Flavors 1,4,5,6-tetrahydropyridines

Chemistry of important flavor compounds, viz., 2-acetyl-l-p)nToline, 6-acetyl-l,2,3,4-tetrahydropyridine, 2-acetyl-2-thiazoline, and 5-ace-tyl-2,3-dihydro-4H-thiazine 06CRV2299. 

C6H9NO, Mr 111.14. Rusty liquid with popcom-like odor, bp. 26-28 °C (10 Pa) rapid darkening of color at 20 °C, limited storage life at-20°C, 1% in pentane olfactory threshold in water 0.1 ppb. TVpical bread and rice flavor. A. is formed from yeast constituents on heating L- ornithine and 2-oxopropanal. The bread aroma 2-acetyl-A A )-tetrahydropyridine [C7H,NO, Mr 125.17,bp. 33-34 °C(0.12 kPa), olfactory threshold 1.6 ppb] is formed analogously from l- proline and 2-oxopropanal. 

Bread flavor. White bread During the baking process the impact substances 2-acetyl-1 -pyrroline and 2-ace-tyl-A (A )-tetrahydropyridine form in the crust, on storage their concentrations decrease rapidly. Precursors are metabolites of yeasL Also of importance are ( )-2-nonenal (see alkenals), 3-methylbutanal (see al-kanals), 2,3-butanedione, and methional. In bread crumbs, degradation products of linolic acid such as ( )- and (Z)-2-nonenaJ and ( , )- 2,4-decadienal predominate. 

One must recognize that microorganisms can accomplish a wide range of chemical transformations thereby potentially producing a host of different oflf-flavors. Earlier in this section, several oflf-notes have been described as being the result of microbes. One can add other sensory notes such as faceal (indole and skatole in rotting potatoes [147]), musty potato (2-methoxy-3-isopropylpyrazine [148]), mousy (wine, 2-acetyl-1,4,5,6-tetrahydropyridine [149]), and musty, dirty dishcloth (machine oils 2,6-dimethyl-3-methoxypyrazine [150]). 

A mousy taint is occasionally detected in fruit and berry wines and, less often, in grape wines. It is thought that the tetrahydropyridines given in Table 20.24, which have been also identified as important flavor compounds of toasted bread (cf. 15.4.3.3.2), contribute to the mousy taint. These compounds might be formed by microorganisms in wine. 

The principal impact aroma compounds of freshly prepared popcorn were determined by Schieberle as 2-acetyltetrahydropyridine, 2-acetyl-1-pyrroline, and 2-propionyl-1-pyrroline (49). The crackerlike aroma of the tetrahydropyri-dine, which exists in two tautomeric forms (2-acetyl-1,4,5,6- and 2-acetyl-3,4,5,6-tetrahydropyridine) was previously identified as the character compound of Saltine crackers (50). The decrease of these compounds during storage was directly correlated with staling flavor. Another popcornlike odor note, 2-acetyl-... 

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