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Drying
Microencapsulation is defined as the entrapment of tiny particles of an active agent inside coating materials [151]. Spray drying is the most commonly used microencapsulation technique, due to its low cost and high efficiency, fast and continuous operation and significant increment of the bioactive shelf life [149–151].
Spray drying has different stages: first, the coating materials are solubilized, generally in aqueous solution and then the bioactive compound is added to this coating dispersion. In case of oils, this dispersion is homogenized to generate a stable emulsion. Then, this emulsion is atomized into the spray dryer chamber where hot dry air circulates and quickly dries the droplets and the powder (microcapsules) is collected in a mechanical cyclone. Drying rate is influenced by the emulsion characteristics, such as viscosity and particle size. Emulsions with high viscosity form elongated and bigger droplets, which interfere with the atomization process, affecting adversely the drying rate [149]. The emulsion viscosity can be modified by varying the feed temperature. Some process conditions that have to be controlled are the inlet and outlet air temperature, atomization pressure, feed rate, the concentration of fed flow [150, 151]. The selection of the atomizer type (nozzle or disc) is also important.
The selection of appropriate coating materials or encapsulating agents has a direct impact on encapsulation efficiency and is critical in order to design microparticles with site-specific release properties and stability during storage. The coating material needs to have high water solubility, film-forming and emulsifying capacity, diffusibility, and low cost. Some of the most common coating or wall materials are gums, starches, gelatines and maltodextrins [152], which are water soluble and the elease mechanism of encapsulated compounds is by dissolution. Some results of vegetable oils spray drying microencapsulation are shown in Table 2.2.
To obtain a gradual release, it is necessary to include other hydrophobic compounds, such as proteins or other polymers in the coating material formulation [148–154]. Chitosan and alginate may be used to generate pH-sensitive encapsulation systems. In the intestinal tract (pH >6) alginate (a polyanionic water-soluble polysaccharide) disintegrates, and the bioactive compounds are released [155]. Although it is extensively used in food and pharmaceutical industry, alginate is barely been used as wall material for spray-drying encapsulation [156]. Moreover, other polysaccharides that are described for their health benefits as prebiotic or bifidogenic, such as inulin, have been used as colonic release polymers. Their glycosidic linkages are stable during the human digestive enzymatic action but they are fermented by colonic bacteria, releasing the bioactive compounds properly [157].
Oleurepin bioaccesibility (from olive leaves) is influenced by the type of the encapsulation system, polymer type and process conditions [158]. In this context, works in this line are scarce but necessary because the digestive mechanisms related to phenolic compounds remain unknown. Moreover, the most consumed compounds are not necessarily the most active in the organism, since their concentrations in the bloodstream depend on its modifications during metabolism in the gastrointestinal tract.
Table 2.2 Vegetable oils microencapsulation by spray drying.
| Coating material | Spray dryer | Process conditions | Results | Ref. | |
|---|---|---|---|---|---|
| Flaxseed oil | MD, GA, WPC, Hi-Cap 100, Capsul TA | Laboratory scale spray dryer | Flow rate 12 ± 2 g/min. I/O T: 180 °C/110 °C. | High encapsulation efficiency: Good stability. | [159] |
| Chia oil | WPC, MG, GA | Nichols/Niro spray-drier, Turbo spray-drier PLA | Feed rate 40 ml/min I/O T: 135 °C/80 °C. Pressure 4 bar | WPC:MG and WPC:GA blends promoted good encapsulation efficiency | [160] |
| Olive oil | MD, agave inulin (IN), AG | Niro Minor pilot scale spray dryer | Flow rate 57.6 g/min I/O T 180 ±5 °C/90 ± 5 °C. Pressure 5 bar | MD-AG and IN-AG blends generate high microencapsulation yield. | [161] |
| Pomegranate oil | Skimmed milk powder | Pilot scale spray dryer, Buchi, B-191 | Feed rate 1.75 ± 0.05 g/min Inlet temperature 150–190 °C Pressure 5 bar | Core to wall material ratio defined the encapsulation yield. | [162] |
| Avocado oil | WPI and MD | Small scale spray dryer, Model SL10, Saurin Group of Companies | I/O T 180 °C/80 °C | Microencapsulated avocado oil showed a good oxidative stability. | [163] |
AG, acacia gum; GA, gum Arabic; Hi-Cap 100 and Capsul TA, modified starchs; I/O, Inlet/outlet MD, maltodextrin; MG, mesquite gum; WPC, whey protein concentrate; WPI, whey protein isolate; T, temperature.
2.3.3.2 Coacervation
Coacervation is a physicochemical microencapsulation method, in which one polymeric solution is separated into two or more liquid phases, one of them rich in the polymeric material. The bioactive agent (core) is surrounded by the coating material, forming particles with a polymeric cover. The particles suspended in the solution are separated from the initial polymeric solution, and subsequently, solidify. The barrier created by the polymers around the core allows the encapsulation of the interest compounds, which can be oils. The coacervation can occur in either aqueous or organic liquid. The factors which have influence on the coacervation process are pH, ionic strength, biopolymers concentration, biopolymers ratio, biopolymers molecular weight, temperature, and homogenization degree [164].
Coacervation may be simple or complex, according to the involved phase separation. In simple coacervation, only one polymer is in aqueous solution, occurring the polymer separation when electrolytes or water-miscible solvents are added, a temperature change is produced, or an inorganic salt is added. In complex coacervation, two or more polymers are in aqueous solution, occurring the separation phenomenon when electrostatically oppositely charged biopolymers are brought together, under certain specific conditions [165].
The coacervation is regarded as one of the simplest, low cost and reproducible micro-encapsulation method for hydrophobic substances (vitamins and oils), due to its high encapsulation efficiency and high oxidative stability [166]. The biopolymers used as wall materials have hydrophilic colloidal properties, adequate charge density, and high water solubility. The coating materials most used for this method are sodium alginate, protein isolates and gum Arabic [165]. In Table 2.3 some studies of vegetable oils microencapsulation by complex coacervation are shown.
Table 2.3 Vegetable oils microencapsulation by complex coacervation.
| Active agent | Wall material | Process conditions | Results | Ref. |
|---|---|---|---|---|
| Chia oil |
Protein and gums |