When Scientists say a tea has a “faint scent,” evidently that means they like it.
The world of tea science is, perhaps surprisingly to a Westerner, flourishing. We don’t hear about all the research being done, but since it is such a major cash crop in East Asia, you can bet that countless teams of researchers in China and Japan are working around the clock to analyze just what makes the best tea possible. One study that came out last year struck me as particularly interesting, especially when considered in the context of matcha.
The study, by a team of Chinese researchers led by Pan Pan Liu, and published in the journal LWT - Food Science and Technology, is entitled “Characterization of stale odor in green tea formed during storage: Unraveling improvements arising from reprocessing by baking.”
In this study, our researchers chose three types of green tea - Basket Fired, Roasted, and Steamed - and aged them over the course of 60 days, testing their aromatic qualities at various points throughout the process. The teas were held at around 100 degrees Fahrenheit, and 40% humidity. Not the normal temperature of most homes, but not too far from room temperature, and certainly an achievable temperature on an ocean freighter making its way across the Pacific in late May and June. The results confirm what we have tasted and experienced for ourselves: with minimal aging, the aromatic qualities of green tea leaves change, often for the worse.
Matcha is made from Tencha, which, like most Japanese green teas, is steamed. Therefore, I’ll be paying particularly close attention to the results of the Steamed Tea (ST) in this study.
The aromatic qualities tracked over the 60 days included “faint scent, flowery, fruity, roasted odor, and stale odor.” Just what is meant by “faint scent” is unclear, but it is referred to here as a positive characteristic of the tea, rather than a flaw (as in, the tea doesn’t smell like much). The researchers rather seem to imply that “faint scent” refers to an aroma of freshness, perhaps a touch of grassiness, like a spring breeze. It’s hard to describe, but if you are familiar with high quality green tea, in particular the Chinese concept of high quality green loose leaf tea, you are likely familiar with the light, refreshing, dewy and slightly sweet sensation that is very, very hard to put into words. It characterizes famous green teas in China like Longjing and Anji Bai Cha.
Anyways, the above is important to grasp, as our “faint scent” category ends up being a pretty big deal in the process of aging green tea leaves. Let’s get into what they found.
For starters, on day zero, the researchers identified the Steamed Tea as having “the highest intensities of faint scent and fruity descriptors.” There are several aromatic compounds linked to these descriptors. The strongest one identified for “faint scent” is (Z)-3-Hexanyl Hexanoate, which is later described, in a table, as tasting “fruity.” Evidently, fruitiness and faint-scent-iness are linked here. Anyways, that’s just the most abundant relevant molecule. Ultimately, 70 separate aroma components were identified in the Steamed Tea.
So, what happened as the teas got older? By day 60, of all of the aromatic qualities, “faint scent” decreased the most. In the Steamed Tea category, it was found to have been reduced by a whopping 89.3 percent! That is, the predominant aromatic compounds of Steamed Tea were nearly 90% reduced after 60 days. What aromas were left in its wake? Well, certain aromas did actually increase over time, namely stale ones. The researchers found that stale odors appeared after just 7 days of storage, getting worse rapidly before day 30, and reaching peak degradation by day 60. Of course, they didn’t test beyond 60 days, so it may have gotten worse after the fact as well.
What exactly is happening to the tea leaves here? The aromatic compounds come from a range of classes: aldehydes, ketones, alcohols, and heterocycles. Aldehydes, in particular, are the largest class of aromatic compounds in green tea, and it is precisely the aldehydes that decreased the most over time. Aldehyde-reductase was found to drive the breakdown of these aldehydes into alcohols, such as α-terpineol, which is “described as an earthy, stale odor, and had the highest AI (Aroma Intensity), of the alcohols." What’s also interesting: aldehydes are produced, in part, by the breakdown of amino acids - something which matcha has in abundance. This could explain why, traditionally, tencha in Japan is aged underground until about November, at which point its aroma has developed fully and the tea is considered ready for milling. This is all conjecture on my part, but part of what may be going on is letting the amino acids in the tencha break down to produce the best aroma possible. Once the tencha is milled, however, the surface area exposed to oxygen is way greater than it is for a loose leaf tea, leading to a rapid increase in the rate of breakdown of all molecules. This may mean that the amino acids break down to form more aldehydes, producing a more aromatic tea a few days after milling occurs. Meanwhile, the researchers in this study found that “stale” aromas increased after only a week. This would be consistent with what I’ve heard from a few people, and found to be my own experience - namely, that the sweet spot for fresh matcha is not right off the mill, but a few days later (about when it arrives at your door, if you’re ordering from us!). At this point, we could surmise that the formation of new aromatic aldehydes is greater than the development of the “stale” compounds.
Okay, looking beyond aldehydes. In the aged tea, the proportion of ketones increased. Carotenoids in the tea leaves broke down over time, creating new ketones like β-ionone and α-ionone. In studying their data, the researchers concluded that “β-ionone and α-ionone contributed greatly to stale odor.” I’ve also already mentioned the alcohol, α-terpineol, which derives from the breakdown of aldehydes and also contributes to a stale flavor.
Finally, in summing up all of their findings, the researchers determined that “For the characterization of stale, the correlation coefficient of aroma compounds was just opposite to that of their faint scent and fruity correlation coefficients.” That is to say, insofar as the tea became “stale,” it lost its qualities of “faint scent,” and “fruity.” As you may recall, those are the two flavors most abundant in steamed tea! So to summarize, the steamed tea lost 90% of its “faint scent” molecules by day 60, and acquired new “stale”-scented molecules in roughly equal proportion.
Whew! That was a lot! If you’re still with me, thank you for reading all of that. It was dense, but here’s the main points: over the course of 60 days in 100 degrees Fahrenheit, unmilled green tea leaves became measurably “staler” in aroma, with fewer “fruity,” and “faint scent” notes. Steamed tea, as that which makes matcha, had the largest decline in desirable notes, at just under 90%. Now consider that the vast majority of matcha in the West is not only from steamed tea, but also pre-milled and shipped across the world, often by slow boat. 60 days at 100 degrees (though most degradation happened just by day 30), is not far off from the storage conditions encountered by most commercial matcha on its way to you. I don’t know about you, but I would really like to find out just what these researchers mean by “faint scent.” Clearly, that can’t be done unless the tea is fresh!
As a fun aside, I went through the data and looked at the molecules that changed the most over 60 days. Here are some summaries:
Link to study here.
Citations:
Liu, P. P., Et al. (2023). Characterization of stale odor in green tea formed during storage: Unraveling improvements arising from reprocessing by baking. LWT - Food Science and Technology. https://doi.org/10.1016/j.lwt.2023.114458