Elsevier

Analytica Chimica Acta

Volume 660, Issues 1–2, 15 February 2010, Pages 232-239
Analytica Chimica Acta

Dissolved oxygen distribution during micro-oxygenation. Determination of representative measurement points in hydroalcoholic solution and wines

https://doi.org/10.1016/j.aca.2009.09.048Get rights and content

Abstract

Red wine tank aging is monitored by organoleptic analysis, therefore, it is necessary to use an objective parameter representing the process. Among the possible parameters to be checked, it stands out the knowledge of dissolved oxygen because it offers the possibility of anticipating undesirable situations that bring about too much oxidation. Dissolved oxygen measurement, with non-intrusive luminescent technology is becoming an effective alternative. Uncertainty arises when trying to choose the measuring point able to represent the entire tank since previous works have considered the existence of gradients throughout the volume of the treated wine.

This paper shows the results obtained from the study of the existence and the quantification of gradients of the dissolved oxygen in a 15% hydroalcoholic solution during the micro-oxygenation process. Different measuring point placements are studied and the solutions to monitor the process by controlling a representative point are set out. A successful monitoring of a red wine tank aging with alternative oak products and adaptative micro-oxygenation has proved that an objective control of the process is, indeed, possible.

Introduction

Micro-oxygenation (MOX) process of wines stored in tanks has become a common practice in wineries all over the world. The oxygen doses added in micro-oxygenation process (with levels similar to the smallest natural ones in barrels, 20–25 mg L−1 year−1) are carried out with just one diffuser, in both small and big tanks. This situation and our previous experiences suggest that the oxygen application at certain and specific point causes the development of gradients in the dissolved oxygen concentration of wine. This gradient is strengthened when temperature gradients appear, reaching up to 1–2 °C [1], [2], in high tanks.

The usual control of the micro-oxygenation processes used in wine aging with alternative oak products (AOP) has been, up to now, the sensorial analysis of wine at different times of the process. This control, which is subjective, analyses wine situation but it does so only at a particular moment of the process so that there is no way of anticipating possible undesirable consequences given away by the existence of an oxygen overdose [3]. Therefore, a control based on an objective parameter is necessary to ensure the possibility of adjusting the process and preventing any possible problem caused by an excessive oxygen dosage. To achieve this objective, important analytical variables, such as the monitoring of free sulphurous, the evolution of ethanal as a sign of potential acetic acid formation, and also, the level of dissolved oxygen available in wine have to be controlled throughout the whole process. In order to do so, a sample must be extracted from the tank, both for SO2 and for ethanal and measured offline in the lab. The dissolved oxygen analysis could be done by extracting a sample, an intrusive method that could interfere with the process, or measuring it online with an external circuit and all possible contaminations [4].

Some authors state that the main antioxidant function of SO2 in wine is to react with H2O2, limiting, thus, the oxidation of ethanol and other saturated hydroxy compounds [5], [6]. Therefore, the sulphite in wine being micro-oxygenated must be controlled because its decrease could indicate oxidation and if so, the dosed oxygen would not integrate in the wine structure and would not react with the phenolic compounds producing then an increase on DO [7]. The uncertainty related to the SO2 measurement according to the official method CEE (9 mg L−1) [8] suggests that it is not an appropriate parameter to be used as a control value. As for ethanal, although it offers an accurate measurement, it is also an analyte resulting from the oxidation process. It is detected when the dosage kinetic is overcoming wine capacity to consume oxygen, capacity that rules the micro-oxygenation process of red wine. The oxygenation is practiced in red wines with a well-balanced phenolic content to stabilize their colouring matter. When a wine has a lack of a phenolic structure, it suffers an unlimited phenolic compounds’ polymerization causing dryness and brownish [9]. The oxygen doses added should be fixed according to the concentration of phenolic compounds, the dynamics of production and the ethanal consumption. The ethanal could in turn react to flavanols to induce the formation of a very reactive carbocation that quickly reacts either with another flavanol molecule or with an anthocyanin, producing ethyl-bridged flavanol–flavanol and flavanol–anthocyanin oligomers. More recently, it has been reported that ethanal also participates in the formation of new pigments such as vitisin B and other pyranoanthocyanins [10], [11]. These reactions are related to the appearance of more stable pigments, which have an impact on wine colour stability. Traditionally, ethanal excess is considered to possess an offensive odour and taste, which brings bitterness and oxidized flavour to wine. Timberlake and Bridle [12] first proposed one of the mechanisms that ethanal could contribute to the formation of dimer and trimer between flavanols (tannins), which was later confirmed by other researchers [9], [13], [14]. The colour of the yellow spectral region and the condensation degree increase [15]. Wildenradt and Singleton [16] showed that the coupled oxidation of ethanol to ethanal took place at an appreciable rate during phenols oxidation process. Therefore, the dosage of small oxygen quantities favours the emergence of the ethanal necessary to cause condensation reactions and polymerization among different phenolic compounds. On the other hand, an overdose of oxygen could produce a lot of ethanal considered a symptom of the beginning of an oxidation process in wine that too often means the loss of that wine. An important role of SO2 in chemical oxidation is to bind, reversibly, acetaldehyde, other aldehydes, and ketones. These mask the sensory perception of aldehydes because sulphites reduce aromas from the aldehydes [17].

The monitoring of the dissolved oxygen, which is available for wine components to interact, is of great interest since it indicates the balance between the consumption and dissolution of oxygen in wine during the aging process. It has to be taken into account that this parameter is, at present, very accurately measured with a very fast, non-intrusive and online method and that it is the main actor and cause of the oxidation processes that are led by the micro-oxygenation technology. Since the micro-oxygenation is practiced in wine aging with alternatives, it is important to point out that the knowledge of the usual levels of dissolved oxygen in barrel aging, a highly valued process, provides the reference values as well as the maximum limits that usually bring about an accumulation of oxygen not consumed by wines [8], [18], [19].

Regardless the parameter used, either the one inducing oxidation or any other indicating oxygen consumption in undesirable reactions (sulphurous and ethanal), it is very important to define the measuring point in the tank. This point becomes a key factor to work properly because it has to represent all the wine stored in the tank in order to carry out an ideal control of the micro-oxygenation.

In previous works of the group has been confirmed a variation in DO values according to wine homogenization and measure point [20].

This paper shows the results obtained in a non-intrusive way from the determination and validation of the reference point and from the online measurement of the dissolved oxygen content in a hydroalcoholic solution during a micro-oxygenation process. This has allowed the accurate control of desirable DO level available by the wine during the aging process with AOP. To this end, an adaptative MOX with variable oxygen dosage has been used.

Section snippets

Trials design

Wine works as a scavenger with the resulting management problem. In order to know how the oxygen is distributed throughout the tank when a low oxygen doses are used, it is necessary to guarantee that no wine compounds are consuming it. Hydroalcoholic solution was chosen to work with and to check the formation and evolution of gradients in the dissolved oxygen concentration. Thus, the tests related to gradients were carried out in a solution of pure water and ethanol at 15% volume, reproducing

Comparison of the measurement systems used

The behaviour of the luminescent system chosen to measure the dissolved oxygen in red wines was checked by comparing it with the electrochemical measurement system commonly used. According to our experiences, both systems are considered the best to measure dissolved oxygen in micro-oxygenated wines with DO levels from 0 to 0.1 mg L−1[4], [30]. They are high accuracy measurement systems [31].

Table 1 depicts the values obtained from repetitiveness, calculated from measures of the tonometer balanced

Conclusions

The luminescent technology suitability to measure DO is guaranteed enough to be used in a study of a MOX process. This non-invasive technology is shaping up as an essential tool to manage dissolved oxygen in tank aging processes.

It was proved that during the micro-oxygenation process significant gradients are produced in the dissolved oxygen concentration of the solution, which indicates that the oxidative process is heterogeneous in the treated volume. By a gentle pumping process, it is

Acknowledgements

Authors are grateful to Spanish Regional Government Project VA030/06 project for financial support and M. Vargas for the wine oxygen measurements. Authors are academic members of O2inWines™ (www.O2inWines.org), an international nonprofit association, promotes scientifically based solutions for oxygen management challenges in wine.

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