Thermodynamically speaking, heating the beans past 100 C (or less at altitude), causes a vaporization (phase change from liquid to gas) of the water in the bean. Vaporization requires a significant amount of energy, and until all of the water has vaporized, the bean remains at 100 C. The amount of energy required to bring the beans to this temperature depends on the starting temperature of the bean, its percent moisture, and the atmospheric pressure.
Following vaporization, the specific pressure of the newly formed water vapor inside the bean increases until what is called “First Crack” occurs, where the pressure exceeds the internal structural strength of the now dry bean. This first crack is only an audible indicator that the beans have reached ~195 C, not a violent or instantaneous chemical reaction. The temperature of the first crack depends on roasting altitude (ambient confining barometric pressure within the bean), internal moisture content of the green bean, and quality of homogenous heat transfer from the air to the bean.
During the initial heat up process (before the bean reaches 100 C) the chemical transformations of unique compounds found in coffee are starting to take place, but it is only after exceeding 100 C that the reaction rates start to accelerate exponentially. The duration of time to reach 100 C, complete vaporization, and rate of temperature climb following 100 C, all play a part in the chemical makeup of the bean at the end of the roast. Sugars and fats break down in varying concentrations, amino acids and sugars form components of compounds that react with each other, and degradation products spark chain reactions. The end result is the fresh formation of new compounds, and reduced concentrations of the original compounds that existed prior to the roast. Those compounds include aldehydes, ketones, furans, pyrazines, pyridines, phenolic compounds, indoles, lactones, esters and benzothiazines. Below are some of the more important chemical reactions that take place during the heat induced reaction, the varying concentrations of these compounds form the main differences in the taste and texture found in different roasts.
- 5-O-Caffeoylquinic acid -> Lactone
- Lactone -> Hydroxyhydroquinone + 2-Furfurylthiol -> 4-[(2-Furylmethyl)sulfanyl]-hydroxyhydroquinone
- Lactone -> Phenylindanes
The color of the bean (determined by the final temperature of the bean when it is dumped out of the roaster) is an indication of the duration and extent of the Maillard reaction which has occurred during the roasting process. The Maillard reaction is a common heat induced chemical reaction that causes foods to brown when heated, as a result of the change in chemical composition of various sugars and proteins.
Now that we understand what drives the properties of the bean after it has been roasted, we can start to play with the variables in the equation until you get your fully customized personal favorite roast.
Scientifically this is pretty straight forward. The adjustment of these variables will cause subtle, but real, changes in the proportions of those flavors and aroma compounds we love so much in coffee!
We have taken a baseline for the following variables as defined by the chart below from a roast that we like, and then allow you to adjust many of the variables up or down, within a reasonable range, and our calculator will predict the total heat applied and retention time to forecast the temperature of the bean at the end of the roast. This will give you a prediction of the darkness of the roast you are likely to end up with.
Carbon Dioxide gas is produced as one of the products of the various chemical reactions included in the roasting process. This gas is present and naturally emanates from the roasted bean for two to three days following the roasting event. The gas naturally helps to slow the oxidation and degradation of the newly formed organic compounds. This off-gassing and oxidation is discussed in further detail under the Brewing Science section.