Destructive Myths: The Dissolved Oxygen Hypothesis

Part I: The Argument

There are a lot of myths and unfounded maxims surrounding the proper preparation of coffee and tea. Most are harmless, or, at worst, detrimental to beverage flavor. But one such myth has resulted in a massive waste of energy and water. I refer to this myth as the dissolved oxygen hypothesis.

The dissolved oxygen hypothesis states that, when brewing tea, one should always use freshly drawn water, and never reboil water in the kettle. The justification given is that water that has previously been boiled has less dissolved oxygen (DO). The result is that many tea drinkers are wasting enormous amounts of energy by dumping leftover hot water from their kettles.

The dissolved oxygen hypothesis rests on two premises: (1) that once-boiled water contains more dissolved oxygen than twice-boiled water; (2) that dissolved oxygen improves the flavor of tea. Both premises are demonstrably false.

Boiling itself does not remove dissolved gases. It is the change in temperature or pressure that affects the amount of gas that a liquid can hold (i.e., the solubility of a gas in a liquid). Solubility decreases as temperature increases. Assuming normal atmospheric pressure and composition, water at 0˚C can hold a maximum of ~15ppm DO, while water at 50˚C can only hold ~5ppm. Once the water reaches 100˚C, solubility is zero. Therefore, if you've brought water even close to a boil, you've removed virtually all the DO. What this means is that neither once-boiled nor twice-boiled water contain significant levels of DO, refuting premise (1).

Studies (Faust & Aly 1998, Pangborn & Bertolero 1972) have found that the level of DO in drinking water does not have a significant impact on its taste. It seems likely that the folk belief that DO improves water flavor results from the fact that running water (e.g., streams) is generally preferred to stagnant water (e.g., lakes), and is also higher in DO.

In principle, DO could soften the tannins in tea, just as decanting a bottle of red wine does. However, decanted wine contains much more DO, on account of its lower temperature, and wine is usually allowed to breathe for at least 15 minutes, compared to the 1-5 minutes that tea steeps for. Furthermore, tea drinkers can control the level of tannins in their cup via manipulation of steeping time, water temperature, and water/tea ratio. In short, a well-brewed cup has no need of oxidation.

Therefore, both premises that lead to the dissolved oxygen hypothesis are false. (1) Tea water does not contain a significant amount of DO, and even if it did, (2) there's no evidence that the level of DO has any impact on the flavor of the tea.

All argumentation aside, I simply cannot tell the difference between tea brewed with once-boiled water and tea brewed with twice-boiled water. I've done the tasting blind, more than once. In part, this post is a challenge to any believers in the dissolved oxygen hypothesis: try a blind triangle test. If you succeed in distinguishing tea made from once- and twice-boiled water, let me know.

All this is not to say that water is unimportant. Water is important. Alkalinity is important. Salt content is important. Minimal iron content is super important. Dissolved oxygen is not important.

Part II: Confounding Results

Here's the twist: I can easily distinguish between fresh tap water and water that has been boiled for an extended period of time. In the process of investigating the dissolved oxygen hypothesis, I boiled a small quantity of filtered water for 5 minutes, then refrigerated it until it matched the temperature of my tap water. I then drew some fresh filtered water, and tasted the two, blind.

There was a clear taste difference between the boiled and freshly drawn water. The freshly drawn water had a clean, crisp finish, while the boiled water had an off-putting twang to it. This was not a hard distinction to make.

I repeated the test using a different pot to boil the water, to make sure there weren't any contaminants in the first pot. Same result: the boiled water tasted worse than fresh water.

Given that DO does not have an impact on water flavor, what could explain the flavor impact of the 5 minute boil? Boiling can have a number of effects besides removing dissolved oxygen. It can also remove chlorine as well as concentrate dissolved minerals by reducing the water. But my water filter removes all detectable chlorine, and the short boiling time did not reduce the amount of water by an appreciable amount. Boiling can also remove calcium and bicarbonate ions (by precipitating temporary hardness), but my water is quite soft, and I have never noticed any scaling (which would indicate precipitation of temporary hardness.)

The only plausible explanation I can think of is that boiling the water would also have removed any dissolved CO2. Dissolved CO2 will form a small amount of carbonic acid, which can significantly lower the pH of very soft water (like mine). The fresh water would then have a lower pH than boiled water. Since most consumable liquids are at least slightly acidic, this might explain why the fresh water seemed to have a clean, crisp finish compared to the boiled water. This explanation predicts that less of a difference would be evident if more alkaline water were used, because alkalinity buffers against changes in pH.

Why then wouldn't removing dissolved CO2 from water also affect tea flavor? Well, the amount of dissolved CO2 in tap water is not enough to have a significant impact on the pH of any solution much stronger than pure (soft) water. A full explanation of this phenomenon would require an in-depth discussion of pH buffering, but this is why the pH of a brewer's mash depends much more on the alkalinity than the pH of the source water.

Even if small amounts of dissolved CO2 did affect the flavor of tea, heating water close to boiling will remove virtually all dissolved CO2, just as it removes DO. For brewing tea, coffee, or any other hot beverage, dissolved gases are irrelevant.

Sources:
http://www.quora.com/Tea/Why-do-tea-instructions-suggest-starting-with-cold-water-You-boil-it-so-does-it-really-make-a-difference
http://www.engineeringtoolbox.com/air-solubility-water-d_639.html
Faust & Aly, 1998. Chemistry of Water Treatment, 2nd Edition. p. 114 (http://books.google.com/books?id=ivLiNH-NjOcC&pg=PA114)
Pangborn & Bertolero, 1972. Influence of Temperature on Taste Intensity and Degree of Liking of Drinking Water. Journal of the American Water Works Association.

Acid Experiments, Part 1: Iced Tea

As I've mentioned in a previous post, I like the acidity that lemon brings to iced tea, but I don't want my iced tea to taste like lemon—I want it to taste like tea. Therefore, I decided to run some trials adding different amounts of various pure acids, to try to achieve a neutral acidity that would complement the tea flavor.

For the first test, I tried malic acid (powder), lactic acid (88% solution), and acid phosphate (a solution containing phosphoric acid partially neutralized by phosphate salts). Malic acid is found naturally in fruit like grapes and (especially) apples. Lactic acid is mainly found in substances that have undergone lactic fermentation, such as yogurt, sauerkraut, sour beer, and some wines and ciders. Acid phosphate was once used at many soda fountains, and has recently made a minor resurgence in the world of craft sodas and cocktails. In the future I will also try citric acid (found in citrus fruits) and straight phosphoric acid (often added to modern sodas).

Since the malic and lactic acids I used are very concentrated, I had to be precise with my measurements. After undershooting and then overshooting my desired level of acidity, I settled on a ratio of 1.3 grams of malic acid or 4.2 ml of lactic acid per liter of tea. I did not much like the taste of the acid phosphate with the tea, so I didn't try very hard to dial it in.

I found that, at similar levels of perceived acidity, lactic acid enhanced the tannic astringency of the tea, while malic acid did not. I therefore preferred the tea with malic acid. I also found that tea acidified with malic acid makes a very nice lightly sweetened tea with (the equivalent of) 3 teaspoons of sugar per liter of tea. And I don't usually like sweet tea!

If anyone wants to give this a try, I'd love to get another opinion. The malic acid was $12 for a pound, and I very much doubt I will ever use all of it.

Note: I do not recommend adding acids to iced coffee. It's weird.

Tea Processing and Brewing

A recent trip to a tea shop made me realize that I no longer have a firm grasp on all of the distinctions between tea processing methods, due to my focus on coffee. Obviously this is unacceptable.

I will limit my discussion here to the five most common types of tea, leaving aside such rarities as yellow tea. I will also avoid discussion of non-tea 'tea', i.e., steeped plant matter that is often called tea but not made from Camellia sinensis (e.g., herbal blends, rooibos, honeybush, yerba mate). The types of tea I'll discuss are: black, green, oolong, and white. All four types are made from the same species of plant, which has two main varieties. Most Chinese, Taiwanese, and Japanese teas are made from the variety Camellia sinensis sinensis, which has a low yield but a smooth, sweet flavor. Most Indian and Ceylon (Sri Lankan) teas are made from Camellia sinensis assamica, or a hybrid of the two varieties. Assamica is higher yielding and stronger in flavor.

Processing

Black tea has the longest tradition of consumption in the western world. The major producers of black tea are India (Assam, Darjeeling, Nilgiri), Sri Lanka (Ceylon), and China (Keemun, Yunnan, Lapsang Souchong). Black tea processing consists of four stages. After picking, the fresh tea leaves are left out in the sun to dry (though not completely), a process known as withering which lasts 8-24 hours. Then the leaves are sent through heavy metal rollers which bruise the leaves, rupturing the cell walls of the tea and exposing its innards to the air. The bruised leaves are left until fully oxidized. The leaves are then fired in a heated drying chamber, which removes the remaining moisture. Ready to drink.

Green tea production and consumption is centered in China and Japan. Some famous Chinese green teas include Long Jin (Dragon Well), Huang Shan Mao Feng, and Bi Luo Chun. Sencha dominates Japanese tea production. Green tea differs from black tea in that green tea is unoxidized. After picking, the leaves are immediately exposed to heat in order to deactivate oxidative enzymes. In China, this is typically accomplished through pan frying or baking, while the Japanese use steam. After de-enzyming, the leaves are rolled into the desired shape, often by hand. The shape of a green tea leaf may identify the variety of tea; Bi Luo Chun, for instance, is rolled into a distinctive spiral. Finally, the shaped leaves are fired to remove the remaining moisture.

Oolong's flavor lies between green and black tea, because oolongs are partially oxidized, with levels of oxidation ranging from 8%-85%. Oolong is mainly produced in China and Taiwan (AKA Formosa). Some famous dark oolongs include Da Hong Pao (Red Robe) and Dongfang Meiren (Oriental Beauty). Some famous green oolongs include Tieguanyin (Iron Goddess) and Dongding. Oolong processing is quite involved. Oolong leaves are allowed to grow larger and more mature before picking, which makes for slower oxidation, allowing the processor a greater degree of control. After picking, the leaves are withered for no more than a few hours. They are then carefully and gently bruised through hand-tossing in baskets. The bruised leaves are left until the desired level of oxidation is achieved, then pan-fried to halt the oxidation process. The leaves are rolled into shape and then fired. Many oolongs are then roasted over low heat for hours or even days, which adds roast flavor, further reduces moisture and improves the tea's longevity.

White tea is produced mainly in China. There are three traditional Chinese varieties: Baihao Yinzhen (Silver Needle), Bai Mudan (White Peony), and Shou Mei, listed here in order of price and prestige. White tea is processed in much the same way that natural processed coffee is: it's picked and then left in the sun until dry (mechanical driers are also sometimes used). A modicum of oxidation occurs during drying. The result is a mellow, sweet, and nutty cup of tea.

Pu-erh is a unique form of tea produced in the Chinese province of Yunnan from assamica plants. Pu-erh comes in two forms: raw and ripe. Raw pu-erh is essentially a green tea that is sun-dried rather than kilned after rolling, resulting in a modicum of oxidation. It tends to taste pretty rough when fresh, so it's usually aged for long periods of time before consumption, which slowly darkens the tea and smooths out the flavor, as well as adding complex, earthy aromas. Ripe pu-erh is a more recent invention, intended to mimic the flavor of aged raw pu-erh. Ripe pu-erh is made from raw pu-erh which is moistened and piled in heaps, then monitored and turned regularly for 6 - 12 months. The damp, warm environment increases microbial activity. Raw and ripe pu-erh are available in both loose leaf and compressed form; pressed pu-erh ages more gracefully.

Brewing

Different types of tea require different brewing techniques. Though all tea should be steeped in hot water, the temperature of the water, the ratio of tea to water, and the length of the steep all vary by the type of tea. To a certain extent brewing practices are culture-dependent—in China, for example, even black tea is generally steeped for a shorter period of time with more tea (compared to Western practice)—so it's worth experimenting with tea/water ratios and steep times, but these traditional methods are a good starting point.

Black tea should be steeped in water of at least 200˚ for about four minutes, though you may want to steep longer if you wish to add milk. About 2 tsp of tea per 8 oz of water should be used; use more if the tea leaves are particularly large, less if they're broken or small. You can steep black tea up to two times.

Green tea should be steeped in ~180˚F water for 2-3 minutes. About 3g / 150ml (2 tsp / 5 oz) should be used. Green tea yields 2-4 infusions.

White tea should also be steeped at ~180˚F, but the steep should be closer to five minutes. About 3g / 150ml (2 tsp / 5 oz) should be used. White tea also yields 2-4 infusions.

Because the processing of pu-erh breaks down most of its tannins, it's near impossible to over-steep. However, it is generally prepared gongfu (see below). In any case, it should be given a quick rinse with hot water before brewing, which removes any impurities on the tea.

Oolong tea can be brewed in the same manner as green tea, but traditionally brewing oolong involves a more complex protocol, known as gongfucha. Water between 180˚ and 200˚F should be used, depending on how oxidized the oolong is. The bare bones of gongfucha are as follows:

1. Pre-heat the tea equipment with hot water.

2. Fill small teapot (3-8 oz) 1/4 – 1/3 full of tea, or 5 – 8 grams / 4 oz. Traditionally, an Yixing clay pot dedicated to brewing oolong is used.

3. Rinse the tea leaves: fill the teapot halfway with hot water, then discard immediately.

4. Fill the teapot full of hot water.

5. Steep for 30 seconds. Pour into a serving vessel, then into several small cups. (This isn't the kind of thing you do alone.) 5 - 8 infusions should be performed in close succession. Increase steep time by ~10 seconds for each subsequent infusion.

Sources:

http://www.esgreen.com/

http://www.youtube.com/watch?v=0Mu0H-Uxasc/

http://www.peets.com/learn/tea_processing.asp
http://www.stoneleaftea.com/roasted-oolong-tea/
http://floatingleavestea.blogspot.com/2011/10/basic-taiwanese-oolong-making-process.html

Iced Tea

Since arriving in Los Angeles, I have found a new appreciation for iced tea. Iced tea inherently lacks the aromatic complexity of hot tea, but when it's over 90 degrees outside, cold beverages have certain advantages.

Fortunately, iced tea is nowhere near as challenging to make as iced coffee. One of the main reasons that my interest in coffee has waxed and my interest in tea has waned is that tea is really quite simple to make. Tea need not—should not—be ground before brewing, as coffee must. Eliminating the grinding step greatly simplifies matters, given how crucial controlling the coarseness of the grind is to brewing coffee.

The second reason that tea is easy to make is that tea seems to be relatively insensitive to extraction (see previous post for info on coffee extraction). Strength is still important, since overly strong tea is unpleasantly tannic, and weak tea is, well, weak. But brewing tea to the same strength with a longer steep time and less tea vs. using more tea and a shorter steep time yields fairly similar results (up to a point). If you tried the same thing with coffee you'd see dramatic differences, due to the differing percentages of the coffee solids that would be extracted. Tea's insensitivity to under- and over-extraction also explains why one can steep tea leaves multiple times, whereas brewing with used coffee grounds... not so tasty.

Since under-extraction is not a big concern, brewing hot tea at double strength and pouring it over ice immediately after steeping yields good results. Brewing tea (regular strength) at room temperature for 8–16 hours also works, provided you protect the tea from oxygen; mason jars work well for this purpose. Hot-brewed and cold-brewed iced teas have different flavor profiles, but they're both agreeable to my palate. I've found cold-brewed tea to be a bit clearer in appearance, but your mileage may vary. One advantage of hot-brewed iced tea is that it can be a more efficient use of tea, if you incorporate a second steep using a longer steep time and adding less ice.

More important than brewing method is the kind of tea used. Some teas make really terrible iced tea. My current favorite iced tea is a blend of two parts Ceylon to one part Keemun. The brisk citrus flavor of Ceylon makes a fantastic base, and the intense floral/incense notes of the Keemun add sweetness and complexity.

Finally, there's the question of sugar and lemon. I prefer unsweetened tea, but sweetened iced tea has a long tradition. I've always been tempted to sweeten tea with honey, since it tastes nice and is (maybe) healthier than sugar. After a few experiments, though, I can't recommend it, unless you really like the taste of honey. The problem is that honey has such a strong flavor that it tends to overpower the flavors of the tea itself. So stick with plain old refined white sugar or nothing at all, unless you're making chai.

I sometimes like lemon in iced tea, because the acidity adds a refreshing brightness and complements the citrus flavors of Ceylon tea. But lemon can easily overpower the tea's flavor, so I usually add only a very small amount—maybe 1/16th of the juice of a lemon. I recently had an unusual idea that I think is worth trying, which is to bypass the lemon and add acid directly to the tea. Homebrew shops sell acid blends for winemakers to use to add acidity to their wines. These blends contain several naturally occurring acids, namely malic, citric, and tartaric acids. Malic acid is found in high concentrations in apples, and citric acid is highly concentrated in (surprise!) citrus fruits. Tartaric acid is a bit less familiar, but wine grapes contain a balance of all three acids. My goal in using an acid blend would be to add acidity to the iced tea without the strong aromatic flavors that lemon also contributes.