Sensory Impressions of Over- and Under-Extraction
What Does Over- and Under-Extraction Taste Like and Why?
Extraction, just like strength, can be communicated with tastes and textures. As an extraction occurs, different compounds will be dissolved at different rates. This is primarily due to their weight — lighter compounds need less energy and time to be dissolved and vice versa for heavier compounds. This evolving mix of compounds is repeatable, and luckily, very obvious to our palates.
Lower extractions, or under-extractions, are quite underwhelming and unsatisfying. Their sweetness is very low or nonexistent and their acidity is generally high and of low quality. They also exhibit unfavourable nutty, vegetal, grassy, salty, and savoury flavours, followed by a fast and empty finish.
It is important to note that the compounds responsible for these undesirable flavours also exist in a well-extracted brew. It is the compounds that are extracted later on which balance or mask these undesirable flavours.
An over-extraction is equally undesirable. There’s a strong association with a dry mouthfeel and an increase in perceived bitterness. Almost every coffee that has ever been extracted contains a mix of all of these levels of extraction.
But what is a good extraction? The goal in coffee extraction is balance. The best balance does not imply equal proportions of taste sensations or aromatic ones, more it suggests a harmonious interaction of all the parts of a coffee beverage. The key activators in achieving a good balance are sweetness and smoothness. Under or over-extracted coffee is not as sweet as it is bitter or sour and some additional sweetness can restore balance. Whether it is over or under-extracted, coffee can dry the palate which is usually undesirable. So when you taste an extraction which has more smoothness and more sweetness than other recipes you’ve tried with the same coffee, you know you are headed in the right direction.