Cold Drip Coffee: Process Parameters and Flow Rate - What's the Ideal Drip Speed?

"Do you have cold drip coffee?"

"Yes, today's cold drip is..."

"How is it ready so quickly?"

"Due to time constraints, cold drip is always prepared in advance and kept in the refrigerator."

"Then how long does it take to make a cup of cold drip coffee?"

This is a conversation that occasionally occurs at FrontStreet Coffee. Regarding cold drip coffee, besides the fact that it's "more expensive" which often invites various comparisons, its lengthy preparation time also adds quite a bit of mystique to this coffee variety.

Although both are coffees extracted through low-temperature water, the preparation steps and principles of cold drip and cold brew are completely different. Cold brew, also known as cold extraction, has a simple and straightforward preparation method - just mix coffee grounds with water, then refrigerate for several hours, and finally filter out the coffee grounds. Therefore, as long as you have coffee beans (or grounds), anyone can make cold brew at home.

Cold drip, on the contrary, requires a specific set of equipment for a standard serving - a cold drip pot. The upper, middle, and lower layers of the container hold ice water, coffee grounds, and the filtered coffee liquid respectively, making it a continuous multiple coffee extraction method.

Furthermore, as we can tell from the name, cold drip coffee not only emphasizes "cold" but also requires "drip" for extraction. Therefore, the upper pot holding ice water needs to be equipped with a valve that can control the water flow to ensure the coffee is always in an extraction state. Finally, when the coffee liquid reaches the target extraction amount or when all the ice water has dripped into the coffee grounds, the extraction is complete. However, at this stage, the coffee liquid cannot be consumed directly. It needs to be refrigerated for a period to allow fermentation and maturation, which will make the taste rich and smooth yet layered.

In summary, to make standard cold drip coffee, we not only need specialized equipment but also need to constantly monitor the water drops to maintain stable extraction, while also mastering various parameters and details, and waiting through a lengthy fermentation process. Obviously, compared to cold brew that "anyone can make with their hands," the preparation elements for cold drip coffee are much more complicated, which is why few people choose to make cold drip at home.

FrontStreet Coffee often mentions in articles that different water temperatures can brew coffee with different flavors, and at low temperatures, the astringent substances in coffee are not easily released. Like pour-over, cold drip is also a type of drip coffee, except it uses ice water below 5°C as the solvent, which can be understood as low-temperature drip coffee.

Based on FrontStreet Coffee's experience, the preparation time for cold drip is mainly affected by the following four factors: water drip speed, coffee grind size, extraction ratio, and maturation time.

1. Coffee Grind Size

The grind size of coffee needs to consider two aspects. If the grind is too coarse, the resistance formed by the coffee bed is smaller, making it easier for water to penetrate and fall into the lower pot. With low extraction efficiency in cold water, this results in weak coffee with insufficient flavor. If it's too fine, that won't work either. Increased resistance can easily cause blockage and water accumulation, and if it soaks for too long, bitter flavors will emerge.

For controlling cold drip grind size, FrontStreet Coffee uses a #20 (0.85mm) standard sieve for calibration. The grind sizes for various common extraction methods, from fine to coarse, are arranged as follows: cold drip (cold brew) > pour-over (siphon) 80% > American drip 75-80% > cupping 70-75% > French press 65-70%. Therefore, cold drip coffee should be ground finer than pour-over coffee, approximately with an 83-85% pass rate.

2. Water Drip Flow Rate

Throughout the entire drip process, new cold water continuously passes through the coffee bed and then filters to form new coffee liquid. Compared to the static cold brew type, cold drip is a dynamic brewing method that extracts and filters simultaneously. Therefore, to prevent extraction interruption, we must ensure that water drops contact the coffee bed evenly to obtain a cup of cold drip coffee with both concentration and flavor.

Thus, controlling the water drip rate is a crucial part of making cold drip coffee. If the water drip rate is too fast, it's easy to under-extract, resulting in weak coffee without aroma; conversely, if the flow rate is too slow, it can cause the coffee to ferment, producing undesirable sour or miscellaneous flavors.

According to FrontStreet Coffee's experimental results, when using light roast coffee beans, the valve can be adjusted to a relatively fast state of 7 drops per 10 seconds to highlight the light floral and fruit notes, such as FrontStreet Coffee's Red Cherry and FrontStreet Coffee's Little Tomato. When making cold drip with medium roast coffee beans, such as FrontStreet Coffee's Sidra and FrontStreet Coffee's Strawberry Candy, FrontStreet Coffee suggests controlling the flow rate at 5 drops per 10 seconds in a slow state, which allows the final coffee to present a rounder body and fermentation notes.

3. Extraction Ratio

The cold drip ratio not only affects preparation time but more importantly determines the coffee's concentration and extraction rate. Considering that the coffee will ultimately be served with ice to maintain its low temperature, FrontStreet Coffee typically uses a 1:10 ratio when making cold drip. For those who prefer a cleaner taste, FrontStreet Coffee suggests using 1:12 to 1:14, which produces iced coffee more suitable for direct consumption.

4. Maturation Time

I believe many people know that even cold drip extracted under the same conditions will have significant flavor differences due to different storage days.

A freshly made pot of cold drip placed in the refrigerator allows the substances within the coffee to enter a slow, controlled fermentation process. After several hours of settling, the coffee's taste becomes clear and mellow. On days 3-5, the fermentation flavor of cold drip is most prominent. Starting from day 6, the flavor substances dissolved in the water continue to change, some aromas have dissipated, and the acidity is no longer bright. Therefore, considering both flavor experience and health safety, FrontStreet Coffee suggests that cold drip coffee is best consumed within 5 days (counting from the preparation day).

How Long Does It Take to Make a Cup of Cold Drip Coffee?

According to FrontStreet Coffee's specification of approximately 200ml for a cup of Red Cherry cold drip coffee, when using a 1:10 coffee-to-water ratio, 20g of coffee grounds are needed, corresponding to 240g of ice water (coffee grounds absorb twice their own weight in water). With a valve flow rate of 7 drops per 10 seconds, it takes approximately 2.5 hours for all dripping to complete.

But since we know that cold drip preparation efficiency is low, most of the time we don't just make one cup, but rather "fill as much as we can." For example, the equipment used at FrontStreet Coffee's store can hold approximately 650ml of coffee liquid in the lower pot, so each batch makes three cups, using 60g of grounds to drip 600g of coffee liquid. With the valve at 7 drops per 10 seconds, the dripping time is about 7 hours. Finally, the coffee liquid is placed in the refrigerator to chill overnight, and it can be sold the next day.

So when asked how long it takes to make a cup of cold drip coffee, saying "a day and a night" seems quite accurate.

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