Check out pressure canning of wort. There are numerous links to threads about it. I've been doing it for quite a while with good results.
I measure dry malt into quart jars and add water by weight to get the specific gravity that I want for starters. Then I run them through the pressure canner and put them away, labeled with date, type of malt and specific gravity. About 9 quarts each time and they last for years.