The influence of freeze drying conditions on microstructural changes of food products

AbstractThe drying of food products can result in significant changes in the chemical composition, morphology and physical properties of foods and can lead to stabilized products with longer shelf-life and easier commercialization. Information on porous structure of foods is very important, characterizing the quality and texture of dehydrated food products. Therefore, structural properties, such as porosity, bulk density and true density of freeze-dried food products were investigated as affected by process conditions. Rice kernels were boiled for different time periods and agricultural products, including potato, mushroom and strawberry, were cut into cubes. The samples were frozen, tempered in liquid N2 and freeze-dried, under various vacuum conditions, using a laboratory freeze-dryer. True density of the products was measured using a helium stereo-pycnometer. Bulk density was obtained by measuring the dimensions of the samples with a Vernier caliper. Simple mathematical models were developed in order to correlate the structural properties with process conditions. The microstructure of food products was also analyzed by Scanning Electron Microscopy and image analysis. Bulk density of freeze-dried materials increased with the applied pressure during freeze-drying, while porosity decreased. In addition, bulk density of freeze-dried rice kernels decreased with the increment of boiling time, while porosity increased. The changes in bulk density and porosity were closely supported by microstructural observations, according to SEM images. The microstructural changes of products, freeze-dried under various vacuum conditions, can be predicted using the proposed models and image analysis