BH App Archive

Flavour Extraction from Ground Coffee

Quick Start – it is not obvious to many coffee lovers that small grains and the outside of large grains experience full extraction, while the interior of large grains shows no extraction. This graphic shows why.

As you increase Time…

the flavour molecules, (of whatever type, red or blue) escape into the water. The % that escapes depends on when water reaches them, and how far inside they are, because it gets harder to escape the nearer they are to the centre. For the small particles there is already some flavour extracted at t=0 because some molecules will already be on the surface and instantly removed.

In the graphic when all the flavour molecules are extracted this is “100%”. In real coffee, the insoluble solids such as cellulose account for about 70%, so maximum extraction is 30% of the total weight. Therefore “100% of flavour molecules” means “30% of total coffee solids”.

This graphic was inspired by the excellent podcast featuring Matt Perger of Barista Hustle. It points out that if you have small coffee grains then they are 100% extracted very quickly. Larger grains are 100% extracted at the outside but can be untouched in the middle, even if the water gets into the middle. So your extraction is not “X%” of the flavour; it is 100% of the small/outer portions down to 0% on the interior. If you have evenly ground fine particles AND if the water can flow nicely past them, then you get 100% extraction of the flavours. If you have large particles then it takes a long time to access the flavour in the middle, which might give time for nasty things to happen to the flavours that have been quickly extracted and are sitting around getting damaged in the hot water.

Or with large particles you might be tempted to hit them with hotter water in the false hope that the flavour will extract faster. Or you try too hard with high-pressure water and it simply flows badly through your espresso bed.

In other words, if you have particles small enough to give good extraction with controlled flow, life is easy and you get the full flavour. If your particles are too big then torturing them won’t help and will kill the flavours that are quickly extracted from the outer portions.

Top simple tip from the podcast? Sieve your ground coffee and put the larger particles back into the grinder.

Made by Professor Steven Abbott, especially for Barista Hustle!

This app was made especially for Barista Hustle and our subscribers by Professor Steven Abbott. His extraordinary career has taken him all around the world, with gigs that have included working with banana growers in the Philippines, printing companies in Colombia and a coffee bag valve company in the USA. Prof Abbott is a world expert on drying science and diffusion. His of apps and ebooks are a go-to for anyone serious about the science of extraction.

See Professor Steven Abbott’s collection of books here!