Cold brew has a reputation here as a coffee affair, but in Japan it is, on the contrary, a fully-fledged tea preparation technique with its own name: Mizudashi (水出し, "water extraction"). It is a method that suits Japanese teas exceptionally well — thanks to their production technology.

Japanese vs. Chinese method: why Japan wins

As soon as a tea leaf is plucked, the enzyme polyphenol oxidase begins to work inside it — the same one that causes browning in a cut apple. To keep the resulting tea green, this enzyme must be quickly deactivated, and this is done with heat. And here, Japan parts ways with the rest of the world.

In Japan, the leaves are steamed. Hot steam passes through the leaves within tens of seconds, deactivates the enzyme, and simultaneously softens and disrupts the cell walls. For sencha, this is followed by rolling, which further breaks down the leaf structure. Leaves processed this way easily release their contents into water, which is why Japanese tea brews easily across a wide range of temperatures — from cold to hot.

In China, dry heat is predominantly used to stop oxidation. The cell walls break down unevenly, creating a denser, more intact structure that releases flavor gradually. For hot water preparation, this is an advantage — the infusion develops over time and bitterness increases gently. For cold water, however, it is a disadvantage: there is not enough force to extract substances from the firm leaf, and much of the flavor remains trapped inside.

Simply put: a steamed leaf is pre-prepared for cold water; a roasted one is not. That is why cold brew works so well with Japanese teas. That is why a good kukicha in cold water will please you more than a Chinese green tea of comparable price.

One more note for the sake of completeness. With fukamushi (longer-steamed), this effect is strongest. The longer steaming disrupts the leaf structure more, and the leaf then releases its contents even faster, so the infusion is denser and cloudier. With asamushi (lightly steamed), it is clearer and more delicate.

What actually happens in cold water: the numbers

Here we get to the most interesting part. Steeping in cold water is not "everything equally slow" — individual substances react to temperature very differently.

Manami Monobe from the tea research division of Japan's NARO institute in Shizuoka measured this on sencha. The reference point was a hot infusion at 80 °C for two minutes, representing 100 %. The water was around 10 °C — exactly what happens in practice with cold brew: fresh tap water is around 10 °C, and in the fridge it settles at 7 to 8 °C. After one hour, the infusion looked like this:

(Measured on sencha, water ~10 °C, after 1 hour. Amino acids were not measured in this experiment — we will get to them with kōridashi.)

The table can be read in one sentence: the bitter component largely stays in the leaf. So this is not a diluted version of hot tea — it is a different tea from the same leaves, with an inverted flavor hierarchy. Theanine, which is responsible for sweetness and umami, is on the contrary a small and highly soluble molecule that extracts more readily from the leaf than catechins. Therefore, it takes center stage in a cold infusion. But "more readily" does not mean instantly — theanine also needs its time.

How it changes over time

Here we need to be precise, because a lot of nonsense has been written on this. Individual substances behave differently over time:

• Theanine increases in the first few hours and then levels off. It is not an instant matter — temperature and time both help it according to measurements, but compared to catechins, cold water leaves its yield incomparably less impaired.
• EGC continues to increase over hours — time is sufficient for it; it does not need temperature.
• EGCG barely increases no matter how long you wait. It lacks temperature, and you cannot make up for that with hours. A fifth remains a fifth.
• Caffeine is somewhere in between. In the leaf, it is partly bound to catechins, so its release follows theirs.

The practical consequence is more important than those four numbers: the total amount of dissolved solids continues to grow long after the first hour, but the ratio between sweet and bitter components is established early and does not change significantly afterward. Therefore, a longer steep makes the infusion fuller, but does not make it more bitter. So with cold brew, you cannot "over-steep" it the way you are used to with hot preparation — and that is the most pleasant thing about this method.

On caffeine honestly: why "less caffeine" is only half true

Claims about lower caffeine content can be found on every other tea website and are taken as a given. They are not. The numbers above support it, but only under three conditions — and these caveats are so significant that they deserve their own paragraphs.

First, the actual temperature matters. The data cited comes from water around 10 °C, which corresponds to the fridge. But if you brew tea on the kitchen counter at 22 °C, extraction is significantly higher and the difference from hot infusion narrows. The difference between the fridge and the counter here is more significant than the difference between three hours and eight.

Second, you dose more leaves. Because cold water extracts less efficiently, cold brew typically uses two to three times as much tea as hot preparation. A percentage saving per gram of leaves therefore does not mean the same saving per cup — the absolute caffeine content in a mug can be comparable to a hot infusion.

Third, and most importantly, this does not apply to matcha at all. Matcha is not steeped — you drink the entire leaf whisked in. So all the caffeine that is in the powder enters your body, whether you whisk it in ice-cold or hot water. Cold matcha is refreshing and tastes excellent, but it is not a low-caffeine choice. Anyone avoiding caffeine should reach for kukicha.

An additional note for the more curious. If you are after a high EGCG content, cold preparation is not the way to go — only about a fifth transfers into the infusion. A cold infusion has a differently set profile, in which the main catechin is EGC. The NARO institute has long been studying the composition of cold infusions; we stick to what can be tasted, and that is unequivocally sweetness and umami.

Which teas for cold brewing: my favorites

General rule: the more amino acids a tea has and the more disrupted its leaf structure is, the better it performs in cold water. In practice, these four have proven best for me.

Kukicha. Stem tea — a blend of stems, petioles and small leaves separated during processing. Theanine is formed in the roots and travels to the leaves through the stem — and significantly more of it remains in the stem than in the leaf, because the stem is not as exposed to light, which converts theanine into catechins. Kukicha therefore has naturally high sweetness, little astringency, and low caffeine content. In cold water, it is the most foolproof of the four — it is practically impossible to ruin. For anyone wanting tea even in the late afternoon, it is the first choice.

Sencha. Steamed, rolled, pressed — exactly what we described above. In cold water, it loses its typical astringency and reveals grassy-sweet notes that you might easily overlook in a hot infusion. Anyone who knows their sencha only hot is in for a surprise.

Matcha. It is not steeped; it is whisked. The simplest way: one teaspoon of matcha into a sealable bottle or shaker, add a little cold water, shake to dissolve the powder, then top up with the remaining water and ice. The flavor is cleaner and sharper than with hot preparation, and the sweetness comes more to the forefront. But beware of caffeine, as described above.

Gyokuro Karigane. Karigane is a stem tea from shaded gyokuro — so it combines two things that enhance sweetness: the stem, where theanine remains, and shading, which slows its conversion into catechins. In cold water, this produces a rich, smooth and noticeably sweet infusion. Of the four, this is the most remarkable experience.

I first saw karigane served cold at Mr. Sakamoto's in Kagoshima. He told me it started at a friend's wedding. The bride was pregnant and could not drink wine — cold karigane was an elegant solution so she could toast with everyone else. For me, this is omotenashi in practice: Japanese hospitality.

Areek's tip: saving tea from last year's harvest

As a seller, this advice probably goes a bit against my own interests, but a lot of work and effort from many people went into growing honest tea, and it would be a shame to let it go to waste.

If you have quality tea from last year sitting at home, its aroma already faded and the hot infusion tasting flat, do not throw it away right away. Try cold preparation — you can get a pleasant infusion even from such tea.

The logic follows from the numbers above: hot water extracts everything from the leaf, including what is no longer in its best form. Cold water extracts selectively and much more gently — it draws out the sweet component and leaves in the leaf what would spoil the infusion in hot preparation. It will not have the freshness of a new harvest, but you will get an infusion you will enjoy drinking, instead of having the tea end up in the bin.

That is my personal experience.

How to do it: step by step

• Dosage. Approx. 10 g of tea per 1 liter of cold filtered water. Use a lower dose for sencha, a higher one for kukicha and karigane, where astringency is not a concern. You will find your ideal dose only through experimentation.
• Vessel. The simplest is to use Japanese bottles designed specifically for this purpose, but a glass carafe with a lid, a sealable bottle, or anything clean will work.
• Temperature and time. Into the fridge. Sencha and kukicha are usually ready in 2 to 4 hours. Overnight steeping is a convenient solution. Going over 12 hours makes little sense.
• Hygiene. Use a clean vessel, always refrigerate, and consume the infusion within 24 hours.
• Serving. Strain the leaves and serve chilled. Before pouring, gently swirl the carafe; amino acids tend to settle at the bottom.

Kōridashi: steeping in melting ice

There is one more variation — kōridashi (氷出し, "ice extraction"). Gyokuro leaves are covered with ice cubes in a shiboridashi and steeped only in the small amount of water that the ice gradually releases.

Conclusion

Cold brew needs almost no equipment, no thermometer, and no experience. The most important things are quality tea without chemicals, a fridge, and a little foresight the evening before.

If you are unsure where to start, write to us. We will be happy to advise you based on what you already have at home and what flavors suit you. And if you have last year's tea at home that you have already given up on, give it this chance.

Areek

From Japanese farmers to your teapot, with the Rishe Tea seal of quality.

Source of data: Monobe, M. (2018): Health Functions of Compounds Extracted in Cold-water Brewed Green Tea from Camellia Sinensis L. JARQ 52 (1), 1–6. Tea Research Division, Institute of Fruit Tree and Tea Science, NARO, Shizuoka.