Innovative Technologies Figure 5: Impact of the molecular weight and the size of the free volume holes on the oxygen uptake by citrus oils encapsulated in maltodextrins of varying DE values. a. The oxygen uptake decreases with increasing DE value, i.e. decreasing average molecular weight of the maltodextrin matrices. b. The glass transition temperature of the maltodextrin matrices decreases with increasing DE value. c. The oxygen uptake by the matrices decreases with decreasing size of the free volume holes. The data in a. and c. is replotted from Refs. 10, 12 Mostly straightforward Challenging Difficult/not possible Pharmaceutical powders Tabletted products Liquid products Nutritional supplements Compressed products High-aw formulations Dairy powders Flour mixes Emulsions Beverage mixes Intermediate moisture foods RTD beverages Dehydrated nutritional products Petfood Ready meals Culinary mixes Cereal bars Frozen foods Table 2: Product areas for the application of glass encapsulation systems. 1/2016 eFOOD-Lab international 27 weight of the matrix, the higher is the degree of protection, which is afforded by the matrix 8. The oxygen uptake by the matrices turns out to be directly related to the hole size of the matrices (Fig. 5c). The downside of a low molecular weight encapsulation matrix is that the glass transition temperature will be low (Fig. 5.b). As discussed below, this leads to significant limitations in the use of encapsulation systems containing for instance high amounts of sucrose. Applications The formulation of encapsulation matrices is thus subject to a trade-off: on the one hand, one aims to maximize the content of low molecular weight compounds (specifically disaccharides), to decrease the molecular hole size and obtain a dense molecular packing, resulting in elevated barrier properties. On the other hand, however, a high content of such low molecular weight compounds results in a low glass transition temperature and thus a limited stability of the particles. Because of this trade-off, glass encapsulation systems have to be optimized for specific applications: for low-moisture products stored at moderate temperatures, one can tolerate a higher amount of, for instance, sucrose, in the matrix than for products, which are characterized by a higher water activity and which are stored at higher temperatures (Fig. 6). Glass encapsulation systems may be successfully used in very many food applications, provided that they are in low-moisture conditions (Table 2). While this may sound to be highly restrictive, this is fact not the case, as many processed foods are actually formulated in such a way that they water content is relatively low. This is principally done in order to increase the shelf life of these products. Naturally, in beverages, liquid foods and in foods, which are processed or stored under extreme temperature conditions, glass encapsulation systems also cannot be used or show only a limited performance. Glass encapsulation systems can be adapted for a better performance under higher moisture conditions. In glass encapsulation systems, one can for instance incorporate delivery vehicles for active ingredients, which remain functional even in liquid state. While these delivery vehicles do not provide high barrier properties against oxygen, they may for instance help to modulate the release of the active ingredient 13. In addition, one can modulate the properties of the glassy matrix, for instance by crosslinking or by phase separation 14. This will lead to glass encapsulation systems, which combine high barrier properties with an improved moisture tolerance. These adaptations are however highly specific and a detailed discussion would go beyond the scope of this article. Conclusions Glass encapsulation systems provide a very high level of protection of active ingredients in many food applications. Because of the very high barrier properties of glassy carbohydrates, they are specifically useful in protecting oxidation-sensitive active ingredients, such as citrus oils and polyunsaturated fatty acids, against oxidation in low-moisture applications. They are however more broadly used, as the conversion of generally liquid active ingredients in a powder provides additional advantages, such as more convenient handling, easier dosing, and improved blending and mixing characteristics.
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