When essential oils are made, attention naturally goes to the fragrant oil that is separated and collected. But the process also leaves plant residues and water. Examining that water reveals the possibility of using constituents other than those found in the essential oil.
In a study published in 2024, Kırkıncı and colleagues extracted lavender essential oil by two methods and compared the constituents of the oils and the water obtained as a by-product. Their central question was whether changing the extraction method changes not only the fragrant oil but also the characteristics of the resources left behind.
Four-hour hydrodistillation versus microwave-assisted extraction
The material consisted of flowers of the lavender Lavandula angustifolia collected in Tekirdağ Province, Turkey, in summer 2023. This was a laboratory-scale comparison using 100 g of dried flowers and 500 mL of water.
The conventional method used four hours of hydrodistillation, heating the plant material together with water. The other method used microwave heating in a dedicated instrument, programmed to reach 100°C in 10 minutes and maintain that temperature for 30 minutes. This was not a test of a procedure for making essential oils in a household microwave.
The researchers separated the essential oils and filtered the by-product water for analysis. Experiments were performed in triplicate, but the comparison did not span multiple growing regions or harvest years. The water discussed here is an aqueous by-product of the research process. This does not mean that every commercially sold hydrosol has the same composition.
Similar major oil constituents, with differences in the details
The essential oils were assessed using techniques including gas chromatography and mass spectrometry to separate and examine their constituents. Terpinolene and borneol accounted for large proportions in both methods, giving similar overall patterns of major constituents. However, the presence of minor constituents and their relative proportions differed with extraction method.
These percentages express instrument signals as proportions of the total. They do not show how many times more essential oil was obtained, nor are they scores for odor intensity or pleasantness. The study did not ask people to smell the oils and compare their aroma quality.
Plant composition can vary with origin, harvest time, plant part, and extraction conditions. The major constituents found here cannot be treated as representative values for all lavender essential oils. The value of the study lies in processing the same material in different ways and examining both the oil and its by-products.
Phenolic constituents remain in the water
The water analysis focused on phenolic constituents and flavonoids, one group within them. These are groups of plant compounds that reveal a different aspect of composition from the volatile constituents collected in essential oil.
Measured total phenolic content was approximately 1.78 mg GAE/g for the conventional method and 3.28 mg GAE/g for the microwave method. GAE expresses the response in terms of an equivalent amount of the reference substance gallic acid. It does not mean that the water contained that amount of gallic acid itself.
Total flavonoid content was also higher with the microwave method. The authors proposed possible explanations including cell disruption that facilitates release of constituents and reduced thermal degradation due to shorter heating. The individual contributions of these mechanisms were not independently established in this study.
The researchers also combined liquid chromatography and mass spectrometry to examine 32 target analytes individually. Some were not detected, so the result does not mean that all 32 were present in the water.
Protocatechuic acid was among the prominent constituents in both methods. Not every individual compound was more abundant with microwave extraction, however. For example, apigenin and luteolin were higher with the microwave method, whereas p-coumaric acid was higher with the conventional method. The preferable process may differ depending on the target compound.
What do the antioxidant assay values show?
The water samples were also examined using three methods: DPPH, ABTS, and CUPRAC. These are antioxidant-capacity assays that measure how strongly a sample acts in particular chemical reactions.
In all three assays, water from the microwave method gave higher values than water from the conventional method. For example, the DPPH result expressed in equivalents of the reference substance Trolox was approximately 5.60 mg/g for the conventional method and 7.62 mg/g for the microwave method. This indicates a difference in reactivity within a chemical assay conducted under matched conditions.
These results do not measure effects from applying the water to skin or drinking it. This experiment alone also cannot establish skin irritation, microbial growth during storage, or stability in a finished product. Finding that constituents remain is a starting point for exploring uses.
Does a shorter process necessarily mean a smaller environmental impact?
Under the settings used here, microwave extraction required less heating time. The study also used indices that assess analytical procedures in terms of reagents, energy, waste, and other factors. One environmental index gave the microwave method a score of 0.57 and the conventional method 0.55. A separate practicality index also rated the microwave method slightly higher.
However, these are composite indices combining selected assessment criteria. They do not measure the environmental impact of an entire product, including factory electricity use, equipment manufacture, wastewater treatment, and purification of recovered constituents. The initial volume of water was the same for both methods in this study.
The authors suggest that combining more efficient extraction with use of by-products could support more sustainable manufacturing. Achieving this will require production-scale evaluation of which constituents can be recovered, at what purity, and with what resource requirements and costs.
Summary
Phenolic constituents remained in the water after lavender essential oil extraction. Under the conditions tested, water from the microwave-assisted method had higher total phenolic content and greater antioxidant capacity in chemical assays. Changes in individual constituents were not uniform, however.
The authors consider this water worth further investigation as a potential ingredient source for foods, cosmetics, and other products, rather than simply treating it as waste. How can plant constituents be used beyond a single bottle of essential oil, including those in the water? This study reconsiders fragrance production through resource use across the whole process.
References
Kırkıncı, S., Gercek, Y. C., Baştürk, F. N., Yıldırım, N., Gıdık, B., & Bayram, N. E. (2024). Evaluation of lavender essential oils and by-products using microwave hydrodistillation and conventional hydrodistillation. Scientific Reports, 14, 20922. DOI: 10.1038/s41598-024-71115-w / Full original article. Original article: © The Author(s) 2024, CC BY-NC-ND 4.0.
