22). Volatiles with the highest odor impact included l,5-octadien-3-one, 2,6-nonadienal, 3-hexenal, and 3,6-nonadienal. 3,6-Nonadienal and 3-hexenal were considered to contribute most to the fatty, fishy flavor in stored boiled fish. 

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