Final answer:
The safest method to cool down a casserole and prevent bacterial growth is by using a cold water bath before storing it in the refrigerator which should be kept at or below 4°C (40°F).
Step-by-step explanation:
To safely cool down a casserole, it is essential to prevent bacterial growth while bringing the temperature down efficiently. The method of cooling should ensure that the food does not stay in the temperature danger zone (between 5°C and 60°C or 41°F and 140°F) for too long. The safest recommendation, to place the casserole in a cold water bath, quickly reduces the temperature and avoids prolonged exposure to temperatures that would allow bacteria to proliferate.
The cold water bath technique is effective because ice has a smaller specific heat than water, meaning it absorbs less thermal energy for a given temperature change, which in turn leads to more efficient cooling of the casserole. Additionally, it is important to ensure that the refrigerator temperature is kept at or below 4°C (40°F) to inhibit further bacterial growth once the casserole is eventually stored in the refrigerator.
Other methods such as placing it directly in the freezer or the refrigerator might cool the casserole slowly, thereby leaving it in the danger zone for too long, or cause quick freezing of the outer layer while the internal part remains warm, both of which can be risky for food safety.