Electrochemical Determination of the "Furanic Index" in Honey

Abstract

5-(hydroxymethyl)furan-2-carbaldehyde, better known as hydroxymethylfurfural (HMF), is a well-known freshness parameter of honey: although mostly absent in fresh samples, its concentration tends to increase naturally with aging. However, high quantities of HMF are also found in fresh but adulterated samples or honey subjected to thermal or photochemical stresses. In addition, HMF deserves further consideration due to its potential toxic effects on human health. The processes at the origin of HMF formation in honey and in other foods, containing saccharides and proteins-mainly non-enzymatic browning reactions-can also produce other furanic compounds. Among others, 2-furaldehyde (2F) and 2-furoic acid (2FA) are the most abundant in honey, but also their isomers (i.e., 3-furaldehyde, 3F, and 3-furoic acid, 3FA) have been found in it, although in small quantities. A preliminary characterization of HMF, 2F, 2FA, 3F, and 3FA by cyclic voltammetry (CV) led to hypothesizing the possibility of a comprehensive quantitative determination of all these compounds using a simple and accurate square wave voltammetry (SWV) method. Therefore, a new parameter able to provide indications on quality of honey, named "Furanic Index" (FI), was proposed in this contribution, which is based on the simultaneous reduction of all analytes on an Hg electrode to ca. -1.50 V vs. Saturated Calomel Electrode (SCE). The proposed method, validated, and tested on 10 samples of honeys of different botanical origin and age, is fast and accurate, and, in the case of strawberry tree honey (Arbutus unedo), it highlighted the contribution to the FI of the homogentisic acid (HA), i.e., the chemical marker of the floral origin of this honey, which was quantitatively reduced in the working conditions. Excellent agreement between the SWV and Reverse-Phase High-Performance Liquid Chromatography (RP-HPLC) data was observed in all samples considered.

Keywords: HMF; RP-HPLC; cyclic voltammetry; furanic acids; furanic aldehydes; homogentisic acid; honey; square wave voltammetry.

Figure 1

Chemical structures of the furanic acids and aldehydes. 2FA, 2-furoic acid; 3FA, 3-furoic acid; HMF, hydroxymethylfurfural; 2F, 2-furaldehyde; 3F, 3-furaldehyde.

Figure 2

CV responses of HMF (1 mmol dm⁻³) in a 0.1 mol dm⁻³ water solution of supporting electrolyte. AE: Pt, RE: SCE. (a) WE: GC, supporting electrolyte: NaH₂PO₄, potential scan rate: 0.10 V s⁻¹ (b) WE: Hg, supporting electrolyte: LiClO₄, potential scan rate: between 0.10 and 1.5 V s⁻¹.

Figure 3

CV responses of HMF (1 mmol dm⁻³) in a 0.1 mol dm⁻³ of LiClO₄ in methanol. WE: GC; AE: Pt; RE: SCE, potential scan rate: 0.10 V s⁻¹, red line; 0.30 V s⁻¹, blue line, 0.50 V s⁻¹, green line; 1.0 V s⁻¹, brown line.

Figure 4

SW voltammograms of: HMF (0.08 mmol dm⁻³), 2F (0.10 mmol dm⁻³), 3F (0.62 mmol dm⁻³), 2FA (0.36 mmol dm⁻³), and 3FA (0.18 mmol dm⁻³) in 0.1 mol dm⁻³ LiCl aqueous solution, WE: Hg; AE: Pt; RE: SCE. Potential scan rate: 0.10 V s⁻¹; pulse height: 4 mV; frequency: 15 Hz; modulation amplitude: 25 mV.

Figure 5

SW voltammograms of (1) a pure strawberry tree honey (50% w/w) in a 0.1 mol dm⁻³ LiCl solution in water; (2) the same solution 1, after the addition of 0.75 mg of HMF, (3) the same solution 1, after the addition of 1.50 mg of HMF; (4) the same solution 1, after the addition of 2.25 mg of HMF. WE: Hg; AE: Pt; RE: SCE. Potential scan rate: 0.10 V s⁻¹; pulse height: 4 mV; frequency: 15 Hz; modulation amplitude: 25 mV.

Figure 6

SW voltammogram of HA (0.1 mmol dm⁻³) in 0.1 mol dm⁻³ LiCl aqueous solution, WE: Hg; AE: Pt; RE: SCE. Potential scan rate: 0.10 V s⁻¹; pulse height: 4 mV; frequency: 15 Hz; modulation amplitude: 25 mV.