Common Chemical Reactions in Coffee Roasting

Cooking coffee involves a large number of chemical and physical alterations, transitioning unroasted coffee into the aromatic roasted coffee that everybody knows. This alteration occurs due to thermodynamics, which in terms of chemistry is an examination of heat, labor, and the chemical and physical alterations that are caused by these impacts. The decisions made by roasters in terms of time and temperature affect the flavors of the coffee as they impact the rate at which moisture is lost, the difference between the outer temperature of the bean versus the inner temperature and the microchemistry of the coffee, according to Ildi Revi, M.Ad.Ed., Director of Performance at Ally Coffee. What are the chemical reactions that contribute to the characteristics of roasted coffee? What steps can be taken to influence the outcome of the roasting process through intentional decisions regarding heat application?

This blog will help you comprehend some of the main changes undergone by your green coffee and how they cooperate to generate the roasted product that you distribute to your customers.

Decoding Flavor Formation

Coffee is much admired for its delightful flavour which results from the combination of two unique sensory experiences, smell and taste. The other advantages include the energizing effect of the beverage, the fun and luxurious feeling of drinking coffee, and the communal components linked to drinking coffee. The trends in the consumption of coffee away from home are constantly shifting, and the demand for gourmet coffee is growing around the globe as the number of coffee connoisseurs grows. Generally, people are more vigilant when it comes to the quality of their coffee, particularly concerning where it originated and how it was roasted. Coffee can be thought of as both an art and a science. Researchers and coffee gurus compare coffee production to winemaking, with the quality of the final product the result of qualifications, skills, and factors such as land, climate, and type of bean, ripeness, fermentation, and aging. drinking coffee is becoming more and more connected with certain events and feelings, much like wine is. Generally, coffee serves as an avenue for exchanging information and expertise with alcoholic beverages like wine and beer.

Many factors can influence the coffee quality. At the start of the process for creating coffee, there are agriculture components to consider, like the number and type of plants, the environmental conditions, and the techniques used after it is harvested. They figure out the mixture of green coffee beans, which influences the quality of the taste. Roasting is undeniably the most critical point in the coffee value chain. It is during this process that key physical and chemical transformations occur, setting the groundwork for the unique roasted coffee flavors. Coffee is transformed from its green state to a roasted form upon heating it above 200°C. Through this transformation, the coffee gets its characteristic color, scent, and flavor. Although the actual taste of the coffee is predestined when the beans are still green, a roaster has the skill to fully reveal its potential by properly applying the ideal conditions. The outcome of the product is largely determined by the level of roasting, the roasting method applied, and the equipment used. It is of great importance to have an understanding of the basis of food flavors, the means by which they are produced, as well as how fast they can be created, in order to be capable of producing items with desirable sensory characteristics. This data should help to create high-grade coffees through a pinpointed selection of ingredients and a more detailed understanding and applications of roast methods. Therefore, deep molecular science research of coffee roasting is essential for gaining knowledge and utilizing it in a coffee roaster’s daily activities.

This article is an overview of the flavor precursors that exist in green coffee, along with the relevant aroma and taste compounds that can be identified with complex analytical techniques. In the following section, we will discuss in more detail the chemical reactions and treatment conditions required for the production of a typical coffee flavor. Finally, we shall look into the development of taste and explore methods of controlling the flavor.

THE MAILLARD REACTION

The Maillard Reaction is a common subject of discussion amongst coffee roasters. This is a type of browning that does not involve enzymes and is caused by the reaction between simple sugars and amino acids, especially when heated. The interaction divides the basic sugars and amino acids, producing a vast selection of new distinctive aromas and flavors.

The Maillard Reaction is known to happen swiftly when temperatures range between 280–330°F (140–165°C), however it can still occur at lower temperatures and will last until the initial ‘crack’ is heard. When the temperature gets to 150° Celsius (302° Fahrenheit), distinct aromas, like the sweet and caramel-like furans, come out of the coffee. As the Maillard Reaction progresses, when the temperature rises to about 320°F, melanoidins form and the coffee begins to take on a darker color. Substances known as melanoidins are responsible for the dark hue of coffee that has been roasted and for the fullness and consistency of the beverage once it has been brewed.

AT THE ROASTER

The quantity of heat you apply and speed at which you go through the Maillard Reaction when preparing coffee will have a major influence on the outcome. Generally, if you roast quickly, the cup will have a stronger acidity as well as a sweeter taste. On the other hand, if you take your time with roasting, the acidity, sweetness and body of the cup will be more rounded overall.

A Few Aromatic Compounds Created During Roasting

CARAMELIZATION

Caramelization of the beans which are being roasted contributes greatly to the taste, fragrance, and color of brewed coffee. The process of caramelization is when sugars, both simple and complex, are heated and turn brown in color. Caramelization and the Maillard Reaction can both produce melanoidins, as well as flavorful and aromatic substances such as ketones, esters, and aldehydes.

The starting point for caramelization variations depending on how quickly the temperature increases. Sucrose, which is one of the main sugars present in raw coffee beans, could begin to caramelize when the temperature reaches 320°F/160°C should the heat increase slightly over time. However, a spike in temperature may cause the process of sucrose caramelization to begin to occur at temperatures higher than 356°F/180°C. Caramelization in coffee roasting can be more complex due to the various levels of heat different sugars take to begin the process, and because the more intricate sucrose is splitting into more basic sugars like fructose and glucose.

In the end, the most important thing that caramelization does for roasted coffee is bring out the sweet, toasty scents from the sugar during the roasting process. These arise from the transformation of sugar through caramelization, and consist of aldehydes, ketones, sugar-based molecules, and pyrazines which emit fragrances including herbal, malty, floral, spicy, fruity, and many more.

AT THE ROASTER

Making the caramelization in your roast profile even can be a challenge. A lack of proper caramelization will make roasted coffee devoid of fragrance and substance, whereas over-burned caramelization can break down the organic acids which produce some of the berry-like flavours of coffee. It is essential to consider not just the temperature but also the total roasting period and the rate at which you raise the heat of the mixture when caramelizing.

STRECKER DEGRADATION

The Strecker Degradation Reaction is not a topic that is referred to often in regards to coffee roasting but it is still very important to the final taste of the roasted coffee. The response involves the transformation of an amino acid to an aldehyde, ammonia and carbon dioxide in the presence of an oxidant.

Strecker Degradation has two major influences on roasted coffee. First, the aldehydes generated during the reaction are fundamental aromatic molecules in brewed coffee, such as a broad range of aromas like fruity, floral, grassy, nutty, and more; for instance, the Strecker-related aldehydes 2-methylpropanal and 3-methylbutanal are the main cause for malt-tasting notes in coffee. The carbon dioxide released during the reaction is what allows darker roast levels to be achieved.

AT THE ROASTER

Unraveling the intricate relationship between the results of Strecker Degradation on coffee and the methodology used for roasting is a complex task. A quicker roasting technique results in a larger amount of Strecker-related aldehydes being implemented in the early stages of the roasting operation. At higher temperatures, the breakdown of aldehydes reduces their ability to contribute an aroma. The Maillard Reaction facilitates the Strecker Degradation process, suggesting that the longer the Maillard phase is extended during the roasting process, the higher the chances will be for the Strecker Degradation to happen during the latter parts of the roasting. Considering the development of your roasting profile can be profitable; prolonging the Maillard phase can bring more chances of powerful aromatic substances existing in your roasted coffee with the use of Strecker Degradation.

PYROLYSIS

The breakdown of substances at high temperatures in a non-reactive atmosphere is known as pyrolysis. Caramelizing sugar is the major transformation that takes place while brewing coffee, yet when coffee beans are roasted beyond first crack and into second crack, pyrolysis may also happen. The roasting process using pyrolysis can give the coffee a darker hue and an increased amount of melanoidins. The taste that is typically associated with this type of process is described as “roasty” or even “charred” in more dramatic cases.

AT THE ROASTER

Pyrolysis usually occurs in the later phases when the roast goes too far, and its consequences on the taste of the coffee become more evident the more intense the expected final temperature is. Roasting green coffee for an extended period of time so that it caramelizes more and goes through more pyrolysis will give the coffee a fuller drink and a pleasantly smooth texture, distinct from the end result of a light roast of the same kind of green coffee. These coffees will be less acidic and will not have as many or as intense of a fruity flavor, but they will have more of a roasted flavor.

Flavor Precursors Occurring in Green Coffee Beans  

The ingredients used in the green beans will affect the smell and flavor that emerges while they are being roasted. Thus, an extensive examination of the components of green coffee was done to determine the quality that could be attained from a certain type of coffee and to use this understanding to refine coffee processing. Cooking by roasting involves a dry heat process, which begins by evaporating moisture and then reaches a temperature between one-hundred and two-hundred and twenty degrees Celsius. During this phase, the flavor components of the food are concentrated and finally, it is cooled down. This table illustrates the complexity of the components that make up Arabica and Robusta coffee in their green state. Green coffee chiefly contains carbohydrates, substances made up of nitrogen (largely proteins, trigonelline, and caffeine), fats, organic acids, and moisture. The majority of the components in green coffee beans can be utilized to create flavors and colors, or become contributing elements in their formation. The amount of water used can have a major influence on the quality of the coffee. From the group of ingredients in green coffee, the main components that increase the flavor are sugars, proteins, free amino acids, trigonelline, and Chlorogenic acids (CGA).

The mixture of Arabica and Robusta species is alike as a whole, but the quantity of each that’s combined varies drastically. Arabica coffees contain elevated levels of carbohydrates such as sucrose, oligosaccharides, mannans), lipids, trigonelline, malic acid, citric acid, quinic acid and 3-feruloyl-quinic acid (3-FQA). In contrast, Robusta coffees include higher levels of caffeine, proteins, arabinogalactans, CGA (minus 3-FQA), total phosphate, ash (e.g., Ca-salts), and transition metals (e.g., Fe, Al, Cu). The varying makeup of these elements is pivotal in determining the unique qualities and characteristics of roasted coffee, which will be further discussed later.

Carbohydrates

The dry basis of green coffee is typically made up of 40-65% carbohydrates, which are divided into types that are soluble and not soluble in water. Polymers made of arabinose, galactose, glucose, and mannose create both the water-soluble polysaccharides and the insoluble fraction. These polysaccharides, together with proteins and CGA, construct the cell walls. Around 45% of the dry weight of coffee beans is composed of cellulose, galactomannan, and arabinogalactan, all of which possess intricate structures. The soluble disaccharide sucrose accounts for the rest.

It is commonly believed that the extractable compounds within green coffee play the most significant role in establishing the smell, flavor, and color of coffee. The ingredients for many different reactions are easily accessible, which is proven by their quick use during the first part of the cooking process. The substances that can be dissolved in water are split into two parts, referred to as the large molecular weight (HMW) fraction and the small molecular weight (LMW) fraction. The two main types of water-soluble high molecular weight polysaccharides are galactomannans and arabinogalactans, which jointly make up 14-17% of the dry matter. Arabinogalactans which are of a green hue are connected to proteins using strong covalent bonds to develop arabinogalactan proteins (AGPs).