Instaurator revisits the theory behind espresso extraction

Instaurator revisits the theory behind espresso extraction

5 February 2014

In the past five or six years, the approach to espresso extraction has progressed enormously, and has been taken to extremes at all ends of various spectrums.

An old-school traditional single espresso shot was considered to be about 30 millilitres including crema, while a single ristretto shot was about 20 millilitres including crema. This was a pretty straightforward way of approximating the volume of a shot when using coffee that was three to four weeks after its roast date and stored appropriately.

Today, of course, espresso shots are being weighed. A typical weight for the liquid of a double shot may be around 30 – 35 grams. That is 15 – 18 grams per single shot. It doesn’t sound like much liquid, and when some baristas use 20 grams of coffee grinds to produce 20 millilitres per gram of coffee liquid, that’s a 1:1 ratio, and it is even less!

Just over five years ago, as my book on espresso, The Espresso Quest, went to print, it almost immediately seemed out of date. And that’s as it should be, because if innovation is not ongoing then we are not progressing.

But that book was talking about espresso extraction in reference to one particular style of extraction, for a very particular style of coffee. The idea was to simply encourage people to explore different flavour outcomes for themselves.

What became known as up-dosing works in a narrow range of circumstances. If a coffee has been packed to eliminate oxygen in an airtight pack, and is then cellared at a constant diurnal ambient temperature, after about three weeks the crema will be far less volatile. The coffee will not have oxidised and it is possible to dose higher without the unpleasant taste of crema that occurs when the coffee is too volatile. This can also produce a much more viscous and smoother style of espresso shot.

By the same token, an espresso shot made with beans only a couple of days after roasting will require quite different dosing and grind particle size to achieve an optimum extraction and flavour outcome. Using the full range of Scottie Callaghan’s dosing tools, for instance, you can perfect a different dose and grind to achieve a constant flow rate for anytime up to 30 days, given that the volatility will be different on each of those 30 days.

The head space to accommodate the particular volatility of the carbon dioxide on any given day has to be affected, because it will never be the same on any two days. This works by allowing more head space between the top of the coffee grinds and the dispersion screen on the machine, when the coffee is less mature and more volatile. Accordingly, you will have to adjust the grind proportionately finer to maintain a consistent flow rate.

There are actually hundreds of different ways of skinning the espresso “cat” in terms of extraction. This follows the explosion of espresso ratios and yields such as: double ristrettos with a yield of 20 grams of liquid from 20 grams of ground coffee, to Matt Perger-inspired drip-filter style espresso ratios of 16.5 grams to 300 grams (millilitres) liquid, bearing in mind that one millilitre of water weighs one gram.

In case you haven’t noticed already, this article may verge on the geeky/nerdy side of coffee. If that doesn’t interest you, move on.

It is understandable why some people refer to the weight (grams) of an espresso shot rather than the liquid (millilitres) and this is because it seems easier to exclude crema as a factor in trying to achieve consistency across shots. The weight of crema can be as much as 5 – 6 grams in a 34-millilitre/gram shot and will be relatively consistent given the same roast batch of coffee, but the volume can be extremely variable.

This may help to explain why the ground-breaking scientific espresso textbook written by Dr Illy has such a vague unscientific definition of recommended espresso volume.

The old school 30-millilitre shot which, incidentally, is still also surprisingly vague in the World Barista Championship (WBC) rules, has to do with the fact that the majority of old-school Italian baristas would have been working with coffee that was at least a few weeks from roast date and you have less crema and more liquid volume in this case: up to 27 millilitres/ grams of liquid.

In the current WBC rules, the same idea applies. The volume of the shot is evaluated by visual inspection looking down, and includes crema. The trouble with this, of course, is that because most competitors are using coffee that hasn’t been cellared for anything like three to four weeks, the crema is way more volatile and so a shot that looks like 30 millilitres including crema and is approved by a judge may hold as little as 18 grams/ millilitres of liquid including crema, as one competitor recently found. This is actually closer, and in some cases less than, an old-school ristretto. An old-school ristretto was 15 – 20 millilitres including crema. So allowing for the old-school crema it would have been maybe 12 – 18 grams of liquid.

Another recent area of improvement to espresso extraction has been the increased precision of filter baskets.

The improved precision of the holes in the filters, in particular, has been a welcome refinement. In my opinion, IMS now produce filters of equivalent, if not better, precision to the VST filter which basically copies the standard 49 hexagonal hole pattern used in common ‘3-cup’ filters. IMS, who have been manufacturing espresso filters and dispersion screens for most espresso machine suppliers for nearly 70 years, now have a whole new Competizione range of precision filters.

They are also providing some really interesting innovations with both filter baskets and dispersion screens that can substantially alter the extraction space. In addition to improving hole precision specifications, they are also experimenting with hole patterns, numbers, shapes, and even coatings like Teflon. All with the aim of improving the taste of espresso.

And improve it does. With a different hole pattern the coffee flavour becomes much sweeter, cleaner and smoother than even the standard VST filter.

Dispersion screen design can also have a great effect on flavour consistency. As any budding barista knows, trapped grinds and dirty water from the previous shot are major factors in extractions. These factors are among the most under-rated but allpervasive taste faults bedeviling espresso shots around the world. Rinsing between shots doesn’t entirely get rid of all the junk from the previous shot with the design of current dispersion screens.

The new IMS smoother, strengthened dispersion screens greatly improve these results.

The question of storage of coffee beans still repeatedly comes up. Many good baristas in cafes are now at least storing their week’s coffee stock in insulated containers to minimize ‘oil-migration’. Oil migration is a term used to describe the movement of the natural coffee oils within the coffee bean, and occurs as a result of day/night (diurnal) variation in temperatures that can occur in your average outlet. When the ambient temperature is higher, the oils will gravitate to the surface of the bean. When the temperature is lowered, the oils will move inwards. As the oils increasingly move through the bean structure, they become increasingly rancid and flat tasting. This can occur very rapidly during extreme summer temperature variations.

All in all, keep experimenting and keep letting taste guide your espresso extraction results. 

This article was also published at BeanScene