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We use a press pot to make coffee at home, and usually after a 3 minute steep, the grounds are floating at the top, but with this one bag of beans we got, they're all sunk to the bottom.

The coffee basically tastes normal. Maybe a little thin, but that could easily be the roast.

Why would most coffee grounds float, but these sink? Is there something wrong with these beans?

We use a burr grinder and hadn't changed the grind setting. Same water, etc. Only change is the beans. "Medium-Dark" roasted Ethiopian Yirgacheffe from a local roaster, roasted about 2 weeks before brewing...

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That is unusual ... usually grounds sinking is because they're really, really old. But you'd taste that ... they'd be nasty and bitter. –  FuzzyChef Dec 27 '11 at 7:19
    
@FuzzyChef: Bag says they were roasted 12.09.11, so was about 2 weeks from roasting to first pot of coffee, which I don't think counts as "really, really old". –  freiheit Dec 27 '11 at 7:32
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1 Answer

The question should really read "What makes coffee sink?" It does not matter if it is in the form of grinds or beans (though it is easier with grinds) nor does the receptacle. The issue is buoyancy.

buoy·an·cy (boin-s, byn-) n. 1. a. The tendency or capacity to remain afloat in a liquid or rise in air or gas. b. The upward force that a fluid exerts on an object less dense than itself.

As the coffee is soaked in the hot water chemicals are extracted from the grind, mostly oils, that have an impact the buoyancy of the coffee. Once, the desirable chemicals are extracted to the surrounding water (again, moistly an oil) the buoyancy of the coffee is altered and the residue sinks to the bottom.

During my time in the U.S. Army field coffee was made quickly by getting a 15 gallon stock pot boiling and just pouring a five pound can of coffee in. Allow to boil for 5 minutes, reduce the heat and hit the side the stock pot with a ladle. Immediately all (ok, most...) of the coffee would sink to the bottom and fresh (if not altogether worthy of Starbucks) coffee could be ladled out. Why whack the pot? This would disrupt boiling of the water and allow the coffee to sink quickly.

Why are your grinds sinking? Well, the best guess is that they came deficient in the compounds that make the coffee normally buoyant. If the brand/roast is not up to your satisfaction you should be able to return it for fresh. If this a brand to which you are accustomed, but this batch isn't 'normal' then it is probably an anomaly, as you said the coffee tastes normal, so I wouldn't not worry to much about it.

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You made 5 POUNDS of coffee???? for how many people? –  Midhat Dec 28 '11 at 15:30
    
It wasn't me doing the making, but rather the cooks assigned to my company in the field. That would make coffee for more than 100 soldiers (15 gallon pot, less some water for working room at the top and the sludge at the bottom, call it 10 gallons) x 16 cups per gallon. About a 1 1/2 cups per cold tired G.I.. If you had a hot cocoa pack from your MRE we might mix that in. This is NOT would I would call GOOD coffee, but strong, hot, fast coffee. –  Cos Callis Dec 28 '11 at 15:49
    
@Midhat - Having tasted US Army coffee, it would be more accurate to say they ruined 5 pounds of coffee :) –  Chad Dec 28 '11 at 20:39
    
@Chad, wasted, no. Army Coffee is not "to be enjoyed on the veranda with breakfast", it is "to warm and energize troops fast and effectively." –  Cos Callis Dec 28 '11 at 21:27
    
@CosCallis - I understand but they could do that with generic beans... But those are actually high quality beans. Not that our troops dont deserve them just that the beans dont deserve to be treated that way :p –  Chad Dec 29 '11 at 14:58
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