THE EFFECT OF POROSITY ON GLASS TRANSITION MEASUREMENT1

Abstract

ABSTRACT Texture is considered a key quality parameter determining the acceptability of many porous foods. Dynamic mechanical thermal analysis (DMTA) is commonly employed in food and polymer science research as a means of measuring the glass transition (Tg) of materials. In the field of food science, Tg measurements have been used to characterize the textural changes of food materials. However the effect of sample porosity and different sample preparation techniques on Tg measurement using a DMTA have not been investigated. The objectives of this study were to compare Tg measurements of identical materials varying only with porosity and to examine the effect of sample preparation technique on Tg measurements. Both starch based extrudates and synthetic polymers with different porosities were tested with DMTA. For comparison, representative samples from the porous specimens were obtained and subjected to one of three methods of Tg determination: 1) ground into a powder and compressed into tablets for testing with DMTA, 2) ground into a powder and tested by differential scanning calorimetry (DSC), and 3) ground into a powder and tested by controlled strain rheometry. Porosity was observed to affect the measurement of the glass transition using DMTA. The extrudate samples containing the lowest porosity exhibited a Tg range 22°C lower than the extrudate samples with the highest porosity. Also, the polymer samples with the lowest porosity showed a Tg range 20°C lower than the polymer samples with the highest porosity. The tablets (no pores) did not show any significant difference in Tg as measured by DMTA nor did the ground samples (no pores) measured by DSC and controlled strain rheometry. This study shows that porosity of the sample affects Tg measurement. Care should be taken when reporting Tg values as porosity may introduce an artifact in Tg measurements using DMTA.