Triglycerides Volatile fatty acids

It is now well established (6,10) that the hydrocarbons, except possibly those having three or four carbon atoms, found in the irradiated meats can come only from the lipid. This hypothesis has been verified in earlier studies, when the volatiles from irradiated methyl oleate (10) were found to contain appreciable quantities of alkanes and alkenes, and now in more detail from studies of both triglycerides and fatty acid esters. 

The carboxylic acids can be subdivided into nonvolatile fatty acids, volatile fatty acids, hydroxy acids, dicarboxylic acids, and aromatic acids (Fig. 3). The nonvolatile fatty acids are molecules with more than five carbon atoms, such as stearic and palmitic acids, which are the degradation products of fats and triglycerides. Three different 18-C fatty acids that are important constituents of plants include oleic and linoleic acids that are abundant in plant seeds, and linolenic acid, which is abundant in plant leaves. Volatile fatty acids are short-chain molecules with one to five carbon atoms, such as acetic and valeric acid, associated with anaerobic metabolism. The hydroxy-acids are common intermediates in biochemical pathways, including the tricarboxylic acid cycle. The excretion of hydroxyacids by algae, such as the... 

When lipids are heated a large variety of compounds are formed. Hydrolysis of the ester linkage occurs, liberating glycerol and volatile fatty acids. It 1s well known that thermal decomposition of triglycerides generates a series of compounds including alkanals, alkenals, alkadlenals, methyl ketones, lactones, and hydrocarbons. 

Gas chromatography is one of the most powerful analytical techniques available. Its only major limitation is that it can not analyse involatile compounds such as fats. The solution in this case is to make a volatile derivative, e.g. the use of fatty acid methyl esters to analyse triglycerides. 

In contrast to the other large cats, the urine of the cheetah, A. jubatus, is practically odorless to the human nose. An analysis of the organic material from cheetah urine showed that diglycerides, triglycerides, and free sterols are possibly present in the urine and that it contains some of the C2-C8 fatty acids [95], while aldehydes and ketones that are prominent in tiger and leopard urine [96] are absent from cheetah urine. A recent study [97] of the chemical composition of the urine of cheetah in their natural habitat and in captivity has shown that volatile hydrocarbons, aldehydes, saturated and unsaturated cyclic and acyclic ketones, carboxylic acids and short-chain ethers are compound classes represented in minute quantities by more than one member in the urine of this animal. Traces of 2-acetylfuran, acetaldehyde diethyl acetal, ethyl acetate, dimethyl sulfone, formanilide, and larger quantities of urea and elemental sulfur were also present in the urine of this animal. Sulfur was found in all the urine samples collected from male cheetah in captivity in South Africa and from wild cheetah in Namibia. Only one organosulfur compound, dimethyl disulfide, is present in the urine at such a low concentration that it is not detectable by humans [97]. 

Clinical Chemistry (see Chapter 10). Gas chromatography is adaptable to such samples as blood, urine, and/or biological fluids. Compounds such as proteins, carbohydrates, amino acids, fatty acids, steroids, triglycerides, vitamins, and barbiturates are handled by this technique, directly or after preparation of appropriate volatile derivatives. 

Table I summarizes the various meats, meat constituents, and other related substances which have been analyzed, including substances reported on previously (6) as well as those for which new data are given. The substances chosen are intended to provide a cross-section of the type of inherently related material from which volatile irradiation odor and flavor compounds might be expected to form. Thus, in addition to several whole meats, the volatile irradiation products from a number of protein and lipid substances have been analyzed. Among the lipid substances included are typical whole fats and separate moieties such as triglycerides, fatty acid esters, and cholesterol, as an example of a steroid. Among the proteinaceous substances included are a protein, a polypeptide, and some individual amino acids. Finally, beef itself has been separated into a protein, a lipid, and a lipoprotein fraction, and these have been separated, irradiated, and analyzed.

Most of the other products found in irradiated meat volatiles except those containing sulfur or aromatic rings may also be accounted for by mechanisms associated with alkyl free radical formation in the fat. Oxygenated compounds are far less abundant than hydrocarbons, but appreciable amounts of a homologous series of n-aliphatic alcohols up to hexanol are found. Of these, only ethanol is detected in the unirradiated controls. Since the water content of meat averages nearly 60%, the formation of alcohols may be thought to occur by reaction of the alkyl free radical with water. Such a mechanism is supported by the fact that only traces of alcohols are found in irradiated dry butterfat and were undetected in irradiated triglycerides or methyl esters of fatty acids. 

Cholesterol is a low-volatile compound, but it is more volatile than the major triglycerides of milkfat. Superheated steam can be bubbled through the oil, heating it indirectly, which provides for the latent heat of vaporization of the distilling compounds and prevents steam condensation. Thus, the temperature and pressure can be varied independently. When the sum of the partial vapor pressures of water vapor and the distillates is equal to the total pressure, water vapor and the low-volatile components, such as cholesterol and free fatty acids, distill over. 

In the late 1970s, Harllee and Lefflngwell (1512, 1513) cataloged cocoa components identified to that date with particular emphasis on the volatile, flavorfnl components common to cocoa and tobacco or its smoke. Both tobacco and tobacco smoke, as well as cocoa, contain numerons fatty acid triglycerides (1512) and many of the same fatty acids (1512). At least nineteen amino acids are common to cocoa and tobacco (1512). Of 352 volatile components identified in cocoa by 1979, 209 (59%) had also been identified in tobacco and tobacco smoke (1513). 

Karahdian et al. (1985a,b) demonstrated the production of l-octen-3-ol, 8-nonen-2-one, 3-octanone, 3-octanol and octanoic acid from linoleic acid and linolenic acid. The use of microbially produced fatty acids for characterization of mold fungi has been suggested (Blomquist et al., 1992). Lipolysis of triglycerides or amino acids may lead to the production of compounds such as 2-methylpropanoic acid, butanoic acid, 2-methylbu-tanoic acid, pentanoic acid, hexanoic acid and octanoic acid (Jolivet and Belin, 1993). The presence of different lipids and lipases in different fungi (Ha and Lindsay, 1993) and under different environmental conditions may explain a great deal of the variation in the volatile compounds produced. 

The overall yield of radiolysis products is relatively low and their distribution will depend on the fatty acid composition of the triglycerides. In meats irradiated in the absence of oxygen, low levels of the free fatty acid, the associated propanedioldiester, hydrogen, and products derived from the triglyceride radical are to be expected. Much lower yields of volatile hydrocarbons are produced that provide insight into other scission processes and reaction pathways [27, 30, 31],... 

Esters are the produets of condensation reaetions between acids and alcohols. The other produet of sueh reactions is water. Fruit esters are volatile eompounds found in fruits. One elass of hpids (triglycerides) are esters formed in the reaction between glyeerol and various fatty acids. 

Beer contains trace amounts of lipids. A Swedish beer (12° Plato) was found to contain (mg/1) triglycerides, 0 1-0 2 diglycerides, 0 1 monoglycerides, 0 1-0 3 sterol esters, 0 01 free sterols, 0 01-0 02 and free fatty acids, C4-C10, 10-15 Ci2-Cx8, 0-0 5 [42]. Similar data were found for American beers. The role that lipids play in head formation is discussed later. The free fatty acids are volatile and more detailed values are given below. Whereas unsubstituted fatty acids are volatile, the addition of a further substituent usually results in loss of volatility. Data for hydroxy-, keto, di-, and tri-basic acids are given in Table 22.7. 

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