Olfactory perception of 6 amino acids by human subjects
Authors: Matthias Laska
Journal: Chemical Senses (2010)
Discipline: NEUROSCIENCE & BEHAVIOR
Keywords: olfactory properties; amino acids; psychophysical tests; detection thresholds; discriminability
Affiliations: IFM Biology, Linköping University, SE-581 83 Linköping, Sweden. 

Abstract:  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. 

Retracted: No

PubPeer comments:
 This study claims to investigate the odor of amino acids. It uses commercial products without further purification, which is problematic, since many commercial chemicals have odors which are due to impurities, not the substances. Likewise, the calculated gas-phase concentrations assumed for the amino acids are unreliable. Amino acids are not really volatile. The paper describes the odor sensations of the amino-acids as follows: (D L)-Cysteine: odor impressions:  sulfur ,  rotten eggs . This is readily explained by the smell of H2S, which is released from Cysteine samples and can be detected at low concentration. (D L)-Methionine: odor impressions:  moldy ,  old potatoes ,  rotten dairy products . Such smells can be due to MeSH and derived products formed from methionine by degradation. The  potatoes smell is due to MeS-CH2-CH2-CHO, from Strecker degradation of methionine. (D L)-proline: odor impressions:  smell of semen, sperm, chlorine . This odor impression is due to 1-pyrroline, an oxidative decarboxylation product from proline. The  chlorine is explained since 1-pyrroline is formed from proline by the action of bleach, and what the test subjects described by  chlorine was not really chlorine smell, but the smell produced by bleach on skin (which contains amino acids), also due to 1-pyrroline. It seems the author has just investigated artifacts due to impurities in the chemicals used. Had he ordered amino acids from a variety of commercial sources, he probably could have noted differences in  their smell easily. I am a bit surprised this study was accepted, as most any specialist in odor flavor chemistry could easily have told the above things to the author and warned him of the pointlessness of the study design. ,