In vitro bioaccessibility and physicochemical properties of phytosterol linoleic ester synthesized from soybean sterol and linoleic acid
Introduction
Phytosterols are bioactive compounds in plant, and also integral components of oil unsaponifiable matter. The composition and content of phytosterols (PS) differ in different vegetable oils, with the most important being β-sitosterol, stigmasterol, campesterol and brassicasterol (Moreau, Whitaker, & Hicks, 2002; Piironen, Lindsay, Miettinen, Toivo, & Lampi, 2000). Studies have shown the importance of PS in reducing cholesterol levels in the serum (Brufau, Canela, & Rafecas, 2008; Wolfs, de Jong, Ocké, Verhagen, & Monique Verschuren, 2006). Linoleic acid (LA) is a common polyunsaturated fatty acid with large amounts found in safflower seed oil, sunflower oil, walnut oil and soybean oil. LA is an essential fatty acid and can also reduce the risk of arteriosclerosis in animal model and humans. Both PS and LA are regarded essential molecules since they cannot be synthesized in the human body. These molecules must be obtained from food sources.
Phytosterols are insoluble in water and their solubility in oil is just about 1% (Yang, Oyeyinka, & Ma, 2016). This trait limits their wide application in food/pharmaceutical industry. In order to enhance the application and improve the bioaccessibility of PS, researchers have utilized the esterification method to produce phytosterol esters from PS and fatty acids. Chemical synthesis and biological synthesis are the two main methods currently employed. Chemical synthesis has several advantages including providing good conversion rate (CR), and results in high productivity. However, it has several drawbacks too. For example, chemical esterification requires the use of catalysts such as magnesium oxide, lanthanum oxide, zinc oxide, aluminum oxide, and aluminum triiodide (Hang & Dussault, 2010; Meng, Pan, & Yang, 2010; Robles-Manuel, Barrault, & Valange, 2011; Valange et al., 2007). The major challenge is the difficulty in separating catalyst from the final product. Another major challenge is the high temperature used during the synthesis, which may lead to the production of by-product. Biological synthesis uses a relatively low temperature, produces no or less by-product, but takes a longer time with low CR products (Villeneuve et al., 2005; Vu, Shin, Lim, & Lee, 2004).
Recently, we synthesized PS esters using PS from soybean and acetic anhydride (Yang et al., 2016). The optimum condition for the production of high yield of PS ester (99.4%) was found to be a temperature of 135 °C for 1.5 h with a mole ratio 1:1 for phytosterol and acetic anhydride, respectively. Furthermore, Fourier transform infrared spectroscopic and gas chromatography-mass spectrometric studies revealed that no other harmful by-products were formed during the process (Yang et al., 2016). With the growing interest in the synthesis of high-quality PS ester products using new technology, it may be necessary to investigate promising alternatives to the traditional chemical methods. Hence, in this paper, PLE was first synthesized from soybean sterol and LA using acyl chloride method in order to optimize reaction conditions. The physicochemical properties and in vitro bioaccessibility of the PLE were thereafter assessed.
Section snippets
Materials
Linoleic acid (≥99%), trypsin, pepsin, sodium taurocholate, as well as lipase (Type II), colipase and cholesterol esterase from bovine pancreas were purchased from Sigma-Aldrich company (America). Acetone and acetonitrile used were of chromatography grade. Hexane, PCl3, NaOH, NaHCO3, NaCl, CaCl2, HCl, KH2PO4 were analytical grade. Soybean sterol (≥95%, seperated and purified from soybean oil deodorized distillate), soybean oil, rapeseed oil, peanut oil, corn oil, sunflower oil were obtained
Effects of reaction conditions on synthesis of PLE
Different mole ratios (1:1, 1:1.1, 1:1.2, 1:1.3, and 1:1.4) of PS and LC respectively, at varying temperatures of 50, 60, 70, 80 and 90 °C and different time (0.5, 1, 1.5, 2, and 2.5 h) were used to optimize the synthesis of PLE. CR of PS and LC to form PLE increased with increasing mole ratio, increasing reaction time and increasing temperature, reaching a maximum value of 97.5, 96.8 and 96.5% respectively (Table 1). It appears that the optimum conditions (temperature of 80 °C for 1.5 h) to
Conclusions
This study demonstrated that PLE can be efficiently synthesized by the chloride method from soybean sterol and LA. The CE of PS was above 96% at 80 °C for 1.5 h with a 1:1.1 mol ratio of PS and LA, this condition was chosen as an optimum method to synthesize PLE. The physicochemical properties of the synthyzed PLE was analyzed by HPLC and FTIR. Melting and boiling points of PLE were significantly lower than PS. Solubility of PLE in oil was higher than PS, and PLE had a good pH stability.
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