Essential oils, such as citrus have a shorter life and others like, patchouli, sandalwood and vetiver get better as they age. The average shelf life of essential oils can vary due to differences in their chemical make up and storage. Essential oils should be stored in light proof, tightly sealed glass container at stable temperature. Under optimum storage conditions most oils will stay fresh for up to 3 years. Citrus oils have shorter shelf lives because they are expressed from the fruit peel and unstable components such as waxes and fatty acids that are present in the living fruit also present in essential oil. The great majority of essential oils are extracted using steam distillation and the heat produced during this process alters and stabilizes the natural components in the plants. Many oil go through a curing process immediately following steam distillation during which additional chemical changes take place before aroma stabilizes. This can take a few weeks in cases such as peppermint. Rich, heavily scented oils like patchouli, sandalwood and vetiver take longer to reach their full potential.