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Dry Processing

PC 5.03 Quality Controls

This dry-processing workflow includes the quality-control measures of winnowing, sifting, and flotation (left side). Best practice is to dry floaters and sinkers separately because they will have different moisture contents. The arrows at the right side of the figure show the process used for unripe, overripe, and dry cherries, which are sent directly to the drying beds. 

Where quality is not a consideration and coffees have been strip picked, they are usually sent straight to the drying beds/patios. Because strip picking leads to a big variation in ripeness between cherries, some cherries that have dried on the tree will be mixed in with some cherries that could take weeks to dry. So, even with commodity grade coffees, winnowing and flotation can improve efficiency at a drying mill — and save money, too.  

When floaters and sinkers are separated prior to natural processing, the final product will be more likely to obtain a uniform moisture content, which is important when it comes time to roast. Moreover, if the sinkers contain a high proportion of unripe cherries, then the use of mechanical dryers (see Chapter 6) needs to be carefully controlled. Producers may justify adding a separation phase into their dry-processing workflow for yet another reason: it’s possible to avoid the formation of black beans amongst the unripes by ensuring that drying temperatures do not exceed 30° C (86° F), thus some millers may decide to slow down drying times for immature lots (J.N. Wintgens, 2004, pg. 622).

When mechanical dryers are used (particularly in locations that have too much humidity to complete the natural process in time), then there can be considerable fuel savings in not drying coffees of different moisture contents simultaneously. Mechanical separators and flotation tanks allow millers to dry floaters and sinkers separately; this may reduce fuel costs. It may also prevent the overdrying of beans that are already dry, in which case they can become brittle and break during hulling (S. Leighton, 2017). 

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