I have only had an issue once.
I generally have at least a gallon or so put back for MMS
is made by the process of fermentation, followed by a few different methods of purification.
I think these processes have a lot to do with whether or not you will get organic growth.
I only use 99.99% food grade, I sterilize all my bottles.
(First with ClO2, then in an autoclave.) I realize most people can't autoclave, but even a bath in boiling water should work. I keep mine in a cool place like a cupboard.
I can't speak for what happens after the final user breaks the seal, but I have kept it for probably 3 months with no visible effect, or loss of acidity.
|The one time I had problems, I bought from a new source, and the crystals weren't as fine, and had a more lemony/sugary smell than what I normally use. It grew mold in a week.