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The Wet Mill

PC 2.01 – Cleaning and Separating

A range of processes are used to remove unwanted objects from agricultural products such as coffee, but the oldest method is almost certainly winnowing. This method of separating ‘the grain from the chaff’ dates back to the ancient world. There is a reference to winnowing in Homer’s ‘The Odyssey’, which dates back to around the eighth century BCE. Winnowing can be as simple as hurling baskets full of cherry up into the air and allowing the breeze to blow light objects out of the way as the cherry fall back into the basket. 

Winnowing machines are often built into mechanical harvesters. These harvesters substitute vacuums for the breeze. The chocolate industry also makes use of winnowers to remove the shells (cascara) that surround the cocoa beans. One unusual difference between cocoa and coffee processing is that cocoa is usually roasted with the cascara still surrounding the cocoa beans. 

Modern coffee harvesters usually include a conveyor belt that moves the coffee towards the vacuum. Before the harvested material gets too close to the mouth of the vacuum, the conveyor belt comes to an end, so that the material falls through the air in front of the vacuum. So long as the vacuum power and position are correctly calibrated, coffee cherry will not be drawn into the vacuum