# Microencapsulation of Anthocyanins--Critical Review of Techniques and Wall Materials
## 2 Microencapsulation Techniques
### Physical Methods
#### 2.1.1 Spray Drying
Furthermore, nonuniform particles with a wide range of size distribution are produced, making the control over the morphology and size a demanding matter [10]. The particle size could generally be influenced by the spray atomizing pressure, nozzle position and size, liquid flow rate, polymeric structure, and solution concentration. The properties of the powder largely depend on numerous factors, such as the drying parameters, including the feed flux, air temperature, flow rate, and the type of wall material, as well as its concentration and viscosity [24]. The loading capacity of microcapsules is estimated using the encapsulation efficiency (EE). Temperature is the main factor that can affect the EE in this technique. High temperatures lead to a decline in the amount of powder that is adhered to the chamber wall, enhancing the EE. Furthermore, high temperatures facilitate the drying rate and formation of a crust, restricting the diffusion of heat in the droplet and the leaching of anthocyanins [25]. [PERSON] et al. [26] concluded that high air temperatures, combined with a high solid concentration of the solution, results in a high EE and preservation of the anthocyanins. In a study by [PERSON] et al. [27], _Oryza sativa_ L. anthocyanins were microencapsulated and sprayed at 140, 160, and 180 \({}^{\circ}\)C. According to the results, as the inlet air temperature rose, the process yield increased from approximately 64% at 140 \({}^{\circ}\)C to 82% at 180 \({}^{\circ}\)C. Still, the total anthocyanin content (TAC), surface anthocyanin content (SAC), and radical scavenging capacity of the microcapsules decreased. It is worth pointing out that both the outlet and inlet temperatures are interrelated and should be considered while using SD because anthocyanins are highly heat-sensitive compounds [28]. A high inlet temperature exposes particles to an increased outlet temperature. In contrast, reduced outlet temperatures extend the drying time, creating a combined effect of temperature and time that directly impacts the loss of anthocyanins [29]. The EE of anthocyanins created using spray drying can also be affected by the core-to-wall mass ratio. Several studies have adopted a typical core-to-wall ratio of around 25% for obtaining the high EEs [30; 31; 32; 33]. In contrast, [PERSON] et al. [34] reported that the core-to-wall ratio of 1:6 resulted in a higher yield (52.90-60.66%) in comparison to the ratio of 1:4 (25.40-43.14%) and 1:5 (35.40-52.08%). This finding might be associated with greater proportions of core materials close to the drying surface, decreasing the air-particle interface's diffusion path length [15]. The solid concentration of the wall material can also affect the EE of anthocyanins. On the one hand, it has been stated that as the solid concentration goes up, the viscosity of the wall material increases, where this phenomenon influences the atomization process and causes the production of large and elongated particles, affecting the drying rate [23]. Moreover, the formation of cracks on the particles' surfaces eases anthocyanins' diffusion from the core to the wall. [PERSON] et al. [25] utilized modified rice starch with concentrations between 5 and 10% (w/v) for the microencapsulation of anthocyanins of purple rice bran. It was reported that the EE dramatically went down once the concentration exceeded 7%. On the other hand, [PERSON] et al. [35], who produced phosphorylated starch loaded with purple maize anthocyanins, reported the highest EE (49.11%) when the concentration was 20%. At the same time, this figure was lower for solutions with 10 or 15% concentrations. The choice of proper wall material for microencapsulation via spray drying fulfills a prominent role in microcapsules' EEs. The selection of encapsulating agent in SD is somewhat limited because it should meet the following criteria: acceptable solubility, low viscosity at high concentrations, emulsifying properties, and film-forming capacity [23].