Polysaccharides as Carriers of Polyphenols: Comparison of Freeze-Drying and Spray-Drying as Encapsulation Techniques

Polyphenols have received great attention as important phytochemicals beneficial for human health. They have a protective effect against cardiovascular disease, obesity, cancer and diabetes. The utilization of polyphenols as natural antioxidants, functional ingredients and supplements is limited due to their low stability caused by environmental and processing conditions, such as heat, light, oxygen, pH, enzymes and so forth. These disadvantages are overcome by the encapsulation of polyphenols by different methods in the presence of polyphenolic carriers. Different encapsulation technologies have been established with the purpose of decreasing polyphenol sensitivity and the creation of more efficient delivery systems. Among them, spray-drying and freeze-drying are the most common methods for polyphenol encapsulation. This review will provide an overview of scientific studies in which polyphenols from different sources were encapsulated using these two drying methods, as well as the impact of different polysaccharides used as carriers for encapsulation.


Introduction
Polyphenols are secondary plant metabolites consisting of an aromatic ring to which one or more hydroxyl groups are attached [1]. These compounds are synthesized through plant development and/or as a plant's response to environmental stress conditions [2]. Even when they are at low concentrations in plants, polyphenols protect them from predators or ultraviolet damage [3]. They are known as natural antioxidants and therefore have many beneficial effects on health (e.g., antimicrobial, anti-inflammatory, antioxidant effect, etc.) [4][5][6]. The health benefits of polyphenols are influenced by the matrix in which they are processed and ultimately consumed [7]. One well-known property of polyphenols is the positive influence on diabetes and obesity due to the possibility of inhibition of digestive enzymes such as α-glucosidase and α-amylase [8]. Anthocyanins are a group of polyphenols responsible for the red, blue and purple color of fruit and vegetables. The major anthocyanin found in most plants is cyanidin-3-glucoside, correlated with reduced reactive oxygen species (ROS) levels and antioxidant potential in in vitro conditions [9,10]. Flavan-3-ols are a subgroup of flavonoids and their main representatives are catechin, epicatechin, epigallocatechin and epigallocatechin-3-gallate. These compounds are abundantly found in green tea, strawberries and black grapes. Studies showed a positive effect of catechin in Alzheimer's and Parkinson's diseases, diabetes and in cancer treatment [11]. Gallic acid, a polyphenol from a group of phenolic acids, a subgroup of hydroxybenzoic acids was the subject of many studies that have proven its significant antioxidant, anticarcinogenic, antimicrobial and antimutagenic effects [12].
Due to the presence of unsaturated bonds in their structures, polyphenols are sensitive to various environmental conditions such as the presence of oxygen, light and water [2]. The presence of water is the most important factor, due to its essentiality in most chemical

Polyphenols
Polyphenols include various different compounds divided into several classes: phenolic acids, flavonoids, stilbenoids, tannins, coumarins, and polymeric lignans ( Figure 1) [18]. Their chemical structure may vary from simple to complex. The largest group of polyphenols is that of flavonoids, divided into several subgroups (flavonols, flavones, flavanols, flavanones, anthocyanidins, isoflavonoids, and chalcones) [19]. Their structure consists of two phenyl groups linked with a three-carbon bridge. According to the degree of oxidation and unsaturation of the three-carbon segment, they differ from each other. Different sugar molecules can be attached to the hydroxyl groups of flavonoids. They are usually in a glycosidic form which improves their solubility in water. Acylation of the glycosides where sugar hydroxyls are derivatized with acid (such as ferulic and acetic acids) is also common. The interconnection between several basic units of polyphenols makes larger and more complex structures, such as hydrolysable tannins and condensed tannins [20]. Phenolic acids represent a large group of hydrophilic polyphenols and they are constituted of a single phenyl ring [14]. Due to the diversity in the structures of polyphenols, they possess different properties (such as solubility and polarity) [20].
Polyphenols exist ubiquitously in vegetables and fruits and their consummation is very desirable [14]. Various positive bioactivities of polyphenols toward pathologic conditions are known, through their antioxidant properties. These molecules are capable of donating hydrogen atoms and electrons [14]. One such activity is the anticancer property of polyphenols [21]. Hollman et al. [22] reviewed the antioxidant activities of polyphenols within the organism and their positive impact on cardiovascular health. In addition, polyphenols contribute to the sensory quality of the products (wine, jellies, juices, chocolate, etc.). They affect color, bitterness, turbidity, etc. [6,23]. Due to their many functional properties, polyphenols are of great interest to the food, chemical and pharmaceutical industries [2]. Polyphenols exist ubiquitously in vegetables and fruits and their consummation is very desirable [14]. Various positive bioactivities of polyphenols toward pathologic conditions are known, through their antioxidant properties. These molecules are capable of donating hydrogen atoms and electrons [14]. One such activity is the anticancer property of polyphenols [21]. Hollman et al. [22] reviewed the antioxidant activities of polyphenols within the organism and their positive impact on cardiovascular health. In addition, polyphenols contribute to the sensory quality of the products (wine, jellies, juices, chocolate, etc.). They affect color, bitterness, turbidity, etc. [6,23]. Due to their many functional properties, polyphenols are of great interest to the food, chemical and pharmaceutical industries [2].

Encapsulation
Encapsulation is the process of entrapment of active compounds into particles to enable isolation or controlled release of given compounds. The main role of encapsulation is the protection of sensitive active compounds from degradation [17]. Various techniques can be used for encapsulation depending on core material, size of required particles, physical state, or sensitivity to high temperature [24]. These techniques include freeze-drying, spray-draying, extrusion, fluidized bed coating, spray-cooling/chilling, coacervation, liposome entrapment, cocrystallization, vacuum-drying, centrifugal suspension separation, nanoencapsulation, molecular inclusion and emulsification [13]. Encapsulation with coacervation is based on phase separation of a hydrocolloid from an initial solution and subsequent deposition of the formed coacervate phase around an active ingredient suspended in media. It is considered an expensive technique but very beneficial for highvalue compounds [25]. Extrusion is a process based on passing a polymer solution with active ingredients through a nozzle (or syringes) into a gelling solution. Usually the used

Encapsulation
Encapsulation is the process of entrapment of active compounds into particles to enable isolation or controlled release of given compounds. The main role of encapsulation is the protection of sensitive active compounds from degradation [17]. Various techniques can be used for encapsulation depending on core material, size of required particles, physical state, or sensitivity to high temperature [24]. These techniques include freeze-drying, spraydraying, extrusion, fluidized bed coating, spray-cooling/chilling, coacervation, liposome entrapment, cocrystallization, vacuum-drying, centrifugal suspension separation, nanoencapsulation, molecular inclusion and emulsification [13]. Encapsulation with coacervation is based on phase separation of a hydrocolloid from an initial solution and subsequent deposition of the formed coacervate phase around an active ingredient suspended in media. It is considered an expensive technique but very beneficial for high-value compounds [25]. Extrusion is a process based on passing a polymer solution with active ingredients through a nozzle (or syringes) into a gelling solution. Usually the used wall material is sodium alginate while calcium chloride solution serves for capsule forming. It is easy to execute on a laboratory scale with long shelf-life capsules, while scale-up of this technique is expensive and demanding with a limited choice of wall materials [26]. Emulsification includes the dispersion of one liquid into the other (two immiscible liquids) in the form of droplets. An emulsifier is required for stabilization and by application of drying, a powder form of encapsulates can be achieved [26]. The molecular inclusion method is also known as host-guest complexation. Apolar guest molecules are trapped inside the apolar cavity of host molecules (such as cyclodextrins) through non-covalent bonds [26]. Cocrystallization techniques include modification of the crystalline structure of sucrose to an irregular agglomerated crystal with porous structure in which active ingredient can be incorporated [25]. Fluidized bed coating is also referred to as fluidized bed processing, air suspension coating, or spray coating. The principle is that coating is applied to the particles suspended in the air. For this technique a wide range of coating materials such as aqueous solutions of cellulose, starch derivates, gums, or proteins is suitable [27]. Spray chilling is also known as spray cooling, prilling, or spray congealing. The basic principle is similar to spray drying with the key difference of using a cooling chamber instead of a drying chamber. Regarding coating materials, only lipid-based materials (fats, waxes, fatty acids, fatty alcohols and polyethylene glycols) are used [27]. Nanoencapsulation is an innovative trend in the field of food technologies. The final results are particles of diameters ranging from 1 to 1000 nm. The term nanoparticles includes nanospheres and nanocapsules. The first one has a matrix-type structure where the active ingredients can be adsorbed at the sphere surface or encapsulated in particles. In nanocapsules, the active ingredient is limited to a cavity with an inner liquid core surrounded by a polymeric membrane. Nanoparticles have a larger surface area, increased solubility, enhanced bioavailability and improved controlled release [25]. Freeze-drying and spray-drying are the most frequently employed methods for removing water from foods with encapsulation effects [28]. A schematic view of these methods is presented in Figure 2.
drying, a powder form of encapsulates can be achieved [26]. The molecular inclusion method is also known as host-guest complexation. Apolar guest molecules are trapped inside the apolar cavity of host molecules (such as cyclodextrins) through non-covalent bonds [26]. Cocrystallization techniques include modification of the crystalline structure of sucrose to an irregular agglomerated crystal with porous structure in which active ingredient can be incorporated [25]. Fluidized bed coating is also referred to as fluidized bed processing, air suspension coating, or spray coating. The principle is that coating is applied to the particles suspended in the air. For this technique a wide range of coating materials such as aqueous solutions of cellulose, starch derivates, gums, or proteins is suitable [27]. Spray chilling is also known as spray cooling, prilling, or spray congealing. The basic principle is similar to spray drying with the key difference of using a cooling chamber instead of a drying chamber. Regarding coating materials, only lipid-based materials (fats, waxes, fatty acids, fatty alcohols and polyethylene glycols) are used [27]. Nanoencapsulation is an innovative trend in the field of food technologies. The final results are particles of diameters ranging from 1 to 1000 nm. The term nanoparticles includes nanospheres and nanocapsules. The first one has a matrix-type structure where the active ingredients can be adsorbed at the sphere surface or encapsulated in particles. In nanocapsules, the active ingredient is limited to a cavity with an inner liquid core surrounded by a polymeric membrane. Nanoparticles have a larger surface area, increased solubility, enhanced bioavailability and improved controlled release [25]. Freeze-drying and spraydrying are the most frequently employed methods for removing water from foods with encapsulation effects [28]. A schematic view of these methods is presented in Figure 2. The wide range of polyphenol biological activities can be restricted due to their low stability, low bioavailability, and unpleasant flavor [14]. Encapsulation of polyphenols improves their stability during storage and can also be used to achieve the masking of Figure 2. Schematic view of freeze-drying and spray-drying process, characteristics of particles and advantages and disadvantages of methods (adapted from Fang and Bhandari, [25]; Grgić et al., [26]).
The wide range of polyphenol biological activities can be restricted due to their low stability, low bioavailability, and unpleasant flavor [14]. Encapsulation of polyphenols improves their stability during storage and can also be used to achieve the masking of unpleasant flavors in foods (such as bitter taste and astringency) [2]. Food with high intensity of bitterness and astringency elicit negative consumer reactions. Compounds responsible for that are flavanols and flavonols. Flavan-3-ol monomers such as catechin, epicatechin, epigallocatechin, epicatechin gallate and epigallocatechin gallate as well as their oligomers proanthocyanidins (condensed tannins) are abundant in wine and tea. It can be said that flavanols are the main compounds that cause bitterness and astringency in tea and red wine (with the exception of caffeine in tea) [29]. The method selected for encapsulation must be based on the polyphenol's characteristics such as chemical structure, thermophysical stability, solubility, affinity to coating material, target properties such as particle size and morphology, among others [14].

Freeze-Drying
Freeze-drying, also known as lyophilization, is a drying technique based on the phenomenon of sublimation. This technique allows the long-term preservation of heatsensitive and oxidation-prone compounds, as well as foods and other biological materials, since it is conducted at low temperatures and under vacuum [30]. This method does have disadvantages such as higher unit cost and processing time [17]. Despite those issues, freeze-drying is widely used to obtain high-value food products and is considered a standard method for encapsulation in most research studies [30].
The freeze-drying process includes the complete freezing of samples, ice sublimation (primary drying) and desorption of remaining unfrozen/bond water (secondary drying). In the first step, the freezing rate determines the formation and size of ice crystals. Large ice crystals formed by the slow rate of freezing can sublimate easily and increase the primary drying rate. In primary drying, the shelf temperature will increase using a vacuum to start the sublimation. It is necessary that product temperature is 2-3 • C below the temperature level of collapse, at which the product can lose its macroscopic structure. The endpoint of the primary drying phase is a key parameter to determine because the increased temperature (in the secondary drying) before the sublimation of all ice could collapse the final product quality [30].

Spray-Drying
In the food industry spray-drying is the most frequently used technique. It is economical, flexible and can be used continuously with easy scale-up [13]. Advantages over the other methods are higher effectiveness and shorter drying time [16]. This technique is based on transforming a material from liquid form into powder form [24]. However, when spray-drying is used for polyphenol encapsulation, conditions must be optimized in order to avoid an accelerated degradation [17]. Depending on process conditions and formulations, spray-drying micrometric capsules can be core-and-shell, multiple core or matrix type [14]. Before spray-drying, a solution or suspension of polyphenols with carriers must be obtained, followed by atomizing into a hot air stream.

Particle Morphology
Spray-dried particles are usually spherical with varying diameters and concavities, regardless of the choice of coating material [17]. Mean size range of such particles range from 10 µm to 100 µm [25]. The formation of concavities is associated with shrinkage of the particles due to dramatic loss of moisture after cooling. On the other hand, powders obtained by freeze-drying have a flake-like structure or one that resemble broken glass. The reason could be the low temperature of the process which results in the absence of forces to break the frozen liquid into droplets. Differences in surface morphology of freeze-dried powders could be due to different coating agents. For example, powder with soybean protein and maltodextrin as coating materials had a spherical porous structure, while powders with only maltodextrin lost their porous structure [17]. The final particle size of freeze-dried powders depends on the grinding procedure, not on the drying process [31].

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-, βand 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].

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. By increasing inlet air temperature from 145 • C to 155 • C, a 4% increase in total polyphenols content occurred, but a further increase in inlet temperature had a negative effect; an increase in the concentration of carriers resulted in a decrease in total polyphenols content (18.96 g/100 g-without carrier, 7.61 g/100 g-for 3 g/100 g of carrier, and 5.43 g/100 g-for 5 g/100 g of carrier)

Observations
Optimal conditions for encapsulation with maltodextrin were 3:1 ratio of core/coating material and 140 • C inlet temperature while for inulin the optimal ratio was 3:1 for encapsulation of pulp extract Higher encapsulation efficiency (around 42%) of total polyphenols was obtained when higher amounts of protein (4%) were used, pointing to the important impact of protein in that target delivery system; encapsulation efficiency with 2% of whey protein isolates was around 28%; type of maltodextrin showed no significant impact on encapsulation efficiency   By increasing the amount of fiber above 1%, a decrease in the concentration of adsorbed polyphenols occurred; complexes with higher amounts of fiber (2% and 4%) had higher retention levels of polyphenols after eight months' storage (70% and 79%, respectively) Raspberry juice polyphenols [72] Carrier Cellulose

Observations
The complex with 2.5% of cellulose resulted in the highest concentration of polyphenols (2.43 g/kg for 15 min of complexation and 1.96 g/kg for 60 min of complexation); higher amounts of the carrier (5%, 7.5% and 10%) negatively affected polyphenols adsorption; the highest retention of polyphenols during storage was observed in powders with 5% and 7.5% of cellulose (from 90-100%) Table 3. Studies of spray-drying (SD) and freeze-drying (FD) applications for encapsulation of polyphenols from different sources.

Observations
The freeze-dried sample with 100% konjac had the highest antioxidant content but with low encapsulation efficiency of 43.6% (anthocyanins located on the surface); a mixture of maltodextrin and gum Arabic provided powders with high antioxidant content and efficiency for both methods of drying (around 95%) Table 3. Cont.

Observations
The higher concentrations of gallic acid in freeze-dried samples were achieved with close to 100% gum Arabic and encapsulant concentration of 10-20% and with maltodextrin concentration of 80-100%; for spray-dried samples the best conditions also included 10-20% encapsulant concentration and maltodextrin:gum Arabic ratio of 50-80% Lemon by-product aqueous extract [17] Carrier Maltodextrin, soybean protein and [16] FD: Not defined

Observations
The freeze-dried sample with 100% konjac had the highest antioxidant content but with low encapsulation efficiency of 43.6% (anthocyanins located on the surface); a mixture of maltodextrin and gum Arabic provided powders with high antioxidant content and efficiency for both methods of drying (around 95%) Grape (Vitis labrusca var. Bordo) skin phenolic extract [73] Carrier Gum Arabic, polydextrose, and partially hydrolyzed guar gum Conditions SD: 0.60 L/h-flow rate; 140 °C-drying air temperature; 3.5 kg/cm 2 -air pressure; 40.5 L/h-air flow rate FD: Dispersions were frozen at −68 °C for 24 h and then freeze-dried at −57 °C for 48 h at vacuum pressure of less than 20 µm Hg

Observations
Between these drying methods, there was no difference in polyphenol retention; the spray-drying method using 10% gum Arabic had the highest retention of phenolic compounds (25.03 mg GAE/g) as well as freeze-drying with 5% gum Arabic and 5% of polydextrose (24.57 mg GAE/g) Coffee grounds polyphenols extract [2] Carrier Maltodextrin and gum Arabic Conditions SD: 108 mL/h-flow rate; 100 °C-air inlet temperature; 75% (28 m 3 /h)-aspiration FD: The samples were previously frozen and then freeze-dried at −60 °C at 0.05 bar for 48 h

Observations
The higher concentrations of gallic acid in freeze-dried samples were achieved with close to 100% gum Arabic and encapsulant concentration of 10-20% and with maltodextrin concentration of 80-100%; for spray-dried samples the best conditions also included 10-20% encapsulant concentration and maltodextrin:gum Arabic ratio of 50-80% Lemon by-product aqueous extract [17] Carrier Maltodextrin, soybean protein and ȷ-carrageenan

Observations
Freeze-dried samples obtained with a combination of maltodextrin and soybean protein achieved the highest encapsulation productivity of total polyphenols content and total flavonoids content (74%); in spray-drying, the best encapsulation productivity for total polyphenols content (67%) was achieved with the same combination of wall materials as for freeze-drying, while for encapsulation productivity of total flavonoids content (58%), no statistical difference was observed between wall materials Acerola (Malpighia emarginata DC) pulp and residue [74] Carrier Gum Arabic and maltodextrin mixture Microencapsulation efficiency for total polyphenols content of freeze-dried samples of both pulp and residue was higher (around 68%) than for spray-dried samples; for microencapsulation efficiency of total flavonoids content, the best results (around 59%) were obtained for freeze-dried acerola residue Table 3. Cont.

Bioactive Material Reference
Star fruit (Averrhoa carambola) pomace polyphenols [41] Carrier Maltodextrin Powders obtained with sugar free cranberry juice and without maltodextrin had almost five times higher contents of polyphenols (6423 mg/kg-FD and 6433 mg/kg-SD) than powders with 15% maltodextrin and cranberry juice (961 mg/kg-FD and 807 mg/kg-SD); in samples with maltodextrin, p-coumaroyl-hexose concentration was higher when spray-drying was applied (180 mg/kg); considering anthocyanins, spray-and freeze-drying equally affected their retention Spray-dried microparticles with maltodextrin and hydroxypropyl-β-cyclodextrin microcapsules had higher total polyphenols content and total anthocyanins content (around 1.65 g/100 g and 1.3 g/100 g, respectively) than freeze-dried microparticles with β-cyclodextrin (around 1.45 g/100 g and 1.1 g/100 g, respectively); total losses of anthocyanins and total polyphenols during drying were lower in the freeze-drying process Apple peel polyphenols [77] Carrier Maltodextrin, gum Arabic, and whey protein concentrate

Application of Encapsulated Polyphenols in Food Products
Due to the increasing awareness of consumers toward health and the consumption of food that promotes health, the enrichment of food products with encapsulated polyphenols and replacement of artificial food additives with natural ones is strongly supported [78]. Over the last few years, applying encapsulated polyphenols in food products has been on the rise. In reviewing scientific papers, those dealing with encapsulated polyphenols with polysaccharides were singled out.
Yogurt has high water content and a low pH value which makes it challenging to incorporate polyphenols with poor solubility. Encapsulated polyphenols into hydrophilic wall materials can overcome these shortcomings [78]. Robert et al. [37] encapsulated polyphenols from pomegranate with maltodextrin and soybean protein isolates and incorporated them in yogurt. Encapsulates with maltodextrin had the lower degradation rate during storage. Moreover, mushroom extract rich in polyphenols, encapsulated with maltodextrin crosslinked with citric acid, was incorporated in yogurt [79]. In a study of El-Messery et al. [77], polyphenols extracted from apple peel were encapsulated with maltodextrin, whey protein and gum Arabic using spray-drying and freeze-drying. The obtained powders were used in supplementing yogurt. Results showed no significant influence of powders on the physiochemical and texture properties of samples. The authors suggested that those encapsulated polyphenols can be used as a functional food ingredient for yogurt. One interesting study dealt with encapsulation of eugenol-rich clove extract in maltodextrin and gum Arabic by spray-drying. These encapsulates were incorporated into soybean oil for increasing antioxidant activity. Potatoes fried in that oil had better sensorial properties than ones fried in oil with butylated hydroxytoluene (synthetic antioxidant) [80].
Encapsulated polyphenols can also be incorporated into bread. Ezhilarasi et al. [81] enriched bread with encapsulated Garcinia fruit polyphenols with maltodextrin and whey protein isolates. Furthermore, green tea polyphenols, encapsulated using β-cyclodextrin and maltodextrin by freeze-drying and spray-drying, were added to bread. Bread quality (volume and crumb firmness) didn't change compared to the control sample [82]. Furthermore, anthocyanins from red onion skins were encapsulated using gum Arabic, soy protein isolate and carboxymethyl cellulose as wall materials, and applying the gelation and freeze-drying techniques. The powder, which has the highest encapsulation efficiency, has been added to crackers and results showed improved antioxidant activity of these enriched crackers. The authors suggest the suitability of applying such additives in bakery products [83]. Table 4 presents some other studies that dealt with incorporating encapsulated polyphenols with polysaccharides using freeze-drying and spray-drying techniques into food products. Table 4. Selected studies on the incorporation of encapsulated polyphenols into food products. Gum Arabic/maltodextrin was the best wall material; jelly with 7% of powder showed better consumer acceptability than commercial jelly; antioxidant activity was increased [88]

Conclusions
Bioactive compounds, such as polyphenols attract a lot of attention from scientists, functional food product developers and consumers due to their health-promoting effects. Most of these compounds are chemically unstable and encapsulation techniques have been widely applied in order to enhance their stability. Nevertheless, freeze-drying is still assumed to be the most suitable for heat-sensitive compounds. The adequate method of encapsulation depends on the type of polyphenol and material for encapsulation, however. Therefore, it cannot be generally said that freeze-drying is better than spray-drying or vice versa. Choosing a polyphenol carrier is important in order to achieve effective encapsulation. While freeze-drying will definitely result in a higher quality and better bioactivity retention compared with conventional spray drying methods, its application is excessively expensive, time consuming and limited in throughput capacity for most commercial applications. Therefore, in order to increase the number and variety of products on the consumer markets, as well as the application range of encapsulated polyphenols, new technologies will need to be researched and if necessary developed. This paper contributes to the insights into previously researched and optimized encapsulating conditions for a large number of polyphenol-rich materials.

Conflicts of Interest:
The authors declare no conflict of interest.