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.