The nature of espresso brewing means that changing the temperature doesn’t always have the straightforward effect on extraction that we might expect. Changing one variable in espresso inevitably changes many other variables too, making it hard to separate out the effect of individual factors. For example, as we saw in Lesson 5.05, changing the espresso flow rate might also have an effect on the effective brewing temperature.
Increasing temperature has been shown to increase extraction (D Albanese et al, 2009; S Anduesa et al, 2003). Recall from Lesson 5.06 that a 10°C (18° F) change in brew temperature would be expected to double the initial rate of extraction, and thus we would expect temperature to have a large effect on extraction. However, experiments using controlled dose and yield show the effect of water temperature on extraction to be smaller than we might expect.
Two examples come from experiments published in blog posts by Five Senses and Compound Coffee Co. Both show that a higher brew temperature increases extraction, but by only a relatively small amount: increasing the temperature by 4–6°C (7.2 – 10.8° F) increased the extraction by only about 0.6%. Both posts point out that the difference in taste is much bigger than this difference in extraction would suggest.
If we compare this to the results predicted by Professor Abbott’s app, the difference is quite striking. Using the default settings for espresso brewing, a change in the temperature results in a substantial relative change in extraction (from 52 to 60 on the chart). While these numbers don’t directly translate to extraction percentages, the difference shown here is equivalent in size to a change in extraction from 17.5% to 20% — much bigger than the 0.6% effect reported in the Five Senses and Compound Coffee Co. experiments.
The effect of a 5°C temperature change on extraction in espresso predicted by the BH Coffee Extraction app.