The stages of coffee roasting: How do chemical changes affect flavor?

A batch of light-roasted coffee can have floral, fruity, and acidic notes. As the roasting continues, caramel, nutty, and chocolate flavors emerge. However, if the coffee is roasted too dark, smoke, ash, and bitter notes can overpower the previous characteristics.

These changes are not simply a color transition from green to brown. Inside the bean, dissolved sugars and chlorogenic acid decrease, while a series of volatile compounds are formed, transformed, or broken down.

A study published on October 8, 2026, in the Foods journal tracked Arabica from Vân Nam from green to eight consecutive roasting stages. Instead of only comparing a light-roasted sample with a dark-roasted sample, the research team combined chemical analysis, gas chromatography-mass spectrometry, and sensory evaluations to track changes throughout the process.

1. Coffee roasting is a continuous chemical transformation process

Green coffee beans contain carbohydrates, proteins, lipids, organic acids, caffeine, phenolic compounds, and many precursors of flavor.

When heated, these components do not change simultaneously. Some reactions begin early, while flavor-creating reactions develop more strongly as temperature and time accumulate.

Important groups of transformations include:

• Maillard Reaction: a reaction between the carbonyl group of reducing sugars and the amino group of amino acids or proteins, forming flavor precursors and brown products.
• Sugar breakdown: sugars are consumed in thermal reactions, forming many intermediate compounds.
• Chlorogenic acid breakdown: changing the composition of phenolic acids and forming metabolites.
• Formation of volatile compounds: creating floral, fruity, caramel, nutty, smoky, and other characteristics.
• Bean structure transformation: increased gas pressure, bean expansion, porosity changes, and extraction capacity changes.

Because reactions occur simultaneously and depend on heat transfer rates, it is not possible to describe a roasting profile fully using only the final temperature.

2. The 2026 study tracked eight roasting stages how?

The research team used Arabica grown in Lujiang Town, Baoshan City, Yunnan Province, China.

Green beans were divided into separate parts and roasted at eight increasing levels, denoted S1 to S8. Unroasted green beans were denoted S0.

Each roasting level was performed three times independently. After roasting, the beans were quickly cooled to room temperature and stored in sealed bags before analysis.

The research team combined three evaluation approaches:

• HPLC to analyze some non-volatile compounds.
• HS-SPME-GC-MS to analyze volatile compounds in the headspace above the sample.
• Sensory evaluation to track changes in aroma, flavor, texture, balance, and overall quality.

3. Soluble sugars and chlorogenic acids decrease progressively during roasting

One consistent finding of the research is that the total content of soluble sugars and chlorogenic acids decreases continuously as the roast level increases.

Soluble sugars

Sugars are an important group of precursors in flavor formation. As temperature rises, a portion of the sugars participates in Maillard reactions and thermal decomposition.

This process generates intermediate compounds, contributing to the formation of caramel, baked goods, roasted grain aromas, and many other flavor characteristics.

However, it should not be assumed that a reduction in sugars equates to a proportionally lower perceived sweetness. The perception of sweetness is also influenced by acidity, bitterness, aroma, body, and overall balance.

Chlorogenic acids

Chlorogenic acids (CGA) are a group of natural phenolic compounds found in green coffee. During roasting, CGA undergoes transformation through various thermal reactions.

Some metabolites may include derivatives of caffeic acid and quinic acid, along with other compounds depending on roasting conditions.

Therefore, the decline in CGA does not mean that all CGA disappears or is converted into a single product.

4. Volatile compounds do not increase linearly

Roasted coffee aroma is composed of a complex mixture of volatile compounds. Some compounds form as roasting progresses, while others continue to decompose or transform as temperature and cumulative time increase.

A 2026 study noted heterogeneous changes:

• The diversity of volatile compounds increases up to stage S5.
• The total concentration of volatile compounds peaks at S7.
• 47 compounds with an Odor Activity Value (OAV) greater than 1 were recorded in the analysis.

OAV (Odor Activity Value) is the ratio between the concentration of a compound and its odor threshold. An OAV greater than 1 generally indicates that the compound is likely to contribute significantly to odor perception under the measured conditions.

This does not mean that every compound with an OAV greater than 1 is perceived independently in a cup of coffee, as odor interactions can alter overall perception.

Five prominent compounds identified

The authors identified compounds capable of clearly distinguishing roasting stages, including:

• Phenylethyl alcohol.
• Furfuryl alcohol.
• Furfural.
• Phenol.
• Pyridine.

These are compounds associated with aroma development during roasting stages, but one should not attribute a single compound to an absolute sensory note.

5. S4 achieves the highest cupping score: Why is darker roasting not necessarily better?

Overall sensory scores in the study increased progressively from S0 to S4, then declined with further darkening of the roast.

At S4, the attributes of aroma, flavor, body, balance, and aftertaste are more harmoniously integrated. This is why the sample achieved the highest total score.

As the roast progresses to S5, certain dark chocolate and spicy characteristics remain present. However, at S6–S8, smoky, charred, and burnt bitter notes become more prominent, causing a decline in complexity and balance.

Notably, S7 exhibits the highest total concentration of volatile compounds, yet it does not achieve the highest cupping score. This indicates that a higher quantity of volatile compounds does not necessarily equate to better flavor.

6. How can roasters apply this research?

Not only by monitoring the end temperature

Roasters should record temperature, time, rate of rise (RoR), First Crack timing, development time, and post-roast bean color. These data points help identify differences between roast batches with the same end temperature.

Sensory evaluation across multiple development levels

Instead of selecting a single profile for evaluation, roasters can experiment with various roast levels within a safe range, then compare balance, aftertaste, and aromatic structure.

Combining roast color and sensory analysis

Agtron measurements track color but do not fully reflect aromatic structure. Roasters should combine color measurement with cupping and thermal profiling.

Ensuring consistency between batches

The key is not to find a single profile for all beans, but to develop profiles suited to the characteristics of each origin while maintaining consistency across production runs.

SAVINA’s professional perspective

Coffee roasting is not a race to achieve a dark brown color or maximize volatile compounds. The goal is to create a balanced aromatic structure that aligns with the raw material’s characteristics and the intended brewing method.

A 2026 study highlights a notable point: sensory quality may peak before the total volume of volatile compounds reaches its maximum. As roasting deepens, the quantity or concentration of certain compounds may continue to increase, but the cup experience can deteriorate.

For roasters, this reinforces a principle: a good profile must be evaluated through a combination of process data, chemical analysis, and sensory assessment, rather than relying on a single parameter.

References

1. Chen Y. et al. (2026). From Chemical Transformations to Cup Quality: Integrated Insights into Key Non-Volatile Compounds, Aroma-Active Compounds and Sensory Evolution During Continuous Roasting Stages in Yunnan Arabica Coffee. Foods, 15(19), 3569. Published 08/10/2026. DOI: 10.3390/foods15193569.
2. Specialty Coffee Association. Professional documentation on specialty coffee evaluation and sensory analysis.