Coffee acidity in different roast level

Can you taste the acidity in coffee? Are they ‘‘good sourness’’ or the ‘‘bad sourness’’ for you? 

In coffee roasting, acidity (or sourness) is a dynamic chemical journey. Unlike sweetness, which peaks and then disappears, acidity is a complex interplay of acids that are present in the raw bean and others that are "born" during the roast.

The acidity you taste in coffee is a race between decomposition and transformation: light roasts are high acidity because they preserve the plant's natural organic acids. Dark roasts lose their acidity because those acids are destroyed by heat (T > 220 ℃), and it leaves behind only the heavier, more bitter byproducts of acid degradation.

The chemistry of acidity: key compounds

To understand the roasts, we first need to identify the "culprits" behind the acidity:

• Organic Acids (Citric and Malic acid): inherited from the fruit. Citric acid gives a "lemony" brightness; Malic acid gives a "green apple" tartness.
• Chlorogenic Acids (CGA): the most abundant acids in green coffee. They contribute to perceived acidity but also astringency.
• Acetic Acid: generated during roasting from the breakdown of sucrose. It provides a "vinegary" or "winey" edge.
• Quinic Acid: generated when Chlorogenic Acids break down. It is a major contributor to the sharp, bitter-sour finish in darker or "old" coffee.
  
  

Roast levels: A chemical timeline

1. Light roast: the peak of ''brightness"

In this stage, the bean has reached the first crack (196 ℃ to 205 ℃).

1.1 Retention: most of the Citric and Malic acids are still intact. This is why light roasts taste like citrus or stone fruits.

1.2 Synthesis: sucrose begins to break down into Acetic acid. In light roasts, this often presents as a pleasant "sparkling" acidity or "viney" complexity.

1.3 Chlorogenic Acids status: only about 45-50 % of Chlorogenic acids have degraded. The acidity is "vibrant" because it hasn't been overwhelmed by the bitter compounds formed later.

2. Medium roast: the "balanced point"

As the roast progresses (210 ℃ to 220 ℃), the chemical profile shifts toward balance.

2.1 Degradation: heat begins to destroy the organic acids. Citric acid levels drop to roughly 50 % of their original concentration. The sharp "bite" of the acidity starts to soften.

2.2 Acetic peak: Acetic acid usually reaches its maximum concentration here before it begins to evaporate (it is highly volatile).

2.3 Formation of Quinic acid: as Chlorogenic Acids continue to break down, Quinic acid levels rise. This adds a heavier, more structured "body" to the acidity, which makes it feel less like juice and more like ''coffee''.

3. Dark roast: the decay of acidity

By the second crack (225 ℃ and above), the chemistry changes drastically.

3.1 Acid destruction: almost all Citric and Malic acids are thermally degraded. The fruity sourness is effectively gone.

3.2 Volatilisation: the Acetic acid (vinegar-like) has largely evaporated due to the high temperatures.

3.3 Quinic acid dominance: while other acids vanish, Quinic acid remains stable or even increases. However, it is now masked by Melanoidins (brown pigments) and carbonisation. Any remaining "sourness" here is usually perceived as a harsh, "chemical" or "metallic" sensation rather than a bright one.

Table: ‘‘the acidity’’ in light, medium and dark roasted coffee beans