Introduction
One of the most consumption and nutritious edible oils in the world is canola oil. The unsaturated acids content in canola oil is higher than other vegetable oils. The ratio of linolenic acid (C18:3, ω3) to linoleic acid (C 18:3, ω6) in this oil is about 1:2 that is desirable from the point of view nutrition (Shahidi., 1990). The high contents of unsaturated fatty acids in canola oil especially linolenic acid effect on its stability and quality (Tokarska et al., 1986). Lipids are more potential to autoxidation than other chemical components of an organic material. Triglycerides oxidation process rate depends on saturation degree and double bonds position in molecule (Yanishlieva-Maslarowa., 2001). One of the most effective ways to prevent oxidation and development off-flavour is addition antioxidants in foods containing fat (Jeong et al., 2004). Antioxidants as food additive are used to develop oils shelf life and fat-based foods during storage and processing (Bitar et al., 2008). The most extensive synthetic antioxidants useable are including tert-butyl hydroxy quinoline (TBHQ), butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA). However, there are too much worry about consumption safety these antioxidants such as carcinogenic effects (Jeong et al., 2004). The demand for using of natural antioxidants instead of synthetic ones have been increased increasing which cause to prevent of diseases and having healthy effects (Zhou & Ly., 2004). Green tea has known as a potent natural antioxidant. This property strongly is related with the total phenolic content of green tea (Langley-Evans, 2000). Green tea leaves are a superior source of polyphenolic compounds that has shown anticancer properties versus several types of cancer (Ahmed et el., 2014), such as lung, skin, esophagus, liver, and stomach (Mandel et al., 2004). A main group of green tea polyphenols are catechins. Many researches have shown, tea catechins are included numerous therapeutic effects such as neuroprotective activity, prevent atherosclerosis, inhibit tumor angiogenesis and modulate cholesterol metabolism (Tang et al., 2013). The most importance catechins of green tea are including: epicatechin (EC), epicatechin-3-gallate (ECG), epigallocatechin (EGC), and epigallocatechin-3-gallate (EGCG) (Rains et al., 2011). Chander et al. reported green tea leaves (Camellia sinensis L .) contain polyphenolic components which have been known as strong antioxidants. One of the most polyphenolic components are catechins. Catechins reduce fatty acids oxidation ability by chelating iron and copper which cause to form catalyzed free radicals by metal (Chander et al., 2005). Ou et al. reported natural antioxidants compounds of green tea such as caffeic acid, cherogenic acid, quercetin, rutin and catechins have been acted as stronger antioxidants than vitamins C and E (Ou et al., 2001). Koketsu and Satoh. studied the antioxidant effect of green tea extract on the lard and soy bean oil. The green tea extract used in 20 and 40 mg/L levels. They stated, green tea polyphenols had stronger antioxidant properties compared with tocopherol in both lard and soy bean oil (Koketsu and Satoh., 1997). Despite the promising therapeutic effects of polyphenolic compounds under in vitro conditions and be contain strong antioxidant activity, their usage is limited due to various reasons including poor absorption and bioavailability (Zhang et al., 2004). In addition to, the tea polyphenols are unstable when exposed to light, heat and oxidants (Shpigelman et al., 2012). An effective method to overcome this problem is entrapment systems such as liposome, nanoparticle and microcapsule (Dsai & Park., 2005). Liposomes possess a spherical structure along with an aqueous core that have surrounded by single or several lipid bilayers. The most important advantage of liposome structures is their ability to encapsulate both hydrophilic and lipophilic bioactive compounds (Liveny., 2015). Noudoost et al. prepared liposomes containing green tea polyphenols with thin film layer method which applied sonication process to reduce of particle size. They reported green tea polyphenols incorporated in liposome had significantly more antioxidant activity than free extract (Noudoost et al. (2015). Rasti et al. studied the oxidative and physical stability of liposomes and nanoliposomes containing docosahexaenoic and eicosapentaenoic acid, prepared by the thin film hydration technique and Mozafari method. They found prepared liposomes by Mozafari method had the most physicochemical stability (Rasti et al., 2012). Lu et al. prepared green tea extract liposome by the thin film ultrasonic dispersion method to increase the bioavailability of tea polyphenol. They reported that liposomes containing green tea polyphenol were stable and suitable for more widespread application (Lu et al., 2011). Considering that harmful effects synthetic antioxidant, the purpose of this study was the evaluation green tea extract as a natural antioxidant on the oxidative stability of canola oil in two forms free and incorporated in liposome during storage at an oven at 60°C temperature and compare it with BHT as a synthetic antioxidant. For this purpose, green tea extract liposomes prepared with using Mozafari method without using any solvents.