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In this study, the graphene oxide was used as adsorbent for the separation (adsorption and desorption) of flavonoids from lemon peel. Properties of desorption extract (total phenolic content, total flavonoid content, scavenging ability of DPPH free radicals) were determined. Antioxidant activity of desorption extract at three levels of 250, 500 and 1000 ppm and BHT synthetic antioxidant at 200 ppm in antioxidant-free frying oil were evaluated by measuring peroxide value, thiobarbituric acid test, conjugated DN and color index. Each gram of lemon peel extract contained 19767.20 μg phenolic compounds and 33.552 μg flavonoids on dry matter. Each gram of desorption extract contained 5861.56 μg phenolic compounds and 3.446 μg flavonoids on dry matter. Peroxide value, thiobarbituric acid, conjugated DN and color index of oil increased with increasing heating time and the highest increase was observed in the control treatment without antioxidants. Treatments containing desorption extract at 500 and 1000 ppm, can compete with BHT antioxidant. The results of this study indicated that desorption extract can be used as a replacement for synthetic antioxidants in oil.

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This study developed carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) as sorbents to extract flavonoids from grapefruit peel. The impact of solution pH and desorption conditions on extraction efficiency was investigated. In addition, Fourier transforms infrared spectroscopy, thermogravimetry, UV-visible spectroscopy, and scanning electron microscopy were used to characterize the carbon nanotubes. After five cycles, the desorption percentage of flavonoids was 82.7%. HPLC analysis indicated that naringin was the dominant flavonoid in the grapefruit extracts, followed by rutin and quercetin. Insights into the adsorption mechanism of naringin to the MWCNT-COOH were obtained using the Freundlich isotherm equation to model the results. The carbon nanotubes developed in this study offer a cost-effective and straightforward method of extracting value-added functional ingredients from food waste, thereby improving the sustainability and economic viability of the food supply.
 

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The purpose of this study was to compare the efficacy of citrus peel phenolic compounds extracted using carboxylated multi-walled carbon nanotubes (MWCNT–COOH) with common synthetic antioxidants [butylhydroxyanisole (BHA) and butylhydroxytoluene (BHT)] in stabilizing sunflower oil (SO) during frying (180 ± 5 °C, 24 h). To evaluate the antioxidant activity of these compounds, total phenolic, and flavonoid contents, reducing power, ABTS scavenging activity, β-carotene bleaching ability, and 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity were measured. Extracts were combined at various concentrations (100-1000 ppm) with SO, and both synthetic antioxidants were standardized at 200 ppm. Thermal-oxidative stability was investigated by assessing the change in peroxide value (PV), color, free fatty acid (FFA) content, conjugated diene value (CDV), and thiobarbituric acid (TBA). Bitter orange peel extract (BPE) displayed a higher content of phenolic compounds and antioxidant activity. At 1000 ppm, it showed a lower FFA content, PV, TBA, and CDV. It can be used as an alternative to synthetic antioxidants. As a result, citrus peel phenolic compounds extracted with MWCNT–COOH can be used as preservatives in frying oils.