Can I have a full definition of under and over extracted espresso?

I hear these phrases thrown around a lot, and have even used them myself, but I've never fully understood what an over extracted and an under extracted espresso really are.I can taste a shot and tell if it's good or not but I don't truly know the difference between over and under extraction. What will an over extracted espresso taste like? What about under extracted?When working with much more experienced coffee professionals, I noticed they can taste a shot and say "it's good but slightly over extracted".I assumed that if I pulled a shot that was longer than the extraction parameters for that particular espresso, it meant it was over extracted, but I want to have a better understanding of these two terms. Specifically taste nuances for both. How can I train myself to be able to tell when something is "slightly" over or under extracted? Any knowledge you guys can give me will be a big help! Thanks!

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I think the best advice I could give here is to tamp a shot really hard and pull it then tamp really soft and pull that. Tastes them side by side and compare. To me over extracted would be bitter and harsh. Under extracted would be dull and sour. What should also be noted is the difference between over dosed and under dosed.

Thanks Jeff, I took your advice and I do think I have a better understanding. I guess now I just need time and experience tasting lots of good and bad espresso before I become a true master. 

Over extracted = bitter astringent flavors

Under extracted = earthy flavors

I never found tamping to have any significant effect on shot time, or extraction for that matter. 

to put it simply over extraction is taking more out of the bean than you meant to. under extraction is not taking enough. according to most professionals proper espresso extraction is somewhere between 18%-22% - that is 22% of the weight of the ground coffee ends up in the cup. however with most contemporary espresso equipment in use today it is very difficult to reach even 20% extraction, which make "over extracting" technically impossible. once you know your coffee though, you're likely to find an area of extraction you enjoy, be it 18%, 15% or even 10% extraction which means extracting more or less than that could be considered over or under extraction. it would be very difficult to identify flavors of over or under extraction until you are familiar with the coffee though. with my coffee, extractions of over 19% tend to be, sweet, balanced and complex but are very hard to get, extraction between 17%-19% are can be very roasty and ashy with both strong bitter and sour flavors,  15%-17% can be very fruity and sweet with no bitter flavors but are also aggressive and can be harsh. below 15% the shots can get very punchy and sour, low on sweetness and unpleasant. as you can see the flavors are not so clean cut and it can get very confusing even when you know your coffee very well.
you can usually manipulate the extraction by either pulling longer or shorter shots (to increase or decrease extraction - up to a point), or change the amount of water you pull through the coffee (more water = more extraction).
hope that helps

Hi Gere,

What do you mean by tampering not affecting the shot time and extraction? For me the relation is obvious. Of course, there are other factors. Grinding being the most important (?).

If your tamper is very consistent, of course, you will find no difference. But try the same espresso machine and grinding with different baristas (or with begginer baristas)...

Hi Tiago,

The water pressure of your espresso machine at 9 bar is equal to around 250kg of pressure. which means you can probably do a hand stand on top of your puck and it would still be far lower than the pressure of the pump. A little experiment for you to try is to tamp as hard as you can and pull a shot and next don't tamp at all just place the tamper on top of the coffee with no pressure and pull a shot. I've done this many times and the difference is insignificant (maybe 2sec difference which is probably more to do with headroom in the basket than actual tamp pressure).

Any difference between Baristas is likely to be more about dose differences or uneven tamping which creates channeling rather than tamp pressure.

If you look at the different factors that influence actual levels of extraction in coffee the number one factor is brew ratio (the amount of water to coffee), next comes Grind/shot time (I put them together because if you dose to the specific requirement of your basket (i.e 22g in VST22) grind changes will change extraction time. Grind changes by it self has only a very small, somewhat insignificant effect on extraction, i.e. 22.5g vs 22g in VST22 extracted to the same brew ratio in the same time will extract almost the same despite different grind setting). I don't include dose in this list because it is only a significant factor if you are extremely overdosing or underdosing your basket (lets say 2g over or under the designated dose), in that case your extraction could only drop. then there is temperature and pressure but they have a rather complex and confusing influences on extraction and flavor which are not completely linear. 


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