Roasting at Home: Mastering the Basics

Key Ideas

• It's easy to get started roasting coffee at home and you can use what you already have.
• Roasting coffee at home ensures you always have access to fresh coffee, allows you to customize your roast, develop a new skill or hobby, and also allows you to save money.
• Green beans are cheaper and can be purchased in bulk at discounted rates because they are shelf-stable.
• If you want more control over the process and a more consistent result, you can choose to invest in some roasting equipment.

Why Roast Coffee at Home?

Coffee is more than just a beverage that gets you up and going in the morning; it's an experience. That's why many people become obsessed and invest hundreds, if not thousands, of dollars, chasing the perfect cup. The enticing aroma of roasted coffee, the complex flavours of the beverage, and the undeniable energizing effects make it a beloved beverage across the globe.

Most people have seen whole-roasted coffee beans, and fewer people have seen what unroasted beans look like (pictured below). These little, dense green beans are the dried seeds of the coffee tree. But how do we turn the green, bitter, unpalatable seeds into the fragrant, flavorful beans used to brew your morning cup? This is where roasting comes in. The best part? It does not need to be expensive. You can use a simple tool found in most kitchens- the oven.

Why would you roast coffee at home instead of relying on local roasters? Well, there are a number of reasons. Roasting your own coffee is similar to baking your own bread- in return for investing a little effort and a little time to understand what's going on, you are rewarded with a beautiful, delicious product that rivals the quality produced at most professional establishments for a fraction of the price. Freshly roasted coffee has an unmatched aroma and flavour that is seldom experienced by the average coffee drinker, simply due to the reality of roasting, packing, shipping, and retailing large amounts of coffee. Roasting at home also allows you to access coffee at peak freshness whenever you want it. Once you know how to roast, you never have to run out of coffee again. Green beans are dried and shelf-stable, meaning they will keep for up to 2 years if stored correctly. Roasted coffee has a limited shelf-life and typically develops off-flavours around one month after roasting or even sooner.

Roasting your own also allows you to fully customize your roast and experiment with bringing out the absolute best in your beans. Many people don't like lighter roasts; others don't like dark roasts. It's important to realize that different coffee will need different roasting profiles to be at their best, and sometimes you may disagree with the choices your local roaster makes. They also roast at a much larger scale and a faster pace, meaning they cannot always ensure that the roast is 100% up to their standards- it will still be delicious but not as good as it could possibly be. Green coffee is also significantly cheaper than roasted coffee. By purchasing unroasted coffee, you can typically expect to save between 30-50% compared to buying the same coffee roasted and even more if you buy in bulk.

Coffee roasting is perfect for those who love coffee and want to access better products for less, those looking for a new hobby, and those concerned with packaging waste. After all, roasting your own eliminates the need for single-use coffee bags and pods, contributing to a more sustainable coffee routine in the long term.

Understanding the Coffee Roasting Process:

Coffee roasting is a transformative but surprisingly simple process involving applying heat to green coffee beans, leading to complex physical and chemical changes. These changes bring out the distinct flavours, aromas, and colours we associate with roasted coffee.

Once in the roaster, the green beans undergo three major steps: drying, browning, and development.

1. Drying:

During the initial phase, the coffee beans lose moisture and begin to change colour from green to yellow. This process is also called desorption. The heat causes the water content to evaporate, creating steam and releasing carbon dioxide. This process is essential to prepare the beans for subsequent chemical reactions. The beans become more fragile and less dense as they lose moisture and emit a grassy or hay-like aroma. The breakdown of longer, bigger carbohydrates into simple sugars like glucose and fructose during saccharification is crucial for developing flavours during the subsequent roasting stages. In this stage, the beans absorb heat energy. The duration of this stage can vary depending on the density of the beans, the moisture content, the ambient conditions, and the equipment used for roasting. Still, it typically lasts for a few minutes until the coffee reaches around 150-165°C (300-329°F).

2. Browning:

As the roasting temperature increases, the green beans undergo further chemical and physical changes that trigger the Maillard reaction. This process is similar to that which occurs when roasting meat or caramelizing vegetables. Amino acids (protein building blocks) within the beans react with glucose and fructose in the presence of heat, leading to the development of fruity, nutty, and caramel notes. This reaction is responsible for the browning of the beans, the development of aromatic compounds, and the formation of desirable flavours. The resulting compounds called melanoidins contribute to the characteristic brown colour of the roasted coffee beans.

The beans expand due to the buildup of water vapour and carbon dioxide. Eventually, the internal pressure becomes too great and the endosperm (the outer, denser layer of the bean) cracks and pops. This stage, called the 'first crack,' is an important milestone during the roasting process and signals the start of the exothermic stage of the roast, where the coffee beans start to release heat, accelerating the roasting process. Monitoring the temperature at this stage is crucial to ensure the beans reach the desired roast level without scorching or burning. The cell walls also start to break down, allowing for the future release of aromatic compounds and the migration of coffee oils to the surface of the beans.

During this phase, the coffee's smell will change from being reminiscent of hay to popcorn or nuts, becoming very aromatic and complex. The length of this phase leading up to the first crack, which occurs at around 195-205°C (383-401°F), is one of the key variables that roasters manipulate to achieve their desired outcome.

3. Development:

In this final stage, the coffee beans continue to undergo internal chemical changes. The oils within the beans migrate to the surface, giving the beans their characteristic satiny shine. At this point, the length and speed of the roast determine the ultimate degree of roast. As the beans continue to produce heat, the heat applied in the roaster needs to be carefully controlled to ensure the coffee continues developing properly and doesn't scorch or burn, which would lead to the creation of off-flavours. This is another crucial stage, one which shows the roaster's mastery and ability to fine-tune the coffee.

The sugars created during the drying stage further caramelize, contributing to the sweetness and richness of the final flavour profile. The acidity of the coffee can also be moderated during this phase, allowing it to harmonize with the other elements of the coffee's flavour. Roasting darker results in the breakdown of organic acids like malic and citric acids, which are naturally present in the beans, reducing the overall acidity and also results in the increased caramelization of sugars, adding sweetness and richness. Furthermore, extending the development phase can temper a high initial acidity and controlling the heat application rate can help preserve a desirable type of acidity and reduce any harshness that may have been initially present.

If you roast long enough, you will reach what is known as a second crack at around 225-230°C (437-446°F). This is another stage at which the coffee will crack and pop audibly; however, in contrast to the first crack, the second crack is quieter, sharper, and faster, reminiscent of the sound of cedar branches placed on a fire. The sound is often attributed to the further breakdown of the cell structure as the cell walls in the coffee burst, producing a quick, sharp sound. The second crack is often used as an indicator of a darker roast (medium-dark, dark, French, Italian etc.). At this point, the beans will be visibly darker, and the oils on the surface will start to pool or puddle, making the beans look slick and wet. Roasting darker results in the production of smoke as volatile organic compounds decompose, burn, and release gasses, and as coffee oils start to vaporize so it's not recommended without adequate ventilation, which is very hard to achieve at home.

Roasting Coffee at Home in the Oven:

Specialized coffee roasting machines provide increasingly more precise control and consistency and are often priced accordingly, with the big commercial machines like Loring or Probat- brand drum roasters capable of nearly fully automatic roasting at a large scale. However, because roasting coffee only requires the careful application of heat, it is possible to roast at home using a basic oven or stove. We recommend using a stove over roasting coffee in a pan on the stovetop as the latter requires a lot more work, and there is less control over the heat being applied. The basic steps are as follows:

1. Preparing the Beans:

Start with high-quality single-origin green coffee beans, ensuring they are free from defects (discoloured, damaged or smelly beans) and of an origin and variety that suits your taste preferences. Measure the desired amount of beans for your roast in grams, considering that you will lose around 15-19% of the total mass during roasting. For example, if you use a recipe that calls for 20 grams of coffee, and you'd like to roast enough for 14 cups to last you through the week, you would need to roast around 330-335 grams of green coffee.

2. Preheating the Oven:

Preheating the oven to 450°F (232°C) is similar to preheating a commercial machine because both aim to establish a consistent and stable roasting environment. Preheating your oven, including the baking sheet or tray you will be using to roast, promotes more even heat distribution along the roasting surfaces and reduces the temperature fluctuations when placing cool beans into a cold oven. Preheating the oven also allows you to track your roasting time more accurately, as it eliminates the time needed for the oven to reach the right temperature.

Preheating the green beans is not necessary or recommended as it can result in uneven roasting or scorching.

Ensuring proper ventilation is crucial for coffee roasting. Turn on your exhaust or range fan and open some windows to promote natural airflow through the space.

If you have an oven with hot spots, there are a number of ways to reduce their impact. First, you can put a baking stone or sheet on a lower rack to absorb and re-radiate heat. You can also use a heat deflector, like a layer of foil, on a higher rack to redirect the heat downward onto the beans. You can also stack another baking tray under the one you're roasting on to create a cushion of air that will buffer and distribute heat more evenly to the beans. You can also try experimenting with the position of the tray on the rack and rotating the tray during the roast.

3. Placing the Beans in the Oven:

Once everything is hot, use a pair of oven mitts to take your preheated tray out of the oven. Do not keep the oven door open for long to avoid losing heat. Spread the green coffee beans evenly on a baking sheet, allowing enough space for the beans to expand. Don't let them overlap or crowd because this will produce a less-even roast. Place the sheet in the preheated oven and set a timer for around 12-15 minutes as a reference. Do not rely on the timer; you need to watch the roast progress.

4. Observing the Roast:

Throughout the process, closely monitor the beans' colour and aroma. After a few minutes, the beans will undergo the drying phase, turning yellow and releasing a grassy aroma. If you'd like, you can note the times at which these changes occur to monitor your roast progression, but this is optional with oven roasting as the process is inherently less precise than roasting on a dedicated machine. The transition in colour from yellow to brown and the release of a nuttier aroma signals the end of the drying phase. You should start hearing the first crack around 10-12 minutes into the roast, but this can happen as soon as 8 and as late as 14 minutes in. The first crack signals the end of the browning phase.

As the roasting progresses, you'll notice the beans transitioning from light brown to medium or dark brown, accompanied by the release of aromatic compounds. The coffee will start to smell like- well- coffee. Once the beans reach your desired roast colour (you can compare them to another roasted bean you like from a similar origin) and you start to see a satiny sheen on the surface, you can take the coffee. This is likely to be just before the second crack starts. If you like a slightly more developed roast, wait for the first few signs of a second crack before removing your beans. We do not recommend roasting dark in an oven because of the large amount of smoke produced. You are likely to set off the fire alarm.

Measuring the temperature of the beans is tricky in an oven. Your best bet is to use an infrared thermometer to measure the surface temperature of the beans, but this necessitates opening the oven door, which results in heat loss. As a result, it's not recommended that you do this until you are well into the development phase after the first crack because, before this point, any fluctuations in heat can result in significant temperature dips and negatively impact the roast. You can take this opportunity to both measure the surface temperature and stir/agitate the beans with a wooden spoon or rotate the tray to even out the roast. It is best not to open the oven door if it can be avoided to maintain a stable roasting environment.

You will have more opportunities to take temperature readings as the coffee approaches your desired degree of roast to help ensure you reach your desired level. Please remember that surface bean temperatures are reliably 10-30°C (18-54°F) lower than internal bean temperatures due to a higher density in the core, allowing the internal portion to absorb and retain more heat and the involvement of various exothermic reactions that constantly release heat throughout the roast. Measuring internal bean temperatures is always a challenge. To the best of our knowledge, there is no technology that can reliably measure the internal bean temperature.

5. Cooling and Storage:

Once the desired roast level is reached, remove the baking sheet from the oven and immediately transfer the beans to a cool surface. Remember, the beans are still producing heat, so if you do not cool them quickly, you will over-roast your coffee. If available, put your coffee in a metal sieve or strainer and stir or toss the beans to speed up the cooling process. You can also use a fan to direct air into the beans while stirring or tossing to speed up the cooling process. You could spray some water onto the beans as soon as they come out of the oven and then toss them in the sieve, but we don't recommend this as the beans could absorb an excessive amount of moisture, potentially affecting the shelf life and the flavour.

Please be advised that since the chaff (the papery outer layer on the beans) had nowhere to go in the oven, tossing the beans will result in them separating from the bean, which can end up on your counters. As such, it is best to do this over a sink or outside, where it will make less of a mess.

Allow the beans to rest for at least 12-24 hours before brewing to allow the flavours to develop fully. This is the same as letting a pie or a loaf of bread cool down or allowing a stew to sit for a bit- it always tastes better when it has had time to rest and come together. Store the roasted beans in an airtight container away from light, heat, and moisture to ensure they stay fresher for longer and consume within 7-10 days from the date of roast.

Some Tips and Tricks from a Professional Roaster:

Here are some insights from a professional roaster that can enhance your home coffee roasting experience:

• Create a roasting profile by noting down the start time and the time of each change (colour/aroma changes, first crack, and possibly second crack) during each roast and temperature, if available. This will help you replicate successful roasts and make adjustments for future batches. At the roastery, we have temperature probes that feed into our roaster and take continuous readings, which are interpreted by a roasting software called Cropster. There is a free alternative called Artisan, but you will likely need to upgrade your roasting setup before using it.
• Remember to preheat your oven or roasting equipment. Maintaining a stable roasting temperature is just as crucial in the roastery as it is at home.
• Measure the roast temperature at the end of the roast. This is the equivalent of the "drop temperature" that we measure when we roast coffee in the roastery and is one of the best ways to describe the degree of roast as colour can be a little unreliable between different types of beans.
• Stirring or shaking the beans on the baking sheet during roasting when you're in the development phase can promote more even heat distribution and avoid scorching. Inside our roaster's drum, paddles help stir the coffee as it turns, ensuring a more even roast.
• Allow the roasted beans to rest for 12-24 hours after roasting to achieve optimal flavour development. At our roastery, the beans get packed once they are cool. Still, they take 24-36 hours to arrive at their final destination and may take longer to be consumed, meaning they have plenty of time to rest. We understand this is a little unrealistic at home, so try to roast before you run out. There is nothing wrong with drinking coffee as soon as it's cooled off, it's just not at its best, but it will still be delicious.
• Cup and evaluate your roasts in a formal way to explore what you like and do not like about your roast so you can improve next time. Plenty of cupping sheets are available online (including from the SCAA and Barista Hustle). If you'd like to know more about cupping, you can read our article Cupping Coffee Explained.

Ready to Upgrade? Where to go Next:

As you explore the world of coffee roasting, consider upgrading your setup to achieve greater control and consistency. Here are a few options for you to consider regarding upgrading to more specialized roasting equipment:

1. Hand-Held Stovetop Roaster: This is the least expensive option and arguably only constitutes an upgrade in that it is portable and is a piece of equipment dedicated to coffee roasting. Using one requires you to constantly shake the roaster to ensure an even roast; speaking from experience, this is hard to do. There is also little place to mount a temperature probe that will provide an accurate reading. A range of options are available, including the Hive and the Zenroast.
2. Popcorn Popper: Another low-cost option. Popcorn poppers have a low footprint on the counter space, and roasting requires little input from the operator, but there is minimal visibility vis-a-vis temperature or roasting environment. It is easier to take temperature measurements, but it's not as precise as the following few options on our list.
3. Air Roaster: small countertop air roasters are an excellent option for people interested in roasting at home and looking for more insight and precision. These can range in price from sever hundred to more than $1000 depending on the number of features and the capacity. Air roasting is most efficient and cost-effective at smaller scales because they can use electricity to heat cols as the heat source rather than relying on gas. We recommend the Fresh Roast line as an entry-level roaster because it's fairly intuitive and inexpensive compared to other roasters. The parts are easily replaceable, and the newer models incorporate temperature sensors. Although making modifications will void the warranty, there are plenty of guides online on how and where to add temperature sensors and start using roasting software like Artisan. The new Kaffelogic Nano 7 air roaster is marketed as splitting the difference between home roasters and professional sample roasting machines. It costs almost two thousand dollars and is, as of the writing of this article- on backorder, but it boasts a host of features and integrations through its software- Kaffelogic Studio, which can export to Artisan and Cropster.
4. Advanced Machines: Consider getting a sample roaster for a more advanced approach. There are many great options on the market, from the Ikawa Home to the one we use at our roastery- the Huky drum sample roaster. These machines are more expensive and will require better ventilation to use. This option is the equivalent of getting a one-group version of a commercial espresso machine for home use- very nice, but not necessary to get a delicious cup of coffee.

We hope that this article has provided you with a few key insights into the process of coffee roasting and that you feel more confident attempting it at home. Just remember to be kind to your neighbours and not set off the fire alarm unnecessarily and watch your beans closely so you don't overshoot.

Remember, at the end of the day, roasting coffee at home is only as complicated and expensive as you want it to be.