What is a clear sign of over extraction in espresso?

Over-extraction in espresso refers to the process where too many compounds are dissolved from the coffee grounds during brewing. This happens when the water flows through the coffee for too long or when the grind size is too fine, leading to undesirable flavors.

Bitterness is a clear sign of over-extraction because as the brewing time increases, more bitter compounds are released from the coffee. These compounds include chlorogenic acids and quinic acids, which can overpower the desirable flavors in the coffee. When brewing is optimal, the resulting espresso should have a balanced flavor profile with a pleasant sweetness and acidity. However, if the brew is over-extracted, the bitterness becomes prominent, indicating that the extraction process has gone too far.

In contrast, bright acidity may be a desirable feature in certain coffee types and not necessarily an indication of over-extraction. A sour flavor is often associated with under-extraction rather than over-extraction. Lastly, a weak aroma typically suggests insufficient extraction or a low-quality coffee rather than a definitive sign of over-extraction. Thus, experiencing bitterness in espresso is the primary indicator of over-extraction.