Fabrice Ducept

Rheological properties of starch suspensions using a rotational rheometer fitted with a starch stirrer cell

The rheological properties of a starch suspension are usually studied through two viscosity measurements-pasting behavior and flow behavior of the resulting starch pastes-performed separately with two different tools and demanding rather high starch concentrations (6–10 wt %). This study focused on the feasibility of using a rheometer fitted with a starch stirrer cell to characterize, in a single experiment, the starch suspension’s behavior during and after pasting, all the while involving only low concentrations (2–4 wt %), more representative of a real-food context. A calibration of the starch stirrer cell in comparison to the coaxial cylinders one was done using model fluids (Newtonian and shear-thinning). A link between torque, rotational speed, and rheological properties was determined through two recalculated conversion factors (shear rate and shear stress). An operating diagram was then set indicating the laminar flow and good sensitivity domain for this cell. The accuracy of those constants to starch suspensions in the concentration range 2–4 wt % was demonstrated. The pasting behaviors of 2 wt % starch suspensions were followed successfully at two selected shear rates (13.5 and 135 s−1). The impact of the level of turbulence on the profiles obtained was stressed, a result that is not limited to low-concentration starch suspensions. Finally, the method developed was used to compare the pasting behaviors of 2 wt % native and modified waxy maize starch suspensions.

Dimensional Analysis: Principles and Methodology

This chapter examines the principles and methodology involved in dimensional analysis, which should be regarded in this context as a tool for establishing the dimensionless numbers linking the causes of the phenomena studied to its effects, using the homogeneity of dimensions.

Objectives and Value of Dimensional Analysis

The phenomena involved in matter transformation can be described by fundamental momentum, mass and energy transport equations, coupled with equations of chemical or biological kinetics and the constitutive and rheological equations. Boundary conditions of the flow domain and initial conditions are associated with this system of equations. Unfortunately, it is usually impossible to resolve this system of equations because: 1) the form of the differential equations is too complex to be integrated over the entire flow domain, especially given the complex geometry of the industrial equipment;

6 – Case Studies

This chapter provides a collection of examples taken from our research in order to illustrate the application of dimensional analysis discussed in the previous chapters. These examples examine the study of momentum, mass and/or heat transfer and their consequences on a target variable of the process. They involve liquids, solids and gases in different types of isothermal and non-isothermal reactors. The dimensional analyses presented: – are first “traditional” in their implementation (sections 6.1–6.3), with a series of examples on the rehydration of powders, the continuous foaming by whipping and the fouling of a plate heat exchanger; – then, in the case of powder mixing (section 6.4), involve an intermediate variable as defined in Chapter 3, with a view to reduce the configuration of the system without restricting the range of validity of the process relationship; – and finally, focus on processes involving fluids with variable physical properties (sections 6.5 and 6.6). These examples deal with the gas–liquid mass transfer in a mechanically stirred tank containing shear-thinning fluids and the ohmic heating of fluids; the variable property of the fluid is the apparent viscosity which depends on the shear rate and the electrical conductivity varying with temperature, respectively.

Dimensional Analysis: A Tool for Addressing Process Scale-up Issues

This chapter aims to illustrate the advantages of dimensional analysis to address issues of process scale-up or scale-down.

Practical Tools for Undertaking the Dimensional Analysis Process

This chapter addresses certain points raised in Chapter 2, where the principles and methods for carrying out a dimensional analysis were discussed. It aims to highlight the difficulties which users may encounter while undertaking the dimensional analysis process themselves. This involves: – facilitating the process of establishing the list of relevant physical quantities which influence the target variable and helping to evaluate the consequences of an omission/error in the compilation of this list (section 3.),

Dimensional Analysis of Processes Influenced by the Variability of Physical Properties

Food processes involve matter (fluid, emulsion, suspension, etc.), which is placed in equipment in order to be transformed. In numerous cases, one (or several) physical properties of the matters change between the inlet and the outlet of the equipment. There is also very often a spatial and/or temporal distribution of these physical properties within the equipment insofar as they are dependent on various potential fields (composition, temperature, etc.). This dependence has a significant effect on the process and must be taken into account. For instance: – the variation of viscosity with temperature which can be observed within an exchanger during thermal processing. This variation in viscosity with temperature modifies the heat transfer performance of the process when compared to a fluid whose thermo-dependence is negligible;