1. What is chlorogenic acid?

Chlorogenic acid (CGA, an abbreviation for chlorogenic acids) is the general term for a group of phenolic compounds naturally present in coffee. Structurally, many compounds in this group are formed from quinic acid linked to hydroxycinnamic acids, most commonly caffeic acid.

In green coffee beans, caffeoylquinic acids (CQA) account for a significant proportion of total CGA. A frequently studied component is 5-caffeoylquinic acid (5-CQA), alongside the 3-CQA and 4-CQA isomers. Coffee beans also contain related groups such as dicaffeoylquinic acids and feruloylquinic acids.

The initial CGA content depends on the coffee species and variety, origin, growing conditions, ripeness, processing, and storage. Therefore, two different Arabica samples or two different Robusta samples may not begin the roasting process with the same CGA content.

2. What does First Crack actually mark?

First Crack is the stage when coffee beans emit cracking sounds as their internal structure changes and pressure from accumulated water vapor and gases increases. This process is typically accompanied by bean expansion, changes in porosity, and gas release.

First Crack has practical value for roasters as it provides an auditory and operational signal to monitor the progress of a roast batch. However, it is not a chemical switch that causes all decomposition reactions to begin only at that moment.

Does First Crack occur at 196–207°C?

This temperature range may appear in some roasting profiles, but it should not be considered a fixed threshold. The temperature recorded by the machine depends on probe position, machine design, heat transfer rate, batch size, and measurement method. The displayed temperature is also not necessarily equal to the uniform temperature throughout the entire bean.

Therefore, when documenting an experiment, it is necessary to clearly distinguish between probe temperature, the start time of First Crack, development time after First Crack, and the final roast level. One cannot rely solely on a single temperature figure to accurately determine the extent of CGA decomposition.

3. Transformation mechanism of chlorogenic acid during roasting

During roasting, CGA does not merely disappear from the bean but also participates in various chemical reactions. Studied pathways include isomerization, acyl migration, lactone formation, dehydration, and secondary reactions with other bean components.

3.1. Isomerization and acyl migration

Isomerization changes the structural arrangement within a molecule without necessarily altering its molecular formula. Acyl migration changes the position of the linking group within the CGA structure.

These transformations are important because different isomers may have different thermal stabilities and transformation tendencies. Therefore, measuring a single form of CGA is insufficient to describe the overall behavior of this compound group during roasting.

3.2. Formation of chlorogenic acid lactones

An important metabolic pathway is the formation of chlorogenic acid lactones (CGL). These are products with a lactone ring structure, produced from chlorogenic acid precursors through intramolecular reactions under roasting conditions.

A study by Farah and colleagues published in 2005 identified numerous chlorogenic acid lactones in Arabica and Robusta using high-performance liquid chromatography (HPLC) and synthetic standards. Under the conditions investigated, some lactones reached their highest levels at light to medium roast levels, rather than continuing to increase steadily with darker roasting.

These results indicate the need to distinguish between two quantities: the remaining original CGA content and the amount of lactone products formed. A decrease in CGA does not mean that all metabolic products increase continuously at the same rate.

3.3. Formation of quinic acid, caffeic acid, and secondary products

Quinic acid and caffeic acid are related to the chemistry of chlorogenic acids. However, one should not describe that every CGA molecule is completely cleaved into these two acids at First Crack. The final products depend on the structure of the initial compound and the reaction conditions.

As roasting continues, intermediate compounds may undergo further decomposition, oxidation, or interactions with other components. These reactions occur simultaneously with changes in sugars, amino acids, and polysaccharides within the bean.

3.4. Interaction with other bean components

Coffee beans constitute a complex chemical system. During roasting, products derived from CGA may participate in or interact with reactions involving carbohydrates and proteins. Studies using mass spectrometry methods have recorded many groups of complex-structured products formed during roasting.

This is why the formation of brown color, roast flavor, or changes in antioxidant activity should not be attributed solely to CGA decomposition reactions.

4. How much does CGA content decrease at First Crack?

CGA content generally decreases as roast level increases. However, the rate of decline at the end of First Crack is not a constant applicable to all coffee varieties, roasters, and thermal profiles.

Some studies on the transformation of bioactive compounds during coffee processing have noted a significant reduction in chlorogenic acids during roasting. The specific reduction varies among samples and experimental conditions. For instance, a review of bioactive compound changes across the coffee processing chain synthesizes results showing that the decline at light roast levels can differ significantly among coffee samples; dark roasting typically reduces CGA more substantially.

Bean mass can distort the interpretation of decline rates

During roasting, beans lose water and volatile substances, resulting in weight loss. Therefore, the content per gram of roasted beans may differ from the total amount of compounds remaining in the entire batch.

To accurately compare before and after roasting, the calculation basis must be defined, such as dry mass, sample mass, or the total amount of compounds in the initial bean quantity. Ignoring mass changes can lead to misinterpretation of the extent of CGA decline.

5. How do CGA and its metabolites affect flavor?

5.1. Bitterness and chlorogenic acid lactones

Chlorogenic acid lactones are compounds that play a significant role in the bitterness of roasted coffee. Chemical studies show that the content of certain lactones peaks at light to medium roast levels under the conditions investigated.

However, coffee bitterness does not come from lactones alone. Caffeine, other CGA metabolites, and compounds formed during roasting can all contribute to the overall perception. Therefore, remaining CGA content alone cannot be used to predict the bitterness of a cup of coffee.

5.2. Sourness and organic acids

Perceived sourness depends on the type of acid, concentration, buffering capacity of the beverage, extraction level, and interactions among taste compounds. Quinic acid is a component related to CGA chemistry, but it cannot be concluded that quinic acid always produces pleasant sourness or a refreshing aftertaste.

At the same roast level, two bean types can differ significantly in sourness due to variety, origin, processing, and initial acid composition. Brewing recipes and extraction ratios also influence the perceived sourness in the cup.

5.3. Aroma and color

The aroma of roasted coffee is formed through multiple reaction groups, including the Maillard reaction, thermal degradation, and reactions involving sugars, amino acids, and other precursors. The brown color of the beans also results from various chemical changes, not solely from the degradation of CGA.

Therefore, extending the roasting time after First Crack can simultaneously alter flavor development, bitterness, acidity, and sensory structure. The specific direction of these changes must be confirmed through cupping, rather than being inferred from a single compound.

6. Factors influencing the level of CGA degradation

6.1. Coffee species and varieties

Robusta typically has a higher initial CGA content than Arabica, but the difference depends on the variety, growing region, and cultivation conditions. This means that while Robusta may start with higher CGA levels, it does not automatically guarantee that it will retain more CGA after any given roast profile.

6.2. Bean moisture, size, and density

The physical properties of the beans affect how heat transfers from the roasting environment to the interior. Initial moisture, size, density, and bean structure can all alter the thermal history of the coffee bean.

Processing methods can also influence the initial chemical composition and physical properties. However, to determine the specific impact of processing on the rate of CGA degradation, samples must be compared under controlled roasting conditions.

6.3. RoR and development time

RoR (Rate of Rise) is the rate of temperature increase derived from the roaster's probe data. It is a useful operational metric for monitoring thermal progression, but it is not a direct measurement of the CGA degradation rate.

Two batches with the same end temperature but different RoR, development times, and heat distribution profiles can result in different chemical compositions. Nevertheless, one cannot assert that a batch with a lower RoR always loses more CGA than a batch with a higher RoR without comparative experimental data.

6.4. Final roast degree

Roast degree is a significant factor related to the remaining CGA content. Generally, CGA decreases as roasting progresses from light to dark. However, roast degree should be evaluated using multiple parameters such as bean color, weight loss, thermal profile, and measurement method, rather than relying solely on probe temperature.

7. Methods for verifying CGA in coffee roasting research

If the goal is to determine the level of CGA degradation at First Crack, experiments must be designed rather than drawing direct inferences from the thermal curve.

1. Standardize raw materials: use beans from the same lot, clearly specifying the variety, origin, processing method, moisture content, and other key characteristics.
2. Sample at specific stages: collect samples of green beans, pre-First Crack beans, samples at defined points around First Crack, and samples at the final roast degree.
3. Record thermal profiles: log probe temperature, timing of First Crack, RoR, development time, and weight loss.
4. Quantify compounds: use appropriate methods such as HPLC to identify individual CGAs and, where possible, lactone products.
5. Standardize the basis for comparison: clearly state whether results are calculated based on dry mass, sample mass, or total compound amount to avoid confusion caused by water loss.
6. Conduct separate sensory evaluation: cup samples under consistent conditions and do not treat sensory results as a direct measurement of CGA content.

HPLC analysis helps quantify target compounds, whereas total phenolic tests generally reflect the response of multiple substances in the sample. These two types of results cannot directly substitute for each other.

8. Key conclusions for roasters

• First Crack is an important operational milestone, not the sole boundary of chemical reactions.
• There is no universal First Crack temperature applicable to all roasters and all bean types.
• CGA decreases as roasting progresses, but the extent of reduction at a specific point must be verified with appropriate data.
• Chlorogenic acid lactones are important degradation products that contribute to bitterness and exhibit distinct concentration trends depending on roast level.
• Acidity, bitterness, and aroma result from the combined effect of multiple compounds and cannot be predicted solely by CGA levels.
• Rate of Rise (RoR), development time, and end temperature help describe the roast profile but do not directly quantify CGA degradation.

9. Conclusion

Chlorogenic acids undergo various chemical transformations during coffee roasting, including isomerization, acyl migration, lactone formation, and other secondary reactions. These changes contribute to alterations in the phenolic composition and sensory characteristics of the beans.

First Crack helps roasters monitor the progress of a roast batch, but neither this milestone nor the displayed temperature alone can accurately determine the amount of CGA degraded. Degradation rates must be correlated with specific studies, quantification methods, and experimental conditions.

For roasting practice, the most reliable approach is to combine thermal profile data, weight loss, sensory evaluation, and chemical analysis when necessary. This helps establish an evidence-based link between roast level, degradation products, and cup flavor.

Scientific references

1. Farah, A. và cộng sự (2005). Effect of Roasting on the Formation of Chlorogenic Acid Lactones in Coffee. Journal of Agricultural and Food Chemistry, 53(5), 1505–1513. https://doi.org/10.1021/jf048701t

   Study on the formation of chlorogenic acid lactones in Arabica and Robusta at different roast levels.

2. Understanding the fate of chlorogenic acids in coffee roasting using mass spectrometry based targeted and non-targeted analytical strategies (2012). Food & Function. https://doi.org/10.1039/c2fo10260a

   Study using mass spectrometry to investigate the metabolic pathways of chlorogenic acids during roasting.

3. From Plantation to Cup: Changes in Bioactive Compounds during Coffee Processing (2021). Đọc toàn văn trên PubMed Central

   Overview of changes in bioactive compounds during coffee processing stages, including the impact of roast level on chlorogenic acids.

4. Role of Chlorogenic Acids in Controlling Oxidative and Inflammatory Stress Conditions (2016). Đọc toàn văn trên PubMed Central

   Overview of the structure, chlorogenic acid groups, and studied biological activities.

5. Thermal Contaminants in Coffee Induced by Roasting: A Review (2023). Đọc toàn văn trên PubMed Central

   Overview of thermal reactions and chemical changes occurring during coffee roasting.