What Does Flow Restriction Mean in Pour-Over Coffee? What's the Purpose of Flow Restriction in Drip Coffee? What's the Difference Between Coffee-to-Water Ratio and Coffee-to-Brewed-Liquid Ratio?

Understanding "Flow Cutoff" in Coffee Brewing

In the coffee brewing process, when we remove the filter while there's still liquid remaining that hasn't fully passed through, this action is what we call "flow cutoff."

Because flow cutoff removes the tail-end coffee liquid, and many people believe that coffee's bitterness comes from this tail-end liquid, this practice has become an essential operation for most people when brewing. However, FrontStreet Coffee wants to emphasize that flow cutoff should be used selectively depending on the situation and brewing method. When used inappropriately, not only will it not make the coffee taste better, but it can also lead to putting the cart before the horse.

Why You Shouldn't Arbitrarily Use Flow Cutoff

First, we need to understand that bitterness doesn't just dissolve in the tail-end. FrontStreet Coffee often mentions that the dissolution of bitter compounds occurs throughout the entire process. It's just because they are large molecular compounds that their dissolution rate is relatively slow. When the sweet and sour compounds have finished dissolving, they continue to dissolve due to their difficult-to-dissolve properties. Only if we continue extraction after the sweet and sour compounds have finished dissolving will the coffee dissolve negative compounds.

But under normal circumstances, we establish our brewing framework before starting, ending the extraction when the sweet and sour flavors are mostly dissolved, which can prevent the release of negative compounds and allow the coffee to achieve excellent performance. However, FrontStreet Coffee's explanation might not be easy to understand, so everyone can try it practically. With a reasonable brewing framework, we can try collecting the tail-end coffee liquid separately during brewing to taste. Then we'll discover that the last dissolved coffee liquid has almost no perceptible bitterness—just lighter sweet and sour flavors with a slight tea-like quality, without the imagined bitterness and impurities. The principle is simple: with reasonable parameters, even the tail-end coffee liquid won't extract negative compounds from the coffee, and its main function is actually to dilute the overall coffee concentration.

When Your Coffee Develops Obvious Bitterness

If your tail-end shows obvious bitterness, then our focus should actually be on the parameters. It's because unreasonable parameters were used that caused negative compounds to dissolve in the tail-end. Otherwise, under normal circumstances (excluding sudden situations like blockages), unless the coffee bean quality is poor, it will be very difficult for us to extract negative flavors with reasonable parameters. Take FrontStreet Coffee's commonly used light roast brewing framework as an example: 15g coffee, ground to sugar granule consistency, 1:15 coffee-to-water ratio, 92°C water temperature, and extraction time of two minutes. If we remove the tail-end before the coffee has finished dripping, this coffee might exhibit these negative characteristics: excessive concentration, incomplete flavor profile, and lack of aftertaste.

Because the framework we established includes this tail-end coffee liquid, when we cut it off, the coffee's concentration increases due to insufficient water for dilution, making it less pleasant to drink. Meanwhile, this tail-end coffee liquid also contains certain flavor compounds. When it's missing, we've effectively lost some flavor expression, including the aftertaste that requires bitter compounds. Therefore, we can see that flow cutoff operations cannot be casually applied.

When Should You Use Flow Cutoff?

So under what circumstances should you use flow cutoff? The use of flow cutoff is mainly divided into two situations. The first is when we proactively make flow cutoff part of the brewing process, incorporating it into the extraction framework from the beginning. For example: when using coffee-to-liquid ratio as the measurement unit. In fact, this flow cutoff operation is not a new technique that has emerged in recent years—it was widely used in coffee circles as early as the late 20th century. But at that time, applying flow cutoff was truly a last resort. Because there weren't convenient measuring tools like there are today—both water temperature and liquid weight had to be estimated based on experience or cup capacity.

Therefore, at that time, people mostly used "coffee-to-liquid ratio" when brewing coffee. The so-called coffee-to-liquid ratio refers to the proportion of coffee grounds to the filtered coffee liquid. Since the amount of water injected couldn't be measured, the liquid weight was calculated by observing the water level of the coffee liquid in the cup relative to the cup's capacity. When the coffee liquid reached the target water level, it was necessary to remove the filter to prevent the remaining liquid in the filter from seeping out. Because any coffee liquid that seeped out afterward would be beyond the target liquid quantity, exceeding the extraction range and likely carrying negative compounds. Therefore, flow cutoff was needed at this point to avoid obvious negative expressions in the coffee.

Passive Flow Cutoff

Another situation is passive selection, a scenario I believe everyone is very familiar with. When coffee beans are too light-roasted, too hard, or contain too many fine particles, blockages are very likely to occur during the brewing process.

The appearance of blockages forces the coffee extraction time to be extended. If immediate appropriate measures aren't taken, the coffee will easily become over-extracted due to the extended extraction time, causing some negative flavors like bitterness, impurities, and astringency to dissolve. Therefore, in situations like this, we are forced to use flow cutoff to reduce the addition of bitter compounds, and then determine whether to add bypass water for dilution based on the coffee's concentration.

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