Quantitative model studies on the formation of aroma-active aldehydes and acids by Strecker-type reactions

Application of aroma extract dilution analysis on the volatiles formed by reacting glucose and L-phenylalanine (30 min, 100 °C) revealed the Strecker aldehyde, phenylacetaldehyde (PA), and, in addition, phenylacetic acid (PAA) as the two key odorants among the volatiles formed. Quantitative measurements on α-dicarbonyl formation revealed that the 3-deoxyosone and glyoxal were formed as the first prominent sugar degradation products, whereas 2-oxopropanal became predominant after ~4 h at 100 °C. Among the four α-dicarbonyls analyzed, 2-oxopropanal proved to be the most effective in generating PA as well as PAA from phenylalanine, but the reaction parameters significantly influenced the ratio of both odorants; for example, at pH 3.0 the ratio of PA to PAA was 3:1, whereas at pH 9.0 the ratio was 1:5. Furthermore, in the presence of oxygen and copper ions the formation of the acid was further increased. 3-Deoxyosone and glucosone were found to be effective precursors of phenylacetaldehyde, but neither was very effective in acid generation. On the basis of the results, a new oxygen-dependent formation pathway of the Strecker reaction is proposed.