Listener Question: Why Did My Squash Soup Go Sour? (2024)

A recent caller reported a mysterious incident with butternut squash soup, which she typically cooks in 3-gallon batches. This time, she altered her normal recipe, adding a handful of brown basmati rice to use as a thickener. After cooking, she cooled the soup overnight on her chilly back porch. The next day, she discovered that the soup was alarmingly tangy, with a strangely fermented flavor reminiscent of alcohol. What happened?

It’s no coincidence that the butternut squash soup had fermented, alcoholic notes. Indeed, fermented rice turns out to be the culprit here. Rice is high in starch, which is relatively easy to ferment. And when rice ferments, it produces alcohol, along with many of the same flavor molecules created during the production of sake (a wine made from fermented rice). When dealing with leftovers, it’s important to be mindful of the storage temperature “danger zone.” This is a temperature range between 40°F and 140°F, within which bacteria grow most easily. In this case, the porch likely was not cold enough to prevent bacterial growth. To make matters worse, in a 3-gallon pot, hot soup cools slowly, giving bacteria ample opportunity to multiply. To prevent this kind of fermentation (or other bacterial growth) in leftovers, it’s best to transfer the food to smaller containers before putting them in the refrigerator—or any other cold, temperature-controlled environment. This allows the food to cool faster and stay at a temperature that prevents bacterial growth.

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Illustration: Ross MacDonald