A Comprehensive Study on Thermophysical Material Properties for an Innovative Coffee Drying Process

In the present work a series of physico-chemical and transport properties of coffee extract were determined that are required for developing a new process of drying instant coffee solutions. The influence of temperature and dry matter content on density, viscosity, thermal conductivity, heat capacity, and surface tension is shown systematically bringing light to the contradictory literature data, especially in the case of heat capacity. To specify feasible conditions of liquid feeding in the drying process, the freezing point of the solution was analyzed as a function of dry matter content. Further, the surface free energy, surface roughness, and thermal conductivity of some belt materials were investigated. In addition, the wettability of these materials was determined applying coffee solutions at different dry matter contents. Due to the presence of surface active components, the wettability was found to depend on the drop age revealing a spreading kinetic. As a consequence, dynamic and static measurements result in different contact angle values.