Which of the following is NOT an enemy of oil?

The correct answer is C, plastic. In the context of oil storage and preservation, plastic is not considered an enemy of oil. When oils are contained in plastic containers that are designed for that purpose, they are usually protected from factors that can degrade their quality.

In contrast, air, heat, and moisture can all negatively impact the integrity of oil. Exposure to air can cause oxidation, leading to rancidity. Heat can accelerate this degradation process and cause oils to break down quicker than they would at cooler temperatures. Moisture can introduce water and impurities into the oil, which can lead to spoilage or even promote mold growth. Therefore, while plastic is generally a safe material for storing oil, the other options pose significant threats to the quality and shelf life of oils.