Notice how dogs enjoy gnawing on bones? Ever been to a restaurant where they serve bone marrow?
Boiling bones in water draws flavor out of them. Most canned broth and stock you buy--beef stock, chicken stock, etc--is just this--water boiled with bones for hours.
Most literature I've read suggests using raw bones, but some recipes call for roasted bones--the ones I've seen most often involve roasted veal bones.
I've also made stock from roasted chicken bones. The stock does still take on flavor. It's easier to get good flavor from unused bones, though.
Additionally, I've found another pitfall. I've tried to make stock from the leftover bones of bbq'd ribs. This was not a good idea. The broth had a savory flavor, as intended. Unfortunately it also had the background taste of bbq sauce. Now, when I do make stocks, I'd consider using leftover bones, but
- there have to be enough bones leftover (otherwise I get very little stock for my time or it's weak on flavor)
- the bones can't be "tainted" by other flavors (like bbq sauce)
To answer your original question, try this:
- start with a pot of plain water
- put about 4 lb of bones in per gallon of water while it's still cold, add ~1 tsp of vinegar per gallon of water
- Once the water comes to a boil, lower the heat so that it's just simmering
- this keeps the stock from getting cloudy/white (which doesn't taste bad, just looks worse)
- leave boiling for about 6-8 hours, minimum. Longer is fine, but you won't get too much more at this point.
- turn off heat, allow stock to cool fully, strain it for the bones, refrigerate
- you can speed up this step by putting the pot in a sink full of cool water
- do NOT put a hot pot in your fridge. It will heat up the fridge significantly and just make the food in there go bad.
Use this to
- make soups
- make sauces (reduce it first)
- as a substitute for water in savory dish preparations (i.e. make rice with stock instead of water. Be creative here)
The main benefits here are flavor and nutrients, but I just do it for the flavor. Cutting bones up does improve the extraction process, but if the marrow is exposed already (most beef/veal bones will be) you're fine. If you save old bones, freeze them until you have enough. Don't bother trying to make stock with the bones from one chicken.
Gazpacho is possibly Spain's most famous chilled soup. The main difference aside form the temperature is that it's raw, meaning that the soup is not actually cooked it's just blended and chopped vegetables and occasionally bread. There is nothing inherently wrong with heating up gazpacho but it would lose its fresh texture and flavour which is why it's chilled and according to Wikipedia was popular with labourers who used it to:
"cool off during the summer and to use available ingredients such as fresh vegetables and stale bread"
The main reason you couldn't just chill a normal soup and call it gazpacho is because gazpacho is made up of by no means just tomato. It contains tomatoes, a bit of garlic, cucumber, occasionally bread, some vinegar for tang and a drizzle of olive oil at the end.
If you wanted to make it your own (after all you're the chef!) you could add some Tabasco, bell peppers, spring onions or croutons at the end, basically anything you might find in a salsa dip. Use your common sense for what not to add but even in Spain they have variations that are not at all like what I would think of as gazpacho: in La Mancha they use it like a stew and add game (usually rabbit) and even wild mushrooms!
Hope this helps and gives you some inspiration, if you want a recipe a quick search on Google gives a multitude of results.
Best Answer
Anything that will keep bacteria and other spoilage microorganisms from growing will extend shelf life. Acidity and salt are the most common historical preservatives, since they create inhospitable environments for many microorganisms (or only allow the growth of ones that are less harmful). In high concentrations, sugar can also function in this way, as in jams or jellies (though this is probably not applicable to soups). Unfortunately, any soup with enough salt and/or acidity to stop bacterial growth entirely will probably be unpalatable.
That said, lemon juice is highly acidic, so it's likely that it could prevent spoilage a little longer than in soups without it. However, unless you're adding a lot of lemon juice, chances are this effect will not be long enough to call it a "preservative" -- but it might make your soup last a day or two longer in the fridge. A fairly acidic soup using things like tomatoes, lemon juice, vinegar, etc. will generally last a little longer than a similar one without those ingredients.
Nevertheless, the normal storage recommendations for soup should still be observed. Soup in general is very perishable. Stocks and broths are a near ideal growth medium for any bacteria that might contaminate them, and various ingredients in soups could contain spore-forming microorganisms that are not completely killed during cooking. More importantly, not all spoilage -- even that from dangerous bacterias -- produces odors or molds that could be easily spotted. Thus, while adding more acid to soup probably makes it safer a little longer, I still wouldn't depend on it to last more than a few days even using proper cooling procedures and stored under proper refrigeration.
As for cabbage, I'm not aware of any significant anti-microbial properties. It's possible that soups which contain more vegetable matter (like cabbage) will also contain less meat and other ingredients prone to fast spoilage, but the cabbage by itself isn't likely to have a significant effect.