What Impact Do Under-Extraction and Over-Extraction Have on Coffee Flavor? Grinding Degree and Water-to-Coffee Ratio in Pour-Over Coffee

For professional coffee knowledge exchange and more coffee bean information, please follow Coffee Workshop (WeChat official account: cafe_style).

For more premium coffee beans, please add FrontStreet Coffee on private WeChat, ID: qjcoffeex.

Extraction refers to squeezing something out or obtaining an extract. In chemistry, extraction means extracting valuable substances (flavors) from raw materials (coffee beans). In coffee extraction, the most common method is using hot water to extract flavors from coffee grounds.

In a single coffee bean, 70% consists of woody fiber, while 30% contains flavor compounds. Among these flavor compounds, 20% are positive, pleasant sweet and sour substances, while 10% are negative, unpleasant bitter substances.

Therefore, coffee extraction doesn't mean extracting all 30% of substances, but rather that coffee has at most 30% of substances that can be extracted. To make a delicious cup of coffee, you only need to extract 18-22% of these substances.

What is Over-Extraction?

Over-extraction refers to when coffee grounds remain in contact with hot water for too long, causing excessive release of negative compounds, where bitterness masks the sweet and sour flavors. There are two possible causes for over-extraction: grinding too fine or water temperature being too high.

Coffee beans with different roasting degrees have structures that become more porous as roasting time increases, making the extraction rate of substances faster. Therefore, coffee beans with different roasting levels need to be paired with different grind sizes and water temperatures for brewing.

According to FrontStreet Coffee's roasting and brewing experience, light to medium roast coffee beans have relatively dense structures, therefore requiring fine grind + high water temperature for brewing. Here we recommend using medium-fine grind/fine sugar size (75-80% pass-through rate with China #20 standard sieve), paired with 90-91°C water temperature for extraction.

Medium-dark to dark roast coffee beans have relatively porous structures, therefore requiring coarse grind + slightly lower temperature hot water for extraction. Here we recommend using medium-coarse grind/coarse sugar size (65-70% pass-through rate with China #20 standard sieve), paired with 88-89°C water temperature for extraction.

Another scenario for coffee over-extraction (when grind size and water temperature are appropriate) is when the water flow rate is too small, increasing the time water takes to pass through the coffee bed, which can easily lead to over-extraction.

What is Under-Extraction?

Under-extraction refers to when coffee grounds are in contact with hot water for too short a time, insufficient to extract all flavors from the coffee grounds. This makes the coffee taste thin, watery, and unbalanced. The possible causes of under-extraction are the opposite of over-extraction: grind too coarse/water temperature too low.

If the coffee still tastes thin and watery when grind size and water temperature are appropriate, it's because the water flow rate is too high, causing water in the filter cup to exceed the coffee grounds capacity, leading to flow along the filter cup walls, which dilutes the already extracted coffee.

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473