Ben's Guide to Cold Brew Coffee

2022-02-21 00:00:00 +0000 - Written by Benjamin Dix-Matthews

Cold brew is unequivocally the the best way to make and drink coffee. It is easy to make, requires less equipment than other forms of coffee, and tastes less bitter than hot brewing methods. Cold brew over ice is particularly excellent over the summer. Hot coffee on a 40 degree day is for masochists

1. Brewing Cold Brew

It is extremely easy to over-think and over-complicate the brewing of cold brew. In reality, it is just two simple steps:

  1. Put 80 g/L of good coffee beans (ground as coarsely as possible) in some water, shake or stir, and then leave for 12 - 24 hours. Big Jars, iced tea jugs and french presses all make excellent brewing vessels.
  2. Filter the coffee through cheese cloth (this is what i do), or a paper filter, or in a pinch any fabric, into a bottle and store in the fridge for up to two weeks. The cold brew can then be served over ice, and i have found it goes well with a slice of orange (thanks Nika).

The ratios, brew-times, bean types, and filtration methods can all be tweaked for personal preference. I have yet to mess the recipe up to an extent that i have not wanted to drink the product. My main word of caution would be on over brewing, this can make the coffee taste pretty bitter.

2. Nitro Coffee

If you are one of the fortunate few who have kegging gear and nitrogen gas you can turn your cold brew into nitro. This makes the cold brew come out a bit like a stout, and adds a creamy texture.

  1. Put the cold brew into a keg, seal and pressurise to ~40 psi.
  2. Purge the air from the keg with at least three quick tugs on the release valve. This should make the shelf life super long (not overly scientific, but i have never had a keg last long enough to check this).
  3. Cool the keg down in a fridge for about a day.
  4. Shake the keg for about 10 minutes at 40 psi to try and dissolve as much nitrogen gas as possible. Nitrogen doesn’t dissolve very well, so doing this while the keg is cold is a must (cold water dissolves gas better).
  5. Ready to serve (also at 40 psi). Serving through a stout nozzle is ideal, but whatever is fine.

3. Caffeine Content of Cold Brew Coffee

Note that all of this information was found from random blogs and would benefit with a tad more scientific rigour. That said, who has the time for scientific rigour? Who am i, some kind of scientist?

The caffeine content of cold brew depends on three parameters:

  1. Bean caffeine content (Bc) primarily depends on the origin and species of the coffee beans. Roast and grind can vary the content by a few percent, but this is essentially negligible. Typical Araciba beans have a caffeine content of around 1.2% by weight.
  2. Extraction efficiency (E) refers to the proportion of the bean’s caffeine that is actually extracted during brewing, and depends on the brewing method. The extraction method for hot brewing is around 90% and some random blog said that cold brewing was around 10% less efficient. Thus, we will assume a cold brew extraction efficiency of around 80%.
  3. Brew ratio (R) refers to the ratio of beans to water and can change a fair amount from person to person. I use 80g/L which corresponds to a ratio of 1:12.5.

Thus the caffeine content of the cold brew can be estimated as the product of these factors:

\begin{align*} C &= B_c * E * R \\ &= 1.2 * 10^{-2} [\frac{g}{g}] * 0.8 * 80 [\frac{g}{L}] \\ &= 0.96\% * 80 [\frac{g}{L}] \\ &= 768 \frac{mg}{L} \end{align*}

Note, these calculations are consistent with a rule of thumb on reddit that the caffeine content is 1% of the dry coffee weight.

As a reference point, Red Bull has a caffeine content of around 320 mg/L, and a shot (30 mL) of coffee is around 64 mg. Moral of the story: cold brew is strong, don’t chug a litre of it.