Does brewing tea for longer result in higher caffeine content?

I recently observed the effect of time of steeping on caffeine content in tea. We used High Pressure Liquid Chromatography to determine the levels of caffeine in Green Tea in samples that were steeped for 1, 2, 4, 10, 15, 30, 60, and 120 mins. The concentration of caffeine over time did not show any significant trends. Our results suggest that caffeine content does not depend on the steeping time and instead the caffeine dissolves in the hot water within a minutes of starting to steep the tea. Hope this helps.

First off, the ISO standard is not intended to produce a good cup of tea. It is designed to produce a consistent one for taste testing, so that no tea manufacturer can claim that his tea wasn't made "properly". It's title is "Tea -- Preparation of liquor for use in sensory tests"

As for the actual tea making, yes, leaving the bag in longer will make a stronger cup of tea. The concentration of caffeine (along with flavour molecules and everything else) will slowly trend towards an equal concentration in the leaf and in the water. The longer you leave the tea bag/leaves in the water, the closer to equilibrium you will get.

There are other factors that affect this, such as the temperature of the water, cut of the leaves, bag versus loose leaf and so on, but the trend is always towards equilibrium as time progresses.

I'm not sure where the upper limit of this lies, but I think that once the cup is cold there's no point in it anyway. Thus, leaving the bag in for an hour is a bit much. I usually steep my tea for 3-6 minutes, depending on how strong I want it to be.

In case you're looking for a scientific approach, this paper has some plots showing the amount of caffeine extracted as a function of temperature and time. Look for the "CA" label in figures 2-5. They steep the bags for 30 seconds at a time and record the relative & cumulative amounts of caffeine (and other ingredients) extracted from the tea.

AFAIK the ISO standard is not for enjoying tea, but for "tasting" it. Most people don't brew their black tea that long

Most commercial teas are in a fine grind state (fannings) and should not be brewed with 95°C+ water for longer than 2 minutes or bitter tastes will become apparent

Whole leaf black tea can be brewed with 95°C+ water for more than 3 minutes to get full flavour

Tea has little bio absorb-able caffeine in it, from 10mg to 70mg, many in the range 20mg to 40mg

Normal brewing gets most of the caffeine out

As I understand it, medically you need 100mg+ to get a physiological dose (an effect)

A cup of coffee is 100mg to 200mg of caffeine

So if you want caffeine drink coffee

There have been a number of scientific studies addressing the question of how much caffeine is extracted depending on brewing time. Perhaps the most cited is from 1996, but a 2008 study (carried out to debunk the myth that tea can be decaffeinated with a 30-second steep) also gives some useful data with a variety of teas.

To address your question specifically, approximately 70-80% of caffeine is removed, on average, in a 6-minute steep with boiling or near-boiling water. It will vary depending on variety of tea (green, black, etc.) and form (whole black tea leaves release caffeine the slowest; black tea bags containing tea fannings the fastest). Regarding the 30-second "decaf method," only about 10% of caffeine is released in that short time, so it's hardly effective. You'd need to steep for at least 5 minutes or so to remove a significant portion of the caffeine.

Given the number above for a 6-minute steep, the maximum amount you could expect to extract from a longer steep for hours or days would be 20-30% of the original caffeine content of the leaves. So you could potentially increase the amount of caffeine in the final brew to maybe 1.25 times of the 6-minute cup or a little more, depending on variety.

However, it should be noted that more than 90% of caffeine will be released by 15 minutes, so steeping for hours or days is not very productive. If, for some reason, you wish to extract the most caffeine from the leaves possible, I would recommend multiple short steeps (5 minutes or less) instead, perhaps with a higher concentration of leaves. Using fresh water periodically will allow faster extraction of caffeine, and you'll also avoid the inevitable bitterness that generally comes from a single long brewing.

(I should note all of the above regards typical brewing with relatively hot water. Brewing tea with room temperature water or with cold water will significantly increase the time it takes for caffeine to be extracted. In that case, brewing for hours may be necessary to allow large portions of the caffeine to dissolve.)

"Stimulant action of tea is strongest when allowed to steep for only 2–5 min. as caffeine dissolves quickly in hot water. Longer steeping times (10–20 min.) will increase the yield of catechins, which decreases the stimulant effect because the polyphenols bind the caffeine."

Source: Clinical Overview - Tea, Black/Green http://abc.herbalgram.org/site/DocServer/Tea.pdf?docID=861

The stimulating quality of tea is due more to Theobromine. I read once on my English tea packet about 'stimulating' versus 'calming' brews being dependent upon length of brewing time.

This seems to bare that out: http://nobleharbor.com/tea/caffiene.html

Personally, 1 bag for 1 cup at 3min is too strong. Instead of reducing brewing time, I increase water by a half cup (stays hotter that way too), getting the full-flavor out of the leaves.

This study claims that the caffeine content doesn't change and reaches its max at 4 minutes (law of diminishing returns?). (Assuming you put in hot water, see link.)

https://slicesofbluesky.com/afternoon-tea-in-laboratory-part-2/

You can make a quick cup of "decaf" tea, by letting the bag steep for under a minute, pouring off the water, then pouring more water over it and letting it steep again. The poured off water will contain about 80% of the caffeine. link

So you're probably not going to get a lot more caffeine from a longer steep time.

No. Caffeine is extremely soluble in water. So all the caffeine comes out within the first 10 seconds of brewing.