The Maillard reaction is one of the most important chemical reactions that occurs during coffee roasting. It is responsible for many of the flavors and aromas associated with roasted coffee. The Maillard reaction takes place when the amino acids and reducing sugars in the green coffee beans are heated during roasting. The amino acids and sugars react to form hundreds of new flavor compounds that give coffee its characteristic aroma and brown color.
Let us first understand what is Maillard Reaction
The Maillard reaction is a chemical reaction between amino acids and reducing sugars that gives ‘browned’ food its distinctive flavor. It occurs when foods are cooked at high temperatures, such as baking, frying or grilling. The Maillard reaction is what gives foods like bread, pretzels, cookies, grilled meats and of course coffee their brown color and complex, savory flavors.
Maillard reaction is named after the French chemist Louis-Camille Maillard who first described it in 1912. It occurs between amino acids (from proteins) and reducing sugars (like glucose and fructose). The amino group in amino acids reacts with the carbonyl group of reducing sugars.
Maillard reaction requires heat and begins at temperatures between 285-300°F (140-165°C). The higher the temperature, the faster the reaction. It’s a type of non-enzymatic browning reaction, which means that enzymes are not required for the Maillard reaction to take place.
Maillard reaction is often described as having three stages: early, intermediate, and advanced. The early stage produces more simple sugars and amino acids. The advanced stage results in darker browning and more complex, bitter flavors. The speed of the reaction depends on several factors, including temperature, pH level, moisture content, and the specific amino acids and sugars present. Higher heat, lower pH, less moisture, and more reactive amino acids/sugars will speed up the reaction.
Maillard reaction is a complex reaction that produces hundreds of different molecules that provide a variety of flavors and aromas. As it progresses, the molecules become more complex and the flavors change. The specific compounds produced depend on the food involved, for example, malty flavors in bread vs. roasty flavors in coffee. The Maillard reaction continues to occur over time, even after the food has been cooked. This is why the flavor of leftovers often changes as they are stored.
Maillard reaction is often considered a positive reaction because of the desirable flavors and aromas it produces. But it can also reduce the nutritional value of some amino acids and proteins. It’s responsible for the appetizing smell of baking bread, cookies, and cakes. The aromas are produced by volatile molecules created during the Maillard reaction.
How is Maillard reaction responsible for coffee flavors and aromas?
The Maillard reaction is what turns green coffee beans into the brown, fragrant beans we know and love.
When coffee beans are roasted, the extreme heat and temperatures between 285-300°F (140-165°C) causes the Maillard reaction to occur. Amino acids in the beans react with natural sugars, resulting in hundreds of flavor compounds that give coffee its signature taste. The longer the beans are roasted, the darker they become and the more complex the flavors get. In the early stages, the reaction produces simple sugars and amino acids. In the later stages, it results in darker browning and more robust, bitter flavors.
The speed and specific flavors of the Maillard reaction depend on several factors, including the type of bean, roasting temperature, and pH level. Higher heat and lower pH will accelerate the reaction. The proteins and sugars in the bean also affect which flavor compounds are produced. For example, the reaction of certain amino acids with fructose creates malty, caramel-like flavors. The reaction with other amino acids and sugars produces roasty, nutty flavors instead.
The Maillard reaction is responsible for many of the desirable flavors in coffee like nutty, chocolatey, and malty. However, it also reduces the nutritional value of some amino acids and proteins in the beans. The overall result is a complex blend of hundreds of molecules that provide the aroma and taste we associate with coffee. Even after roasting, the Maillard reaction continues to slowly occur, which is why the flavor of coffee beans changes over time.
In summary, the Maillard reaction is what gives coffee its signature brown color, fragrant aroma, and savory flavor during the roasting process. By creating a complex set of molecules, it transforms the simple green coffee bean into a flavorful, delicious ingredient that forms the basis for one of the world’s most popular beverages. The next time you enjoy a cup of coffee, you have the Maillard reaction to thank for its taste and smell.
Compounds formed by Maillard Reaction in Coffee Beans
The Maillard reaction requires a combination of amino acids, reducing sugars like glucose and fructose, and heat. As roasting temperatures increase, these precursors react to create an array of new compounds that include:
Melanoidins:
Brown-colored compounds that give coffee its characteristic color. Melanoidins also contribute to mouthfeel and aroma.
Furanones:
Compounds with sweet, caramel-like aromas. Furanones are responsible for many of the aromas associated with medium and dark roasts.
Maltol:
A compound with a sweet, malty aroma. Maltol contributes to the aroma of light and medium roasts.
Pyrazines:
Compounds with nutty, toasted aromas. Pyrazines are especially prominent in medium and dark roasts.
Aldehydes:
Compounds with floral, fruity aromas. Aldehydes are most abundant in light roasts where they contribute to a bright, acidic aroma.
Ketones:
Compounds with sweet, spicy or floral aromas. Ketones are found in all roast levels but decrease in darker roasts.
Lactones:
Compounds with sweet, buttery aromas. Lactones are most prominent in medium and dark roasts where they enhance body and mouthfeel.
Acids:
Compounds like acetic acid that contribute to tartness and brightness. Acids decrease in darker roasts as they break down during prolonged heating.
The Maillard reaction is a complex process that produces hundreds of new flavor and aroma compounds from the precursors found in green coffee beans. By creating desirable compounds like melanoidins, furanones and pyrazines, the Maillard reaction is responsible for the flavors and aromas that we associate with a perfectly roasted cup of coffee.
How Maillard reaction is different from Caramelization?
The key differences between Maillard reaction and Caramelization are:
- The Maillard reaction involves a chemical reaction between amino acids and reducing sugars, while Caramelization involves the thermal decomposition of sugars only.
- The Maillard reaction is non-pyrolytic, whereas Caramelization is a pyrolytic process[1].
- The Maillard reaction typically starts at lower temperatures than Caramelization.
Both processes contribute to the browning and flavor development in foods, but they involve different chemical reactions and substances. Our article Coffee Roasting: The Science And How It Affects Flavor introduces the other factors contributing to the flavor profile of the coffee you enjoy.
[1] Pyrolytic process is the breaking down of complex organic compounds (sugars in case of coffee caramelization) into simpler compounds through the application of high temperatures only and does not need any other compound, whereas in case of a non-pyrolytic process two compounds react to form other compounds (e.g. amino acids and reducing sugars reacting in the Maillard reaction).
