Short-chain volatile aldehydes

Chemical Reactions of Aldehydes in Grapes and Wines. The short chain, volatile aldehydes are quite reactive making accurate analysis and quantitation difficult. In addition, many of these reactions are equilibrium reactions with the concentration of unreacted and reacted aldehydes being lughly dependent on the analysis conditions. 

This reaction involves fission of the chain and can occur on either side of the free radical. The aldehyde that is formed can be a short-chain volatile compound, or it can be attached to the glyceride part of the molecule in this case, the compound is nonvola-... 

Each hydroperoxide can produce two aldehydes of which the short-chain volatile member is more significant in this context. The other aldehyde is a polyfunctional compound attached to an ester (glyceride) function. Aldehydes produced from natural fats will be complex mixtures because of the large number of hydroperoxides from which they can be produced. This number may be greater still with partially hydrogenated fats because of the large number of double-bonds positions possible in such compounds (Section 10.1). Most of these aldehydes have a very low flavour threshold level so they need be present only at minute levels. For example, the deca-2,4-dienal produced from linoleate 9-hydroperoxide produces a deep-fried flavour at... 

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. 

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]. 

Formaldehyde, along with other short-chain aldehydes such as acetaldehyde, is a low molecular weight, volatile, reactive contaminant that can be present at low levels from a variety of sources (e.g., excipients such as polyethylene oxide, polyethylene glycol (64,65), or from carbohydrate degradation (66), solvent contamination (51), packaging materials (52), etc.). Formaldehyde is known to react with amines (Fig. 33) to form a reactive N-hydroxymethyl compound (a hemiaminal) that can further react with other nucleophiles. Reaction of formaldehyde with amino acids (67) can cause... 

The main classes of volatile compounds, which are considered to contribute significantly to the overall flavour are lactones, fatty acids, aldehydes and methyl ketones. As noted earlier, there are very small amounts of hydroxy acids, esterified to triacylglycerols, in milk fat. These act as precursors of flavoursome 7-lactones and 8-lactones. It has been reported that three lactones, 8-octalactone, 8-decalactone and y-dodecalactone, are important flavour components in milk fat (Widder et al., 1991 Schieberle et al., 1993). Siek et al. (1969) identified the short-chain fatty acids, 4 0 and 6 0, as key flavour components of milk fat. However, while they may contribute to the overall flavour of milk fat at very low concentrations,... 

Table 1. Flavor characteristics of volatile, short chain aldehydes.

It is known that volatile compounds found in fruits are mainly derived from three biosynthetic pathways in many plants [25] The formation of the hedonically important short-chain aldehydes and alcohols, such as di-3-hexenol, takes place through the action of lipases, hydroperoxide lyases, and cleavage enzymes on lipid components, followed by the action of alcohol dehydrogenases [26]. 

The large number of precursors of volatile decomposition products affecting the flavor of oils has been discussed in Chapter 4. Only qualitative information is available on the relative oxidative stability of hydroperoxides, aldehydes and secondary oxidation products. As observed with the unsaturated fatty ester precursors, the stability of hydroperoxides and unsaturated aldehydes decreases with higher unsaturation. Different hydroperoxides of unsaturated lipids, acting as precursors of volatile flavor compounds, decompose at different temperatures. Hydroperoxides of linolenate and long-chain n-3 PUFA decompose more readily and at lower temperatures than hydroperoxides of linoleate and oleate. Similarly, the alkadienals are less stable than alkenals, which in turn are less stable than alkanals. The short-chain fatty acids produced by oxidation of unsaturated aldehydes will further decrease the oxidative stability of polyunsaturated oils. For secondary products, dimers are less stable than dihydroperoxides, which are less stable than cyclic peroxides. 

Steps B through E result in the production of short chain saturated or unsaturated aldehydes, ketones, and alcohols. These primary reaction products can undergo further oxidation if unsaturated or secondary reactions to yield a host of off-flavor volatiles (see Figure 7.3). These final products are generally aldehydes, ketones, acids, alcohols, hydrocarbons, lactones, or esters (Table 7.5). The unsaturated aldehydes and ketones have the lowest sensory thresholds and are, therefore, most often credited with being responsible for oxidized flavors. The metallic taint in oxidized butter, for example, has been attributed to oct-l-ene-3-one [71]. 

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