Designing a Chill Bath for Hot Soup

Sunday Soup

Every Sunday, Leilani Fonoti and her uncle Sefa cook a big pot of chicken soup and freeze most of it in containers for school lunches. Food-safety guides advise cooling cooked food quickly before it goes in the freezer or refrigerator, because bacteria grow fastest between about 4 °C and 60 °C. Leaving a hot pot on the counter can keep soup in that range for hours. Uncle Sefa also did not want to put a hot pot straight into the small freezer, which would warm everything else inside it.

Leilani decided to design a chill bath: a way to set the soup in cold water or ice water so it cools faster. She set a goal and a limit. The goal: 1 L of soup must cool from 60 °C to 25 °C or lower within 30 minutes. The limit: each batch can use no more than 2 kg of ice, because that is all the freezer makes in a day.

To test safely, she used hot tap water in place of soup. For each trial, Uncle Sefa poured 1 L of water at 60 °C into the container, and Leilani read a kitchen thermometer every 5 minutes. Every ice bath used the same large sink tub filled with 4 L of cold tap water at 15 °C plus the ice. The kitchen stayed at 22 °C. She ran each design twice and recorded the average. When she stirred, she used the same spoon and stirred for 10 seconds each time.