Drying at Sub-zero Temperatures: Case Study on In-store Drying of Grains

Abstract

Drying using ambient subzero temperatures is of potential interest for thermosensitive products. Existing theoretical drying models have been used to predict the response of the system to different aeration systems. The model is based on enthalpy balances and includes water freezing and deposition of water on the surface of the commodity. It uses thermophysical properties of the commodity (i.e., maize in this study) and ambient weather data collected from northeastern China. Water within the grain is modelled as bound, free or frozen. The physical state of water under subzero temperatures has been investigated using a differential scanning calorimeter and nuclear magnetic resonance spectrometry. It has been established that the quantity of bound water was around 17%. Thermophysical properties characterizing the drying behavior of maize kernels cv. Huangmo 417, the most common variety grown in northeastern China, were determined under a wide range of moisture contents and drying temperatures. Those were: particle and bulk density, porosity, thermal conductivity, specific heat, thin layer drying, and sorption isotherms. It could be established that the thermal conductivity and specific heat were strongly dependent on temperature and relative humidity and that the sorption isotherms followed the 5-term Guggenheim-Anderson-de-Boer model. The industrial-scale in-store drying experiments in northeastern China have demonstrated the feasibility of in-store drying under subzero conditions. Advantages in terms of reduced susceptibility of maize to mould formation have been established, resulting in improved quality and financial returns to the processor.