Chemical Reactions in Roasting

Drying, Browning & Caramelization

 “At the start of your roast, the main reaction occurring is drying. You’re drying the free moisture out before you can start a lot of those chemical reactions that create flavors,” Ibby says.

Once most of the moisture has been dried out of the bean – typically about halfway through the roast time, Ibby says – you’ll start getting browning. When the browning starts, the roasting process goes through the main reaction stages. “Then, it’s a balance to get the coffee to a good, stable place where the desirable chemicals are preserved and not broken down.”

Joseph explains that “the main three reaction stages happening during roasting are the drying, browning (Maillard reaction, caramelization) and Development (from first crack.)” Maillard is the chemical reaction between amino acids and reducing sugars and is where many of the main flavor compounds are formed.  First crack is where the steam build-up is released and the main development time starts.

Effects on Flavor

How long a roast stays in each reaction stage impacts the end flavor result. “The Maillard reaction is where many of the main flavor compounds are formed. A shorter Maillard reaction generally leads to a sweeter cup profile, and a longer time can lead to a more bitter profile as the volatile compounds creating flavor gradually get burnt off,” Joseph says.

Similarly, the caramelization stage (which occurs at the same time as the Maillard stage) also affects sweetness, but in a different way. Like in candy-making, caramelization and sugar browning is the start of the reduction of perceived sweetness. If taken too far, the sugars become burnt and acrid. If too many of the dissolvable sugars are burned off, the cup will have much less sweetness and will have unpleasant, bitter or off-flavors from the burned sugars.

First crack typically marks the start of the development time, the most important final stage for the creation, development and refining of the main flavor compounds we taste in coffee. Too short a development time and you have a flatter, under-developed, less flavorful coffee. Too long and you risk ending up with burnt, bitter or baked flavors, or often all three.

At the end of the day, though, “it really depends on the coffee you’re roasting, too. No matter how hard you try, you’re not going to get some super floral and red berry flavors out of a low-grade stock lot from a region where those aren’t common notes,” Joseph points out.

Rate of Rise

Rate of rise (the rise of temperature at regular intervals along a roast curve) also impacts the end flavor. The general rule of rate of rise, Ibby says, is that “your bean temperature should always be increasing, but the rate of rise should be decreasing.” In other words, while the temperature during the roast continues to rise throughout the entire roast, the speed at which it is increasing should slow over time.

“One of the biggest causes of a baked, bready flavor in coffee is that during development time, the rate of rise starts to increase again. There’s too much heat exposure, and that ruins all the delicate flavors,” Joseph says.

This is one of the reasons that temperature probes and roast logging software is so helpful. “Usually in a roast, you’re tracking bean temperature, air temperature and then the acceleration/deceleration of each of those temperatures,” Ibby says.  “It’s crucial for a good roaster to have logging software that can help you track these.”

Second Crack and Specialty Coffee

In Specialty roasting, most coffees will be dropped (removed from the roaster) before reaching second crack. But what is second crack, how does it affect coffee flavor and what can we learn from it? Joseph and Ibby have thoughts about that, too.

“The technical term is pyrolytic reaction,” Joseph says. “It happens from about 220 degrees Celsius, and it’s the heat-induced chemical reaction that leads to the release of CO2 from the bean.”

The resulting flavors are very bitter and roasty, often with notes of charcoal and dark chocolate, depending on how far past second crack the roast is taken. Through this process, the coffee loses body and also weight as the bean becomes more brittle and soluble.

Roast Method and Extractions

Roast degree also affects compound solubility and, in turn, impacts the best brewing method for the coffee. “In an espresso, the water is being pushed through the coffee under extreme pressure and is being forced through the coffee over a shorter period of time compared with a filter brew method, so you want the coffee to be more soluble,” Joseph says. This is why espresso blends tend to be darker, because a darker roast makes the bean and resulting flavor compounds more soluble (less bound within the structures of the bean).

With a pour over or drip coffee, “the water is in contact with the coffee particles for a much longer time. A lighter roast will be less soluble and will gradually extract overtime,” Joseph says. If you use a darker roast with a drip method, you’ll often get an over-extracted, bitter taste due to the higher solubility of the compounds in a darker roast. Conversely, use a lighter roast in an espresso machine and, in most cases, you’ll get a thin, sour and under-extracted-tasting espresso with a lack of sweetness and body.

There’s still a lot the industry doesn’t know about chemical reactions in coffee roasting, so the most important thing is to keep the general principles in mind and go from there. “You’ve just got to really trust your own knowledge and experience. Make sure you’re following the general principles and trust your own palate to identify where changes need to be made,” Joseph says. “Cup, cup and cup some more.”