Impacts of Basket (Espresso Portafilter) Shape on Extraction.
In this article, the second edition of our espresso basket (portafilter) series, we thought that it would pertinent to look into the difference between basket shapes and their effects on extraction.
The two baskets pictured below, whilst being different in shape, have the same basket volume. Meaning if we were to fill both these baskets with water to the lip, they would, irrespective of shape, hold the same amount. (For an explanation on determining your baskets volume, and why we use water, not ground coffee, click here on how to determine basket volume)
Despite the fact the basket on the right looks taller, the volume of both baskets is the same. How they extract coffee though, is very different.
In layman’s terms, the basket on the left has straight wall, while the one on the right, has a tapered wall. To explain the effect the basket shape has on extraction, we need to delve into physics…… again.
Basket Shape and Reactive Compression
Figure 1. Two basket shapes and their compression effect.
Forces of compression are gradually dissipated the further away from the tamp. Reactive forces reflect the remaining energy at that point. (image not to scale)
When tamping a straight walled basket, the tamp will exert a downward force through the ground coffee to the base of the basket, and to a lesser extent, back upwards again to the tamp. The pressure returning back however will not be the same as the initial downward pressure exerted. Due to compression mechanics (the depletion of energy through the act of exerting these forces), the base of the puck will be of lesser compacted density than the top of the puck.
When tamping a tapered basket, different effects occur from these same energies. The same forces are applied at the top of the puck but the directions of these forces change in the region of the taper. The reactive forces create a lateral compressive effect, like squeezing from the sides toward the middle as well as the base. How much pressure turns inward, depends upon the angle of the basket wall and the distance from the top that it commences. The steeper the gradient (up to 45°), the more aggressive the inward pressure. This inward pressure now helps to create an even tamp pressure distribution throughout the entire puck, something not achieved with a straight walled basket.
While tamping pressure is of itself and less important, the dynamics caused by these reactive forces on puck formation is critically important.
Galleries, Pressure, and Fines Migration
In espresso, we aim for uniformed galleries throughout the puck. By “galleries” we mean the air spaces between the ground coffee particles. The size of these galleries, is very important. In fact, it is debatably the most important aspect to espresso puck stability, as it has flow on effects to quality, Total Dissolved Solids (TDS), and extraction yield. To get the full picture we must view these galleries in context to fines.
Fines are the smallest of your ground coffee particles. A discussion on the purpose of fines, whether they are good or bad, or their effects on temperature and the like, is beyond the scope of this article. However, basket shape and fines migration do bear mentioning. Fines migration is the term used to describe the movement of fines within the basket during water flow. When the galleries, or passages between grinds are great, fines will slide thru, or “migrate” during water flow toward the bottom of the basket. When these galleries are reduced, fines are restrained in the galleries of the puck. Needless to say, that gallery consistency throughout the puck helps to maintain stable fines migration.
Because of varying densities in the puck of a straight walled basket, these baskets will always be more prone to fines migration than a tapered basket. The density consistency within a tapered basket will help retain fines within the puck, those that do migrate will be encouraged toward the centre of the basket, helping to prevent fines migration to the bottom, and by default will give a more consistent extraction.
The two pictured baskets above were filled with the same amount of coffee and extracted at the same machine pressure and the same grind. Naturally because of the lack of inward pressure the straight walled basket with the same grind particle size, ran almost 10 seconds faster. So logically in order for a straight walled basket to maintain a similar flow rate to a tapered basket. It will require a finer grind setting.
Just to clarify. When our two example baskets pictured were tested with the same grind setting, the straight walled basket ran 10 seconds faster than the tapered basket for the same shot volume. In order to deliver a comparable flow rate, the straight walled basket must have a finer grind setting.
By fining up the grind in a straight walled basket we can minimise the gallery size and help to limit fines migration. This however comes at a cost. A finer grind creates a higher puck density.
Basket Shapes Influence on Puck Density
Density is mass divided by volume. To use an example, a bucket of sand because of its finer grind size, will have a higher density than the same bucket filled with gravel. This same analogy can be applied to coffee particle size. The finer the grind, the denser the puck becomes. Straight walled baskets, which need a finer grind to maintain gallery integrity and prevent fines migration will naturally create a denser puck. This can become a problem, especially with lighter roasted coffees. As lighter roasted coffees often have a naturally higher roast density anyway, by fining up the grind, this already high-density bean makes even greater densities in the puck which can lead to a channelling
But what about TDS and Yield?
Strictly speaking it is hard to compare TDS and extraction yields between the two baskets. As mentioned prior, if the two baskets were to utilise the same grind setting, the straight walled basket will produce a faster flow rate, and naturally have a lower TDS and Extraction Yield. By fining up the grind in the straight wall basket to make a comparable flow rate to a tapered basket, we are no longer comparing apples with apples. The new finer grind setting of the straight walled basket has created a higher puck density and greater surface area of the resulting coffee particles. Naturally with a higher particle surface area you would expect to have a higher TDS as a result. Interestingly this was not the case. Shots pulled through both baskets, with the same water flow rate, heralded the tapered wall basket with a consistently higher TDS and extraction yield.
Not all baskets are created equal.
Great effort was made to ensure the baskets used in this experiment were the same volume, in order to make the experiment as accurate as possible. We did however find that the angle of the base of the tapered basket, together with its start position on the basket wall is the most important aspect of these baskets. Experiments on several other tapered baskets didn’t herald as greater benefits as the one pictured. This in itself should make basket manufactures perform more experiments about the pitch of this basket wall, and at what height on the wall, the basket starts to angle.
And the all-important taste?
Taste and flavour has been deliberately left out of these conclusions. Not because the tapered wall baskets delivered less flavour. In fact, quite the contrary. I enjoy the taste better, and use these baskets on many of our customer machines. But as humans we tend to put our subjective taste above all else, and forgo the natural physics that make up this taste. When being an industry professional, understanding how coffee is created is initially more important than the resulting taste. If we put taste first at the cost of process, it becomes hard to duplicate. Only after we fully understand coffee dynamics, can we create a process to deliver the desired taste.
In our next article we shall look at fines migration and its effect of temperature in the coffee puck.
Cheers,
Bush and Bush Coffee Systems
Glossary:
Galleries is the air spaces between the ground coffee particles in the portafilter after tamping.
Fines are the smallest particles of your entire ground coffee particles.
Density is equal to mass (m) divided by volume (V).