I know almost nothing about the subject of "Botulism", but I thought I'd mention it for someone else to talk about it. Just playing it cautious.
edit: from BYO Magazine:
Before answering this question about malt extract storage I want to remind our readers that there are no safety issues concerning the storage of wort that has been properly canned in a pressure canner. This whole topic began in 2006 when an article was published in the September issue of Brew Your Own describing canning wort using a boiling water bath instead of a pressure canner. Although the topic of botulism was addressed in the original article, Brew Your Own received a reader letter with stronger warnings and this was printed in the November 2006 issue. To sum all of this up, if you want to can wort and use it for yeast starters, go buy yourself a pressure canner and you will be just fine. I have written about the many uses of pressure canners in previous columns and think every serious brewer and cook should have at least one of them!
Now with that out of the way lets discuss why brewers do not to spend any time at all worrying about the growth of Clostridium botulinum in the malt extract. Malt extract, whether liquid or dry, is concentrated by removing water. One key attribute of food products used to gauge their susceptibility to spoilage is a property known as water activity or AW. Pure water has a water activity of 1.0 and as solids content increases the AW decreases. The definition of AW is not important here, but relates to equilibrium relative humidity. If you want to read more there is a bunch of information about water activity online and in food science books.
At any rate, Clostridium botulinum is not a problem in foods with an AW less than 0.93 because it doesnt grow. The water activity of liquid malt extract (LME) is somewhere around 0.60 depending on its concentration. Honey has an AW between 0.55 and 0.60, so it stands to reason that liquid malt extract with a similar concentration is going to be in the same range. Dried malt extract has an AW of about 0.20 making it very shelf stable from a microbiological view. You are correct that liquid malt extract is not pressure canned because there is no safety concern requiring it to be.
Molds and yeasts can grow on the surface of containers of liquid malt extract that have been opened. One would figure that if the fungi can grow on the surface they should be able to grow throughout the bulk of the LME, but they dont. The reason for this is that the AW of foods products is not homogeneous once the package has been opened because water from the air (humidity) changes the AW at the food-air interface. This is why LME can have mold colonies form on the surface. Covering the surface with vodka is one way to keep the surface clean. Another method is to repackage your 15 kilograms of LME in convenient sized portions using zipper storage bags so that the air can be eliminated from the headspace of the bag, keeping the AW homogeneous.
So from a safety stance you can store LME indefinitely, although the quality may change. To be realistic, if you store it in a clean refrigerator that does not contain a lot of smelly food that could impart odors into the LME, the shelf life is likely to be well over a year. If you really want to toss your LME into a freezer because you have more freezer space than refrigerator space you will certainly do no harm to it and will completely eliminate the possibility of any mold growth.
