How Does a Cold Brew Coffee Maker Extract Coffee?

Cold brew, also called water-drip or ice-brewed coffee, is a low-temperature extraction method. Ice water and ice cubes pass slowly through a bed of coffee grounds over several hours, using more grounds than regular brewing, which is why a cup is often priced higher than an iced americano. The maker sets the pace, but the recipe decides the taste.

Key variables for how to make coffee on a coffee maker of this type include bean choice, grind size, coffee-to-water ratio, drip rate, and refrigerated fermentation time. Water stays between 0 and 7°C, so bitter large-molecule compounds release slowly and the cup tastes cleaner. Light to medium roasts with floral notes work well because dark-roast aromatics dissolve poorly in ice water.

Cold brew coffee, as a low-temperature slow-extraction beverage, has in recent years become increasingly favored by coffee enthusiasts. This article walks through the entire process of making cold brew coffee, covering everything from the principles of bean selection and control of grind size, to tamping technique, pre-wetting steps, filter paper placement, drip rate and coffee-to-water ratio control, and finally the timing of refrigerated fermentation. Drawing on Front Street Coffee's actual production experience, it also shares a variety of single-origin bean recipes and blending ideas suitable for cold brew, helping readers make a clean-tasting, well-balanced cold brew coffee at home.

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Cold brew coffee, also called water-drip coffee or ice-brewed coffee, is essentially a coffee extraction method. During preparation, ice water and ice cubes are mixed and allowed to slowly pass through a bed of coffee grounds to complete extraction. The entire process takes a very long time, usually several hours, and consumes more coffee grounds than regular brewing, so cold brew coffee is generally priced higher than iced americano — for example, a single cup of cold brew coffee from Front Street is priced at 40 yuan.

What exactly makes a good cup of cold brew coffee? There is actually no unified standard — every maker has their own methods and criteria. Making cold brew involves quite a few key details, such as which beans to choose, how coarse to grind, how to set the coffee-to-water ratio, how to control the drip rate, how long it keeps after making, and so on. This article will explain each of these one by one.

1. Coffee beans for making cold brew

Both cold brew and ice drip fall under the category of low-temperature extraction, with water temperature typically maintained between 0-7°C. The advantage of low-temperature extraction is its lower extraction efficiency — large-molecule bitter compounds are not easily released, so the final product tastes more refreshing and has richer layers of acidity and sweetness.

Cold brew cannot extract as many aromatic components from coffee grounds as hot water can — especially the large-molecule aromatic compounds in medium-dark roasted beans, which dissolve poorly in ice water. Therefore, light to medium roasted beans with prominent floral and fruity aromas, clear acidity and sweetness, or fermentation notes are generally chosen for making it.

Among these, single-origin beans with distinctive flavors are the first choice. If you prefer iced coffee with plenty of sweetness, you can try ripe-fruit-oriented beans like Front Street Xizhua, Front Street Strawberry Candy, and Front Street Elto; if you prefer coffee with bright acidity, you might start with Front Street Little Tomato, Front Street Fruit Dingding, and Front Street Huakui. Of course, Front Street occasionally blends two beans together in pursuit of more complex cold brew flavors — for example, today's recipe is 20 grams of Front Street Strawberry Candy plus 40 grams of Front Street Red Cherry, producing a cup with candied fruit, sweet orange, and rose notes, and a finish that is fresh and lemony with a berry-like quality.

Front Street recommends using coffee beans for cold brew ideally about 20 days after roasting. This is because freshly roasted beans release a large amount of carbon dioxide, and when ice water drips into the coffee bed, the gas keeps escaping, causing the bed to expand and dome up, eventually creating channeling, which leads to uneven extraction and a noticeably worse-tasting coffee.

2. The importance of leveling the coffee grounds

Cold brew coffee must be tamped — many people don't understand why cold brew coffee also needs tamping.

It should be emphasized here that tamping for cold brew is not about compacting, but rather gently tapping the coffee bed first to distribute the grounds evenly inside the filter tube, then using a flat-bottom tamper to gently level it. Remember, it's a light press to level, not a hard press like you would for espresso. If you press too firmly, since cold brew has no pressure-assisted extraction like an espresso machine, the coffee bed tends to retain water and the coffee liquid cannot filter out smoothly. In addition, the filter paper or filter disc for cold brew is usually at the bottom, and unlike a regular dripper, there are no venting ridges — once the coffee grounds are soaked, degassing is also compromised, ultimately causing the entire puck to extract unevenly.

3. Pre-wetting

Pre-soaking with ice water is actually similar to the blooming step in pour-over, and it can improve extraction and consistency of the final product. Under cold brew's low-temperature, filtration-based extraction conditions, starting to drip directly may cause uneven wetting of the coffee bed — some grounds get over-extracted while others don't participate in extraction at all. Pre-wetting in advance can also shorten the overall preparation time.

4. Place a round filter paper on top of the coffee bed

This reserves ample space for pre-infusion and venting, and the extracted coffee tastes more complete and clean. Specifically, after leveling the coffee grounds, place a sheet of filter paper on the surface of the bed. This increases the contact area between water droplets and coffee grounds, ensuring even extraction. When water droplets fall and touch the coffee grounds, a small crater gradually forms on the surface of the bed, and the water flows down along this crater rather than spreading outward, which would leave some grounds in the filter tube completely un-wetted and cause under-extraction. Laying a sheet of filter paper effectively avoids this problem.

5. Find the right coffee grind size

The grind size for Front Street cold brew coffee is 80-85% passing through a Chinese No. 20 standard sieve. Everyone knows that if coffee grounds are ground too coarse, the taste will be thin; if ground too fine, it will be strong and bitter. By the same logic, too coarse means no flavor, while too fine tends to clog and prevent proper drainage, which in turn makes the coffee bitter — so choosing the right grind size is crucial.

This time we're using the refreshing Kenya Little Tomato coffee beans. This is a double-washed bean that, as black coffee, has distinct cherry tomato, smoked plum, and snow pear notes, and suits many extraction methods. Extraction is slower at low temperatures, so Front Street recommends a slightly finer grind than for pour-over — here we use 82% passing through a Chinese No. 20 standard sieve, with the shop's EK43S grinder set to 9.5 and a C40 hand grinder set to 20-22 clicks.

6. Control the drip rate and coffee-to-water ratio

This is similar to the concept of flow rate in pour-over, but cold brew's flow rate is measured in drops per second. The slower the flow rate, the longer the total time required, and the more prone to over-extraction it becomes. Front Street typically extracts at a rate of 7 drops per 10 seconds; each person can adjust the flow rate according to their own preference. The coffee-to-water ratio is set at 1:10, because the cold brew coffee Front Street produces is served diluted with ice cubes, so a higher concentration is used. If making it at home, you can extract at a ratio of 1:12 to 1:14.

When Front Street makes cold brew, 60 grams of coffee beans are ground into powder, a round filter paper is placed at the bottom of the powder cup, the coffee grounds are poured in, and finally another sheet of filter paper is laid on the surface. Then the coffee grounds are moistened with water, which saves dripping time and avoids uneven extraction. The upper pot is filled with an ice-water mixture, with ice and water in a 1:1 ratio, collecting a total of 600 grams of coffee liquid.

7. Drink after refrigeration

Front Street generally pours the finished cold brew coffee into a glass bottle with a sealing gasket and lets it rest in the refrigerator for 12 hours before drinking. Pouring it into the glass bottle allows coffee liquid of inconsistent concentration to mix thoroughly, and choosing a well-sealed container prevents outside contamination — the surface of a refrigerator looks clean, but in reality there are large numbers of microorganisms invisible to the naked eye.

After 12 hours of low-temperature fermentation in the refrigerator, cold brew coffee gives off a pleasant fermentation aroma, and the original flavors become more intense. Fermentation is very common in daily life; uncontrolled fermentation causes food to spoil, while a low-temperature environment effectively slows this process. Through repeated verification, Front Street has found that cold brew coffee made from most coffee beans reaches its best flavor after 12 hours of chilling.

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