Understand the ‘‘Sweetness’’ in coffee

Can you taste the ‘‘sweetness’’ from the coffee? Are they the same ‘‘sweetness’’ in light, medium and dark roast?

No, the "sweetness" in light, medium, and dark roasts is not the same.

From a chemical perspective, coffee sweetness is a complex paradox. Green coffee beans are rich in sucrose, but roasting destroys most of it (due to the high roasting temperature).

The "sweetness" we perceive is a combination of residual mono/disaccharides, olfactory-gustatory integration (sweet-smelling volatiles tricking the brain), and macromolecular mouthfeel.

The roasting level: Light vs Medium vs Dark

The nature of the sweetness changes because the chemical profile shifts drastically along the reaction coordinate.

1. The chemistry during the light roast: (often just dropped right after "first crack”)

Source of sweetness:

1.1 Residual sucrose: light roasts retain the highest percentage of actual, unreacted sucrose (though still significantly less than green beans).

1.2 Fruit esters: high concentrations of Esters (such as Ethyl butyrate) and Aldehydes. These provide fruity, floral aromatics.

1.3 Acidity sources: light roasts have high organic acids (like Citric and Malic acid). The sweetness here is often perceived as a "sweet-tart" balance, similar to a ripe orange.

Sensory profile: Clean, fruit-forward sweetness. It is sharp and swift.

2. The Chemistry during the Medium roast: often treated as the time just finished first crack and before second crack. Where the Maillard reaction is maximised (the "sweet spot")

Source of sweetness:  

2.1 Cyclotenes & Maltol: these are potent flavour compounds resulting from caramelisation that taste/smell like cotton candy and burnt sugar.

2.2 Furanones: such as 4-hydroxy-2,5-dimethyl-3(2H)-furanone (Furaneol), which has a "fruity" to "caramel" threshold.

2.3 Balance: the sharp acids have degraded, and the bitter carbon compounds haven't formed yet.

Sensory profile: this is the "classic" coffee sweetness. Chocolate, caramel, nougat, and honey notes. It is rounder and heavier than light roast sweetness.

3. Dark roast: the "Viscous" Illusion. In this stage (approaching or past "second crack"), pyrolysis is dominant. Almost 99% of the original sucrose is destroyed.

Source of sweetness: If you taste sweetness here, it is chemically fascinating because it shouldn't exist. :p

3.1 Macromolecular depolymerisation: Polysaccharides (like Galactomannans) break down into shorter chains, increasing the viscosity of the liquid. A thicker body/texture is often psychologically associated with sweetness/richness.

3.2 Lipid migration: oils migrate to the surface.

3.3 Olfactory association: in this stage, you might smell "vanilla" or "dark chocolate" notes (from phenols and dry distillation byproducts), which triggers a sweet expectation, even if the taste is bitter.

Sensory profile: bittersweet. Any sweetness is fleeting and often masked by pyrroles (bitter/smoky compounds).

4. The ''smell'' and "taste"of sweetness: volatiles vs. non-volatiles

As a chemist, it is crucial to distinguish between Gustatory (taste) and Olfactory (smell).

4.1. The taste (gustatory): mostly comes from non-volatile residual sugars. This decreases linearly as roasting gets darker.

4.2. The aroma smell (olfactory): comes from volatile thiols, esters, and furans.

Ethyl 2-methylbutyrate: contributes fruity/sweet.

Vanillin: provides sweet/creamy.

Sotolon: maple syrup, caramel (high impact in roasted coffee especially dark roast).

Table: the ‘‘sweetness’’ in light, medium and dark roast beans