Determination of ascorbic acid in natural fruit juices using digital image colorimetry

This work proposes a procedure using digital image colorimetry for the determination of ascorbic acid in natural fruit juices. The method was established by measuring the color of the iron(II)-1,2 ortho-phenanthroline complex. The digital images obtained during the experiments were stored in JPEG format, and the region of interest (ROI) was submitted to decomposition in color values according to the RGB additive color model. However, the analytical procedure recommended the employment of the blue channel because it presented higher sensitivity than the red and green channels. The optimization of the parameters of the colorimetric reaction was performed employing a full two-level factorial design. The influence of the distance from the webcam to the cuvette on the sensitivity and precision of the method was also evaluated. This experiment, demonstrated that there was no significant difference between the slopes obtained in the calibration curves under the conditions studied. When utilizing the optimized conditions, the method allows the determination of ascorbic acid in fruit juices with limits of detection and quantification of 0.005 and 0.015 mg L−1, and precision expressed as the relative standard deviation of 1.2% for lemon juice with an ascorbic acid concentration of 24.8 mg/100 g. So, the procedure was applied for the determination of ascorbic acid in lemon, orange, grape, cashew, lime, pineapple, tangerine and mango natural juices. The concentrations found varied from 4.5 to 182.5 mg/100 g. The fruit juice samples were also analyzed by molecular absorption spectrophotometry using a classical method established using 1,2 ortho-phenanthroline. The results obtained were compared using the t-test, which showed that there was no significant difference between the ascorbic acid contents found by the two methods.