Study on Ultrasonic Technology in Extraction of Plant Active Ingredients

Driven by the industrial goal of achieving green extraction, new extraction technologies such as microwave extraction, supercritical fluid extraction, and ultrasonic extraction have emerged. These new technologies have greatly promoted the commercial development of cash crops. In recent years, ultrasonic extraction technology has developed rapidly in the field of food industry. This technology can not only improve the quality of products, but also reduce production costs and improve production efficiency and safety. Ultrasonic waves can effectively prevent heat-sensitive substances from being destroyed in high temperature environments in the process of increasing mass transfer rates. It continuously stimulates intracellular glands through cavitation effects, mechanical effects, etc., thereby promoting the rapid release of active ingredients. This technology can reduce the use of organic solvents, improve the purity of bioactive compounds, and save processing time and operating costs. Therefore, ultrasonic-assisted extraction technology meets human needs in green production, sustainable development, and environmental protection. The application of ultrasound in the extraction of plant active ingredients has attracted increasing attention due to its unique advantages. Researchers are also constantly trying to combine ultrasonic technology with various other extraction equipment in order to achieve better extraction performance. In addition to the more common ultrasonic-solvent extraction method, there are also ultrasonic-Soxhlet extraction, ultrasonic-homogeneous extraction, ultrasonic-water distillation extraction, ultrasonic-microwave extraction, and ultrasonic-supercritical carbon dioxide extraction.

1 Principle of ultrasonic extraction

Ultrasound is defined as a sound wave with a frequency greater than 20 khz, which exceeds the limit of human hearing detection. Ultrasound is a mechanical wave with a high energy density. Its sound energy output source is usually a vibrating object, which can cause the surrounding medium to vibrate and then transfer energy to other adjacent particles. When ultrasound passes through the medium, it causes the longitudinal displacement of the particles. These dense molecular effects cause cell wall damage and accelerate the mass transfer rate of effective substances in the medium, thereby achieving the purpose of improving the extraction rate. Ultrasonic extraction does not rely on a single mechanism of action, but works continuously or simultaneously through multiple physical mechanisms such as mechanical fragmentation, thermal effect, and cavitation effect. In the homogenate of solid-liquid mixture, the micro-beams and micro-turbulence generated by acoustic cavitation in the liquid medium will cause strong mechanical disturbances, thereby intensifying the collision between particles, which can easily lead to the decomposition and local rupture of some brittle materials. On the other hand, due to the reduction in particle size, the mass transfer rate of solid particles and the contact area between solid and liquid phases are increased, which are conducive to accelerating the dissolution of the contents in the sample matrix.

The cavitation effect is a unique and complex physical phenomenon caused by the propagation and vibration of ultrasound in liquid. It generally refers to the process of formation, expansion and rupture of cavitation bubbles in liquid. Simply put, when high-intensity ultrasound is applied, the attraction between medium molecules may exceed the critical value, thereby generating high shear stress in the liquid and subsequently forming cavitation bubbles. 1 Cavitation bubble is formed near the surface of the matrix. After undergoing continuous compression-rarefaction cycles, the cavitation bubble will rupture during the compression cycle and generate short-term heat energy, thereby forming high-speed microjet fluid on the surface of the matrix and generating strong shock waves. This process can make the surrounding local temperature as high as about 5,000 K, and the instantaneous pressure can reach 50~1000 atm. The high-pressure and high-temperature environment formed will destroy the cell wall of the plant matrix, thereby releasing intracellular substances into the solution. From the scanning electron microscope images of basil leaves taken by CHEMAT and others before and after oil extraction, it can be observed more vividly: before extraction, the glands on the leaves are smooth and full; after extraction, they begin to shrink, but the gland structure remains intact; and after ultrasonic-assisted extraction, the glands are completely broken and all their contents are released.

2 Application of ultrasonic combined technology in the extraction of plant active substances

Ultrasonic combined solvent extraction method

Ultrasonic solvent extraction method generally uses organic solvents as energy transmission media, that is, solvents of different polarity are selected according to the properties of the target compound to be extracted, the solvent is fully mixed with the sample matrix, and then ultrasonic intervention is applied. This method does not require the participation of other equipment. The solid-liquid mixture is directly placed in an ultrasonic device for extraction. The ultrasonic wave transfers energy evenly to the sample matrix through the liquid medium, thereby achieving the purpose of improving the extraction rate. This is the most traditional, simple and economical method in ultrasonic extraction.

There are two types of ultrasonic extraction equipment, namely ultrasonic water bath and probe ultrasonic equipment. Both systems are based on transducers as ultrasonic sources. Ultrasonic water baths usually operate at a frequency of about 40 kHz and are equipped with temperature control devices. The equipment is relatively cheap and can process a large number of samples at the same time. However, the water contained in the ultrasonic bath and the glassware used will greatly weaken the transmitted ultrasonic energy. Probe-type ultrasonic systems are usually the first choice for extraction applications. Since the ultrasonic energy is transmitted through a small surface (the probe tip immersed under the liquid surface), the generated ultrasonic energy is directly transmitted in the extraction medium, so the ultrasonic energy loss is small. The intensity of the ultrasonic waves transmitted by the probe system to the liquid medium will cause the temperature of the solid-liquid mixture to rise rapidly, so it is necessary to use double-layer shell condensed glassware for extraction.

Many domestic scholars have used this simple and economical method to extract various active substances from plant samples and achieved good results. Liu Yanyan used ultrasound-assisted extraction of Huoshan Dendrobium polysaccharides. Under the optimal extraction parameters, the polysaccharide yield can reach 19. 96 mg/g; Niu Sikun used ultrasound-assisted extraction of golden ear coumarins, and the extraction rate of total coumarins in golden ear mycelium reached a maximum of 0. 85%. Compared with solvent extraction, ultrasonic solvent extraction can significantly improve the extraction efficiency of the target product, reduce the consumption of organic solvents and is not easy to destroy the activity of the extract. Nevertheless, this method still consumes a certain amount of organic solvents, which will cause a certain degree of environmental pollution, and the organic residues in the obtained extract will greatly affect the product quality. Therefore, in recent years, some scholars have begun to try to use low eutectic solvents to replace conventional extraction solvents, combined with ultrasonic-assisted methods to extract various active ingredients from plants, and have achieved good results. Low eutectic solvents are a new type of environmentally friendly ionic liquid, a eutectic mixture formed by the combination of hydrogen bond acceptors and hydrogen bond donors, with a lower melting point than a single component. Low eutectic solvents are non-toxic, inexpensive, simple to prepare, and have good biodegradability. They are ideal solvents for extracting plant active ingredients. Wang et al. used ultrasound-assisted low eutectic solvent to extract echinacea glycosides and oleuropein from small leaf cloves. When choline chloride/glycerol (1:2, molar ratio) was used as the low eutectic solvent, the solid-liquid ratio was 20 g/mL, the temperature was 68 ℃, the water content was 20%, and the ultrasonic wave was 200 W for 45 min, the extraction rates of echinacea glycosides and oleuropein were 80.04% and 86.21%, respectively, which were significantly better than the extraction results of traditional organic reagents. Ni Yujiao et al. used ultrasound-assisted low eutectic solvent to obtain phenolic substances from sea buckthorn seed meal. The results showed that under the same extraction time, the polyphenol yield of this method was 1.6 times that of the polyphenol yield of hot reflux extraction.