Caffeine magazine

Time for tea?

Matcha, tea in Japan, frothy tea
A Matcha tea in Japan. A lot to contemplate here.

A recent article in Caffeine magazine caught my attention. Emilie Holmes of Good and Proper Tea was writing about the joys of appreciating loose leaf tea. While tea is a little diversion from coffee, January is traditionally a time to look forward as well as back and maybe, BeanThinking should occasionally cross over to the tea side. It was one line in particular of that article that puzzled me. Writing about the ‘naturally “slow” nature of the tea ritual’, Holmes observed that while brewing loose leaf tea you would be able to see “the leaves in a glass pot emit wisps of colour as they infuse…”

It was great to read someone who clearly had spent time carefully observing their tea. And yet that sentence prompted a series of questions in my mind. It was not that I doubted the observation, indeed, thinking back to teas I have made and enjoyed, I realise that I have seen these wisps before. It was more a question of why would it happen, why would the brewing tea emit lines of colour from the leaves? These lines must be telling us something.

diffusion, convection, tea brewing
A tea bag in hot water. The lines of tea are difficult to see in the photo, you’ll just have to do your own experiments, but, streaming from the bottom of the bag, you can see wisps of darker tea-water.

We need to think about how tea brews. A first mechanism would be through turbulence. Hot water poured onto a bed of tea leaves would stir them up and the resulting movement within the pot would mix the leaves with the water leading to a properly brewed cup of tea. This is very much the lazy tea brewers bag-and-cup method (which I can share). It would lead to a brewed tea, but it could not lead to a situation in which you could sit back and see wisps of colour. That requires calm and the quiet moments of a pot of tea brewing while you can enjoy the process.

A second mechanism would be through diffusion. Ultimately the same mechanism as the principle behind how LEDs work, diffusion is where the soluble parts of the tea leaves would travel, through the process of a random walk, throughout the water of the pot. This is a very slow process and we would expect that the concentration of colour would be most intense around the leaves and then fade out gradually with distance from the leaves. We would not expect ‘wisps’ nor lines of tea, that suggests something else.

It suggests the third mechanism of the tea brewing: a mix of diffusion and then convection within the hot water of the pot. The lines of tea are indicating that within the cup, regions of the hot water are at slightly different temperatures. Owing to the hot water being in contact with cooler air surrounding it, the surface of the water is cooling down and sinking, leading to a convective motion within the water inside. As the water moves it carries the diffused tea with it into new areas of the water, a movement of hot water to cooler water and back again. The tea is carried in a line because the convection patterns are occurring in small cells within the tea pot, small regions where hot tea is moving towards cooler tea which is warmed and itself moves. The convection does not happen as if the hot water is one big mass but a series of smaller ‘cells’. We see similar cells on the surface of the Sun. The lines are telling us of the movement in the tea pot and the amount and speed of their movement reveals more about how hot the water is relative to the air outside the pot.

diffusion only
A tea bag in cold water: This time, there are no wisps of tea as the drink brews. Instead, there is a slow diffusion of tea infused water from the bag outwards.

Testing this idea I required tea bags. My tea pots are opaque and so would not help me to appreciate this detail of brewing a cup of tea and so it was back to the bag-in-cup method. However, in order to avoid turbulence, I poured the water (hot or cool) into the mug before adding the tea bag. It was not the best way to make a tea, apologies to tea lovers, but it was a tea that I do not enjoy anyway, so it was good to use it up. Sure enough, when the tea bag was put into the hot water, within a very short time, wisps of coloured water formed lines curling underneath the bag. Why did they flow down? Was it because the tea in the bag was slightly cooler than the hot water and so, as the tea diffused out of the leaves it moved with convection downwards because of gravity and the fact that cooler water is denser? A tea bag in cool water however behaved differently. The water in the cup had been taken from the tap and then left in the cup for a couple of hours so that the water was definitely at the same temperature as the room. This time, the tea bag first floated and then sank to the bottom of the cup. There was no obvious infusion of the tea-coloured water into the plain water but slowly the region around the bottom of the tea cup at the bag turned browner with the tea. As time went on, this region expanded to give a tea layer and a water layer.

The wispy lines of tea only happened when using hot water. Which suggests a further experiment. How do these wisps change when brewing for black teas as opposed to green teas (which use a lower brewing water temperature)?

After about five minutes the tea brewed in hot water (left) was fairly evenly distributed throughout the cup whereas the tea brewed in cold water (right) showed a distinct layering between concentrated tea at the bottom of the cup and plain water above that layer.

One last observation with these tea bags in the hot water. Some of the tea floated within the bag, some sank, as time went on, more tea leaves fell towards the bottom of the bag (which was itself floating). What was happening there? Maybe if you experiment with your tea, you can let me know in the comments below, on Twitter or on Facebook. There are definite advantages to slowing down and brewing a proper cup of tea.