Brew Temperature and the Chemex

November 9th, 2007

I remember reading early in my coffee days that someone had done an experiment where they had compared heat loss in porcelain espresso cups of different thicknesses and found that the heat retention of the porcelain didn’t much matter because all the heat went out of the top anyway. Like a lot of what I read on the internet at the stage I didn’t question it, didn’t want to see data but instead found it interesting, tucked it into its little trivia box in the brain and moved along.

After some comments in the thread about using scales to brew my Chemex on brew temperature I decided to perform a few rounds of experiments. Simple really, I just placed a probe (K-type, not ideal but it will do) into the bed of coffee, poured over the water and waited to see what would happen. The first test gae me an unexpected result.

Brew temperature of a Chemex

Brew temperature of a Chemex

What I expected was a steady decline in temperature during the extraction, and over a couple of minutes losing a fair few degrees, due to the area of steeping coffee being quite large. When the probe was in at the top of the brew the total heat loss was about 2C. Not much at all. What was interesting also was the temperature gradient throughout the liquid. At the start it was a couple of degrees from the top of the brew to the point of the filter cone and over brewing that stretched out to about 4 or 5C.

I’ve only done this experiment a few times so if you have the kit and a few extra minutes to spare I’d love to see it replicated – the coffee coming out isn’t much affected by the probe so it isn’t a wasted cup in the morning.

To completely and utterly hypothesize – it seems the floating crust of grounds does wonders for insulation, possibly with the help of CO2. In the next round I’d be tempted to stir and skim like a cupping bowl to see if this accelerates the cooling.

Thoughts anyone?

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