Just to clarify, it IS possible to store fresh (boiled) wort in the fridge for extended times. I have proof of that, after over 8 weeks it was still uninfected and smelled and tasted (very small bit) fine, no sourness or funky aromas and flavors whatsoever. I used a quart take-out soup container, with its well sealing lid.
The problem, however, and why you shouldn't do it, is that in an anaerobic (no oxygen) environment it allows the deadly botulism to grow. Only one surviving spore is enough.
Now dropping the pH to 4.6 or below eliminates the botulism risk. But that's too acidic and thus not suitable for growing starters, as is. You could restore the pH to around 5.2 and then pitch the yeast, all should be fine. I'd still boil it before use though. So all in all, it is hardly worth the effort. Making fresh starter wort with DME at a 1:10 (weight) ratio to water is so easy.
I boil the starter wort in a stainless pot that has a well fitting lid, then let it chill in the sink or a plastic dishwash tub with cold water, before pouring into a flask, jug or 64 oz pickle jar.
