Soup

Soup keeps 3–4 days refrigerated and 2–3 months frozen. The difficulty with soup is not the window but the cooling, and it is the single most common place home kitchens get leftovers wrong.

A large stockpot of hot soup can take six hours or more to reach a safe temperature in a refrigerator, and most of that time is spent in the bacterial growth range. Putting the whole pot in the fridge is not a solution — it is the problem, and it warms everything else in there besides.

Divide soup into shallow containers no more than two inches deep and refrigerate within two hours. For a very large batch, cool the pot in a sink of ice water first, stirring to move the hot interior to the outside. A frozen water bottle stirred through the pot works nearly as well.

Most soups freeze well for 2–3 months. Cream- and milk-based soups separate on thawing — freeze the base and add the dairy when reheating. Potatoes turn grainy when frozen.

Reheat soup to a full boil rather than merely warm.

Soup's risk profile is dominated by physics rather than biology: it is the thermal mass problem in its purest form.

A gallon of soup holds a great deal of heat and sheds it slowly, especially in a tall narrow pot where the surface-to-volume ratio is poor. The USDA's two-hour rule and the 40–140°F danger zone were written for exactly this situation, and a stockpot cooling on a counter or crammed hot into a fridge violates both for hours at a stretch.

What grows during that time depends on the soup, but two organisms matter most. Bacillus cereus, whose spores survive boiling and germinate on cooling, is a risk in any soup containing rice, pasta, grains, or vegetables. Clostridium perfringens is the more specific concern for meat and poultry soups — it is one of the most common causes of foodborne illness in the US, it grows in the oxygen-poor interior of a large pot, and it is closely associated with big-batch cooking cooled too slowly.

Both are addressed by the same thing: fast cooling into shallow containers.

Reheating to a rolling boil kills C. perfringens cells, but it does not destroy the heat-stable toxin of B. cereus, so cooling remains the real control.

Signs of a soup that has gone: a sour or fermented smell, bubbles or fizzing when it has not been heated, a cloudy or ropy texture in a clear broth, and any mold on the surface.