Is coffee over-extracted whenever the cup tastes bitter?

Bitterness alone does not mean a coffee is over-extracted. Using Golden Cup theory and Front Street's daily practice, extraction above 22 percent counts as over-extraction. The typical signs are burnt bitterness, astringency and off-flavors, which differ from the pleasant bitterness of dark chocolate.

Common causes include water that is too hot, grinding that is too fine, long contact time, forceful stirring and too many fines. Two practical fixes are adjusting brewing parameters and discarding the tail end of the pour. Re-brewing used coffee grounds is also addressed, since the soluble material has largely already been extracted.

Many people, upon tasting a cup of coffee that leans bitter, immediately think, "Is this over-extracted?" But bitterness cannot be directly equated with over-extraction. This article revolves around over-extraction, first explaining what it is—according to the Golden Cup theory and the daily practices of Front Street Coffee, an extraction yield exceeding 22% is considered over-extraction. Then, from a sensory perspective, it outlines three typical manifestations of over-extracted coffee: burnt bitterness, astringency, and off-flavors, and analyzes their causes one by one, such as excessively high water temperature, too fine grinding, prolonged contact time, excessive stirring force, and too many fines. Finally, it offers two practical strategies to avoid over-extraction: adjusting brewing parameters and discarding the tail end, while also addressing whether coffee can be brewed repeatedly, helping coffee enthusiasts more accurately assess and improve their brewing results.

"Huh, this coffee is so bitter, is it over-extracted?" Many friends believe that a bitter cup of coffee means over-extraction. Actually, that's not the case. In this edition of Barista Development Diary, we'll discuss the manifestations of over-extracted coffee.

What is over-extraction? We know that the water-soluble substances in coffee beans account for about 30% of the bean's weight, while the remaining 70% is woody fiber and other materials that cannot be extracted. And it's not that the more of this 30% extracted, the better. According to the recommended values of the Golden Cup theory and what Front Street Coffee (FrontStreet Coffee) has found in daily practice, the recommended extraction yield for coffee is between 18% and 22%. Therefore, what we call over-extraction is an extraction yield exceeding 22%.

What are the manifestations of over-extracted coffee? Sensory-wise, the manifestations of over-extracted coffee fall into the following categories:

① Burnt bitterness. Note that the bitterness here is burnt bitterness, which is a type of defective flavor. Many friends think that bitterness in coffee means over-extraction, but there is a distinction between good and bad bitterness. For example, the bitter notes of dark chocolate are a good flavor in a cup of coffee; this bitterness doesn't linger in the mouth for long and is followed by a sweet aftertaste. Burnt bitterness, on the other hand, is an unpleasant bitterness that is immediately noticeable upon sipping. This bitterness stays at the back of the tongue and in the mouth, causing dryness, a burnt sensation, and other discomforts. The primary cause of brewing burnt bitterness is excessively high water temperature.

② Astringency. Astringency is a tactile sensation. It mainly occurs when large molecules of "dicaffeoylquinic acid" are extracted and bind with proteins in saliva, creating a puckering, rough sensation on the oral epithelium. "Dicaffeoylquinic acid" is also one of the sources of bitterness. Therefore, if too much "dicaffeoylquinic acid" is extracted into the coffee, it results in what we call astringency. This flavor generally arises from grinding coffee too fine, prolonged contact time between coffee grounds and water, and vigorous stirring during brewing.

③ Off-flavors. Off-flavors generally occur when the coffee bed is unevenly extracted, which can also lead to localized over-extraction and localized under-extraction. Off-flavors typically make the flavor profile unclear and mixed with defective tastes. In pour-over brewing, a common cause is excessive coffee fines, which cause localized over-extraction during brewing. Vigorous stirring during brewing also easily leads to off-flavors.

So how can over-extraction be avoided? ① Adjust parameters. Observe the results and process of the previous brew to adjust brewing parameters. For example, first confirm whether the water temperature used previously was too high. Front Street recommends a brewing temperature of 90-93°C for light to medium roasts and 87-89°C for medium-dark roasts. You can use this as a reference and adjust accordingly.

Also observe whether the previous brew time was too long. At Front Street, brewing 15g of coffee grounds takes about 2 minutes. If you believe over-extraction is caused by brew time, you can appropriately adjust the grind size.

② Discard the tail end. Discarding the tail end depends on the situation, because during over-extraction, small molecules like acids and sugars are extracted first, followed by large bitter molecules. Therefore, during brewing, if you suspect over-extraction might occur, you don't need to let all the coffee liquid flow into the server—the last 20-30ml in the dripper can be discarded to prevent astringent and bitter flavors from ending up in the coffee you drink.

The above content is compiled by CoffeeHunters, a coffee news website.