The flavors of your tea: cold versus hot brewing

Have been asked to give a talk at next year’s World Tea Expo about the differences between hot and cold brewed tea. Friend of Pairteas Marzi Pecen was recently in Japan, where she especially enjoyed a cold brewed oolong—enjoyed it even more than hot brewed! — so I was curious to find out how cold brewing might affect the composition of the resulting tea.

To create a tea with cold brewing you rely on the water solubility of compounds at roughly room temperature. By contrast, many compounds that are not soluble at room temperature become more soluble and are released from the tea leaves at hot temperatures. One major group of compounds that are barely soluble at cooler temperatures are the catechins and polyphenols. Their release from tea requires near boiling to boiling temperatures, and a longer steep even then. For a cold brew to become significantly bitter, the brewing time has to be quite prolonged and the proportion of leaf to water high.*

Another compound of interest in oolong is dimethyl sulfide. That is the compound that gives the tea, whether green or oolong, a seashore/oceanic/marine flavor, and is one of the two major compounds in oolong that make people dislike the tea. It is insoluble at room temperature. The other compound is indole, which at very low concentrations is pleasantly floral, but at higher concentrations (or if you are sensitive to it) has an odor that can be politely called animalic. Interestingly, indole is barely water soluble at room temperature, so it will appear in a cold brew at very low concentrations if properly done—in other words at the concentration range where it is pleasant. 

What will appear in a cold brew are the delightfully malty, fruity, and floral compounds in oolong, for example nerolidol, which is considered the hallmark of high quality oolong, and (R)-(-)-linalool, which gives the tea a lavender and sweet basil-like aroma, together with the jasmine-related compounds that make really good oolongs so distinctive.**

Above is nerolidol — the double lines indicate double bonds between carbon molecules, with each carbon at the juncture between lines and at each end. This compound is in the form of a relatively short carbon chain, with an OH (oxygen-hydrogen) group, which help make it more soluble in water—the molecule can let go of the hydrogen, which is then "replaced" by the hydrogen of a water molecule.
Contrast this structure with indole, below, which has two linked circles of carbon atoms—these circle structures are much less soluble in water, because they tend to bond to themselves in a stack, and water can't get in between. Images from Wikipedia.

So if you like your tea bitter and astringent, or if you want catechins and polyphenols for health reasons, hot brewing is the way to go, but if you want to appreciate the deliciously delicate flavors that oolongs offer, it may be best to cold-brew them!

* Sheng-Dun Lin, Joan-Hwa Yang, Yun-Jung Hsieh, En-Hui Liu, Jeng-Leun Mau. Effect of Different Brewing Methods on Quality of Green Tea. Journal of Food Processing and Preservation . 38 (2014) 1234–1243. doi:10.1111/jfpp.12084. 

** Zhu J, Chen F, Wang L, Niu Y, Yu D, Shu C, Chen H, Wang H, Xiao Z. Comparison of Aroma-Active Volatiles in Oolong Tea Infusions Using GC-Olfactometry, GC-FPD, and GC-MS. J Agric Food Chem. 2015 Sep 2;63(34):7499-510. doi: 10.1021/acs.jafc.5b02358. Epub 2015 Aug 19.