Influence of process control strategies on drying kinetics, colour and shrinkage of air dried apples

• Development of quality attributes significantly depends on control strategies.
• Online measurements show that information can be used for process control optimisation.
• Product temperature controlled drying is gentler at comparable drying times.

The influence of two different control strategies, constant air temperature and constant product temperature, on product quality and drying behaviour of apples was investigated. The interactions of the drying parameters air temperature (35–100 °C), product temperature (35–85 °C), dew point temperature (5–30 °C) and air velocity (2.0–4.8 m/s) with drying time, colour changes and shrinkage were measured continuously and determined for both strategies. Based on these results the two strategies were compared with regards to their effect on drying performance.The results show that the drying strategy has a significant influence not only on the development and duration of the drying process but also on the development of colour changes and shrinkage. Controlling product temperature led to shorter drying times and generally lower colour changes. Furthermore, it was shown that the product temperature develops characteristically; two stages and a clearly visible transition period can be detected. This potentially can be used to control the process and, therefore, improve its performance regarding duration and quality aspects.