A Brew Temperature Experiment

A question has burned a hole in me for many years now –

Why (specifically) does a change in brew temperature result in a change in the discernable flavour in the cup? What is actually happening in this process that has such a dramatic influence over the flavours – allowing a skilled brewer to find a very appealing ‘sweet spot’ of flavour from a specific roasted bean?

Let’s list off some possible thoughts from the ether on that one…

Being a coffee nut (putting it mildly), I love to know why things happen. Guessing drives me nuts – because I’m nearly always wrong (well, mostly – perhaps sometimes – maybe once – not that I can recall… lol).

So, it’s not a new ‘experiment’. The information is out there if you look in the right places – but, after running the experiment, I actually revealed something I didn’t expect – read on…

The important part of any experiment is to eliminate enough variables to allow a conclusion as to the reasons why when you change the brew temperature, the coffee’s flavour in your cup changes too – quite dramatically…

I created 5 samples. Each had 2 grams of caster sugar mixed with 1 gram of Acetic Acid and 1 gram of Tartaric Acid (all weighed to the gram + or – 0.05 gram (I have fancy scales)). Both of these acids are found in a roasted coffee bean. I would have preferred to use the 2 most prolific found acids in a roasted coffee bean (Quinic Acid and Caffeic Acid – by products of Chlorogenic Acid breaking down during the roasting process) – but, one of them is $4 per gram and the other $27.50 per gram… so I stuck with the ‘supermarket’ priced acids…

The reason behind adding sugar to the samples was so I didn’t swallow my lips after tasting the samples… to add a bit of balance.

I warmed up my machine sufficiently, conducting a session of shots before commencing the trial – only to ensure the machine was sufficiently stable in its brew temperature of course… I took one (or 5) for the team.

I created a measured 30ml extraction (using double filtered (down to 0.5 micron) rainwater) from each identical sample at 90°C, 92°C, 94°C, 96°C and 98°C. None of the receptacles into which the sample was extracted was pre-warmed. All receptacles were identical and all were washed thoroughly in said rainwater and dried thoroughly before commencement utilising the same clean wash cloth.

Whilst the samples were still relatively hot, I tasted them both going up in the temp order and reverse, aspirating and swishing before expelling. I even missed steps in both directions to try and gain a more distinguishable difference in what I was detecting between samples.

At this point, I should give you some small confidence that I have a measure of a palate that is capable of discerning between different beans flavours, brew temperatures and roast levels of said beans without too much trouble. I drink a ristretto style shot. I would average between 4 and 8 shots on a regular day – up to 20 or more on a testing day. I roast (currently) 3 days per week with anything between 1 and 5 separate roast batches each day – tasting a sample from each batch. I roast anything between 2 and 5 different bean origins each week. I would have tasted close to 100 different bean varieties from around the world, each one many times over at different roast levels and at different brew temperatures. It would be fair to say that I enjoy experimenting – and I don’t drink coffee to be sociable – it’s all about what’s in the cup and how it translates on the palate – I take notice. This doesn’t make me a world class judge (or any type of judge for that matter), but I hope it demonstrates that coffee means something to me and I can tell a Guatemalan from another Guatemalan…

Regardless, after trying all of these samples of the acid/sugar/water solution whilst still very warm, there was no detectable difference of note whatsoever at either end of the spectrum and all the way in between.

I must admit, I was a bit disappointed (a lot really) – as I truly hoped this would reveal some results that would create some simple findings that could be transferred to the coffee making process to some degree or another.

The truth is – that at this point of the ‘professionally run’ experiments that I have read up on, there is a difference – when more of the available acids found in coffee are used in the samples. These acids absent in my experiment react differently (relative to their volatility) when brewed at different temperatures – resulting in the significant differences in flavour that you taste in your cup.

Anyhow – back to my experiment effort –

I returned to the samples approximately 30 minutes later when they were all cold to toss them out – I hadn’t done it straight away as I was distracted by my kids clowning around.

Before ‘sinking’ them, I thought I’d give them one last try – and I’m glad I did…

Lower registers (90°C – 94°C) very very slightly transitioned between pleasant sour/sweetness to fractionally more sweet – a bit like lemon juice with a touch of sugar. But, as I moved further up the register (96°C to 98°C), the difference was significant – to the point where after tasting the 98°C sample, my lips almost touched my tonsils from the bitterness and my eyes nearly closed over for good…

A result!

So, what can be drawn from this?… Sugar is not a volatile product (to any reasonable measurable amount) at these temperatures – yes, it changed state (dissolved), but for the most part, what went into the solution is what remained there throughout. But, the acids I used were volatile. Things changed relative to brew temperature as other variables that would reasonably influence this result had been engineered out. One acid more than the other – and quite frankly, who cares which one – as I’m not out to work out exactly what is happening – I’m purely trying to show that a reaction (or change) has occurred – that that change was relative to the temperature it was brewed at – BUT – that change (in the acids I used) was not immediate. The acid(s) changed over time – dramatically changing the cup taste.

So, in summation… the compressed coffee puck contains a variety of acids of varying levels within it prior to extraction. A hotter extraction should yield a greater level of dissolved solids (very simply because more of the ground coffee ‘dissolves’) – and with it, greater flavour – but these total dissolved solids of themselves do not give rise to an actual flavour change. The most significant tastes we detect in an extraction are acids and sugars (flavours are a different thing from taste as they include your smell and taste combined. Here, we are concerned mainly with salt, sweet, sour, bitter, etc).

Specifically, due to the volatility of the acids, what we detect in each extraction changes as the brew temperature changes – acids will ‘boil off’ due to their inherent volatility – whether during the extraction or for a period afterwards (as was demonstrated in my experiment) – while other acids will become more prominent as balances within the extraction change. At certain points (both in brew temperature and time displacement from when the extraction is initiated), the extraction will taste either pleasant or toilet to the palate. These parameters will also change from bean origin to bean origin and in each bean’s level of roast – therein lies the art of both roasting and brewing…

Give it a crack.

Add any thoughts below – that’s if you’re still awake…

(Just for info – if you include any sort of link back to your website without seeking approval, your comment will end up in the spam folder. I’m chasing constructive comments please. Cheers).

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