Currently, common extraction methods of plant extracts include solvent extraction, ultrasonic extraction, microwave extraction and enzyme extraction, while supercritical fluid extraction and microwave-assisted extraction are widely used as new extraction techniques.
The solvent extraction is to extract an active ingredient from a solid raw material by using a solvent, and the solvent used must have a property of being miscible with the extracted solute. After the plant materials are pulverized, they are placed in a suitable container, and several times of the solvent is added, and the extraction can be carried out by dipping, percolating, boiling, refluxing, and continuous extraction.
In the extraction process by the solvent extraction, the concentration of the solvent, the ratio of material to liquid, the extraction temperature, and the extraction time will directly affect the yield of the active ingredient. Cristina Juan et al. extracted ochratoxin A from rice by solvent extraction, and determined the content of OTA via fluorescence detection and liquid chromatography. The study showed that the content of the exact OTA was up to 4.17 ng/g under the optimum ratio of material to liquid, extraction temperature and extraction time. Monte D. Holt et al. extracted alkylresorcinol from raw and cooked wheat seeds by using solvent extraction. The experiment showed that solvent extraction can save extraction time.
Ultrasonic extraction is to use the strong vibration and cavitation effects generated by ultrasonic waves to accelerate the release, diffusion and dissolution of substances in plant cells into the solvent, while maintaining the structure and biological activity of the extracted substances unchanged. The principle of ultrasonic extraction is mainly a physical process, and it is a relatively new extraction method that has been paid more and more attention in recent years. For most of the ingredients, the ultrasonic extraction can greatly shorten the extraction time with less solvent consumption and higher leaching rate, compared to conventional solvent extraction. Accordingly, it enjoys relatively high extraction efficiency.
In the process of ultrasonic extraction, the selection and concentration of the solvent, the ratio of material to liquid, the extraction temperature, and the extraction time will directly affect the yield. Ling Zhou et al. extracted Schisandra chinensis by using ultrasonic extraction, and mainly studied the influencing factors of the yield of ultrasonic extraction. The experimental study showed that the yield increased with the increase of temperature and increased with the increase of power. Hong Van Le et al. extracted the vitamin E and phenolic compounds in cherries by using ultrasonic wave, which mainly compared the difference in extraction time and yield between ultrasonic extraction and enzymatic extraction. The experimental results showed that the ultrasonic extraction was 6 times shorter than the enzyme extraction, the yield of ultrasonic extraction was 2 ~ 3 times that of enzyme extraction. Zhong Aiguo et al. extracted the chlorophyll from fresh bamboo leaves by ultrasonic extraction, and used a spectrophotometer to quantitatively determine the content of extracted chlorophyll. The results showed that compared with the commonly used organic solvent extraction, the ultrasonic extraction not only has high yield, high speed and high efficiency, but also is carried out at room temperature without heating and saves energy.
Supercritical fluid extraction
Supercritical fluid extraction (SFE) is a new type of extraction and separation technology, generally using CO2 as an extractant. The principle of supercritical fluid extraction is to use the unique dissolving capacity of the supercritical fluid and the characteristic that the solubility of the substance in the supercritical fluid is very sensitive to changes in pressure and temperature to separate the dissolved substances in the supercritical fluid by means of temperature rise and pressure drop (or both), so as to achieve the purpose of separation and purification. It has the functions of rectification and extraction, and enjoys the advantages of active ingredients with less deactivation, high product quality, simultaneous completion of extraction and separation processes, which is considered to be an environmental-friendly high and new separation technology, especially suitable for the separation and purification of unstable natural products and physiologically active substances.
In the mid-1980s, supercritical CO2 extraction technology was gradually applied to the extraction and separation of plant active ingredients, which was a new successful technology in research and application. Ruey Chi Hsu et al. extracted the active ingredients of Ganoderma lucidum, with CO2 and ethanol as solvents, by means of supercritical fluid extraction. The study results showed that the supercritical fluid extraction ensured the fluidity of Ganoderma lucidum extract which was not affected by temperature. Monica Waldeb.ck et al. extracted squalene and α-tocopherol from olives by using pressurized fluid extraction technology. The experimental results showed that the extraction effect was the best when the ethanol was used as the solvent, the extraction temperature was 190 °C, and the extraction time was 10 min. YI QI ANG GE et al. extracted natural vitamin E from wheat germ by use of supercritical CO2 extraction technology, which mainly studied the effects of pre-extraction treatment and extraction conditions on yield. The experimental study showed that the yield was the highest when the particles were 30 mesh and the pressure was 4000 ~ 5000 psi, the extraction temperature was 40-50 °C, and the CO2 fluid flow rate was 2.0 mL/min.
Microwave-assisted extraction (MAE) is a new technology that uses microwave energy to improve extraction efficiency. Microwave-assisted extraction is a method of selectively extracting target ingredients in materials by use of the characteristics of microwave heating. The target ingredients can be effectively heated by adjusting the parameters of microwave, so as to facilitate the extraction and separation of target ingredients. The principle of microwave-assisted extraction for plant extraction is that plant samples absorb a large amount of energy in the microwave field, while the surrounding solvent absorbs less, thus generating thermal stress inside the cells. The plant cells are broken due to internal thermal stress, so that the substances inside the cells is directly contacted with a relatively cold extraction solvent, which accelerates the transfer of the target product from the interior of the cells to the extraction solvent, thereby enhancing the extraction process. Microwave-assisted extraction uses thermal energy in the same way as immersion and filtration in technology principle, but the speed of extracting plant extracts is much faster than traditional methods, avoiding the destruction and degradation of valuable plant extracts while reducing extraction time.
At present, the microwave-assisted extraction is a powerful tool for the extraction of natural plant active ingredients with rapid extraction speed and better extract quality, but the microwave-assisted extraction is selective for internal heating and requires the materials to be treated to have good water absorption. In other words, it is required the position of the product to be separated is easy to absorb water, otherwise the cells are difficult to absorb enough microwave to break themselves, and the product will be difficult to be released quickly. For liquid extraction systems, the solvent substance is required to be polar, and the non-polar solvent is not sensitive to the effect of microwave. Ting Zhou et al. extracted flavonoid and coumarins compounds from medicinal plants by use of microwave-assisted extraction. The effects of sample size, ratio of material to liquid, extraction temperature and time on yield were studied by orthogonal experiments, and the experimental study showed that: the yield was 98.7% under the optimal extraction process conditions. Li Haibin et al. extracted mogroside from dried Momordica grosvenori by use of microwave-assisted extraction. The results showed that the yield of microwave-assisted extraction was 70.5%, which was 45% higher than that of the conventional water extraction and the time was 50% shorter.
Microwave ultrasonic synergistic extraction
Microwave is a kind of non-ionizing electromagnetic radiation. The polar molecules of the irradiated substances are quickly turned and aligned in the microwave electromagnetic field, resulting in tearing and mutual friction to generate heat, which ensures fast transfer and full utilization of energy. Microwave is highly efficient and energy saving without industrial pollution, but its penetration depth is limited (the same order of magnitude as its wavelength), and its mass transfer function is not significant during the process of enhancing the extraction. Ultrasonic wave is a kind of high-frequency mechanical wave with turbulent effect, perturbation effect, interfacial effect, and energy-concentration effect, but the thermal effect produced by ultrasonic wave is not significant and is limited to a very small range around cavitation bubbles. Combining the two of them, the synergistic effect is beneficial to the release of the broken wall components, that is, a high-cost, inexpensive and non-polluting extraction of biological active substances can be obtained by the microwave-ultrasonic synergistic enhanced extraction technology. HeJT et al. extracted the water-soluble biological active ingredients from traditional Chinese medicines by the microwave-ultrasound field synergistic method, and achieved good results. Luo Feng et al. extracted flavone from Glycyrrhiza uralensis by microwave ultrasonic synergistic extraction. Ma Lihua et al. studied the effects of traditional distillation method and microwave-ultrasonic synergistic extraction on the yield of carotenoid in Arctium lappa, and determined the optimal extraction conditions by orthogonal experiment. Bai Hongjin et al. extracted the aloe, with absolute ethyl alcohol distilled water and absolute ethyl alcohol -distilled water (1:1 ratio by volume) as solvent, by means of microwave-ultrasonic synergistic extraction, and the anti-oxidant activity of the extracts on rapeseed oil, lard, cottonseed oil and sunflower oil was determined by use of an edible oil oxidation stability tester.
The cell walls of natural plants are composed of cellulose, in which the active ingredients of the plant are often encapsulated in the cell walls. The enzyme extraction is a method which employs cellulase, pectinase, protease, etc. (mainly cellulase) to destroy the cell walls of plants to promote the maximum dissolution and separation of the active ingredients of the plant. In the extraction process of enzyme extraction, the selection of enzyme, concentration of enzyme, pH value, enzymatic hydrolysis temperature and enzymatic hydrolysis time will affect the yield of plant extracts.
E. BARZANA et al. extracted carotenoid from Tagetes erecta by enzyme extraction, and mainly studied the effects of solid-liquid ratio, enzyme concentration, enzymatic hydrolysis time and temperature on the yield. The study results showed that the optimum extraction process: set solid-liquid ratio 1:4, enzyme concentration 0.3%, extraction time 1.5 hours, and temperature 25°C. Zhang Xiaoqing et al. extracted the active ingredient - flavone from Ginkgo biloba by enzyme extraction, and found the optimum conditions affecting the yield, such as enzyme concentration, pH value, enzymatic hydrolysis temperature and time, by orthogonal experiment.