We know distillation as the method of producing essential oil and its side product, hydrolat or aromatic water. But we often overlook that those are not the only distillation products. There are two additional products we do not hear much about in the phytotherapeutic world: residual plant material and residual distillation water. They are quite valuable from the environmental perspective and present a problem as well as a challenge for the distillation industry.
In fact, distillation is a very specific method for extracting bioactive compounds. Besides volatiles, aromatic plants produce a variety of non-volatile secondary metabolites which cannot be distilled into essential oil or hydrolat. On average, only about 1% of dry plant mass is extracted by distillation, and everything else is simply lost and thrown away.
According to Grand View Research, global production of essential oil was 180 000 tons in 2015 and is predicted to reach 370 000 tons by 2024, with 8.4% annual growth. Imagine all the resources needed for growing plants and distilling such amounts of essential oil.
By introducing the four distillation products, my goal is to put distillation into a wider context and re-think it from an environmental perspective.
1. ESSENTIAL OIL
An essential oil is usually the primary product of distillation. Essential oils are mixtures of volatile lipophilic compounds from plants and are used in the food, pharmaceutical, cosmetic and perfume industries, and in aromatherapy. You can learn more about essential oils in my guide How to find reliable essential oil information.
Hydrolat (hydrosol, aromatic water) is the distilled water enriched with plant volatiles. It’s misleading to think of it as water saturated with essential oil, as the composition of its aromatic fraction (usually between 0,001% and 0,1%) typically differs quite a bit from the corresponding essential oil. Thus hydrolat is an independent distillation product.
Essential oil industry traditionally treated hydrolats as distillation by-products, discarding them or re-distilling to increase the yield of essential oils. However, in the last 15 years, they have witnessed a drastic increase in popularity. Many home distillers, as well as professional producers, distil them as their primary products, aimed for use in aromatherapy and natural cosmetics.
3. RESIDUAL PLANT MATERIAL
The distilled plant material is commonly treated as a waste without economic value. It can be used as animal feed, energy resource, or for soil replenishment. The latter poses a specific challenge, as the antimicrobial activity of the residue may prolong decomposition time and affect its microbial structure.
Lavender and lavandin straws, for example, can be used in mulching around trees, vineyards and horticultural crops to maintain soil moisture and restrict the growth of certain weeds or pathogenic organisms (Lesage-Meessen et al. 2015). However, distillers usually produce far more significant amounts than that, and not all materials may be suitable for mulching.
To put it into a perspective of how much material we’re talking about: one kilo of fresh rose flowers produces two kg of wet residual material. For one kg of essential oil, we need about 3-4 tons of flowers, which yields about 6-8 tons of the residual matter.
In India alone, about 3 million tons of residual material is produced each year, which then typically rots and decomposes in the fields. Due to lack of suitable disposal surfaces and stricter regulations – while facing continuously increasing demand for essential oil – it is clear that this is becoming a problem, a burden not only for the distilleries but the environment as well.
Patchouli has been one of the model plants for research in this regard. It was shown to be suitable for vermicomposting: with the right combination of microorganisms and earthworms, the residual material can decompose into humus in less than 100 days. Humus is rich in nutrients and beneficial microorganisms and can be used to enrich soil with minerals, optimize its pH value and increase water retention capacity, while reducing consumption of additional fertilizers and fungicides. In a field experiment, the patchouli crop treated with the patchouli vermicompost had up to 60% higher yield of essential oil, compared to standard chemical fertilizer (Singh et al. 2013).
On the other hand, many biologically active substances are still present in the residual matter. They can be further extracted by additional extraction methods and used, for example, as antioxidants in the food and pharmaceutical industry. For example, plants from the mint family (Lamiaceae) such as rosemary, thyme, oregano, mint, basil, lemon balm and sage, produce phenolic compounds with a broad spectrum of biological activity:
- Residual sage (Salvia officinalis) contains significant amounts of valuable flavonoids (apigenin and luteolin). Its water and ethanolic extracts retain antioxidative, antibacterial and antifungal activity (Veličković et al. 2007).
- Residual lavandin (Lavandula x intermedia) and spike lavender (Lavandula latifolia) are rich sources of phenolic compounds, such as phenolic acids and flavonoids, many of which are strong antioxidants and antimicrobials (Méndez-Tovar et al. 2015; Torras-Claveria et al. 2007). 34 volatile constituents were identified in lavandin, including considerable amounts of coumarins with potential economic value (Tilliacos et al. 2008).
- Extracts of residual rosemary (Rosmarinus officinalis), savory and oregano species are also a good source of natural antioxidants, especially phenolic acids such as caffeic and rosmarinic acid, and a phenolic diterpene carnosic acid (Oreopoulou et al. 2018).
- From the residual clary sage (Salvia sclarea), a diterpene sclareol can be extracted, which is an important precursor for the synthesis of ambrox, the crucial aromatic compound in ambergris and a staple in the contemporary perfume industry.
4. RESIDUAL WATER
The residual distillation water is mainly produced in water distillation, where plant material comes in direct contact with water, and in smaller amounts in steam distillation.
Large amounts of residual water are produced in the rose distillation industry because rose is one of a few plants of highest economic significance where water distillation is employed. Each distillation of 500-1000 kg rose petals produces 4000 litres of residual water that contains high amounts of poorly degradable polyphenols and should, therefore, be treated as a biopollutant (Rusanov et al. 2014). However, an extracted fraction rich in quercetin, kaempferol and ellagic acid was found to be a potent tyrosinase inhibitor and could be used as an active cosmetic ingredient for reducing hyperpigmentation (Solimine at al. 2016).
Searching for natural sources of antioxidants is a prospective research field, relevant primarily for the food and cosmetics industry, where the two common synthetic antioxidants BHA and BHT turned out as inappropriate for long-term use and may even have carcinogenic activity.
Mielnik and colleagues (2008) incorporated the residual water of rosemary, sage and thyme into marinades for turkey thighs. Antioxidants contained in the residues decreased lipid oxidation and inhibited the development of rancid off-flavours in stored meat. Interestingly, the residual waters were found to be more powerful antioxidants than the corresponding hydrolats.
Perhaps even more intriguing, greenhouse experiments (e.g. Zheljakov et al. 2010, 2011, 2012) indicate that residual waters from certain aromatic plants (foliar application) can influence biosynthesis of terpenes as well as the yield of essential oil. For example, the residual rosemary water increased synthesis of l-limonene in peppermint, while the residual lemon balm water decreased menthofuran and increased menthyl acetate production. While these are indeed interesting observations, more experimentation is needed to elucidate whether (and how) re-use of some of these waters may be employed in practice.