How to Distinguish Between Over-Extraction and Dark Roast Bitterness in Coffee? How to Control Pour-Over Coffee Parameters?

Why Coffee Tastes Bitter Despite Proper Flow Restriction

Yesterday, FrontStreet Coffee received an interesting question from a customer friend who asked FrontStreet Coffee why his coffee still tasted bitter even though he had clearly implemented flow restriction.

Excellent, this is another friend who has been misled by flow restriction~ The truth is quite simple: the final stage of extraction is not the main source of bitterness. Moreover, bitterness comes in various types, and different types of bitterness create distinctly different sensations. So today, FrontStreet Coffee will share the reasons why coffee becomes bitter during brewing, along with corresponding solutions to avoid it~

Inherent Bitter Compounds in Coffee Beans

Many times when we mention "coffee has extracted bitterness," it's basically caused by over-extraction. Coffee beans themselves are agricultural products that naturally contain bitterness. Chlorogenic acids, caffeine, and trigonelline within the beans are the main contributors to bitterness. However, besides bitter compounds, coffee beans also contain other flavor substances like acid, sweet, and salty notes, which is why we don't extract all the bitter compounds right from the beginning.

Acidic and sweet substances are small molecular compounds that dissolve quickly, so at the beginning of extraction, they are the primary substances that dissolve in large quantities. Bitterness, on the other hand, belongs to large molecular compounds, so its dissolution rate is very slow. Although slow, it persists throughout the entire process. From start to finish, it continues to release. Because of this slowness, when the acidic and sweet substances have been completely extracted, bitterness continues to release. When the extraction process reaches this critical point but doesn't stop, all the extracted bitterness and undesirable flavors will join the coffee, resulting in明显的bitter and astringent tastes! This is what we commonly call "over-extraction."

Since this critical point occurs at the end of extraction, it has led to the misconception that the final stage is the main release point for bitter compounds. Therefore, we can understand that the reason for flow restriction isn't because the final stage extracts bitterness. Rather, it's because coffee extraction has reached a critical point where over-extraction is about to occur. Continuing extraction will only yield bitterness, astringency, and undesirable flavors, which is why we implement flow restriction. Typically, flow restriction is applied either due to sudden blockage situations that increase extraction efficiency, or when the planned extraction liquid weight is reached and no further extraction is needed.

Solutions for Bitter Coffee

Speaking of this, I believe everyone now understands why coffee becomes bitter! "Over-extraction" is the main culprit behind this problem. Grinding too fine, having too many fine particles, water temperature that's too high, or extraction time that's too long—all these factors can significantly increase extraction efficiency. Therefore, if your brewed coffee frequently shows明显的bitter and astringent flavors, you should pay attention to your extraction parameters during brewing and make adjustments to reduce extraction efficiency, which will help minimize the occurrence of bitterness.

"Bitterness" from Roasting Process

In addition to the inherent bitterness from coffee beans' genetic makeup, the roasting process also adds certain bitter compounds to coffee beans. Currently, commercially available roasted coffee beans are mainly divided into two roast levels: dark roast and light roast. When beans are roasted lighter, more of the beans' natural acidic compounds are preserved. The high proportion of acidic compounds will make the brewed coffee predominantly feature rich acidity; whereas when beans are roasted darker, acidic compounds decrease with deeper roasting while bitter compounds increase. With less acid and more bitterness, bitter flavors thus become dominant.

It should be noted that the "bitterness" FrontStreet Coffee refers to here is not the unpleasant, hard-to-swallow astringent bitterness like that from over-extraction. Rather, it's the mellow, aromatic bitterness of caramel and chocolate that comes from the caramelization reaction during long roasting times. If coffee beans carry these flavors when roasting is complete, these flavors will also join the coffee during extraction. Although this represents positive flavor expression, for friends who dislike bitterness, this type of bitterness isn't significantly different from over-extraction bitterness. Therefore, in such cases, it's best to choose coffee beans with lighter roasts and flavor descriptions featuring floral and fruity acidity when purchasing. Their abundant acidic compounds make them less likely to extract bitter compounds.

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