Effective experimentation

January 17th, 2011

Like a lot of people in coffee I tend to do little experiments from time to time. Side by side brews of different grinds, or different espresso recipes. I take the results of these experiments and incorporate them into my understanding of coffee, which is still pretty fragmented (to say the least).

For every complex problem there is an answer that is clear, simple, and wrong.
- H.L. Mencken

As an industry we tend to set up simple A/B experiments, or sometimes something a little more complicated. We usually think we’re testing one variable (but with coffee brewing I am not sure if this is often the case) and we get results. Most of the time we get more correlation than we do causation. Just about everyone one of our experiments could be improved by a better understanding of method. This is where I fall down too.

There are, fortunately, some very smart people who read this blog from time to time and I hope they weigh in here. I have a little theory, I’d like to propose some experiment but I’d be interested to get some input on how to do it and see if people would also like to join in to share the data. (I know a small data pool doesn’t invalidate an experiment, but if we’re looking for statistically significant results then a larger data pool would be good to prove or disprove the theory.)


Brew temperature in espresso can be used to help negate the damaging effects of hot coffee grinds resulting from a grinder under heavy use.


The coffee bed absorbs heat from the brew water during the extraction process. The amount of heat lost here would depend on three factors: the amount of coffee, the initial temperature of the coffee and the flow rate of water through the coffee. 1

We know brew temperature has an effect on an espresso’s cup quality. 2  If we reduce the brew temperature, because the coffee is going to loose less heat, this may improve the espresso.

Experiment 1:

The first experiment should be identifying whether hot grinds are as bad as we think.  I propose a hot vs. cold grinder experiment.  Shots to be pulled on a machine that can deliver the same pressure and temperature profile on both groups.  Shots should be pulled to the same spec in terms of weight of dose in, weight of dose out and brew time.  In order to be compared shots from both the hot and cold grinder must fall within acceptable tolerances of each other (open to suggestion here but I would say 0.1g, 0.2g and 1s respectively).

They should be tasted blind by a panel of tasters.  I’d be interested to know how many times this would need to be run in order to get some significant data?

Assuming that a hot grinder produces consistently worse espresso we could move on to the next experiment.

Experiment 2:

The goal here would be to measure the impact of coffee cake temperature on brew temperature.  My idea would be to use a naked portafilter and measure temperature of the liquid as it exits the basket.  If we know our brew temperature (and our machine is stable) then we should see a variation in loss here that correlates to coffee grounds temperatures.

Ideally coffee beds with temps from 20°C up to 40°C would be measured.  Would using a simple IR thermometer be sufficiently accurate to get a reading on the coffee bed before brewing?  A consistent flow rate and dose would be extremely important here so the same brewing specs as Experiment 1 would be observed.

Experiment 3:

A machine with individual brew boilers would be necessary here.  Cool coffee would be brewed in one group at a standard temp, and hotter grounds would be brewed in the other group at a lower temperature so that the exit temperature of liquid from both baskets was the same.  Again, espressos tested would need to be brewed to exacting specifications.

These would then be tasted blind by a panel to see if any preference is observed.


How could these be improved?  Is it worth testing?  Any good suggestions for introduction to the necessary statistics?  If this seems viable would other people be up for joining in and sharing the data?

  1. We’ve all had painfully hot espressos that had a lot more to do with being a fast extraction, than the machine’s brew temperature.  ↩︎
  2. On a personal note my belief is that about 1°C is the minimum that people can distinguish with temperature as the only variable. The latter part of the sentence does make things very difficult I know. Couple that with the range of accuracy of most probes, despite the fact that they may read down to .1°F – it doesn’t meant they are absolutely accurate to that degree.  ↩︎

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