Some thoughts on brew temperature

August 3rd, 2006

I am fully expecting to have this picked to pieces or just generally rubbished, but I thought I would jot down a few thoughts having read a post over at Coffee Aspirations

Let’s treat something going into solution as a chemical reaction (because it is), and let us also assume that this means that increased heat will result in more chemical reactions, or more compounds going into solution. (This comes under the banner of Arrenhius’ Equation I believe, and no I am not going to try and explain it right now!).

So – the hotter my brew water the more I will dissolve from the surface of the ground coffee particles, and this seems to manifest itself to me in the cup. Typically we see it as a bell curve wth cup quality increasing quite suddenly around a certain temperature and dropping off quite radically either side.

Here is a hypothetical espresso extraction, with brew temperature and also exit temperature measured:

(please note this is a very approximate graph – it is just for illustration)

We would expect to see an exit temp lower than the brew temp – energy having been lost through heat loss (radiation, conduction etc) as well as having been spent dissolving lots of wonderful things for us to drink. The shaded section’s surface area should correspond to energy spent.

So if we increase the brew temperature by a small amount (be it 0.1F or 1C) then we are inputting more energy into our little graph:

Question 1 – Do we expect the exit temp to rise by that amount also?

If so – then surely the amount of energy spent (the difference between input and output) would be the same, so therefore would there have been the same reactions occuring regardless? Or would more reactions have occured in the top section of the puck, where the coffee is exposed to the most heat?

Question 2 – Can we compensate? If we are stuck with a lower brew temp can we come close to achieving a similar result by creating a brew time that gives a matching surface area or would the different dose or grind necessary influence the quantity of solubles going into the cup too much?

Other thoughts:

Occaisonally it seems ridiculous that a 0.1F difference in brew temp can change an espresso’s cup quality. Yet if you read about the aromatics present in coffee (and we, as an industry, often talk about the incredible numbers of volatiles in roasted coffee, as if the cells of the bean can barely contain them) you realise that whilst there may be several hundred different ones there are not present in very great quantities. They are capable of having being a character impact odorant (one of the ones that give a product a distinctive taste) at concentrations of parts per million or part per billion.

This being the case then suddenly a subtle shift in chemistry from a subtle shift in temperature could easily mean a big chance in perception. Though perhaps this begs the question of – if I get red fruit notes at 91C but chocolate tones at 93C, why do I not also get a stronger red fruit note at the higher temperature?

Perhaps it is down to the complex relationship between what we taste and the flavours we percieve. Paul Songer briefly spoke to me about how the level of percieved acidity can influence the flavours/aromas you are better able to pick out crediting the brain’s expectation for the difference.

And yet if espresso really is this picky, the cup quality balanced on such a knife edge – how then do people have similar taste experiences with a blend? Perhaps they don’t – I know this is something that is bugging Stephen Morrissey at the moment. Or perhaps the cleverest blends are ones that contain character impact odorants in great abundance, so wherever you hit the shot (within a certain sensible tolerance) you get strong tastes of caramel or whatever they intend.

I also think that the taste of a coffee – its balance of sweetness, bitterness, acidity and on occaison saltiness – is less temperamental. I often find that a coffee will exhibit the same feel and taste in the mouth from brew to brew whilst the flavour/particular aromatics may vary. (This could easily end up spiralling back into earlier thoughts though….)

I seem to have posted more questions that musings, but that is inevitable with any post about espresso these days….

[tags]coffee, espresso, food science, aroma chemistry, taste perception, barista[/tags]

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