William R. Watt wrote:

> I've had homemade apple vinegar develope a solid plug in the neck over
> time in the cold room downstairs. No idea what it's composition, but it
> was a very tough consistency.

Rubbery? Likely a mother. Most commercial vinegars are pasteurized so
they don't do that and scare off consumers. Homemade ones aren't usually
pasteurized.

Pastorio