Title: "MICROSTRUCTURAL MODELLING OF ICE CREAM RHEOLOGY"
Author: AITOR LUQUE GÓMEZ
Date: 2008-04-02
Directors: Dr. Javier Aldazábal Mensa and Dr. José Manuel Martínez Esnaola
Abstract
When we eat ice cream, we feel its sensory properties. This means that our lips, teeth and tongue can perceive the characteristics of that ice cream. Not only those referred to its temperature, but also to its texture. The quality of the texture is the result of the combination of three elements: the ingredients of ice cream (water, sugars and fats, basically), the production method and the storage conditions (time and temperature).
In this thesis, we study the elements that determine the quality of ice cream, namely: the state of the microstructure, its mechanical behaviour and its rheology. This study has been carried out with both an experimental approach and a numerical approach. For the latter, we have developed and/or used models of microstructural evolution, of fracture and rheological behaviour, correspondingly.
First, we have studied the evolution of the microstructures of water and sucrose, as a function of temperature and time. The analysis has been done in terms of Ostwald ripening and sintering. For the former, we calculate the mean ice particle size in the microstructure, at different times. For the latter, the necks appearing between some ice particles are characterized accounting for their sizes in relation to the size of the particles that form it. We have developed two models of microstructural evolution, based on the Monte Carlo technique. The difference lies on whether the sucrose is taken into account in the model or not. The simulations performed with these models allow reproducing the experimental observations.
Later, we have analysed the mechanical behaviour of ice and some sucrose aqueous solutions. For that, the tensile tests of menisci of the corresponding formulations have been carried out. The sample geometry has been chosen due to its similarity with the geometry of the necks appearing between ice particles. These tests permit calculating relevant mechanical properties, such as ice fracture stress. This property in particular is incorporated into finite element simulations. These simulations include some special elements, the so-called cohesive elements, which allow reproducing the fracture process observed experimentally.
Finally, we have studied the rheological behaviour of ice cream. Performing some stepped and oscillatory rheological tests, we have been able to relate the state of the ice network to the hold time at a certain temperature. Special attention has been paid to the value of the apparent yield shear stress of the ice structure. Precisely, this parameter is the one that we can obtain with the rheological model that we have developed. This model is based on the beam theory, enriched with some special elements, namely, the connectors, which are used to establish the fracture criterion associated to the neck appearing between two particles.
