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March 15, 2019 /
What Effect Does Bed-Depth Have When Increasing Your Dose to Make More Espresso?

Dose is one of the fundamental variables of any brew recipe, yet probably the one that is most often overlooked. Most of us probably tweak the grind size or yield very regularly, but always use the same dose.

In fairness, this is often for a good reason — it makes sense for a cafe to choose a fixed dose, in order to offer consistency in strength and size of drinks. For this reason, at Barista Hustle we recommend choosing a fixed dose that works for your cafe and equipment and sticking to it.

However, the dose you choose definitely has an impact on the flavour and extraction, even if you keep other parameters (such as brew ratio, brew time, or total extraction %) the same.

 

Choosing a Dose

The most important thing to consider in a cafe setting is how big a drink you want to make. To understand the importance of this, read this classic primer from the archives. That article also explains why you should choose a dose that fits your baskets — or better yet, choose baskets that fit the dose you want to use.

In this post, we’re going to dig deeper, and look in detail at the knock-on effects of that choice, and how it impacts flavour and extraction.

There are three key parameters that the dose will influence: vertical evenness of extraction; grind size; and the risk of channelling. If you’re trying to optimise a dose for flavour, then there’s a major trade-off to make between these three factors.

 

Vertical Evenness of Extraction

When you brew an espresso, fresh water continually enters the group above the puck, and passes through the coffee bed, picking up coffee solids along the way. By the time the water reaches the lower part of the puck, it’s already loaded with solubles, reducing its extracting power considerably.

This means the coffee at the top of the puck will extract much faster than the coffee at the bottom. The bigger the distance between the top and the bottom, and the more coffee the water has to pass along the way, the bigger that difference in extraction will be.

More even extractions generally taste better, and allow you to use less coffee to get the same strength. So reducing the dose can have a really positive effect both on flavour and your bottom line.

 

Grind Size and Darcy’s Law

Changing the dose also affects how quickly water flows through your puck. An increased dose presents more resistance to flow, which means a coarser grind size is needed to compensate.

Darcy’s Law is the equation which describes flow through a porous medium (like a coffee bed). Put simply, this law states that the flow rate of liquid through the bed is proportional to the permeability of the bed and inversely proportional to the distance it travels. In other words, if you double your dose, and therefore double the distance the water has to travel, you halve the flow rate.

The permeability in this case is controlled by grind size — grind finer, and the bed becomes less permeable, with a corresponding decrease in flow rate. This explains why, to achieve a given shot time, a lower dose needs a finer grind size.

For espresso, in most cases the finer the grind size, the better the results. Espresso extraction occurs mainly through erosion from the surface of particles (M Petracco, 2005) — there simply isn’t long enough for water to travel into the centre of particles, pick up solubles and diffuse back out. This means a high surface area (a fine grind and/or plenty of ‘fines’) is essential for good extraction in espresso.

A lower dose therefore allows you to grind finer, which can increase extraction and again will most likely improve flavour. However, there is a major limiting factor: channelling.

 

Channelling

Most of us experience channelling in espresso, most of the time. Some channels may form only transiently, and some are too small to see, while others can blast right through the puck, giving us the characteristic ‘blonding’ or spray from the basket that baristas are trained to look for. Whether you can see the channels or not, they’re there — and there’s no doubt they make espresso less tasty — by causing areas of localised over-extraction, and resulting in underextraction from the rest of the coffee bed.

A lower bed depth makes it easier for channels to form (M Petracco, 2005). This places a limit on how low a dose can go before the channelling really starts to affect the flavour.

Incidentally, this is the reason some lever machines use narrower baskets, which give you a deeper bed for a given dose. The pressure in some lever machines can reach a peak somewhat higher than 9 bars, which creates an opportunity for channels to form, so the deeper bed counteracts this.

A Side-Note on Preinfusion

The most effective pre-infusion allows the entire bed to become wet before the pressure increases so a bigger dose will benefit from a longer pre-infusion time. If you’re able to adjust your pre-infusion time, try setting it so that the pre-infusion ends just as the first drops start to collect on the underside of your filter basket.

The headspace (the space between the puck and the shower screen) also has an effect here — in a traditional 9 bar espresso machine, a bigger headspace may allow a longer time for the grinds to saturate before full pressure is reached, and this can slightly increase extraction. In a pre-infusion machine on the other hand, a bigger gap means you’ll need extra time to fill the headspace before preinfusion is complete.

 

So What’s the ‘Ideal’ Dose for Best Flavour?

Assuming that you aren’t restricted by the size of drink you need to make, then adjusting the dose can be a useful tool to tweak the flavour of your shots. Bringing these factors together, we can see that while reducing the dose allows you to grind finer, and makes the extraction more even vertically, it increases the risk of channelling.

So the ideal dose is essentially the lowest you can use before channelling becomes an issue. Where that point lies depends on many things such as your grinder, your technique, and the brew pressure. And since even channels too small to be seen will have a big effect on flavour, the only way to determine the ideal dose is by trial and error, and lots of tasting.

Reference:

M Petracco, 2005. Percolation. In: A Illy and R Viana (Eds), Espresso Coffee: The Science of Quality, second edition (pp 259-287)

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Qawiem
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Qawiem

Can you comment on the weight-ratings of certain baskets and how that relates to ideal/optimal dose for a particular basket? and when dealing with different types of beans of different densities, which parameter should I prioritise when choosing dose – bed depth or mass?

maartendejager
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maartendejager

What is the effect of pressure with regards to channel formation, and would a different (lower) pressure also have an effect on the extraction? I’ve read that there is something to be said for brewing at 6 bar instead of the standard 9 bar.

My intuition would be that lower pressure would have a similar effect as a higher bed depth; reducing channeling. But I could also imagine that it would have negative effects on extraction.

Insightful article, thanks!

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