Demonstrating the Minimal Impact of Cultivation Conditions on Antioxidants in Fruits and Vegetables by Differential Pulse Voltammetry

Electrochemical Evaluation of the Total Antioxidant Capacity of Yam Food Samples on a Polyglycine-Glassy Carbon Modified Electrode

Background:

The total antioxidant capacity of yam food grown in southern African regions was investigated by a polyglycine-glassy carbon modified electrode. The modified electrode was fabricated using glycine solution on glassy carbon electrode by electrodeposition method. The proposed modified electrode is found to be nearly 3.15-fold more sensitive than the bare electrode. For the measurement of the total antioxidants of yam, differential pulse stripping voltammetry (DPSV) was employed with standard quercetin compound.

Methods:

The total antioxidant capacity of yam was deduced by DPSV and cyclic voltammetry (CV) methods. The basic parameters for the stripping technique such as pH, accumulation time and accumulation potential were optimized as 20 s, 200 mV and a pH of 3 Britton-Robinson (B-R) buffer solutions in 0.5 mg quercetin/L, respectively.

Results:

In the optimization condition, the linear working range was determined between 5.0 μg/L and 80.0 µg/L for the quercetin. The detection (LOD) and quantification (LOQ) limits of quercetin were found to be 0.39 µg/L and 1.39 µg/L on the modified electrode by DPSV, respectively. The procedure was also applied to natural yam samples and total antioxidant capacity of 0.1 kg of yam was determined as 96.15 ± 0.85 µg/L of equivalent quercetin at 95% confidence level with the relative standard deviations of 0.88%.

Conclusion:

Sensitive and selective voltammetric method was developed for the determination of total antioxidant capacity in yam. Moreover, the modified polyglycine-glassy carbon electrode was constructed more selectively for quercetin. As a result, a simple, sensitive and rapid new voltammetric method for the determination of antioxidants has been developed using the modified electrode.

Antioxidant activity of commercial food grade tannins exemplified in a wine model

Although commercial tannins are widely used in foods and beverages, an improved understanding of the structure and composition of vegetable tannins is needed to promote the exploitation of agri-food by-products and waste and their valorisation in more sustainable industrial applications. This study aims to characterise the phytochemical composition and antioxidant activity of 13 food grade tannins using multiple analytical approaches, including spectrophotometry and HPLC-ECD to determine the amount of targeted polyphenolic compounds. Moreover, the antioxidant activity of tannins was assessed in terms of radical scavenging activity (DPPH• assay), reducing power (FRAP assay), and redox properties (cyclic voltammetry, CV). A statistical univariate and multivariate correlation analysis was performed on 17 parameters including tannin content (range: 0.71-1.62 mM), gallic acid, (+)-catechin, syringic acid and (‒)-epicatechin. The compositional profile of tannins was related to their chemical moiety, antioxidant activity and the botanical origin of the extracts. In particular, the CV signal at 500 mV was highly correlated with DPPH• value due to the catechol ring of flavonoids and trigalloyl moieties of gallic acid-based compounds. Practical examples of tannins application in winemaking are discussed.