P 3.04 – Turbulence

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In the Coffee Brewing Handbook, Ted Lingle comments: ‘For uniform wetting and extraction of the coffee flavouring material to occur, the brewing water must lift and separate each coffee particle.’ However, he also states, ‘Feeding water to the ground coffee must be done uniformly and gently to ensure that the entire area of the coffee bed receives equal treatment.’ In mechanical applications, to ensure a gentle turbulence, it is our advice that the sprayhead should have as many outlets as possible. This will diffuse the turbulence from each incoming steam from each outlet as much as possible. (We will explore the subject of sprayheads in greater detail in Lesson 4.02)

To investigate the behaviour of the grinds as they come into contact with flows, both from pouring kettles and from the sprayheads of mechanical brewers, we conducted a series of experiments. Based on the results of these experiments, we consider the amount of turbulence caused by a single flow to be underestimated in the industry.

Our first experiment to observe the impact of flow rates was to extract coffee grinds up to a very high level — approaching maximum extraction. By doing this, we were able to get a better visual impression of what’s going on under the surface as we pour into the liquid.

In our second experiment, we laced a batch brewer with a layer of glitter, of the sort used for art projects, against the base of the paper filter. We then observed the spread of the particles throughout the coffee bed after the extraction process was completed. To our surprise, rather than having been trapped in the bottom regions of the coffee bed, the glitter had managed to disperse itself throughout the coffee bed quite uniformly.

In our third experiment which you can see in the video below, we used strong lighting and a see-through V60 cone to view into the slurry.

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