I'd like to get 2 full boils from 1 mash. Can I keep the 2nd runnings overnight for boiling the following day? If so, what is the best way to store it.
In general, how long can wort be kept after mashing before it's boiled?
Short answer, a mash-out should be enough. Just hitting a boil is more than enough.
Mashing-out in this case would be key. You'll want to stop all enzymatic activity anyway, so that your wort is stable overnight. This consists of holding the wort at 170°F(76°C) for ten minutes. Here's a pasteurization curve for milk.
On the UK standard, an additional 5°C equates to one order of magnitude reduction in the time necessary to kill. That is, a pathogen that's killed at 69°C in one minute will die at 64°C in ten minutes.
HTST (high-temperature, short time) pasteurization for milk is conducted at 161°F(71°C) for 15-20 seconds. Mashing out happens at 170°F(76°C) for ten minutes. HTST pasteurization acheives a 5-log, or 99.999% reduction in pathogens. By this math, in the first 15 seconds of your mash-out, you're killing ten times as many pathogens as milk pasteurization. If you extend your mash-out to fifteen minutes, you're applying 100 times the thermal killing power that we do to milk. (It should be said, though, that the point of milk pasteurization is to kill pathogens that might harm humans, not to kill bacteria that might contribute to off flavors. But a hundredfold increase is worth something.)
If you want to get in to the science of it, here's a paper on beer pasteurization, which gets into the survival of various bacteria, including Lactobacillus delbruckii.
People want something easy to remember. "Heat it to boiling and you're fine." But killing micro-organisms happens over a time-temperature curve, not at a particular temperature. That's why chefs who use the sous vide method of cooking are able to hold highly spoilage-prone materials like beef short rib at a pathogen-friendly 130°F for 36 hours(!) with no ill effects. This kind of cooking gives the health inspectors the howling fantods, but it's not some sort of home-grown wisdom. It's science.
Short answer, a mash-out should be enough. Just hitting a boil is more than enough. In cooling down, you might generate some DMS, but it will be driven off when you re-boil it later.
If you can bring all the wort up to 185 or better yet just to a boil, you'll decrease the chance of some spoilage.
Grains are covered in spoiling bacteria and yeasts. The primary culprit is lactobacillus a souring bacteria. So over time you may get a little bit of souring if you store the wort to long.
In the end, the short answer is to try it. I have seen where several brewers try it and do it with success. I am mildly dubious, but if it works it works. Give it a try and then provide your own answer or an update in your question.
The cool thing is that you are making the same beer with the two collections then you can compare the overnight stored wort beer to the beer made "in process". That may not be a bad idea for the first time you try it.
The guy at my homebrew shop was telling me that he mashes and lauters at night, then boils the next morning.
Billions of cells of yeast specifically bred for beer can take a couple days to start fermenting. Leaving it in a covered, sanitized container overnight and then boiling it for an hour seems like it would be fine.
You can, but baddies will be chowing down on your delicious sugary wort overnight. I've heard of people using the brew in a bag method doing this though.
One thought would be to boil it for 15 minutes, put the lid on the kettle. I would think this would work well for a day or so. If you can fit your whole kettle in an extra fridge this would help as well.
Some will get in there, but with a starter, the yeast will quickly overwhelm them with low pH and alchohol.