The Role of Voltammetric Analysis in the Wine Industry ^†

Abstract

Voltammetry has been used in many industries and has been a popular method for food analysis in recent years. Its high sensitivity, rapid analysis, usability in a wide variety of concentrations and temperatures have positive potential in the wine industry. With standardized methods for food analysis being too work-intensive and time-consuming for the fast market demand, the main purpose of this study was to gain an understanding of the possible benefits that voltammetry can provide to the wine industry. This review paper focused its discussion on the different voltammetric methods, parametric conditions, and electrochemical behavior of essential compounds from wine as the main substance of interest. In particular, the limit of detection (LOD), initial voltage peak, pH levels, working electrodes, and their respective reagents were taken into account to determine the applicable methods that can be utilized in the wine industry. This study could serve as a point of reference for future research into the topic.

1. Introduction

Voltammetry is an analytical method that gives us information about the “analyte through electrical currents obtained from a potential variation” [1]. This was first brought about in the 1920s by an experiment conducted by Heyrovský, which measured the surface tension of mercury to provide the information about the nature of the liquid–metal interface. This capillary electrometer could also serve as a sensitive null-point detector for balancing potentiometric bridges [2].

Today, voltammetry has expanded to greater variations such as linear sweep, hydrodynamic, stripping, and pulse and cyclic voltammetry (CV). The most common voltammetry methods used in the food industry are cyclic and pulse voltammetry. Cyclic voltammetry is an electrochemical technique which measures the current that develops in an electrochemical cell under conditions where the voltage is in excess of that predicted by the Nernst equation. On the other hand, pulse voltammetry utilizes a regularly increasing pulse height that is applied at periodic intervals. These methods have been utilized to locate, accurately measure and characterize substances like polyphenols in wine products [3]. Food analysis has used systems that were put into operation for the guarantee, standardization, and safety of its consumers. There were two prominent methods for food analysis, namely gas (GC) and liquid (HPLC) chromatography, which are used alongside statistical tools to examine the correlation between samples. Although both GC and HPLC techniques are widely used in wine analysis and are based on traditional separations that are frequently coupled online with other analytical devices, the main drawback is their unreliability in analyzing non-volatile substances [4]. Recent advance technique like capillary electrophoresis and supercritical fluid chromatography have also been used for wine determinations, but they are still in an early stage of application [5].

The recent demand for voltammetry in the food industry is largely due to the demand for faster and more efficient systems that can provide faster and more precise results with a higher sensitivity to environmental monitoring as the world moves forward in technological advancements [5]. Aside from its usage in the food industry, voltammetry has also been used in analytical chemistry and pharmaceutical industrial processes.

The numerous methods for voltammetric analyses have sparked a large number of studies using different samples, especially food substances. The polyphenols and antioxidants that are commonly found in wines have been the subject of research, as wines also possess potentially harmful chemicals such as sulfides which are used to preserve the freshness and taste of red wines [6]. Other properties such as the oxidation state and oxygen consumption can be monitored by voltammetry [7]. Due to the flexible nature and low cost of common voltammetric set-ups, this determination system has many advantages in the wine industry. This study benefits those that will utilize the findings to further the study of voltammetry as it provides researchers with an overview of the past and present trends of voltammetry, serving as a point of reference for future research.

This paper will only cover voltammetry in the wine industry, including its possible uses, effects, and outcome. The studies that utilized voltammetry in the analysis of main substance not related to wine production is not included in the review. Finally, only papers accessible in the literature covering the year 2000 to 2021 has been the focus of this review article.

2. Methodology

In this review article, the systematic literature review protocol proposed by Okili and Schabram (2015) [8] was adapted. Shown in Figure 1 were the collected and screened studies used in this paper. The four main steps were Planning, Selection, Extraction, and Execution, and all included papers were extensively reviewed for inclusion in the study. Planning involves determining the proper keywords to be extracted from the paper; this includes ensuring that the publication date fits into the given timeframe. The selection of papers was conducted automatically through technology-based databases, and then papers were manually selected if they fit the criteria for the systematic review. The extraction of information was then conducted manually for each paper and cross-referenced to ensure that the information was accurate and aligned with the objectives of the paper. The execution of the systematic literature review was then conducted with the data retrieved from the papers, highlighting the common trends and factors.

The Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines were also observed for the data collection procedure of this study, with the final version shown in Figure 2. The extensive screening was performing to ensure that only relevant and important papers were used in this study; sixty-nine papers were identified from online records. An initial screening was accomplished using the keywords of ‘voltammetry’, ‘wine’, ‘substances’, ‘sensor’, and ‘electrode’; duplicate papers were manually removed from the database alongside papers that do not possess relevant information on the given topic. Once the papers passed the initial screening, they were retrieved for further analysis; only one paper was excluded from the retrieval process (n = 1). The final screening for eligibility was performed by individually reading the abstracts of the screened papers; four papers were excluded from eligibility. This was due to the fact that two papers did not explicitly use voltammetry to identify the compound, another paper did not use wine as the main substance of interest, and the final paper was excluded since the paper solely focused on oxidation levels, instead of tangible substances. To determine the most suitable method of determination for each compound, factors such as voltametric methods, working electrodes, the limit of detection (LOD), and pH levels were reviewed from each paper.

3. Results and Discussion

3.1. Electrochemical Detection of Components

According to Cordova and Sumpio (2009) [6], a generic bottle of red wine is composed of approximately 1.8 g/L of total polyphenols (TP), which is an important component of wine products that needs to be detected to ensure quality, while a bottle of commercial white wine consists of only about 0.2 g/L to 0.3 g/L of TP. Thus, a glass of red wine would contain around 200 mg of polyphenols, and a glass of white wine would have 30 mg. In red wine, the majority of the phenols are flavonoids.

2,4,6-trichloroanisole (TCA) is a low threshold sensory metabolite originating from the chlorophenol family, usually by means of natural fungal strains in corks. As cork stoppers are widely used by a plethora of wine companies, ‘cork taint’ is a common defect associated with the spread of TCA throughout the bottle; on average, this costs the wine industry over USD 10 billion in losses worldwide [9].

Sulfur dioxide (SO₂) is one of the most common compounds found in wine. Wines that contain more than 10 mg/L of SO₂ should not be consumed on a regular basis, as too much SO₂ may cause adverse side effects. Much like voltammetry, SO₂ can be easily influenced by pH levels; as the pH in wine increases, there is a higher SO₂ content and vice versa [10]. The number of trace minerals and metals are low in wines, with the most prominent trace mineral being manganese. Manganese (II) concentrations range from 0.4 to 7.8 mg/L for red wines, while white wines have, on average, 1.3 mg/L [11].

3.2. Method of Determination

Of the thirteen studies reviewed for wine components, a total of six studies made use of cyclic voltammetry. As shown in Figure 3, two used differential pulses, four used square waves, and one used stripping voltammetry.

In determining the polyphenol content of wine, cyclic voltammetry (CV) was used most frequently. However, Vilas-Boas et al. (2019) [12] claimed that differential pulse voltammetry (DPV) is a better voltammetric method to use between DPV and CV because it has a lower sensitivity of 64% to sulfur dioxide—which may interfere with results for polyphenol activity [12] compared to the 141% measured by CV.

For detection of metals, such as manganese (II), stripping voltammetry (SV) was utilized as the voltammetric method. Sulfur content and sulfur dioxide were utilized in three studies, two of which used square wave voltammetry (SWV) and the others used CV. Ascorbic acid was found in two studies using both CV and SWV. Lastly, 2,4,6-trichloroanisole (TCA) was found in one study with CV as its method of choice.

3.3. Comparison of Electrode Performance

As seen in Table 1, the pH levels for sulfides were reported to be >1.0 by Ramos et al. (2016) [13] which are relatively low, while total sulfur dioxide was recorded at pH 3.3 by Makhitkina and Kilmartin (2010) [14] who also recorded peak voltages at 1120 mV using CV. The limit of determination (LOD) for free and total sulfur dioxide was recorded at 3 mg/L, while sulfides were recorded at 0.4 mg/L, showing how flexible SWV can be. Both studies noted that their respective LODs can be easily increased or decreased depending on the extraction time utilized [15].

On the other hand, all pH levels for flavonoids and polyphenols (Table 2) were reported to be between 3.4 and 3.76. Catechins show a common cathodic peak at around 400–500 mV using CV and increased to around 730–750 mV when DPV and SWV were employed. The LOD was lowest at 1.77 mg/L with DPV [16,17]. With the DPV voltammograms having narrower potential ranges, it can be inferred that CV is less efficient than DPV in differentiating between wines with diverse compositions; DPV is not susceptible to residual current, while cyclic is sensitive to it [12].

Ascorbic acid (AA) (Table 3) was determined using CV and SWV. Despite identical LOD and pH levels, which were 4.14 mg/L and 7.0, respectively, CV displayed a higher first peak at 596 mV, while for SWV, it occurred at 450 mV [18,19]. The manganese (II) content was determined through SV, showing a peak of 550 mV with a low LOD of 0.6 mg/L.

Table 1. Performance of electrodes in the determination of metals and sulfur-based compounds found in wine.

Working Electrode	Component	Method	Peak	LOD	pH	Reference
AgCl/Ag (3M 78 KCl)	Free and total sulfides	SWV	*n.r.	3 mg/L	n.d.	[20]
Glassy carbon (GC) electrode	Free sulfur dioxide	CV	1120 mV	*n.r.	3.3	[14]
Graphite Electrode	Manganese (II)	SV	550 mV	0.6 mg/L	5–5.2	[17]
Screen-printed carbon	Sulfides	SWV	*n.r.	0.4 mg L	>1.0	[13]

*n.r.—no reading.

Table 1. Performance of electrodes in the determination of metals and sulfur-based compounds found in wine.

Working Electrode	Component	Method	Peak	LOD	pH	Reference
AgCl/Ag (3M 78 KCl)	Free and total sulfides	SWV	*n.r.	3 mg/L	n.d.	[20]
Glassy carbon (GC) electrode	Free sulfur dioxide	CV	1120 mV	*n.r.	3.3	[14]
Graphite Electrode	Manganese (II)	SV	550 mV	0.6 mg/L	5–5.2	[17]
Screen-printed carbon	Sulfides	SWV	*n.r.	0.4 mg L	>1.0	[13]

*n.r.—no reading.

Table 2. Performance of electrodes in the determination of polyphenols and flavonoids found in wine.

Working Electrode	Component	Method	Peak	LOD	pH	Reference
Ag (Silver)	2,4,6-trichloroanisole	CV	*n.r	0.08–0.16 mg/L	n.d.	[9]
Glassy/screen-printed carbon electrode	Catechin	CV	151 mV	*n.r.	3.6	[19]
Glassy carbon (GC) electrode	Catechins	DPV	750 mV	1.77 mg/L	n.r.	[16]
Glassy carbon (GC) electrode	Polyphenols	CV	450 mV	*n.r.	3.3	[14]
Glassy carbon (GC) electrode	polyphenols	SWV	395 and 730 mV	**n.r.	3.46–3.74	[15]
Glassy carbon (GC) electrode	Total polyphenol (TP) content	DPV	440–475 mV	1.77 mg/L	3.6	[21]

*n.r.—no reading.

Table 2. Performance of electrodes in the determination of polyphenols and flavonoids found in wine.

Working Electrode	Component	Method	Peak	LOD	pH	Reference
Ag (Silver)	2,4,6-trichloroanisole	CV	*n.r	0.08–0.16 mg/L	n.d.	[9]
Glassy/screen-printed carbon electrode	Catechin	CV	151 mV	*n.r.	3.6	[19]
Glassy carbon (GC) electrode	Catechins	DPV	750 mV	1.77 mg/L	n.r.	[16]
Glassy carbon (GC) electrode	Polyphenols	CV	450 mV	*n.r.	3.3	[14]
Glassy carbon (GC) electrode	polyphenols	SWV	395 and 730 mV	**n.r.	3.46–3.74	[15]
Glassy carbon (GC) electrode	Total polyphenol (TP) content	DPV	440–475 mV	1.77 mg/L	3.6	[21]

*n.r.—no reading.

Table 3. Performance of electrodes in the determination of ascorbic acid (AA) found in wine.

Working Electrode	Component	Method	Peak	LOD	pH	Reference
Glassy-carbon (GC) electrode	AA	CV	596 mV	4.14 mg/L	7	[18]
Glassy-carbon (GC) electrode	AA	SWV	450 mV	4.14 mg/L	7	[18]

Table 3. Performance of electrodes in the determination of ascorbic acid (AA) found in wine.

Working Electrode	Component	Method	Peak	LOD	pH	Reference
Glassy-carbon (GC) electrode	AA	CV	596 mV	4.14 mg/L	7	[18]
Glassy-carbon (GC) electrode	AA	SWV	450 mV	4.14 mg/L	7	[18]

4. Conclusions

There are different optimal methods and factors when determining different components and in varying situations. Working electrodes, reagents, LOD, and methods of voltammetry all play important roles in the determination of components and should be analyzed further in future studies. Lead (Pb) and gold (Au) both have impressive electrocatalytic abilities but have lower oxidation peaks and there is a risk of contamination of the sample by oxidation and/or reduction. The most common working electrode is a glassy carbon electrode (GEC) due to the fact that it possesses the widest electrochemical window. CV is most suitable for polyphenols because of its accessibility to researchers and parameters of detection, while DPV may be better suited if the situation calls for a higher degree of sensitivity or flexibility.

With this information in mind, future studies and research should be able to utilize a wider spectrum of voltammetric methods to identify different common wine components more efficiently. Finally, pulse techniques, such as DPV, are more advantageous since they are more sensitive than the linear sweep methods because there is minimization of the capacitive current.