You have made a large pot of vegetable beef soup using ground hamburger and vegetables for a family gathering. After the soup has been cooked to the proper temperature, you cover the pot and put it in the refrigerator. When you are ready to serve the soup the next day, you get it out and warm it using several smaller containers. You then serve it using a warming table that holds it at the proper temperature. Which part of this process may have caused the soup to make someone ill?