Techniques to extract bioactive compounds from food by-products of plant origin Hilde Wijngaard, Mohammad B. Hossain ⁎, Dilip K. Rai, Nigel Brunton Teagasc Food Research Centre Ashtown, Ashtown, Dublin 15, Ireland abstract article info Article history: Received 17 April 2011 Accepted 27 September 2011 Keywords: By-products Carotenoids Polyphenols Pomace Pressurised liquid extraction Supercritical CO 2 extraction Pulsed electric fields By-products of plant origin represent an abundant source of bioactive compounds. However, to exploit these resources commercially relevant strategies for their extraction must be developed. This review focuses on the extraction of bioactive compounds from food by-products of plant origin by a number of novel methods, in- cluding pressurised liquid extraction and supercritical CO 2 extraction. In general supercritical CO 2 extraction is most effective for apolar compounds such as carotenoids, while pressurised liquid extraction can be used to extract more polar compounds such as polyphenols. Both techniques are sustainable and green techniques. In addition, pre-treatment of plant by-products by novel non-thermal processing techniques in order to en- hance extraction will be highlighted. In general the selection of an appropriate extraction strategy is depen- dent on the type of compound to be extracted as well as the potential up scaling of the technique. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Processing of foods of plant origin generates vast quantities of by- products. Disposal of these by-products represents both a cost to the food processor and a potential negative impact on the environment. Re- search over the past 20 years has revealed that many of these by- products could serve as a source of potentially valuable bio-active com- pounds. Despite this the vast majority of by-products are currently not exploited as sources of these compounds. This is in part due to the lack of appropriate techniques for extraction of these compounds. In recent times a number of novel extraction techniques have been used to opti- mise extraction of bioactive compounds from by-products. In fact con- siderable advances in this area have been made recently and while some reviews exist (Herrero, Cifuentes, & Ibañez, 2006; Schieber, Stintzing, & Carle, 2001) no update has been made of the research carried out in the last 5 years despite the considerable advances in extraction techniques. Pressurised liquid extraction (PLE) and supercritical CO 2 ex- traction (SC-CO 2 ) are extraction techniques that are gaining popularity due to their ability to increase target molecule specificity and reduce waste solvent production. Therefore the primary focus of the present re- view will be critically evaluating the use of PLE and SC-CO 2 on the recov- ery of carotenoids and polyphenols in food by-products of plant origin. Both PLE and SC-CO 2 usually employ heat in combination with other parameters to enhance extractions of target molecules. A number of other non-thermal approaches are now available such as pulsed electric field (PEF) extraction and ultrasound, which may be particularly effec- tive for thermally-labile compounds. Therefore a secondary focus will be on the use of non-thermal techniques to enhance extraction of these compounds. The use of microwave assisted extraction (MAE) to enhance the extraction of bio-active compounds of plant origin is also reviewed and critically evaluated. For all extraction techniques a princi- ple objective of the present review is to put forward a consensus on op- timal conditions required to gain maximum recovery of target compounds such that reader can benefit from research already carried out. 2. Pressurised solvents Traditionally, the nature and amounts of target compounds in nat- ural products are determined after exhaustive extraction of the sam- ple using solid–liquid extraction techniques. Pressurised solvents use elevated pressures and sometimes temperatures which drastically improve the speed of the extraction process. In ideal solid–liquid ex- tractions, the desired compound should have high solubility in the solvent employed while other compounds from the solid matrix should not be solubilised during extraction (Pronyk & Mazza, 2009). In reality however, this is rarely achieved, and therefore much re- search has been carried out on optimising conditions, such as solvent-to-feed ratio, particle size, modifier concentration extraction temperature, pressure and time and flow rate, to enhance the recov- ery of bioactive compounds from food by-products of plant origin (Kaur, Wani, Oberoi, & Sogi, 2008; Ku & Mun, 2008; Spigno, Tramelli, & De Faveri, 2007; Wijngaard & Brunton, 2010). However, Food Research International 46 (2012) 505–513 Abbreviations: GAE, Gallic Acid Equivalent; MAE, Microwave Assisted Extraction; PEF, Pulsed Electric Field; PLE, Pressurised Liquid Extraction; RSM, Response Surface Methodology; SC-CO 2 , Supercritical CO 2 Extraction; SLE, Solid Liquid Extraction; SWE, Subcritical Water Extraction; UAE, Ultrasound Assisted Extraction. ⁎ Corresponding author. Tel.: + 353 1 805 9500; fax: + 353 1 805 9550. E-mail address: mohammad.hossain@teagasc.ie (M.B. Hossain). 0963-9969/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2011.09.027 Contents lists available at SciVerse ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres