Essential oil chemical changes

Essential oil chemical changes is usually attributed to such general reactions as oxidation, resinification, polymerization, hydrolysis of esters, and to interreaction of functional groups. These processes seem to be activated by heat, the presence of air, of moisture, and catalyzed by exposure to light and in some cases, possibly by metals.
There is no doubt that oils with a high content of terpenes (all citrus oils, pine needle oils, oil of turpentine, juniper berry, etc.) are particularly prone to spoilage, due probably to oxidation, and especially resinification. Light seems to be of lesser importance as a factor causing deterioration, than is moisture.
Essential oils containing a high percentage of esters (bergamot, lavender, etc.) turn acid after improper storage, due to partial hydrolysis of esters. The aldehyde content of certain oils (for example, lemongrass) gradually diminishes as the essential oil chemical changes take place. Fatty oils, with a few exceptions, are very prone to oxidation, but such spoilage can be retarded or prevented altogether by antioxidants. Certain types, especially those containg alcohols (for example, geranium), are quite stable and stand prolonged storage. Still others, patchouli and vetiver, for instance, improve considerably on ageing; in fact, they should be aged for a few years before being used in perfume compounds.
As a general rule, any essential oil should first be treated to remove metallic impurities, freed from moisture and clarified, and then be stored in well-filled, tightly closed containers, at low temperature and protected from light. A layer of carbon dioxide or nitrogen gas blown into the container before it is sealed will replace the layer of air above the oil and thereby assure added protection against oxidation. Nevertheless, moisture seems to be one of the worst factors in itsspoilage and, therefore, any moisture must be removed prior to storing.
The effect of essential oil chemical changes is on their physical properties such as increase or decrease in their viscocity, color change, and loss of scent.

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