Causes and consequences of channelling and how to prevent it

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Taring, grinding, weighting, levelling, tamping. I previously wrote about how important is to focus on every single action we took when making shots. Minimising any mistake help us improving our consistency, maintaining high standard of quality for our customers. If we don’t, we are going to have a neverending fight against channelling that we cannot deal with.

ABOUT CHANNELLING

What is and how we can avoid the channelling phenomenon?

Channelling is simply occurring when water forms a channel in the coffee bed finding an easier way to go through within the coffee puck without extracting all the grounds coffee, causing under extraction (we are going to under extract the coffee in the channels and over extract the surrounding area), a weak shot without flavours and crema with a bitter finish.

There are different variables that bring us to channelling. Here below some tips to avoid it:

This is an example of cracks in a coffee puck made by tapping with the tamper and hitting the group head.
  • remove the wet puck of the previous shot, ensuring to clean and dry the basket ready for the next coffee dose: a wet basket would let the water following exactly the damp paths;
  •  use the correct dose of coffee for the basket we are going to use;
    less than a usual dose and we are going to under extract, too much and we are going to overfilled the basket, over extracting and also dirty the gasket of the group head; if we are going to use much less dose in a basket (i.e. 16 grams in a 19 grams basket) we increase the chances of channelling because of two factors: the particle size of the puck is too coarse and there is too much gap between the puck and the shower screen;
  • do not tap with the tamper on the side of the portafilter with the purpose of even the puck: we are going to create too much cracks where water can easily pass through (we would get the same result as when we hit the portafilter to the group head) (it doesn’t make any sense to re-tamp to create a more compact puck; we can only knock it out and grind another dose)
  • use a leveller to create a flat surface coffee puck ready to be press evenly;
  • use a manual tamper or an automatic one like the puqpress: the action of tamping has to be made with enough pressure to create the right resistance in the grounds (we don’t need to tamp hard, there is no significant gap (in extraction time and flavours) about how much pressure we use when we tamp;
  •  be focused to not hit the filter holder on the group head otherwise we are going to make fractures on the coffee puck.

Furthermore, a naked filter helps us to recognise straight away if a channelling shot is occurring: in this case we are going to see the water coming by a side and spurt from different part of the filter.

The OCD leveler made by the World Barista Champion Sasa Sestic. I highly recommend this leveler for the consistency it gives in both distribution and taste result.

And lastly, prewetting minimise our mistakes about the actions we previously made: it’s basically a preinfusion at low-pressure which improves the extraction saturating the coffee ready to be extracted uniformly and evenly. It also decreases fines migration, a phenomenon in which fines grounds go to the bottom of the basket and mix together with other particles forming a layer that is going to obstruct the water flow. This leads to a uneven extraction (channelling) resulting in a bad quality cup.

Last but not least, whatever equipment we use, we have always to remember to regularly replace it with a new one: baskets, portafilter, gaskets, shower screens and burrs need to be replaced depending on the volume of the business.

 

 

 

 

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