Microstructure droplet-matrix

A second class of models directly relates flow to blend structure without the assumption of an ellipsoidal droplet shape. This description was initiated by Doi and Ohta for an equiviscous blend with equal compositions of both components [34], Coupling this method with a constraint of constant volume of the inclusions, leads again to equations for microstructural dynamics in blends with a droplet-matrix morphology [35], An alternative way to develop these microstructural theories is the use of nonequilibrium thermodynamics. This way, Grmela et al. showed that the phenomenological Maffettone-Minale model can be retrieved for a specific choice of the free energy [36], An in-depth review of the different available models for droplet dynamics can be found in the work of Minale [20]. 

Wood Hill (1991b) induced phase-separation in the clear glasses by heating them at temperatures above their transition temperatures. They found evidence for amorphous phase-separation (APS) prior to the formation of crystallites. Below the first exotherm, APS appeared to take place by spinodal decomposition so that the glass had an intercoimected structure (Cahn, 1961). At higher temperatures the microstructure consisted of distinct droplets in a matrix phase. 

Figure 5.14 The microstructure of the set cement is clearly revealed by Nomarski reflectance optical microscopy. Glass particles are distinguished from the matrix by the presence of etched circular areas at the site of the phase-separated droplets (Barry, Clinton Wilson, 1979).

A typical characteristic of many food products is that these are multi-phase products. The arrangement of the different phases leads to a microstructure that determines the properties of the product. Mayonnaise, for example, is an emulsion of about 80% oil in water, stabilized by egg yolk protein. The size of the oil droplets determines the rheology of the mayonnaise, and hence, the mouthfeel and the consumer liking. Ice cream is a product that consists of four phases. Figure 1 shows this structure schematically. Air bubbles are dispersed in a water matrix containing sugar molecules and ice crystals. The air bubbles are stabilized by partial coalesced fat droplets. The mouthfeel of ice cream is determined by a combination of the air bubble size, the fat droplet size and the ice crystal size. 

In some foods, the crystalline microstructure forms networks between individual crystals. These network structures may also interact with other elements of the food matrix, including emulsion droplets and air cells. The nature and strength of these... 

The ingredients and processing create the microstructure, which is shown schematically in Figure 1.2. It consists of ice crystals, air bubbles and fat droplets in the size range 1 pm to 0.1 mm and a viscous solution of sugars, polysaccharides and milk proteins, known as the matrix. The texture we perceive when we eat ice cream is the sensory manifestation of the microstructure. Thus, microstructure is at the heart of the science of ice cream, and forms the central theme running through this book. 

A typical ice cream consists of about 30% ice, 50% air, 5% fat and 15% matrix (sugar solution) by volume. It therefore contains all three states of matter solid ice and fat, liquid sugar solution and gas. The solid and gas are small particles - ice crystals, fat droplets and air bubbles -in a continuous phase, the matrix. To understand the creation of the microstructure during the manufacturing process we must first introduce some concepts from the physical chemistry of colloids, freezing and rheology (the study of the deformation and flow of materials). 

Figure 1.1 showed the links between the formulation, the process and the texture. The first step is to understand how the formulation and process affect the microstructure. This requires microscopy techniques to visualize the ice crystals, air bubbles, fat droplets and matrix and image analysis to quantify their sizes, shapes and locations. The next step is to measure the mechanical, rheological and thermal properties and to relate them to the microstructure. The final stage is to relate these physical measurements to the sensory properties. This chapter describes the techniques used to make these measurements. 

The microstructure of ice crystals, air bubbles, fat droplets and matrix is central to the physical and hence sensory properties of ice cream. Considering the components separately is sufficient for some properties, but for many others it is impossible to treat one component in isolation from the rest. Many mechanical, thermal and rheological properties depend on the whole microstructure this necessitates a materials science approach. Furthermore, the texture experienced during consumption depends on the manner in which the product is eaten, and the way in which the microstructure breaks down. Only when all these factors are combined is it possible for the ice cream scientist to link the ingredients and the process through the microstructure to the texture. However, the current understanding of these links is far from complete and this remains an active area of research. This chapter is only a short overview of a very complex subject, and interested readers are referred to the Further Reading for more detailed treatments. 

In order to identify the influence of microstructural parameters on the nonlinear behavior of dilute emulsions, specifically on the intensities of the higher harmonics and their strain amplitude dependence, we have employed modelling approaches [36, 37], For dilute emulsions the individual droplets can be regarded as independent of each other, therefore, the measured shear stress can be obtained from a linear superposition of the matrix contribution and the contributions originating from each droplet and their interface [5, 16], as predicted by Batchelor [2] ... 

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