Odor Strecker aldehyde

Methionai [3-(methylthio)propanal]. H3CS-(CH2)2-CH0, C4H8OS, Mr 104.17, bp. 165 °C LD50 (rat p. o.) 0.75 g g. A Strecker aldehyde formed thermally or enzymatically from methionine with an odor resembling, when diluted, bouillon and boiled potatoes olfactory threshold 0.2 ppb. M. occurs in many foods prepared thermally or by fermentation and is important for, e. g., the flavor of bread, meat, potatoes (boiled, chips), cheese, and tomato (paste). It is used as a flavor substance especially for meat and cheese flavors. 

Chocolate represents a highly complex flavor system for which no single character impact has been identified. Vanillin and Furaneol contribute to the sweet, caramel background character of milk chocolate (57). 5-Methyl-2-phenyl-2-hexenal provides a deep bitter, cocoa note, and is the aldol reaction product from phenylacetaldehyde and 3-methylbutanal, two Strecker aldehydes formed in chocolate (58). 2-Methoxy-5-methylpyrazine and isoamyl phenylacetate have chocolate, cocoa, nutty and cocoa-like notes, respectively, and both are used in synthetic chocolate flavors (59). Systematic studies of key odorants in milk chocolate were performed using aroma extract dilution analysis however, character impact compounds unique to chocolate flavor were not reported (57,60). 

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. 

Not only aldehydes (cf. 5.3.1.1), but also amines are formed in the Strecker reaction (cf. 4.2.4.4.T). The odor thresholds of these amines (examples in Table 5.26) are pH dependent. The enzymatic decarboxylation of amino acids produces the same amines as the Strecker reaction the precursors are shown in Table 5.26. Both reactions take place e. g. in the production of cocoa, but the Strecker... 

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