Factors affecting Caffeine in Tea Leaves
Caffeine in tea, also known as theine, is one of three alkaloids1 contained in tea leaves. It accounts for 3~5% of tea leaves’ total solid contents.2 Caffeine is a stimulant that has a bitter taste but no smell. It contributes to a tea’s bitter taste and overall flavor.3 It also functions as part of the plant's defense system against the external environment.
Factors affecting the caffeine level in tea leaves include:
(1) Parts of the tea plant plucked and used: Tender young leaves contain more caffeine than mature leaves. Leaf position on a pluck also matters. According to a study by Taiwan’s tea research agency, the first leaf (closest to the bud) contains almost 40% more caffeine compared to the second to the sixth leaf (away from the bud). Caffeine amount is even lower in tea stems or stalks – about 10% ~ 50% of that in leaves. If the tea leaves are actually consumed (e.g. for matcha, tea leaves are grounded into fine powder and consumed), the caffeine intake is much higher compared to using the leaves to brew tea.
(2) Harvest season: The caffeine amount for the four seasons, from highest to lowest, are:
Summer > Spring* > Fall* > Winter
(3) Cultivation methods: Cultivated plants tend to have more caffeine than those growing in the wild.
(4) Production Process: Studies have shown that oxidation levels do not significantly affect caffeine level. However, post-production roasting can make a big difference. Caffeine dissolves readily in hot water and evaporates away with water during roasting. After roasting, water content in tea leaves is reduced from 5-6% to 2~3%.
When I was in Taiwan observing tea makers during the roasting process, it was not uncommon to see a thin layer of white powdery and threadlike substance on top of roasting ovens. That white substance, as you can guess, was caffeine.
Generally green and black teas are not roasted. For oolong teas, tea leaves may undergo different degrees of roasting, or even none at all. TungTing and TieGuanYin (Iron Goddess) are two Taiwan oolongs noted for its extensive post-production roasting. Roasting also results in “maillard reaction” which enhances the sweetness and “throat lingering” of these types of oolong teas.4
(5) Tea Plant Variety/Cultivar: FuoShou (Buddha Hand) and Wuyi varieties are lower in caffeine, while Assam, ChingXing Oolong, ChingXing DaMao varieties are slightly higher. However, the difference is less significant than that caused by post-production roasting.5
Based on the above, you can make certain adjustments when purchasing tea if you are concerned about caffeine intake. Try to avoid young tender buds and leaves (e.g. BaiHao Silver Needles and white teas), avoid consuming the leaves (e.g. matcha), avoid summer harvest teas, and try to purchase teas that have undergone slow heavy roasting.
If your tea consumption is based mainly on teabags or blended tea leaves, it is much harder to control for the above factors. Most major brands need to maintain a consistent flavor profile, and thus have little option but to blend their teas.
(1) The other two are: theobromine (also found in cocoa) and theophylline. In total, alkaloids make up 3~6% of tea leaves’ solid contents. Caffeine is the major alkaloid found in tea leaves.
(2) For comparison purposes, tea polyphenols make up 18~35% (of which catechin is 10~25%), proteins 20~30%, and amino acids 1~5%.
(3) Other solids contributing to the bitter taste: anthocyanidin, saponin, and arginine.
(4) Based on an interview with Professor Chen You-Ren at Taiwan’s National Taiwan University, formerly head of Taiwan’s Tea Research and Extension station. The interview was published in Taiwan’s Common Health magazine, September-2015.
(5) Based on same interview as (4).
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Based on the above, do you wonder why many studies show different caffeine levels for green (non-oxidized), oolong (partially oxidized), and black (fully oxidized) teas? This seems to conflict with those concluding that oxidation does not significantly affect caffeine levels. My hypothesis is that many studies, using generic teabags purchased from supermarkets, do not account for the factors discussed above. Therefore the difference may be due to the different varieties of tea plants used. Green teas are mainly from Japan and made from local Japanese varieties, while India produces the largest amount of black tea and use mainly Assam and other large-leaf varieties. However, as tea makers become more experimental in their tea-making (for example, Taiwan tea maker using local varieties to make black teas, while Indian tea makers experiment with oolong tea), perhaps caffeine difference across different categories of tea may become further blurred.
Another hypothesis, suggested by a fellow tea drinker on Reddit tea forum, is that the difference between black and green teas is due to brewing. Suggested brewing for green tea is often lower temperature and shorter steeping time, in order to reduce bitterness. This makes a big difference in the caffeine that dissolves into the tea (something I discussed on a previous blog post).
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