Friday, January 5, 2018

Sniffing oolong

While I was looking for something else today, an article about aroma interactions caught my attention: "Evaluation of the Synergism among Volatile Compounds in Oolong tea."*

A fundamental problem in understanding the interactions of aromas has been whether a compound present at sub-threshhold concentrations can influence the overall aroma of a tea. By sub-threshold concentrations I mean concentrations at which you cannot smell the chemical by itself in a solution.

What the authors this paper did was to have trained smellers sniff the aroma of a Tie Guan Yin with and without added (E)-2-hexenal at a sub-threshold concentration. (E)-2-hexenal is characteristically abundant in green teas. The smellers graded the aroma according to degree of roast, sulfur, sweet, floral, and green and grassy aroma.

As you can see in the diagram below, the "roast" and "sulfur" quality of the tea aroma decreased and the "green and grassy" quality increased markedly in the tea with (E)-2-hexenal added.

To me, (E)-2-hexenal has a pungent "green and grassy" smell by itself, indicating that it activates the cool/cold TRPM8 and TRPA1. In fact, the authors of the paper say that (E)-2-hexenal is referred to as having a "fresh note," a term applied to compounds that activate these receptors.

Among the compounds in oolongs that give a "roast" quality is 2,5 dimethylpyrazine, that activates the "hot" receptor, TRPV1, while the sulfur quality is conferred by dimethyl sulfide, that activates the warm receptor TRPV3.

What I think is happening here is that by activating the cool/cold receptors, (E)-2-hexenal even at this sub-threshold concentration is turning off to some degree the warm and hot receptors. While the overall impression remains that of a moderately oxidized oolong, the tea's aroma profile shifts towards that of a pouchong.**

When Friend of Pairteas Kristin van Eetvelt and her sister-in-law were with me in December we had fun sniffing some of these chemicals and then sniffing an oolong tea (a Tie Guan Yin if memory serves). We all declared that sniffing (E)-2-hexenal (obviously over threshold), followed by sniffing the tea was quite unpleasant, and that the beautiful aroma of the tea was severely dampened. We quickly went back to sniffing one of the floral compounds characteristic of moderately oxidized oolongs. After that the tea itself smelled good again!

* JianCai Zhu, Feng Chen, LingYing Wang, YunWei Niu, ZuoBing Xiao, Evaluation of the synergism among volatile compounds in Oolong tea infusion by odour threshold with sensory analysis and E-nose, In Food Chemistry, Volume 221, 2017, Pages 1484-1490, ISSN 0308-8146,

** Worth noting for you food chemists or just curious people out there: the researchers also subjected the samples to analysis by "electronic nose," which showed that the oolong sample with and without added (E)-2-hexenal had different profiles. These results raise the question to me of whether the addition of (E)-2-hexenal actually altered the content in aroma chemicals in the air above the samples by preventing certain compounds from becoming volatile. However electronic noses using metal oxide systems (such as the one used by the authors of this article) cannot identify specific compounds, because compounds (and especially aldehydes such as (E)-2-hexenal) activate more than one of the electronic nose sensors. Electronic noses of this type are good at determining whether two or more products, say ripe and green tomatoes, are different, but cannot say what exactly those differences are. The question remains in my mind, but I still think the explanation above of the aroma change with (E)-2-hexenal can simultaneously be correct.

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