Olfactory perception of 6 amino acids by human subjects.

The olfactory properties of 6 amino acids were assessed in 20 human subjects using psychophysical tests of detectability, discriminability, and chemesthesis. Mean olfactory detection thresholds were found to be 10 microM for D-methionine, 80 microM for L-methionine, 200 microM for L-cysteine, 220 microM for D-cysteine, 75 mM for D-proline, and 100 mM for L-proline. When presented at clearly detectable and intensity-matched concentrations, the subjects readily discriminated between the odors of the L-forms of cysteine, methionine, and proline, whereas they failed to distinguish between the L- and D-forms of a given amino acid. The subjects also failed in localizing the side of monorhinal stimulation with all 6 amino acids when presented at the same concentrations as in the discrimination tasks. These results suggest that amino acids may contribute to the flavor of food not only as taste stimuli but also as olfactory stimuli perceived via ortho- or retronasal smelling. In contrast, it is unlikely that amino acids contribute to flavor perception via chemesthesis. Given that the odors of 4 of the 6 amino acids tested here were detected at concentrations lower than their corresponding taste detection thresholds, this may have important implications for the widespread use of amino acids as food additives as well as for the evaluation of off-flavors caused by amino acids.