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in that very little water is used. As a consequence, 26 1/2016 eFOOD-Lab international Innovative Technologies however, the temperature during processing is very high, and can be up to 120 °C. Together with the high shearing forces, this may lead to significant degradation of the bioactive compound. Another difference of extrusion over the other processes is that the extruded particles are large and dense (Table 1). This on the one hand provides for very good barrier properties, because of the long diffusion distances for oxygen from the external atmosphere. On the other hand, however, the large particles are very conspicuous in many food applications, which could be undesirable. The particle size may be reduced by grinding; this however gives rise to significant amounts of surface oil (Table 1). Principles of glass encapsulation Whereas the shapes and sizes of the particles as produced via the various glass encapsulation technologies are highly distinct (Table 1), the various technologies know a number of important similarities. All glass encapsulation systems consist of a continuous matrix, which in most cases consist of carbohydrates in the amorphous state. In the amorphous state the matrix molecules are randomly packed and do not show any systematic organization. This stands in contrast to the crystalline state, in which the molecules are all neatly positioned on a crystal lattice. The main advantage of the amorphous state is that in the rubbery above the glass transition temperature (Tg) it can be shaped in almost any form, and therefore can be used to embed small droplets or particles of bioactive compounds (Fig. 2). In the glassy state below Tg, the matrix then solidifies in a hard, brittle solid. During shelf life, the glass encapsulation system should thus be stored below Tg in order to ensure stability 2. In recent years, we have however discovered that it is not enough that the glass encapsulation systems are stored in the glassy state, but that for optimal performance the matrix also needs to be as densely packed as possible 8. This dense packing is to be achieved at the molecular level, and may be visualized by the minute "holes" occurring in the randomly packed, amorphous matrix. These so-called "free-volume holes" are schematically outlined in Fig. 3a. Even though at the level of the fundamental physics, the relation between the hole size and the diffusion of molecules such as oxygen, is not straightforward 7, 8, one may intuitively understand that the smaller the holes between the molecules making up the Figure 3: a. Depiction of free volume holes. b. Scanning electron micrograph of pores in a spraydried particle. Free volume holes are typically a factor 10,000 to 100,000 smaller than pores. Figure 4: Dependence of the volume of the free volume holes on the disaccharide content of a maltodextrin-based encapsulation matrix. The higher the disaccharide content, the lower the average molecular weight of the matrix and the smaller the size of the free volume holes. glassy matrix, the more difficult it is for other molecules to migrate through the matrix 8. It should be noted here that the free volume holes are distinct from the pores, which may occur in glass encapsulation systems: a free volume hole has a diameter of typically 0.2 to 0.4 nm, whereas pores as they occur in e.g. spray dried particles are typically between 10 and 100 μm in size (Fig. 3). The molecular hole size in glassy carbohydrates depends on a number of factors. Important for the use in encapsulation is that the hole size decreases with decreasing molecular weight of the encapsulation matrix 9- 11. This can be seen in Fig. 4, where the hole size in maltodextrin matrices is found to decrease with increasing disaccharide content. This explains why typical matrix formulations as used in glass encapsulation contain low molecular weight carbohydrates, generally disaccharides and polyols, as well as higher molecular weight carbohydrates, such as maltodextrins 2. The positive impact of decreasing molecular weight on the protection of encapsulated oxidation-sensitive compounds is observed in Fig. 5a: the higher the DE value of the maltodextrins used as encapsulation matrix, and consequently the lower the average molecular weight of the maltodextrins, the lower the oxygen uptake by the spray-dried particles. Therefore, the lower the molecular


eFOOD-Lab_International_01_2016
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