Dump the Solvents. Switching to Subcritical Water Extraction
Extracting oils from botanicals has often relied on toxic solvents like hexane, pentane, and methylene chloride. However, these approaches carry significant problems, such as complex regulatory compliance requirements and health risks due to solvent residues left in the end product, as well as environmental pollution. Subcritical water extraction (SWE) offers a healthy, robust, and sustainable alternative. This green technology utilizes nothing more than purified water, heat, and pressure to replace harsh chemicals for a safer and more responsible extraction process.
Let’s look at how it works, why it can replace solvents, what you can expect to achieve with it, and how, when paired with other advanced clean technologies, it can become the best solution for companies involved in botanical extraction.
The Mechanisms Behind Subcritical Water Extraction
Subcritical water extraction, also known as pressurized hot water extraction, operates on the simple principle that heating water (changing its properties) to temperatures between its boiling point (100°C) and critical point (374°C) under sufficient pressure keeps it in a liquid state.
When you put water in a subcritical state, its fundamental nature transforms. When room temperature water is a highly polar solvent, it has a high dielectric constant (ε ≈ 80), which makes it an effective solvent for dissolving polar compounds. As the temperature of subcritical water increases, the hydrogen bonds that give water its polarity weaken, resulting in a dramatic drop in its dielectric constant. At 250°C, water’s dielectric constant falls to approximately 27, which is comparable to that of organic solvents like methanol or acetone at room temperature.
This phenomenon is known as “tunable polarity,” which is what makes SWE effective among many applications. By adjusting the temperature, operators can selectively target different compounds. Polar molecules (molecules that have positive and negative ends) are recovered at lower subcritical temperatures, while less polar compounds like flavonoids, essential oils, and terpenes become more soluble at higher temperatures. The extraction mechanism disrupts the weak forces, such as Van der Waals forces (weak attractions between molecules) and hydrogen bonds, that bind solutes to the plant matrix, allowing them to be dissolved and washed out by the water. The reduced viscosity and tenfold increase in diffusivity at 250°C enable rapid mass transfer, resulting in significantly shorter extraction times compared to traditional methods.
Extract Without Introducing Toxins to the Product or the Environment
Replacing solvents with Subcritical Water Extraction improves product purity by eliminating the risk of solvent residue and pollution, and may also increase the speed of product output.
Adding subcritical water extraction, a universally recognized safe method, to your arsenal or completely switching from other solvents ensures your extracts are entirely free from toxic solvent residues, such as hexane and methylene chloride, which are known to be toxic or carcinogenic. Given the US government’s recent efforts to eliminate toxins, many nutraceutical, pharmaceutical, and food manufacturing companies are looking to future-proof their businesses with this technology.
Sub-critical water is also an inherently sustainable and environmentally friendly technology. Water as a solvent avoids health hazards, environmental pollution, and the high costs associated with hydrocarbon and synthetic solvents. This method creates a circular economy by enabling agro-industrial wastes, such as fruit pomace, seeds, and coffee grounds, to become functional ingredients post-extraction.
Implementing SWE in your operation will set your business up for success in the long run. Compared to some traditional solvent-based extraction methods that can take hours or days, SWE is more efficient. A study on garlic extraction found that SWE achieved a higher concentration of the bioactive compound alliin in just 10 minutes compared to 2 hours for Soxhlet extraction. While you may have to pay more up front for subcritical water extraction equipment, the long-term operational costs are lower due to faster processing, reduced energy consumption, and the elimination of solvent-related expenses. This should inspire optimism and a forward-thinking approach to adopting SWE.
More Solventless Equipment for Post-Extraction Purification
Subcritical water extraction becomes a more powerful player when post-extraction supports the same principles of purity. Using Supercritical CO₂ extraction to purify an extract further is one option for downstream processes.
Supercritical CO2 Extraction
For certain botanicals, we can use SWE to capture water-soluble compounds, then use a supercritical CO2 extractor to process other valuable oils, lipids, and aromas left behind by SWE. This manufacturing process enables the maximum utilization of the raw material.
Supercritical CO2 Dryer
Once the extraction takes place, you may consider using CO₂ as a drying agent to preserve the valuable raffinate, a proven technique for applications like the decaffeination of coffee. This method utilizes CO₂ at a high pressure (e.g., 2000 psi) to absorb and carry away water from the wet material. The pressure is then lowered (e.g., 500 psi), causing the CO₂ to release the water and return to its gaseous state, leaving the material completely dry. Because the process operates at low temperatures, it effectively preserves heat-sensitive compounds.
Falling Film Evaporators
In the case you find yourself with water or a solvent in your extract, a falling film evaporator, which creates a thin liquid film over a heated surface and evaporates the solvents or water at low temperatures, can be effective in preserving the integrity of heat-sensitive flavor, aroma, and bioactive compounds while removing what’s unwanted. Fractional separation also allows for the selective removal of undesirable molecules while retaining desirable ones.
Radiant Energy Dryers
For a final drying step, a radiant energy belt dryer offers a fast, low-temperature method to produce a shelf-stable powder while protecting nutritional content and flavor.
Applications of SWE
This list of applications helps demonstrate the numerous bioactive compounds it can extract from a wide range of natural sources. Because so many businesses have seen success with this technology, we have confidence in its potential to revolutionize industries not even mentioned here.
Coffee
SWE combined with Supercritical CO₂ is effective for extracting phenolic compounds like quinic acid and caffeine from spent coffee grounds and coffee cherry pulp, with the pulp showing lots of bioactives.
Apple Pomace
A byproduct of juice production, apple pomace is a great source of polyphenols, chlorogenic acid, and quercetin glycosides, which are recovered from the wet pomace without organic modifiers using SWE.
Orostachys japonicus (Rock Pine)
This medicinal herb is another example of an extract that prefers SWE over traditional methanol or ethanol extraction. Its tunable nature is particularly useful, as higher temperatures (around 220°C) are needed to extract less-polar triterpene saponins.
Mucuna Beans
L-DOPA from Mucuna Beans has been successfully extracted using SWE and other allelochemicals. Research shows us that incorporating a static hold time before the dynamic extraction phase enables water to more effectively penetrate the biomass, thereby increasing the yield of target compounds.
Roses (Rosa alba L.)
Use SWE to extract essential oils rich in phenylethyl alcohol and linalool, as well as phenolic acids and flavonoids from roses. Potentially a better alternative to steam distillation for the fragrance and cosmetic industries.
Pesticides and Pollutants
Beyond nutraceuticals, SWE is a powerful tool for environmental remediation, capable of removing persistent organic pollutants like PAHs, PCBs, and pesticides from contaminated soils and sediments effectively.
How to See Whether SWE is Right For You
Implementing SWE requires planning, with the primary consideration being the initial capital investment, which typically pays off within a few months of operation. When evaluating whether subcritical water extraction is suitable for you, it’s best to talk to an expert to help you determine the optimal temperature, pressure, residence time, and particle size to consider for each botanical you wish to or currently process before purchasing.
Pilot Runs
Find a company willing to do pilot-scale development runs, so you can validate the technology using your own botanicals and quantify yields. Mitigating risks is crucial in your innovation pipeline, and this way, you will have the data necessary to justify a larger investment in a cleaner, more efficient future.
Talk to an Expert
If you’re interested in subcritical water extraction equipment or any other piece of equipment mentioned, connect with an extraktLAB representative. We’d be happy to discuss your project and see where we can assist you.
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