How to check and maintain the quality of a hydrosol?

Pure and natural hydrosols should only be composed of… hydrosol. Many manufacturers add additives or preservatives to ensure better product stability. These hydrosols cannot be used in hydrolatherapy, particularly orally. Due to their composition, their use is reserved for the skin, mainly for cosmetic uses. Cosmetic regulations require the presence of a list of ingredients on the product, also known as the INCI list (International Nomenclature of Cosmetic Ingredients). This is the list written in English and Latin. Often indecipherable, it nevertheless provides very interesting information! In the case of hydrosols, it is this list which indicates the presence of preservatives. No INCI list is indicated? It is then a pure hydrosol, which does not contain any preservatives.

The yield of a hydrosol is much higher than that of an essential oil. When several hundred kilos of material are sometimes necessary to obtain only a few milliliters of essential oil, in the case of hydrosols several tens of liters can be obtained with a single small kilo of material. The reference method for obtaining a quality hydrosol is to respect the 1:1 ratio. This means that to collect 1 L of hydrosol, only 1 kg of plant will be distilled. This condition makes it possible to obtain a product with a powerful odor, of excellent quality and of great therapeutic effectiveness. Some producers do not hesitate to increase the ratio, until obtaining 10 or even 50 L of hydrosol for just 1 kilo of plant, out of economic interest. This has the effect of diluting the hydrosol, as well as the active ingredients that compose it, and therefore reducing its effectiveness and necessarily its quality.

Several terms can be used to talk about a hydrosol. Only the terms “hydrosol” and “floral water” make it possible to ensure that it was obtained by steam distillation. What is the difference between the two? None, except that “floral water” designates hydrosols obtained from the distillation of flowers, while “hydrosol” does not distinguish between those obtained from flowers and those obtained from leaves, for example.

Other terms, however, can be confusing. That of “hydrosol” for example. If this is indeed the translation of the term "hydrosol" into English, a hydrosol in France is a solution in which water is the dispersion medium. It is therefore not a product resulting from steam distillation. The names flavored water or aromatic water designate products resulting from different processes as well.

Since hydrosols are mainly composed of water, the risks of contamination by bacteria, fungi or yeasts are very high. Although the hydrosols are sterile when leaving the still, since the temperature is around 100°C, these germs can develop during storage. To limit these risks, it is necessary to microfilter the hydrosol, that is to say, to pass it through a filter membrane whose pore diameter is between 1 and 0.2 microns. This microfiltration makes it possible to separate the hydrosol from the potential germs it contains. At the time of packaging, the bottles intended to receive the hydrosols can also be sterilized, with alcohol, UV, or another process. Pasteurization should be avoided so as not to impact the intrinsic quality of the product. Finally, microbiological analyzes are systematically carried out during the different stages of manufacturing to verify the absence of contamination.

Like essential oils, hydrosols are very sensitive to light and heat, but they are also susceptible to oxidation and microbial infections. They keep well between 12 and 15°C but this temperature range is quite difficult to respect at home. The best place to store them is therefore the refrigerator, which combines darkness and freshness, and which thus limits the development of micro-organisms. After opening, it is advisable to consume them fairly quickly, preferably within the following 3 months. White deposits may form at the bottom of the bottle but they are completely harmless. Other deposits may indicate contamination. A hydrosol that has gone bad will have an unpleasant odor, due to the degradation of aromatic molecules by microorganisms. It is then best not to use it and throw it away.

Essential oils and hydrosols are obtained by steam distillation or hydrodistillation. The water vapor will pass through the plant material and take on all the “information” of the plant, all its aromatic and water-soluble molecules. In addition to good plant quality, water quality is therefore inherent to the quality of a hydrosol. In fact, this will be, by definition, distillation water loaded with active compounds. This means first of all that it must be drinkable, filtered and sanitized to allow the products to be used in hydrolatherapy, but also from a source, preferably pure and natural.

Hydrosols contain the entire water-soluble molecular fraction of the plant, that is to say the molecules capable of dissolving in water. As for aromatic molecules, they only contain 0.1 to 2%. The few analyzes carried out on hydrosols show that only certain aromatic molecules known from essential oils are found in hydrosols. This is explained by the composition of the molecule itself, by the functional groups it contains, which will have more or less affinity with water. Indeed, in nature there are so-called polar molecules and others called non-polar. This polarity is defined by the distribution of positive and negative charges within the molecule, and is governed by the geometry of the molecule and the electronegativity of the atoms. In short, it's complex. What you need to remember is that two polar molecules will attract each other very strongly, as is the case for two nonpolar molecules. Water is a polar solvent, which will attract polar molecules, unlike essential oils.
In practice, here are the main aromatic molecules found in hydrosols:

• Acids, due to their carbonyl and alcohol functions
• Phenols: carvacrol, thymol and eugenol
• Alcohols: partially for some like linalool, alpha terpineol or geraniol, totally for others like lavandulol
• Coumarins
• Ketones: camphor, verbenone, thujone, etc.
• Aromatic and terpene aldehydes
• Oxides: notably eucalytol (or 1,8-cineole)
• Linalyl acetate and some ethers such as methylchavicol or anethole.

This does not include all other water-soluble molecules in the plant. At present, very few chromatographic analyzes are carried out on hydrosols. So they still have a lot to teach us! Since essential oils and hydrosols have different compositions, they are very complementary since together they represent a large part of the molecular fraction of the plant.

However, the concentration of aromatic molecules alone does not represent the power of hydrosols. They also have an action at the psycho-emotional and energetic level thanks to their vibrational dimension.

Cohobation and bi-distillation are two different processes but both based on the use of the hydrosol during a second distillation.
Firstly, cohobation is a process aimed at increasing the yield of an essential oil. For this, a first distillation is carried out to produce the hydrosol and the essential oil which will then be separated, as is customary to do. Then a second distillation is carried out, this time injecting the hydrosol obtained during the first distillation, in the form of steam. This allows a greater quantity of essential oil to be extracted from the plant. This process is notably practiced by certain Rose producers, to increase the yield of its precious essential oil.
Bi-distillation also reuses the hydrosol obtained during a first distillation, but this time to carry out a second distillation on this same hydrosol, which then becomes the raw material. This process is mainly used in the manufacture of spirits, but also makes it possible to increase the stability of certain hydrosols from fragile plants, in order to make them less sensitive to natural alteration phenomena such as oxidation.