The Role of Hydroxy Amino Acid Degradation in Coffee Aroma and Flavor

The degradation of hydroxy amino acids, such as serine and threonine, is a crucial factor influencing coffee's aroma and flavor. These amino acids are integral to the Maillard reaction, a chemical process that occurs during roasting and is responsible for producing coffee's complex flavor compounds and characteristic aroma. Changes in the availability or stability of these compounds during roasting directly affect the sensory qualities of the beverage.

During roasting, hydroxy amino acids undergo thermal degradation, producing volatile compounds that shape coffee's flavor profile. For example, serine degradation generates pyrazines, contributing nutty or earthy aromas, while threonine breakdown forms aldehydes, imparting sweet or fruity notes. However, excessive degradation can diminish desirable flavors and introduce off-flavors, compromising the coffee's quality.

The extent of hydroxy amino acid degradation depends on roasting parameters, such as temperature, duration, and the bean's chemical composition. High roasting temperatures or prolonged roasting can accelerate amino acid breakdown, reducing their contribution to the Maillard reaction. Conversely, gentler roasting conditions may preserve these amino acids, allowing for a fuller development of desired flavor compounds.

Recent advancements in coffee chemistry research highlight the importance of tailoring roasting techniques to optimize hydroxy amino acid retention. For instance, studies using advanced analytical methods, such as gas chromatography-mass spectrometry (GC-MS), have revealed specific degradation pathways and their impact on flavor compounds. These insights enable coffee producers to refine roasting profiles, balancing temperature and time to maximize flavor complexity while minimizing off-flavors.

Additionally, emerging trends in coffee processing, such as anaerobic fermentation and precision drying, have shown promise in influencing the initial composition of hydroxy amino acids in green coffee beans. These methods can enhance the precursor levels available for the Maillard reaction, further elevating the sensory quality of the roasted product.

Understanding the role of hydroxy amino acid degradation offers a pathway to enhancing coffee quality. By leveraging scientific knowledge and innovative processing techniques, coffee producers can craft exceptional flavor profiles, ensuring a more consistent and enjoyable sensory experience for consumers worldwide.
The Role of Hydroxy Amino Acid Degradation in Coffee Aroma and Flavor

Degradation of trigonelline during coffee roasting

Non-volatile compounds degrade when they undergo thermal processing involving roasting. Chlorogenic acid and trigonelline are rapidly degraded during roasting, and phenolic compounds and pyridines/pyrroles are produced, respectively.

Trigonelline (TG; N-methylnicotinate) is a metabolite of nicotinamide involved in plant cell cycle regulation and oxidative stress.

Trigonelline is found in green coffee beans, and its content depends on the coffee species and origins. The amount of trigonelline in arabica is higher than that in robusta green coffee beans, and thus it can be used as a marker compound to distinguish the coffee bean species.

Trigonelline is a well-known precursor of flavor/aroma compounds in coffee and undergoes significant degradation contribute indirectly to the formation of desirable flavor products, including furans, pyrazine, alkyl-pyridines, and pyrroles, during roasting.
Degradation of trigonelline during coffee roasting

Flavor and aroma of coffee

The plant produces red cherry-like fruits containing two seeds, which, after being separated from the fruit pulp, are known as ‘green coffee’. When received, they are blended by manufacturers with green beans from other origins and roasted to achieve the characteristic flavor and color associated with coffee beverage of their brand.

There are two main species of coffee: arabica and robusta. As a general rule, arabica reveals a sweet, suave, fragrant, fruity, often acidic flavor. Conversely, Robusta is full-bodied, tonic, with a less pronounced aroma but stronger in caffeine.

Chemical composition of lipid, protein and carbohydrates in coffee contains the precursors function for developing flavor, aroma composition of the coffee beverage.

The polysaccharides which make up ~60 % of the green bean’s dry weight, consist of three major types: mannans or galactomannans, arabinogalactan-proteins (AGPs) and cellulose. In addition, there are small amounts of pectic polysaccharides. Carbohydrates have one of the crucial roles in coffee quality – forming aroma, flavor and color in the roasting process.

The lipid content in coffee grounds ranges from 10 to 17%. It has been reported that fatty acids composition in coffee is important to bring mouthfeel characteristics (body, texture) and essential aroma, flavor compounds to the coffee beverage. The aroma and flavor perceived in food are usually influenced by the type and concentration of lipids. Lipids also influence the mouth feel of several foods

The fatty acid (FA) fraction of triacylglycerols releases byproducts of oxidation, which are induced by temperature and mainly comprise aldehydes that react with intermediates of the Maillard reaction, providing additional flavor and aroma to the coffee.

Proteins are known to important flavor precursors in the coffee bean roasting process. It is because of protein-aroma compound or protein flavor compound binding. During roasting, proteins are denatured and fragmented. Proteins have a positive correlation with phenolic compounds and coffee mealnoidins, which can be explained by protein and phenolic compound involvement in coffee melanoidin formation.
Flavor and aroma of coffee

Coffee bean roasting process

Raw or green coffee has no flavor or aroma and has an unpleasant taste. For use as a beverage, it is roasted, powdered and brewed and the aqueous extract used as a beverage with or without the addition of milk, sugar and other substances.

Coffee beans are usually roasted in large batch dryers, which spin and heat them evenly at temperature that reaches 550 ° F. During roasting process, about 20 percent of the water content of the green beans evaporates and gases are released. In addition, the beans’ starch content is converted to sugar.

Roasting releases the oils and acids that give each coffee its unique flavor. The volatile oils and acids that give coffee is tempting aroma and delicious flavor are developed during the roasting process.

The flavor of roasted coffee, to a large extent depends upon the manner and extent of roasting. The flavor and aroma of coffee are best when it is freshly roasted and deteriorate on standing.

Sucrose which is the most abundant simple carbohydrate present in green coffee, acts as an aroma precursor during roasting, generating several classes of compound, such as carboxylic acids furans, and aldehydes, which will affect the flavor of coffee. Sucrose is found to be destroyed quickly at the early stages of roasting.
Coffee bean roasting process

Coffee flavor compounds

Coffee aroma and flavor determine about 80% of the quality of a roasts coffee beverage. Green coffee beans from the tree and after processing, do not yet have the color and coffee flavor or aroma of roast beans. They are formed during the roasting process – normally at 210 °C for 6-10 mins.

Reichstein and Staudinger made an outstanding contribution by characterizing more than 70 flavor compounds in roasted coffee. Since 1960 the list of constituent increased to 300 in green coffee, and to 850 in roasted coffee, respectively.

Many natural flavor compounds were first identified in roasted coffee and were later characterized as key compounds in certain processed foods. The pleasant aroma arising from roasted coffee beans during grinding is as attractive as the aroma of fresh brewed coffee for coffee flavored prepared foods.

However, because these pleasant aromas are especially highly volatile and unstable compounds, these are easily lost during the industrialized processing and storage of coffee products such as beverages.

The protein content in green coffee was reported as 8.7% - 12.2%. More importantly, there exists a considerable amount of amino acids in the forms of peptides and free amino acids,, which is essential to generation of characteristics roasted coffee flavor. Free amino acids in green coffee play central roles in formation of roasted coffee flavors although their contents are generally at a level of 0.15% - 0.25% in green coffee bean.
Coffee flavor compounds

Coffee aroma

The enticing aroma of coffee cannot be characterized by a single chemical component but is a combined response to many different chemical component.

Coffee aroma and flavor determine about 80% of the quality of a roast coffee beverage. Generally, most of the aroma compounds are generated at medium roast. Aroma formed during the roasting process normally at 210 °C for 6-10 minutes.

The coffee oil, which comprises about 10% of the roast beans carries most of the coffee aroma. More than 800 volatile compounds have been found in roasted coffee.

Many of the aromatic components of roasted coffee and particularly of coffee oil itself, are extremely susceptible to deterioration by the action of moisture and oxygen.

The staleness of coffee is due to the oxidative changes that take place with certain coffee constituents. This is prevented by the presence of carbon dioxide in roasted coffee. However, on storage, carbon dioxide is lost and so are the flavor and aroma.

In commercial practice, therefore roasted coffee has to be packed in an atmosphere of low oxygen content, either by CO2 gas-flushing or vacuum packing.
Coffee aroma