Modification of hydrophilic properties of starch film by simultaneous use of oleic acid and UV ray

Introduction: Today, the most of the food packaging materials are based on petroleum-derived polymers. They have caused many environmental pollutions in the world. One of the solutions to deal with the problem is development of edible and biodegradable films as substitutions of the petroleum based packaging materials. Among the bio-based materials which used in the food packaging, starch is interested as an ecofriendly material. It is due to acceptable film forming properties, easy access, renewability and low cost of starch. Nevertheless, high hydrophilic properties and weak mechanical properties have been limited its utilization as a commercial packaging material. The presence of hydroxyl groups in the starch chains create hydrogen bonds between starch and water. As well as the considerable free space between the starch chains facilitate the movement of the water molecules through this space. Many researches have been done to reduce hydrophilic properties of starch using chemical modification, irradiation, and composition with nanoparticles and other biopolymers. Most of the methods are based on blocking the hydroxyl groups of starch chains with hydrophobic agents, formation of emulsions and blocking of the pores.
Other alternative procedure which used to improve the water barrier properties of hydrophilic films is producing composite films with adding hydrophobic components such as lipids and fatty acids.
The starch based films have poor resistance to water vapour transmission. Their water vapor barrier properties might be improved by adding hydrophobic materials, such as lipids, into the film forming solutions. Fatty acids, such as oleic acid (OA) are lipid derivatives which have potentially improve the moisture barrier properties of hydrophilic films. OA is a yellow liquid at room temperature hence, it is easily miscible with biopolymer without needing to further heating treatment. As well as due to the polar nature of biopolymers, OA need to the surfactant to homogenous distribution in the matrix of biopolymers. On the other hand, OA is not very sensitive to oxidation which increases its safety in food packaging uses. In this study, the effects of different concentrations of OA and UV irradiation were investigated to improve the hydrophobicity of starch film.
Material and methods: In order to preparation of starch film, starch (5 wt. %) was added to distilled water. The resulting mixture was stirred at 85 ˚C for 15 min until the starch was gelatinized. Then, glycerol (40 wt. % of dry base) was added to the solution as a plasticizer. OA 0.6%, 1% and 1.4% (wt% of dry base) and Tween 80 as emulsifier (10% of the OA concentration), were mixed in 50 ˚C for 10 min.  Then, 10 ml of distilled water was added to the solution, and then sonicated by ultrasonic homogenizer (Dr. Hielscher, Teltow, Germany) for 7 min. To prepare the uniform distributed film solution, the fatty acid emulsion was gradually added to the gelatinized starch solution. The resulting suspension was homogenized by magnetic stirrer and ultrasonic homogenizer for 10 and 7 minutes, respectively. Film forming solution was irradiated with UV-C as stirred during 60 minutes at a distance of 5 cm from the UV-C lamp. 43 g of each solution was casted into flat mold with a diameter of 15 cm. The samples were finally dried for 48 hours at room temperature. All films were conditioned at 50-55% relative humidity and room temperature for 48 hours before subjected to any tests.
Results and discussion: The contact angle of the starch based films was increased with increasing the OA content up to 1 wt%. After that it was decreased with increasing the OA. It was due to the plasticizing effect of OA at high concentration. The contact angle of the UV irradiated starch-OA were increased with increasing the OA content. Generally, the contact angle of the UV irradiated starch-OA films was drastically higher than virgin starch-OA films. However, the water vapor permeability (WVP) of the starch based films was decreased with increasing the OA content up to 1 wt% and increased at the high OA content film (i.e. 1.4 wt% dry based). But there was no significant difference between starch film and the starch-OA films. The WVP of the UV irradiated starch-OA were decreased with increasing the OA content. The WVP of the UV irradiated starch-OA films was higher than virgin starch-OA films. Nevertheless, there was no significant difference between WVP of UV irradiated starch-OA and starch film.
Moisture content (MC) of the starch-OA film at the lower OA content (i.e. 0.6 wt% dry based) was the lowest. With increasing OA content MC of the starch-OA film increased that way there was no significant difference with starch film. But the MC of the UV irradiated film decreased with increasing the OA content up to 1 wt%. Moisture absorption (MA) of the starch-OA and UV irradiated starch-OA films were shown significant difference with virgin starch film. MA of the starch-OA films were independent from UV irradiation except starch-OA (0.6 wt%) which was increased with UV irradiation.There was no significant difference between the solubility of the starch, starch-OA, and UV irradiated starch-OA films in water except UV irradiated starch-OA contain 1.4 wt% OA.
Conclusion: Regard to results of the current study the low OA content in starch based film can be used as the appropriated method to solve the moisture sensitivity of starch based films. Furthermore, UV irradiation intensify the effect of OA on the moisture sensitivity of the starch based films. The most of characterizations of UV irradiated starch-OA films had been influenced by the content of formed the mono and disaccharides and the development of random cross-linkages during UV exposer process. The applied procedure is useful to develop more biodegradable and profitable packaging material, generally.