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One of the most fundamental variables to understand about roasting is that the heat source and transfer in the roasting machine has a lot to do not only with how the final product tastes, but also how that taste is controlled and achieved over the course of the green-to-brown process. There are three basic ways that heat can be transferred to the seeds in order to create the sugar-browning and caramelization reac- tions that allow coffee's flavor to develop and its compounds to become more water soluble: radiation, conduction, and convection. Radiation is when heat is transferred indirectly—like a tray of fast-food burgers sitting under a heat lamp. The size and turning-speed of a roaster's drum will influence its radiant effects, as well as the materials out of which the machine is made. Conduction is the form of heat trans- ferred by direct contact, like the sizzle on the side of a steak currently being seared in a skillet. Any time a bean actually touches the hot surface of a roaster, it accepts conductive heat, again relative to the drum speed, size, and temperature, as well as the batch size inside the drum. Rounding out the list, convection is the transfer of heat through a fluid medium, such as air or water. Popcorn poppers, which are actually relatively common home-hack coffee roasters, rely on con- vection. (The difference between a convection oven and a conventional oven, then, is that while a conventional oven heats the air around the food, a convection oven has a fan that deliberately moves the heated air within the oven, distributing the heat faster.) Most commercial coffee roasters are either designed with station- ary fluid-bed chambers, or large rotating drums; the former relies largely on convective heat, while the latter combines all three trans- ferences during the process of the roast. In every case—and in every roaster—there are myriad variables the person operating the machine not only needs to consider, but also needs to manipulate when trying to achieve that perfect shade and perfect profile. When Ian Picco, roastmaster for Topeca Coffee in Tulsa, Okla., put his first batches into the company's drums four-plus years ago, he didn't know much about the details. "When I started, I under- stood the fundamentals of what was going on, but we were small, and we didn't have a great lab or protocol," he says. "We didn't really cup and look back the next day at the roast and see how the differ- ent variables affect the cup quality. It was really hard to know what effects airflow was having on the coffee, or the external factors like humidity and temperature were having on green, or even the green itself changing throughout the year. It was kind of like roasting blind or roasting in the dark." Once Topeca started to grow both in vision and in size, Ian set out to learn as much as he could about every aspect and every influence that affected his work. "I started going to Roasters Guild events and learning more, and that really got the ball rolling on understanding the different variables and the science and the ther- modynamics. I really started to pay attention more to airflow, and roasting curve, and different beans, and roasting different ways to achieve different effects—like roasting coffee for a drip versus roasting for espresso," he says. For Jon Ferguson, however, much of his understanding about those variables flew out the window when he stepped out from behind a drum roaster and took his place at an air roaster instead. "It was somewhat of a challenge to just jump right into air roasting," says Jon, who's currently the director of quality control and green buying for the Roasterie in Kansas City, Mo., after years of experience on the floors at Dogwood Coffee, Collectivo Coffee, and Zoka. "You can't bring all of your history, or I guess even preconceived notions about air roasting—it's not the same." Jon says that the Roasterie prefers air roasters to maintain consis- tent distribution of temperature over the beans: Because the batch is in constant motion through the air bed, "there's very little chance of scorching or tipping, because it doesn't have a lot of contact with direct heat or hot surfaces. There's this exceptional amount of even distribution of heat throughout the roasting process, so you have a lot more consistency." ? In addition to the way the beans accept the heat, the source of the heat itself can have a big impact on the techniques a roaster needs to master. While most commercial roasters are gas-powered, the team at Mr. Espresso in Oakland, Calif., have spent the past 30-plus years mastering a more primordial heat: wood fire. John Di Ruocco, the family-run com- pany's green-coffee buyer and quali- 74 barista magazine