I occasionally make soured mash to add to batches of beer for several reasons. I would most often make a simple starter, sour it over a couple days and then pitch it into the kettle for flavor. However, I wanted something that would be more consistent, for experimentation, and that I could add either into the mash (for ph adjustment), to the boil kettle (for flavor). I also wanted to be able to add to the fermenter, primary or secondary, if I chose, while still knowing how much sourness I would get and without worrying about the antiseptic nature of the hops slowing down the process.
I started with a gallon starter of about f 1.044 gravity (1 pound of Pale DME to one gallon of water just to keep things simple).
I cooled it to about 100 degrees, then transferred it into a small cooler that I keep for souring wort. I added a couple ounces of raw grain to this to inoculate with lactobacillus and then covered it with plastic wrap (directly on top of the liquid) to keep out oxygen.
I let this sit for three days and opened it daily to check the temperature and add a small amount of boiling water to bring the temperature back up to around 85 to 95 as needed.
Once it had soured nicely I canned the liquid in small pint mason jars. Though a gallon is eight pints, I only filled seven jars because my canner can process only seven jars at a time. You will notice that I used a water bath canner instead of a pressure canner. Do not do this with regular wort. Regular wort is not acidic enough and you can end up with something toxic! The Department of Agriculture guidelines for canned foods sets a threshold of 4.6 ph. Anything below this can be canned in a water bath canner. Anything above this needs pressure to hit the proper temperatures. My wort was well below 4.6 (thank you lactobacillus) so I opted for the easier method--since I don't own a pressure canner.
I now have seven pints jars of soured wort that I can safely add at anytime in the process and because it was canned there is no need to boil or otherwise sanitize it.