What are green coffee beans?
Green coffee beans are the seeds of the coffee cherries produced by coffee trees. They are obtained by separating (processing) from cherries and they get their brown color after going through roasting process. The green beans start out pale in color, with a grassy aroma and unpleasant flavor. Through the application of heat, they develop into fragrant, brown beans with different flavor profiles and notes of caramel, fruit, or spice.
Coffee trees belong to the genus Coffea, with the two most common species being Coffea arabica and Coffea canephora (robusta). Arabica beans contain more sugars and lipids, resulting in greater flavor complexity. Robusta beans contain more chlorogenic acids, resulting in a harsher, more bitter flavor. The environment, including soil, rainfall, sunlight, and altitude, also impacts the chemistry of the beans. Beans grown at higher altitudes often have more complex flavor due to slower ripening.
Their chemistry depends on the variety of tree and the environment in which the cherries were grown. As the beans are roasted, their pale green color transforms to brown through a series of complex chemical reactions that create new compounds responsible for the enticing coffee flavor, aroma, and color of coffee we all know and love. By controlling these reactions, the roaster can design a specific flavor profile for any given batch of beans.
The Chemistry of Green Coffee Beans:
The green coffee beans contain hundreds of chemical compounds, including reducing sugars like glucose and fructose, amino acids, chlorogenic acids, and lipids. The most important precursors for flavor are the reducing sugars and amino acids. When heated, these compounds undergo the Maillard reaction and caramelization, creating new volatile and non-volatile compounds that produce coffee’s characteristic flavor, aroma, and brown color.
The Maillard reaction occurs between amino acids and reducing sugars, producing melanoidins for color and mouthfeel, furans and pyrazines for aroma, and acids for brightness. The specific compounds depend on the precursors present and the degree of roasting.
Caramelization involves the breakdown of sugars into simpler forms and their recombination into new compounds. It creates volatile furans, maltol, and cyclotene for sweet, fruity notes, and non-volatile melanoidins for color, aroma, and viscosity. It also reduces acidity by breaking down organic acids.
Light roasts experience more Maillard reaction, preserving acidity and volatiles. Medium roasts achieve a balance of Maillard reaction and caramelization, with rounded, caramel-like flavors. Dark roasts undergo extensive caramelization, resulting in low acidity, bold, bittersweet flavors, and an oily surface.
In summary, the flavor of roasted coffee depends on the chemistry of the green beans, which varies based on species, origin, and environment. During roasting, the green beans are transformed through the Maillard reaction and caramelization into the fragrant, flavorful brown beans we all enjoy.
Compounds in green coffee beans and their contribution to flavor:
Green coffee beans contain many chemical compounds that contribute to the flavor of the roasted coffee. The main compounds include:
Carbohydrates in coffee beans
Green coffee beans contain polysaccharides, like cellulose and starch, that provide structure to the beans. When roasted, these carbohydrates break down into simple sugars that add sweetness to the coffee.
Proteins in coffee beans
Green coffee beans contain amino acids that participate in the Maillard reaction during roasting. The Maillard reaction produces hundreds of new flavor compounds that create the characteristic coffee aroma and brown color.
Lipids in coffee beans
Green coffee beans contain fats and oils, especially triglycerides of palmitic, linoleic, and oleic acids. During roasting, these lipids break down and oxidize, releasing volatile compounds that contribute to coffee aroma.
Chlorogenic acids in coffee beans
Green coffee beans contain several types of chlorogenic acids, which are phenolic compounds that contribute to bitterness and acidity. Chlorogenic acids break down during roasting, reducing bitterness and acidity.
Other organic acids in coffee beans
Green coffee beans contain citric acid, malic acid, and quinic acid. These acids are responsible for the bright, tart acidity of lightly roasted coffees. As roasting progresses, these acids break down, decreasing overall acidity.
Volatile compounds in coffee beans
Green coffee beans contain volatile aroma compounds like aldehydes, ketones, alcohols, furans, and pyrazines. These compounds are altered or created during roasting through reactions like the Maillard reaction and caramelization, producing hundreds of new volatile compounds that create coffee aroma.
Minerals in coffee beans
Green coffee beans contain several minerals, including potassium, magnesium, and copper. Some minerals are lost during roasting, while others like potassium remain and add a mineral tang to the flavor of roasted coffee.
In summary, green coffee beans house a variety of complex chemical compounds that are transformed during the roasting process into the flavor, aroma, and color of roasted coffee. The roasting reactions create an intricate web of new compounds from the precursors found in the raw green beans.
