Wine bottle overcapping wax: is it an effective barrier to oxygen or just an aesthetic element?

Abstract

The use of waxes and resins to seal wine bottles dates back several millennia, as shown by archaeological evidence. In modern winemaking, wax overcapping is still practiced for aesthetic, branding, or perceived bottle sealing benefits. However, the effectiveness of wax overcapping as an oxygen barrier has received limited scientific attention. This study aimed to evaluate the oxygen barrier performance of wax overcapping, alone and in combination with cork-based stoppers, under controlled storage conditions (25 °C, 50 % RH). The first part of the study focused on processing conditions, particularly bottleneck temperature during wax application. Glass–wax adhesion was assessed via extraction force measurements, while oxygen barrier performance was evaluated through one-year oxygen permeation analysis using a chemiluminescence method. Preheating bottlenecks to 60 °C or 70 °C before applying wax at 120 °C significantly increased maximum extraction forces, indicating improved wax–glass adhesion. This enhanced adhesion was associated to low oxygen transmission rates (OTRs), comparable to those of the wax material itself. These results demonstrate that higher bottleneck temperatures during application improve adhesion and further limit oxygen transfer at the wax–glass interface. After optimizing wax deposition, oxygen transfer was monitored over one year in bottles sealed with natural and microagglomerated cork stoppers, with and without wax overcapping. Some stoppers were left without surface treatment to simulate leakage with high oxygen transfer occurring at the glass–cork interface. Corks were inserted 3.5 mm below the bottleneck, and wax was applied forming a solidified layer. For surface-treated corks, wax application was delayed by one month to avoid deformation due to cork outgassing. Untreated corks were waxed immediately after insertion. Results showed that wax effectiveness depends on the intrinsic oxygen barrier properties of both the wax and the stopper. For the low-permeability blend wax studied, barrier performance was similar to that of microagglomerated stoppers, indicating a mainly aesthetic role. For natural corks, material variability made it difficult to assess the wax contribution. In worst-case conditions, with untreated corks, wax could prevent gas leakage at the glass–cork interface, but only when applied immediately after insertion, a condition practically achievable only after pressure equilibration, typically after at least one month.