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This is a fermentation tank - there are quite a few at the Ruiz mill to keep coffees from different estates and different quality levels isolated as distinct lots. Fermentation breaks down the mucilage layer coating the seed so it can be washed away. Think "rotting fruit". You ferment depending on altitude and temperature, usually 18-24 hours. The dark stuff in the tank is residual peel. Fermentation should not "flavor" coffee at all - it is simply to get rid of the mucilage layer.