I posted a few days ago on an article in a trade publication based on an MW dissertation by Alison Eisermann-Ctercteko that examined the extent of screwcap damage in the retail chain, and the implications for wine quality. This caused quite a storm, because it claimed that 26% of all caps showed some physical damage, and over 8% of screwcapped wines showed sufficient cap damage to cause significant chemical changes.

The IMW kindly sent me a copy of the dissertation so I could see the data, and after having spent some time examining it, I think this figure of 8.2% is misleading.

Eisermann-Ctercteko examined over 10 000 bottles for damage, and then purchased 600 bottles of a 2012 Sauvignon Blanc for chemical analysis. She did a formal trial of 444 bottles with increasing cap damage, which she classified from A-D in ascending severity. There was also a class E which was where the cap was undamaged but hadn’t been applied correctly.

There were 84 controls and 72 wines in each category, with the damage inflicted artificially on each damaged bottle to the appropriate level. The wines were stored for 10 weeks, and a number examined chemically at two week intervals, with the largest sample being at the end of the 10 weeks when the remaining bottles were tested. The chemical analysis was for absorbance at 420 nm (looking at browning, a marker of oxidation) and free and total sulphur dioxide (SO2, again a marker of oxidation). Sensory analysis was also carried out.

Now let’s look at the data. Damage categories B-D all result in significant chemical changes, which is where the figure of 8.2% comes from (8.2% of bottles in retail have damage levels in these categories). But while these changes are statistically significant by some statistical tests, they are not very large, with the exception of damage category D.

Looking at the error bars (and let’s focus on the results at week 10, the most important because of the large sample size and because the analysis was by the AWRI), the only striking result is at damage level D.

Let’s now look at free SO2.

Let’s concentrate on week 10. The free SO2 levels are 33.9 (control), 32.8 (A), 32.7 (B), 32.6 (C) and 33.4 (E). D is the only significantly different level at 29.65. However, the technique used to examine the statistics, REML, does show significance for A-D at 10 weeks, apparently. I find this hard to see from the graph. Irrespective of this, the differences in free SO2 in A-C and E are nothing at all troubling. Even the level for D is pretty healthy, and not a cause for alarm.

The conclusion I would draw from these data is that damage level D is the only one I’d be worried about, and even then, I wouldn’t be too upset. Screwcaps in this setting seem pretty robust. And even D isn’t as bad as cork taint. In the sensory analysis, no significant differences were found by the panellists for any of the damage categories.

Here’s a picture illustrating just how badly damaged level D is:

From Alison’s data on the incidence of level of damage found in retail stores, that figure of 8.2% now becomes 0.06% (the incidence of level D damage found). It’s a much less alarming figure.

It should be added that this work forms just part of the dissertation, and while I disagree with the interpretation on this point, the experimental method and the work involved is very impressive, and cost a good deal of money (she wasn’t funded in any way). The survey data on the number of damaged screwcaps in retail stores are important, and while this doesn’t seem to be resulting in many sensory changes in the wine over the short term, it points out a potential issue that the supply chain and retail trade needs to address.