Why is a pig-skin like substance floating on top of my vinegar? Apologies if this is off-topic.
Every year, I make about 25 litres of cider vinegar. I wash the apples in a bucket of tap water, press them, add a sachet of yeast and leave the fermenting vessel in the garage. The juice is very acidic so not very pleasant to drink. It does however make tasty vinegar.
I have several fermenting buckets in the garage each with a different vintage of vinegar. Floating in each is a jelly-like mother of vinegar. Today I discovered that the yeast in one of the vessels (18 month vintage) has gone a bit wonky. The mother is about an inch thick. It's mostly jelly-like but the very top has the texture of pig skin, is very tough, and smells like cheese. The vinegar has lost its acidic taste and also has a hint of cheese.
Can anyone tell me what might have happened? Why has the mother become so tough? Why does it smell like cheese? Is it safe to eat?
That looks like a symbiotic culture of bacteria and yeast -- a SCOBY.
As to why a SCOBY developed in one vessel and not the others, only guesses can be made. I found a blog post that may offer an explanation:

So a SCOBY is typically a SCOBY for all starting vinegar and kombucha fermentations when slow processes like vat or barrel fermentation are used. Both terms can work at this point. However, for vinegar around 1% acidity the yeast die off and at higher acidities the lactic acid bacteria die off as well leaving only acetic acid bacteria to feed on alcohol. At this point it is only a mother of vinegar containing one type of organism. Kombucha acidity usually ends between 0.5-1% while vinegar is 4% minimum and usually 5%. So calling a fermenting vinegar mother a SCOBY isn’t strictly accurate.

It may be the case that the yeast grew faster than the lactic acid bacteria and the solution never reached &gt;1% acidity in that vessel.  I would be interested to know if the pH of that vessel is higher than the others.