# Microencapsulation of Anthocyanins--Critical Review of Techniques and Wall Materials
## 2 Microencapsulation Techniques
### Physical Methods
#### 2.1.2 Freeze Drying  
Freeze drying (FD), unlike SD, applies low temperatures, making it a potential candidate for the dehydration of water-soluble and thermosensitive bioactive compounds, particularly anthocyanins. The FD method's fundamental principle is sublimation, during which the moisture is directly converted from a liquid to a gas phase, and finally, a powder is formed [43]. Since this process is performed in a vacuum and at a low operating temperature, the number of deterioration reactions is dramatically reduced. Regarding this method's drawbacks, it is a time-consuming process (between 24-48 h) and requires a high amount of energy [44]. The major limitation of this approach is the production of a porous structure due to ice sublimation, paving the way for anthocyanins to easily contact oxygen through pores on the particles' surface [10]. Many studies have made a comparison between FD and SD microencapsulation of anthocyanins. Most of them have indicated no significant difference in the total anthocyanin content of microcapsules produced using both methods [45; 46; 47]. In FD, anthocyanins are mainly degraded due to grinding and oxidation processes. In contrast, in SD, the loss of anthocyanins is attributed to the high temperature, oxidation, and ultimately, polymerization and degradation [46]. However, some studies detected notably higher anthocyanin content in freeze-dried samples when compared to SD. In this regard, [PERSON] and [PERSON] [27] found an approximately 72% greater quantity of black rice anthocyanins in microcapsules produced using FD. Furthermore, the process yield varied from 64.07 to 82.16% for SD but above 85% for FD. [PERSON] et al. [48] also reported a 22-25% higher anthocyanin content for freeze-dried microcapsules loaded with black carrot juice, with the EE ranging from 80.81 to 98.51% for FD and 60.04 to 82.48% for SD. [PERSON] and [PERSON] [49], who attempted to microencapsulate rose residue anthocyanins, showed that the anthocyanin and total phenolic compound retention were, respectively, 91.44% and 95.12% for FD while 75.85% and 86.00% for SD. The morphological properties (size and shape) of powders are the most noticeable difference between the FD and SD microcapsules. According to the survey by [PERSON] and [PERSON] [50], FD resulted in irregular agglomerates and a thin porous sheet-like material. SEM observations of spray-dried particles also showed that high inlet temperatures led to small-sized spherical particle formation with a smooth surface, whereas low temperatures generated shriveled ones. Considering these properties, freeze-dried microcapsule had 19-100% lower storage stability than spray-dried samples at all temperatures due to their morphology and oxygen permeability [50]. In contrast, according to [PERSON] et al. [45], no difference was observed in the half-life of microencapsulated maqui juice through FD and SD despite the different morphological properties. The stability of powder during storage could also be evaluated using the T\({}_{\text{g}}\) value. This parameter can be influenced by the moisture content, chemical structure, and molecular weight of the carrier agents. Hence, the most stable powder would have the highest T\({}_{\text{g}}\) value. Therefore, to ensure stability, the powder's temperature should be lower than T\({}_{\text{g}}\). In this case, molecular mobility is limited since the material remains in the glassy state, which controls the chemical, physical, and biological changes [43]. [PERSON] and [PERSON] [46] reported T\({}_{\text{g}}\) values ranging from 11 to 26 \({}^{\circ}\)C for grape skin extract microencapsulated via FD, whereas this figure for SD was between 42 and 52 \({}^{\circ}\)C. Thus, freeze-dried samples had lower stability, mainly due to the high moisture content. The anthocyanin-to-polymer wall material ratio in lyophilization has a role to play in the EE and anthocyanin content of microcapsules. [PERSON] et al.