Dye leakage of the conventional oxygen indicator film based on water-insoluble zein polymer (5%) was examined by immersing the film in distilled water. The leaching of thionine into water as a variation of time is indicated in Fig. 2 (open circle). The dye leached from the film into water quite rapidly in the first hour of immersion, and after 6 h the dye leakage reached 80.80 ± 0.45%.
After the immersion in water, the zein-based film became so light-coloured even in the air that it was not suitable for oxygen indicators. Even though zein is insoluble in water except in the presence of alcohol or high concentration of alkali (pH 11 or above) (Shukla & Cheryan, 2001), thionine, cationic dye is so soluble in water and small in size that it can diffuse out readily. The new oxygen indicator film was prepared using alginate solution (0.25%) and then tested in the same manner. As shown in Fig. 2 (closed circle), the dye leakage was only 19.65 ± 0.42% even after 24 h in water. It was very interesting that alginate, hydrophilic natural polymer could prevent the dye from leaching out much better than hydrophobic zein polymer. This is attributed to ability of alginate to form insoluble gels with divalent cations