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.