Multiple variables come into play when determining which extraction method to use, but a new one is pushing to the forefront for both manufacturers and consumers. How “green” or sustainable the manufacturing process is may be just as crucial a factor as price and availability. By keeping this goal in mind, there are multiple extraction techniques that have minimal environmental impacts that can also increase efficiency.
Freeze-Thaw Assisted Extraction
Freeze-thaw assisted extraction is an effective way to efficiently break down dense plant materials reducing the use of solvents and the maceration time.
Recently this method was applied as alkali extraction pretreatment for hemicellulose extraction from bamboo. This plant has a particularly dense structure with robust cell walls.[1] After exposing the dried plant material to -30 °C for 12 hours, it was thawed and treated with a 7% alkaline solution at 75 °C for 90 minutes. The extraction rate of the hemicellulose was 64.71% and the purity rose from 82.63% to 89.45% compared to traditional alkali extraction withouth freeze-thaw pretreatment. Allowing for a more effective extraction without any negative consequences on the environment.
Pressurized Hot Water Extraction
Pressurized Hot Water Extraction (PHWE) is also a green physical extraction which relies on hot water to separate organic compounds.[2] This method has with wider applications than the freeze-thaw extraction method, and is useful for extracting materials from plants, food and soil samples. There are multiple variables that can affect extraction rate, but the most important is temperature. When heating the water above the boiling point, it leads to “high diffusion, low viscosity, and low surface tension.” However, it is critical not to overheat the water, as higher temperatures may also lead to termolabile compound degradation.
Intensification Methods Assisted by Ultrasonication and Microwaves
Similar to PHWE extraction, it is possible to intensify the extraction efficiency through the application of ultrasounds or microwaves, maximizing the yields and the extract purity while shortening the extraction time.[3] Using microwaves or ultrasonic frequencies (ranging from 20-1000 kHz) increases pressure, and thus heat, to break down the plant’s cell walls prior to extraction. This method is entirely sustainable and can be done at lower temperatures reducing the maceration time of the plant material. To make an example, when extracting anthocyanin and phenolic from blueberries, researchers were able to produce a 41.6% yield in 10 minutes, increasing the yield 2.5 and 3.2-folds higher than conventional solvent extraction wihtouth using ultrasounds.
Supercritical Fluid Extraction (SFE)
SFE is one of the most ubiquitous extraction methods with applications in cosmetics, pharmaceuticals, food products and more. In Supercritical Fluid Extraction (SFE) non organic solvents reach the supercritical stage by tuning pressure and temperature in order to maximize the separation of the extractants from the plant matrix . Sometimes the extraction involves the use of co-solvents such as methanol or ethanol. The most common supercritical solvent used is carbon dioxide (CO2) because of its low price and its low toxicity. Moreover, CO2 low viscosity, high diffusivity and low surface tension allow an easier mass transfer for a relatively environmentally friendly extraction compare to other conventional methods. Generally closed loop systems are used in SFE in order to recycle the solvents and use them in repeated extraction processes.
References:
[1] Li, J. et al. Green, efficient extraction of bamboo hemicellulose using freeze-thaw assisted alkali treatment. Bioresource Technology, (2021); 333, 125107. https://doi.org/10.1016/j.biortech.2021.125107
[Journal Impact Factor = 9.642 ] [Times cited = 12]
[2] Teo, C. C. et al. Pressurized hot water extraction (PHWE). Journal of Chromatography A (2010); 1217(16), 2484–2494. https://doi.org/10.1016/j.chroma.2009.12.050 [Journal Impact Factor =4.17] [Times cited = 529]
[3] Panda, D., et al. Cavitation Technology—The Future of Greener Extraction Method: A Review on the Extraction of Natural Products and Process Intensification Mechanism and Perspectives. Applied Sciences, (2019); 9(4), 766. https://doi.org/10.3390/app9040766
[Journal Impact Factor = 2.679 ] [Times cited = 80]
[4] Herrero, M. et al. Supercritical fluid extraction: Recent advances and applications. Journal of Chromatography A, (2010); 1217(16), 2495–2511. https://doi.org/10.1016/j.chroma.2009.12.019
[Journal Impact Factor = 4.17] [Times cited = 725]
Image: https://www.pexels.com/photo/bamboo-stick-lot-in-gray-galvanized-buckets-405034/
