Why does my coffee taste bitter?
A guide to Coffee Problem Solving.
For the moment let us assume that you have selected and are buying good coffee. Reasonably fresh coffee sourced from a roaster who knows his trade. Coffee that has been over-roasted is usually quite easily noticed. Look for a bean that is very carbonised in that it crushes easily into black dust. This will quite often be particularly dark and oily. An occasional bean with an oily surface is acceptable in an Italian or very dark roast, but this should not show on the entire batch.
Be aware however that there is an element of bitterness in most coffee. The caffeine content itself has a bitter taste to it which, when combined with some delicate acidity, which we are looking for, creates a bitter taste. There are also chemical reactions that take place during the roasting process – these build up the (welcome) antioxidants and yes, also help to create an element of bitterness. The beauty of a good coffee is in the balance between this slight bitterness, a little acidity or “puckering,” and of course the underlying sweetness. Arabica beans show less evidence of bitterness than the Robusta bean. We do not use Robusta in our coffees, but this bean is regularly used where a lower-cost option is demanded. Some roasters put forward a case for a Robusta content of up to 10% but this remains a matter of some debate.
By default, the cause must be in your brewing techniques
If you are managing your own extraction as one might in a restaurant-style espresso machine, there is a good chance that you are over-extracting. This takes place if too much water passes over the coffee while in the porta-filter. As a general rule, if one either pushes through more than 30ml, or 60ml for a double shot, the added water will extract unwanted bitter flavours from the coffee. I have even seen some baristas pull double the shot while preparing an Americano instead of adding the hot water before or afterwards. All they are doing is delivering a bitter mess.
Or using the incorrect grind size
The finer the grind, the more bitter the coffee. If, for example, you have a light hand in tamping, you might inadvertently set the grind size too small to pull your shot within the ideal range of between 22 and 28 seconds. In doing so, you are preparing a more bitter coffee than you would have if you’d had the correct grind size along with a firmer tamp pressure. The acceptable tamp pressure should be around 13-20kg. Many writers refer to a 23-second shot and a 15 kg tamp as barista law’s ‘holy grail’. Whatever works for you is fine, but I would exercise extreme caution if this is under 10kg. If you are experiencing bitter coffee and tamping lightly with a finer grind, this could be a possible cause.
Heat essentially ‘overcooks’ the coffee. Excess heat can come from many causes, the machine thermostat could be incorrectly set – the correct water temperature should be between 90 and 95 C.
Another cause might be if you leave your porta-filter in place while you prepare your milk. In addition to keeping the porta-filter hot, this also prevents your group head from cooling between coffees.
Old coffee stuck to the group head or in an unwashed porta-filter.
This sounds very basic, but if used coffee has stuck to the group head for any reason (including the barista not purging properly), this coffee is then used again and will essentially be the same as an over-extraction.