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Procedia Food Science

Procedia – Food Science 00 (2011) 000–000

Procedia Food Science 1 (2011) 647 – 654

11th International Congress on Engineering and Food (ICEF11)

The influence of freeze drying conditions on microstructural changes of food products
VasilikiP. Oikonomopouloua*, Magdalini K. Krokidaa, Vaios T. Karathanosb
a Department of Chemical Engineering, National Technical University of Athens, Zografou Campus, Athens, 15780, Greece e-mail:, b.Department of Nutrition, Harokopio University, Kallithea, Athens, 17671, Greece e-mail:

Abstract The drying of food products can result insignificant 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 offreeze-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 ahelium 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 theapplied 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, canbe predicted using the proposed models and image analysis.
© 2011 Published by Elsevier B.V. Selection and/or peer-review underpeer-review of 11th International Congress ICEF11 © 2011 Published by Elsevier Ltd. Selection and/or responsibility under responsibility of on Engineering and Food (ICEF 11) Executive Committee. Executive Committee Members.
Keywords: agricultural products; freeze drying;rice; SEM; structure

* Corresponding author. Tel.: +0030 7723149; fax: +0030 7723155. E-mail address:

2211–601X © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of 11th International Congress on Engineering and Food (ICEF 11) Executive Committee. doi:10.1016/j.profoo.2011.09.097


Vasiliki P. Oikonomopoulou et al. FoodScience 00 (2011) 000–000 647 – 654 Author name / Procedia – / Procedia Food Science 1 (2011)

1. Introduction Dehydration operations are widely used for the preservation of foods, since the removal of water minimizes the microbial spoilage and prevents the physical and chemical reactions of the foods’ compounds during storage [1, 2]. Dehydrated food products are easily obtained and maintain thecharacteristics of natural products [3]. Drying procedure comprises of simultaneous heat and mass transfer, which cause significant changes in the physical and chemical composition as well as in the structure of food products, depending on the transport mechanisms applied. Therefore, the microstructure and morphology of foods and as a result the quality of the final product, are related to the...