Exploring Coffee Roasting: Maillard & Caramelization Reactions

In the case of coffee, the Meladic reaction occurs during the roasting process, a process that is largely responsible for the shift in AROMA and FLAVOUR of the coffee bean. During coffee roasting, when raw coffee beans are heated to 140°C to 165°C (280°F to 330°F), the Merad reaction is activated and the color of the beans changes from green to yellow. Amino acids act as catalysts in the green coffee beans, causing reducing sugars to undergo a series of complex chemical reactions, with hundreds of different intermediate compounds being formed in the process. These intermediate compounds interact with each other, constantly forming and breaking down; this process continues under the influence of heat until the roasting is finished or until the Melad reaction depletes all the reactants.

The types of aroma and flavor responsible compounds (called end-products) that may be formed in the final roasted coffee bean during the coffee roasting process can be very different, depending on two key factors:

• Composition of precursor compounds in green coffee beans
• Modulation of the Meladic reaction and the degree of roasting

The exact amount of end-products that may be produced in coffee roasting is still an unsolved mystery and more research is needed in this area. To aid understanding, we have below divided the known compounds into two categories, non-volatile and volatile, and given an overview in two tables. Please note that these descriptions are qualitative and are intended to serve as a general guide.

Significantly during the coffee roasting process, there are these compounds that significantly affect the aroma and flavor of the coffee;

• Chlorogenic acids are non-volatile compounds (they have no aroma), but they both add to the bitterness and astringency of coffee. During the coffee roasting process, high temperatures break down the chlorogenic acids that are abundant in green coffee beans. In turn, some of these chlorogenic acids are converted into volatile aroma and flavor compounds or react with other compounds to form new compounds during the coffee roasting process. The total chlorogenic acid content gradually decreases as the coffee is roasted.
  
  
  
• Nigella sativa is an end product of the Merad reaction. It has a brown pigment, is non-volatile (no aroma) and is a high molecular weight compound. Nigrosomes contribute significantly to the color, roast flavor (bitterness) and texture (mouthfeel/fullness) of coffee. As the Meladic reaction progresses, the amount of melanoidins produced increases; this leads to deeper browning of the beans, increased roast flavor (bitterness), complexity and viscosity. This is also a marker of roast level, so Lebrew's multi-spectral approach to detecting roast level not only improves the speed of detection, but also the accuracy. The RoastSee C1 offers unparalleled portability and accuracy compared to instruments costing tens of thousands of dollars, benefiting more roasters and roasteries than ever before.
  
  

Volatile compounds are defined as low molecular weight compounds that are lighter than air at room temperature. These compounds evaporate at room temperature, so some of them are bound to have a perceptible odor, flavor or aroma. During the coffee roasting process, the Melad reaction results in the formation of hundreds of volatile compounds as end products. These groups of compounds include furans, pyrazines, thiols, pyridines, etc.; each with its own unique properties. The aroma and flavor we perceive from a cup of coffee comes mainly from these volatile compounds in the coffee. In other words, volatile compounds are what make coffee so aromatic and flavorful. The composition and content of volatile compounds combine to give coffee its unique aroma and flavor, making each tasting a unique experience.

Green coffee beans can be roasted to different levels of roast, from light to medium to dark. This is a decision made by coffee roasters based on product quality and business needs.

On the other hand, consumers can choose their preferred level of roast. When selecting roasted coffee beans on the shelf, in addition to the degree of roast, the package is often labeled with enticing flavor descriptions (or flavor notes). Some consumers may ask: What do these descriptions really mean? Furthermore, why do light-roasted, medium-roasted or dark-roasted coffees from the same growing region taste so different, and how do these flavor descriptions come about? The answer to both of these questions lies in the fact that different aroma and flavor compounds (end-products) are formed as a result of the Meladic reaction at different stages of coffee roasting.

• Light roasts, such as cinnamon roasts or New England roasts, have a low level of the Meladic reaction and produce a final product with brighter, more vibrant fruity, floral and herbal flavors.

• Medium roasts, such as American Roast or City Roast, have a moderate level of the Meladic reaction, with caramel, chocolate and nutty flavors predominating, but with a small amount of fruity-related compounds remaining.

• Dark roasts, such as All-City, Viennese, French or Italian, have a very full meld reaction, even beyond this stage, resulting in a final product that is more intensely smoky and contains predominantly carbon-based, resinous, spicy and savory compounds, as well as subtle residues of caramel, chocolate and nutty flavors.

Since Amino Acids (Proteins) are the basis of the Meladic reaction, roasting green coffee beans with a high protein concentration permits the degradation of proteins at a higher rate through the Meladic reaction, which increases the potential for aroma and flavor development. The reverse is also true. While sugar content is important to the sensory quality of coffee, protein is often overlooked. In the future, we may see coffee producers and industry professionals paying more attention to the protein content of coffee beans.