Formulations based on polysaccharides and proteins have proved
their excellent selective permeability to O2 and CO2. However, because
of their hydrophilic nature they exhibit poor moisture barrier properties,
which can be improved by adding hydrophobicmaterials such as natural
waxes, acetylated monoglycerides and surfactants through emulsion or
lamination technology. That is why edible films usually are heterogeneous
in nature.
Literature data indicate that O2, CO2 and water vapor permeability
of edible coatings are lower than the conventional plastic films. Starch
films ability for food protection is by controlling and reducing oxygen
transport, thus extending the shelf life of the food.
However, the incorporation of antimicrobial agents to starch films increases
O2 gas permeability thus incorporation of potassium sorbate to
sweet potato starch films, led to higher oxygen permeability [52]. The
mass transfer of oxygen in a semi-crystalline polymer is primarily a function
of the amorphous phase, because the crystalline phase is usually
assumed to be impermeable. Fama et al. [53,55] showed that the incorporation
of potassium sorbate decreased the crystallinity of tapioca-starch
edible films causing an increase in oxygen permeation (OP) of the starch
films. The incorporation of potassium sorbate weakened inter-molecular
forces between adjacent starch polymeric chains, facilitated chain mobility
and increased the free volume between starch molecules, which
promoted oxygen permeability [132].However, a better oxygen transportation
barrier property was obtained with the starch films when chitosan
was incorporated. With 15% of chitosan, sweet potato starch film had a
significantly lower OP (1.94±0.95×10−6 cm3×m/(m2×d×kPa)) than
that of the control starch film. Chitosan could forminter-molecular hydrogen
bonds with starch, which limited the inter-molecular chain mobility
and decreased its free volume, contributing to the decrease of OP. Moreover,
Xu et al. [31] reported that chitosan film possessed low permeability
to oxygen.
Mathew and Abraham [43] determined the water vapor transmission
(WVT) of films gravimetrically. The films were fixed on to the
circular opening of permeation cell-containing anhydrous calcium
chloride (0% RH) using melted paraffin. The cups were then weighed
and placed at 92% relative humidity and 37 °C in a humidity chamber.
The cups were weighed at 1-h intervals until the change in weight became
constant. The water vapor transferred through the films at different
time intervals were determined from the weight gain of the
cups. Changes in the weight of the permeation cell were recorded
and plotted as a function of time. The slope of each line was calculated
by linear regression and WVT rate was calculated from the slope of
the straight line (g/h) divided by the transfer area (m2).
Oxidized ferulic acid incorporated films were found to significantly
decrease the WVT compared to the control blend films, probably due