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Characterization of chlorogenic acids in coffee by square-wave voltammetry

Chlorogenic acids (CGAs) are one of the most important dietary polyphenols in human diet and the coffee is the major source of CGAs. The most abundant CGAs in the coffee are caffeoylquinic acids (CQAs) and the dominant is 5-O-caffeoylquinic acid (5-CQA). In this study, the characterization of CGAs in different coffee samples was performed by square-wave voltammetry (SWV) and 5-CQA was used as a standard. The effect of concentration of 5-CQA on the SWV responses was investigated. Calibration curve has shown linearity in the concentration range 5-50 μmol L-1 (LOD=7.7·10-7 mol L-1). Coffee extracts (green beans, roasted beans, ground coffee, instant coffee) were diluted in 0.1 mol L-1 PBS pH 7 in order to perform SW voltammogram. Oxidation/reduction peaks of coffee samples were well expressed and the net peaks potential was observed in range of 0.223-0.229 V, which is very close to the oxidation peak potential of 5-CQA (0.225 V). Anodic to cathodic peak separation (ΔEp) of coffee samples is 26 mV and the ratio of anodic to cathodic peak currents (If / Ib) is -1.7, which probably indicate reversible electrode reaction according to the SWV criteria of reversibility. In order to identify the profile of CGAs in the coffee samples, the spiking was performed with standard solutions of CQAs and diCQAs. The total CGAs content in coffee samples was calculated from the net peak current values of SW voltammograms by using calibration curve of 5-CQA as a standard. The total CGAs content was expressed as mg 5-CQA equivalents per 100 g of coffee (mg 5-CQAE/100g coffee). The CGAs content decreases as follow: green coffee, ground coffee, instant coffee, roasted beans. This indicated that the roasting process and the coffee processing have an influence on the loss of CGAs. SWV results are in the very good agreement with the results obtained by HPLC method. Therefore, SWV was shown as very sensitive and selective method for the determination of CGAs of coffee samples.

chlorogenic acids ; square-wave voltammetry ; coffee

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