Optimization of antioxidant properties of sour orange seed coat extract using microwave-assisted extraction

Food wastes and losses are produced during all phases of food life cycles. The highest wastes belong to the processing of fruits and vegetables. Bioactive compounds have the potential to be extracted from the by-products of fruits and vegetables which can be used in the food processing. Extraction of flavor compounds, phenolic compounds, enzymes, and organic acids from wastes of fruits – pomace, peel, and seeds of citrus fruits, pomace and leave of apple, seeds of grape, and peel of kiwifruit – and vegetables – pomace of carrot, husk of garlic, skin of onion, peel of potato, and skin of tomato – have been reported (Sagar et al., 2018).Sour orange, Citrus aurantium, is one of the species of citrus fruits. Sour orange seeds contain fats, protein, and bitter compounds which affect citrus processing (Ye et al., 2017). Naringin, neohesperidin, flavon, caffeic acid, p-coumaric acid, ferulic acid, and sinapic acid have been detected in methanolic extract of sour orange seeds (Bocco et al., 1998).Bioactive compounds are recovered from food wastes through various conventional and novel extraction techniques. Microwave-assisted extraction is one of the most used novel and environmentally friendly extraction methods. Advantages of microwave-assisted extraction over conventional extraction techniques include automated operation, more effective and selective heating, and less extraction time (Vinatoru et al., 2017).The objective of this study was optimization of microwave-assisted extraction of sour orange seed coat extract in terms of microwave power level, extraction time, sample quantity, and solvent volume on yield, bioactive compounds (total phenolic content and total flavonoid content), and antioxidant activity (free radical scavenging activity, ferric ion reducing antioxidant power, cupric ion reducing antioxidant capacity, and ferrous ion chelating). Also, optimum conditions of microwave-assisted extraction was compared to that of conventional magnetic stirrer-assisted extraction method.

Sour orange seeds were purchased from Limondis Company (Beyza, Fars province, Iran). Microwave-assisted extraction conditions including microwave power level (100, 200, and 300 W), extraction time (5, 10, and 15 min), sample quantity (5, 10, and 15 g), and solvent (methanolvolume 100, 150, and 200 mL) were optimized. Yield, bioactive compounds (total phenolic content (Habibi et al., 2015) and total flavonoid content (Habibi et al., 2015)), and antioxidant activity (free radical scavenging activity (Habibi et al., 2015), ferric ion reducing antioxidant power (Rekha et al., 2012), cupric ion reducing antioxidant capacity (Pascu et al., 2014), and ferrous ion chelating (Oyetayo et al., 2009)) of sour orange seed coat extract were evaluated. After determining the optimum conditions of microwave-assisted extraction, yield, bioactive compounds (total phenolic content and total flavonoid content), and antioxidant activity of sour orange seed coat extract were compared to those of conventional magnetic stirrer-assisted extraction method. Design Expert software (Version 10, Stat-Ease, Minneapolis, MN) was employed for analyzing four variables – microwave power level, extraction time, sample quantity, and solvent volume – at three levels consisting 30 experimental runs. Response surface methodology concerning central composite design (6 center points, quadratic model, and face center = 1) was applied.

Optimum conditions of microwave-assisted extraction were microwave power level of 200 W, extraction time of 12 min, sample quantity of 5 g, and solvent volume of 200 mL. Under optimum conditions, yiled, total phenolic content, total flavonoid content, IC50, ferric ion reducing antioxidant power, cupric ion reducing antioxidant capacity, and ferrous ion chelating were11.57%, 15550.50 µg gallic acid equivalent/g, 1476.22 µg quercetin equivalent/g, 11.33 mg/mL, 7.12 mg ascorbic/g, 6.44 mg ascorbic acid/g, and 0.43 mg EDTA/g, respectively. Intermediate microwave power level (200 W) can be more suitable from an industrial perspective and energy consumption (Jokić et al., 2012). Further increase in microwave power level, i.e. higher than 200 W, causes thermal degradation of bioactive compounds (Dahmoune et al., 2013), decreasing total phenolic content, total flavonoid content, and antioxidant activity of sour orange seed coat extract. The highest extraction time gives the bioactive compounds a chance to diffuse and release from the cell matrix to the surrounding environment (solvent). The highest solvent volume was selected as the optimum extraction condition. By increasing solvent volume up to 200 mL, meaning a greater gradient in bioactive compound concentration, mass transfer was also improved (Dahmoune et al., 2013). Also, the minimum sample quantity (5 g) was determined in optimum conditions. Increasing sample quantity (while the solvent volume remained constant) reduces the surface area available for the solvent to penetrate the sample matrix. As a result, higher sample quantity caused lower extraction of bioactive compounds (Ballard et al., 2010). There were no significant differences between yield, bioactive compounds, and antioxidant activity of extract obtained by conventional-assisted extraction method in comparison with those of microwave-assisted extraction. In conclusion, microwave-assisted extraction, as a green and fast method, can be proposed as a suitable and practical method for extraction of bioactive compounds from sour orange seed coat.