Rapid Determination of the Antioxidant Capacity of Lettuce by an E-Tongue Based on Flow Injection Coulometry
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This work proposes the use of an electronic tongue based on flow injection coulometry for the rapid determination of the antioxidant capacity of fresh lettuce. The e-tongue consisted of a series of 16 porous carbon electrodes, each poised at a fixed potential from +100 to +850mV. Each injection leaded to a characteristic hydrodynamic voltammogram, whose profile reflects the composition of antioxidants. The correlation between the peak area recorded by each sensor and the 1,1-Diphenyl-2-picryl-hydrazyl (DPPH) assay was maximum in the range of potentials between +400 and 750mV (R2>0.97). Accordingly, the charge measured provided a direct and simple index of the antioxidant capacity. The practical utility of such index was initially demonstrated by determining the best extraction conditions. This consisted in freeze-drying of lettuce followed by methanolic extraction. Later, the e-tongue was used to evaluate the effect of storage (one week at 5 degrees C) on lettuce. The e-tongue revealed that lettuce lost up to 25% of their initial antioxidant activity during storage. However, when lettuce samples were pre-treated with fast cooling or vacuum cooling, the decrease of the antioxidant index was limited to 14 and 15%. Overall, the e-tongue is a rapid, simple and sensitive method for the determination of the antioxidant capacity of fresh lettuce samples. Indirectly, these findings suggest also that lettuce may serve as potential dietary sources of natural phenolic antioxidants.