How is Chlorogenic Acid Decomposed During the Roasting Process? The Relationship Between Moisture and Chlorogenic Acid? What is Produced?

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FrontStreet Coffee: Brief Overview of Roasting Process and Chlorogenic Acid Decomposition

When moisture is heated, chlorogenic acid begins to decompose. However, during the roasting process, moisture continuously dissipates. The speed and extent of moisture loss both affect flavor development (Maillard reaction), but here I will focus specifically on chlorogenic acid.

The Relationship Between Moisture and Chlorogenic Acid

When there is sufficient moisture (high bean water content, or slow dehydration process before the first crack), chlorogenic acid can fully decompose into caffeic acid and quinic acid. This process is what Mr. Akita Kouchi refers to as the "hydrolysis" reaction on page 94 of the "Complete Guide to Specialty Coffee." In terms of flavor, this results in sour-bitter (quinic acid) and astringent (caffeic acid) notes.

After the first crack, the coffee beans' moisture is essentially depleted (referring to the free-flowing water within the raw beans). At this point, if heating continues, molecular dehydration within the chemical structure is forced. Quinic acid then undergoes a dehydration reaction to become quinic acid lactone. This substance loses its acidic characteristics because the hydroxyl group of its original acid-functional group (carboxyl group) combines with another hydrogen to form water. In terms of mouthfeel, it tends toward bitterness.

Meanwhile, caffeic acid undergoes decarboxylation (removal of CO2) to become vinyl catechol. The vinyl structure of this substance is most concerning, as continuous heating can lead to polymerization, forming what Taguchi described as "bad bitterness" - vinyl catechol polymers. Looking at its phenolic structure and polymeric nature, this substance is not only bitter but also astringent, affecting the root of the tongue (bitterness) and the throat (choking sensation).

When moisture is insufficient (low bean water content, or fast dehydration process before the first crack), the moisture within the raw beans is inadequate (free water is depleted prematurely). In this case, chlorogenic acid cannot fully decompose into caffeic acid and quinic acid. Instead, chlorogenic acid undergoes a "dehydration reaction," where the quinic acid portion of the chlorogenic acid molecule dehydrates to form chlorogenic acid lactone.

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