Scanning Electron Microscopic Study of Microstructure of Gala Apples During Hot Air Drying

Abstract

Microscopic changes that occur in plant food materials during drying significantly influence the macroscopic properties and quality factors of the dried food materials. It is critical to study the microstructure to understand the underlying cellular mechanisms to improve the performance of food drying techniques. However, there is limited research on such microstructural changes of plant food material during drying. In this work, Gala apple parenchyma tissue samples were studied using a scanning electron microscope for gradual microstructural changes as affected by temperature, time, and moisture content during hot air drying at two drying temperatures: 57 and 70°C. For fresh samples, the average cellular parameter values were as follows: cell area, 20,000 µm²; ferret diameter, 160 µm; perimeter, 600 µm; roundness, 0.76; elongation, 1.45; and compactness, 0.84. During drying, a higher degree of cell shrinkage was observed with cell wall warping and an increase in intercellular space. However, no significant cell wall breakage was observed. The overall reductions in cell area, ferret diameter, and perimeter were about 60, 40, and 30%. The cell roundness and elongation showed overall increments of about 5% and the compactness remained unchanged. Throughout the drying cycle, cellular deformations were mainly influenced by the moisture content. During the initial and intermediate stages of drying, cellular deformations were also positively influenced by the drying temperature and the effect was reversed at the final stages of drying, which provides clues regarding case hardening of the material.

Keywords:

• Cell deformations
• Food drying
• Microstructure
• Plant cells
• Scanning electron microscope
• Shrinkage

ACKNOWLEDGMENTS

This research was conducted at the experimental facilities of the Queensland University of Technology (QUT), Brisbane, Australia. The support given by the Faculty of Engineering, University of Ruhuna, Sri Lanka, is also gratefully acknowledged.

Notes

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4πA/p ².

Major axis length/minor axis length.

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^a SD = Standard deviation as a percentage of the corresponding parameter average value.