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Porosity and the Effect of Structural Changes on the Mechanical Glass Transition Temperature

Kasapis, S., Sablani, S.S., Rahman, M.S., Al-Marhoobi, I.M., Al-Amri, I.S.
Journal of agricultural and food chemistry 2007 v.55 no.6 pp. 2459-2466
apples, dried fruit, glass transition temperature, porosity, mechanical properties, drying, food processing, thermal analysis
Continuing an investigation on the fundamentals and applications of a recently proposed concept, i.e., the mechanical or network glass transition temperature, we now report data on the macrostructural changes in dehydrated apple tissue in relation to apparent porosity. Care was taken to keep the moisture content of the matrix constant (approximately or equal to 81%) while the volume fraction of total pores ranged from 0.38 to 0.79. Reproducible mechanical profiles identified the first derivative of shear storage modulus as a function of temperature to be the appropriate indicator of the mechanical Tg at the conjunction of the William-Landel-Ferry/free volume theory and the modified Arrhenius equation. Information on the microstructural characteristics and morphology of porous apple preparations was also made available via modulated differential scanning calorimetry and scanning electron microscopy. The work reveals and discusses discrepancies in the Tg-porosity relationship obtained from calorimetry and mechanical analysis attributable to the different extent to which the two techniques respond to degrees of molecular mobility.