Coffee bean degassing

coffee, Roast House

Coffee from the Roasting House, one light roasted one dark roasted. They were roasted within an hour of each other.

How long do freshly roasted coffee beans take to  degas? Should you let the beans lose the carbon dioxide inside them for 24h, 72h, one week, more? Do dark roasted beans degas for fewer days than light roasted beans? As readers of Bean Thinking will hopefully know, one of the aims of Bean Thinking is to bring science, and particularly experimental science, onto everybody’s coffee tables. Is there an experiment (or experiments) that you can do to measure the amount, and duration, of degassing with equipment that you will have in your kitchen?

To help me in my coffee bean degassing experiments, I got in contact with the very helpful people at Roasting House. Based in Nottingham (UK) they will deliver freshly roasted beans to you by bicycle if you live in the Nottingham area or, for the rest of us, by Royal Mail. Together with the cycling aspect of their business, they also have a commitment to supporting those people who produce the coffee. It is important I think, not just that coffee tastes good, but that everybody involved in the coffee process (from grower to consumer inclusive) gets a good deal. Lastly, and very importantly for the degassing experiment, Roasting House offer their beans roasted to the degree that you specify. While they helpfully recommend a particular style of roasting for each bean (dark roast for one bean type, a lighter roast for another), they do give you the option of choosing which you would prefer.

They are also very knowledgeable about their coffee. As I was discussing the degassing issue with them, they suggested a coffee (Daterra, Bourbon Yellow) that they thought would degas quite a lot. Not just that, but the coffee concerned would taste great as both a dark and a light roast (I do drink the coffee after all). All in all, this experiment could not have been done without the help and input from Roasting House and I am very grateful to them for their support in my little project. So, onto the experiment.

The Experiment:

water acidification via coffee beans

Red cabbage liquid approx 96h after roasting and then being sealed in a jar with coffee beans. Note the colours.

To discover the time period over which the beans degas, I decided to utilise an effect that (for reasons unconnected to coffee beans) is currently having an alarming environmental effect: the acidification of water by carbon dioxide. Carbon dioxide dissolves in water to form carbonic acid. With the rising atmospheric levels of CO2, this is leading to ocean acidification, which is another factor in the “global weirding” phenomenon. For the degassing experiment however, if the roasted beans are sealed in a jar with some water, appreciable CO2 degassing will lead to the water becoming acidic, something that is easily measurable.

Experiment 1 – Red Cabbage, Do the coffee beans really degas CO2?

red cabbage, acidity, indicator, natural indicator, coffee bean degassing

The colours of the red cabbage liquid on tissue. Control sample is on the left, light roast in the middle, dark roast on the right

An acidity indicator that you may well have in your kitchen is red cabbage. Liquid extracted from red cabbage is initially purple but will turn blue in the presence of an alkali or red if it is exposed to an acid. For the experiment, three (identical) jars were prepared each containing 60 ml of red cabbage indicator and three (identical) shot glasses. Each shot glass contained either 10g of dark roast, 10g of light roast or nothing (as a control). The coffee beans were kept dry and out of the water by placing them in the shot glasses. The jars were closed, sealed with sellotape and then left. On opening the jars, (approximately 96h after roasting) the two that had contained the coffee beans had turned red (indicating acidity) while the control jar remained purple – see pictures. It is a pretty way of showing the acidification of the water by carbon dioxide and confirms that the beans are degassing. To establish the duration of degassing, it would be necessary to refresh the red cabbage liquid and measure for a further period of time.

Experiment 2 – testing the pH more systematically.

I headed off to a pet shop to get a pH indicator used by people who keep fish (Nutrafin Test). As with experiment 1, the coffee (10g) was sealed in jars (with 30 ml of water) together with a control. When the jars were first opened (at the same time as the red cabbage jars), the jars containing the coffee showed really low (acidic) pH values (approx 6.0 – 6.5). The control water was neutral or slightly alkali (approx 7.5). The water in each jar was then emptied, the jar rinsed and the water replaced with 30 ml of fresh water which was then sealed in the jar, again for 48h. The picture below shows the evolution of the pH with time (measured as hours after roasting) for the jars containing both roasts. The jar containing the dark roast showed a reduced acidity by 192h (8 days) after roasting (the test tube in the picture is greener), compared with about 288h (12 days) for the light roast. Even after this amount of time however, the water was still becoming slightly more acidic than the control, indicating that the beans were still degassing a little.

pH testing, coffee bean degassing

Testing the pH of the water exposed to the coffee bean degassing. The light roasted beans are on the top row, the dark roast on the bottom row. The ‘hours’ is the number of hours after roasting. The pH is measured by comparing the colour of the liquid in the tube to the colour chart.

Experiment 3 – using a bubble system to ‘catch’ the CO2

A third experiment to try to ‘catch’ the CO2 degassing from the beans (in an adaptation of this experiment) sadly did not work on either occasion that I tried it. If you try it and get it to work with with equipment that you can find around the house, please let me know via the comments section below.


The coffee tried here, Daterra, Bourbon Yellow, degassed significantly for 6 days after the roasting date. The time over which the beans degassed, was dependent on the roast type, with the dark roast degassing for less time, consistent with the thoughts expressed here. Degassing certainly continued for many days after the critical ’72’ hours. Even 10 days after roasting, some degassing was still occurring. To be pedantic about things, the gas was not identified in these experiments. However, the acidification of the water in proximity with the coffee beans is consistent with the gas being CO2.

Please do try this at home and send me your results and pictures. Let me know what you find out, whether you use red cabbage or a bubble system that works. One thing that these experiments did not do at all of course was monitor how the beans tasted over a similar time frame to the degassing experiment. Perhaps you have thoughts on this. Please send your comments via the form below, comments are moderated but will (hopefully) be approved pretty quickly after you submit them.

Thanks again to Roasting House for being very efficient about sending me freshly roasted coffee and also to Tyla for helping to independently test the red cabbage experiment.

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