Nanostructured Lipid Carriers
Nanostructured lipid carriers (NLCs) are believed as the new generation of SLNs. NLCs are produced from a solid matrix enclosing various liquid nanocomponents whereas SLNs are produced from pure solid lipids (Muller et al., 2002). NLCs possess good biocompatibility, controlled release, production on a large industrial scale compared to SLNs. The limitations of SLNs such as limited loading capacity, recrystallization potential, and expulsion risk during storage can be solved by the NLC technique. NLCs represents an alternative method for SLNs to transport lipophilic substances for food applications (Fu et ah, 2016).
The NLCs technique is receiving interest as an effective encapsulation technology in the cosmetic, food, and pharmaceutical industries. The distorted matrix structure of NLCs produces more room to entrap active agents. Thus, NLCs can be applied to accommodate ultraviolet filters in sun care products, which makes it superior in cosmetic applications (Chu et ah, 2019). NLCs prevent the expulsive release of active agents from the structure, compared to SLNs, as shown in Figure 13.5 (Rezaei et ah, 2019).
Nanocapsules are the capsules presented in nano-size, ranging from 10 to 1000 nm, and consist of natural or synthetic wall materials. Nanocapsules are vesicular systems in which the active agents are entrapped in the centre by a nanocarrier membrane. Microencapsulation may assure the outstanding protection of essential oils toward degradation or evaporation and, at the same time, does not influence its antimicrobial activity. On the other hand, nanoencapsulation may promote passive cellular absorption mechanisms, thus lowering mass transfer resistances and increasing antimicrobial activity (Donsi et ah, 2011). There are various techniques to produce nanocapsules, which include coacervation, ionic gelation, and spray-drying.
FIGURE13.5 Structures of solid-lipid nanoparticles and nanostructured lipid carriers after storage. Source: Modified from Rezaei et al. (2019).
Complex coacervation is a recognized technology based on the electrostatic attraction between two or more oppositely charged biopolymers. It is usually produced from the combination of protein and polyanions of polysaccharide to achieve high encapsulation yields, mainly related to the high loading capacity. Complex coacervation normally occurs via the reciprocal interaction of two oppositely charged wall materials, resulting in the phase separation and formation of a coacervate outer part around the core substances (Koupantsis et al„ 2016). Hydrogen bonding and hydro- phobic interactions are other weak interactions devoted to complex coacervation (Ach et al., 2015).
Gelatin is the most common protein used in the complex coacervation, and gum arabic, sodium dodecyl sulfate, pectin, or chitosan are used as the anionic polymers. However, more researches are needed to explore the alternative protein sources from soybean, peas, and other plants to replace the use of gelatin. This is due to the limitation of the use of animal protein in certain phenomena (Dias et al., 2017). Formaldehyde and glutaraldehyde are the common cross-linking agents used in the complex coacervation process. However, these chemicals are considered toxic and forbidden in food applications. Therefore, transglutaminase, tannic acid, and genipin were used as the cross-linking agents in the complex coacervation process due to their non-toxic property and environmental friendliness (Peng et al., 2014).
The ionic charge, pH, the ratio of protein to the polysaccharide, concentration and molecular weight of wall materials, charge density, flexibility and conformation of wall materials, stirring, pressure, and temperature are the factors that affect the attractive or repulsive interactions of the two biopolymers in an aqueous environment (Yang et al., 2012). Microcapsules or nanocapsules formed by complex coacervation have an outstanding controlled release behaviour, are heat resistant, and have promising encapsulation efficiency (Yang et al., 2014). A previous study reported that the nanoencapsulation of capsaicin by complex coacervation achieved 81.2% of encapsulation efficiency and exhibited good dispersion performance (Xing et al., 2004).
Sodium alginate is the most common wall material used in ionic gelation because of its chemical stability, low toxicity, low immunogenicity and can form a rigid gel in the presence of Ca2+ (Chew and Nyam, 2016). Ionic gelation is carried out by incorporating the active agents into the alginate solution and then using the extrusion method by dropping droplets of the mixture solution into a hardening bath. A pipette, a syringe, an atomizing disk, a jet cutter, a vibrating nozzle, coaxial airflow, or an electric field can be used as the dripping tool (Nedovic et al., 2011). Hardening solution likes Ca2+ from CaCl2 solution neutralizes the repulsive charges of carboxylate groups and results in the cross-linking of the alginate chains (Sun-Waterhouse et al., 2012). The low temperature used in this technique represents its advantage to prevent unavoidable reactions and product volatilization.
Besides that, co-extrusion that uses an encapsulator equipped with a concentric nozzle allows alginate solution and core solution pumped into the encapsulator simultaneously to produce uniform-size microcapsules. Vibrating nozzle technology is applied using an encapsulator, whereby a laminar liquid jet splits into droplets by a superimposed vibration on the nozzle and dripped into a CaCl, solution. This process can be performed under mild, non-toxic conditions and can easily be scaled up. The selection of wall material, concentration of wall material, nozzle size, flow rate, vibrational frequency, electrode tension, and drying methods are factors that affect the co-extrusion process. However, highly viscous fluids and the flow rate are limited by the nozzle, which presented as the limitations of this technique (Chew et al.,
This technique avoids the use of organic solvent, high temperature, and extreme pH conditions. Besides, the selected wall materials can improve the antimicrobial activity of encapsulated essential oils via enhancing the cellular interactions exhibited with the pathogen. The previous study showed that the nanoencapsulation of essential oils by ionic gelation helped improve the stability of essential oils and enhanced antifungal activity (Mohammadi et al., 2015). Also, the thyme essential oil-in-water emulsion prepared by ionic gelation technique helped suppress the growth of Enterobacteriaceae and Staphylococcus aureus (Ghaderi-Ghahfarokhi et al., 2016).
Spray-drying is based on the principle of the atomization of emulsions into a high- temperature drying medium which leads to rapid evaporation of water and the formation of quick crust and quasi-instantaneous entrapment of the core material. Spray-drying is the most common technology used for the encapsulation process in the food industry due to its ability to cope with heat-sensitive materials, low cost, and available equipment. The spray-dried powders produced can have good reconstitution characteristics, have low water activity, protect the core material against undesirable reactions, and are suitable for transportation and storage (Phisut. 2012). Spray-drying consists of three steps, which are feed atomization, droplet drying, and powder recovery. The liquid feeds are pumped into the drying chamber via an atomizer which will atomize the feed into an enormous number of tiny droplets by the nozzle. This serves to establish a maximum heat transferring surface between the dry air and the liquid in a short time. As droplets of the atomized feed are released through the nozzle, they come in contact with the hot air, where evaporation of water begins. Thus, the liquid feeds are turned into dried powder form at this stage of droplet-air contact or droplet drying. Following the completion of drying, the dried powder can be separated from the drying air by cyclones, recovered, and collected at the collecting vessel (Patel et al., 2009).
Many factors affect the spray drying process, which includes atomization airflow, inlet air temperature, liquid flow rate, aspirator suction velocity, wall materials, and solid concentration (Roccia et al., 2014). The common wall materials used in the spray-drying process, which include polysaccharides (starches, maltodextrin, corn syrup, and gum arabic), lipids (stearic acid, mono- and diglycerides), and proteins (gelatin, casein, whey protein, soy, and wheat) (Saenz et al., 2009). Maltodextrin is the most commonly used wall material in the spray drying process. However, the high glycemic index (>130) of maltodextrin has discouraged its utilization as wall material for developing functional food products. Dietary fibre is encouraged for use as wall material in the encapsulation process due to its health advantage and protective advantage on the active agent (Chew et al., 2018).