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Dry Processing

PC 5.04 Sugars in Natural Processing

Do Sugars Travel from the Mucilage into the Bean? Natural-processed coffees are typically sweeter and more full bodied than washed coffees, and they often have distinctive fruity flavours. It seems intuitive that the sweetness, body, and characteristic flavours of a natural coffee should come from prolonged contact with the sugary, sticky flesh of the coffee fruit  — but is there evidence for this? And if not, what’s happening?


Do Natural Coffees Contain More Sugar?

The main form of sugar in coffee beans is sucrose, making up anywhere from 5–9% of the seed and more than 90% of the total sugar content. The amount of sucrose found doesn’t vary with the processing method; instead, it is dependent on how the coffee was grown. However, naturally processed green beans do indeed contain more fructose and glucose than fully washed, so they contain slightly more sugar overall, while pulped naturals lie somewhere in between (S. Knopp et al., 2006).

But if sugars from the pulp can cross the parchment into the seeds of drying fruit, why hasn’t this already happened on the tree? And, since sugars themselves are mostly destroyed during roasting, where does the added sweetness come from? The first clue to what’s happening is that the fructose and glucose contents of the beans before processing are also higher than in the washed coffee. This does not imply that sugar levels increase during processing of natural coffee — rather, it suggests that sugar levels actually decrease in processing of washed coffees.



Why Do Washed Coffees Have Less Sugar?

As part of the washing process, beans spend some time submerged in water, and early studies suggested that sugars were being dissolved by the water (Wootton, 1973). Since then, however, the same changes have been found to take place in mechanically washed coffee, as well as on the drying bed, so the water can’t be the culprit (M. Kleinwächter and D. Selmar, 2010).