Rayki Goh, MSc

Jul 1011 min read

The Chemistry of Coffee Bean Roasting – Explained!

Coffee consumption continues to rise globally, reflecting diverse cultural preferences and evolving trends. Did you know that 67% of American adults drink coffee daily, the highest rate in over 20 years? This surge underscores the demand for both traditional and specialty coffee, with unique roasts and brewing methods becoming increasingly popular. What drives your coffee preference: the rich flavours or the complex chemistry behind the roast?

Coffee Bean Roasting. How to Roast Coffee at Home?

Dear food people,

It seems almost impossible to quit that non-negotiable morning cuppa that gives us the push we need to power through a day packed with meetings. In 2024, coffee consumption remains robust and continues to grow, reflecting evolving preferences and cultural trends across various regions. According to the latest National Coffee Data Trends (NCDT) report, 67% of American adults have had coffee in the past day, marking the highest level in over 20 years. This figure represents a significant increase from 49% in 2004, with the rise in daily coffee consumption primarily driven by individuals aged 25 and older, particularly those aged 60 and above.

Specialty coffee has seen notable growth, with 57% of American adults reporting that they have enjoyed a specialty coffee in the past week. Beverages like lattes, espressos, and cappuccinos are especially popular, reflecting a growing preference for unique and premium coffee experiences. This segment has experienced year-on-year increases, indicating a shift towards more diverse and high-quality coffee options.

Singapore

In Singapore, coffee consumption is characterised by a unique blend of traditional and modern influences. The city-state consumes around 15,000 metric tonnes of coffee annually, translating to approximately 2.6 kg per capita. The market is expected to grow steadily with a projected compound annual growth rate (CAGR) of 3.2% from 2022 to 2027. Historically a tea-drinking nation, Singapore has seen a rise in coffee consumption, particularly with the local favourite, Kopi, a heavy-bodied, butter-and-sugar-roasted coffee. The proliferation of specialty coffee shops and the increasing popularity of at-home brewing methods, such as coffee pods and small-scale roasters, are significant drivers of this growth.

United Kingdom

In the United Kingdom, coffee consumption continues to thrive, bolstered by a strong café culture and an increasing interest in specialty coffee. The UK market has seen significant growth in the sale of ready-to-drink coffee beverages and a steady rise in at-home brewing, driven by the popularity of coffee pods and machines. This trend towards premium coffee experiences is expected to persist, with consumers seeking high-quality options both in cafés and at home.

Australia

Australia's coffee culture is renowned for its high standards and vibrant café scene. In 2024, coffee consumption remains robust, with a significant portion of Australians drinking coffee daily. The specialty coffee market is particularly strong, with consumers showing a preference for artisanal and high-quality products. There is also a growing trend towards sustainable and ethically sourced coffee, with many consumers willing to pay a premium for such products. Australia's café culture is set to continue thriving, driven by the demand for unique and high-quality coffee experiences.

Canada

Canada also sees steady growth in both traditional and specialty coffee consumption. Canadians are known for their high coffee consumption, with many drinking coffees daily. The market has seen a rise in the popularity of specialty coffee shops and premium products. Additionally, there is a growing trend towards convenience, with ready-to-drink coffee and single-serve pods becoming increasingly popular. The Canadian coffee market is expected to continue its growth trajectory, with a focus on quality and convenience driving consumer preferences.

In addition to the growing consumption and evolving preferences in coffee, an appreciation for different types of coffee roasts is also commonly observed amongst coffee drinkers. Most people are familiar with the concept of flavour notes in coffee, which are developed through the roasting process. These flavour notes can range from nutty-earthy to fruity-sweet, offering a diverse spectrum of taste experiences.

If you are curious about the science behind the formation of flavour notes in roasted coffee beans, read on as I break down the complex science into bite-sized information.

The chemistry behind these flavour profiles is fascinating. It involves the formation of various chemical compounds during the Maillard reaction, a complex process that occurs at different concentration levels during heating. This reaction is crucial in developing the rich and varied flavours that coffee enthusiasts seek out and enjoy.

Through this lens, coffee becomes more than just a beverage; it is an intricate interplay of science and art, where each cup showcases the skill of roasters who expertly balance time and temperature to unlock the full potential of each bean.

How to Roast Coffee Beans from Fruit to Bean?

The journey of coffee beans begins with careful cultivation in tropical regions around the equator, known as the "Coffee Belt." Here, coffee plants, typically grown from seeds, are nurtured in nurseries for several months before being transplanted into fields. Major coffee-producing areas include Central and South America, Africa, and Asia. There are two primary species of coffee plants: Arabica (Coffea arabica) and Robusta (Coffea canephora). Arabica beans are celebrated for their sweet, complex flavours, while Robusta beans offer a more bitter taste with higher caffeine content.

Once the coffee cherries mature, usually taking three to four years after planting, the harvesting process begins. Harvesting typically occurs once a year, although regions with varying altitudes and climates may experience two harvest seasons. There are different methods of harvesting: hand-picking, which is selective and ensures only ripe cherries are chosen, though labour-intensive and time-consuming; and strip-picking, which is faster but less selective, stripping all cherries off the branch regardless of ripeness.

After harvesting, the coffee cherries undergo processing to extract the beans. Several methods are used for this. The wet (washed) process involves pulping the cherries to remove the fruit, fermenting the beans to strip away the mucilage layer, washing them thoroughly, and finally drying them to reduce the moisture content to about 10-12%. The dry (natural) process skips the pulping and washing steps, instead spreading the whole cherries out in the sun to dry before hulling. The honey process is a hybrid, leaving some of the mucilage on the beans during drying to produce a sweet, fruity flavour.

The next step is milling, where beans are hulled to remove the parchment layer (for wet-processed beans) or the dried husk (for dry-processed beans). Polishing is optional and removes any remaining silver skin from the beans. The beans are then graded and sorted by size and weight, with defective beans removed to ensure consistent quality.

Green coffee beans are then packaged in jute or sisal bags and stored in cool, dry conditions to prevent spoilage before being exported to coffee-consuming countries worldwide. Green coffee beans can be stored for an extended period without losing their quality, typically up to 12 months or more, ideally stored between 15-21°C (60-70°F) maintaining in low humidity levels (around 60%) to prevent mould growth and away from exposure to light to preserve quality.

How to Roast Coffee at Home?

Upon reaching their destination, the beans are roasted to develop their flavour. Roasting involves heating the beans to temperatures between 188°C to 282°C (370°F and 540°F). This can also be done from home! During roasting, the beans undergo a first crack as moisture evaporates and internal pressure builds up. Roasting time and temperature are carefully controlled to achieve the desired flavour profile. A second crack may occur for darker roasts, resulting in a stronger, bolder flavour with less acidity.

After roasting, the beans are quickly cooled to prevent over-roasting and then packed in airtight containers to preserve freshness. Some packages include one-way valves to allow gases to escape while keeping oxygen out. The final step is grinding the roasted beans to the desired consistency based on the brewing method, whether it be coarse for French press, medium for drip coffee, or fine for espresso. The ground coffee is then brewed using methods such as drip coffee, where water drips through the ground coffee to extract flavours, espresso, where pressurised hot water is forced through finely-ground coffee, or French press, where coarse-ground coffee is steeped in hot water before being separated by a plunger.

Fun Fact!

The amount of caffeine in dark roast coffee can vary depending on the type of coffee bean, the brewing method, and the serving size. Generally, a standard 8-ounce (240-milliliter) cup of dark roast coffee contains approximately 70-120 milligrams of caffeine.

It's a common misconception that dark roast coffee has more caffeine than lighter roasts due to its stronger flavour. In reality, the roasting process slightly reduces the caffeine content, so dark roasts may have slightly less caffeine than lighter roasts, but the difference is typically minimal.

Factors such as the coffee bean variety (e.g., Arabica vs. Robusta), grind size, and brewing time also influence the caffeine content in your cup of coffee.

Coffee Bean Chemistry

Coffee beans, in their raw form, contain carbohydrates (mainly sugars like sucrose), proteins with amino acids, lipids, water, and other minor components. During roasting, the intense heat triggers the Maillard reaction between amino acids and reducing sugars, developing numerous flavour compounds. This reaction produces pyrazines, contributing to nutty and earthy flavours, and furans, which add sweet, caramel-like notes. The specific flavours depend on the roasting conditions, such as temperature and time.

Beyond flavour, the roasting process enhances the aroma and richness of coffee. A significant result of this process is the formation of melanoidins (pronounced as "mel-uh-noy-dinz"). These compounds not only impart a rich colour and taste to coffee but also possess antioxidant properties. Research has shown that coffee consumption is linked to various health benefits, partly due to these antioxidants formed during roasting.

Melanoidins What?!

If you are curious to know more about the compounds formed from the Maillard reaction, here are some of the key antioxidant molecules identified in roasted coffee:

Melanoidins

They are large, brown polymers formed during the later stages of the Maillard reaction and are known for their antioxidant properties. Melanoidins contribute significantly to the colour and viscosity of coffee.

Melanoidin Precursors

Intermediate compounds in the formation of melanoidins, such as:

Amadori Compounds: Early-stage products of the Maillard reaction that can act as antioxidants.
Heyns Compounds: Similar to Amadori compounds but derived from different sugars.

Phenolic Compounds

While not directly formed by the Maillard reaction, phenolic compounds in coffee are transformed during roasting and contribute to its antioxidant activity. Examples include chlorogenic acids and their derivatives.

Maillard Reaction Products (MRPs)

These include a variety of small molecules with antioxidant properties. Some MRPs in coffee are:

Pyrazines: These contribute to nutty and earthy flavours and have been shown to have antioxidant activity.
Furans: These compounds add sweet, caramel-like notes and also possess antioxidant properties.
Pyrroles: These contribute to the roasted flavour and have antioxidant potential.
Strecker Aldehydes: Formed from the reaction between amino acids and reducing sugars, these compounds also exhibit antioxidant properties.
Heterocyclic Compounds: These include various nitrogen and oxygen-containing compounds formed during roasting, such as pyridines and quinolines, which contribute to both flavour and antioxidant activity.

Advanced Glycation End Products (AGEs)

Some AGEs formed during the Maillard reaction can have antioxidant properties, although they are more commonly known for their potential negative health impacts.

Hydroxycinnamic Acids

Such as caffeic acid and ferulic acid, which are transformed during roasting and contribute to the antioxidant capacity of coffee.

Caramelise Products

Although distinct from the Maillard reaction, caramelise of sugars also occurs during roasting and can produce antioxidant compounds like hydroxymethylfurfural (HMF).

How Can Coffee Producers Ensure Perfect Roasting Quality?

The precise composition of antioxidant molecules in coffee can vary depending on factors such as the type of coffee bean, roasting conditions, and brewing methods. This complex mixture of antioxidants contributes to the health benefits associated with coffee consumption, including reduced oxidative stress and potential protection against various diseases.

Quality control checks for assessing the degree of roasting and ensuring that time and temperature are applied correctly involve a blend of visual, sensory, and technical methods. One of the most immediate indicators is the colour of the roasted coffee beans. Light roasts are typically light brown, medium roasts are medium brown, and dark roasts are dark brown or almost black.

Tools such as the Agtron scale or colour comparators provide objective measurements of the bean colour, allowing for precise control and consistency. Here are the various methods to conduct quality control assessment to verify the effectiveness of roasting according to specification:

Sensory Evaluation

Sensory evaluation plays a crucial role as well. Cupping, a standardised method of tasting coffee, allows professional cuppers or quality control experts to assess the flavour profile, aroma, and other sensory characteristics. This involves tasting the brewed coffee and evaluating attributes such as acidity, body, flavour, and aftertaste to ensure it meets the desired profile. Smelling and tasting the beans and brewed coffee help identify any off-flavours or undesirable characteristics indicating issues in the roasting process.

Physical Analysis

Physical analysis is another critical aspect. Measuring the moisture content of roasted coffee beans ensures they have been roasted to the correct degree, with beans typically losing 10-20% of their weight due to moisture loss. Moisture analysers or gravimetric methods determine the moisture content. Additionally, the density and weight of the beans are checked, as roasted beans should have specific density and weight loss consistent with the desired roast level. Density meters or displacement methods measure the bean density, while weight loss is calculated by comparing bean weight before and after roasting.

Chemical Analysis

Chemical analysis provides further insights. Colourimeters or spectrophotometers measure the colour of ground coffee, offering an objective assessment of the roast degree by measuring light reflectance or absorbance. Gas chromatography analyses the volatile compounds in roasted coffee, identifying specific flavour compounds and ensuring consistency in the roasting process.

Time and Temperature

Time and temperature monitoring is fundamental. Detailed roasting profiles are recorded for each batch, tracking time and temperature throughout the roasting process. Roasting software and data logging tools monitor and replicate desired roasting curves, ensuring consistency across batches. Thermocouples or infrared sensors measure the temperature inside the roasting drum and the beans themselves, providing real-time data that allows roasters to make precise adjustments.

Consistency Checks

Consistency checks should be performed through regular sample testing from different batches to ensure consistency in the roasting process. Samples are taken at various stages of roasting and subjected to the above quality control checks to verify that the roast degree matches the desired profile.

Can I Roast Coffee Beans in an Air Fryer?

Roasting coffee beans in an air fryer is simple and convenient. Start by preheating the air fryer to 350°F to 375°F (175°C to 190°C). Measure out half a cup of green coffee beans and place them in the air fryer basket in a single layer for even roasting.
Set the timer for 10-15 minutes. Stir the beans every 2-3 minutes to ensure uniform roasting. Listen for the first crack around 5-7 minutes for a light to medium roast. Continue until the second crack for a darker roast, being careful not to over-roast.
Once done, transfer the beans to a colander or baking sheet to cool, stirring to remove any remaining chaff. Allow the beans to rest for 12-24 hours to develop their flavours fully. Store them in an airtight container away from light and heat.
Ensure your kitchen is well-ventilated as roasting can produce smoke and strong aromas. This method provides a straightforward way to enjoy freshly roasted coffee beans.

Wrapping Up!

The chemistry of coffee beans plays a crucial role in shaping the flavour, aroma, and overall quality of the final brew. Central to this is the Maillard reaction, where amino acids and sugars interact during roasting to create a myriad of flavour compounds. Understanding this process allows you to appreciate the meticulous balance of time and temperature that roasters employ to achieve desired flavour profiles. By learning about the chemical changes during roasting, you can make informed choices about your coffee preferences and level up your coffee conversation, recognising the artistry and science behind each cup.

And hey, feel free to drop us a message at me@obsideonmedia.com if you've got something to say or a question to ask. Let's keep the conversation going and make food education a part of our daily habits together.