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English to Thai: Antioxidant properties of ascorbic acid in bulk oils at different relative humidity General field: Science Detailed field: Chemistry; Chem Sci/Eng
Source text - English Chapter 1
Introduction
Oxidation products from lipids can cause deterioration in the sensory attributes and nutritional values of foods, which in turn decreases consumers’ acceptance of food products. The degree of lipid oxidation is greatly influenced by many factors including the content of unsaturated fat, matrix types such as oil-in-water (O/W) emulsions or bulk oils, and the presence of pro-oxidative metal ions and antioxidants (Chaiyasit, Elias, McClements, & Decker, 2007; Choe & Min, 2006; McClements & Decker, 2000).
Ascorbic acid has an enediol-lactone resonant structure, which provides reducing ability. Due to its hydrophilic characteristics, the antioxidant capacities of ascorbic acid have been extensively studied in oil-in-water emulsions (Jayasinghe, Gotoh, & Wada, 2013; Kim, Decker, & Lee, 2012; Kim, Park, Kim, & Lee, 2013), matrices containing a high proportion of water such as milk (Van Aardt et al., 2005), or olive oil containing intentionally added moisture to carry the ascorbic acid (Mosca, Ceglie, & Ambrosone, 2008). Ascorbic acid can effectively retard the formation of lipid oxidation products in bulk oils (Shahidi & Zhong, 2011) and in linoleic acid model systems (Watanabe, Ishido, Fang, Adachi, & Matsuno, 2005).
Generally, ascorbic acid shows antioxidant capacities in bulk oils and pro-oxidant properties in O/W emulsions (Frankel, Huang, Kanner, & German, 1994; Kim et al., 2012). This phenomenon is called the ‘antioxidant polar paradox’, which states that hydrophilic antioxidants decrease the rates of lipid oxidation most efficiently in non-polar media such as bulk oil systems, whereas lipophilic antioxidants show better antioxidant capacities in more polar media such as O/W emulsions and liposomes. Recently, this theory has been re-evaluated in both O/W emulsions (Laguerre et al., 2010; Lee et al., 2013; Lucas et al., 2010) and in bulk oils (Shahidi & Zhong, 2011). Shahidi and his research group (2011) proposed that the antioxidant capacities of chemicals in bulk oils are greatly influenced by the polarity and concentration of the antioxidants.
The moisture content in bulk oils plays an important role in lipid oxidation through the association of colloids with amphiphilic compounds including free fatty acids (FFAs), phospholipids (PLs), diacylglycerols (DAGs), and monoacylglycerols (MAGs) (Chaiyasit et al., 2007; McClements, 2004; Schwarz et al., 2000). The surfaces of association colloids are major sites of lipid oxidation (Frankel et al., 1994; McClements & Decker, 2000). Park, Kim, Kim, and Lee (2014) reported that increases in the rates of lipid oxidation occur concomitantly with increases in moisture content in oils treated at 100 and 140 C under airtight conditions. Also, moisture in lipids may participate in the formation of volatiles during lipid oxidation under airtight conditions (Kim, Kim, & Lee, 2014).
However, the effect of moisture content in bulk oils on the antioxidant capacities of hydrophilic compounds has not been reported in the literature. Changes in antioxidant capacities in bulk oils according to the concentration and polarity of compounds as suggested by Shahidi and Zhong (2011) could be due to the available moisture content in bulk oils because antioxidants should be located near the surface of association colloids. In this study, ascorbic acid was selected as a representative hydrophilic antioxidant and conditions of different moisture content were generated using saturated salt solutions.
The objective of this study was to determine the effects of relative humidity (RH) and ascorbic acid concentration on the oxidative stability of stripped bulk oils. Double vials containing saturated salt solutions were designed to generate the desired relative humidity conditions.
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