Comparison of Spectrophotometric and Electrochemical Methods for the Evaluation of the Antioxidant Capacity of Buckwheat Products after Hydrothermal Treatment

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Abstract
This paper reports the use of spectrophotometric and voltammetric methods for the determination of the antioxidant capacity of buckwheat and its products originated from a technological line of a buckwheat roasted groats producer. 80% methanol extracts from raw and roasted buckwheat and groats and hulls obtained from roasted buckwheat were used. The spectrophotometric methods included (1) free radical scavenging activities of the extracts against ABTS•+ radical cation (TEAC) and 2,2-diphenyl-1-picrylhydrazyl radical (DPPH RSA) and (2) determination of reducing capacity by the means of Folin−Ciocalteu reagent (FCR) application. The radical scavenging activities of the extracts were also investigated using a voltammetric assay. Moreover, the flavonoids profiles of the studied materials were provided. Buckwheat roasting caused a decrease in TEAC, DPPH RSA, and FCR reducing capacity by 70%. The lowest TEAC, DPPH RSA, and FCR reducing capacities were noted for roasted groats. Both DPPH RSA and TEAC methods were highly positively correlated with the FCR reducing capacity assay (r = 0.98 and r = 0.99). Cyclic voltammograms of analyzed buckwheat extracts were useful for evaluation of the antioxidant capacity. The total charge below the anodic current waveform was correlated with the data obtained by TEAC (r = 0.770), DPPH RSA (r = 0.88), and FCR reducing capacity (r = 0.81). The changes in the antioxidant capacity of buckwheat and its products followed the changes in flavonoids composition. In particular, the concentration of flavonoids was related to measurements by cyclic voltammetry. Keywords: Whole buckwheat; hydrothermal treatment; roasted groats; hull; antioxidant capacity; spectrophotometric methods; cyclic voltammetry; flavonoids

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