Never ending soup; is it actually safe?

It takes quite a while for a pot of hot soup to cool down to 40°F in the fridge. Several hours, sometimes, depending on the shape of the pot and the volume of soup. If you're heating and re-chilling the same soup daily, it's going to spend a lot of time in the danger zone. From a safety perspective, you'd be much better off making a pot of soup every few days and then reheating just the portion that you're actually going to eat.

I still recommend this money saving idea to current students

Soup is a great food for stretching a dollar, but I don't see how it's any more expensive to make a fresh pot twice a week and it shouldn't take much time either. I understand that you were adding scraps each night, but you could as easily save those scraps for a day or two until you make the next pot of soup.

Officially, its unsafe.
This is largely due to the fact that the time spent in "the danger zone" is cumulative. You may be killing off bacteria, but during their lifespan they may release toxins and spores that you may not kill. Every time you reheat the bits of leftovers, they're adding up time in the 'danger zone'.

_{Addionally, soup is only good in the fridge 3-4 days - so unless it was fully reheated every 3-4 days, its unsafe that very first 5th day , irrespective of any other debate. Bacteria still grow in the fridge, just slower.}

Ok, I have been making never ending soup for many years. I do not put it in the refrigerator at all. We really eat very luxuriously. So, it is not the savings that we are after. There is nothing like hot soup whenever you want it, if the soup is good enough. We do not put left overs from other people into the soup (it’s safe because of the temperatures, we just don’t like to) except from time to time I freeze bones until I have enough to make bone broth. For our never ending soup if we really like it, we keep it at 180°F. It hardly changes and I don’t worry about the top being hotter than the bottom or having to stir it as you do when at 140°F. If adding ingredients for over night cooking I keep it at 200°F. If adding ingredients to be done in 1/2 to 1 hour 210°F (simmer). Although, you can keep it going indefinitely because 180°F is like freezing it, I usually keep it going only about 5 days. I find the fresh soup seems to taste a bit better then to keep adding to the old soup, although I have done it many times and have had it going for months, over time I found 5 days is enough.

The good thing about soup is it is hot enough to kill all potential pathogens. It is also unlikely to be a low-acid low-oxygen environment long enough for botulism to rear its head. Sounds completely practical and the safest possible way to make use of left overs and questionable cuts (the other way is curries I guess).

Seems to me a "cup of brown" was available from soup vendors in the distant past. Maybe that's just in fantasy novels :-).

The danger zone is between 40 and 140 F, where bacteria can grow. While simmering, it's above 140. After the fridge chills it, it's below 40. Theoretically, you could perpetually chill it or perpetually simmer it and it would be preserved, but that's not practical (too expensive to keep it simmering all the time, and on the other hand you wouldn't want to eat it cold). Also, the time any part of it spends between those temps is where bacteria grows, so the shape of the container you're chilling it in is also important. Never put the pot in the fridge, as that will leave the soup in the center of that shape retaining some heat longer -- as the periphery chills, the center will be in the danger zone too long. So it's best when refrigerating to transfer it to flatter or smaller containers to eliminate this warm center. This is why restaurants transfer soup from the pot to flatter pans like those in a steam table, not because they need the pot for the next batch, but because a pot of warm soup in the fridge becomes a bacteria incubator.