A key part of the wet process of coffee fruit is overnight fermentation, to break down the fruit (mucilage) layer that tenaciously clings to the coffee seed, so it can be washed off. Fermentation must be done soon after picking the cherry from the tree, and lasts 12 - 24 hours depending on temperatures and other factors. When you feel the slimy coffee and the parchment layer feels rough like sandpaper, the coffee is ready to wash. Good fermentation and subsequent drying can lead to the cleanest coffee flavors in wet-process lots. Note that when I talk about fermentation, I don't mean to imply that the coffee seed is subject to fermentation. That would create defective coffee. The fruit coating the outer parchment skin is broken down with the action of peptic enzymes in the coffee. Cacao is fermented, coffee is not.