Wednesday, September 09, 2009

Afternoon Tea in the Laboratory: Part 2

A few weeks ago, I reviewed an academic paper on tea brewing — how brewing technique and tea brand affect caffeine and dissolved solids levels. In that paper, the authors demonstrated that the highest level of dissolved solids (which correlates with flavor and possibly anti-oxidants) come from loose leaves or a well-agitated tea bag, and that caffeine pours out of the tea most quickly in the early moments of brewing.

During the search for academic papers about tea brewing that led me to the previously reviewed on, I ran across an impressive body of work by Michael Spiro, Deogratius Jaganyi, and several colleagues that spans several decades. While working at the University of Natal (South Africa) or the Imperial College of Science, Technology and Medicine (London), they have published at scores of scientific papers on the subject of tea, including a 15 part series on "kinetics and equilibria of tea infusion." (The word "kinetics" in this context refers to transfer or reaction rates — e.g., how fast is caffeine extracted, how chemical reactions progress. The word "equilibria" refers to the end state of the brewing process — after a long time, what is the stable configuration.)

Spiro and Jaganyi's 1992 paper in Food Chemistry is only a few pages, but contains a lot of insights into tea brewing, so I'll use it to as the main resource. Having the unwieldy title of "Kinetics and equilibria of tea infusion: Part 9. The rates and temperature coefficients of caffeine extraction from green Chun Mee and black Assam Bukial teas," the article explores how brewing temperature affects caffeine concentration. To explain their findings, I've taken their measurements and modeling and created new charts.*

Time, Temperature and Color
This first figure shows Spiro and Jaganyi's measurements and calculations for black tea at three different temperatures (using 4 grams of tea leaves and 200 cm3 of distilled water). The x-axis is time — how long has it been since water was poured over the tea — and the y-axis is the concentration of caffeine in the water (in millimoles per cubic decimeter). The black diamonds are the actual measurements made in the 80 °C experiment; the lines are calculations of caffeine concentration the well-established mathematical model** presented in the article for 70, 80 and 90 °C water (158, 176, and 194 °F). Although the curves show non-zero caffeine concentration at time zero, that is an artifact of the mathematical model, not representation of real behavior.


A few things are readily apparent in the chart. First, for reasonably short brewing times hotter water means more caffeine. For a four minute brew, for example, increasing the water temperature from 80 to 90 °C increases the caffeine content by 36%. Second, for the lower brewing temperatures, the caffeine rises almost linearly during first few minutes, so increasing the brewing time will significantly increase the caffeine level. For instance, lengthening the brew time from 3 to 5 minutes at 70 °C ups the caffeine level by 26%. Third, for water at nearly the boiling point, almost all of the caffeine is extracted in a few minutes: at 90 °C increasing the brewing time from 3 minutes to 5 minutes adds only 13% more caffeine. Fourth, given enough time, the caffeine content will be independent of the brewing temperature — the system will reach an equilibrium point where all of the available caffeine will be extracted from the tea leaves***. However, according to many tea experts, all sorts of unpleasant flavor compounds will also be extracted.

The paper also has data for green tea, which shows similar behavior: hotter water leads to more caffeine, most of the caffeine is extracted in the first few minutes, and so on.  The equilibrium level for caffeine is quite a bit lower: the teas used in the study have equilibrium caffeine concentrations of 2.7 vs. 4.2 millimoles per cubic decimeter for green and black tea at 90 °C and 92 °C, respectively.)

Brewing while the Water is Cooling
This is all interesting, but what happens in a tea pot at home is not what happens in the lab. In many home tea pots (or the Pyrex measuring cup that I use), the water temperature decreases during brewing. I measured mine and found a 46 °F (25 °C) drop over 5 minutes (I have since started covering the measuring cup when brewing tea to reduce heat losses).

The first figure showed a strong dependence of caffeine extraction on temperature, so as water temperature drops, we would expect less caffeine to be extracted. In other words, the "rate constant" decreases.  Using data in the paper and my kitchen measurements, I was able to write an equation that gives the caffeine concentration over time as the temperature of the water decreases.  The next figure shows the results. The smooth curves are based on the data in the Spiro and Jaganyi paper; the large black arrows illustrate what is happening as the tea brews while the water cools. The first arrow shows the approximate progression during the first minute: from zero caffeine to a level that corresponds to water at about 85 °C. In the next minute the temperature drops to 80 °C, and so the arrow progresses at a slightly lower slope. Over the next three minutes the temperature continues to drop, with a final temperature of about 75 °C. My calculation results in approximately 3.3 mmol/dm3 of caffeine, quite a bit less than what one would obtain from a brew at a continuous 90 or 100 °C (about 4 mmol/dm3).



Summing Up
To sum up, the highest rate of caffeine extraction is in the early part of the brewing, higher temperatures lead to higher caffeine levels, and heat loss during brewing will reduce the overall caffeine content. So, for the most potent cup of tea without without brewing for so long that unpleasant flavors are extracted (e.g., 15 minutes), use the hottest water possible and minimize heat losses during brewing.




* A question about chart making:  I make my charts in Excel, take a screen capture, crop in Picasa, export, and then upload to Blogger. During this process, lines lose clarity. Is there a better way to get a chart into Blogger?

** The concentration of caffeine at any time can be expressed as ln [ c / (c - c) ] = k t + a, where c is the current caffeine concentration, c is the caffeine concentration at equilibrium, t is time, k is the rate constant, and a is the intercept. In the paper described above, k and a were determined using linear regression of the experimental data.

*** I'm skeptical that caffeine will reach equilibrium levels in a reasonable amount of time at low temperatures, such as in a refrigerator, but my only evidence is anecdotal. A few years ago, inspired by a segment on The Splendid Table, I tried a cold brewing method  — where you add tea leaves to cool water and let it sit in the refrigerator overnight. During that short-lived experiment, I was incredibly tired, dozing off in the afternoon. My guess is far less caffeine was dissolved in the cool water than for my usual brew.




Random link from the archive: Edible Communities

Technorati tags: Tea : Drinks : Caffeine

4 comments:

Michael Kwun said...

You can dynamically generate charts using the Google Chart API:

http://code.google.com/apis/chart/

I haven't tried it, so couldn't tell you how well it works, and how easy it would be to generate comparable charts.

Crash said...

The free google documents has a spreadsheet with charting, but they might not be as flexible or look as good. I think the ones you did look great.

Nick Weber said...

Your post is very helpful and nicely researched. It was quite helpful for me, because unlike you I prefer to have as little caffeine in my tea as possible. I enjoy quite a large amount of tea while working primarily for its flavor and the other health benefits. However, caffeine is a negative attribute that I have to tolerate. I have bought some decaffeinated tea, but I am unable to find as wide a variety of aromatic teas or different tea types that are decaffeinated. Consequently, I tend to prefer green or oolong teas for their reduced caffeine content. I tried your experiment of brewing a cup of oolong tea at a lower temp (approx 60-70C, as estimated by feel), but unfortunately noticed that the large tea leaves did not open up hardly at all from the crumpled up formation that they have when dry, and the flavor of the tea was pretty weak. I assumed this was surely a consequence of the reduced brewing temperature. I imagine that an attempt at reducing the caffeine content also results in a reduction in many of the other extracts that are obtained from the brewing process (including antioxidants and flavors). What a pity.

So, here's my question for you:

Why is caffeine such an integral drug to human culture? Am I in the minority in that I do not like the effects that caffeine has on my mental or physical state, or the effects it has on my sleeping habits? Is the primary attraction to this drug simply that it awakens you from a drowsy state or is there something else to it? Could it be that in reality it is a method of in some way compensating for human's increased (physical) inactivity due to an ever increasing sedentary lifestyle? In other words, do humans that replace walking, hunting, gathering, and building with sitting at a desk, typing on a computer and talking on a telephone in order to obtain their alimentation and safety have a greater desire and enjoyment of caffeine than their ancestors who had to rely on their physical efforts to survive did? And lastly, is caffeine addictive? How does it compare to other addictive drugs like nicotine, alcohol or narcotics?

I guess those are more than just one question. Just food for thought.

Marc said...

Nick --

Thanks for your comment and questions.

From my reading of the literature, the diffusion of taste components from the tea to the water is closely tied to the diffusion of caffeine, so if you can't cut out one element without cutting out the other. But since the early period of brewing sees the most caffeine release, perhaps you could try pouring boiling water over the tea leaves, waiting one minute, draining the water, then pouring 70 C water over the leaves. The initial minute will extract some of the caffeine and open up the leaves. You'll lose some flavor, of course, but it is worth a try.

"Am I in the minority in that I do not like the effects that caffeine has on my mental or physical state, or the effects it has on my sleeping habits? Is the primary attraction to this drug simply that it awakens you from a drowsy state or is there something else to it?"

I suspect that you are in the minority.

"Could it be that in reality it is a method of in some way compensating for human's increased (physical) inactivity due to an ever increasing sedentary lifestyle?"

The use of caffeine as a stimulant goes back a long time. One of origin stories of tea drinking is related to a monk who found that tea helped him stay away during long meditation sessions.

Caffeine consumption via beverages was well established before the West became a predominantly sedentary culture. According to Tom Standage's fascinating "A History of the World in Six Glasses," caffeine consumption is closely tied to the Industrial Revolution of the 18th and 19th century. Tea drinking was promoted in factories because it allowed workers to work longer shifts. Standage's book goes into this in great detail. In the United States, coffee became the drink of choice, and has been important beverage for assembly line and office workers alike for decades. Unfortunately my copy of the book is out on loan, so I can't provide any more details. The World Affairs Council of Northern California has an audio file of Standage talking about his book, available for streaming or download, and he might get into the subject of tea and the industrial revolution (I listened to is a long time ago and can't remember).

"And lastly, is caffeine addictive? How does it compare to other addictive drugs like nicotine, alcohol or narcotics?"

Caffeine is definitely addictive and result in withdrawal symptoms when the supply is cut off (really bad headaches), but I don't know how it compares to other drugs. You can probably find some authoritative information at websites like mayoclinic.com or webmd.com, or maybe in the archives of the New York Times health section (I have a vague memory of someone writing a piece about caffeine withdrawal there in recent years).