2009_Book_FoodChemistry

368 5 Aroma Compounds
Fig. 5.21. Formation of precursors of 2-acetyl-2-thiazoline (according to Hofmann and Schieberle, 1995)
Fig. 5.22. Metal catalyzed oxidation of 2-(1-hydroxyethyl)-4,5-dihydrothiazole and 2-acetylthiazolidine (according
to Hofmann and Schieberle, 1995)
produces the pleasant popcorn aroma of certain
types of rice consumed mainly in Asia. In gas
chromatography, Apy appears predominantly in
the imine form shown in Table 5.23, whereas
2-acetyltetrahydropyridine (ATPy) appears as
the eneamine and imine tautomers. Model experiments
show that 1-pyrroline is the precursor
of Apy and ATPy. 1-Pyrroline is formed by the
Strecker degradation of both proline (cf. Formula
5.14) and ornithine (cf. Formula 5.15).
In the baking of white bread, ornithine comes
from yeast where it is found in a concentration
about four times that of free proline.
In addition, triose phosphates occurring in
yeast have been identified as precursors. They
yield on heating, e. g., 2-oxopropanal from di-