Flavor Contribution and Formation of the Intense Roast-Smelling Odorants 2-Propionyl-1-pyrroline and 2-Propionyltetrahydropyridine in Maillard-Type Reactions

Application of aroma extract dilution analysis on two different model mixtures of proline and glucose, reacted under aqueous or dry-heating conditions, revealed 2-propionyl-1-pyrroline (PP) and 2-propionyltetrahydropyridine (PTHP; occurring in two tautomers), besides 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine, as important roast-smelling odorants in both mixtures. A comparison of the isotope distribution in PP and PTHP formed from either ¹³C₆-labeled or unlabeled glucose suggested a formation pathway for both odorants from the same intermediate, 1-pyrroline, when reacted with either 2-oxobutanal (yielding PP) or 1-hydroxy-2-butanone (yielding PTHP). 2-Oxobutanal, a possible precursor of 1-hydroxy-2-butanone, was shown to be formed in high yields (29 mol %) by reacting acetaldehyde and glycolaldehyde, two well-known degradation products of carbohydrates.