three cold brew coffee experiments

The Science of Cold Brew Coffee

This is a guest post by a fellow food scientist with ample experience in various categories of new product development: Nathan Silva. Feel free to connect with him on LinkedIn.

Cold brew coffee has been taking over the U.S. coffee industry in recent times. Whether it be in coffee shops, ready-to-drink cans, or even in ice cream shops & restaurants, cold brew coffee seems to be everywhere. Dependent on who you ask, cold brewed coffee has a number of perceived benefits over its hot brewed counterpart. But are any of those benefits real?

What is cold brew coffee?

Let’s start by discussing the process of making cold brew coffee. The starting ingredients are as simple as can be: coffee grounds and water. To make cold brew, you simply mix the two ingredients together, put them in a container, and let them steep for a number of hours. From there, you simply remove the coffee grinds by pouring through some sort of filter (a coffee filter usually works out great). For me personally, I find that allowing for an overnight steep (around 12 hours) produces a great cold brew coffee, but you can find recommendations for everything from 8 to 16 hours.

making cold brew coffee

How is cold brew coffee made?

You make cold brew coffee simply by soaking ground coffee in cold (or lukewarm) water for several hours. While you’re soaking the ground coffee, you’re extracting the flavor, aroma’s and mouthfeel from the coffee. To make a great cold brew, you want to extract just the right amount of flavor and aroma from the beans. The science behind coffee extraction is actually the same as that of making vanilla extract.

Under-extracting coffee grounds would mean that you’re left with a weak cup of coffee that is missing the characteristic notes (think a cheap cup of gas station coffee). You’ve not extracted enough of the flavors and aroma. Over-extracting coffee grounds would mean that you left with a cup of coffee that is unbalanced, overly bitter, and acidic.

So what does this have to do with cold brew coffee? Like it’s hot brew counterpart, a cold brew coffee can be either under or over extracted. With all of the counteracting information out there, this is an extremely common mistake. With so many variables involved, including:

  1. The temperature of the water
  2. The grind size
  3. The roast and age of the coffee grinds
  4. The coffee bean origin
  5. The temperature that the cold brew is brewed at

There is really no “one size fits all” recommendation to make the perfect cup of cold brew. And let’s be honest here, there is no such thing as the perfect cup of coffee! Everyone has different preferences, and what may taste great for one person may not work for the other. My suggestion is to do a bit of experimenting, using some of the insights we share below, and find what works best for you!

Grind Size

Grind size has a large effect on the end cup of coffee, especially when you transition away from the traditional hot brewing method. Whether it be pour over, cold brew, espresso, or french press, each has a coffee grind size that is optimal for the method. A general rule of thumb is the longer the contact time, the larger the grind size. There are two main reasons why the grind size is so important: surface area and resistance.

Surface area applies to how long the coffee grind takes to dissolve into the water. Think of it like rocks at the beach: the larger the rock, the longer it takes for the salt water to corrode it down into sand. The same applies to coffee. If the water is quickly passing through the coffee grounds (think espresso or hot brew), you’ll want a finer grind size to allow for the best extraction. If the water is slowly passing through or staying with coffee grounds (think cold brew or french press), you’ll want a coarser grind size to allow for a slower extraction.

Resistance applies to how easily the water flows through the coffee grounds. Coffee grounds, much like a filter, can prevent liquids from flowing through them. This particularly applies to brewing methods that utilize a vertical flow of water, like pour over or espresso. The finer the grind size, the longer it takes for the water to flow through the coffee grounds. This can be used to your benefit, however can also cause over extraction. So make sure that your grind size is appropriate for your brewing method!

Coffee Bean Roast

The roast of a coffee bean has a TON to do with what kind of flavor and aroma you will be able to get out of it. There are four different types of roast commonly found in the market:

  1. Light
  2. Medium
  3. Medium-Dark
  4. Dark

Light roasts are milder in flavor, have more acidity, are light brown in color, and should not have any oil on the surface. Medium roasts are stronger in flavor, a bit more balanced and sweeter, medium brown in color, and should not have any oil on the surface. Medium roasts are typically referred to “American” roasts, as they are what can be found in most mainstream US brands (think Folgers). Medium-Dark roasts are darker in both flavor and color. There should be some oil found on the surface of these beans, as we are now getting to roast times that are long enough to allow for the oil to be expelled from the bean. Dark roasts are as dark in color and flavor as they come. They have a pronounced bitter/smokey flavor, and are always found with an oily surface.

Coffee Bean Origin

So let’s briefly speak about coffee bean origin. Ethiopia and Columbia are two countries that come to mind when talking about where coffee comes from, however, it is grown all over the world. Each origin, each farm, and each bean has its own characteristics. Much like clothing or where you live, you may have to try different things until you find what you like. Luckily, there are a ton of websites out there that will send you a variety pack of coffees from different origins to try!

three cold brew coffee experiments
Make a few small batches to see what works best for you!

How does cold brew coffee differ from hot brew?

There are two main differences between hot brewing and cold brewing: temperature and exposure time. Hot brewed coffee utilizes hot water that passes through the coffee grinds quickly, whereas cold brew coffee utilizes cold or room temperature water that is left with the coffee grinds for multiple hours. Just how the two differ from one another depends a lot on exactly which parameters you used for making either the hot or cold brew.

There are a ton of different perceptions around how cold brew coffee is “better” than hot brew. One of the main reasons is that people find the cold brewing process to produce a less bitter and acidic end product.

What’s with all of the hoopla around acidity?

The main benefit most people will tell you there is with a cold brewed coffee is the reduction in acidity. For a lot of everyday coffee drinkers, the acidic profile of most hot brewed coffees is an undesirable trait. This has caused cold brew coffee to boom in popularity. But is this “benefit” actually real?

If you ask Megan Fuller, Ph.D., and Niny Rao, Ph.D., of Philadelphia University, the answer is: kind of. Through their research, they found that the same coffees brewed by both hot and cold methods, had a fairly significant difference in pH – an analytical measurement used to measure the acidity of a product. However, it was found that the roast of the coffee bean itself had a larger pH differential than the brewing method. So, choosing a good roast is has more impact than changing over brewing method. But if you’re stuck with one bean, the brewing method can help you get the desired acidity!

Caffeine Content

Coffee is commonly consumed not only as part of your morning ritual, but also for the jolt provided by the caffeine content. So what role does the brewing method have on the caffeine content? Let’s head back to Dr. Fuller and Dr. Rao’s research. They found that although the brewing method plays a role in the amount of antioxidants in the end cup (hot brewed coffee had about 30% more), the brewing method did not play a role in the end caffeine content. It was actually found that the caffeine content is fairly stable across various roasts as well, which is really interesting!

making cold brew coffee

Cold brew coffee

Yield: 1 strong cup
Prep Time: 5 minutes
Additional Time: 12 hours
Total Time: 12 hours 5 minutes

Time for some experimentation!

Everyone prefers their coffee slightly different, so use this recipe as a starting point for your experiments to find your perfect brew!

Ingredients

  • 22g ground coffee*
  • 150ml water**

Instructions

  1. If you have small filter bags, fill a bag with your ground coffee. Place the bag in a jar/container that you can close on top.
  2. Pour the water through the filter to wet all the coffee into the jar. Push the filter into the water and leave to rest in the fridge for 8-16 hours. The longer you store it for, the stronger the coffee and the more you extract.
  3. Drink over ice, diluted with some cold water. Again, experiment to find your ideal ratios!

Notes

*Your ground coffee will have a major impact on the cold brew as you could read in the article. Experiment with different grinds, roasts, etc. If you're grinding your own coffee it's best to do so with a coffee or spice grinder. As you can see below, a food processor won't do the job, it leaves a lot of larger pieces.

When it comes to trying to find the right coffee bean for you, I would suggest doing a little experiment. First, get a few different kinds of coffee that you’re interested in (if you're in the US consider ordering a variety pack from Driftaway coffee or Trade coffee, or from Edgcumbes in the UK). Whether it be origin, roast, or both, just make sure that they’re different. Now brew a cup of each using your favorite brewing method, and set them up side by side. Taste each one in one direction, and then taste them backward. Which was your favorite, and why? Which was your least favorite, and why? We’d love to see your comments below!

ground coffee beans in food processor

**Experiment with cold, warm, or even boiling water when making your coffee!

Want to read more articles written by Nathan? Check these out:

References

American Chemical Society, Using chemistry to unlock the difference between cold- and hot-brew coffee, April 2, 2020, link

Emma Betuel, Crucial Difference Between Hot Coffee and Cold Brew Revealed by Chemists, Inverse, March-2018, link

Howard Bryman, Research Into Acidity and Antioxidants in Cold Brew vs. Hot Coffee Yields Surprising Results, April 15, 2020, Daily Coffee News, link

Camano Island Coffee Roasters, The Art of Roasting Coffee and Why It Matters, link

Coffeebros, Everything you need to know about coffee grind size and brewing, June 4, 2019, link

Meghan Grim, Niny Rao, Megan Fuller, Role of roast on chemical characteristics of cold brew coffee, Apr 01, 2020, ACS Spring 2020 National Meeting & Expo, link

Maciej Kasperowicz, The Complete Guide to Coffee Grind Size, August 07, 2019, link

Sampo Latvakangas, Coffee Roasting Basics: Developing Flavour by Roasting, 30-May, 2017, link

NCA, Coffee roast guide, link

Jennifer Ouellette, The chemistry of cold-brew coffee is so hot right now, 9-April, 2020, link

Rao, N.Z., Fuller, M. Acidity and Antioxidant Activity of Cold Brew Coffee. Sci Rep 8, 16030 (2018). https://doi.org/10.1038/s41598-018-34392-w, link

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