Causes and consequences of channelling and how to prevent it

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(Updated 2019 August the 27th)

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 a less resistant way to go through within the coffee puck without extracting all the coffee grounds, 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. And that’s what we don’t want to achieve. To avoid channelling, we need to have a clear idea of what brings to it. Mainly, it’s about our skills. Let’s have a look together.

Different variables bring us to channelling. Here below some tips to avoid it:

Causes and consequences of channelling and how to prevent it 1
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 overfill 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;
  • while making a dose of coffee ensure to distribute the coffee grounds in the filter. People usually underestimate or are not aware that we can just wait for the grinder to grind for us. We should try to distribute the coffee grounds as soon as they come out from the spout. An even distribution makes easier the way we are going to even the surface with the distributor and will result in a tastier extraction;
  • 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 many 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 distributor 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. We don’t need to tamp hard, there is no significant difference (in extraction time and flavours) about how much pressure we use when we tamp. We just need to make sure to tamp on a flat surface, not on a slant;
  • be focused to not hit the filter holder on the group head otherwise, we are going to make small cracks on the coffee puck;
  • do not use any clogged filter basket to prevent uneven water flow.

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 out by a side and spurt from different part of the filter. In the case of a normal portafilter ending with two spouts, coffee comes out from a spout rather than the other one (ideally, coffee should come out from the spouts at the same time).

Causes and consequences of channelling and how to prevent it 2
The OCD distributor made by the World Barista Champion Sasa Sestic. I highly recommend this distributor 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 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 with other particles forming a layer that is going to obstruct the water flow. This leads to an 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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