You’re absolutely right, raised corks can either be a problem (if they’re too high, or too high of a percentage from bottle-to-bottle) or it could be nothing at all. The devil is in the details and finding out why. Bear with me while I break down a few possible causes and their implications of what you discovered in your mixed case:
Possible causes
1. Uneven screwing-down of the plunger guides on the bottling line.
This happens when the plungers (controlled by the automatic machinery on the bottling line) don’t quite push the cork in all the way. Usually a slightly raised cork is caught by the workers overseeing the bottling line, and they make adjustments during the run to get the corks to line up with the lip of the bottle (or a few millimeters below) for the remainder of the bottling run. Bottling lines consist of many pieces of machinery that must operate in synchronicity and every commercial bottling start-up is an art and science of making minute adjustments until all the parts (rinser, filler, corker, capper, labeler, packer, etc.) run in concert. There will be natural variations in every bottling run and every bottling line, it’s just a question of how quickly everything is “run in” to spec by the crew.
-Verdict: No big deal unless there is a high percentage of raised corks per case or pallet or some of the corks are sticking out so much that the bottle tops look like mushrooms, out of desired specifications. It is purely a visual packaging issue and doesn’t mean the wine is spoiled or compromised in
any way.
2. Wine was exposed to heat during transport.
Higher temperatures over time during transport can cause the headspace gas (usually a mix of air and nitrogen, depends on the bottling line sparging treatment) in the bottle to expand, pushing corks outwards. If it appears, this defect usually presents in transport batches, where for instance an entire truckload will be affected to a similar degree. This defect is less likely if there is high variation of degrees of raised corks within a case or at random over a pallet. To investigate if heat exposure was the cause, clues can be found if there is evidence of entire batches being raised, i.e. investigate if a pallet or truckload was broken down to discover if there was homogeneity in heat exposure.
-Verdict: Potential that wine quality has been negatively impacted. You need to taste the wine to see if it tastes cooked or visually has turned brown, all signs of premature heat oxidation in the bottle.
3. Wine is experiencing microbial activity in the bottle.
If the wine was not sterile filtered, or if the sterile filter on the bottling line is compromised at some time during the run, corks can push due to re-fermentation (could be bacterial or yeast, yeast is more likely) in the bottle. Since you’re not experiencing this in every bottle in a batch, batch sterile filtration failure is likely not the cause. Further investigation/information is needed as to %, number, and distribution of pushed corks. It’s possible random bad corks (maybe poorly re-expanding in bottle) have caused oxygen to enter in only some of the bottles, triggering a microbial bloom if there is a food source for those organisms, as follows: It’s possible in the case of a sweet wine or a wine that is not ML complete because in each of these cases, the wine is offering “food” (sugar, malic acid) to microbes. If conditions are right, random refermentations are possible.
-Verdict: If a re-fermentation is present, this is a potential issue because you may note if the wine is fizzy or cloudy due to the refermentation and corks may push further out and more bottles may become affected over time. Microbial analysis and sensory analysis of a good sampling of multiple bottles would help to settle if this was the cause or not, but since you bought a mixed case you’re not about to go back to the store and do this, right? Since you can’t tell if every bottle of that particular kind is affected, more information is needed to investigate the source/reasons.
I wouldn’t be surprised to see a few bottles in any given large bottling run have corks slightly above the bottle lip but it’s certainly a problem if corks have pushed over time, or if it’s an unacceptable incidence/percentage. As you recognize with your home winemaking and home hand-corking, it’s easy to get bottle-to-bottle variation. However, if for those wines that have pushed corks all of the bottles from the shop are like that, it’s an indication something may be wrong with those bottles from the get-go. Since you bought your wine from your local shop, you might want to give your shop owner some feedback, especially if you know him or her and especially since it sounds like you’re a repeat customer.
Maybe they can contact the producer and do a little digging for you. At the very least, I as a winemaker would appreciate the feedback if all of a sudden my corked bottles started pushing up in the marketplace!
