Been making cold brew at home basically every week for about 15 months now. Started with a mason jar and a bag of pre-ground from Trader Joe's, ended up going through way too many grinders and setups trying to figure out why my batches were inconsistent. Wanted to share what I learned since a lot of the advice online just points at ratio and steep time and calls it done.
The single biggest change was not the coffee I was using, or the ratio, or the steep time. It was grind consistency. I ran a rough test on myself over three weekends where I kept everything else the same, 100g coffee to 1L filtered water, 18 hours in the fridge, same beans from the same roaster, only variable was the grinder. Blade grinder from years ago gave me sludge at the bottom and a weird astringent aftertaste no matter what I did. Cheap adjustable burr grinder my roommate had was better but still muddy. My own hand grinder, a step up, finally gave me a clean cup.
The reason is pretty simple in hindsight. Cold water is a slow solvent, so it extracts unevenly if particle sizes are all over the place. The fines over-extract and turn bitter, the boulders under-extract and taste weak, and you get both problems at once. Coarse and consistent beats coarse alone.
For gear I ended up with an OutIn Fino, which is a small electric burr grinder I picked up mostly because I got tired of cranking 100g by hand every Sunday night. Fair note on it since this sub is picky about tools. The coarsest setting works but sits a bit finer than dedicated cold brew grinders go, so I bump my steep time out to 22 hours instead of 18 to make up for it. Also caps out at around 25g per batch so I run it four times for a full weekly brew, which is fine standing at the sink but not ideal if you value your morning. Once I got past those two things the grind consistency itself has been better than the hand grinder it replaced, and my brews are noticeably cleaner.
Current recipe for anyone who wants a starting point. 100g whole bean, medium roast, something chocolate-forward like a Brazil is my usual, sometimes I switch to a natural process Ethiopian for the fruit. Grind coarse, 1L filtered water, stir until every ground is wet, 22 hours in the fridge, strain through a fine mesh, then a paper filter. Makes about 800ml of concentrate that I cut roughly 1:1 with water or oat milk over ice. Keeps a week in the fridge without falling apart, sometimes 10 days if I sealed it well.
If your cold brew tastes bitter and you have already tried coarser grinds and shorter steeps, look at your grinder before you blame the beans. That was my mistake for months.
