Original article Comparison of rheological models for determining dark chocolate viscosity Emmanuel Ohene Afoakwa, 1,2 * Alistair Paterson, 1 Mark Fowler 2 & Joselio Vieira 2 1 Centre for Food Quality, SIPBS, University of Strathclyde, 204 George Street, Royal College Building, Glasgow G1 1XW, UK 2 Nestle Product Technology Centre York, P. O. Box 204, Haxby Road, York YO91 1XY, UK (Received 7 August 2007; Accepted in revised form 26 November 2007) Summary Parameters in chocolate rheology, namely shear viscosity and yield stress, are important in manufacture and directly influenced by product particle size distribution (PSD) and composition. The Casson model was the standard confectionery industry strategy to quantify rheological properties of molten chocolate until in 2000, the International Confectionery Association recommended the use of interpolation data to describe viscosity. The two strategies are compared and correlated in defining rheological properties of molten dark chocolates prepared using different PSD, fat and lecithin content. Rheological parameters were determined using a shear rate-controlled rheometer and data examined using correlation, regression and principal component analyses to establish their inter-relationships. Correlation and regression analyses showed high correlation (r = 0.89–1.00) and regression coefficients (R 2 = 0.84–1.00). The newer International Confectionery Association technique gave higher correlation and regression coefficients than the Casson model, but multivariate principal component analysis showed that the two models were highly related and either could effectively quantify dark chocolate viscosity parameters. Keywords Chocolate composition, particle size distribution, shear viscosity, yield stress. Introduction The rheological properties of molten chocolate are important for confectionery quality assurance and accurate weight measurements. Molten chocolate be- haves as a non-Newtonian liquid, exhibiting non-ideal plastic behaviour with a yield stress, related to amount of energy required to initiate fluid flow, and plastic viscosity, energy required to keep fluid in motion (Beckett, 1999; Ziegler & Hogg, 1999; Afoakwa et al., 2007a). Plastic viscosity relates to pumping characteris- tics, filling of rough surfaces and coating properties, yield stress to shape retention, pattern holding, inclined surface coating and bubbles in processing (Seguine, 1988). Afoakwa et al. (2007b) concluded that rheolog- ical properties are important in chocolate manufactur- ing for quality-control purposes and can be related to composition, processing strategy and solid particle size distribution. Apparent viscosity in oral aqueous solu- tions influences flavour attribute perception (Ziegler et al., 2001); thus, rheological parameters often give information related to sensory character in chocolate. Important models that have been used to characterise chocolate rheology include the Herschel-Bulkley, Cas- son and Bingham forms (Chevalley, 1999; Beckett, 2000; Sokmen & Gunes, 2006), following the equations: Herschel-Bulkley: s ¼ s 0 þ g pl ð _ cÞ n ð1Þ Casson: p s ¼ p s CA þ p g CA  p _ c ð2Þ Bingham: s B ¼ s 0 þ g B  _ c ð3Þ [s, shear stress; s 0 , yield stress; g pl , plastic (shear) viscosity; s CA , Casson yield value; g CA , Casson plas- tic viscosity; s B , Bingham yield value; g B , Bingham plastic viscosity; _ c, shear rate; g, viscosity of the suspension; n, flow viscosity index]. Since 1973, the International Confectionery Associa- tion (ICA) has accepted rheological measurement of molten chocolate using rotational viscometers with concentric cylinders (bob and cup geometry) and Casson equation calculation of the parameters (IOCCC (International Office of Cocoa, Chocolate and *Correspondent: E-mail: e.afoakwa@strath.ac.uk (Presented at the First International Chester Food Science and Technology Conference, Chester, UK, April 2007) International Journal of Food Science and Technology 2009, 44, 162–167 162 doi:10.1111/j.1365-2621.2008.01710.x Ó 2008 Institute of Food Science and Technology