Water, heat and vessels: the science of a good cup

What temperature actually does

Tea leaves hold many flavour compounds, and they don’t all dissolve at the same rate. The sweet, savoury amino acids (L-theanine is the best known) come out easily, even in cool water. The catechins, which bring bitterness and a drying astringency, and caffeine, which is also bitter, dissolve much faster as the water gets hotter.

That is why delicate green teas and gyokuro are brewed cool: you keep the sweetness and hold back the bitterness. Black teas, roasted oolongs and pu-erh have been changed by oxidation, roasting or fermentation, and they need hotter water to open up fully.

The water itself

• Hard water (lots of calcium and magnesium, and usually bicarbonate) makes tea darker and duller, and often leaves a film on the surface.
• Completely pure water, distilled or reverse-osmosis with nothing added back, tastes flat and hollow in tea.
• Soft water with a little mineral content is the sweet spot. Many tea drinkers use filtered tap water or a low-mineral bottled spring water.
• Fresh water, boiled once. Many tea drinkers find repeatedly reboiled water makes a flatter cup.

Try this: brew the same tea side by side with your tap water and a low-mineral bottled water. It is the quickest way to hear what your water is saying.

Vessels and heat

A small vessel holds very little water, so it cools quickly, and the vessel’s material decides how fast.

• Porcelain is dense and glazed. It holds a moderate amount of heat and adds no flavour, so it is honest with every tea.
• Unglazed clay (such as Yixing-style pots) is slightly porous and holds heat well. Over time it absorbs oils from the tea, which is why people keep one pot for one family of tea.
• Glass shows the leaves and colour but loses heat fast. Lovely for greens and flowering teas.
• Thick stoneware and cast iron hold heat longest: good for black teas and herbals.

Whatever you use, warm it first. Pouring 95 °C water into a cold cup can drop it to 80 °C in seconds.

Leaf-to-water ratio

More leaf in less water gives a stronger infusion in less time, with less bitterness than a long steep. That is the whole idea behind gongfu brewing, and it is why, when a tea is weak, the answer is usually more leaf rather than more minutes.