ReviewIntensification of extraction of natural products using ultrasonic irradiations—A review of current status
Highlights
► The present work describes an exhaustive overview of ultrasound assisted extraction (UAE) of various natural products. ► Mechanism of UAE and recommendations for optimum operating conditions of UAE has been reported for maximizing the yield. ► Possible intensification of UAE by coupling with solvent and supercritical extraction processes has also been discussed. ► Effective application of UAE for intensification of the extraction of important constituents from natural products are discussed.
Introduction
Solid–liquid extraction using solvent is used in many chemical, biotechnological and pharmaceutical industrial processes for recovery of valuable ingredients. The pharmaceutical industry, perfume manufacturing or pesticide industries deal with the recovery of active components from plants/vegetable seeds. Vegetable materials contain only a small amount of active solute, but in most cases high value of active compounds justifies the development of high-performance separation processes [1]. The need for effective extraction of biologically active components from plants without any loss of activity and high purity has resulted in development of newer processes of extraction. Conventionally different mechanical and chemical processes such as solvent extraction [2], steam distillation [3], high hydrostatic pressure extraction [4], pulse electric field process [5], high pressure process [6], etc., are used for the extraction of products from plant materials. The selection of the method to extract active components with maximum yield and highest purity mainly depends on the nature of compounds and thermal stability and nature of raw material to be processed. Conventional extraction techniques are often limited by the mass transfer resistances due to involvement of more than one phase in the system [7]. These methods are time consuming and may require fairly long time depending on the diffusion rates of solvents through the tortuous paths of the materials. High energy consumption is also one of notable disadvantage of conventional extraction processes [8]. In the case of heat sensitive active compounds extraction carried out at higher temperatures, may damage the quality of the extract. In certain cases, the active molecules may be destroyed due to use of severe pressure and temperature conditions [9], [10], [11], [12]. Generally speaking mechanical processes presents poor efficiencies and low yields, while chemical extraction methods involve use of large quantity of organic solvents which are harmful to human and environment. Solvents which are used in conventional methods exhibit lower efficiency due to decreased molecular affinity between solvent and solute [13]. Government, health and environmental regulatory requirements on the use of organic solvents have encouraged active research on clean extraction techniques [14], [15], [16], [17]. The confines of the conventional techniques make obstinate to search for the new techniques which are equally competent and at the same time economically viable. Recently, the application of ultrasound techniques in solvent extraction has attracted more attention. Use of ultrasound results in ‘cavitation’ commonly described as the formation, growth, and collapse of gas–vapour filled bubbles in a liquid. Sonochemistry, specifically involves cavitation induced by the pressure fluctuations generated by ultrasound waves in a liquid medium [18]. Cavitation generated using ultrasound is known to produce physical effects such as liquid circulation currents and turbulence which can lead to a significant increase in the mass transfer rates. Ultrasound can be effectively used to increase the yield and rate of mass transfer in several solid–liquid extraction processes. The mechanism of intensification of extraction process has been attributed to cavitation phenomena resulting in intense turbulence and liquid circulation currents in the system [7], [19], [20], [21]. The earlier work on ultrasound assisted extraction focused either in terms of the experimental work on a specific system or the effects of use of ultrasound in analytical techniques. The current work aims at providing an in depth analysis of different aspects related to intensification using ultrasound assisted extraction process and also provides an overview of different applications related to food, pharmaceuticals, perfumes, cosmetics and other industries. One of the novel contents of the present work is that the combinatorial extraction schemes based on the use of ultrasound have been highlighted which can further intensify the process and result in economical operations even at commercial scale operations. Also the need for research work required for development of large scale efficient reactor have been highlighted based on a detailed analysis of the existing literature related to the ultrasound assisted extraction which has not been highlighted in any of the earlier work.
Section snippets
Ultrasound assisted extraction (UAE)
Among the various modes of generating cavitation, acoustic and hydrodynamic cavitation are frequently used due to the ease of operation and the generation of the required intensities of cavitational conditions suitable for different physical and chemical transformations. In the case of extraction of natural products; however, only ultrasound based cavitation has been reported successfully; for example extraction of vanillin, almond oils, herbal extracts (fennel, hops, marigold, mint), ginseng
Overview of different applications
The different substances that have been extracted using ultrasound include essential oils, aromatic compounds, citrus compounds, sugars, proteins, acids, natural dyes, pigments, etc. In addition to the beneficial effect of ultrasound on the yield and kinetics of extraction, it has been observed that extraction can be carried out at much lower temperature and pressure, which can result in substantial decrease in the overall cost of the operation. It is however important that the sonication time
Guidelines for optimum operating conditions for ultrasound assisted extraction
After discussing the variety of applications of ultrasound assisted extraction, which has clearly established the effectiveness and excellent benefits of UAE, it is worthwhile to make some recommendations for the optimum operating parameters so that the overall operation becomes economically viable. The operating parameters that need to be properly selected include the sonochemical reactor configuration, operating frequency, intensity of irradiation, sonication time in addition to the common
Possible combined approaches for intensification of extraction operation
Environmentally friendlier preparations of chemical compounds and organic or inorganic materials are generally accompanied by the concept of saving resources by optimizing reaction conditions and/or introducing new process technologies [76]. With this objective, combining two or more techniques, which can give synergistic effects based on the similarity in the controlling mechanisms or supplementary roles, can be a viable option with possible commercial applications. Ultrasound-assisted
Path forward and research needs
A detailed overview of the work related to ultrasound assisted extraction of natural products have indicated that significant intensification in terms of reduction in the treatment time, enhanced extraction yields and possibility of operation at lower temperatures, can be obtained with the use of ultrasound either alone or in combination with other technologies. However, it has been observed that majority of the studies have been based on the use of simple designs of sonochemical reactors such
Conclusions
The discussion presented in the current work has clearly established the superiority of ultrasound-assisted extraction compared to the conventional extraction processes. Major achievement of ultrasound-assisted extraction is the reduction of processing time for the same yield in addition to other benefits such as operation at lower temperature, requirement of lower amounts of solvents and sometimes higher recoveries with enhanced purity of the final recovered products. UAE technology can
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Current address: Sinhgad College of Engineering, Pune, India.