Applying quality-by-design to develop a coffee freeze-drying process

• A coffee freeze-drying cycle has been designed taking into account the energy utilization efficiency.
• A mathematical model has been used to calculate the design space of the process.
• Experimental investigation has been carried out to determine model parameters.
• The model has been used to carry out the exergy analysis of the process.
• The operating conditions that allow minimizing the exergy losses have been identified.

This paper aims at investigating the design of a freeze-drying cycle taking into account the energy utilization efficiency. A coffee freeze-drying process is considered as case study. The analysis is focused on the primary drying, as this stage accounts for most of the energy consumption. A simplified mathematical model is used to calculate the design space of the process, with the goal to point out the operating conditions (temperature of the heating shelf and pressure in the drying chamber) that allow satisfying the constraints of the process. Experimental investigation is required to determine model parameters, namely the coefficient of heat transfer to the product, and the resistance of the dried cake to vapor flow. The same model, coupled with equations describing the dynamics of the freeze-dryer, is used to carry out the exergy analysis of the process, thus pointing out the operating conditions that allow minimizing the exergy losses and maximizing the exergy efficiency.