How Does the Roasting Process Affect the Chemical Composition of Coffee Beans?

The roasting process significantly affects the chemical composition of coffee beans, leading to changes in the levels of various compounds that contribute to the flavor, aroma, the antioxidant activity, and overall quality of the final product. The specific changes depend on factors such as roasting temperature, time, and the species of coffee bean used.

Here are some key ways the roasting process impacts the chemical composition of coffee beans:

1. Maillard Reaction: The Maillard reaction, a complex set of chemical reactions, occurs when heat is applied to coffee beans, typically starting at around 150°C. This reaction produces colored melanoidins that contribute to the color, body, and flavor of the roasted beans.

2. Degradation of Chlorogenic Acids: Chlorogenic acids, which are abundant in coffee beans, are broken down during the roasting process. At high temperatures, chlorogenic acid is converted into caffeic acid and quinic acid, which are responsible for the bitterness and astringency in the final cup of coffee.

3. Formation of Volatile Compounds: The roasting process leads to the formation of over 900 volatile chemical compounds, with around 40 being directly related to aroma. These include aldehydes (contributing to fruity and green aromas), furans (caramel aromas), guaiacol (smoky and spicy notes), and ketones. These volatile compounds are trapped by the oils that coat the roasted beans, which helps retain the flavor and aroma.

4. Degradation of Sucrose and Other Carbohydrates: The roasting process leads to the decomposition of sucrose and other carbohydrates in coffee beans. This breakdown contributes to the formation of various compounds that impact the flavor and aroma of the roasted beans.

5. Changes in Antioxidant Activity: Different roasting conditions can affect the antioxidant activity of coffee beans. The causes of changes in antioxidant activity are associated with the degradation of chlorogenic acids and the formation of Maillard reaction products.

We now dive deeper on each aspect mentioned above.

1. ABOUT THE MAILLARD REACTION AND HOW IT AFFECTS THE CHEMICAL COMPOSITION OF COFFEE BEANS DURING ROASTING.

The Maillard reaction is a crucial chemical process that occurs during coffee roasting, leading to the formation of numerous flavor compounds and contributing to the characteristic brown color and rich aroma associated with roasted coffee beans. This reaction occurs between amino acids (building blocks of protein) and reducing sugars when exposed to heat. The Maillard reaction releases hundreds of flavor compounds from green coffee beans, enhancing the complexity of aromas in roasted coffee.

During the Maillard reaction, nitrogen in the amino acid bonds to the carbon chain of the sugar, resulting in the formation of glycosylamine. This unstable molecule undergoes further transformations, leading to the production of melanoidins, dark brown compounds that provide color and contribute to flavors such as roasted, malty, bready, bitter, and burned notes in coffee. These compounds also play a role in forming and stabilizing crema in espresso and providing body to brewed coffee.

The Maillard reaction is temperature-dependent and becomes significant in coffee roasting from about 140°C upwards. As the reaction progresses, a wide range of flavor compounds is formed, including roasted, bready, bitter, floral, fruity, and caramel notes. The reaction is crucial for the complexity of aromas in roasted coffee and is influenced by factors such as temperature, time, and the types of amino acids and sugars involved in the reaction.

2. ABOUT THE DEGRADATION OF CHLOROGENIC ACIDS.

During the coffee roasting process, chlorogenic acids, which are abundant in green coffee beans, undergo significant degradation. This degradation leads to the formation of various compounds that impact the flavor, aroma, and antioxidant properties of the roasted coffee beans. Here's a more detailed explanation of the degradation of chlorogenic acids during roasting:

1. Conversion to Caffeic Acid and Quinic Acid: At high temperatures, typically above 200°C, chlorogenic acid is converted into caffeic acid and quinic acid. Caffeic acid contributes to the bitterness and astringency in the final cup of coffee, while quinic acid is a dominant acid in green coffee beans and is also a product of chlorogenic acid degradation.

2. Isomerization: The roasting process also leads to the isomerization of chlorogenic acid isomers, such as 5-O-caffeoylquinic acid (5-CQA), 3-O-caffeoylquinic acid (3-CQA), and 4-O-caffeoylquinic acid (4-CQA). During roasting, 5-CQA tends to isomerize to 3-CQA and 4-CQA, while 4-CQA can also be converted to 3-CQA.

3. Formation of Volatile Compounds: The degradation of chlorogenic acids during roasting leads to the formation of various volatile compounds that contribute to the aroma and flavor of roasted coffee beans. These compounds include aldehydes, furans, guaiacol, and ketones, which impart fruity, caramel, smoky, and spicy notes to the coffee.

4. Antioxidant Activity: The degradation of chlorogenic acids during roasting can lead to the formation of potent antioxidants. Studies have shown that roasted coffee beans have higher antioxidant activity compared to green coffee beans, and this is attributed to the formation of antioxidant compounds from the degradation of chlorogenic acids.

3. ABOUT THE FORMATION OF VOLATILE COMPOUNDS.

The formation of volatile compounds during coffee roasting is a complex process that involves the breakdown of various chemical compounds present in the green coffee beans. These compounds, including carbohydrates, amino acids, and lipids, undergo thermal degradation and react with each other to form new compounds with distinct flavors and aromas.

Main Classes of Volatile Compounds

The main classes of volatile compounds formed during coffee roasting include:

• Aldehydes: These compounds are responsible for fruity, floral, and green aromas in coffee.
• Ketones: Ketones contribute to sweet, caramel-like flavors and aromas.
• Furans: Furans are formed from the thermal degradation of carbohydrates and are associated with sweet, caramel-like flavors and aromas.
• Pyrazines: Pyrazines are nitrogen-containing compounds that contribute to nutty, earthy flavors and aromas.
• Pyridines: Pyridines are also nitrogen-containing compounds that contribute to bitter, astringent flavors and aromas.

Factors Influencing Volatile Compound Formation

Several factors influence the formation of volatile compounds during coffee roasting, including:

• Roasting Temperature and Time: Increasing the roasting temperature and time leads to the formation of more volatile compounds, particularly those associated with roasted, caramel-like flavors and aromas.
• Grind Size: The grind size of the coffee beans affects the extraction of volatile compounds during brewing, with finer grinds leading to higher levels of volatiles.
• Water Type: The type of water used for brewing also impacts the extraction of volatile compounds, with filtered water leading to higher levels of certain volatiles compared to tap and bottled water.
• Coffee Bean Variety: The variety of coffee bean used can also impact the formation of volatile compounds, with Arabica beans generally producing more complex and nuanced flavor profiles compared to Robusta beans

4. ABOUT THE DEGRADATION OF SUCROSE AND OTHER CARBOHYDRATES.

The degradation of sucrose and other carbohydrates is a crucial process that occurs during coffee roasting, leading to the formation of various compounds that contribute to the flavor, aroma, and color of roasted coffee beans. Here's an explanation of how sucrose and other carbohydrates are degraded during the roasting process:

Sucrose Degradation
Sucrose, the main carbohydrate in green coffee beans, undergoes significant degradation during roasting. This degradation occurs through two main pathways:

• Hydrolysis: Sucrose is partially broken down by hydrolysis into its constituent monosaccharides, glucose and fructose.
• Pyrolysis (Caramelization): The remaining sucrose is degraded by pyrolysis, also known as caramelization, which occurs at higher temperatures (around 180°C).

The Maillard reaction, which occurs at lower temperatures (around 130-140°C) and in the presence of amino acids, is favored over caramelization in breaking down sugars. The Maillard reaction produces many volatile aroma compounds, acids, and non-volatile melanoidins.

Degradation of Other Carbohydrates

In addition to sucrose, other carbohydrates present in green coffee beans, such as polysaccharides (arabinogalactan, galactomannan, and cellulose), are also degraded during roasting.

The polysaccharide content is reduced due to degradation into low molecular weight carbohydrates (mono- and oligosaccharides), making them more extractable. This degradation leads to the formation of various compounds that contribute to the flavor and aroma of roasted coffee.

Impact on Flavor and Aroma

The degradation of sucrose and other carbohydrates during roasting has a significant impact on the flavor and aroma of the final product. The volatile compounds formed, such as furans, aldehydes, and ketones, contribute to the characteristic flavors and aromas of roasted coffee.

Additionally, the remaining sucrose and its degradation products, such as fructose, contribute to the sweetness and caramel-like flavors in the roasted beans. The aliphatic acids formed, including formic, acetic, glycolic, and lactic acids, also contribute to the overall acidity and flavor profile of the coffee.

5. ABOUT THE CHANGES IN ANTIOXIDANT ACTIVITIES.

Changes in antioxidant activity during the coffee roasting process are influenced by various factors, including the degradation of phenolic compounds, the formation of new antioxidant compounds, and the overall impact of roasting conditions on the chemical composition of coffee beans. Here is an explanation of the changes in antioxidant activity based on the provided sources:

1. Degradation of Phenolic Compounds: Phenolic compounds, such as chlorogenic acids, are key antioxidants present in coffee beans. During roasting, these compounds are degraded, leading to a reduction in the overall antioxidant activity of the coffee. The loss of polyphenolic compounds due to thermal degradation contributes to the decrease in antioxidant activity observed in darker roasts.
2. Formation of New Antioxidant Compounds: Despite the degradation of phenolic compounds, the roasting process also leads to the formation of new antioxidant compounds, such as Maillard reaction products, which can contribute to the overall antioxidant capacity of the roasted coffee beans. These newly formed compounds may offset some of the losses in antioxidant activity resulting from the degradation of phenolic compounds.
3. Effect of Roasting Degree: The degree of roasting plays a significant role in determining the antioxidant activity of coffee. Lighter roasts tend to preserve more phenolic compounds and exhibit higher antioxidant activity compared to darker roasts, which undergo more extensive degradation of antioxidants. The antioxidant activity is inversely correlated with the degree of roasting, with lighter roasts generally showing higher antioxidant activity
4. Variability in Antioxidant Activity: The relationship between antioxidant activity and the degree of roasting can vary depending on the type of coffee, roasting conditions, extraction method, and the specific antioxidant assay used. Different studies have reported varying results regarding the impact of roasting on antioxidant activity, highlighting the complexity of this relationship.