Coffee grinding is what allows us to taste the flavours developed in a coffee roast. It is vital for determining the strength of your brew.
It applies the same chemical principle that our human cells use to survive: increasing surface area and minimising volume improves the rate at which molecules can pass in and out.
Grinding the beans into tiny fragments increases their surface area and decreases their volume. This increases the rate and number of flavour molecules that can move out of the bean. The finer the grind, therefore, the stronger and tastier your coffee.
Popular wisdom is that grind quality is determined by three factors: moisture, roast and origin of the beans. The theory is that the drier the bean the easier it is to cut into small pieces, thus producing more flavour; a darker roast means more brittle beans, which break more easily into smaller particles; and the origin of the bean affects its physical properties, including how well it dries out and its ability to be cut.
Like much received wisdom, however, these ideas have been challenged by science. Research into the factors affecting coffee-grind quality has found the pivotal issue is temperature.
“Grinding colder coffee beans produces a more uniform particle distribution, with a decreased particle size,” concluded a 2015 study by an international team of scientists led by Christopher Hendon of the University of Bath. “While the decreased particle size will tend to speed up extraction due to the larger surface area, the increased uniformity should minimise the amount of wasted bean, which is discarded without being extracted to completion.”
A reason, perhaps, to go for the trendy ‘cold brew’ over the traditional hot espresso!