(Academicwino) - Natural cork stoppers are the historical fan favorite for closing bottles of wine, and though while nowadays there are many more options, natural cork still remains a popular choice for many wineries, particularly higher price point bottles.

One of the reasons why some wineries choose to go with synthetic corks or screw tops (or some other type of closure) is that the natural variability in cork in terms of allowable oxygen ingress is too great to justify the higher price. In other words, due to natural variations in the bark of the tree, natural cork closures can have a variety of pore sizes within them, with some allowing for greater oxygen ingress than others. Too much oxygen through the cork over time can lead to several faults in the wine, most commonly oxidation. This can lead to wines tasting a little off (like an overripe/browned fruit) and can also have negative effect on the color of the wine. TCA is another major fault found in wines closed with natural corks, though that’s another story for a different post.

Natural cork can vary in terms of the sizes of pores within the material for a variety of reasons, including insect activity, microbial activity, and wetcork spots. Insects can cause larger pore spaces within the cork material by digging around to lay their eggs and grow their developing larvae. Specific pests known to do this in cork are the beetle Coroebus undatus F. and the ant Crematogaster scutellaris. The beetles in particular will lay their eggs inside the cork and the larvae will dig around and develop for upwards of 2 to 3 years.

Wetcork is an area within the cork with a high percentage of water content (400-500%!) compared with a normal area of cork that has about 25% water content present. These wetcork sections take a lot longer to dry and thus longer processing times than normal areas of cork, though if found after the cork has been harvested, a drying kiln is often used to speed the process up. During the drying process, the areas wetcork will shrink upwards of 30% due to the compression of cells and pores in the area.

A new study in the Journal of Food Engineering aimed to explore this issue of variability in the porosity of natural cork, and utilized a never-before used technique for this purpose to examine it further: Xray tomography. The goal was to examine how wetcork, beetle, and ant infestation affects the porosity of cork using the non-destructive X-ray method, and to compare to that how much oxygen ingress there is into a bottle of liquid using the damaged corks.

Brief Methods

150 natural corks were used in the X-ray portion of this study: 50 with wetcork defects, 50 with C. undatus (beetle) activity, and 50 with C. scutellaris (ant) activity. 30 corks were used for the oxygen ingress portion of the study.

The surfaces of each cork were imaged and analyzed for: porosity coefficient, average pore area, area of largest pore size, largest pore aspect ratio, average elongation, and largest pore elongation using AnalySIS computer software.

Images were compared with X-ray tomography images taken of the same cork surfaces. 3D images were created using the 2D X-ray tomography images.

Oxygen ingress was determined by utilizing a non-destructive colorimetric procedure. According to the study, the procedure used:

“an aqueous indigo carmine solution that is totally reduced by adding sodium dithionite in a controlled excess (the excess corresponds to the quantity necessary to consume the oxygen that enters into the bottle in the bottling operation) leading to a colour change from indigo blue to yellow. Once the excess of sodium dithionite was consumed, the reduced indigo carmine starts to consume the oxygen that is introduced into the bottle in known amounts, resulting in a colour change that was measured with a colorimeter apparatus.”

In other words, as oxygen enters into the bottle through the cork, there is a corresponding color change in the solution which can be quantified using a colorimeter.

After the solution was added to the bottles, they were corked and left upright for 24 hours. After the 24 hours, bottles were placed horizontally and “aged” for 12 months.