I made a vegetable soup with peas, onion, garlic, potatoes, celery, cubed chicken stock, salt, and olive oil. I put it in two containers, one open and the other airtight, and in the fridge. After 6 days, the soup in the open container had turned sour and had a vinegar taste, while the one in the airtight container had a normal taste. So I presume that oxidation plays a role in the vinegar taste.
Why does a vegetable soup turn sour?