SUBCRITICAL WATER EXTRACTION

What is subcritical water extraction?

According to the review by Cravotto et al. (2022), subcritical water extraction (SWE) is one of the technologies that involve an appropriate physical treatment of water, alongside in situ plant water extraction (ISPWE). The review describes SWE as exploiting the change in the dielectric constant of water under subcritical conditions. Knierim et al. (2024) write that the pressurized hot water extraction (PHWE) they consider uses subcritical water for extraction. According to the same authors, hydrodistillation, for the selective extraction of essential oils, and PHWE, for the extraction of secondary metabolites, fibres, lignins, cellulose and hemicellulose, can be used as a water-based cascade utilisation of side streams.

How does temperature change water as a solvent?

Knierim et al. (2024) describe the way PHWE works as a change in the physical properties of water as the temperature rises, their focus being the change in polarity, described by the dielectric constant. As the temperature rises, the dielectric constant shifts toward the nonpolar range and reaches values similar, for example, to those of methanol or ethanol, which according to the authors makes it possible to extract nonpolar components without organic solvents. The review by Hedayati et al. (2025) describes substantial changes in the properties of water in its subcritical state, which “can be attributed” to reduced intermolecular hydrogen bonds between water molecules.

In their introduction, Filly et al. (2016) write that a higher extraction temperature can increase solvent–matrix interaction, but also transfer, solubility and diffusion. According to the same passage, the macroscopic dielectric constant of a solvent characterises the polarity of the medium and controls the ionic dissociation of salts. The dielectric constant of water decreases at high temperatures and pressures, which, the authors write, increases water diffusivity.

Why is temperature the variable that is adjusted?

According to the review by Cheng et al. (2021), pressure has minimal influence on the dielectric constant of water, so in actual extraction processes the temperature of water is adjusted to control its dielectric constant in order to mimic various organic solvents. The abstract of the same review states that the density, ion product and dielectric constant of subcritical water can be adjusted by temperature.

How does subcritical water select compounds?

According to the abstract of Cheng et al. (2021), these tunable properties allow subcritical water to carry out class-selective extractions, such as extracting polar compounds at lower temperatures and less polar ingredients at higher temperatures. The review, which calls the technique SBWE, states that its main extraction mechanism follows the “like dissolves like” rule. Polar compounds can be extracted efficiently by water at “lower” temperatures, while less polar analytes require higher temperatures to achieve a reasonable extraction efficiency; the less polar the analyte, the higher the temperature required. According to the same passage, larger and more complex molecules also require higher temperatures. The review adds that pressure does not have a significant effect on SBWE efficiency as long as it is high enough to keep water in the liquid state.

How is the extraction run?

According to Cheng et al., there are mainly two SBWE modes, static extraction and dynamic extraction. In static extraction, subcritical water and the sample are added to an extraction vessel, then heated to the desired temperature under a moderate pressure that keeps water in the liquid state. Dynamic SBWE is a continuous process: once the samples are in the extraction vessel, water is continuously fed into the extractor with a pump, at a fixed temperature or under gradient temperature conditions. The review states that the efficiency of static extraction is “normally” lower than that of dynamic extraction.

What limits the technique?

The abstract of Cheng et al. names potential organic degradation under high temperature conditions as another challenge for the application of SBWE. According to the review by Cravotto et al. (2022), SWE has been shown to be a suitable method for the additional extraction of lipophilic compounds, but requires further purification because of its low extraction selectivity. The review relates this to the high extraction efficiency of SWE, through which primary metabolites (proteins, fats, carbohydrates and soluble fibres) and secondary metabolites are extracted in a very efficient way.

What happens to the essential oil in a PHWE extract?

According to Knierim et al. (2024), when PHWE is used as an alternative to hydrodistillation, the essential oil does not settle on the surface after the extract has cooled, “as is the case with extraction using organic solvents”. One of their figures shows a PHWE extract of chamomile in a separation funnel with no visible essential oil.

If a PHWE extract is subsequently hydrodistilled, the essential oil becomes visible above the hydrolate again. To understand these phenomena, the authors analyse phase diagrams of the hydrodistillation and PHWE processes for the extraction of essential oils from Iris pallida Lam. In these diagrams, the composition of the PHWE extract lies in the single-phase region, where the essential oil is dissolved, while that of the hydrolate lies in the two-phase region, where the solubility of the essential oil in water is exceeded and the oil and water phases separate.

What did a model-based study of PHWE report?

Knierim et al. (2024) optimise the PHWE process for a new material system, which the passage does not name, using a digital twin. The results generated with the digital twin in their MFAT studies are analysed statistically with the JMP software, using linear regression. Among the parameters analysed, temperature was found to have a significant effect on yield, and an increase above what the authors call a critical temperature of 130 °C has a negative effect on yield. The authors attribute the sharp decrease in yield above 130 °C to the thermal decomposition of the target component. For the material system shown, the parameters the authors call ideal for the highest yield are a temperature of 130 °C, a flow rate of 600 rpm, equal to 1.5 mL/min, a particle size of 1 mm and a quantity of plant material of 1 g.

What did one study on Aquilaria malaccensis wood report?

In the study by Samadi et al. (2020), wood of Aquilaria malaccensis Lam. was collected from the garden of the Forest Research Institute Malaysia (FRIM). The wood was air-dried in a dark room away from sunlight, which the authors relate to avoiding the loss of volatiles due to light and temperature, and stored in a bag at a cool room temperature (4 °C). It was then cut into smaller pieces with a saw, shredded with a small knife and crushed in a grinder (Panasonic miller).

How was the wood extracted?

The comparison hydrodistillation used a Clevenger-type apparatus described by the European Pharmacopoeia. The wood was first soaked in water for seven days, in order, according to the authors, to speed up the release of essential oil; 100 g of air-dried, size-reduced wood and an appropriate amount of distilled water were then placed in a heating mantle attached to the Clevenger and boiled at 100 °C and 1 atm.

The subcritical water extraction was performed in a static-mode batch laboratory-size system with a 20 L heating bath, using silicon oil for temperatures of 100 to 170 °C or a potassium nitrate/sodium nitrate salt for temperatures of 180 to 271 °C. The ground, dried sample was loaded into a stainless-steel reactor of 30 mL (65 mm long, 16 mm internal diameter), which was flushed with argon gas to remove trapped air, then sealed. Once the bath had reached the set temperature (100 to 271 °C), the reactor, filled with sample and water at 0.08 to 0.22 gr/mL, was immersed in it; on completion of the extraction, within 1 to 34 min, it was pulled out and immediately dipped into cool water.

The liquid part, containing water and essential oil, was poured into a separation funnel, and the essential oil was removed by liquid–liquid extraction with 5 mL of n-hexane, replicated twice, after which the hexane was removed. The yield of essential oil is expressed in % v/w, as the volume of collected essential oil (mL) relative to the weight of plant material (g).

Which parameters changed the yield?

In the response surface of Figure 3a, with extraction time held at 17.5 min, temperature (up to 250 °C) and the sample-to-water ratio were positively associated with the yield of essential oil. A further increase in temperature led to a decrease in yield. According to the same passage, the sample-to-solvent ratio has only a minor effect on the yield of essential oil from wood, and the optimum observed was 190 to 250 °C and 0.13 to 0.2 gr/mL. In Figure 3b, at 0.15 gr/mL, a rise in temperature brings a higher yield of essential oil, while extraction time has a somewhat minor effect, especially at temperatures above 200 °C. According to the authors, the parameter with the greatest impact on the yield of A. malaccensis wood oil is temperature. The Figure 3b passage places the highest yield at a temperature of around 200 to 260 °C, with an extraction time equal to or greater than 5 min, while the Figure 3a passage gives a decrease in yield beyond 250 °C and an optimum of 190 to 250 °C.

How do the two oils compare?

By GC/MS, the authors identified 43 (“Forty-three”) chemical components, constituting 92 % of the dark greenish wood oil obtained by hydrodistillation (HD), and 50, constituting 96 % of the light orange/brownish oil obtained by SCWE.

According to the text, the major compounds of the hydrodistilled oil were n-hexadecenoic acid (17.238 %), 2-butanone, 4-phenyl (10.732 %) and agarospirol (7.618 %), and those of the SCWE oil furfural (14.36 %), guaiacol (13.504 %) and 2-butanone, 4-phenyl (12.042 %). In Table 4 ("%Presence", HD and SCWE columns), agarospirol reads 7.618 and 3.52, α-eudesmol 1.887 and 0, and hexadecanoic acid 17.238 and 10.104; the totals are 92.159 and 96.618. The text names n-hexadecenoic acid with the value 17.238 that Table 4 gives to hexadecanoic acid. Table 4 lists furfural at 14.36 and guaiacol at 13.504, the values the text gives for the SCWE oil.

How do the authors explain the differences?

In their abstract, the authors state that, in comparison to conventional methods, the subcritical method involves a shorter processing time and delivers a higher oil yield and quality. According to the authors, their FTIR and SEM results verify the inability of HD to effectively break down the cell walls of A. malaccensis wood and extract its essential oil, which in their view explains the lower yield of HD in comparison to SCWE.

What do the authors write about unwanted reactions?

Because SCWE runs at high temperatures and pressures in the presence of water, Samadi et al. (2020) write that unwanted reactions such as hydrolysis, degradation and charring are “inevitable”.

References

• Cravotto C., Fabiano-Tixier A.-S., Claux O., Abert-Vian M., Tabasso S. et al., « Towards Substitution of Hexane as Extraction Solvent of Food Products and Ingredients with No Regrets », Foods 2022;11(21):3412, doi:10.3390/foods11213412 — https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=pmc&id=9655691&rettype=xml
• Knierim L., Uhl A., Schmidt A., Flemming M., Höß T. et al., « Pressurized Hot Water Extraction as Green Technology for Natural Products as Key Technology with Regard to Hydrodistillation and Solid–Liquid Extraction », ACS Omega 2024;9(29):31998-32010, doi:10.1021/acsomega.4c03771 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC11270720/fullTextXML
• Cheng Y., Xue F., Yu S., Du S., Yang Y., « Subcritical Water Extraction of Natural Products », Molecules 2021;26(13):4004, doi:10.3390/molecules26134004 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC8271798/fullTextXML
• Hedayati S., Tarahi M., Madani A., Mazloomi S.M., Hashempur M.H., « Towards a Greener Future: Sustainable Innovations in the Extraction of Lavender (Lavandula spp.) Essential Oil », Foods 2025;14(1):100, doi:10.3390/foods14010100 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC11720256/fullTextXML
• Samadi M., Zainal Abidin Z., Yoshida H., Yunus R., Awang Biak D.R., « Towards Higher Oil Yield and Quality of Essential Oil Extracted from Aquilaria malaccensis Wood via the Subcritical Technique », Molecules 2020;25(17):3872, doi:10.3390/molecules25173872 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC7503260/fullTextXML
• Filly A., Fabiano-Tixier A.S., Louis C., Fernandez X., Chemat F., « Water as a green solvent combined with different techniques for extraction of essential oil from lavender flowers », Comptes Rendus Chimie 2016;19:707–717, doi:10.1016/j.crci.2016.01.018 — https://comptes-rendus.academie-sciences.fr/chimie/item/10.1016/j.crci.2016.01.018.pdf
• Govaerts R. (ed.), World Checklist of Vascular Plants (WCVP), Royal Botanic Gardens, Kew — https://sftp.kew.org/pub/data-repositories/WCVP/wcvp.zip