Effects of air–drying on the shrinkage, surface temperatures and structural features of apples slabs by means of fractal analysis

Efectos del secado convectivo en el encogimiento, temperaturas superficiales y características estructurales de placas de manzana mediante análisis fractal

V. Santacruz–Vázquez^1,2, C. Santacruz–Vázquez^1,2, J. Welti–Chanes¹, R.R. Farrera–Rebollo¹, L. Alamilla–Beltrán¹, J. Chanona–Pérez¹ and G.F. Gutiérrez–López^1,*

¹ Escuela Nacional de Ciencias Biológicas—IPN. Departamento de Graduados en Alimentos. Carpió y Plan de Ayala 11340, México, D.F. * Corresponding autor. E–mail: gusfgl@gmail.com Fax: (555) 7296 000 ext. 62463

² Benemérita Universidad Autónoma de Puebla, Facultad de Ingeniería Química, Ciudad Universitaria, 72570, Puebla, Pue., México.

Recibido 13 de Julio 2001
Aceptado 4 de Abril 2008

Abstract

Shrinkage–deformation phenomenon of apple slabs during convective drying was studied, aiming to analyse its causes. Apple slabs were dehydrated in an experimental drying tunnel using a factorial design 4². Independent variables were airflow (1, 2, 3 and 4 m/s) and drying air temperature (50, 60, 10 and 80 °C). Relative shrinkage (A/Ao) was evaluated during drying as the ratio of the projected (top view) area of slab at any time during the process (A) to the initial area (Ao) of the same slab. In five punctual measuring zones on the surface of the slab, surface temperatures (ST) and moisture contents (MC) were evaluated. ST and MC were different for each measuring zone, causing irregular dehydration. Fractal analysis was used to evaluate the non–linear (A/Ao) observed pattern and ST distribution. Fractal dimensions for (A/Ao) ratios (FD_A) and ST (FD_ST) were 1.08–1.30 and 1.12–1.54 respectively. Temperatures of 60° and 10°C, and airflows of 2 and 3 m/s caused the highest irregularity of shrinkage and highest FDA and FDST values. Scanning electron microscopy images for measuring zone were captured and fractal dimension of texture of images were obtained (FD_SDBC) it was observed that nonlinear shrinkage was related to irregular microstructure given by FD_SDBC.

Keywords: shrinkage–deformation, power law, fractal dimension.

Resumen

Se describe el encogimiento–deformación de placas de manzana durante su deshidratación convectiva y se analizan sus probables causas. La muestras se deshidrataron en un túnel de secado, siguiendo un diseño factorial completo 4². Variables: velocidad (1, 2, 3 y 4m/s) y temperatura (50, 60, 10 y 80°C) del aire de secado. Se determinó el encogimiento (A/Ao) de las placas durante la deshidratación. A es el área (vista superior) proyectada de la placa a cualquier tiempo y (Ao) el área inicial. Se determinaron en 5 zonas de medición las temperaturas superficiales (ST) y contenidos de humedad (MC) durante el secado. ST y MC variaron en cada zona causando deshidratación irregular, relacionándose ésto con la no linealidad del encogimiento. Con análisis fractal se evaluó el comportamiento de (A/Ao) y de ST, obteniéndose valores para la dimensión fractal de la relación de áreas (DF_A) y de la distribución de temperaturas superficiales (DF_ST) entre 1.08–1.30 y 1.12–1.54 respectivamente. Mayores irregularidades del encogimiento del área y altos valores de DF_A y DF_ST se observaron a 60 y 10°C y 2 y 3m/s. Se obtuvieron fotomicrografías de microscopía electrónica de barrido de las zonas seleccionadas y su dimensión fractal de textura (FD_SDBC). La no linealidad del encogimiento fue relacionada con la microestructura irregular descrita por FD_SDBC.

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Acknowledgments

We acknowledge the financial support from the PROMEP–SEP program, CONACYT and ENCB–IPN Mexico.

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