Françoise et al ([1], [2]) have identified OR2AG1 as a receptor associated with green pea, beany, grassy, galbanum, and bell pepper aroma characters of substituted pyrazines. This class of aroma compounds tends to have nutty, earthy, cacao, roasty, and potato type odors, and the compounds are almost all OR5K1 agonists (Marcinek et al, 2021), although a subclass of pyrazines exhibits the “green” effect. This subclass is associated with strong 2AG1 agonist activity, while non-“green” pyrazines are associated with weak or absent 2AG1 agonism. Yet, confusingly, combining a non-“green” pyrazine with a 2AG1 agonist, such as phenirate or isoamyl acetate, does not produce the “green” effect (PrimaryOdors internal research). Evidence from electronic nose experiments may indicate that an earthy component is also required in order to build a perceptual greenness from 5K1+2AG1.
To date, at least three earthy-smelling aroma compounds have been found to activate separate olfactory receptors: 2-ethyl fenchol is an agonist of OR11A1 (Mainland et al; Adipietro et al; Dunkel et al); 2-methylisoborneol activates OR3A4 (Son et al); and geosmin can be detected by both OR51S1 (ibid.) and OR52B4 (Ashti Baghaei, K. (2015)). The presence of multiple unrelated receptors associated with different compounds with similar smells is not unusual; compare caramellic furaneol’s OR7A5 ([3]) and OR5M3 ([4]) vs. caramel furanone’s OR8D1 (ibid., Mainland et al, Adipietro et al) or the lack of any known common receptors for mintiness between (R)-carvone, menthol, and methyl salicylate.
But the paper by Son et al shows something interesting: OR3A4 is a receptor for at least two of the known “green” pyrazines, while OR51S1 seems to be a weak receptor for one such pyrazine. This suggests the possibility that 5K1+2AG1+earthy may be a combinatorial code for greenness. It would not be the only such combinatorial code since cis-3-hexen-1-ol is not known to be an agonist for any of these receptors, yet does have a perceptually very similar greenness to that of the green pyrazines.
To test the hypothesis of an earthy contribution to pyrazine odors, the author combined the following on test strips: 1.) equal parts trimethylpyrazine 10%, isoamyl acetate 10%, and geosmin 10%; 2.) equal parts trimethylpyrazine 10%, isoamyl acetate 10%, and 2-ethylfenchol 100%; 3.) green smelling 2-methoxy-3-isopropyl pyrazine 0.1% as a control. Mixture 2 resembles the control in smelling green and beany, though differs in smelling sweatier and more nutty. Mixture 1 smells nutty and earthy, like peanuts mixed with topsoil, and somewhat green.
Certainly a better experiment would be to mix 2-methylisoborneol with the trimethylpyrazine and isoamyl ester. But the evidence presented here does support a role for at least OR51S1 as part of the green note observed in some pyrazines.