Microstructure elements affect the mass transfer in foods: The case of convective drying and rehydration of pumpkin

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Rojas M.L.
Augusto P.E.D.
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Academic Press
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Drying is typically evaluated considering a pure diffusional process in an isotropic sample. However, biological materials are anisotropic, and their structural elements can affect the water flow during processing. In this work, the effect of microstructure elements on sample deformation, drying and rehydration processes (kinetics and mechanisms) was evaluated, using pumpkin cylinders (formed mainly of parenchyma and xylem tissues). The cylinders were obtained considering a transversal (Tc) and longitudinal (Lc) cut orientation. The orientation of the xylem vessels did not affect the drying kinetics (both orientations showed similar k and parameters of Page model). However, it had a significant impact on deformation. The Lc-samples showed more uniform appearance than the Tc ones. Tc showed a tendency towards black in the grey intensity histogram and higher density in the X-ray analysis. The Lc-samples presented the highest rehydration rate (k1 of Peleg model: 2.87 ± 0.16 min d b−1). In contrast, higher equilibrium moisture was observed in samples with Tc (k2 of Peleg model: 0.078 ± 0.003 d b−1). The results showed the anisotropicity of biological materials, and how the microstructural elements affect the mass transfer. Additionally, a water transport mechanism by capillarity through the xylem vessels during drying and hydration was demonstrate
The authors are grateful to the São Paulo Research Foundation (FAPESP, Brazil) for funding the project nº 2016/18052-5; to the National Council for Scientific and Technological Development (CNPq, Brazil) for funding the project nº 401004/2014-7; and to Cienciactiva for the M.L. Rojas Ph.D. scholarship (CONCYTEC, Peru; Contract 087-2016-FONDECYT), from the ‘‘Consejo Nacional de Ciencia, Tecnología e Innovación Tecnológica”. The authors are also grateful to the “Laboratório de Análise de Imagens” (LPV-ESALQ/USP) for the support and facilities for X-ray analysis.
Palabras clave
Xylem vessels, Biological materials, Cylinders (shapes), Deformation, Flow of water, Hydraulics, Mass transfer, Microstructure, Shrinkage, Thermal processing (foods), X ray analysis, Equilibrium moisture, Food drying, Intensity histograms, Kinetics and mechanism, Microstructural elements, Microstructure elements, Water flows, Drying