The control and measurement of the oxygen permeability of the packaging is one of the most important aspects to take into account in order to maintain the quality of food products during the manufacturing, packaging and conservation process. Food oxidation is related to the loss of organoleptic properties and reduced durability.

In the case of the wine sector, the evolution of the wine is extremely dependent on the amount of oxygen that the wine receives. Oxygen is crucially involved in the oenological process, from the collection of the grapes to the opening and consumption of the wine itself (Zaldivar E et al. 2017). The processes in which oxygen intervenes have been studied and analyzed for years, reaching the conclusion that, to a greater or lesser degree, oxygen determines the development of the brewing process and defines the final sensory characteristics of the wine Oxygen management and oxidation-reduction reactions are part of the main challenges winemakers face during wine production and aging (Laurie and Clark, 2010).

The processes by which the wine can have a contribution of oxygen occur when there are movements of the wine, filtrations and bottling (table 1 -Palacios A. et al 2017- and see also figure 1). However, moderate exposure to oxygen is known to be beneficial as a color stabiliser, reducing astringency and bitterness (Singleton 1987, Castellar et al 1998, Atansova et al. 2002).

Table 1. Wine movement operations and their relationship with oxygen supply

Figure 1. Supply of oxygen throughout the winemaking process presented at AWITC workshop by Olav Aagaard

Bottling is a complex and delicate process involving many factors that must be controlled. In addition, it is known that the amount of oxygen that is introduced at the beginning and at the end of the bottling process is greater and describes a U-shaped curve. According to the quality director of Constellation Brands, the bottling process should start with a dissolved oxygen (DO) lower than 1mg/L, and with an increase lower than 0.30mg/L. When filling, the oxygen concentration in the headspace of the bottle (HS) can range from 1.5 to 2.50mg/L (Nygaard, M. 2010). Ken Fugesang, professor of oenology at the University of California  recommends total oxygen (TPO) levels within  of the bottle below 1.25mg/L for red wines and below 0.60mg/L for rosé wines (Letaief H. 2016). Wine Quality Solutions researchers recommend a TPO of between 2 to 4mg/L and a tank dissolved oxygen before bottling of approximately 0.50mg/L.

There are various data and recommendations about the bottling process, some of them contradictory. This fact indicates that every wine has one bouquet determined, more oxidizing or more reductive characteristics in accordance with its nature and, therefore, different capacity to accept more or less oxygen throughout its production and bottling process. At the same time, measure and control the TPO during bottling  it is essential to ensure the quality of the wine and its useful life.

After bottling, exposure to oxygen depends on the efficiency of the closure. The permeability of each cap is measured as the oxygen transfer rate (OTR) or oxygen that enters the bottle over time: either through the cap itself, from exchange with the environment, or through air passage between the cap and the walls of the bottle (figure 2). Another reference measurement is that of the total oxygen of the container (TPO) which corresponds to the sum of the dissolved oxygen in the wine (DO) and the oxygen in the headspace of the bottle ( HS). It is documented that OTR, TPO and wine storage conditions are key parameters to ensure the correct organoleptic status of wine (Godden et al. 2005).

Figure 2. Ways of oxygen entering the bottle during wine aging. (1) Diffusion through the pores of the cap. (2). Diffusion between the interface between the cap and the bottle. (3). Oxygen expelled by the cap, outgassing. Lopes P. et al (2007) Main Routes of Oxygen Ingress through Different Closures into Wine Bottles)

The oxygen permeability of the stoppers is a parameter that must be taken into account when assessing the stopper-wine interaction. It is now known that a slight continuous and constant supply of oxygen seems to be beneficial for the maturation of red wine, given that the phenolic oxidative reactions enhance the color and reduce the astringency. However, when it comes to the production and storage of white wine, it is believed that any entry of oxygen is considered negative. Until a few years ago, alternative caps could not compete, in terms of oxygen permeability, with corks. Several studies supported the fact that the best companion of a good wine was the cork, since screw caps could give reduced compounds, given that they allowed little oxygen to enter, and on the contrary, synthetic corks could give compounds oxidizers because they allowed too much oxygen to enter. The cork was the only one that had a constant and prolonged supply of oxygen that favors maturation. 

There are several methodologies to determine the oxygen permeability of a cover, either through the measurement of the oxidation-reduction potential, the measurement of dissolved oxygen by polarographic probe and the measurement of the gas composition at the headspace of a flask using gas chromatography. The problem with these techniques is that they are destructive techniques. For this reason the most recent studies use non-destructive techniques such as the Mocon, a colorimetric method based on the color change of the oxidation-reduction reaction of indego carmine or the NomaSense TM O₂.

Currently not having a proven and standardized methodology  due to the measurement of OTR it is difficult to assess the values of OTR for different coverings. It is essential to know both the matrix of the wine and its oxidative capacity as well as the OTR of the cap.

ICSURO has the technology to measure the OTR, specifically a Nomasense P300 PSt6 and PSt3 sensors to make oxygen measurements by non-destructive oxo-luminescence with a detection limit of 0.02hPa and 0.31hPa respectively. Partial pressure units are easily converted to mg O2 as a function of time.

Maria Verdum
Department of R+D+i
Catalan Institute of Cork Foundation