Abstract: Hydrolates, the aromatic waters obtained as co-products of essential oil distillation, have gained increasing attention in cosmetic formulations due to their composition characterised by low abundance of volatile compounds, pleasant scent and biological activity. Containing small amounts of oxygenated volatile compounds, hydrolates exhibit antioxidant, antimicrobial and anti-inflammatory effects, making them suitable for natural skin care products, particularly for sensitive skin. Despite their long traditional use, scientific data on hydrolates remain limited, with challenges related to their chemical variability, lack of standardisation and inadequate regulation. Most used analytical technique which enables detailed profiling of hydrolates is gas chromatography coupled with mass spectrometry or simply GC-MS. Hydrolates therefore represent a promising yet underexplored category of sustainable ingredients for future natural and functional cosmetic formulations.
Ravnikar E. Scientific perspective on hydrolates in cosmetics. Cos ACTIVE J. 2025;3:43–51
INTRODUCTION
Plants have long fascinated humans not only for their medicinal properties but also for their pleasant aromas. These have traditionally been extracted through steam distillation, a process originally focused on obtaining essential oils, while hydrolates were once considered merely as secondary distillates. Today, however, hydrolates have gained increasing attention as valuable ingredients in cosmetics recognised for their natural origin, mildness and functional properties.
THE DEFFINITION – WHAT ARE HYDROLATES?
Hydrolates are transparent, aromatic solutions obtained by the hydro- or steam distillation of aromatic plants, yielding two products: an essential oil (lipophilic phase) and a hydrolate (hydrophilic phase) (1, 2). According to ISO standard ISO9235:2021 (3), a hydrolat is defined as the aqueous distillate remaining after distillation and separated from the essential oil.
HOW DO HYDROLATES DIFFER FROM ESSENTIAL OILS?
Essential oils obtained by distillation contain 100% volatile compounds, mainly monoterpenes, sesquiterpenes and their oxygenated derivatives (typically 20 to 60, and sometimes up to 300 different components) (4). In contrast, hydrolates contain only traces of these, primarily more polar, water-soluble compounds. In terms of chemistry, these molecules are mostly oxygenated mono- and sesquiterpenes, which means that they contain oxygen atoms that enable hydrogen bonding, increasing their solubility in water. Despite their low concentration (typically below 0.1% volatile compounds or less than 1 g/L), hydrolates exhibit a distinctive scent, mild taste and biological activity (2, 5).
The profile of volatile compounds in hydrolates is often comparable to the chemical profile of essential oils, but there are exceptions with significantly different chemical profiles, depending on the ratio of hydrocarbons to oxygenated compounds in the original plant. When oxygenated compounds predominate, the similarity is greater; when hydrocarbons prevail, the difference is more pronounced (2). Due to these compositional differences, the sensory properties of hydrolates and essential oils from the same plant species usually differ (2, 6). In the case of bog myrtle (Myrica gale), for example, the essential oil contains mainly p-cymene (13%), α-pinene (12%), limonene (11%), selina-4(15),7(11)-diene (10%) and cis-nerolidol (6%), while the hydrolate is dominated by 1,8-cineole (29%), α-terpineol (16%) and terpinen-4-ol (14%) (5).
ALLERGENS IN HYDROLATES – REGULATORY REQUIREMENTS AND RISKS
Hydrolates are used in cosmetics either as standalone products or as ingredients. Like all cosmetic ingredients, they must comply with Regulation (EC) No. 1223/2009 on cosmetic products (6) which ensures consumer safety, harmonised EU market rules, traceability, transparency and a ban on animal testing. The regulation also defines lists of permitted, prohibited and restricted substances, including fragrance allergens (7).
It should be noted that the presence of allergens in hydrolates is a natural and expected consequence of their volatile composition, as many small aromatic molecules are classified as allergens. They may trigger hypersensitivity or allergic reactions in sensitive individuals, such as allergic contact dermatitis (type IV reaction). This occurs when haptens (small molecules that are not immunogenic on their own) bind to skin proteins to form a complex, i.e. allergen, and trigger an immune response, manifested as redness, itching, swelling, blistering or skin peeling (7, 8). However, this does not imply that hydrolates are unsafe, but it underlines the importance of identifying and quantifying these compounds through appropriate analytical methods to ensure transparency and consumer safety.
Despite legal requirements, not all manufacturers clearly label allergens; general terms such as “perfume,” “aroma,” or “fragrance” are often used, which complicates the identification of potential irritants (9). Due to the high prevalence of allergic contact dermatitis in the EU, associated with fragrance allergens in cosmetics, food, detergents and other products (10), the European Commission seeks to protect consumers through primary prevention (prohibition or restriction of hazardous substances) and secondary prevention (mandatory allergen labelling) (6). Fragrance allergens must be declared when their concentration exceeds 0.001% (10 mg/L) in leave-on products or 0.01% (100 mg/L) in rinse-off products. Specific limits are also set for certain allergens depending on product type (6).
The list of fragrance allergens defined in Regulation (EC) No. 1223/2009 was recently updated by Commission Regulation (EU) 2023/1545, which entered into force on 16 August 2023. Although the newly listed allergens are not yet necessarily reflected on all cosmetic labels, manufacturers must ensure full compliance by 31 July 2026 for newly placed products and by 31 July 2028 for products already available on the market (11).
REGULATION, ANALYSIS AND STANDARDISATION OF HYDROLATES
The growing global interest in sustainable, safe and non-toxic cosmetic ingredients has driven research into natural alternatives to synthetic compounds. Among these, hydrolates stand out as stable, multifunctional aromatic waters suitable for cosmetic, pharmaceutical, food and aromatherapeutic use (5). However, for effective, safe and standardised application, it is essential to understand the factors influencing their chemical composition and biological activity (2, 12, 13). In this context it is important to emphasise that the lack of regulation, analytical control and standardised production or testing methods makes quality comparison difficult.
CHEMICAL ANALYSIS OF HYDROLATES
The volatile composition of hydrolates is commonly analysed by gas chromatography (GC) and gas chromatography coupled with mass spectrometry (GC-MS). Before a GC-MS analysis, hydrolates containing very low concentrations of volatiles can be extracted (e.g. using liquid-liquid extraction (12)) to enhance peak resolution and intensity. Hydrolates can also be directly analysed by GC-MS without any pre-treatment or extraction, which enables a more accurate evaluation of their volatile profile and helps detect hydrophilic compounds that may otherwise be lost during solvent extraction. However, direct GC-MS analysis of hydrolates is associated with lower detection sensitivity, as the volatile content in hydrolates is typically very low. Therefore, the choice between extraction and direct analysis should depend on the analytical goal, whether to maximise sensitivity or to preserve compositional integrity. For example, for detecting adulteration with hydrophilic compounds, direct analysis is most suitable (14). In contrast, advanced methods such as HS-SPME (headspace solid-phase microextraction) and P&T-ATD (purge-and-trap with automated thermal desorption) enable analysis without prior extraction (5).
Furthermore, factors such as the plant material, amount of plant material, distillation type, temperature and distillation time also affect hydrolate composition, while analytical results depend on the extraction and quantification methods, as explained previously (12, 14). Establishing criteria for standardised analytical protocols would improve consistency, traceability, safety and effectiveness, and strengthen trust in the hydrolate industry (15).
QUALITY OF HYDROLATES ON THE MARKET
Hydrolate quality is directly linked to the concentration of dissolved volatile compounds, which varies with the plant material, environmental conditions (temperature, rainfall, soil composition), harvest time and method, plant material storage and the distillation process (13). Despite their centuries-long use in cosmetics, gastronomy and traditional medicine (5), chemical and microbiological analyses of hydrolates remain limited. Unlike essential oils, for which extensive literature, analytical methods and regulatory frameworks exist, hydrolates are poorly studied and non-standardised (14)
Moreover, limited awareness among producers and users contributes to inconsistent product quality and frequent mislabelling or adulteration. Some commercial hydrolates even contain undeclared preservatives or solvents, raising safety concerns for oral or dermal use. Due to insufficient analytical control, many low-quality or adulterated products are available, undermining consumer confidence (14–16).
COMPARISON WITH ESSENTIAL OILS
Unlike essential oils, hydrolates have been studied significantly less. The physicochemical and biological properties of essential oils are well documented, and numerous monographs by the European Medicines Agency are based on traditional and well-established use. Essential oils generally exhibit antibacterial, antiviral, anti-inflammatory and antioxidant effects and are commonly used to relieve digestive and dermatological conditions (17).
It is important to note that, due to the markedly different chemical composition of hydrolates and essential oils, the biological effects observed for essential oils cannot be directly attributed to the corresponding hydrolates. While both originate from the same plant, hydrolates contain only trace amounts of volatile compounds and differ significantly in their polarity and concentration. Consequently, the therapeutic or cosmetic properties established for essential oils should not be automatically extended to hydrolates, despite such claims occasionally appearing in commercial descriptions. Each hydrolate must therefore be individually analysed and evaluated to determine its characteristic composition and the biological effects that can realistically be expected from its use.
HYDROLATES IN COSMETICS – RESEARCH OVERVIEW
Traditional use and general properties
Hydrolates have a long history of cosmetic use due to their non-concentrated composition, pleasant aroma and biological activity. Their diverse effects are attributed to the volatile compounds that dissolve in the aqueous phase after distillation (12, 15). These include antibacterial, antiseptic, antioxidant, anti-inflammatory, soothing and wound-healing properties (2, 5, 18, 19). Researchers have focused primarily on the antioxidant, antimicrobial and anti-inflammatory effects (2, 5, 20) demonstrated in numerous in vitro studies (1, 2, 12, 18–21). These findings indicate potential applications in products aimed at slowing skin ageing, reducing pigmentation, moisturising and softening, and promoting wound healing (1, 22).
Biological activity – In vitro and clinical studies
One of the key mechanisms of skin ageing is the accumulation of reactive oxygen species also known as ROS, leading to oxidative stress, loss of elasticity and wrinkle formation. The presence of natural antioxidants in formulations is therefore crucial (1, 20). Hydrolates of field marigold (Calendula arvensis), citrus species (Citrus sp.) (5), thyme (Thymus vulgaris) (23), rosemary (Salvia rosmarinus) (23) and lavender (Lavandula angustifolia) (24) have all shown notable antioxidant activity.
Hydrolates are highly susceptible to microbial contamination (as discussed later), however, following high-quality production and storage standards, volatile compounds in hydrolates may express antimicrobial effects. Hydrolates have shown promising antimicrobial potential (2, 20, 21, 23, 25), particularly for oily and acne-prone skin or for the development of natural antiseptics. In vitro studies confirmed antimicrobial effects of peppermint (Mentha × piperita) (26), rosemary (Salvia rosmarinus) (5) and damask rose (Rosa × damascena) hydrolates, the latter also showing anti-inflammatory properties (27). Despite encouraging results, however, few in vivo studies have validated their effectiveness, and additional clinical research is urgently needed (1, 2, 5, 19).
Reports on clinical studies are extremely rare; we identified two studies that investigated the biological activity of hydrolates. In a pilot study, Yayla et. al. (28) evaluated the effectiveness of a mouthwash prepared from sage, thyme and peppermint hydrolates as an adjunct therapy for periodontal disease. Participants used the hydrolate-based mouthwash after professional dental cleaning. The study found that rinsing with 15 mL of the mouthwash for 30 seconds, 30 minutes after basic oral care, 4 times a day for 14 days, reduced bacterial counts in the oral cavity and decreased inflammatory signs such as bleeding and plaque index. In a clinical study by Teymuri et al. (29), lemon balm (Melissa officinalis) hydrolate demonstrated a positive effect on the treatment of acne, inflammation and the regulation of oily skin. 20 people were included in the experiment for 3 months. They sprayed lemon balm hydrolate or hydroalcoholic extract on the skin, three to five times a day. Scientists observed that 75% of clients using lemon balm hydrolate improved skin pores and 92% recovered of hypersecretion of sebaceous glands.
FORMULATINGS WITH HYDROLATES
General considerations
Herbal and floral aromas, combined with bioactive properties, make hydrolates attractive cosmetic ingredients. Their antibacterial and anti-inflammatory effects benefit skin health, making them valuable in natural cosmetic formulations (21, 30–32). Although hydrolates are considerably gentler than essential oils and can usually be applied directly to the skin (5, 19), they may still contain fragrance allergens in non-negligible concentrations, as previously discussed. Since they are often incorporated into cosmetic formulations in higher amounts to provide a mild natural aroma, their allergenic potential should not be overlooked, particularly in leave-on products intended for sensitive or allergy-prone skin.
Technological aspects of formulation
Effective formulation with hydrolates requires an understanding of their chemical composition, stability and microbial sensitivity, as they are natural, minimally processed aqueous extracts (1, 2). They can be used as stand-alone cosmetic products, either alone or with additional cosmetic ingredients such as preservatives and water-soluble cosmetically active ingredients, or as part of the aqueous phase in emulsions (33). Hydrolates are commonly found in cleansers, toners, creams, products for problem skin, facial and hair masks, and even perfumes, where they serve as natural fragrances, mild antiseptics, antioxidants, and skin soothing and moisturising ingredients (2, 5, 12). Considering their composition, they are generally not expected to initiate incompatibilities with other ingredients in a formulation.
pH and microbiological stability
Most hydrolates have a slightly acidic pH, typically 4.0 to 6.0 (2, 20), which is compatible with the skin’s natural pH (1, 23). This makes them suitable for formulations that help restore the skin’s natural balance (1) and prevent dehydration while providing a refreshing effect (34). Proper pH also influences preservative stability (33).
Although their natural acidity and monoterpene oxides contribute to mild antimicrobial protection (5), hydrolates remain susceptible to microbial growth due to their high water content. Therefore, aseptic production, sterile packaging and cool storage are crucial (2, 5). Preservatives are often added in compliance with Regulation (EC) No. 1223/2009 (6). Recently, however, interest in preservative-free formulations has increased; in this case, microbiological stability can be maintained through sterile filtration (0.2 μm membrane) (35) and aseptic packaging (2).
IMPORTANCE OF PACKAGING
The choice of packaging material plays a key role in maintaining product stability, quality and safety (33, 36, 37). Volatile compounds in hydrolates are susceptible to oxidation, isomerisation, polymerisation and hydrolysis (4, 5); therefore, packaging must prevent oxygen exposure and protect against UV light. Hydrolates should ideally be stored for up to a year after distillation (5) in sterile, airtight, dark glass containers at cool temperatures (2, 5, 12).
A study on peppermint (Mentha × piperita) and rose geranium (Pelargonium graveolens) hydrolates demonstrated that packaging material significantly affects stability: samples in plastic containers (PE and HDPE) lost almost all volatile compounds after six months, while dark glass containers best preserved quality, especially under refrigeration (2–8 °C) (38).
HYDROLATES AS RAW MATERIALS: WHY SPECIFICATIONS MATTER
While many factors influencing hydrolate quality have been addressed in previous sections, one critical aspect requires special attention: product specifications, particularly the availability of GC-MS reports. These analytical profiles are essential for verifying chemical composition and ensuring consistency and transparency across distillation batches. Although such analyses are not yet standard practice, it is imperative that suppliers and manufacturers be required to provide them. By demanding transparency and traceability, we can drive the industry of hydrolates toward higher standards and ensure that hydrolates are treated as scientifically validated raw materials.
CONCLUSION
Hydrolates represent a valuable yet underexplored class of natural aromatic products with great potential in cosmetics. Their appeal lies in their pleasant aroma and mild effects, including skin-beneficial activities such as antioxidant, anti-inflammatory and antimicrobial effects, making them ideal for the development of sustainable and eco-friendly formulations.
Despite their long history of use, the scientific understanding of hydrolates remains limited. Standardisation of production, analytical methods and quality criteria are lacking, hindering cross-study comparability and product consistency. Further in vivo and clinical studies are essential to confirm effectiveness and safety. The establishment of clear regulatory guidelines, standardised analytical protocols and traceable production practices would ensure reliable identification, labelling and safety of hydrolates as raw materials in the production of cosmetics. Only through coordinated efforts among researchers, manufacturers (distillers) and regulators can the full potential of hydrolates as high-quality natural raw materials be realised.
Eva Ravnikar, M. Pharm.
Modern CosmEthics, Velenje, Slovenia
eva.ravnikar17@gmail.com
References
Please click on the references below for more information.
4. Tisserand R, Young R. Essenatial Oil Safety: A Guide for Health Care Professionals. 2nd ed. Edinburgh [etc.]: Churchil Livingstone/Elsevier; 2014. 69-186 p.
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