ReviewGas transfer through wine closures: A critical review
Graphical abstract
Introduction
Throughout food human history, cork and wine have been two complementary elements. Since the Antiquity, cork has been used as a stopper to preserve wine, first with amphora but today with glass bottle. In the 19th century, the production of cork stoppers increased a lot and has become mechanized along with the glass bottle industrialization. Even though cork has been used on the industrial scale for so long, the first study on the characterization of its alveolar structure (Fig. 1) and mechanical properties only dates back from the 80's (Gibson, Easterling, & Ashby, 1981), and the first one dealing with gas transfer from the middle 90's (Waters, Peng, Pocock, & Williams, 1996). As an alternative to natural cork stoppers and a good way to valorize cork waste (representing 65 to 85 hundred tons annually (Karade, Irle, & Maher, 2006)), stoppers made of cork particles agglomerated with food grade polyurethane adhesives were developed in the beginning of the 20th century (King & Dardig, 1968). These stoppers were first designed for sparkling wine, in order to respond to economic issues. Indeed, sparkling wine stopper diameter is about 31 mm (while for still wine it is around 24 mm) and tubing cylinders of such a diameter imposed to wait for a sufficiently thick cork bark (Gil, 2000). Agglomerated stoppers are generally categorized as a function of the cork particle size. The term “technical stoppers” has been attributed to closures made with the smallest particle size (less than 2 mm diameter), also called microagglomerated cork, in opposition to macroagglomerated cork (with particle size ranging from 2 to 8 mm) (Fig. 1). Nowadays polyurethanes used to agglomerate cork particles are composed of pre-polymers (resulting from the polymerization of polyol and low molecular weight isocyanates). Isocyanates used are usually aromatic, such as toluene diisocyanate (TDI) or methylene bisphenylisocyanate (MDI). But some aliphatic isocyanates can also be used such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI). Since agglomerated cork is a composite material, the properties of the resulting stopper depend on the particle size and the chosen adhesive. Furthermore, since the 90's, microagglomerated corks have also benefited from the development of new technologies such as CO2 supercritical treatment (T > 31 °C and pressure > 73 bar) (Aroso, Duarte, Pires, Mano, & Reis, 2015; Lumia, Perre, & Aracil, 2000). It has been set up at industrial scale for small size cork particles in order to extract the molecules responsible for off-flavor in wine, such as trichloroanisole, often pointed out for cork taint (Amon, Vandepeer, & Simpson, 1989; Taylor, Young, Butzke, & Ebeler, 2000). Nowadays, cork-based stoppers share 70% of the wine closures market, including natural cork stoppers and agglomerated stoppers (Serra, 2017). Even though natural cork stoppers represent a low mass quantity compared to the production of agglomerated cork (18 000 vs 135 000 tons exported in 2015 from Iberian peninsula, respectively), the added value is rather high with a selling price of around 30 $.kg−1 vs 4.3 $.kg−1 for agglomerated cork (Sierra-Pérez, Boschmonart-Rives, & Gabarell, 2015).
Screw caps were developed in the 70's with the aim to prevent the cork taint problem and to reduce the oxygen permeation for wines sensitive to oxidation. They were first used for specific markets such as Californian and Australian wine and then extended worldwide to reach around 14% of the wine bottle market (Cahill, 2007). Screw caps are principally made of aluminum with an inner joint to ensure the barrier property. Two joints are mainly used: (i) Saran joint composed of successive layers of expanded polyethylene (PEE), polyethylene (PE), tin, and polyvinylidene chloride (PVDC); (ii) Saranex joint composed of PEE inserted between two PE/PVDC/PE multilayers (Silva, 2011) (Fig. 1). The latter has been made to improve the gas barrier properties (Obadia, 1994). PVDC is used for its good barrier property to oxygen, PE as a water vapor barrier and PEE for the mechanical property of the joint. The tin layer ensures a good sealing of the system. Another alternative to cork-based closures are the synthetic stoppers, or stoppers made of synthetic polymers, which were commercialized since the end of the 20th century, especially to avoid cork taint problems in wine (Fig. 1). These stoppers are produced either by molding or by co-extrusion processing. They represent 16% of the worldwide wine closure market (Cahill, 2007). They are generally made of low density polyethylene (LDPE) for co-extruded stoppers, and styrene-butadiene-styrene (SBS) or styrene-ethylene-butylene-styrene (SEBS) for molded ones (Gardner, 2008). These polymers are commonly used for food packaging. A last category of stoppers can also be mentioned, considering crown capsules used during the bottle fermentation and ageing of sparkling wines. In this case, the barrier property to gas transfer is ensured by the gasket composed of LDPE/ethylene vinyl acetate (EVA) blend (White, 1996). It is noteworthy that despite of the alternatives offered by screw caps and synthetic closures, cork-based stoppers remain the most appreciated by consumers (Barber, Taylor, & Dodd, 2008, 2009). Thus, they are driving the worldwide market, which was estimated to be around 17 billion of closures sold annually (Amorim, 2017; Giunchi, Versari, Parpinello, & Galassi, 2008). Indeed, 86% of the worldwide consumers declared favoring cork stoppers, while they consider screw caps as well as synthetic closures dedicated for cheap wine (Serra, 2017). However, a few specific markets turned from cork closures to screw caps, like New Zealand market, where 80% of the bottles are sealed with screw caps (Choi, Garcia, & Friedrich, 2010).
Oxidation is the main issue related to the shelf life of wine. This is especially true for wine, which has the specificity to be sold without any shelf-life indication. Wine can therefore be stored for a very long time and the optimum for tasting can be debatable. Many studies published during the last forty years have displayed an interest in the field of wine oxidation (Karbowiak et al., 2009; Li, Guo, & Wang, 2008; Oliveira, Ferreira, De Freitas, & Silva, 2011; Singleton, Trousdale, & Zaya, 1979), which is more newsworthy than ever (Roullier-Gall et al., 2017; Roullier-Gall et al., 2016). Wine oxidation is mainly driven by the ingress of oxygen through the wine stopper, which acts as a barrier to prevent oxidation (Roullier-Gall et al., 2016; Silva, Julien, Jourdes, & Teissedre, 2011). However, the barrier properties of wine stoppers to gases and the mechanisms involved in the mass transfer are not yet well identified. Since no strong consensus exists concerning the closures performance as a barrier to gas transfer, it is rather complex to predict the resulting amount of oxygen entering the wine as a function of the closure used, and even more the impact on the shelf life of the product. Due to these problems of oxidation and the associated economic impact for winemakers, some research works investigated the gas barrier properties, first on natural stoppers and then extended to the other types of closure. To that purpose, several methods, either derived from food packaging or specifically dedicated to this application, were developed. However, regarding the corresponding literature, the comparison of the results remains very difficult due to the high number of parameters considered for gas diffusion determination.
The main objectives of this paper are, firstly, to expose the scientific background on gas transfer and its application to wine closures. Secondly, the different experimental methods used to measure gas transfer through wine stoppers will be presented. Then, an overview of the different studies dealing with gas transfer through stoppers will be given. Finally, a critical appraisal will be made on the methods and the closures performance comparison.
Section snippets
General background on gas transfer
One of the most useful properties of the materials used for packaging is their barrier property to gases, vapors or liquids, acting as a protection from the external environment and determining the shelf life of the packed product. The more the material is permeable to mass transfer, and the less efficient is its barrier property. The determination of the material permeability to mass transfer (Crank, 1975; Cussler, 1986) and especially to gases, such as oxygen, or to vapors, such as water, is
Experimental methods used for measuring gas transfer through wine stoppers
Since the 90's, various methods have been developed to determine gas transfer through stoppers. It has particularly been focused on oxygen transfer because this is the main factor limiting the shelf life of wine. These different techniques are briefly described subsequently in order to bring a clear view of what is experimentally measured before giving pros and cons of the different methods.
How to assess and to compare gas transfer data through wine stoppers
The main objective of this review was to collect all data available in the literature related to gas transfer through wine stoppers and to use them for calculating the diffusion coefficient D (in m2.s−1), which is the most relevant parameter to characterize the intrinsic gas transfer property of the material. When the calculation of D was not possible, the GTR (in mol m−2.s−1) and/or the permeability (in mol.Pa−1.m−1.s−1) were reported after conversion of the data into the same units. In order
Conclusion
This paper reports a full overview of gas transfer through stoppers with application to wine conservation. It highlights the difficulty to establish a relevant comparison between both the different methods and the stoppers used because each study often relies on very specific conditions (in terms of interface, compression, thickness, pressure gradient …) and lack of experimental details. To make comparison between the closures easier, it is necessary to clearly indicate all the information
Acknowledgment
The authors gratefully acknowledge Prats & Bonany, Relvas and the French Federation of Cork (FFL) and the French National Association of Research and Technology (ANRT) for their financial support and the PhD grant (CIFRE) of Kevin Crouvisier Urion. This work was also supported by the Regional Council of Bourgogne - Franche Comté and the “Fonds Européen de Développement Régional (FEDER)”
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