Polyunsaturated fats are the most fragile. Oils that are high in polyunsaturated fats (such as flaxseed oil) must be refrigerated and kept in a dark container. Cooking with polyunsaturated fats, as well as exposing them to heat, light or oxygen, damages the fats, depleting their nourishment and creating oxidized fats and toxins. Store these oils in a dark airtight container in the fridge, use them immediately after opening and don’t keep them for long periods of time. You can usually taste when they start to change.