The role of free amino acids present in yeast as precursors of the odorants 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine in wheat bread crust

Recently we established bakers' yeast as a potent source of precursors for the roast-smelling odorants 2-acetyl-1-pyrroline (ACPY) and 2-acetyltetrahydropyridine (ACTPY) in wheat bread crust. To reveal their role in the formation of both odorants, the concentrations of free amino acids occurring in baker's yeast were determined. The 11 amino acids present in concentrations above 60 mg/100 g dry yeast were separately reacted with 2-oxopropanal in model solutions and the amounts of ACPY and ACTPY formed, determined by a stable isotope dilution assay (SIDA). ACPY was formed from proline and ornithine, while ACPTY was exclusively liberated from proline. The remaining amino acids were ineffective. Further experiments revealed that the formation of ACPY from ornithine proceeds via 4-aminobutyraldehyde and 1-pyrroline as intermediates. The amount of free ornithine in yeast was more than three times the amount of free proline. Furthermore, additions of either proline or ornithine to wheat doughs enhanced the amounts of ACPY in the bread crust by a factor of two or four, respectively. The data led to the conclusion that ornithine is the most important precursor for the formation of ACPY during baking.