Coffee Roasting

Coffee roasting turns green coffee into the dark brown aromatic beans used to make your cup of morning cup of java. The green coffee beans you start out with are spongy and smell a little like grass. With coffee roasting a series of chemical processes take place that totally change the flavor and aroma of the coffee bean.

More than a hundred different chemical and physical changes happen during coffee roasting. And, more seem to be discovered with each passing year. The basic changes are these.

  • Decomposition of sucrose
  • Loss of free water
  • Decrease in total protein
  • Loss of chlorogenic acid
  • Decomposition of trigonelline
  • Formation of melanoidins, lactones, aliphatic acids, aromatic components

(Science Direct)

Coffee Roasts

Coffee roasting at successively higher temperatures and for more time creates successive changes that alter the quality of the roast. The finished roast is classified as mild, medium, medium-dark, or dark.

Light Roasts

This level of roast has a light-brown color. They are removed from the roaster before any oil breaks through the surface of the bean. Use this level of roast for milder varieties of coffee in order not to lose their subtle taste and aroma.

  • Light City
  • Half City
  • Cinnamon

Medium Roasts

A medium roast has a deeper brown color and stronger flavor than a light roast. It also has a non-oily surface. This is the level of roast most often preferred in the USA and is also referred to as the “American Roast.”

  • City
  • American
  • Breakfast

Medium-dark Roast

With this roast some oil comes to the surface of the dark brown bean. The taste of this roast is deeper and richer. And, a medium-dark roast is where you will begin to experience a bitter-sweet aftertaste.

  • Full City

Dark Roasts

At this level of roast the almost-black bean is shiny due to coffee oil that has broken through to the surface of the bean. Dark roasts are noticeably bitter. As a rule, coffee becomes less acidic with successively darker roasts. This level of roast ranges from a very dark brown bean to a charred bean!

  • High
  • Continental
  • New Orleans
  • European
  • Espresso
  • Viennese
  • Italian
  • French

(National Coffee Association)

The level of roast you aim for in your home roasting will vary with your taste. Your results will depend on your level of experience with the coffee roasting process.

Some folks believe that richer flavored dark roasts have more caffeine. Actually, a bit of caffeine is lost with a dark roast. Your caffeine concentration is highest with a light roast!

Coffee Roasting Milestones: First and Second Crack

First Crack

When you pre-heat the coffee roaster and then add the beans, the temperature falls. The beans are absorbing the heat! As the beans heat up their water also starts to evaporate. The chlorophyll, anthocyanins, and other green plant constituents break down and the bean loses its green tint to become more golden or yellow. At this time the coffee loses its “grass-like” aroma and smells more like toasted bread or even popcorn.

When the interior of the beans reaches the boiling point of water, 100 degrees Celsius or 212 degree Fahrenheit, the remaining water in the coffee beans comes to a boil and creates steam and pressure. As more and more beans come to this point you can hear many of them “pop” all at the same time. This popping is called the “first crack.”

This is when the bean gets larger to as much as twice its original size. You will start to smell aromas that you normally associate with coffee. The bean moisture content is now down to the three to five percent range (from its original ten to twelve percent).

If you want to be really fancy, at this point the coffee has an Agtron value of around 90. This is a way to measure the color of the bean in the near-infrared end of the light spectrum and is used in industry for quality control purposes. (Coffee Review)We really do not think you will want to go this far into the high-tech world in your home coffee roasting!

Second Crack

While the first crack is caused by steam building up in the coffee bean, the second crack comes from the production of carbon dioxide, carbon monoxide, nitrous oxide and other gases. These are produced during the chemical breakdown of fats, proteins, and sugars within the coffee bean.

The second crack occurs around 225 to 230 degrees Celsius or 437 to 446 degrees Fahrenheit. Here is where the beans start to become shiny as coffee oils break through to the surface of the coffee bean.

How far you take your coffee beans in this process depends on the level of roast that you want. When you get to the roast level you like, you turn off the heat and transfer the beans to a cooling container or surface. This needs to be done promptly because otherwise the beans will continue to a darker roast than you intended.

(Coffee Chemistry)

Ideally, you want to grind and brew your coffee as soon as possible after roasting. Roasted whole bean coffee retains a reasonable degree of freshness for up to six weeks when properly stored. But, the best and freshest coffee is right after you have roasted it.

Roasting Your Own Coffee at Home

It is equal parts science and performance art when you are roasting coffee. You need to know what you are doing any why. And then, every time you roast a few beans, you need to listen, be aware of the changes in aroma, and move safely and promptly to cool the beans to stop the roast at the appropriate time.

Professional coffee roasters do this for years and many grew up families in the coffee business, like Juan Fernando Hoyos Alzate in Manizales, Colombia.

Coffee roasting requires that the roaster checks the temperature, smells the roasting coffee, and listens for first and second crack.

Roasting Coffee in Manizales, Colombia

A few seconds delay can turn a great batch of gourmet coffee into motel and institutional coffee! Not paying attention to the first and second crack or changes in aroma, as well as the temperature, can absolutely ruin a batch of perfectly good beans.

You are certainly allowed to go right out and buy a home coffee roaster if you like. But, a good way to learn the basics of coffee roasting is to start with the basics and then progress to a more automated approach.

Manual Coffee Roasting: The “Popcorn Popper” Route

In places like Ethiopia and the mountains of Mexico, they traditionally roast the coffee beans in a pan over an open fire, grind them up, and brew coffee all in one sitting. So, when you decide to go the manual coffee roasting route, you are in good company.

Use the same sort of device you would use to pop popcorn. No, not the microwave! You will need to shake or stir the pot or cast iron skillet or use an old popcorn popper with a mechanism for stirring.

Manual coffee roasting really is a bit like popping popcorn. Just don’t use the microwave and don’t add any cooking oil!

Note: You can get some smoke when you roast coffee, especially when aiming for a dark roast. It is a good idea to roast your coffee in a well ventilated area, with open windows, or with an exhaust fan turned on.

The skin or parchment that covers the bean dries and detaches when you roast. Some may fly around when you roast manually and, when you use a home coffee roaster, you need to routinely remove it to prevent fires!

Like popcorn, coffee gets bigger when you roast it and becomes lighter due to water loss.

Manual Coffee Roasting Equipment

  • Kitchen scale
  • Clock, watch or timer
  • Thermometer able to read up to 500 degree Fahrenheit
  • Oven mitts or hot pads
  • Cooling rack
  • Roasting pan or metal colander
  • Paper and pencil to write down the route you took to the perfect roast
  • Airtight container for storing coffee

Manual Coffee Roasting Steps

Measure out the beans

  • Use three to four ounces of green coffee
  • Remember that beans will expand to as much as three times their original volume
  • You will want the coffee beans to be no more than three layers deep while roasting
  • If you are using a popcorn popper with a crank stirrer, eight ounces is good

Keep track of what you are doing

  • Method used
  • Heating method and settings
  • Time to first crack and duration
  • Time to second crack and duration (only for dark roasts)
  • Roasting container temperature of or beans at each step
  • Aromas and color at critical times
  • Time spent cooling the coffee
  • Color and roast level

The Coffee Roasting Process

  • Turn on your stove to medium which is just over 400 degree Fahrenheit.
  • Add the beans once you have achieved roasting temperature
  • Shake or stir the beans with a wooden spoon or crank the popcorn popper
  • Watch the temperature
  • Watch the beans for color changes
  • Listen
  • Note the changes in aroma
  • For most home roasting you will remove after the first crack

When to Remove from Heat

  • The color is right (from experience)
  • The aroma is right (from experience)
  • The sound is right (first crack or second crack)
  • Temperature is right (from experience)
  • Expected time has elapsed (from your notes)

Now, use your oven mitts and pour the roasted coffee into a colander or large pan to cool. Stir occasionally while the beans cool off. Blowing a small fan across the cooling beans will help remove any remaining chaff.

When the beans are down to room temperature, move them to an air-tight container. Ideally use something like a zip lock bag that can expand. Let them rest for at least four hours and for as long as three days. Having said that, you are absolutely allowed to grind a few of your newly roasted coffee beans to brew a cup, or two, or three of coffee!

Roasting coffee the manual way can be fun and even an adventure. But, with time we will get tired of this approach and start looking for a home coffee roaster and a more “automated” and “reproducible” approach to coffee roasting.

(Home Roast Coffee)

Roasting with a Home Coffee Roaster

Using a coffee roaster allows you to more easily control temperature and roasting time. And, most home roasters let you keep track of your favorite roasting profiles so that you do not have to guess each time you roast a new batch.

There are two basic types of coffee roasters, fluid bed and drum.

A drum coffee roaster consists of a rotating chamber that tumbles the green coffee beans as they roast. A fluid bed coffee roaster, also called a hot air roaster, forces very hot air through the roasting chamber. The air will typically enter from the bottom of the chamber and thereby lift and continually mix the beans for an even roast. Or the air can enter from the side of a rotating chamber such as with the Gene Café CBR models.

Coffee roasting with a home roaster takes the coffee through the same roasting steps as when you roast manually. What you are looking for with a home coffee roaster is an easier, more automated process that you can reproduce each time you want to roast a batch of coffee.

The ideal home coffee roaster lets you record several roasting profiles that you can return to time and time again for the various roasts you like the best.
With a home coffee roaster, however, you need to do more maintenance. These devices can even catch on fire if not routinely cleaned properly!

Fluid Bed Coffee Roasters vs Drum Roasters

Any roaster where forced air moves and heats the coffee beans is a fluid bed roaster. Many believe that you reliably get a “brighter” roast with a fluid bed as opposed to a drum roaster.

Drum roasters are more common in large commercial settings or in specialty coffee shops. They consist of a large drum that rotates and mixes the coffee as it roasts. According to Michael Sivetz who invented the fluid bed roasting method, the chaff that separates from the beans and remains in the drum carbonizes and forms potentially carcinogenic (cancer causing) chemicals. The chaff is blown out of the roasting chamber with a fluid bed system. (This theory is yet to be proven true.)

Which is better for you?

Drum Roasters

The design of a drum roaster is relatively simple and this contributes to the fact that it is the more-economical method of coffee roasting. A rotating cylindrical drum is heated from under the drum or in the center via a pipe or conduit. The heat source can be electrical or natural gas. The advanced models in use today have “profile controllers” that set temperature and timing for any given roasting profile.

This roaster heats partially by conduction of heat from the wall of the drum directly to the coffee beans (about 25%). But most of the heating is via convection of hot air and beans mixing and creating air movement within the drum (75%). A typical drum roaster measures two temperatures, the flame or electric heating element temperature and the temperature (using a thermocouple) within the roasting chamber. Here is a basic drum roaster diagram courtesy of Coffee Chemistry.

Coffee roasting with a drum coffee roaster is cost efficient and can produce large batches

Drum Coffee Roaster Diagram: credit, Probat

Drum roasters are typically very reliable and can produce vast amounts of roasted coffee in commercial settings.

Problems with drum roasters happen with extreme temperatures and uneven distribution of heat within the drum. Scorching of part of the coffee batch can be a problem. In unusual circumstances, an uneven distribution of the beans can cause tipping as well. Obviously, this could be a dangerous issue with an extremely hot machine in your home. Setting the drum to rotate too rapidly and excessively high heat settings are risky as the centrifugal force may push the beans against one side of the drum and scorch them and make the roaster unstable.

Commercial drum roasters can roast as much as 5,000 pounds in each batch. A unit designed for home coffee roasting often roasts 200 to 500 grams (up to a pound) of beans.

Fluid Bed Coffee Roasters

This type of coffee roaster has been around since the 1970s. This is a tall cylinder in which the coffee beans are heated by forced hot air entering from the bottom. The coffee beans are lifted and mixed by the constant stream of hot air allowing for a uniform distribution of heat. All of the heating in this system is from convection. People who have used both methods often prefer the roast characteristics of this method.

Nice features of a fluid bed roaster are ease of cleaning, reduced roast time (about half), and better uniformity of the roast. Although supporters of this coffee roasting method say it makes a better cup of coffee, blind comparisons (where both methods are tested side by side) fails to confirm this opinion.

(Coffee Chemistry)

Here is a diagram of a fluid bed roaster courtesy of Mt. Hood Roasters.

Coffee roasting with a fluid bed roaster often results in a "brighter" roast

Diagram of a Fluid Bed Coffee Roaster

Both roasters need to be cleaned routinely. If you do not clean out the drum, the residual chaff and coffee oils give the next batch a bitter taste. If you do not clean out the chaff collector (chaff “can” in the diagram) you will get smoke and even fires. It is not a mistake that when you read the fine print that comes with your new coffee roaster, it says to keep a fire extinguisher handy!

When learning the art and science of coffee roasting, it is not a bad thing to start by roasting a few beans in a cast iron skillet or old popcorn popper. Many find that they can better appreciate the coffee roasting process without having too much technology in the way.

Whether you decide to go with a drum roaster or the fluid bed design, read the directions carefully and make sure you are setting up correctly. These devices by their very nature generate a lot of heat. Even with a push button highly computerized roaster, keep oven mitts handy and be careful, both when loading green beans into a pre-heated roasting chamber and when removing the roasted coffee beans.

The biggest advantage of home coffee roasting with a real coffee roaster instead of the trusty popcorn popper lies in the programming. With most modern home roasters you can try out various roasting profiles and even record them for future use. Your home coffee roaster will let you learn the results of changing the temperature, timing, and even fan levels. And when you get the roast that you like, you can reliably achieve that roast again and again.

(Coffee Chemistry)

Good Coffee Roasting Deserves Good Coffee

But, don’t forget that it all starts with the coffee. Don’t spend hundreds of dollars, or thousands of dollars, on a home coffee roaster only to roast old, stale Robusta coffee beans. The best gourmet coffees brands are almost all Arabica coffees from countries all around the world. Take a little time to learn about great coffee since you are now going to the trouble, and expense, of roasting your own at home.

While it is easy to walk into any grocery store and buy roasted coffee, either whole bean or ground, you will rarely find any green coffee beans. You can find green coffee beans online, but very often the price is rather high. A local coffee roaster has green coffee beans and may be a reliable source for you. Luckily, green coffee beans retain their freshness for a lot longer than roasted coffee beans, providing that you store them in a cool and dry place. This gives you the option of buying green coffee in larger amounts and usually for a better price per pound.

If you have questions about where and how to get green coffee beans suitable for home roasting, feel free to contact us at by leaving a comment on our site or sending an email to or We can help you find suppliers in the USA and can even help you get Arabica coffee beans shipped directly from Colombia from the heart of the coffee growing “triangle.” Whenever you have a question about coffee, coffee making, or coffee roasting, let us know and we will be pleased to help.

Leave a Reply