Adding sucrose lowers the average molecular weight of the
blend, thus restricting the visco-elastic function of the starch. This
was found to result in less die-swelling and thus a lower average
pellet diameter. It is conceivable that higher sucrose quantities
disrupt the gelation behavior of macromolecular starch chains.
Sucrose might have an opposite effect as water (Spies and Hoseney,
1982), limiting the disentanglement of polymer chains, thus
limiting the transition from the brittle to rubbery state. As already
mentioned above, DSC measurements show that corn starch
matrix systems are able to dissolve around 25% sucrose. That the
sucrose remnants appear crystalline after the extrusion process
can confirm this theory.
The extent of the die-swelling phenomenon can be quantified
using the volumetric expansion index (VEI) (Alvarez-Martinez
et al., 1988) (Table 4). This parameter combines calculated
sectional expansion index (SEI) and longitudinal expansion index
(LEI) of the extruded materials (see Eqs. (6)–(8)).