Alcohol from lipid oxidation

The volatiles from cooked meat contain large numbers of aliphatic compounds including aldehydes, alcohols, ketones, hydrocarbons and acids. These are derived from lipids by thermal degradation and oxidation (J7) and many may contribute to desirable flavor. In addition, the aldehydes, unsaturated alcohols and ketones produced in these reactions, as well as the parent unsaturated fatty acids, are reactive species and under cooking conditions could be expected to interact with intermediates of the Maillard reaction to produce other flavor compounds. 

Citation of the classic chain reaction for lipid oxidation persists even though, as product analysis and studies of mechanisms have become more sophisticated, there is now considerable evidence that only Reactions 1, 2, and 5 (and perhaps also 6) of Figure 1 are always present. Research has shown that, although hydrogen abstraction ultimately occurs, it is not always the major fate of the initial peroxyl or alkoxyl radicals. Indeed, lipid alcohols from H abstraction are relatively minor products of lipid oxidation. There are many competing alternative reactions for LOO and LO that propagate the radical chain but lead to different kinetics and different products than expected from the classic reaction sequence (5, 6, 21). A more detailed consideration of each stage shows how this basic radical chain sequence portrays only a small part of the lipid oxidation process and products, and a new overall reaction scheme for lipid oxidation is needed. 

Peroxyl Radicals Secondary peroxyl radicals, as are found in most lipid acyl chains, recombine rapidly (2k = 10 -10 M s ) (192, 362) to form a variety of products, including alcohols and ketones (Reaction 67) (361, 362, 366), ketones and alkanes (Reaction 68) (60, 292), or acyl peroxides and peroxyl radicals (Reaction 69) (264, 367, 369). The alcohols thus produced are indistinguishable from H abstraction products of an original LO, but the ketones and dialkyl peroxides are unique to recombination reactions. As any R3OO and RO released from Reaction 68 or Reaction 69a react further, peroxyl radical recombinations also have the potential for propagating lipid oxidation (Section 3.1.4). 

During lipid oxidation, the primary oxidation products that are formed by the autoxidation of unsaturated lipids are hydroperoxides, which have little or no direct impact on the sensory properties of foods. However, hydroperoxides are degraded to produce additional radicals which further accelerates the oxidation process and produce secondary oxidation products such as aldehydes, ketones, acids and alcohols, of which some are volatiles with very low sensory thresholds and have potentially significant impact on the sensory properties namely odor and flavor [2, 3]. Sensory analysis of food samples are performed by a panel of semi to highly trained personnel under specific quarantined conditions. Any chemical method used to determine lipid oxidation in food must be closely correlated with a sensory panel because the human nose is the most appropriate detector to monitor the odorants resulting from oxidative and non-oxidative degradation processes. The results obtained from sensory analyses provide the closest approximation to the consumers approach. Sensory analyses of smell and taste has been developed in many studies of edible fats and oils and for fatty food quality estimation [1, 4, 5]. 

Alcohols in peas are mostly formed from enzymatic oxidation of lipids. Physical damage, storage and processing of seeds could lead to the formation of alcohols (Eriksson, 1967 de Lumen et al., 1978 Oomah Liang, 2007). Volatile alcoholic compounds have distinct characteristics and they could therefore affect the taste and flavour of peas. For example, 1-propanol has an alcoholic odour and a fruity flavour 2-methyl-l-propanol has a wine odour, 3-methyl-l-butanol has a fruity, banana, sweet odour with a bittersweet taste 1-hexanol has an herbaceous, mild, sweet, green fruity odour and an aromatic flavour 1-heptanol has an aromatic and fatty odour and a spicy taste, whereas 1-octanol has a fresh, orange-rose odour and an oily, sweet taste (Burdock, 2002). 

The most important reactions taking place on the hydroxy groups of alcohols are 0-H bond cleavage and C-O bond cleavage. With the O-H bond cleavage, reactions with strong acids proceed, as do oxidations of primary alcohols to aldehydes, secondary alcohols to ketones and reactions with organic acids (formation of esters). In foods the last three reactions are particularly important, and are usually enzymatically catalysed. Other important reactions are dehydration and the opposite reaction, hydration, which yield unsaturated hydrocarbons from alcohols and isomeric alcohols from unsaturated hydrocarbons, respectively. These reactions are particularly important in terpenic alcohols. In oleochemistry, oxidation and esterification reactions are used for the production of various lipid derivatives. 

The formation of lipid components in an aqueous phase at temperatures from 370 to 620 K was studied by Rushdie and Simoneit (2001), who heated aqueous solutions of oxalic acid in a steel vessel for 2 days the yield of oxidized compounds reached a maximum (5.5% based on oxalic acid) between 420 and 520 K. A broad spectrum of compounds was obtained, from n-alkanes to the corresponding alcohols, aldehydes and ketones. At higher temperatures, i.e., above 520-570 K, cracking reactions competed with the synthetic reactions. 

Accumulation of cytoplasmic NADH and glycerol 3-P may also contribute to lipid accumulation in alcoholic liver disease. Free fatty acids released from adipose in part enter the liver where P-oxidation is very slow (high NADH). In the presence of high glycerol-3P, fatty acids are inappropriately stored in the liver as triglyceride. 

It should be noted that all three are lethal, however, and so the acute toxicity of these compounds is not entirely due to C-40 substituent effects. Potency does follow the oxidation series from alcohol to aldehyde to acid vivo, suggesting that perhaps these substituents influence the degree of accessibility of each lipid-solvent soluble toxin to its membrane site of action. Being that the toxins in their natural forms are so soluble in non-polar solvents, and tend to bind to or solubilize in the lipid components of membrane... 

The CLD methods for HPLC using isoluminol (190) with microperoxidase catalysis, for determination of lipid hydroperoxides in clinical fluids, have been reviewed. Determination of phospholipids hydroperoxides by luminol (124) CL has been reviewed . A fast RP-HPLC method (retention times 1 to 2 min) for determination of hydroperoxides and other peroxide compounds includes UVD, which is not always effective, and CLD, attained on injection of luminol (124), the CL reagent (Scheme 3), hemin (75a), a catalyst, and NaOH to raise the pH of the solution. A FLD cell may act as CLD cell if the excitation source is turned off. The selectivity of CLD is of advantage over UVD in industrial analysis thus, for example, UVD of a sample from a phenol production line based on cumene oxidation (equation 13) shows peaks for cumyl hydroperoxide (27), unreacted cumene, cumyl alcohol and acetophenone, whereas CLD shows only the 27 peak. The... 

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