Analyzing Defects and Oxygen Ingress Rates of Natural Cork Using X-ray Technology

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.

Photo by Flickr user Holly Lay

Photo by Flickr user Holly Lay

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

Photo by Flickr user Júlio Gaspar Reis

Photo by Flickr user
Júlio Gaspar Reis

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.

Brief Results

Cork Characteristics Using X-Ray

  • Average lateral surface porosity coefficient was:
    • 7% for ant damaged corks.
    • 4% for beetle damaged corks.
    • 8% for wetcorks.
  • Insect-damaged corks had average porosity coefficients in the range comparable to standard porosity coefficients in poorer quality natural corks.
  • Wetcork had average porosity coefficients less than that of the average porosity coefficients reported for premium natural corks (1.7-2.0% compared with 2.4%).
  • Corks damaged by beetle larva had the largest pore sizes.
  • The numbers and features of smaller pores were similar between the three cork types.
  • The number and features of larger pores were significantly different between the three cork types (specifically between the two insect damaged corks and the wetcork).
    • Insect-damaged corks contained 2-3 pores greater than 10mm2 compared with wetcorks.
    • Insect-damaged corks in general had larger pores than wetcorks.
  • X-ray showed ant-damaged pores were typically empty, while beetle-damaged pores sometimes had excess material built up within them.
  • X-ray showed that wetcork had higher density due to compression of cell/pore walls during the drying process.

Oxygen Ingress

  • 21% of all the corks studied reached the limit of oxygen measurement defined by the method used by the end of the 12 month study (i.e. the method used couldn’t detect any more oxygen coming in after a certain point even though there likely was oxygen getting through).
    • 27% of these corks were ant-damaged.
    • 23% of these corks were beetle-damaged.
    • 13% of these corks were wetcorks.
  • Wetcorks showed similar oxygen ingress rates to corks with no known defects.
  • Insect-damaged corks had higher oxygen ingress rates than wetcorks and corks with no known defects.
  • For all corks, there was a significant ingress of oxygen within the first month after bottling, particularly within the first few days, then stabilized after the 3rd
  • Oxygen ingress rates for insect-damaged corks within the first month was 66.5μg/day for the ant-damaged corks and 63.1μg/day for the beetle-damaged corks, while oxygen ingress rates for wetcorks during that first months’ time was only 39.9μg/day.
  • Image/X-ray analysis and oxygen ingress data confirm insect-damaged corks have larger pores and larger oxygen ingress rates than wetcorks.
  • After 4 months, oxygen ingress rates were:
    • 2μg/day for ant-damaged corks.
    • 5μg/day for beetle-damaged corks.
    • 9μg/day for wetcorks.
  • Oxygen ingress rates were only significantly different between the cork types during the first three months, then stabilized and were statistically similar 4 months and after.

Conclusions

This study utilized X-ray tomography to image variability in natural corks caused by three

different types of damage: two insects and wetcork. These X-rays were then compared with oxygen ingress rates to find any correlations that may have been between the two.

The results suggest that insect damage to corks significantly increases pore sizes within the cork material, which corresponds to a significant increase in oxygen ingress during the first three months after bottling. Specifically, when compared with data on natural cork in general, insect-damaged corks corresponded with the poorest quality natural corks in terms of their pore sizes and oxygen ingress rates.

On the other hand, wetcork damage resulted in smaller pore size and oxygen ingress rates similar to corks with no known defects present. Compared with known data on natural cork, results from the wetcork were similar to pore sizes and oxygen ingress rates of the highest quality natural corks present on the market.

This study suggests that using X-ray tomography may be a good non-destructive way to analyze defects in natural cork, and also suggests that natural cork with wetcork “damage” actually improves oxygen ingress rates within the first three months of bottling due to pore shrinkage during the drying phase of processing. Despite this improvement in oxygen ingress, I wonder if wetcork damage could cause other problems for the bottle of wine not examined in this study. Further research is certainly needed.

What do you all think of this study? What future studies do you think need to be done here? Feel free to share your comments below!

Source:

Oliveira, V., Lopes, P., Cabral, M., and Pereira, H. 2015. Influence of cork defects in the oxygen ingress through wine stoppers: Insights with X-ray tomography. Journal of Food Engineering 165: 66-73.

6 comments for “Analyzing Defects and Oxygen Ingress Rates of Natural Cork Using X-ray Technology

  1. Terrytenley@comcast.net
    December 4, 2015 at 9:53 pm

    I have done my own bench tests the corks made from pieces hold up the best period. I have been testing this out for 8 years. Seems to many inconsistencies with high quality cork. The cork made from pieces has a constant level of protection. Why not test for this type od cork,?? It’s a green option.. Throw away cork. What’s left after making good corks.. It’s the best for me. Go test it please.

  2. Terry Tenley ..Sunrockwine.
    December 4, 2015 at 9:57 pm

    Many of my corks made from pieces preform very well. But never get consistent results from high quality cork. Even the veneer corks fail. Try a new approach. Test out the green cork. Pieces.. Be well all.

  3. Tom Stutz
    December 8, 2015 at 12:34 am

    Please explain how anything can be 400-500%. How can you have 4 to 5 times unity? Do you mean that it is 4-5 times the normal range of the moisture, which you state is 25%. This still means that you have 100+(!)% water and that there is nothing other than water present. I am confused…

    • WineKnurd
      December 10, 2015 at 6:18 pm

      It’s a weight/weight %, based on the average “dry weight” (or equilibrium weight) of the cork itself. Corks and other porous material like sponges and such can easily absorb liquids in multiples of their dry mass. Becca probably should have listed it as %w/w for clarity.

  4. December 10, 2015 at 5:08 am

    Interesting study but I would not entirely agree with conclusions. When speaking of OTR we should not take into account only the oxygen getting into the bottle through the cork itself or through the interface cork-glass but the oxygen released by the cork cells when subject to compression upon corking. I wonder if that much higher OTR during the first few days up to 3 months would not have more to do with the oxygen existing in the cork cells that is slowly released into the wine due to enormous pressure after reducing original cork diameter from 24mm or so to average 18mm bottle mouth diameter. Just a thought.

    • WineKnurd
      December 12, 2015 at 10:46 am

      Interesting thought Jose. If a wine bottle is laid on its side so that the “wine end” of the cork is completely covered by liquid, the majority of the compressed air in the cork would exit out the dry end, under the capsule. I wonder if this slow release of air from the wet end to the dry end of the cork creates a partial vacuum the “sucks” wine into the channels sometimes seen when uncorking a wine.

Comments are closed.