The sweetness in coffee comes from saccharides, better known as carbohydrates. Saccharides relevant to coffee are mono and disaccharides. The monosaccharides are composed of glucose and fructose, with the disaccharides made up of sucrose. The roasting process brings about a significant degradation of sucrose content in coffee beans with scientific literature reporting numbers ranging from up to 97-99% degradation of sugars. (Refer to page 184.108.40.206) One suspicion we have of existing scientific literature is that a lot of research on this subject is carried out using low scoring coffees which could suggest some of these figures are understated.
James Hoffmann walks you through the processes which contribute to a reduction in sucrose content in his very succinct post on Maillard Reaction, Strecker Degradation and caramelisation.
One major part of the extracted mass, formed as a result of the degradation of sugars is so complex as to befuddle scientists. This is the melanoidins. This brown coloured high molecular weight group of compounds are formed as the result of the Maillard reaction. In a coffee brew, melanoidins were estimated to account for up to around 29% (w/w) of the dry matter (Adams et al.;2005). Roaster and Author Rob Hoos speculates that they contribute to the body of a cup more than it’s sweetness, but we know for sure they add to the brown colour of the drink.
Inasmuch as a lot of sugar is lost in the roasting process, there is little doubt that great coffees come from perfectly ripe coffee cherry which has higher sugar levels than unripe or partially ripe coffee. The growing environment for coffee plants plays a large role in optimising sugar levels, with most coffees benefiting from a daytime temperature range between 17–23°C. Agronomist Leonardo Henao discusses here in an interview with Tom Owen (from 4min to 6min) that daytime temperature is the key factor in optimising coffee metabolism and the storage of sugars in the seed.