*3.4. Polyphenol Carriers*

Materials used for encapsulation, which makes a protective shell, should be biodegradable and food-grade. They also must be able to establish a barrier between an internal phase and its surroundings [13]. Selection of coating material influences encapsulation efficiency and encapsulates stability [24]. Commonly used materials are carbohydrates such as maltodextrin, cyclodextrins, gum Arabic and modified starch. These materials lead to an increase in the glass transition temperature of the dried product. By trapping a bioactive compound, they enable its preservation against stickiness, temperature, enzymatic and chemical changes [13].

Maltodextrin is one of the most often used carriers, having a low bulk density and viscosity and high solubility at high solids contents [16,24]. It is obtained by partial hydrolysis of starch using an enzyme or acid. Maltodextrin has the ability to retain volatile compounds. Its disadvantages, such as low emulsibility, are often overcome by combining maltodextrin with other materials [16].

Cyclodextrins are safe for food applications and broadly studied as hosts for encapsulation [3]. The commonly used cyclodextrins are α-, β- and γ-cyclodextrin. They possess a hydrophobic central cavity and a hydrophilic external part [32]. Due to this structure, guest molecules (different organic and inorganic molecules) are able to be accommodated into the cavity. The hydrophilic external surface provides aqueous solubility [3,24]. The application of cyclodextrins in spray-drying is limited due to their low water solubility (1.8%). This limitation can be overcome by cyclodextrin modification. An example of such modification is hydroxypropyl-β-cyclodextrin which has a higher water solubility (60%) and thus can be subjected to drying techniques [24].

Gum Arabic consists of galactose, rhamnose, arabinose, 4-O-methyglucuronic acid and glucuronic acid and its natural source is the acacia plant (stems and branches). Furthermore, its low viscosity and high solubility, stable emulsions and high retention rates of volatile compounds have enabled a broad range of applications of this polymer [16]. On the other hand, some disadvantages such as low production yield and a consequently higher price make it less accessible [33].

#### **4. Application of Spray-Drying and Freeze-Drying for Encapsulation of Polyphenols**

During freeze-drying of foods rich in polyphenols, cells are disrupted, and therefore exposed to an increased enzyme activity (polyphenol oxidase and peroxidase enzyme) upon thawing, and the degradation of the polyphenols can occur [34,35]. However, some studies have shown that the amount of polyphenols may increase after this process. The flavonol content in freeze-dried onions increased, which can be attributed to the release of polyphenols from the matrix [36]. Wilkowska et al. [24] observed that freeze-dried powders had 1.5 times higher retention of anthocyanins than spray-dried ones. From studying the values of total polyphenols content, it has been noticed that when applying spray-drying, 73% of compounds were lost. Encapsulates of polyphenols in coffee grounds extract achieved by freeze-drying and spray-drying with maltodextrin, gum Arabic and maltodextrin:gum Arabic (1:1) as carrier materials were evaluated for total polyphenols content and flavonoid content. The results showed that freeze-drying was a more effective technique for retention of polyphenols and flavonoids and maltodextrin a more efficient carrier. On the other hand, spray-dried particles possessed a higher antioxidant activity than freeze-dried ones [2]. One interesting investigation was conducted on developing novel protein ingredients fortified with blackcurrant concentrate. As a source of protein, they used whey protein isolate and freeze-drying and spray-drying techniques. Encapsulates obtained by spray-drying possessed a higher total polyphenols content, anthocyanins content and encapsulation efficiency compared to the freeze-dried ones [28]. Robert et al. [37] encapsulated polyphenols from pomegranate juice and ethanolic extract by spray-drying, and observed a higher encapsulation efficiency when soy protein isolates were used compared to the maltodextrin. On the other hand, capsules with maltodextrin, stored at 60 ◦C in an oven for 56 days, resulted in a lower degradation of polyphenols and anthocyanins. Wu et al. [38] investigated the physicochemical properties and nutritional characteristics of functional cookies with incorporated encapsulated blackcurrant polyphenols. For the preparation of encapsulates, whey protein isolate and blackcurrant concentrate were used and the applied encapsulating techniques used were freeze-drying and spray-drying. The results of total polyphenols content were higher for enriched cookies with freeze-dried encapsulates than for those with spray-dried encapsulates.

Ersus and Yurdagel [39] observed that during spray-drying, maltodextrins with higher DE (equivalents of dextrose) are more sensitive to higher outlet air temperatures. Heating could lead to structural deformations due to shorter chains and oxidation of free glucose functional groups at the open ends. Their results confirmed the encapsulation of black carrot anthocyanins using maltodextrin DE 20-21 for 20% feed solid content and

160–180 ◦C drying temperatures. Gomes et al. [40] obtained a higher retention of papaya pulp polyphenol and flavonoid compounds in spray-dried products than in freeze-dried products. Vanillic acid had an enormous decrease of 76% after freeze-drying. Enzymatic reactions with the action of peroxidase and polyphenol oxidase are likely to occur in the freeze-drying process. Moreover, the disrupted material structure caused by the formation of ice crystals and lower exposure to oxygen can cause the liberation of these enzymes. Furthermore, artepillin C concentration increased three times after drying, which can lead to the false-positive results of the spray-drying technique. It is known that processes at high temperatures may release more bound polyphenols which cannot be detected in fresh samples [40]. Saikia et al. [41] encapsulated polyphenols from *Averrhoa carambola* pomace using maltodextrin and freeze-drying and spray-drying methods. The authors obtained a higher encapsulation efficiency in freeze-dried encapsulates. It might be that some polyphenols are destroyed during spray-drying, due to their sensitivity to heat. During the spray-drying process, fine misty droplets with increased surface are obtained and due to that higher surface, they are more exposed to heat. Moreover, during atomization, some amount of carrier can be eliminated from the core material and the partially covered capsules thus obtained can be destroyed by heat. During the freeze-drying process, no atomization or heat exposure are present. These authors also observed a decrease in surface polyphenols with higher maltodextrin content [41]. On the other hand, high inlet temperatures applied in spray-drying are short-lived, so this technique is less destructive for bioactive compounds compared to the other conventional thermal processes [42]. It has to be taken into consideration that in the freeze-drying process, obtained powders are ground and this process increases the possibility of contact with air, resulting in oxidation reactions [42,43]. In the freeze-drying process, the formation of a sawdust-like form is usual, leading to a lower surface area/volume ratio. Additionally, by the spray-drying process, smaller sized microspheres with a larger surface area were obtained using the spray-drying process (for the same amount of material as for freeze-drying), which led to the deterioration of the surface polyphenols [2]. Considering the short time of exposure to high temperatures during spray-drying, this technique seems good for the encapsulation of polyphenols. Dealing with thermosensitive and highly valuable materials, freeze-drying is a suitable method [33]. Tables 1–3 present the studies on the encapsulation of polyphenols using freeze-drying and spray-drying techniques.


**Table 1.** Studies of spray-drying application for encapsulation of polyphenols from different sources.




**Table 2.** Studies of freeze-drying application for encapsulation of polyphenols from different sources.



**Table 3.** Studies of spray-drying (SD) and freeze-drying (FD) applications for encapsulation of polyphenols from different sources.




