the bitternes in espresso and some solutions about it

my experience tells me that bitterness in espresso coffee is most often the result of over-extraction. over-extraction, the extended length of time that the hot water runs through the ground coffee, allows the naturally occurring but bitter tanic acid to be extracted along with the desirable coffee oils. tannic acid is a pure astringent which causes a bitter and constringing action when it comes in contact with the mucous tissues of the mouth. it is the same acid that causes that dry and puckery feeling in the mouth when you eat unripened fruit or drink red wine. 

below demonstrates the color and extraction volume that you should be aiming at. there is no evidence of the bitter tanic acid in this extraction. here is an espresso coffee extraction with no evidence of the tannic acid and so containing no bitterness/sourness:

the tanic acid is evidenced by the change in creme color from the rich golden brown to a lighter straw color as shown below : 

this image shows the point where the desirable coffee oils give way to the bitter tanic acid with the straw colored creme in the center. this extraction was stopped just in time to avoid the bitterness from being transfered to the cup.

in below picture, you see over-extracted espresso. first and second photograhs have tannic acid point. third photograph is extremly over-extracted espresso.

another reason may be that you are no longer extracting the coffee oils because the extraction pressure is being lost via the worn rubber seals that connect the portafilter with the group head. espresso coffee needs about 8-10 bars of pressure to properly extract the desirable oils. maybe replace the seals or in the short term jam the portafilter harder onto the group head.

also, bitterness in espresso coffee can be caused by the extraction temperature not being hot enough. optimum extraction temperature is just short of boiling, at about 94 degrees c (200 degrees f). you may need to adjust your boiler thermostat or give the espresso machine more time to reach the required temperature (often showing on the boiler 

maybe consider the effect of the 'burnt' coffee oil residue that will build up over time on the back of the portafilter filter basket and the shower head in the espresso machine. removing the filter for cleaning from the portafilter and back-flushing the group head shower may also help reduce some of the bitterness experienced in the coffee taste.

the "rule of thumb" is that sour shots mean the temperature is too low, and bitter shots mean the temperature is too high. but, you should make sure you've got everything else dialled in before you start adjusting the machines brew temp.

old stale coffee is very hard to get non-bitter shots from, same with pre-ground beans.  ıf you're buying coffee from a supermarket, "you're doing it wrong". find a local specialty coffee roaster and ask them for something roasted for espresso.

bitterness starts from the poor grinding. no matter how expensive or how user-friendly your coffee machine is, but, if you are not aware of its grinding procedure, you will end up with the bitter coffee. grind size of the coffee seeds (or powder), and its interactionwith the water, decides the taste of the coffee. bad grind size and lack of consistency will lead to uneven distribution of the flavors, due to which, there will be a cup of bitterness in your hands. if i'm having bitterness problems, i'll start by upping the dose to 18 or 20g for a double, "overdosing" hides a lot of flaws but also makes it much easier to pull acceptable shots. tweak the grind till you get a 25-30 second shot of 44-59 ml (by volume, 30-45 g by weight).

if you can't find a non-bitter extraction in there, it may be that your machine is set too hot.

also, make sure your portafilter is connected to the machine ( i.e. hot ) and not resting on the machine ( and cold ). if it's cold, you'll get bitter espresso !

but, don't forget it : bitter coffees are mood killers; surprisingly there are people who like the bitter coffee too.

by peter baskerville, iain chalmers and aurick shaffer