I work in the science behind nutrition and one of the first things you learn is that a huge amount of nutrients from your food goes down the drain when you throw out the cooking water. In the water you use to cook broccoli, chickpeas, tomato, basically any vegetable, you will find a significant fraction of the vitamins as well as some carbohydrates and fats.
I once read something about a "perpetual stew", a stew that people in the Middle Ages were keeping all year long on the fire. Whenever you needed to cook something in water, you threw it in the stew: not only it added the flavor of the stew to your meal, it also changed the taste of the stew.
I like this concept, and I was thinking that each time I cook something in boiling water, I could save that water to prevent wasting all the nutrients it contains (and likely pesticids, too, but that's another story). Keeping something on the gas at all times is a crazy ideas in this day and age, and unrealizable in my apartment. But I was thinking of buying a big jar and keeping it in the fridge at all times, filled with the broth from whatever I cook (mostly vegetables and starchy foods, the idea of putting meat in a perpetual stew grosses me out a little bit). I would fill up the jar with the water while it's still hot, and immediately close it.
Do you think it's a good idea? Would it improve the taste of my food in addition to increase its nutritional value? And would keeping it in the fridge be enough to prevent contamination and bacterial/fungi growth?
Edit: my question does not concern soup per se. I'm not planning on leaving any chunk in there, no solid food, only slightly-flavored water. This limit the amount of carbohydrates contaminants could use as food, and render it pretty low. Besides, I have specifically mentioned not wanting to elt the soup on the fire at all time.
In the comments of the first answer, I have concluded that I will probably be cleaning the jar every time I use the broth, and boil it before placing it in the jar again. This makes my question really different from the soup one.
Best Answer
I like this idea, a lot actually, it would be an interesting experiment, but you would have to keep it on the gas.
Ignoring the safety concern of accumulating pesticides, which you mention, this would be like any stew which a quick Google search tells me should last about 3 to 4 days in the fridge. At least you don't want to cook meat in it. Bringing it to a boil over and over again might help but you're also adding a new risk of contamination every time. Definitely not a long term solution.
In terms of flavors, you're basically making a stew out of anything; it could turn out great or horrible depending on what you cook but over time it would surely develop some strange off-flavors.
If you want to reuse your cooking water, I would do so in the same dish; for example, in the sauce. Or maybe you could keep it for a soup you want to make in the next day few days.