Cold Bruer: Experiments With Grind Size | Bruer | Cold Brew Coffee Maker
December 10, 2013

4 comments


Cold Bruer: Experiments With Grind Size

One of the most frequently asked questions we get is:

"What is the best grind for Cold Bruer?"

Great question! Those that are asking have probably tasted how the grind size influences the quality of their hot brewed coffee. Too fine, and it's an over-extracted, bitter cup. Too coarse, and the result is an under-developed, watery substance. While the slow-drip cold brewing method has some fundamental differences from hot brewing techniques, grind size will still play a role in the quality of the cold brew. In this post, we'll take a look at three different grind sizes and the resulting influence on extraction and taste.

 It's difficult to describe grind sizes. What exactly is coarse? Or fine? Grind size is relative, so the easiest way is to compare them visually. Above, the three grind sizes we tested are shown with table salt and a penny for reference. The espresso grind is really fine. It's finer than the table salt (though it looks similar), and almost feels like flour. The drip grind is much coarser than both the table salt and espresso grind. It's got a definite gritty feel between the fingers. Reminds me of the beach sand in Santa Cruz. This is also the grind we do most of our cold brews with. The coarsest of the three is something we use for our french press. It's definitely got some big chunks.

You'll notice that each grind does not consist of all the same sized particles. What we've got is a range of particle sizes with the "average" being our desired grind size. All coffee grinders (including the burr grinders we use) will produce a distribution of particle sizes. The only way to get a uniform grind size is to use a sieve to sort out the desired size. But it's not really necessary. As long as our grinder is producing a narrow particle size distribution  (the vast majority of particles are close in size), having varying sized particles is no a significant problem. The most important thing is that we're getting consistency from grind to grind.  Coffee grinding is a fascinating subject. Expect a future post on grinders where we'll dive more in depth.

 

The Setup

For each Cold Bruer, we used 50 grams of coffee and 600ml of water. This gives us the 1:12 brewing ratio that's been our go-to ratio anytime we're trying out a new coffee. We also set the drip rate to about 1 drip every 2-3 seconds. For these tests, we're using Verve's Streetlevel espresso blend. It's a coffee we've used extensively in the past for testing Cold Bruer.

The coarse grind was the quickest to begin dripping, and the fine grind was the slowest. The finer the grind, the more difficult it is for the water to begin passing through. 

Water pooling above finely ground coffee bed

 

The fine grind was restricting the water so much that it began pooling up above the coffee bed! It never got more than 3-4mm deep, but we expect that it may have risen more had we used a faster drip rate

 

 

On to the good stuff! We set up a blind tasting of each brew, and jotted down our tasting notes on the post-it by each cup. After we were sufficiently caffeinated, we took some guesses about what cups had what cold brews, then we looked under the cups to see if we were right. The Fine brew was an easy one to pick out. The tasting notes for that one were "harsh", "bitter" and a "slight metallic taste". Not very pleasant in general. Everyting we'd expect from an over extracted brew. The Medium and Coarse were a little more challenging to pick out. In the end, our guesses were correct. The Coarse had the familiar sweetness that we've come to love about cold brew, but it was less pronounced than in the Medium brew. While the Medium brew had a long lasting dark chocolate sweetness that stayed with you, the Coarse brew left the palate quickly: An indicator of under extraction.

 

 

Hands down, blind tasting is the best way to evaluate the quality of coffee. But it's sometimes nice to correlate some numbers with changes made in the experiment. Here's where the coffee refractometer comes in to play. It's a device that measures the total dissolved solids in the brew telling us the brew strength and giving an indicator of the extraction yield (how much was extracted from the coffee). The refractometer deserves a dedicated blog post of its own, but we'll give a quick run-down of the results. 

 

The Fine brew had 2.17% Total Dissolved Solids (TDS) correlating to a 23.76% extraction yield. 

The Medium brew had 1.96% TDS correlating to a 21.46% extraction yield.

The Coarse brew had 1.86% TDS correlating to a 20.36% extraction yield.

 

These are the basic equations for determining the extraction yield of the coffee. There's a slick App that crunches the numbers called VST Coffee Tools from the same company that makes the refractometer, but sometimes it's nice to work it out on paper to see what's going on. The below numbers are for the fine brew. The 2.17% is the TDS measured with the refractometer, the 520g is the weight of the brewed coffee (we started with 600g of brew water, but about 80g of it was absorbed by the coffee bed). The dry mass is the mass of ground coffee, but you'll notice that 47.5g is used in the calculations when we had measured 50g of coffee. This reduced amount takes into account the moisture and CO2 content of the ground coffee (about 5% of the total mass) that will not contribute to the extraction. 

 

 

There's been extensive research done to determine the ideal Brew Strength and Extraction range for conventional hot brewed coffee. The SCAA calls for a TDS range of 1.15%-1.35% and an extraction of 18%-22%. These values are the result of polling people's preference for coffee flavor and brew strength. Cold brew is relatively new, and there's little information on what the accepted standards are (though the only thing that really matters is what tastes good to you). From our experience, we've found that brew strengths above 2% tend to develop some bitterness, so it's no surprise that the fine grind brew at 2.17 had that harsh bitterness. We're looking forward to using the refractometer in future experiments to help build our quantitative understand of cold brew.

Conclusion

Grind size is definitely an important variable that will influence the quality of the cold brew. Grinding too fine will over extract leading to a harsh and bitter result. Too large of a grind and the brew is underdeveloped and weak.  There's definitely a sweet spot for the grind size that will bring out the coffee's sweetness. A good starting point is to choose a grind that has the feel of sand and then adjust from there. If it's too weak/watery, try a finer grind. If it's tasting bitter, move to a coarser grind. Half the fun is experimentation, so enjoy!

 

Future Blog Posts

We've got more experiments in the works, and we're looking forward to sharing them with you here. If you've got any questions about this post, or have suggestions for future posts, we welcome your comments below. 

Happy brewing!

-Gabe


Bruer LLC
Bruer LLC

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4 Responses

Courtney
Courtney

October 08, 2016

Thank you! You just saved me from wasting the time/money/coffee of doing my own test on fine ground.

Falko
Falko

October 15, 2014

The Bruer is version of a cold dripper is a fantastic breathe of fresh air compared to the big Tower from Hario and Yama. We from coldbrewtest.com are quite interested in how the device (bruer, hario, etc.) affects the taste of the Cold Bre Coffee at the end. If you’re interested have a look at our page where we compare different devices with each other!

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Peter Ehrmanntraut
Peter Ehrmanntraut

July 10, 2014

Hello. I find your coffee experimentation very interesting. I have always preferred precision espresso machines to prepare the most flavorful coffee. My machine seems to be able to get the variables to the equation right: right amount of coffee, right amount of water, right amount of grind fineness, right amount of pressure, right amount of temperature, right amount of pre-infusion, right amount of extraction time, right amount of post-infusion. I have tried coffee prepared through an inverted Aeropress and just doesn’t have the flavor profile and sweetness the espresso machine produces no matter how I play with the variables. I have made delightful homemade ‘frappucinos’ using freshly made espresso and it is delightful. I have decided also try cold brew for another cold drink option and to compare it’s taste profile to that of perfectly made espresso. My question is how does the water drip setting affect the taste from your 3 hour to 12 hour settings? Does the 4 hour drip produces a milder, weaker under-extracted brew and does the 12 hour drip create an over-extracted brew? Would you use coarser grind for longer drips and ‘steeping’? Thanks.

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