Pretreatment of Macadamia Nut Shells with Ionic Liquids Facilitates Both Mechanical Cracking and Enzymatic Hydrolysis Wuan Xin Teh, Md. Mokarrom Hossain, Trang Quynh To, and Leigh Aldous* School of Chemistry, UNSW Australia, Sydney, New South Wales 2052, Australia * S Supporting Information ABSTRACT: The effect of ionic liquids upon the mechanical and (bio)chemical integrity of macadamia nut shells (from Macadamia integrifolia) has been investigated. Whole macad- amia nuts-in-shell are notoriously difficult to crack, and the Australian macadamia nut shells used in this study required 2240 ± 430 N of force to crack. Ionic liquids were screened for their solubility values, with 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) able to dissolve 5.5 ± 0.5 wt % macadamia nut shell. Treatment with small quantities of [Emim][OAc] resulted in weakened whole nut-in-shells that could be cracked with only ca. 46% of the displacement (0.67 ± 0.16 mm), ca. 34% of the force (760 ± 240 N) and ca. 15% of the energy (0.25 ± 0.10 J per shell) relative to no treatment. Further treatment by dissolution and precipitation of macadamia nut shell, followed by enzymatic hydrolysis with cellulase, resulted in the release of 80 ± 15% of the expected glucose content, relative to 1.3 ± 1.0% before any pretreatment. KEYWORDS: Macadamia, Nuts-in-shell, Facilitated biomass processing, Weakening biomass, Ionic liquids ■ INTRODUCTION Ionic liquids (ILs) are increasingly difficult to define, due to their increasing diversity in chemistry, structure and function. A general definition of ILs is that they are liquid below 100 °C, and at this point, it should be an ionic compound. A staggering number of IL structures are possible. 1 Of these, a relatively small number of ILs possess the ability to act as nonderivatizing (to a degree 2 ) near-universal solvents for a wide range of lignocellulosic biomasses, spanning from soft and hard woods to rice husks, bagasse, straws and grasses, as examples. 3 With weight percentage solubility values exceeding 50% for lignin 4 and cellulose 5 in ILs, and with complicated biomass samples dissolving relatively intact either at room temperature 6 or within minutes at elevated temperatures, 7 comparable solvents for biomass processing do not exist. The negligible volatility of most ILs implies theoretically zero volatile organic components (VOCs) pollution during application and ease of recyclability. When combined with tunable toxicity and functionality, 8 tentative “green” credentials are apparent in the potential replacement of other solvents with ILs. Nuts (encased in nut shells) require processing to isolate the edible components from the waste lignocellulosic components. Biorefinery processing of nonedible lignocellulosic biomasses typically aims to significantly disrupt the holocellulosic components (e.g., to facilitate enzymatic accessibility) and to remove and isolate the lignin. 9 ILs have been highlighted as significant pretreatment media that can facilitate grinding of wood, 10 the subsequent enzymatic hydrolysis of the wood, 11 the selective dissolution of individual fractions of wood 12 as well as the complete dissolution and subsequent fractionation of wood into cellulosic, 13 lignin 13 and hemicellulosic 14 fractions. As noted above, ILs have since been demonstrated to be near universal solvents for lignocellulosic biomass, although to the best of our knowledge they have not been investigated with respect to nut shells in general, and macadamia nut shells in particular. Macadamia is the generic term referring to a number of species of trees that are native to Australia. Macadamia integrifolia trees are now extensively cultivated for their edible, round seeds that are encased in a tough, smooth casing. These seeds (and tough seed coat) are popularly mistaken as nuts (and nut shell). 15 Due to the widespread adoption of this misconception, they will be referred to as nuts, nut-in-shell and nut shell throughout this paper. Macadamia cultivation has now expanded to much of Australia, as well as the United States of America (Hawaii and California), South Africa, Brazil and several other countries. From 2006 to 2011, Australia accounted for roughly 38% of the worldwide production of madacamia nuts (ca. 40 000 metric tonnes of shelled nuts per annum from Australia alone), with South Africa accounting for 23% and the USA for 16%. 16 Macadamia nut shells have been described as an “isotropic wood” 17 and possesses many remarkable physical properties. Compared to annealed aluminum, the shells possess half the Received: February 17, 2015 Revised: March 18, 2015 Research Article pubs.acs.org/journal/ascecg © XXXX American Chemical Society A DOI: 10.1021/acssuschemeng.5b00126 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX