Cold Brew Coffee: Traditional vs. Hot Bloom Experiment #1

Earlier this week I did a little experiment comparing two mostly-identical cold brews, and I wanted to quickly share some results of this experiment. And, yes, this is an n=1 so you can’t draw too many conclusions from this experiment, but it was interesting nonetheless. My experiment involved making two cold brew coffees, but using a hot bloom on one of them. The differences were very apparent!

I used some River City Roasters Yirgacheffe Aricha that I reviewed a little while back because I wanted a coffee with some brightness. Acidity and brightness is usually the parameter most people claim is boosted when a hot bloom is used. On both, I used the same 1:7 ratio of coffee to water, the same water, the same coffee, the same grinder, same amount of time… everything was the same except one got a hot bloom.

The criticism of cold brew is that it doesn’t extract many of the desirable acids that hot brewed coffee does. It’s like seafood without a squeeze of lemon. It can be dull and boring. A “bloom” is when hot water is added to the dry coffee grounds, just enough to saturate them all with water and begin the extraction process. The coffee, if fresh, will puff up and bubble and burp some as gas escapes. Cold brew artistes have been experimenting with doing a hot bloom using, like I did, 20% of my water at 205°F to bloom for about a minute, and then adding the remaining 80% of cold water as with the traditional process.

The idea behind the hot bloom is that it will extract some of those acids to liven up the resulting cold brew, but I wanted to test it out for myself.

The Results

I noticed a big difference in the physical properties of these two coffees right away. My cold brew looked like it always does, dark. The hot bloom had a distinctly red cast to it, was murkier and had a cruddier looking surface, like Lake of the Ozarks after big storms! LOL

I filtered both coffees through a Clever coffee dripper with well-rinsed paper filters. The cold brew filtered decently well, slowing down and eventually stopping toward the end like it always does. When I filtered the hot bloom I thought it would run through faster because I saw fewer grounds go into the filter, but in fact, the opposite happened. That coffee clogged and stopped filtering almost immediately and it wasn’t from grounds that got into the cup, but presumably from more dissolved solids in the coffee itself!

I eventually had to sort of manipulate the filter and squeeze it to get the coffee to go through but that caused a little hole in the filter and so that wasn’t going to work because some fines could get through and create an unfair comparison.

After that I decided to pour them both through my AeroPress using rinsed filters. The cold brew filtered through as expected, but the now ground-less hot bloom again jammed the AeroPress filter almost immediately. It wasn’t dripping slow… it was just stopped. This was a big surprise to me. I ended up putting the plunger in and pressing the coffee through. I inspected the filter afterward, couldn’t see a thing, so again, this lack of filtering was due to dissolved stuff in the coffee. Crazy!

The resulting coffee was still a clear, deep brown-black for the cold brew with minimal highlights on the edge of the glass while the hot bloom was murky and had tons of red highlights on the edges. Visually, they were very different coffees.

Taste testing revealed two pretty different coffees, too. The cold brew is pretty bright and fruity and has sort of a spike of acidity in the front of the sip that mellows out in the back end. It comes off almost a little sour. Overall, not bad, but I like a chocolatey/caramelly cold brew myself. Body was a bit thinner than I would expect from a 25 hour brew.

The hot bloom, weirdly because of it’s clogging problem, felt much thinner in mouthfeel. I expected the opposite. It overall seemed almost a little less acidic than the cold brew, but where the cold brew had this spike of acidity the hot bloom version remained bright from beginning to end. The hot bloom had more sustained acidity, but the magnitude of that acidity seemed less than the peak acidity of that spike I got from the cold brew. The flavors were pretty much the same, but they had different dynamics for sure.

I have an inexpensive TDS meter (TDS = Total Dissolved Solids) and I don’t know how accurate or meaningful this is, but for fun I metered both coffees while at the same temps and the cold brew came out as having 138ppm and the hot bloom as having 151ppm, which made a lot of sense because the point of the hot bloom is to dissolve some acids into the coffee in addition to what the cold brew pulls out during the extraction.

If you follow TDS testing you’ll know that those numbers don’t make a ton of sense on their own, and heck, even my tap water measures around 190ppm. What happens on some of these cheap TDS meters is that they “turnover” when they hit a certain level, kind of like an odometer that rolls over to “1” after it hits one million miles! So, the TDS meter is a decent comparison of the two coffees, but disregard the actual amounts because who knows where (or even how many times) the meter turned over? It makes sense that the hot bloom had more dissolved solids as we expected that in the extraction and it sure acted that way in filtering.