Electronic noses

One of the big problems with using trained panelists for tea aroma evaluations lies in the variability of the human sense of smell, both between individuals and across time for any single individual. 

A solution to this problem would be to develop an electronic nose, which would be able to detect the different chemical constituents of a mixture of volatile chemicals that make up an aroma. The ideal electronic nose would be able to detect the individual voltiles, identify them, then tell you what the resulting combo of volatiles would smell like. An example of the latter problem: two quite different chemicals—ethyl isobutyrate, which has a sweet, fruity, slightly garlicky smell, and ethyl maltol,  which is sweet with a strawberry jam aroma—when put together together give a distinct pineapple aroma.

Existing electronic noses rely on the ability of volatiles to bind to electrodes coated with specific compounds that bind different categories of volatiles. Once the volatile is bound, a current can go through the electrode, resulting in a signal that a detector can recognize. So far these “noses” are not able to perform the tasks listed above, but they can detect differences among different samples, or detect certain individual volatiles that may be indicative of the quality of a tea.

All if this by way of introduction to a paper from Italy about the comparison between volatiles in the processed leaf and ones in the cup among a sampling of Chinese teas.* Here I am going to mention one observation (among many) that I find quite fascinating, and that confirmed what I have thought might be the case for a long time, namely that oolong and white tea were more alike than you might believe when you just consider the processing of each.

The white was a Pai Mu Tan, consisting of a leaf bud and two youngest leaves (W in the graph below). The oolong was a Wuyi Shui Xian, second and third leaves from the top (O in the graph below). The other teas were greens (G), black (B), yellow (Y), and pu-erh (P).

The graphs are designed to show the patterns of volatiles in each of the teas, using a statistical technique called principal component analysis. I’ve circled in red the white and the oolong—the left panel represents the volatiles emanating from the leaf and the right panel the volatiles emanating from the infusion.

While there are obvious differences between leaf and infusion, the white and the oolong teas cluster together in each graph, suggesting that the pattern of volatiles in them is similar.

These similarities, as Torri and her colleagues suggest, are probably due to the long withering process each type of tea undergoes. This allows the leaf to produce all the injury chemicals that we find so delicious!

The typical suggestion is that white tea is ultra delicate, which it may be, but the richness of volatiles that the long withering creates makes the delicacy extremely complex. You often see white tea matched with cucumber or mint. Yes, the volatiles in white tea do activate the cold receptors, so cucumber or mint might seem to be a good match. But the compounds in cucumber and mint are so dominant you lose the wonderful complexity of the tea. 

A better suggestion? perhaps a lightly lemoned pound cake? Let me know what you think. 

* Torri, L., Rinaldi, M. and Chiavaro, E. (2014), Electronic nose evaluation of volatile emission of Chinese teas: from leaves to infusions. Int J Food Sci Technol, 49: 1315–1323. doi:10.1111/ijfs.12429.