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Background:

Summer is upon us. After resuming my cold-brewing regimen, I'm finding that cold-brewing coffee isn't working as well as I seem to remember it did last summer.

For those unfamiliar with the process, you take ground coffee and let it sit in cold water from 12 to 24 hours, then strain it. You should end up with a coffee concentrate, to which you add hot water.

The main problems are that the coffee is significantly weaker than it was a year ago. I'm using the same proportions, the same volume of water, and, unless I'm misremembering, the same amount of brewing time.

The only way I can get a reasonably strong concentrate is by brewing for 12 hours, replacing the grounds with fresh ones, then brewing for another 12 hours. (I haven't tried a straight 24-hour brew yet.)

The question:

I've used three kinds of ground coffee, and the best results are from pre-ground Chock-full-o-nuts. (A local brewer's beans and Peets beans both produced weak, watery coffee that can't be diluted. Both used fresh beans.)

Aside from the obvious (use moar coffee, etc), what can affect the strength of the concentrate? Might the quality of our water have changed? Are there environmental factors to consider?

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Is the ambient temperature cooler, perhaps? It doesn't sound like you moved, but maybe it's a cooler beginning of summer, or you're using more AC? (Also, are the ground beans which produced weak concentrate ones you used last year, or something new? Seems like a coarser grind might cause problems too.) –  Jefromi May 11 '11 at 19:28
    
Cold-brewed coffee is so much less acidic that, to me, it will always feel "weak" compared to drip/press prepared coffee - what I did notice though, was that the coffee was just as dark/opaque after diluting. Are you noticing that your coffee is more translucent, as well as tasting weak? –  overslacked May 11 '11 at 20:55
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You're using a fine grind? I've always found grinding it myself to extra-fine works best for cold-brew. –  Bruce Alderson May 11 '11 at 22:55
    
Using a coarse grind for the beans - they were fairly fresh. Will try a finer grind. It is a little cooler than it was when I was doing this last summer. No A/C in the kitchen, now or then. –  Neil Fein May 11 '11 at 23:01
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If the beans themselves have been sitting on the shelf for longer - it means they have lost a lot of their fresh, volatile flavours. May not be the only reason - but it definitely affects the flavour you'll get. –  Taryn East May 17 '11 at 7:18

3 Answers 3

Grind

You're using a fine grind? I've always found grinding it myself to extra-fine works best for cold-brew. – Bruce Alderson

and

I pour it through a standard mesh a few times, then a mesh + cone filter. The resulting coffee is thick, dark, and has very low acidity (very nice over ice with a spot of condensed milk). – Bruce Alderson

Bean Age

If the beans themselves have been sitting on the shelf for longer - it means they have lost a lot of their fresh, volatile flavours. May not be the only reason - but it definitely affects the flavour you'll get. – Taryn East

Temperature

Is the ambient temperature cooler, perhaps? It doesn't sound like you moved, but maybe it's a cooler beginning of summer, or you're using more AC? [...] – Jefromi

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It seemed like the best answers were in comments, so I turned this into a community wiki answer –  Ray May 18 '11 at 1:46

I used to work in a coffee shop and we would regularly make cold-brew coffee. How we did it was to grind whole beans on the finest setting we had (Turkish coffee) and to fill a porous bag, tie it off, submerge in water, cover, and place in the fridge for 12 hours. After the 12 hours waiting period, take the water-soaked bag out of the bucket, and place in a colander (preferably with a handle) over the bucket until most of the water drains out. Squeeze out the remaining water and dump the grounds. With about 3-3.5 pounds of ground and a 5 gallon bucket filled 3/4 of the way (with the bag inside), we did not dilute the mixture after brewing.

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From everything you've said, it sounds like a temperature thing.

Did you leave the coffee sitting in a sunny window last year? This could have warmed the water significantly. Maybe you kept it closer to a hot oven or something? Those are the only environmental changes that I can think of that would affect your brew.

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