I have for a long time thought to share some knowledge that I know that a lot of bubble lovers are puzzling over. What's up with the corks that looks like mushrooms in the base. You know the one like the picture above that just has not expanded properly. Many years ago I asked one of my friends in Penedès that works as winemaker for one of the high end producers. This was his explanation.
This mushroom shape is a defect related to the cork quality, known as "green cork" (direct translation). As you know corks for sparkling wines are usually composed of two different parts: the upper part that is agglomerated cork (grinded natural cork that has been extruded-pressed and glued with different silicones...), and the part that is in contact with the wine. This second part is made of 1 to 3 natural cork layer-discs. These discs have the desired and typical mechanic performance that you desire when closing a bottle. As you well know when fitting the cork in the bottle, you stretch-press it in order to fit inside the neck, afterwards the material expands itself making the bottle "hermetic". If this natural cork is not extracted from the proper tree sections of the cork-oak, or has not had a minimum time of "maturation" (time needed for the bark to achieve the desired mechanical properties), then you may have a problem with the closures. The problem is caused by a fungus in the cork, that makes water penetrate into the cork cells, and this in turn distort the elasticity and the natural ability of this material for recovering its original size after being pressed down the bottleneck. Sparkling wines with is cork are likely to have a quicker evolution - oxidation of the wine since oxygen gan get into the bottle.
So now you know why, and what to expect of the wine next time you come across a "mushroom cork".
Big hugs! /Anna