Rice flour films and rice starch films plasticized with the same
concentration of glycerol presented similar equilibrium moisture
(Fig. 2a). Films plasticized with sorbitol presented lower equilibrium
moistures when compared with the films plasticized with
glycerol. This behavior has been observed by other authors (Mali
et al., 2005; Mu¨ller et al., 2008; among others). According to these
authors, glycerol and sorbitol have similar structures because they
are both straight-chain molecules. However, the glycerol molecule
presents higher water affinity, demonstrated by absorption
isotherms reported in the literature (Leung, 1986). Moreover, since
sorbitol is more similar to the molecular structure of glucose units
than glycerol, the chances of sorbitol to interact with polymeric
starch chains are higher, thus sorbitol-containing films present
higher intermolecular forces and show lower capacity to interact
with water. Films prepared with rice starch and rice flour presented
similar water sorption isotherms, even for different sorbitol
concentrations (0.20 or 0.30 g/g raw material). Data presented in
Table 1 showed that plasticizer content did not have a significant
influence on the monolayer water content (m0) for each kind of
film. Rice starch films plasticized with glycerol gave the highest
values for monolayer moisture. The parameter k is the corrective
constant taking into account properties of multilayer molecules.