ReviewMicro and nano encapsulation, retention and controlled release of flavor and aroma compounds: A critical review
Introduction
Encapsulation is a common technique for creating an external membrane or coating of one material over another material, which is applied for the protection and/or preservation of bioactive, volatile, and easily degradable compounds from biochemical and thermal deterioration. It is also used for masking undesirable flavors and aromas. Encapsulation was first developed around 60 years ago as a technology for coating solids, liquids and gaseous compounds. The coating enables controlled release of compounds at specific rate under certain conditions (Desai & Park, 2005; Shishir, Xie, Sun, Zheng, & Chen, 2018). The encapsulated materials can be pure substances or a mixture, which are also called the coated material, core material, actives, fill, internal phase or payload. On the other hand, the coating materials are known as packing material, capsule, wall material, film, membrane, carrier or outer shell (Fang & Bhandaria, 2010). The coating materials are usually made of natural or modified polysaccharides, gums, proteins, lipids, and synthetic polymers (de Souza Simões et al., 2017). The selection of coating material depends upon the nature of the core material, encapsulation process, and final use of the product. The morphology of the resulting microcapsule depends upon on the arrangement of core material and deposition process of the coating material, which can be divided into mononuclear, polynuclear, and matrix. Mononuclear capsules contain an outer shell around the core, while polynuclear capsules have many cores which are entrapped into a shell, and matrix capsules have the core material uniformly distributed within the shell material. In addition, it is possible to have a multi-shell mononuclear capsule or a capsule with non-spherical shape/irregular shape (Kim et al., 2004). Fig. 1 represents different forms of capsules.
Nano-capsules and microcapsules are the most functional and desirable size in encapsulation processing. Even though, nano scale and micro scale refers 1–1000 nm and 1–1000 μm, respectively, nano-encapsulation ranges the capsule size from 1 nm to several hundred nanometer in diameter and microencapsulation ranges from 1 μm to several hundred micrometers in diameter (Katouzian & Jafari, 2016; Rodríguez, Martín, Ruiz, & Clares, 2016; Shishir et al., 2018). Furthermore, particle size in between the range of nano and micro encapsulation is called as submicron particles, while the particle size above the microencapsulation range is referred as macro-particles (Koo, Cha, Song, Chung, & Pan, 2014; Lević et al., 2015; O'Toole et al., 2012). Microencapsulation is the most common and widely used in food and pharmaceutical industries. Recently, nanoencapsulations have attracted rising interest for their unique feature in terms of efficient encapsulation, enhanced stability, and better controlled release of capsulated materials (Katouzian & Jafari, 2016; Shishir et al., 2018).
Flavors and aromas are organic molecules having low molecular weight. They are relatively volatile and very sensitive to air, heat, light, and moisture (Bakry et al., 2016). Therefore, encapsulation is a common practice in the flavor and fragrance industry to preserve both the flavor and aroma. Depending on the applied encapsulation technique, the materials obtained from the encapsulation process can be a powder, a paste or a liquid (Prost, Poinot, Rannou, & Arvisenet, 2012). In processed food products the degradation of flavor and aroma occurs during processing and storage of the product. To reduce the level of degradation and preserve the originality of flavor and aroma, pre-encapsulated volatile ingredients can be used in foods and beverages (Madene, Jacquot, Scher, & Desobry, 2006). However, the type of flavor and aroma preservation technique depends on the type of product it is going to be used in or the processing conditions through which it will pass through. For example, in the bakery industry, powder forms of encapsulated flavor and aroma compounds are mostly used as it is very stable, easy to weigh and store. In bakery applications, starch, wheat flour or soy flour are used as carrier substances (Cappelle, 2002). Liquid forms (e.g. emulsion) of encapsulated compounds are also used in different bakery products as it helps to improve the texture of the final product along with preserving the flavor and aroma compounds (Cappelle, 2002). In frozen products such as ice cream, flavorings are added directly to the other ingredients in the ice cream mixture followed by passing through homogenizer and forms an oil-in-water type emulsion, or flavor is added to the ice cream mixture during agitation/maturation before freezing (Clark, 2009; Fiol et al., 2017). These emulsion particles preserve the flavor compounds and the temperature during processing and storage also assist in retaining the flavor until consumption. In beverages, flavor and aroma compounds are generally encapsulated in the form of micro and nano-emulsions before mixing with other ingredients. Different antioxidant compounds are also used to preserved flavor and aroma compounds in food products by preventing oxidation and off-flavor production. However, due to the taste profile and intense colors of some antioxidants it limits their use in food industry (Weerawatanakorn, Wu, Pan, & Ho, 2015).
To date, a variety of encapsulation techniques have been reported as novel and promising for the encapsulation of flavor compounds. These techniques, in the form of micro-capsule, micro-emulsion, nano-emulsion, nano-capsule, nano-fiber, nano-tube, solid lipid nanoparticle, inclusion complex, and bead encapsulation offer numerous benefits over conventional methods (e.g. spray drying) (Fuciños et al., 2017; Hussein et al., 2017; Koupantsis, Pavlidou, & Paraskevopoulou, 2016; Lević et al., 2015; Rodenak-Kladniew, Islan, de Bravo, Durán,& Castro, 2017; Saberi, Fang, & McClements, 2016; Tampau, Gonzalez-Martinez, & Chiralt, 2017; Zhang et al., 2017). Therefore, in this review, we comprehensively discuss the recent progress on micro and nano-encapsulation of flavor and aroma compounds. Several recent review articles focused on the encapsulation of food bioactive compounds and bioactive oils (Bakry et al., 2016; de Souza Simões et al., 2017; Katouzian & Jafari, 2016; Rodríguez et al., 2016), but from our best observation, there is particularly no recent review on the advances in micro and nano-encapsulation of flavor and aroma compounds. Hence, the aim of this review is to highlight the suitability of micro and nano encapsulation approaches for the retention of flavor and aroma compounds including emerging techniques, new formulations and novel encapsulate systems. Additionally, we aim to illustrate the controlled release mechanisms of encapsulated flavors, and to expose the industrial applications for encapsulated flavor and aroma compounds. This review enhances our understanding of the production of flavor and aroma capsules for industrial applications, and therefore provides some insight for future potential.
Section snippets
Roles of encapsulation for flavor and aroma compounds
Food flavor is a sensory attribute of food material. Chemically it is the combination of taste and aroma. Flavors and aromas are the most important attributes of food material. They may present in the food inherently, may develop during processing or be added during processing according to consumers' demands. Aromas and flavors give the first impression of a food material, which can change the consumers’ intention for further use or consumption. Flavor stability with an appropriate level of
Factors affecting flavor and aroma retention during encapsulation
Flavor retention during processing and flavor stability in processed products is a major concern in the food industry. Flavor represents both the unique characteristics and the quality of a food product. Almost all flavor and aroma compounds are very active and highly volatile in nature. They mix easily with air molecules and diffuse very quickly. Therefore, flavor demands significant attention during processing of food products, since it is very susceptible to degradation and/or loss in the
Encapsulation approaches
Various techniques are available for the encapsulation of food ingredients into the micro or nano scale. The methods include spray drying, spray chilling or spray cooling, freeze-drying, fluidized bed-coating, extrusion, co-crystallization, emulsification, molecular inclusion, rotational suspension separation, and coacervation (de Souza Simões et al., 2017). Broadly, these methods are divided into three categories, i.e. chemical methods (e.g. in situ polymerization, molecular inclusion);
Flavor release-affecting factors and release mechanisms
Flavor release or release rate may be defined as the migration of flavor molecules from one environment or state to another environment or state over a certain period of time. Release rate of flavor and aroma compounds is very important for food products. Even before the food is placed in the mouth, people can estimate the food's quality and taste. The volatile flavors mixed with air molecules can enter the nasal cavity, which comes into contact with receptor cells in the olfactory epithelium
Application of micro and nano-encapsulated flavor and aroma compounds in food industry
Flavor and aroma compounds are the most important active food components. The enhanced retention and controlled release of flavor and aroma is the highest priority of a food processor. Therefore, encapsulation is a common practice in food industries for the preservation of flavor compounds (Saifullah et al., 2016). However, there are a variety of encapsulation methods available that produce different forms of encapsulates, such as paste, powder, capsules, granules, and emulsion droplets. Most
Conclusion and future trends
Flavors and aromas are the most important bioactive food ingredients, which represent the taste and quality of a food product. Encapsulation allows flavors to be in a stable form and makes them suitable for application in a wide range of food products. Nano-encapsulation approaches provide a greater loading capacity and encapsulation efficiency, enhanced stability, and better control on flavor release profile compared to microencapsulation. In recent, spray chilling is gaining increasing
Conflicts of interest
The authors declare that there are no conflicts of interest.
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