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On our home espresso machines we extract shots of espresso every day. But have you ever stopped to think about what is actually happening in the portafilter when you extract a shot of espresso? 

When we extract espresso we are essentially forcing hot water, under pressure, through a dense layer of finely ground coffee to create a concentrated coffee brew. We are aiming for the water to be in contact with the coffee for just the right amount of time in order to extract the right amount of coffee oils and solids (minute fragments of the coffee bean), which is where the luscious flavour lies. 

Once we lock in a dosed and tamped portafilter into the group of the espresso machine, we immediately start extracting the shot by engaging the pump and getting water flowing through the dense layer of ground coffee. This allows the water to make its way through the coffee grinds, washing away the coffee oils and solids, which then pour into the cup. The high pressure generated by the espresso machine emulsifies a small amount of the oils to create the dense layer of foam that sits on top of the espresso shot, called the crema. 

When we look at an espresso pour there are three pretty distinct stages. The first phase is the start of the pour, where the coffee oils and solid are most concentrated. The pour should be a deep brown colour and quite viscous. The streams of espresso should hang from the spouts quite straight, similar to the look of honey pouring from the back of a spoon.  The second phase is where the colour of the pour will change as the amount of solids and oils are diluted. Overall there will be more brown colour, but you will start to see the colour lighten, which we call ‘blonding’. The third phase of the pour is when all of the coffee solids and oils are depleted and you are left with a thin, watery stream that is very light in colour. The streams of espresso will start to thin out at this point and will slightly pull inwards, forming what we call ‘mouse tailing’. This part of the pour is very watery and bitter in flavour. We therefore want to have as little of this end up in the cup. 

The rule of thumb for espresso extraction is that it should take around 25 – 30 seconds to pour 25 - 30ml of espresso.  Make the brew time too short and you won’t extract enough of the oils and solids, leaving you with a coffee that lacks flavour and body, as well as being quite acidic. We call this an ‘under-extracted’ shot. Make the brew time too long and you’ll extract too many of the oils and solids, leaving you with a coffee that will be quite bitter and unpleasant. We call this ‘over-extracted’.  In a perfectly poured espresso you should find a harmonious balance between acidity, sweetness and bitterness, as well as it being round, smooth and full bodied.