BINDING OF VOLATILES TO EXTRUDED STARCH AT LOW WATER CONTENTS

The binding of volatiles to extruded wheat starch samples in the glass y and rubbery states was studied to determine whether there were subst antial differences in binding behaviour between the two states. Theory suggests a significant change in the mobility of some molecules betwe en the rubbery and glassy states which may affect the uptake of volati les. Extruded samples containing 19-43% water were prepared and charac terized by DSC and X-ray diffraction to ensure that samples were compa rable in terms of starch gelatinization and had not retrograded. The p hysical state of samples (glassy or rubbery) was determined by calcula tion from the Couchman-Karasz equation. Uptake of volatiles was follow ed by measuring headspace concentration over time using automated samp ling, which reduced variability to 5%. Volatile uptake was followed fo r periods ranging from 2 to 50 hours in starch samples of different wa ter contents in the glassy and rubbery states, with a range of surface areas and also in starch/sucrose extrudates. The binding curves showe d rapid initial uptake of diacetyl followed by a further, slower uptak e. Heptane and 3-methylbutanal were bound to a lesser extent. The glas sy state samples bound diacetyl more slowly than the rubbery samples a lthough there was no available water (as measured by DSC) in either th e glassy (19% water) or rubbery (26% water) samples. This suggests tha t the physical nature of the glassy state, as well as water content, h as an effect on volatile binding. (C) 1998 John Wiley & Sons, Ltd.