The Science of Cold Brew
Often, when looking at how to brew cold brew, there are certain protocols in place to achieve the “perfect” cup. Time of brew, the temperature of the water, the amount of grounds, grind size, etc. These factors are all very important, and if you follow protocols you will achieve a nice cup of cold brew. But at Blue House, we love making quality coffee accessible to people, and what better way to do that than to explain why we do what we do? Instead of throwing protocols in the form of a brewing guide at you today, we want to help you understand cold brew, from the inside out. Why does grind size matter? What makes one temperature of brewing better than another? Today, we will guide you through these queries and more, by taking you through the different factors that go into brewing cold brew!
The Grind
Why is grind size important? The first and most obvious answer is that it contributes to the total amount of surface area in which water in the coffee can interact. To understand this better, let’s first look at jolly ranchers. How much time does it take to dissolve a jolly rancher? How about when you bite it into 1 million smaller pieces? Although jolly ranchers and coffee are obviously very different, if you can picture this idea, this will give you a basic understanding of how grind size affects the outcome of cold brew.
When it comes to the grind size, in order for these compounds to become soluble in the water, there needs to be some exposure to the liquid. A smaller grind size leads to an increase in surface area with a greater ability for water to reach those compounds.
That being said, we still prefer coarse grounds for cold brew. The question of the day then is this: why don’t we use fine grounds when they clearly have a better extraction potential? There are a few reasons why this is the case.
Reason 1: Stray Particles
Fine particles can actually get through the filter and hang around in the finished cold brew. This can create a few issues. First, you could end up with a cold brew that lacks that nice clear color that we all love. Additionally, these particles can get in our mouth as we drink the coffee and feel unpleasant. Lastly, too many of these particles can mean that the cold brew will actually stay fresh for a lesser time, as these insoluble particles can break down into unpleasant taste.
Reason 2: The complex relationship between solubility and grind size
When the particles get too small, often they will clog the pores of whatever filter medium is being used for cold brew. We all can remember a time when our pour-over or Mr. coffee machine had grinds that were too small put in, creating a mud-like medium that leads water to flow around the grounds instead of through. In this case, because the water is not actually touching the grinds, you can get an under-extracted brew from too fine of grounds.
We just explained why too fine of a grind is far from ideal. But why do we consider a coarse grind to be the best?
Cold brew is brewed between 18 and 24 hours, which is plenty long enough for the water to fully saturate the bean, even the inner layers. In addition to this, a coarse grind allows the water to flow freely between the fragments of the coffee beans during the course of the brewing time, allowing for an even extraction. The water is able to reach the beans at the center of the filter just as easily as the beans close to the edge, allowing for all of the beans to be extracted the same as the others. This allows for a nice clean flavor profile of the cold brew.
Temperature and Time
When looking to understand how the temperature of brewing and time of brewing work together, the first step is to understand how temperature affects the solubility of coffee beans. Higher temperatures lead to an increase in solubility and a quicker brew of hot coffee. Lower temperatures during brewing lead to a decrease in solubility, which is why cold brew is brewed for so long, so as to maintain proper extraction of the beans at a lower temperature.
In other words, time and temperature work together in an inverse fashion. When the temperature of brewing increases, the time necessary for proper extraction and brewing decreases. And in the opposite manner, when the temperature of brewing decreases, the time necessary for brewing increases.
Think about if you were to go to your kitchen and make some hot coffee versus make some cold brew. Brewing hot coffee can happen in a matter of minutes. Why? The temperature at which brewing takes place is quite high. In contrast, you might plan to make yourself a batch of cold brew for the next day, but you wouldn’t be able to just go and “make yourself a cup of cold brew.” Cold brew takes time. Why? The temperature at which cold brew is brewed is so much lower than the temperature at which hot coffee is brewed.
This correlation helps to understand both the general brewing process for cold brew, as well as the impact that certain brewing methods can have on the resulting cold brew. Brewing at room temperature as well as brewing in the fridge will both produce quality cold brew, but there is a difference in temperature between these brewing methods that must be accounted for with the time of brewing. A decrease in temperature will decrease the rate of extraction, which will, in turn, require a longer amount of time for the cold brew to be properly extracted. This is why brewing cold brew in the fridge requires a longer amount of time than brewing cold brew at room temperature. As far as specific time guidelines for brewing, we’d suggest brewing cold brew for somewhere between 16 and 24 hours, with twenty being the sweet spot. But taking into consideration temperature, this time may be on the lower end of the range with a room temperature brew and may be on the higher end of the range with a brew below room temperature.
Water Content
Brewing coffee is sometimes described as an art, but it is also most definitely a science, and more specifically, it’s chemistry. When viewed from this perspective, it is clear that the composition of the water used for brewing cold brew is important. When we think water, we think H2O, plain and simple. However, the water we find in our home is often full of many extra chemicals and minerals. Anything in our water simply adds to the chemical processes that occur in brewing cold brew and affect the final flavor of the brew.
So, how do the various components of the water affect the final brew?
Noticeable Flavors
Some of the components in common tap waters leave a noticeable flavor difference in water which can negatively affect the cold brew. One example of this is chlorine. If a water has enough chlorine in it, you can taste it in the water by itself. This can become problematic if you use this water to brew cold brew. This is because there is the possibility of having that chlorine flavor stick around. The flavor of the coffee will definitely mask the chlorine flavor, but it will prevent the crisp and clean profile of cold brew which we aim for.
Other Minerals
While some components of water are more obvious and noticeable in the way they affect cold brew, others components like the minerals calcium and magnesium aid in the extraction process of the coffee beans. That being said, even minerals that help in the brewing process are only meant to be present in specific amounts. Anything outside this will negatively affect the brewing process.
There is so much variation when it comes to water composition, and there are a few ways to navigate this to end up with a crisp cold brew. For more information on how to deal with water variations in your kitchen to end up with the best brew, click here.
