## 1 Introduction  
The mode of heat transfer in coffee bean processing has a considerable impact on the taste of brewed coffee [1; 2]. In the coffee roasting process, heat can be transferred to coffee beans via three methods: convection, conduction, or radiation [3]. The most important variables for each type of roasting are the type of roaster, the temperature of the coffee beans, and the duration of the roasting process [4]. Convection heat transfer is based on hot air rising from warm surfaces towards coffee beans, whereas conduction heat is transferred via direct contact of the coffee beans with the warm surfaces of the rotating drum. Roasters based only on conduction heat do not ensure a homogeneous temperature distribution in the coffee beans due to their non-homogeneous contact with the drum surface in the roasting process. Heating via radiation is based on the emission of light and heat waves that penetrate the coffee bean; hence, very little energy is lost during the process, rendering this heating method the most efficient. A problem in most roasters is the instability of the roasting process [3]. In fluidized-bed roasters, coffee beans \"levitate\" in a stream of hot gas, and the air velocity is difficult to control or cannot be controlled. Even in the most popular drum roasters, which lift and pass coffee beans through hot air, adjustment to one variable may cause large fluctuations in other parameters [5]. Convection-conduction-radiation (CCR) roasters differ from other roasting machines in the presence of burners with low nitrogen oxide emissions, which ensures very clean combustion. These roasters have over 50% higher efficiency than blue-flame burners, which substantially reduces energy costs and ensures much cleaner roasting. The CCR roaster method combines the most effective technologies, being based on several types of heating process. Heat exchangers provide the roasting process with clean hot air drawn via convection [6]. After the flame is turned off, the exchangers ensure a stable air temperature for 4-6 min. The carbon steel drum first radiates heat, which is dependent on the temperature of the metal and the conductivity coefficients of the coffee beans and the metal with which they come in contact. Next, the conductive heat acts via direct contact with the coffee beans. The CCR roaster operator can change the airflow rate by modifying the drum energy and temperature during the subsequent stages of the roasting process and depending on the size of the coffee bean batch. Therefore, CCR roasters can roast 250 g of coffee with the same precision as a full 90 kg batch. The independent control of convection and conduction heat and the precise control of energy transfer contribute to the maximization of the capacity of roasting profiles [7]. In contrast to CCR roasters, which do not use contaminated combustion gases as energy carriers [8], the method with combustion gases is still being used in convection-conduction (CC) devices, as in most drum roasters. However, in the coffee roasting process, such heat exchange has a considerable disadvantage. The ambient air is usually contaminated and saturates the coffee beans with nitrogen oxides, carbon oxides, carbon dioxide, and other gases that can make the taste of the coffee impure and less palatable [9; 10]. Gas burners, which do not evenly distribute the heat around the drum leading to hot and cold spots, are still used in CC roasters. They do not ensure precise control of the airflow through the coffee bean-containing drum during the roasting process. This may result in various types of defect in the roasting process. Several defects that may occur during the coffee roasting process yield baked, underdeveloped, overdeveloped, and scorched coffee beans [11; 12; 13]. The baked coffee defect occurs when coffee beans are heated too long at an insufficiently high air temperature without reaching the first crack.