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 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, and instead is dependent on how the coffee was grown. However, natural processed green beans do indeed contain more fructose and glucose than fully washed, so they do 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 content of the beans before processing is also higher than in the washed coffee. This implies that it’s not sugar levels increasing during processing of natural coffee, but rather that the sugar levels actually decrease in washed coffees.

Why Do Washed Coffees Have Less Sugar?

As part of the washing process, beans spend some time submerged in water, so 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, and on the drying bed, so the water can’t be the culprit (M Kleinwächter & D Selmar, 2010).

Instead what researchers found is that the coffee seeds are starting to germinate. This process consumes the sugars stored in the seed, and uses up the simplest sugars (glucose and fructose) first. As well as affecting the sucrose levels, germination in washed coffees increases the concentration of certain amino acids in the seed. Amino acids are some of the most important aroma precursors in green coffee, and so this contributes to the complexity of aromas found in washed coffees (S Knopp et al, 2006).

Why Do Only Washed Coffee Seeds Germinate?

Coffee seeds are essentially always ready to germinate. Unlike many other seeds, they don’t have a dormant phase, for example when they are dried. This is one of the reasons it’s hard to store coffee seeds in a seed bank; instead coffee varieties must be kept in living collections.

While the seeds are still in the fruit, something in the pulp, whether a plant hormone or just the presence of water, prevents the seed from germinating. As soon as the pulp is removed during fermentation, however, the seed’s metabolism begins to change as it prepares for germination. With naturals, the pulp inhibits germination until the cherry is dried, at which point the seed’s metabolism shuts down entirely. This preserves the sugar content in the seed (D Selmar et al, 2014).

What Gives Natural Coffees Their Sweetness and Fruity Aromas?

While it’s now clear that little if any sugar passes into the seed during processing, there are some compounds that do seem to be absorbed into the seed. These are not the aromas from the fruit itself, but the volatile compounds, particularly esters, that are created by microbes as the fruit starts to ferment (GV de Melo Pereira et al, 2014). These compounds  — unlike the majority of volatile compounds found in green coffee before processing  — survive roasting, and can contribute floral and fruity aromas to the roasted coffee.

Some of these esters, such as ethyl butyrate, also contribute to the perception of sweetness (D. Labbe et al, 2006). While the sugars themselves are mostly destroyed during roasting, they are also important precursors to many aroma molecules created during caramelisation, such as furans, that also make natural processed coffee seem sweeter.

References

M Kleinwächter & D Selmar, 2010. Influence of drying on the content of sugars in wet processed green Arabica coffees. Food Chemistry, 119(2), 500–504. doi:10.1016/j.foodchem.2009.06.048 

S Knopp, G Bytof, D Selmar, 2006. Influence of processing on the content of sugars in green Arabica coffee beans. European Food Research and Technology, 223(2), 195–201. doi:10.1007/s00217-005-0172-1 

D Labbe, A Rytz, C Morgenegg, S Ali, N Martin, 2007. Subthreshold olfactory stimulation can enhance sweetness. Chemical Senses, 32, 205–214. doi: 10.1093/chemse/bjl040

GV de Melo Pereira, DP de Carvalho Neto, AI Magalhães Júnior, ZS Vásquez, ABP Medeiros, LPS Vandenberghe, & CR Soccol, 2018. Exploring the impacts of postharvest processing on the aroma formation of coffee beans – A review. Food Chemistry. doi:10.1016/j.foodchem.2018.08.061

D Selmar, M Kleinwächter, G Bytof, 2014. Metabolic responses of coffee beans during processing and their impact on coffee flavor. In: RF Schwan & GH Fleet (Eds.), Cocoa and Coffee Fermentations (pp431-476) ISBN 9781439847916

AE Wootton, 1973. The dry matter loss from parchment coffee during field

processing. Cinquième Colloque International sur la Chimie des Cafés Verts,

Torréfiés et leurs Dérivés, Lisbonne, Portugal, pp. 316–324