Five proportions of WPI/PUL were chosen to study the
effect of pullulan addition on the moisture content (MC)
and the solubility of WPI-based edible films (Table 4).
The control film had the lowest MC (16.48 ± 1.5%). The
MC increased with increased content of PUL and reached
its maximum when the proportion of WPI/PUL was 1. The
results showed that MC of WPI-based films differed significantly
(p < 0.05) with change of PUL content in the mixture.
The reason why pullulan is more readily
disintegrated by absorbing water and having poorer barrier
properties could be its hygroscopic property and its high
hydrophilicity.
The results showed that the control film was not dispersed
after 24 h immersion in water and had no visual loss
in integrity. This was related to the relatively low solubility
value (50.58 ± 4.8%) when no PUL was added and reached
the highest value (97.98 ± 2.2%) at WPI/PUL = 1. The relatively
low solubility of WPI films in water has been reported (Fairley et al., 1996; McHugh & Krochta, 1994b).
Low solubility could be explained by the high interaction
density and presence of intermolecular disulfide bonds as
a result of the heat treatment (McHugh et al., 1993;
McHugh & Krochta, 1994b). Perez-Gago et al. (1999) confirmed
that native whey protein produces water-soluble
films, but the heat-denatured solution of WPI produces
films whose protein is insoluble.