Binding of volatiles to extruded starch at low water contents

Abstract

The binding of volatiles to extruded wheat starch samples in the glassy and rubbery states was studied to determine whether there were substantial differences in binding behaviour between the two states. Theory suggests a significant change in the mobility of some molecules between the rubbery and glassy states which may affect the uptake of volatiles. Extruded samples containing 19–43% water were prepared and characterized by DSC and X-ray diffraction to ensure that samples were comparable in terms of starch gelatinization and had not retrograded. The physical state of samples (glassy or rubbery) was determined by calculation from the Couchman–Karasz equation. Uptake of volatiles was followed by measuring headspace concentration over time using automated sampling, which reduced variability to 5%. Volatile uptake was followed for periods ranging from 2 to 50 hours in starch samples of different water contents in the glassy and rubbery states, with a range of surface areas and also in starch/sucrose extrudates. The binding curves showed rapid initial uptake of diacetyl followed by a further, slower uptake. Heptane and 3-methylbutanal were bound to a lesser extent. The glassy state samples bound diacetyl more slowly than the rubbery samples although there was no available water (as measured by DSC) in either the glassy (19% water) or rubbery (26% water) samples. This suggests that the physical nature of the glassy state, as well as water content, has an effect on volatile binding. © 1998 John Wiley & Sons, Ltd.