Optimal design of single-screw extruder for liquorice candy production: a rheology based approach Domenico Gabriele, Stefano Curcio, Bruno de Cindio * Department of Chemical Engineering and Materials, University of Calabria I-87030, Arcavacata di Rende (CS), Italy Received 21 February 2000; accepted 14 August 2000 Abstract The aim of this work was to investigate the rheological behaviour of liquorice extract to design properly and to optimise a single- screw extruder dedicated to liquorice production. A stress±shear rate law was determined and a simpli®ed analysis of the extruder performance was carried out. The velocity pro®les in the extruder channel were determined by using a numerical method, while few geometrical parameters, like the channel depth and the helix angle, were optimised to obtain the maximum ¯ow rate as a function of dierent operating conditions. A power law relationship between either the ¯ow rate or the power consumption and the screw speed was established. The deviation from linearity was expressed by an exponent which, in all the cases examined, was found to be almost constant and ranging from 1.22 to 1.30. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Liquorice; Extruder; Design; Non-Newtonian ¯uid 1. Introduction Liquorice extracts (in pharmacy called Succus liquir- itiae) are largely used in dierent areas of food industry. So far, all the process operations have been designed rather empirically and without accounting for the complex rheological behaviour of the treated substanc- es. This leads, quite often, to several practical problems, as for instance the blocking of the extruder head, or the diculty to pump the extract. To carry out a better process design capable to overcome these problems, a rheological characterisation of liquorice extracts has been performed and the results were applied to the de- sign of a liquorice plant with the aim of improving the traditional process. A typical traditional process for li- quorice production, essentially based on four dierent unit operations, is sketched in Fig. 1. Raw materials, i.e., the roots of two plants, called Glycyrrhiza Glabra and Rigulizia Cordara, are ground in a hammer mill. About ®ve parts of water for any part of liquorice are added to coarse root powder to obtain an aqueous mixture that is then fed to the steam extraction plant to extract the most valuable compounds, i.e., Glycyrrhizin and sugars, proteins and mineral salts. By operating the extraction plant at about 110°C it is possible to obtain a solution containing 40 g l 1 of liquorice. The solid residue (li- quorice-husks) is pressed and may be used as a biolog- ical fertiliser or as animal feed ingredient. Prior to concentration the liquorice solution is centrifuged to remove all suspended solid particles. Then, the super- natant is primarily concentrated in multiple eect evaporators and ®nally in heated stirred tanks. After primary concentration, carried out in two stages at 110°C and at 60°C, respectively, the liquorice concen- tration is about 55°Bx. In these conditions, thermal damage may occur as a consequence of partial dena- turation of the more thermal sensitive compounds. For instance, the change of liquorice colour from yellow, typical of the root, to dark brown of the ®nal solution, is due to the oxidation of some compounds contained in the raw material. In the secondary concentration plant, working at a temperature of about 95°C, a higher con- centration is achieved. The liquorice solution ¯owing out of this plant is the so-called liquorice extract with a water content of 12±15%. The average composition is reported in Table 1. The liquorice extract is then cast, thus yielding the so-called liquorice loaves. These may represent either the ®nal product of a liquorice ®rm or the feed material for the last unit operation of the pro- cess, viz., the extrusion. Owing to the extract complex rheological behaviour, the extrusion is probably the Journal of Food Engineering 48 (2001) 33±44 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +39-098-4492080 ext. 2035; fax: +39- 098-4492058. E-mail addresses: rheo.lab@unical.it (D. Gabriele), stefano.curcio@ unical.it (S. Curcio), bruno.decindio@unical.it (B. de Cindio). 0260-8774/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 1 4 0 - 0