Chocolat

Pages: 13 (3080 mots) Publié le: 30 janvier 2011
Study of the rheological behaviour of chocolate
Mélanie Bécu, Joan Rius Fonoll, 1Sophie Liégeois, Benoît Haut, Frédéric Debaste Transfers, Interfaces and Processes (TIPs) –Chemical Engineering Unit, Université Libre de Bruxelles, Av. Franklin Roosevelt, 50 CP 165/67 B-1050 Brussels, email: mbecu@ulb.ac.be; 1 Chemical Engineering Department, Institut Meurice, Institut Meurice, Av. Emile Gryson, 1B-1070 Brussels, Belgium. Introduction In the food industry, the production process is often established in an empirical way, according to rules of good practice. These methods present gaps, in particular at the level of the production regularity. To model and optimize the process, it is highly useful to determine the physico-chemical properties of the product. In this work, chocolate is studied,aiming direct industrial application and a general enhancement of rheological mechanism understanding. Indeed, the chocolate is a suspension of aggregative solid particles in cocoa butter. Rheological behaviour of this fluid is therefore relying on different key phenomena. In this work the flow behaviour of chocolate is characterized and an attempt of a mathematical model is developed. Theapplied, industrial, goal is to model the tempering process. Tempering is an important step in the process to obtain a quality product. A temperature programme is applied to chocolate to lead to a good crystallization, which is the usual tempering process. A well tempered chocolate have a good snap, is shiny and can be well preserved. The tempering process is affected by many parameters such asviscosity, shear rate, particle concentration, and thermal history. For example it is possible to induce a good crystallisation by a high shear rate [1]. To improve this process, a better understanding of the rheological behaviour and the physical properties of liquid chocolate is required. Chocolate is a suspension of solids particles (cocoa, sugar and milk powder) in a continuous fat phase. It is aconcentrated suspension; solid content can reach 75%. A lot of works have already studied simple concentrated suspensions [2]. A common basic model is the Krieger and Dougherty one. Its equation shows that suspension’s relative viscosity depends only on the solids particles volume fraction and the maximum packing. However, viscosity measurements following the International Office of Cocoa, Chocolateand Sugar Confectionery (IOCCC) recommended method shows that chocolate has a slightly thixotropic behaviour [3]. A possible explanation of this behaviour is the presence of sugar particles in chocolate. They can stick with other sugar particles and also with other kind of particles. The agglomerates formed are progressively broken with higher shear rates. To confirm the rheological behaviour ofchocolate describe in the literature, a test following the IOCCC method on dark chocolate is realised. Then, to enhance the comprehension of this thixotropic behaviour, the effect of each ingredient (sugar, milk powder and cocoa) on chocolate’s viscosity is studied. Therefore viscosity measurements of various content of sugar, milk powder and cocoa solids particles added to cocoa butter are made.Flow curves are described by a power law and the parameters of this law are manually adjusted. A mathematical law describing the rheological behaviour of chocolate in function of composition, temperature and shear rate is thus developed.

Materials and methods The measurement equipment and products The viscometer used is a controlled rate SEARLE VT 550 (Thermo Fisher Scientific, Breda, TheNetherlands) with a temperature control unit. A DIN bob and cup geometry with 1,64mm gap is used. Dark chocolate and other ingredients are provided by Puratos – Belcolade (Erembodegem, Belgium). This chocolate has the following composition (mass fractions): 48,5% sugar (saccharose), 28,5% cocoa butter and 22,5% cocoa (solid). The milk powder is a roller dried one containing 26% fat. Cocoa is present...
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