Choosing the appropriate wall materials for spray-drying microencapsulation of natural bioactive ingredients: Taking phenolic compounds as examples
Graphical abstract
Introduction
Recently, the utilization of natural plant extracts or their bioactive ingredients for health promotion has become an increasing trend, due to their link between nutrition and medicine. Natural bioactive ingredients are those substances that can bring about beneficial health effects through cellular activities as well as physiological activities, [1] and phenolic compounds are one of the most popular natural bioactive ingredients because of their potential health benefits. Incorporation of natural bioactive ingredients into various functional food systems is a major approach to improve their nutritional value. However, there are also many difficulties encountered limiting their applications, including: (i) conditioned solubility, (ii) inferior stability, (iii) unpleasant taste, and (iv) limited bioaccessibility and bioavailability. [2]
Microencapsulation, which has been employed as a promising technique to package natural bioactive ingredients, is a widely used economical strategy to alleviate these limitations. It is defined as the entrapment of a substance (core material) within an immiscible substance (wall material), [3] which can build a barrier to protect the core material away from undesirable surrounding conditions and enhance the beneficial properties, bioavailability and efficacy. Various techniques, including spray drying, freeze drying, fluid bed coating, ionic gelation, thermal gelation, emulsion and so on, have been applied for microencapsulation and each technique presents unique advantages and disadvantages. [4] The selection of a microencapsulation technique depends upon encapsulation efficiency, complexity in procedure, process cost and the choice of wall materials.
Although most commonly considered as a dehydration process, spray-drying is also a pervasive and sound technology for microencapsulation. [5] Compared to other microencapsulation methods, spray drying possess the benefits of simple operation and low cost. And it is suitable for heat-sensitive substances like phenolic compounds, as the spray drying temperature is relatively low (generally the inlet temperature is lower than 200 °C) and the residence time of the droplet/particles is very short (in a matter of seconds). The cooling effect caused by the solvent evaporation helps the temperature of the dried product does not rise above its wet bulb temperature. [6] Spray drying involves complex interactions of equipment components, physical properties of feed, and processing parameters, [7,8] which all have influence on the final product characteristics. It is noteworthy that the difference in drying characteristics between wall materials and core compounds rules this microencapsulation process, [9] which is possible to affect the powder properties like size, shape, flowability, compressibility, bulk density, stability, solubility, process efficiency, moisture content and the degree of protection for the core material. In a word, the wall composition and the microencapsulation techniques may determine functional properties and potential applications of the encapsulated components. [10] In this sense, the main focus of microencapsulation of natural bioactive ingredients is the improvement of their encapsulation efficiency, bioavailability or nutrition value by selecting and modulating the type and proportion of wall materials in the spray drying process. The application of spray drying for microencapsulation has experienced an appreciable increase since the late 1950s. Taking phenolic compounds as an example, the proportion of papers related to spray drying microencapsulation of phenolic compounds significantly increased over the past 15 years, and the percentage of published papers increased from 0.43% in 2004 to 9.07% in 2019, with an almost 210% growth.
Section snippets
Phenolic compounds
Phenolic compounds, ubiquitous in plants metabolites, are an important part of natural bioactive compounds that are widely used for functional foods, nutraceuticals and pharmaceuticals. [11,12] A large amount of research data indicates that phenolic compounds possess a high spectrum of biological activities, including antioxidant, anti-inflammatory, anti-allergenic, antibacterial, antiviral, and even antitumor functions. [[13], [14], [15]] Therefore, some researchers have related an inverse
Wall materials
Spray drying has been proved to be an effective method for the drying and microencapsulation of phenolic compounds. For spray-drying microencapsulation, the selection of appropriate wall materials is the crucial step. Wall material is an important part of the microencapsulation and the characteristics of wall materials, including molecular structure, wall material film properties, emulsification stability and rheological properties, are decisive factors affecting the functional properties and
Selection of wall materials for spray-drying encapsulation of phenolic compounds
The encapsulation of phenolic compounds by spray-drying involves (i) preparation of the feed solution, (ii) atomization, and (iii) dehydration. Normally, for the preparation of the feed solution, wall materials are dissolved in solutions with stirring and core materials are added to the solutions with or without the addition of an emulsifier, depending on the water solubility of core material and emulsifying properties of the wall materials. For hydrophilic phenolic compounds, there is no need
Discussion and conclusion
The microencapsulation afforded to phenolic compounds during spray drying is reviewed, and it is obvious that the choice of suitable wall material is an extremely important first step. In a nutshell, to achieve the best microencapsulation, appropriate wall material should be selected and relied on the physical-chemical properties of core material and the application intended for the product powder. The overview of the microencapsulation strategy of phenolic compounds was shown in Fig. 1.
Declaration of Competing Interest
The authors declare that they have no conflict of interes.
Acknowledgements
The authors would like to acknowledge financial support from National Natural Science Foundation of China (Grant No. 81503023) , Natural Science Foundation of Hunan Province (Grant No. 2020JJ5788) and the Fundamental Reseach Funds for the Central Universities of Central South University (Grant No. 2021zzts0989).
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