Innovative Technologies 1/2016 eFOOD-Lab international 25 tive compounds in carbohydrate glasses. This is followed by a brief discussion of the principles underlying the protection of sensitive active ingredients by carbohydrate glasses. Based on the understanding of these principles, I will then discuss how carbohydrate-based encapsulation systems can be used in foods, taking into the account the limitations but also the opportunities offered by these versatile systems. Technologies for the encapsulation in carbohydrate glasses Over the decades, a broad range of technologies has been developed for the encapsulation of active ingredients in carbohydrate glasses, giving rise to particles displaying a large variation in shape, size and various functional properties (Table 1). One of the most commonly used technologies is the encapsulation of sensitive active ingredients by spray drying (Fig. 1a). In the flavor industry, spray drying is one of the first established and still most widely used technologies for the encapsulation of flavor compositions and essential oils 3. In addition, spray drying is widely used for the encapsulation of nutrients such as vitamins, carotenoids and polyunsaturated fatty acids (PUFAs) 6. The typical spray drying operation is rather simple. First, a solution of the matrix material, commonly consisting of starches, maltodextrins and sucrose, and an emulsifier, generally gum arabic or octenyl succinic anhydride modified starch (OSA starch) is prepared. These solutions, which are often at a solids content of around 40 wt. %, are heated to temperatures in the range between 40 °C and 70 °C in order to promote complete dissolution of the matrix materials. This is of particular relevance for starches and maltodextrins with low DE values (DE = dextrose equivalent). A second benefit of heating the solutions is that the viscosity will be lower, facilitating the handling and spraying of the concentrated solutions. The active ingredients, which are mostly liquid at the dissolution temperature of the matrix constituents, are dispersed into the matrix solution by a high-speed homogenizer, forming a fine emulsion (typical droplet size < 1 μm) of the bioactive compound in the matrix solution. The emulsion then spray dried, using either a nozzle or a rotary atomizer (Fig. 1a). Such spray drying operations give rise to a very fine powder (typical particle size between 5 and 50 μm) (Table 1). This fine powder is dusty, shows a poor flowability, is generally difficult to mix with other powders and reconstitutes poorly in water. To minimize these issues, most spray-dried powders are agglomerated. This can be done in principally two different ways. First, one may recirculate the smallest powder particles into the spraying tower (Fig. 1a). In this way, somewhat larger aggregated clusters are obtained (Table 1). Alternatively, the agglomeration can be carried out using a fluidized bed or a belt dryer after the spraydrying operation (Fig. 1.a). In both cases, it is of importance that the fine powders are softened to just above the glass transition temperature (Tg) of the matrix, so that they stick together in a controlled way. This can be achieved either by increasing the temperature of the powder, or by slightly humidifying the surface of the particles. Other technologies that have found significant use in the encapsulation of bioactive compounds are freeze drying and extrusion. In freeze drying, which is little used in the food industry but which is the technology of choice in the pharmaceutical industry for the encapsulation of delicate bioactive ingredients 7, a concentrated solution of the matrix constituents and the active ingredient is quickly frozen, and then dried under reduced pressure at very low temperatures below the Tg of the matrix. During the socalled primary drying phase, the ice crystals sublimate, leaving a porous structure (Table 1). During the so-called secondary drying phase, the water in the cryo-concentrated matrix is evaporating, gradually increasing the glass transition temperature of the matrix to above room temperature. In this way, a stable, but porous and mechanically fragile powder is obtained. A major advantage of freeze drying over spray drying is that the exposition of active ingredients to oxygen is minimized. It is however an expensive technology, which is one of the reasons it is little used in the food industry. In the encapsulation by extrusion, a low moisture melt of the matrix components, commonly starches and maltodextrins, is extruded using a twin-screw extruder (Fig. 1b). Often, extrusion encapsulation is carried out without the use of an emulsifier, so that the inclusions or droplets of the active ingredient are rather large, around 10 μm, instead of around 1 μm as in spray drying. The extrusion process is furthermore different with respect to the other encapsulation processes Figure 1: Process schemes of a. Encapsulation by spray drying and b. Encapsulation by melt extrusion. Figure 2: Structure of a spray-dried particle containing a hydrophobic active ingredient.
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