Understanding the Stability and Degradation of Essential Oils: A Comprehensive Review

Essential oil is a product obtained from plant raw material by distillation or other physical extraction processes. They posses antimicrobial, antiviral, nematicidal, antifungal, insecticidal, and antioxidant properties, making them highly valuable as perfuming and flavoring agents. Recognized as safe for consumption (GRAS - Generally Recognized as Safe), essential oils can be utilized in medicinal products for both humans and animals. With the growing awareness and preference for natural ingredients in food, household, cosmetic, and pharmaceutical items, essential oils have gained significant popularity [1].

The chemical structure of essential oils renders them susceptible to degradation caused by factors such as light, heat, oxygen, metal contaminants, and water content. Oxidation, isomerization, cyclization, and dehydrogenation can occur, leading to the formation of degradation products. It is essential to delve into the chemical composition of essential oils and identify the factors influencing their stability [1]. The degradation of aroma chemicals within essential oils can impact their sensory attributes and potentially act as allergens when in contact with the skin [2]. Let us explore the various chemical classes of essential oils [3]:

Example cases of essential oil degradation:

1. Geraniol: Geraniol (2, trans-3,7-dimethyl-2,6-octadiene-1-ol), is an important fragrance terpene, and is widely used because of its fresh flowery odor. Studies have shown that air-exposed geraniol and its oxidation products (i.e., the aldehydes geranial, neral, epoxygeraniol, and geranyl formate) exhibit a significantly higher sensitizing potency than pure geraniol, emphasizing the importance of understanding the oxidation process of fragrance terpenes [2].

2. Sweet fennel oil: Sweet fennel oil, containing trans-anethole, can be completely oxidized to anisaldehyde or isomerized to cis-anethole. Exposure to UV rays or high temperatures can convert trans-anethole, the main component in sweet and bitter fennel oil, into the more toxic cis-anethole, which is 10 to 12 times more toxic than its trans-isomer.

3. Linalyl Acetate. Thermal degradation of linalyl acetate in bergamot or lavender essential oils to β-myrcene, trans- and cis-β-ocimene occurred through the elimination of acetic acid. The dehydration of linalool also contributes to the increased level of myrcene and ocimene.

Terpenoids, characterized by volatility and thermolability, are easily oxidized or hydrolyzed depending on their specific structures. The thermal isomerization of unsaturated mono- or sesquiterpenoids can cause double bond migration, affecting their overall structure [4]. Moreover, monoterpenes have been observed to degrade rapidly under the influence of light [1]. Consequently, proper storage of essential oils in tightly closed, dark bottles, and away from direct sunlight is crucial.

Understanding the stability and degradation of essential oils is vital for preserving their beneficial properties and ensuring their optimal use in various applications. By recognizing the factors that influence their stability and the potential consequences of degradation, users can make informed decisions about storage, handling, and utilization. Further research is needed to deepen our knowledge of essential oil degradation and enhance the longevity of these valuable natural products.

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References:

[1] Stability of Essential Oils: A Review - Turek - 2013 - Comprehensive Reviews in Food Science and Food Safety - Wiley Online Library

[2] Mechanism of Air Oxidation of the Fragrance Terpene Geraniol | Journal of Chemical Theory and Computation (acs.org)

[3] jpsr13072109.pdf (pharmainfo.in)

[4] Thermolabile essential oils, aromas and flavours: Degradation pathways, effect of thermal processing and alteration of sensory quality - ScienceDirect