How much sugar is in coffee with natural processing? During natural processing, does sugar move from the outer layer into the beans? 

Naturally processed coffee is usually sweeter and fuller than wet (washed) processing and often has a distinctive fruity flavor. It seems intuitive that the characteristic sweetness, body, and flavor of natural coffee comes from prolonged exposure to the sticky pulp and honeydew of the coffee fruit – but is there evidence for this? ? And if not, what happens to the sweetness?

 Naturally processed coffee contains more sugar than usual?

The main form of sugar in coffee beans is sucrose, which makes up 5-9% of the bean and more than 90% of the total sugar. The amount of sucrose found did not change with the processing method, and instead depended on how the coffee was grown. However, naturally processed green beans actually contain more fructose and glucose than wet-processed (washed), so they contain slightly more sugar (S Knopp et al, 2006).

But if the sugar from the rind can penetrate the rind into the dried beans, why didn’t this happen while the cherries were still on the tree? And since the sugar itself is mostly destroyed (caramelized) during roasting, where does the sweetness come from?

The first clue to what happened is that the fructose and glucose content of pre-processed green coffee beans is also higher than in wet-processed coffee. This implies that the sugar content is not increased during natural coffee processing, but that the amount of sugar actually decreases in wet-processed coffee.

Why is wet-processed coffee low in sugar?

Because during wet processing, green beans spend time in water (as part of the process), early studies suggested that sugar was dissolved in water (Wootton, 1973). However, at that time, similar changes were found in coffee that was mechanically processed and dried on a rig, so the water was not necessarily the culprit.

Instead, what the researchers found was that the beans were starting to sprout. This sprouting process consumes the stored sugars in the beans and uses up the simplest sugars (glucose and fructose) first. As well as affecting sucrose concentrations, sprouting in wet-processed coffee increases the concentration of certain amino acids in the bean.

Amino acids are among the first, the most important aromatic compounds in green coffee, and thus contribute to the complexity of aromas found in wet-processed coffee (S Knopp et al. al, 2006).

 

Why only wet-processed coffee sprouts?

 Basically, coffee beans are always ready to germinate. Unlike many other beans, they do not have a dormant period, for example when they are dried. This is one of the reasons it can be difficult to store coffee beans in a bean bank; instead coffee varieties must be kept in live collections.

While the beans are still in the fruit, something in the rind, plant hormones, or just the presence of water, prevents the seeds from germinating. However, as soon as the flesh is removed during fermentation, the metabolism of the bean begins to change as it prepares for germination. In a naturally processed coffee, the rind inhibits germination until the cherries are dried, at which point the bean’s metabolism is completely shut down. This preserves the sugar content in the beans.

What gives naturally processed coffee a sweet and fruity taste?

Although it is now clear that very little of any sweetener enters the bean during processing, there are several compounds that appear to be absorbed into the bean. These are not aromas from the fruit itself, but volatile compounds, especially esters, that are produced by the bacteria as the fruit begins to ferment (GV de Melo Pereira et al, 2014). These compounds (unlike most volatile compounds found in pre-processed green coffee) persist during roasting and can contribute floral and fruity aromas to roasted coffee.

Some of these esters, such as ethyl butyrate(C6H12O2) also contribute to the perception of sweetness (D. Labbe et al, 2006). Although sugars are mainly destroyed during roasting, they are also important precursors to many aroma molecules produced during caramelization, such as furan(C4H4O), which also makes naturally processed coffee appear sweeter.

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