Edible oil-based switchable-hydrophilicity solvent liquid–liquid microextraction prior to smartphone digital image colorimetry for the determination of total curcuminoids in food samples

Abstract

Edible oil-based switchable-hydrophilicity solvent liquid–liquid microextraction was coupled with smartphone digital image colorimetry for the determination of total curcuminoids. Images of the colored extracts were captured in a laboratory-made colorimetric box, which were then split into their red–green–blue channels. Optimum extraction conditions were achieved using 550 μL of almond oil as the extraction solvent and 0.40 M sodium hydroxide for hydrolysis of the oil to the salt of its fatty acid. Phosphoric acid (2.0 mL, 4.0 M) was used as the hydrophilicity-switching trigger, while pH of the sample solution adjusted to 5.50 and extraction time of 1.0 min, were found to be optimum. Optimum detection conditions were achieved at a distance of 7.0 cm from the detection camera, a region of interest of 175 px², a detection wavelength of 420 nm and 50.0% brightness of the light source. The limit of detection was found to be 0.020 μg mL⁻¹. A good linearity was achieved as indicated by coefficients of determination above 0.9965. The proposed method was used for the determination of total curcuminoids in tea and turmeric samples with percentage relative recoveries of 95.0–105.0% and percentage relative standard deviations below 8.7%.