# Advances in the Application of Microcapsules as Carriers of Functional Compounds for Food Products
## 1 Introduction  
Research on microencapsulation began in the late 1930s. However, it was only in the 1950s that the microencapsulation process was first applied in industry, when the National Cash Register Company developed \"carbonless carbon paper\" using the coacervation technique. Currently, different industrial sectors such as pharmaceuticals, cosmetics, agriculture, textiles, and food use the process of microencapsulation for various purposes [1; 2]. Moreover, as the encapsulation field is expanding rapidly, international groups and organizations such as the Bioencapsulation Research Group ([[http://bioencapsulation.net/](http://bioencapsulation.net/)]([http://bioencapsulation.net/](http://bioencapsulation.net/))) have been established. The aforementioned group has been active in the field of encapsulation since 1990, playing an important role in the development and improvement of encapsulation technologies and providing knowledge through conferences and training schools. Concerning the food industry, the growing consumer interest in functional food products with nutritional quality, safety, improved shelf-life, and which can offer health benefits, has shifted the interest of researchers and companies to the use of artificial substances to natural bioactive compounds from fruits, vegetables, pulses, roots, and other plant sources [3; 4]. However, many natural bioactive ingredients are unstable, being prone to oxidation, which is increased by exposure to light, and is also affected by heat, pH, and moisture content. As such, the food industry is interested in stabilization technologies for the preservation of the functional properties of bioactive materials during processing and storage, in modifying the physical properties of bioactive compounds to allow easier handling, in designing the release at the desired time and to specific targets, and in the increase of their bioavailability [5; 6; 7]. Microencapsulation is a process in which small particles or droplets of liquid are wrapped or coated by a polymeric material to produce small particles, which are called microcapsules or microspheres. The difference between them arises from their internal structure and morphology: microcapsules--the object of this review--are hollow internally possessing a reservoir system, while microspheres are dense matrix systems. This process allows the active ingredient, also named the core material, to be protected from adverse external environmental conditions by the coating, which is called the encapsulating agent or wall material [8; 9; 10]. The microparticles can exhibit different morphologies, being dependent on the properties of the core, wall material, and microencapsulation technique. Figure 1 shows a representation of the main types of microcapsules that can be formed, including multi-wall and single-wall systems, with single or multi-cores, with the core entrapped inside the capsule or within the wall, and with spherical or irregular shapes. A large variety of works have been reported in the literature regarding the microencapsulation of natural bioactive compounds such as antioxidants present in aqueous extracts (e.g., phenolic compounds [11; 12; 13; 14; 15], carotenoids [16; 17; 18; 19; 20; 21]), organic extracts, or essential oils [22; 23], as well as living cells [24; 25; 26; 27; 28]. Microcapsules produced for the purpose of incorporation into food products must be formed by a food-grade wall material, and edible polymers such as maltodextrin, inulin, arabic gum, and starch, among others, have emerged as candidates [29]. The present work highlights the recent advances in stabilization strategies for the production of microcapsules loaded with bioactive natural materials for food applications. It includes the most commonly used encapsulation processes, the bioactive materials studied as core materials, and the food grade biopolymers used as wall materials.