Applications and opportunities for ultrasound assisted extraction in the food industry — A review

Abstract

Ultrasound assisted extraction (UAE) process enhancement for food and allied industries are reported in this review. This includes herbal, oil, protein and bioactives from plant and animal materials (e.g. polyphenolics, anthocyanins, aromatic compounds, polysaccharides and functional compounds) with increased yield of extracted components, increased rate of extraction, achieving reduction in extraction time and higher processing throughput. Ultrasound can enhance existing extraction processes and enable new commercial extraction opportunities and processes. New UAE processing approaches have been proposed, including, (a) the potential for modification of plant cell material to provide improved bioavailability of micro-nutrients while retaining the natural-like quality, (b) simultaneous extraction and encapsulation, (c) quenching of the radical sonochemistry especially in aqueous systems to avoid degradation of bioactives and (d) potential use of the radical sonochemistry to achieve targeted hydroxylation of polyphenolics and carotenoids to increase bioactivity.

Industrial relevance

The application of ultrasonic assisted extraction (UAE) in food processing technology is of interest for enhancing extraction of components from plant and animal materials. This review shows that UAE technology can potentially enhance extraction of components such as polyphenolics, anthocyanins, aromatic compounds, polysaccharides, oils and functional compounds when used as a pre-treatment step in a unit process. The higher yield obtained in these UAE processes are of major interest from an industrial point of view, since the technology is an “add on” step to the existing process with minimum alteration, application in aqueous extraction where organic solvents can be replaced with generally recognised as safe (GRAS) solvents, reduction in solvent usage, and shortening the extraction time. The use of ultrasonic for extraction purposes in high-cost raw materials is an economical alternative to traditional extraction processes, which is an industry demand for a sustainable development.

Introduction

The application of ultrasound as a laboratory based technique for assisting extraction from plant material is widely published. Several reviews have been published in the past to extract plant origin metabolites (Knorr, 2003), flavonoids from foods using a range of solvents (Zhang, Xu, & Shi, 2003) and bioactives from herbs Vinatoru (2001). A limited number of publications have included continuous ultrasonic process development and pilot-scale applications. The range of published extraction applications include herbal, oil, protein and bioactives from plant materials (e.g. flavones, polyphenolics), summarised in Table 1 and outlined in more detail in the following Applications section. Much of the work is empirical in nature and explanations of the mechanisms have been proposed. Some workers also discuss both the mechanisms involved in UAE and the likely issues for potential for scale up. The review by Vinatoru (2001) outlines a program of work where industrial scale up was attempted under an EU Copernicus grant (ERB-CIPA-CT94-0227-1995). They highlight that while it is relatively easy to achieve extraction on the laboratory bench it is very challenging to attempt extraction on an industrial scale. Several key issues and observations relating to UAE have been identified, as follows, (1) the nature of the tissue being extracted and the location of the components to be extracted with respect to tissue structures, (2) pre-treatment of the tissue prior to extraction, (3) the nature of the component being extracted, (4) the effects of ultrasonics primarily involve superficial tissue disruption, (5) increasing surface mass transfer (Balachandran et al., 2006, Jian-Bing et al., 2006), (6) intra-particle diffusion, (7) loading of the extraction chamber with substrate, (8) increased yield of extracted components and (9) increased rate of extraction, particularly early in the extraction cycle enabling major reduction in extraction time and higher processing throughput (Moulton and Wang, 1982, Caili et al., 2006).

Living tissues where the desired components are localized in surface glands can be stimulated to release the components by relatively mild ultrasonic stressing (Toma, Vinatoru, Paniwnyk, & Mason, 2001). In tissues where the desired components are located within cells, pre-ultrasound treatment by size reduction to maximise surface area is critical for achieving rapid and complete extraction (Riera et al., 2004, Balachandran et al., 2006, Vinatoru, 2001). Where pre-hydration is necessary to achieve extraction, ultrasound effectively accelerates the hydration process (Vinatoru, 2001). Ultrasound induced cavitation bubbles present hydrophobic surfaces within the extraction liquid (Grieser, personnel communication) thereby increasing the net hydrophobic character of the extraction medium. Thus it is possible to extract polar components into otherwise hydrophilic aqueous extraction media, reducing the need for generally undesirable hydrophobic or strongly polar extraction media. The disruption of tissue surface structures is revealed with microscopic examination by Vinatoru (2001), Chemat, Lagha, AitAmar, Bartels, and Chemat (2004), Haizhou, Pordesimo, and Weiss (2004), Balachandran et al. (2006). Several of the authors in the work cited below highlight concerns due to the potential for ultrasonic cavitation to propagate free radicals, in particular hydroxyl radicals. Where the potential oxidative damage is a concern radical production can be quenched by the addition of small amounts of ethanol to lower the temperatures within the cavitation bubbles and extinguish the chemistry involved (Sun et al. unpublished work in progress).

This paper provides a compilation of food-related UAE applications, highlighting the application approaches and performance. Following this, a more detailed discussion is given on UAE mechanisms, process development, equipment design and future opportunities.