About ten years ago, when I was distilling tons of aromatic plants for research, I didn’t think much of hydrolats. In fact, I threw away tens of litres before I even knew them as valuable products on their own. A couple of years later, an encounter with orange flower hydrolat roused my interest and initiated an ever-growing fascination.
If you’re into essential oils, chances are you like hydrolats as well. They’re more readily available than ever, and it’s also trendy to buy a small still and start experimenting on your own. The boundary between traditional use of herbs and modern innovation has become blurred. With a general deficit of research and literature, it’s easy to succumb to unrealistic expectations regarding the use of hydrolats. Throwing just about any plant in an alembic and expecting to obtain a miracle healing water that depicts the whole plant’s properties is surely misleading.
HYDROLATS, HYDROSOLS, AROMATIC WATERS?
To the aromatherapist, all terms refer to the same thing and can be used interchangeably. I prefer the term hydrolat, as it’s the least ambiguous and is also more common in Europe. Hydrolat denotes the whitish, milky appearance of fresh distillate which, over time, clears up and becomes transparent.Hydrosol, on the other hand, is a scientific term for a particular type of colloid with dispersed solid particles in water. The distillate, once the initial emulsion clears up, is a solution (a single-phase mixture of compounds) rather than a colloid (a stable two-phase mixture).
The ‘water’ terminology (herbal, flower, aromatic, essential waters) is also somewhat vague because it’s often used for artificially aromatised water that has not been produced by distillation.
What, then, is so alluring about these fragrant waters? Is it their attractive scent, a ready-to-use form of a medicinal plant, their gentle nature, suitability for use in 1001 different ways, or a pinch of mystery surrounding their still poorly understood composition? Or is it something more profound, some alchemist archetype of humankind that has been repressed for centuries but continues to push to the surface in the world of materialism in which we lost wonderment?
If there’s anything to this and since many of us feel the need to be modern-age alchemists, let’s start at the beginning – with distillation. Hydrolat is one of the four products of distillation, and the two key parameters needed for its understanding are volatility and solubility.
Distillation is a type of separation where compounds are separated on the basis of differences in their volatility. In distillation of aromatic plants, we typically aim to separate all volatile organic compounds (VOCs) from the remaining residue, by using water and modulating temperature and pressure.
In hydrodistillation and steam distillation, VOCs vaporise then condense into the distillate together with water, and hydrolats are almost pure water (see diagram below). Vitamins, minerals, amino acids, tannins, flavonoids, carotenoids, bitters, alkaloids and many other compounds generally do not evaporate.
Why? Because they are too heavy or too strongly bound to water molecules via electric forces, and therefore their kinetic energy is too low to be able to pull out from the water phase. If we evaporate the same amount of hydrolat and tap water from two glasses, only the latter will contain white mineral deposit at the bottom.
The amount of total Volatile Organic Compounds (VOCs) in a hydrolat strongly depends on the distilled plant species, the ratio between water and plant material and distillation time, and can vary by a factor of more than 1000. Typically, there are between 10 and 1000 mg of VOCs per 1 L of water, which corresponds to 0.001% and 0.1% on the mass-per-mass basis (essential oils are at the other extreme, close to 100% VOCs). The diagram illustrates an approximate upper limit of VOCs (0.1%) for most commercial hydrolats.
However, due to intensive vapourisation of water during distillation, bigger molecules and particles can get caught in the aerosol of water droplets (steam) and travel within all the way to the condenser, ending up in the distillate (Labadie et al. 2015). The amount of distilled non-volatiles is low (probably in traces), and it is unlikely that it contributes to biological activity. In general, the more aggressive the distillation, the less pure and more tea-like hydrolat will be obtained.
The distillate spontaneously separates into polar or hydrophilic phase (the hydrolat) and non-polar or lipophilic phase (the essential oil). Polarity in large part determines the extent to which individual constituents will distribute to either phase. The more polar the compound, the higher proportion will distribute to the hydrolat, and vice versa.
Unoxygenated compounds (terpene hydrocarbons such as limonene, pinenes, β-caryophyllene) are non-polar molecules – they don’t contain oxygen and therefore do not form hydrogen bonds with the strongly polar water molecules. Consequently, they don’t mix with water and tend to distribute almost exclusively to the essential oil.
On the other hand, many oxygenated compounds such as alcohols, aldehydes or phenols are slightly more soluble in water. Thus hydrolats from thyme, savory, oregano, cinnamon, cloves or eucalyptus will typically have higher total amounts of dissolved volatiles and qualitatively resemble essential oils, while the same does not apply to hydrolats from conifer leaves or citrus fruits.
The most straightforward definition of a hydrolat is that of a water product of plant distillation, saturated with essential oil. According to this definition, there should be no qualitative difference between a genuine hydrolat obtained by plant distillation, and a fake one obtained by distilling the essential oil directly or dissolving it in water to the point of saturation (which is indeed a way of producing artificial hydrolats). Will such a product really be indistinguishable from a true hydrolat?
Comparison of essential oils and hydrolats obtained from the same distillation clearly shows that they are independent products. Some constituents are present in both products but with different proportions, and some can be found only in essential oil or hydrolat. In a study on lemon balm (Melissa officinalis), 30 constituents were exclusive to essential oil and 24 to hydrolat, while 11 were identified in both products (Garneau et al. 2014).
Comparison between essential oil and hydrolat composition obtained from the same distillation. (top) Lemon balm (Melissa officinalis), 240 mg/L identified VOCs; (middle) Lavender (Lavandula angustifolia), 602 mg/L; (bottom) Cypress (Cupressus sempervirens), 26 mg/L. Constituents below 1% in both products are not shown for clarity. Data were obtained from Garneau et al., 2014 (a) and provided by Histria Botanica (b,c); EO = essential oil; HY = hydrolat
It’s easy to see from the charts above that the distribution of constituents between essential oil and hydrolat varies significantly from plant to plant. The majority of constituents is present only in one of the products, most of them in minute amounts.
It is also possible to calculate their qualitative similarity – how much of identified VOCs profiles is shared between essential oil and hydrolat. In the sample of lavender, the products had 24.2% similarity of their volatile profiles, mostly on account of linalool. In the case of cypress, similarity was only 1.3%, which shows that hydrolat and essential oil are almost completely different products.
As noted above, we can expect higher degree of similarity for plants rich in relatively more polar compounds that dissolve in water more readily. Thus in a sample of bay laurel (Laurus nobilis; data provided by Histria Botanica, not shown on chart) essential oil and hydrolat were 41.1% similar, mostly due to shared 1,8-cineole and linalool.
The lack of research has been the main culprit for equating essential oil properties with herbs. With increasing amount of research the situation is slowly beginning to change for essential oils, but when it comes to hydrolats, many sources still rely on traditional herbal medicine.
Despite the lack of research, the basic knowledge of phytochemistry enables us to predict what we can and cannot find in hydrolats, and thus assess their value more easily. Let’s see some examples of bioactive constituents you won’t find in there (or at best, only in traces) either due to low volatility or low solubility in water.
- Calendula (Calendula officinalis) distillates won’t contain triterpenic alcohols known for their wound healing properties, as well as antioxidative carotenoids.
- There will be no alkylamides with proposed immunomodulatory properties in the distillates of purple coneflower (Echinacea spp.); hypericin and hyperforin with antidepressive action in St. John’s Wort (Hypericum perforatum); artemisinin, a sesquiterpene lactone with antimalarial and antitumor properties, in sweet wormwood (Artemisia annua).
- β-caryophyllene (BCP) is a sesquiterpene and a phytocannabinoid with a tremendous pharmacological potential. It’s a significant constituent of cannabis (Cannabis sativa), copaiba (Copaifera spp.), lemon balm (Melissa officinalis), black pepper (Piper nigrum), cloves (Syzygium aromaticum) and many other essential oils. However, it’s almost completely insoluble in water, and you won’t find it in the hydrolat (see the example of lemon balm above).
- Distillation of rosehip fruit (Rosa canina) for vitamin C is nonsensical as it does not evaporate. Even if it could, it would quickly oxidise due to the presence of oxygen and high temperature.
- Tannins importantly contribute to biological effects of plants such as sage, yarrow, witch hazel, juniper, cypress, oak and many others due to their astringent action. However, you will get tannins only with the use of properly prepared water infusions (decocts). Hydrolats and essential oils do not contain tannins or other astringents and therefore do not have astringent properties.
While essential oils are distilled from aromatic plants, it is possible to produce hydrolat from any plant you can imagine. No plant is devoid of volatile compounds – including those considered non-aromatic – and you will find some in the hydrolat. Although such products may have some limited use, it is questionable whether large amounts of collected plant material can always justify their distillation.
In many cases, hydrolat just cannot replace a cup of good old tea! Every type of herbal preparation has its advantages and disadvantages; education is key when deciding which is best suited for the particular case.
It should be obvious by now that hydrolats are neither a diluted form of essential oil nor a concentrated form of tea. More appropriately, we can describe them as water products of plant distillation with dissolved plant volatiles. While many volatiles are characteristic of essential oils, others may predominantly distribute to hydrolats. So what exactly are those constituents?
This is the least explored part of hydrolats and one of the reasons for the somewhat mysterious appeal surrounding them. As we saw, those are not the usual non-volatile suspects (at least in significant amounts) but derivatives of fatty acids, amino acids and other metabolites of non-terpenic origin: alcohols, aldehydes, ketones, esters and acids.
The presence of organic acids makes hydrolats acidic (pH < 7). Most acids originate either from hydrolysis of esters or oxidation of aldehydes. Acetic acid, a product of acetate hydrolysis, is likely one of the most common acids in hydrolats. But if you ever saw a hydrolat analysis report you may have noticed that organic acids are rarely listed.
The reason is that, for hydrolat analysis, VOCs must first be extracted and concentrated with an apolar solvent, and acids are hard to extract from water because polarity makes them more soluble in water than other constituents. Also, some acids are highly volatile, which further complicates their detection due to overlapping solvent peak in the chromatogram (Dr Benoit Roger, personal communication).
Over time, many post-distillation processes may occur in the presence of water. One interesting example is the formation of an antibiotic Turbomycin A, an indole derivative which gives an orange hue to sunlight-exposed orange flower hydrolat (Roger et al. 2016). There’s a lot to explore about the diversity, dynamics and bioactive properties of these compounds.
NON-TERPENIC CONSTITUENTS OF HYDROLATS
Some examples of non-terpenic volatiles are phenyl ethyl alcohol, methyl anthranilate, indole, cis-3-hexenol (a.k.a. leaf alcohol).
Phenyl ethyl alcohol is one of the major constituents of the rose hydrolat, but there’s only about 1% of it in the essential oil. It’s also the major constituent of the rose absolute, which is why it has a higher yield, lower cost and is closer to the fragrance of fresh petals. Yet, the smell of essential oil is stronger, brighter and more alive, considered by many (me included!) to be superior to the absolute.
In addition to phenyl ethyl alcohol, methyl anthranilate and indole (amino acid derivatives) are characteristic to orange flower absolute and hydrolat, lending them heavy and narcotic feel, while they’re absent or present in very small amounts in the fresher and brighter essential oil (neroli).
Leaf alcohol (cis-3-hexenol), together with its ester (cis-3-hexenyl acetate) and aldehyde (cis-3-hexenal) are representatives of green leaf volatiles (GLVs), derivatives of fatty acids. They are present almost ubiquitously in green leaves and have a powerful scent of freshly cut grass – which is indeed what we smell when mowing or tearing fresh leaves.
Biologically, GLVs act as signalling molecules. Plants release them when attacked by herbivores to signal their presence to the predators or to signal nearby plants to mount a further line of defence.
GLVs are often detected in small amounts in hydrolats obtained from leaves and may contribute to some extra freshness to their fragrance (see the chart of lemon balm above). Leaf ester and especially leaf alcohol are classical perfume ingredients, used to build fresh green-floral accords in the top notes.
This is another mysterious aspect of hydrolats. Some authors (Price and Price 2004, Catty 2001) describe hom*oeopathic properties of hydrolats, their ability to retain some form of memory or life force from the plants. Cellular water (water from the plant material that gets distilled into hydrolat) is considered an important element for mediating these properties, which is one of the reasons why high-quality hydrolats should be distilled from fresh plant material (Harman 2014).
As we know, this is a controversial topic in science. Cellular water indeed has some interesting properties, as it exists in a highly ordered state around charged biological polymers. Since the cell interior is crowded with proteins, lipid membranes and charged particles, it wouldn’t be a surprise if it turns out that the majority of cellular water is ordered at least to some degree.
The extent of structured interfacial water and its biological significance have been a subject of debate for a long time. Its role in the shape of proteins and DNA, as well as for facilitating their interactions, is well established. Some hypotheses further predict interfacial water could mediate specific long-range interactions within cells, such as transfer of protons and electrons or even molecular vibrations and electromagnetic fields, thus actively contributing to the dynamics of biochemical processes (Chaplin 2006, Cifra et al. 2011, Ho 2014).
Some researchers go even further, and others are sceptical of any other role of cellular water beyond that of a mere solvent. It’s not impossible, in principle, that some of these predictions could be underpinned by macroscopic quantum effects, observed recently to play a role in specific biological processes. This fascinating subject has received broad interest from the mainstream research community in the last ten years (Huelga and Plenio 2014).
All that said, I have some concerns about the significance of cellular water for hydrolat quality. First, hom*oeopathy is based on a different set of principles, such as sequential dilution, potentiation and the ‘like cures like’ rule, which do not depend on cellular water. Second, as soon as water molecules are heated to the boiling point and released from cells (as in distillation), their ordered structure is lost, and they should start to behave as bulk water. And third, the actual amount of cellular water in hydrolats is likely very low (you can get far more significant amounts from eating fresh fruits and vegetables).
Many aroma-enthusiasts like to stress we don’t know much about the composition and biological properties of hydrolats. The research does exist, especially for products of high economic value. Their potential applications are in the food and cosmetics industries, with emphasis on their antimicrobial, antioxidative and anti-inflammatory properties.
Without a doubt, they’re far less researched than essential oils, and their use is mainly based on tradition and individual experience. Hydrolats do have advantages over essential oils: you don’t have to dilute them for topical use, you can use them with small children, as flavourings in food and drinks, in cases where you would normally use plain distilled water, in diffusers, in natural cosmetics, as a mouthwash, and many other possible ways.
Hydrolats can be ingested, but make sure they are microfiltered and microbiologically tested, and follow storage recommendations (best to use them as fresh as possible). In fact, many hydrolats have a very pleasant and natural taste, when properly produced and diluted. Sometimes they also smell superior to corresponding essential oils due to a higher proportion of oxygenated compounds. They are wonderful natural products exactly as they are, without the need for mystification.
REFERENCES
Catty S. 2001. Hydrosols: The Next Aromatherapy, Healing Arts Press.
Chaplin, M. 2006. Do we underestimate the importance of water in cell biology? Nature Reviews Molecular Cell Biology, 7(11), 861-866.
Cifra, M., Fields, J. Z., & Farhadi, A. 2011. Electromagnetic cellular interactions. Progress in biophysics and molecular biology, 105(3), 223-246.
Garneau, F. X., Collin, G., & Gagnon, H. 2014. Chemical composition and stability of the hydrosols obtained during essential oil production. I. The case of Melissa officinalis L. and Asarum canadense L. Am. J. Essent. Oils Nat. Prod, 2, 54-62.
Harman A. 2015. Harvest to Hydrosol, Distill Exquisite Hydrosols at Home, botANNicals.
Ho, M. W. 2014. Illuminating water and life. Entropy, 16(9), 4874-4891.
Huelga, S. F., & Plenio, M. B. 2014. Quantum biology: A vibrant environment. Nature Physics, 10(9), 621-622.
Labadie, C., Ginies, C., Guinebretière, M. H., Renard, C., Cerutti, C., Carlin, F. 2015. Hydrosols of orange blossom (Citrus aurantium), and rose flower (Rosa damascena and Rosa centifolia) support the growth of a heterogeneous spoilage microbiota. Food Research International, 76, 576-586.
Price, L., & Price, S. 2004. Understanding Hydrolats: The Specific Hydrosols for Aromatherapy: A guide for Health Professionals. Churchill Livingstone.
Roger, B., Burger, P., Baret, P., Chahboun, J., Cerantola, S., Fernandez, X., & Jeannot, V. 2016. Identification of antibiotic and antiproliferative compounds in natural orange blossom water. Journal of Essential Oil Research, 28(2), 89-95.
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