Intensification of extraction of natural products using ultrasonic irradiations—A review of current status

Abstract

Extraction of active chemical compounds from natural products is one of the most important research areas for pharmaceutical and chemical industries. Traditional techniques used for the solvent extraction of natural products are associated with longer extraction times and lower yields, use of large amount of organic solvents and poor extraction efficiency. Ultrasound can be effectively used to improve the extraction rate by increasing the mass transfer rates and possible rupture of cell wall due to formation of microcavities leading to higher product yields with reduced processing time and solvent consumption. The present work presents an exhaustive overview of different aspects of ultrasound assisted extraction (UAE) of various natural products. Mechanism of UAE has been discussed and recommendations for optimum operating conditions have been reported for maximizing the yield. An overview of different applications of UAE has been also presented. Possible intensification of UAE by coupling with traditional solvent and supercritical extraction processes has also been discussed. Overall it appears that ultrasonic irradiations can be effectively used for intensification of the extraction of important constituents from natural products.

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.