Which is more important: physical changes or chemical reactions?

Roasting is generally divided into three stages: drying, Maillard (caramelization) reactions, and flavor development. These terms actually describe different stages of chemical and physical changes.

A) Drying

The drying process begins at the turn-around point, which occurs when green beans are placed in the roaster. The internal heat of the machine initially drops before rising again, and the point at which the temperature starts to climb is called the turn-around point. During the drying stage, the moisture in the green beans begins to evaporate, and pressure starts to build up inside the beans.

B) Maillard Reaction

When the coffee beans start to turn brown, it indicates the beginning of the Maillard reaction, which occurs at around 150°C. This process produces various gases, including carbon dioxide, water vapor, and volatile gases. When the internal pressure becomes strong enough to break through the cell walls, the beans expand—this is known as the first crack.

Flavor development also occurs simultaneously during the Maillard reaction. In addition to the change in bean color, it influences the final flavor of the coffee.

C) Flavor Development Stage

After the first crack, the roasting process shifts from an endothermic reaction to an exothermic reaction. During this stage, physical changes continue—the surface pores of the beans increase, oils move from the core to the surface, and the color continues to darken.

Physical changes involve alterations in volume and porosity.

The cell walls of coffee beans are among the strongest in the plant kingdom, with a tough outer layer that enhances their rigidity and strength. When coffee is roasted, the rising temperature and the conversion of moisture into gas increase the internal pressure of the beans. These conditions transform the structure of the cell walls from rigid to rubbery (also known as vitrification), due to the polysaccharides (bound sugar molecules) in the beans.

Internal substances are pushed toward the cell walls, leaving a gas-filled void in the center. This means that as the mass decreases, the volume of the beans expands, and most of the gas accumulation is the carbon dioxide released after roasting.

Roasting also increases the porosity of the beans, reducing their density and enhancing solubility. Of course, this also significantly contributes to making them into a delicious beverage.

Changes in Oils

Coffee beans contain oils, and during roasting, the internal high pressure causes these compounds to move from the center of the cells to the surface.

Oils help retain volatile compounds within the cells. Volatile compounds are chemicals with high volatility at room temperature, and these substances are essential for producing the aroma and fragrance of coffee. Without oils, these molecules might dissipate quickly.

The longer the roasting time, the more pronounced the structural changes, the lower the density of the beans, and the more gases are produced. The longer the roasting time, the more surface oil the beans will have.

These developments explain to some extent why dark-roasted coffee tastes different from light-roasted coffee, but there are also important chemical changes that affect the roasting results.

Different roasting methods affect the final flavor, aroma, and mouthfeel of the coffee, as they occur at different times and temperature levels.

Chemical Reactions

The Impact of Caramelization and Flavor Profiles

Temperature Levels Determine Flavor Directions

　 A. Light Roast Highlights Enzymatic Reactions

· Floral notes are rare, citrus notes are brilliant

· Onion and garlic notes are startling, while sugarcane notes are enchanting

B. Medium Roast Highlights Caramelization and Maillard Reactions

· "Creating caramel sweetness"

· "Maillard Reactions" Outshine "Caramelization"

　　 C. Dark Roast Highlights Dry Distillation

· Cellulose is the source of all aromas

· "Thiols" Are the Key Contributors to Deep Roast Richness

Chemical Reactions of Components During Roasting

Sucrose: Melting point 187.8°C → Sugar compounds → Caramelization → Water and CO2 escape, causing the first crack phenomenon... Medium bean temperature in the roaster drum at 190–196°C

Cellulose (Lignin): Component of coffee cell walls, breaks down at 230°C, damaging cell walls → Second crack phenomenon... Medium bean temperature in the roaster drum at 211–216°C

Trigonelline: Pure crystal begins to degrade at 217.9°C, starts degrading at 192.2°C, and at 229.4°C → 85% degraded. The degree of Trigonelline degradation is a key indicator for determining the optimal roasting reaction ratio.

Therefore, the Trigonelline loss rate (degradation rate) after roasting is 50%–80%, decomposing into various compounds, including non-volatile nicotinic acid and 29 volatile substances, 9 of which contain coffee aromatic compounds.

Nicotinic Acid: Nicotinic acid in green coffee beans is often found within cellulose. During roasting, nicotinic acid derivatives form soluble substances. These derivatives contribute desirable acidity and a clean finish to the coffee, making the nicotinic acid derivative rate another key indicator for determining the optimal roasting reaction ratio...

Environmental Temperature (ET): Specific chemical reactions in roasted coffee occur within a certain temperature range, producing favorable flavor reactions. This temperature range is the environmental temperature.

Therefore, the low-molecular-weight, highly volatile floral and fruity acidity produced by enzymatic action in light roasts.

Floral notes are the rarest flavors in specialty coffee, primarily consisting of coffee flower and jasmine aromas. Due to coffee variety or terroir, enzymes produce high concentrations of floral aldehyde compounds during metabolism, resulting in captivating floral scents.

The fruitiness of coffee is also prominent, mainly featuring citrus and berry notes. Ethiopian Yirgacheffe and Panamanian Geisha are typical examples of citrus flavors, especially the Panamanian Hacienda Esmeralda Geisha, which is the king of citrus. This is attributed to the esters and aldehydes in the coffee.

Light-roasted coffee often exhibits spicy aromas like cinnamon and cardamom, which are due to volatile compounds such as aldehydes, esters, ketones, alcohols, and others. Light to medium-roasted coffee contains furanone (Furaneol), a fruity component also found in strawberries and pineapples. Onions, garlic, or green onions, when uncut and with intact cell walls, do not emit pungent odors. Once cut, the cellular structure breaks, and enzymes immediately react with previously odorless precursor aromatic compounds.

Onion and garlic notes are categorized under herbal undertones in the flavor wheel. Occasionally, onion and garlic-like components emerge during the roasting catalytic process. When evaluating dry and wet aromas, people often describe scents like pizza, seafood sauce, onion and garlic, or beef broth... Almost everyone’s experience differs.

Diluted durian-like fermented fruit or fermented tofu flavors, both sweet and slightly spicy, are unique "regional characteristics" of coffee-producing regions. Additionally, premium washed beans with bluish-green hues often emit a refreshing sweetness combining hay and sugarcane, both of which are captivating herbal notes.

Light roasts most effectively highlight the volatile fruity acidity of esters, aldehydes, and organic acids stored during the bean’s development stage.

As the process moves into medium roasting, the Maillard reaction accelerates, caramelizing carbohydrates, degrading nitrogenous components and lipids, and pyrolyzing sugars, amino acids, and trigonellines to produce complex, unique aromatic compounds.

Color, aroma, and flavor are inseparable, vividly manifested under the effects of Maillard and caramelization reactions. Maltose, for instance, involves sugar caramelization, where sugar content is proportional to roast color depth. It also includes the browning reaction.

Thus, nutty, almond, buttery, and chocolatey aromas during this process stem from Maillard reactions, not caramelization. In other words, if coffee only undergoes caramelization without Maillard reactions, it would only have a monotonous bitter-sweet taste, rather than a richly diverse flavor. Classifying nuts and many sweet aromas in the SCAA flavor wheel as solely caramelization reactions somewhat oversimplifies the formation of sweet flavors. Incorporating Maillard reactions brings the actual results closer to reality. The "aroma spectrum" under caramelization and Maillard reactions can be divided into three major profiles: nutty, caramel, and chocolatey.

　 (Note 1) Caramelization Reaction: The carbohydrates or sugars in coffee beans undergo caramelization between 170°C and 205°C. Sucrose dehydrates, releasing water vapor and carbon dioxide, and its color changes from colorless crystals to brown, producing fragrant substances: "Diacetyl" (a component of butter with a buttery-sweet aroma); HMF-Furans (with a caramel taste); HAF-Maltol (commonly known as sugar-flavored spice). Caramelization accounts for 17% of the weight of roasted beans, with a taste that is bitter yet sweet.

(Note 2) Maillard Reaction: Protein degradation and polymerization. This reaction is not solely a chemical one but involves a series of complex polymerization and degradation processes between amino acids and reducing sugars like glucose, fructose, lactose, and maltose under continuous heating. This process does not occur at a fixed temperature but roughly happens between 185°C and 240°C for coffee. Recent studies have found that 10% of coffee’s bitterness comes from Maillard reactions.

At higher roasting temperatures, the process enters [Dry Distillation].

A) Deep Roast Enthusiasts Love the Resinous Aromas of Smokiness, Heaviness, Pungency, and Alcohol—These Are Products of Maillard Reactions and Dry Distillation. (Resinous Components: Conifers like pine and fir secrete terpene compounds such as pine resin, which has a pungent aroma to deter pests or squirrels.) "Dry Distillation" refers to the process of heating solids or organic matter in isolation from air until completely carbonized. Isolating air prevents oxygen from aiding combustion or causing explosions.

B) Heavy-roasted beans, when removed before burning, undergo dehydration, pyrolysis, dehydrogenation, and carbonization—processes similar to dry distillation. Medium-roasted beans also generate many smoky or pungent aroma components. Therefore, the aromas of deep roasts can be attributed to dry distillation.

C) Aromas of light and medium roasts belong to low and medium molecular weights. But in the deep-roast world after the second crack, carbonization intensifies, caramelization disappears, but Maillard reactions continue. Amino acids and polysaccharide fibers of cellulose continuously degrade and polymerize, producing more high-molecular-weight bonding compounds. The interpretation of aroma shifts from caramelization to being dominated by Maillard and dry distillation reactions, characterized by smokiness, heaviness, and pungency.