Written by Kieran Lobo
Giving Coffee Flavour: Roasting and the Maillard Reaction
Flowery, nutty, bitter, mellow, fruity, sour, and sweet. Does your cup of morning brew actually contain these flavour compounds? The short answer is yes. Coffee flavour is complex, and a multitude of factors contribute to a great-tasting cup: geography, climate, method of cultivation, processing, and roasting. However, when you break it all down, the answer to why your coffee tastes like it does can be found in the chemical process known as the Maillard Reaction.
The Nobel Laureate Jean-Marie Lehn called the Maillard Reaction “the most widely practiced reaction in the world,” and rightly so. From toasting bread to grilling meat—and an integral part of the coffee roasting process, the Maillard reaction is ubiquitous in our daily lives.
Discovered by the French chemist Louis Chamille Maillard, the reaction explains how reducing sugars and amino acids react to the application of heat, giving food—and coffee—its unique flavour. In 1912, Louis Chamille noticed a distinct yellow-brown colour formed when this happened. In the coffee roasting process, the reaction occurs between 140 and 165 degrees celsius.
Roasting and the Maillard Reaction (How Nangoo does it)
The principles of the Maillard Reaction are imperative to producing a batch of great tasting coffee and to ensure consistency across our batches of roasted coffee. We believe that better understanding of the reaction means better correlation between the application of heat during the roasting process and the development of flavour.
Roast temperature, therefore, is key. We begin roasting coffee by charging the roaster—effectively preheating it before adding the green beans. Too low a temperature could result in underdevelopment of flavour, while too high a temperature could burn your coffee beans.
Soon after, the green beans enter the “yellowing phase.” This is where the Maillard Reaction begins, and continues through the rest of the roasting process. Reducing sugars react with amino acids, producing the distinct brown colour of roasted beans.
During the development phase—which includes the first and second crack of the bean—the coffee attains its flavour profile. When the sugar and amino acids react, the nitrogen in the amino acid bonds with the carbon chain present in the sugar to produce a molecule of water. This water molecule is unstable, and further reacts to produce a range of different flavours. For example, the popular “buttery” flavour is due to diacetyl, a product of the Maillard Reaction. At higher temperatures, the sugars in the coffee bean decompose to form furans, which provide the caramel-like flavour and aroma in coffee.
When the sugar and amino acids react, the nitrogen in the amino acid bonds with the carbon chain present in the sugar to produce a molecule of water. This results in the Amadori Rearrangement, where the formed molecule is unstable and changes its structure before reacting again in one of three ways:
- it could lose more water molecules and turn towards caramelisation.into caramel.
- it could break down into short-chain molecules and result in diacetyl, which imparts a distinct buttery flavour.
- Or, the formed molecule could react with amino acids again to form melanoidins, which, apart from stabilising the crema atop your espresso, is also said to produce the antioxidants that make coffee a healthy beverage.
The different paths this reaction can take, coupled with the numerous potential combinations of amino acids and sugars, means an enormous number of possible tasting notes (think coffee flavour wheel!). Among the flavour compounds that occur as a result of the Maillard reaction, the sugars and amino acids react to form 2–furfurylthiol, which is responsible for the “roasty” aroma in your cup of coffee.
Sugar in your coffee
At around 170 degrees celsius, the sugars present in the coffee bean begin to caramelise, producing the aromatic and acidic compounds. Sugar in the coffee bean decomposes during roasting to form fFurans, which are largely responsible for the caramel-like flavour and aroma in the cup., however, caramelisation occurs only after the “drying phase” of roasting is complete, as the initial reaction tends to release water molecules. Sugars also give coffee the dark brown colour it is associated with. Due to its ability to easily react with other compounds, sugars produce a large amount of polymers which eventually create the familiar browning.
At around 195 degrees celsius during the roasting process, coffee beans emit a distinct sound similar to popcorn crackling. Roasters refer to this as “the first crack,” when water present in the bean is converted into steam and the consequent pressure presses on the inner walls of the coffee bean. This loss of moisture is a result of the Maillard reaction and occurs after the caramelisation of the bean, and the crack alerts roasters that the beans will soon achieve a “light roast.”
The Next Step in Coffee Science
While the coffee industry agrees that roasting is an incredibly scientific process and that the Maillard Reaction is at the heart of it, research is still somewhat sparse. In all probability, we’ve only scratched the surface of the potential flavour profiles that can be produced during the roasting process. Anja Rahn, a researcher at the Coffee Excellence Centre in Zurich, Switzerland, says that there is a gap between the theory of the Maillard reaction and applying this to coffee and the roast process. “Our inability to predict the Maillard reaction in food systems suggests that there is still value to be gained through Maillard reaction research. Knowledge-based food formulation should be the goal.”