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One of the more common questions involves the typical shelf life of an essential oil. Many essential oils vary in shelf life, and some actually smell better as they age.
Oils are spoiled due to oxidation. Oxidation is a process of chemical reactions with free radicals that produce byproducts, eventually making the oil unsuitable, or in some cases better.
The degradation of an oil can be detected by physical properties such as color, texture, and odor. Thankfully, antioxidant products are able to minimize the reactivity of oxygen and free radicals, preserving the overall integrity of the oil.
A study performed via Arcoleo et al delved into the oxidation of olive oil when using cold pressed lemon essential oil as a preservative to improve its shelf life. Recently there has been a trend in using essential oil as preservatives due to their high antioxidant content. Additionally, the use of such products can be used to improve overall flavor.
Virgin olive oil is an excellent dietary source due to its fatty acid composition and mild antioxidant power. 70% of the oil is composed of oleic acid (a fatty acid) with the remainder components being antioxidant compounds such as alpha-tocopherol, phenolic compounds, and carotenoids. These compounds are known to prevent the formation of free radicals in the body. Unfortunately, the natural antioxidant products in the oil are not enough to keep the oil from being susceptible to oxidation and ultimately “going bad.”
Cold pressed lemon essential oil on the other hand is regarded as one of the best essential oils in the world due to its aroma and chemical profile. Lemon essential oil is known to be a powerful antioxidant as a result of its high limonene content and an excellent antimicrobial that allows it to be used successfully in cleaning blends and products. Its high content of monoterpenols is the primary reason for its high antioxidant power, as they are excellent sources known to reduce free oxygen radicals.
Cold pressed lemon essential oil and virgin olive oil were made into several samples for the study. They made a control sample of virgin olive oil that contained no essential oil, in addition to olive oil samples containing 0.4% and 0.8% of lemon essential oil. Samples were poured into amber bottles and stored in dark cellars in order to simulate normal home storage. Over the course of the study, 6 samples of each oil were taken over 10 months in order to analyze the chemical and sensory parameters.
Chemical tests of olive oil included the identification of the chemical compounds of the oil and evaluation of antioxidant activity by free radical scavenging activity tests and super oxide anion scavenging activity tests. Additionally, this study included a sensory analysis of the oil in order to define the attributes characterizing the products. The sensory analysis was placed in a spider plot in order to investigate the sensory changes of samples during the storage period of ten months.
In the first spider plot, the chemical and sensory index values of the initial parameters of virgin oil illustrate a high quality oil. Due to the presence of monoterpenes found in lemon essential oil, it was hypothesized that lemon essential oil can protect olive oil from oxidation. This is because monoterpenes can easily be oxidized into peroxides, making the oil more oxidatively stable.
Initially the only significant differences between the olive oil samples were sensory attributes, such as olive odor and lemon odor. During storage, it is shown that the non-treated olive oil had further degrees of oxidation than those treated with the lemon essential oil.
During the first phase of oxidation, olive oil treated with lemon essential oil showed the formation of peroxides. Ultimately, this prevented degradation changes in the olive oil that eventually lead to rancidity. When compared to untreated samples of olive oil, the first phase of oxidation produced hydroperoxides. Interestingly, the differences in oxidation were not as obvious in sensory parameters, if at all.
No significant difference was observed from samples that contained 0.4% or 0.8% lemon essential oil and the differences in odor were still detectable. As one can imagine, olive oil treated with lemon essential oil contained more of a lemon-like aroma and flavor. Additionally, the pungency of the oil did not increase in those treated with lemon essential oil, whereas the untreated oil had a noticeable more pungent odor at the end of 10 months.
Overall the study concluded that small amounts of lemon essential oil improves the shelf life of virgin olive oil because it provides oxidative stability.
What we liked about this study was the insight it gave us with regards to the oxidative process in oils. Although in this case, oxidation was a sign of degradation that ultimately leads to the rancidity of olive oil, it was interesting that products in lemon essential oil still underwent oxidation, but produced a byproduct more favorable that prevented the loss of integrity of the oil. This is an indicator that oxidation products can be used to our advantage. For instance in other oils such as sandalwood, patchouli, or helichrysum, when the oxidative degradation products are favorable and produce a higher quality oil.
What we did not like about the study was the lack of discussion about the sensory parameters. It left it rather vague and unclear with the inclusion and exclusion criteria for particular attributes. Furthermore, they didn’t explain what sensory changes occurred, to what degree, or compare the major differences observed. Rather, they left it to the spider plot visual to do the explaining.
All in all it was an excellent study that was able to illustrate sensory changes as a result of oxidation and how some oxidation products can in fact be favorable.