Grinding on the dance floor.

Last week we talked all about the science of milk, this week we are moving onto the other important ingredient in coffee – the espresso shot.

The shot is one of the vital parts of the drink. Messing this up will either leave you with a sour or bitter taste in your mouth (good milk can cover this up but only so much).

The shot process begins by grinding the beans into one of the portafilters.


The portafilter is what sits in the machine and what the shot pours out of.

The actual grind (how big or small the individual coffee grinds are) of the beans can be adjusted into different levels of coarseness through the grinder.

When the espresso shot pours it will take a certain amount of time. The ideal for this is 25-30 seconds. As the water comes down on the grinds (at about 130 pounds of force) it extracts the flavours from the grinds. However, if it is not timed right this will ruin your coffee shot.

If the shot pours too quickly then as the water runs through it does not absorb the oils or flavours in the coffee and you are left with a weak, thin espresso which tastes sour.

However, if the shot pours for too long then it is because the water sticks around for longer than it should, as it can’t get passed to the desired escape route of the coffee cup. This causes it to absorb not only the flavour but also other undesirable chemicals leaving it tasting strong and bitter.


A shot is poured

The best way to fix the extraction time is to change the grind. Water will have to force its way through a fine grind, making it take longer whereas water can rush through a coarser grind much faster. If you imagine a container of rocks and a container of sand – it takes the water a lot longer to get through the sand than the rocks as there is less space for it to get through.


It is easier for liquid to pass through larger, more coarse objects.

You can also adjust your tamping pressure as well. Tamping provides a nice flat compact bed of coffee for the water to hit and compresses the coffee grinds. A harder tamp will compact more, meaning a longer extraction time.


Tamping allows the water to hit a compact, flat surface.

Coffee extraction in its 3 stages

  1. Dark thin trickle with a strong aroma, flavour and colour that is released into the shot.
  2. The caffeine and other bitter compounds begin to be extracted from the grinds.
  3. Stream lightens in colour and then the more unpleasant, more bitter acids begin to be extracted.

So next time you get your coffee and it tastes bitter or sour – or just not quite right, you now know why! (I don’t know if that will make you feel better about it or not though).



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