Sour cold brew

[Vernholio]

Hey guys,

I bought a full set up from Keg Outlet about a year ago to cold brew coffee and nitrogen pour it from my keg fridge. I've made 10 or so batches in that time. I'm using a 1 to 1.5 ratio of lbs. of ground coffee to RO water, as suggested by the Cold Brew Guide that Keg Outlet offers. (I use a 2+ lbs. bag of beans, and roughly 3-ish gallons of water, based on what the grind yields.) I typically get a good end result. But, with the last couple of batches, I noticed the coffee was a bit sour. So, I did a bunch of research after the first sour batch and read about under absorption due to too little time in the brew, and also the size of the grind (too large) both contributing to sourness. I also thought maybe it was the coffee I used (Costco Medium Roast), but I'd used that successfully before. I also like to add about a 1/2 cup of simple syrup (brown sugar and RO water) to my ~2.25 gallon batches after the brew to mellow it out. I read that that might also be contributing to the sour. So, I changed everything up... I used better coffee--Starbucks medium roast, I used a slightly smaller grind and left it in a little longer in the fridge to brew (24 hrs instead of 20), and no simple syrup. I did all the things you're supposed to do: forced all of the oxygen out of the keg via the nitrogen line, and I'm using a Quick Cascade Nitro Coffee Keg Lid... and guess what? I got sour coffee again.

This morning, I came across yet another article about sour coffee and cold brew, and it talked about the gas used. This one talked about a "beer gas mixture" of nitrogen and how it can: 1. Create sourness and 2. Affect mouthfeel. I'm assuming that by "affect mouthfeel" they're talking about a slight fizziness, due to the CO2 that's in it, correct? And yep... my coffee does have a slightly fizzy mouthfeel.

So, after that LONG-WINDED explanation--I wanted you all to share in my pain--I figure I'm just looking for a confirmation on this from someone with experience: Could it be that I accidentally got a can of "beer gas mixture" nitrogen from Airgas? (And, in the future I need to tell them I need 100% nitrogen?) It'd be REALLY nice if the solution is this simple... I was thinking about throwing in the towel on cold brewing, 'cause I couldn't figure out why my batches were sour!

Thanks!

 

[Vernholio]

Bringing an old post of mine back from the dead, but I figured I should share my findings in case anyone actually reads this. (Kinda doubt it, based on the lack of replies, but what the heck... Maybe I'll help someone else out.)

It was exactly as I assumed... I had a bottle of "beer mix" gas. 50/50 nitro and CO2. Next, and subsequent trips into Airgas, I always make a point of saying, "I need 100% nitrogen. No 'beer gas' mix." Problem solved... Coffee's excellent again!

 

[Ruint]

I know that CO2 will sour cold brew coffee, especially with extended exposure. I have not taken the time to set up a solely nitrogen tap for my cold brew yet, and have only used the gallon size vessel to do nitro coffee. Whether it is nitrogen or nitrous oxide. I have noticed that nitrous oxide makes the cold brew seem a little sweeter, so maybe you might try that instead of a simple syrup. At 2 days under pressure, with nitrous, haven't gotten any sour notes. With nitrogen, it is more of a cascade of bubbles effect with the coffee than anything else. I do 5 to 6 gallon batches at a time, and roughly do the same ratio as you. I usually go towards the heavy side, as I like being cursed by friends and family when it's interrupting their sleep pattern!! I do give them fair warning though!