For films prepared with the same material (rice starch or rice
flour), the values of Kw were increased by about 50–80% when the
glycerol changed from 0.20 to 0.30 g glycerol/g rawmaterial. Values
of b1 (related to aw1 ¼0.75) and b2 (related to aw2 ¼ 0.02) for all the
film samples are also presented in Table 2. The driving force that
causes water transfer in the film is given by the side-to-side
moisture difference, i.e., ps(b1 aw1 b2 aw2), given in g water/g
dry solid. The comparison between the values of b1 aw1 and
b2 aw2 shows that the higher RH determines the driving force that
causes mass transfer through the film. Inside the diffusion cell,
where RH ¼ 0.02, the films’ surface moistures ranged from about
0.025 to 0.065 g water/g dry solid, while outside the cell, where
RH ¼ 75%, the film’s surface moistures ranged from about 0.38 to
0.89 gwater/g dry solid. The comparison of samples SG20 and SG30
data shows that the increase of Kw was caused partially by the
increase of the diffusion coefficient, Dw (that depends on the film
structure), but mainly by the increase of the solubility coefficient b1
(that depends on the film hygroscopicity). The same reasoning can
be used to explain the Kw increase for samples FG20 and FG30