Dibutyryl-cAMP, sodium salt Besides above mentioned UV visib
Besides above-mentioned UV–visible and fluorescence spectroscopy methods, electron paramagnetic resonance (EPR) spectroscopy, the only analytical method that detect compounds having unpaired electrons known as free radicals, was rarely used in antioxidant capacity measurement (Amarowicz et al., 2004, Bartoszek and Polak, 2012, Polak et al., 2015, Polak et al., 2013, Weil and Bolton, 2006). Although all of other methods screen the scavenging ability of antioxidant compound, ESR method monitors the free radicals itself, which makes this method superior to other assays. With this approach, the antioxidant capacity of a Dibutyryl-cAMP, sodium salt was determined by the monitoring of changes in the intensity of the spectrum of stable free radicals such as DPPH and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) through the reaction of those radicals with antioxidant (Harbour et al., 1974, Polak et al., 2015). As a historical view, all of these approaches assumed that all antioxidants in foods could be extracted completely by various extraction procedures. Besides the conventional chemical extraction methods, other non-conventional techniques including microwave-assisted, ultrasound-assisted, pulsed electric fields, high voltage electrical discharges, pressurized liquid extraction and supercritical fluid extraction have been started to develop to improve the extraction efficiency of antioxidant compounds (Barba et al., 2016, Dahmoune et al., 2015, Liu et al., 2015, Povilaitis et al., 2015, Xu et al., 2017). It may be true that these all techniques have the capacity to increase the extraction of antioxidants by inducing the mass transfer. Nonetheless, it was obvious that there was no unique solvent and mixture or extraction method to dissolve all the antioxidant compounds present in food structure, and the resulting extracts were not representative of total antioxidant capacity of the food in question. It was highlighted that comparison between antioxidant properties of different food matrices on the basis of a standardized database was not possible as the results differed from each other depending on the used extraction procedures or measurement assays (Pérez-Jiménez et al., 2008, Pérez-Jiménez and Saura-Calixto, 2006). In addition, it was important to bear in mind that measurement performed with different extraction solvents (methanol, acetone, hexane, dichloromethane) could not represent the physiological effects of antioxidant compounds. Our research group demonstrated a simple and direct assay for antioxidant capacity measurement called “QUENCHER” (Quick, Easy, New, CHEap, Reproducible) (Gökmen et al., 2009, Serpen et al., 2007). This method gave opportunity to determine the antioxidant capacity without extraction and hydrolysis of food samples. It was demonstrated that both soluble and insoluble bound, or hydrophilic and lipophilic antioxidants present in food came into contact with radicals via liquid-liquid and solid-liquid types of reaction (Table 3). Although the QUENCHER approach was introduced only 10years ago, there now exist more than 100 research articles using this method adapted to other assays (ABTS, DPPH, FRAP, CUPRAC, ORAC) to determine antioxidant capacity of foods, especially those containing large amounts of insoluble bound antioxidants (Cömert & Gökmen, 2017). On the basis of the QUENCHER approach, a new methodology called Global Antioxidant Response (GAR) was proposed to determine the total antioxidant activity of insoluble and soluble fractions obtained after a simulated gastrointestinal digestion (Delgado-Andrade et al., 2010, Pastoriza et al., 2011). According to this method, in addition to the antioxidant activity of soluble antioxidant compounds that released and became bioaccessible after digestion procedure, the antioxidant activity of remaining non-digestible fractions could be evaluated using classical methods and the QUENCHER assay, respectively (Cömert & Gökmen, 2017). More recently, Pérez-Burillo et al. (2018) improved the GAR+ method including both in vitro digestion and fermentation procedures to reflect the antioxidant activity of antioxidants through the entire gastrointestinal digestion system. This method could be evaluated as one of the most physiological-resembling method since it was able to eliminate the organic solvent extraction procedure by means of QUENCHER approach, and include both the in vitro fermentation and digestion protocols.