2-5% of wines under natural cork are returned at competition as being TCA-infected. Most judges hit a TCA infection between 3-10 parts per trillion. Most corked wine (3-5%) would have 5 ppt TCA infection.
" In a 2005 study of 2800 bottles tasted at the Wine Spectator blind-tasting facilities in Napa, California, 7% of the bottles were found to be tainted." Let's assume the TCA taint rate of those 7% were 3-4 ppt.
.5 ppt TCA can change a wine, mute its nose and change the mouth feel. Now if you read the literature, especially that funded by cork companies, they will stress that most consumers can't tell a wine is corked under 3-4 ppt.
What they don't say is that even slight TCA infections (>.5 ppt in my 20 year pro wine and judging experience) DO CHANGE a wine's aroma, bouquet, mouthfeel and finish.
So if 7% of wines are corked to where almost anyone can tell, (5ppt), we can assume a ten times higher incidence of .5 ppt taint, which could be as high as (gulp!) 70%. I believe the 25%-40% rate I mention is a very low and conservative number.
I might even go so far as to say that MOST wines under natural cork are somehow influenced negatively by TCA. But I'll wait to get some more data from ETS to go on the record on that one.
Regardless, I think we can agree that there is no reason in the world for any cautious, sane winemaker to close a bottle of wine with natural cork.