Porosity evolution in food


A number of factors including quality of raw material, internal microstructure, the method of preparation, pre-processing treatments and drying conditions affect the quality of dried food. The structure of the food materials goes through deformations due to the simultaneous effect of heat and mass transfer during the drying process. Pore formation and evolution lead to changes in many quality attributes and directly affects physical properties including the mass diffusion coefficient, thermal conductivity, thermal diffusivity, and microstructure. A considerable amount of research work has been done to investigate the characteristics of pore during drying. However, the research up to now has examined porosity formation during traditional drying methods like convective drying and freeze-drying. Microwave assisted convective drying is an advanced area of drying and research shows that intermittent microwave-convective drying (IMCD) increases both energy efficiency and product quality. However, researchers have not investigated pore formation and evolution in IMCD yet.

Plant-based food materials are complex in nature as these have heterogeneous, amorphous, hygroscopic and porous properties. During drying, water from hygroscopic porous media is migrated in different phases (e.g. liquid water and vapor). Only multiphase transport models consider transport of liquid water, vapor, and the air inside the food materials separately and, therefore, are more realistic. However, currently, there is no multiphase model for the IMCD process.
This research, therefore, first develops a pore formation and evolution model and investigates the relationship between the porosity and mechanical and other properties of food. The model takes both process parameters (e.g. drying temperature) and sample properties (e.g. glass transition temperature) into consideration. The model can predict dynamically the changes in porosity, shrinkage and case hardening of food samples during drying. A multiphase model for IMWC drying has been developed taking porosity evolution and shrinkage over the course of drying into account. In this model, solid matrix, liquid water and gas (water vapor and air) phases, pressure-driven flow as well as capillary diffusion, binary diffusion, and evaporation are considered.

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