Photo-Modification of starch-oleic acid by TiO2: investigation of food packaging properties

Among the bio-based materials which used in the food packaging, starch is interested as an ecofriendly material. This interest is mainly due to its acceptable film forming properties, easy access, renewability and low cost. Nevertheless, high hydrophilic and weak mechanical properties have been limited its utilization as a commercial packaging material. 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. Fatty acids, such as oleic acid (OA) are lipid derivatives that can potentially improve the moisture barrier properties of hydrophilic films. OA is a yellow liquid at room temperature and miscible with biopolymer without further heating treatment. Due to the polar nature of biopolymers, OA needs to surfactant to get 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 applications. Titanium dioxide (TiO2) is an inert, cheap and nontoxic material with broad application as a food pigment and potential activity against a wide variety of microorganism due to its photocatalytic activity.TiO2 is the most commonly used semiconductor, photocatalyst which activated by UV ray. The main purpose of the current study was modification of starch hydrophilicity using OA, nano-titanium dioxide and UV ray. In this study OA and TiO2 added at the optimized condition and the solution was irradiated by UV ray. 

A starch solution 5 (wt. %) in distilled water was prepared. This solution was mixed and heated (85 ˚C for 15 min) until the starch gelatinized. Thus, plasticizer (glycerol, 40 wt. % of dry base) was incorporated into the solution to achieve more-flexible films. OA 1(wt. % of dry base) mixed with Tween 80 as emulsifier (10 wt. % of the OA). This solution was mixed and heated (50 ˚C for 10 min). Then, 10 ml of distilled water was added to the solution, and homogenized by ultrasonic homogenizer (Dr. Hielscher, Teltow, Germany) for 7 min. The suspension of TiO2 nanoparticles in water was prepared so that the final TiO2 content in film specimens was 2 (wt % of dry base). The suspensions were stirred for 10 min, and homogenized by ultrasonic homogenizer for 60 min. The TiO2 suspension was added to starch solution gradually and mixed for 10 min. Afterward, OA- Tween 80 solution was added to starch solution gradually and mixed for 10 min.The starch solution containing TiO2 and OA was homogenized by ultrasonic homogenizer for 7 min. Film forming solution placed under three UV-C lamps (8w, Phillips, Holland) at a distance of 5 cm. The solutions were stirred simultaneously with UV irradiation. After 0, 30, 60 and 90 min of UV exposure, the air bubbles were removed and the solutions were casted into flat, leveled, non-stick disposable 15 cm-Petri dishes. Each of the dishes were contain 43 g which dried for 48 hours at room temperature. All the film specimens were conditioned at 50-55% relative humidity and room temperature for 48 hours before subjected to further analysis.Moisture content (MC), moisture absorption (MA), solubility in water (SW), water vapor permeability (WVP), surface properties, visual properties and mechanical properties of the film specimens were measured. The chemical structure of the films was investigated by FTIR spectroscopy.

The results shown that water contact angle of the starch film was increased by adding OA and TiO2. But it was decreased after UV exposure of the starch-OA-TiO2 solution. WVP of the starch based films was decreased by its composition with OA and TiO2. But no change was observed after UV exposure of the starch-OA-TiO2 compositions. MC of the starch-OA-TiO2 film had no change, simultaneously. However, the tensile strength and Young’s modulus were decreased by adding OA-TiO2. Elongation at break and tensile energy to break were increased, simultaneously. However, the mechanical properties of the starch-OA-TiO2 nanocomposites were changed by UV exposure but these changes did not follow a specific trend.Although, whiteness index (WI) and total color difference (ΔE) were not changed by OA-TiO2 composition. But yellowness index (YI) was changed, simultaneously. After UV irradiation ΔE and YI were increased and WI was decreased. The results, demonstrated that the virgin starch-OA-TiO2 composition was the best modification method to decrease the sensibility of starch based films to moisture as a packaging material. UV irradiation at short time (30 min) was the optimum condition to modify sensibility of the films to moisture and mechanical properties among the UV cured films.