How is it that coffee still tastes sweet, even though in scientific literature we’re told all — or almost all — the sugars have been caramelised. Is the literature wrong?
Green coffee contains as much as 9% sucrose (A. Farah) as well as numerous polysaccharides (multiple sugars joined in long chains). However, only the simple sugars and the very shortest chain polysaccharides have a sweet taste.
During roasting, two chemical reactions use up the sugars in green coffee: caramelisation and Maillard reactions. Maillard reactions happen at 140-165°C, and sucrose caramelises at 160°C. Given that the final temperature of coffee beans in the roasting process typically reaches 200°C or higher, you would expect very little sugar to remain at the end of the process, and indeed more than 99% of the sugars in green coffee are degraded in roasting (Coffee Flavour Chemistry by von Flament, 2002)
Even at high temperatures, it takes time for caramelisation to occur. As a result, roasted coffee still contains detectable amounts of simple sugars, including fructose, arabinose, and glucose (U. Kroeplien, 1974). However, the amount of sugar remaining is well below the taste threshold (Food Chemistry by H.-D. Belitz, W. Grosch, P. Schieberle).
If there’s virtually no sugar remaining, what brings the sensation of sweetness to a good brew? The answer lies with the aromas, and the way these influence our sense of taste.
Strictly speaking, sweetness refers only to a taste on the tongue and should be evaluated separately from odours (by blocking the nasal passages). But many compounds found in coffee have an aroma that is commonly described as ‘sweet’ – from sweet-fruity smelling esters and aldehydes, to the sweet-caramel notes of furans.
Some aromas also enhance our perception of sweetness, where sugars actually are present. For example, the aromas of furaneol and ethyl-methylbutyrate, both found in roasted coffee, can make fruit juices taste sweeter (C. Barba, N Béno, E. Guichard, T. Danguin, 2018). A similar compound, ethyl butyrate, enhances the perception of sweetness even at concentrations so low you can’t smell it directly. (D. Labbe, A. Rytz C. Morgenegg S. Ali, N. Martin, 2006)
It’s certain aromas in coffee that can give the impression of sweetness, as well as make us more likely to taste whatever actual sweetness may remain in the brew.