Physicochemical and control releasing properties of date pit (Phoenix dactylifera L.) phenolic compounds microencapsulated through fluidized‐bed method

Abstract This study aimed to investigate the effect of different ethanol/water solvents on phenolic compound extraction and microencapsulated extract of date pit powder. The highest and the lowest amounts of total phenolic compounds were 742.37 and 236.07 mg GAE/g dm, respectively, observed in water–ethanol composite solvent (25% W: 75% E) and water solvent (100% W). In this regard, the highest and lowest values of IC50 were 6.83 and 0.90 μg/ml, measured in water solvent (100% W) and water–ethanol solvent (25% W: 75% E), respectively. In the second phase, using maltodextrin (10%, 20%, and 30% W/V) as the first layer, date pit extract was microencapsulated. Alhagi maurorum gum (10%, 20%, and 30% W/V) as the second layer and medium‐chain triglycerides (MCT oil) (15% W/W) as the third layer were used by a fluidized‐bed drying technique. By increasing temperature, the microencapsulated extract powder solubility was increased as well. In contrast, the moisture content, bulk density, tapped density, and compressibility index decreased. By increasing temperature, the maltodextrin and A. maurorum gum concentration, the coating efficiency, and the loading capacity of the samples increased initially and decreased eventually. Moisture content, powder solubility, bulk density, and compressibility index increased, with increasing maltodextrin concentration, however, tapped density decreased. The optimal physicochemical properties of the phenolic compounds’ microcapsules were determined at 45°C and at a concentration of 20% of each of the maltodextrin and A. maurorum gum. According to scanning electron images, the powder particles were spherical and had a relatively smooth surface. Notably, the release rate of phenolic compounds reached its maximum (64%) after 24 h.


| INTRODUC TI ON
Date trees (Phoenix dactylifera L.) are widely cultivated in arid and semiarid environments. They are considered an important crop in North Africa and the Middle Eastern region (Chao & Krueger, 2007).
The industrial processing of this fruit results in the rejection of considerable quantities of waste, represented mainly by the pits (Messaoudi et al., 2013). Many practices have been established already to valorize this agroindustrial by-product, mainly as a precursor to producing activated carbon or used in animal feed.
Antioxidant activity is closely related to the phenolic content of plants (Awulachew, 2021;Oladzad et al., 2021). In recent years, great interest has been focused on using natural antioxidants in food products since the studies have indicated that possible adverse effects may have been related to the consumption of synthetic antioxidants (Hasheminya & Dehghannya, 2020;Lourenço et al., 2019;Negi, 2012). Phenolic compounds are unstable and sensitive to high temperature, light, pH, and oxidative and degradative enzymes, which affect the phenolic profile of extracts. Therefore, finding a strategy to protect phenolic compounds and preserve their biological activities and properties seems vital. Encapsulation is a common technique for creating an external membrane or coating of one material over another material and provides both stabilization and a controlled release of the entrapped materials, resulting in the conversion of volatile liquid into stable solid-encapsulated products, protection of the active compounds against environmental factors (such as oxygen, light, moisture, and pH), reduction in flammability, and increasing water dispersibility, thereby, improving handling ease, safety, and applicability to various products.
The coating enables the controlled release of compounds at the targeted site and specific rate (Budinčić et al., 2021;Li et al., 2020;Martínez-Ballesta et al., 2018). In the case of antioxidants, the active agent should be released slowly and continuously at an optimal threshold in the food by encapsulation.
The release kinetics of the active elements within the microcapsules depends directly on the processes and formulation parameters which are specifically designed to release components when subjected to certain parameters (Hameed et al., 2020). The active ingredient can be released through two methods; forced and controlled release. The forced release is obtained by rupturing the microcapsule membrane under thermal and/or mechanical conditions, such as friction. The controlled release is based on the diffusion of the encapsulated active element through the membrane or its degradation. Various processes, such as extrusion, coacervation, cocrystallization, lyophilization, spray drying, and fluidized-bed coating, are employed to encapsulate and coat food ingredients or additives, in fluidized-bed encapsulation, while the core material particles are suspended, the wall material is atomized into the chamber, depositing on the core. When the particles reach the top of the ascending column, they are released into a descending column of air that releases them back into the fluidized bed, where they are again coated, dried, and hardened, ensuring a uniform coating. The simplest encapsulation technique consists in making soluble ingredients in a solution containing the wall material, followed by spray drying. There are many published works on spray-drying encapsulation of phenolic extracts from plant sources, such as pomegranate peel (Šavikin et al., 2021), berries (Etzbach et al., 2020), bay leaves (Medina-Torres et al., 2016), carrots (Wagner & Warthesen, 1995), olive leaves (Dobrinčić et al., 2020), sour cherry pomace (Başyiğit et al., 2020), soybean (Poomkokrak et al., 2015), and date pit oil (El-Massry et al., 2019).
According to studies, no attention has been paid to date fruit wastes, such as pit, which is a rich source of phenolic compounds.
Microencapsulation of extracts containing phenolic compounds, as a natural antioxidant, is one of the approaches to prevent oxidation and extend its shelf life.
This study aimed to investigate microencapsulation and controlled release of phenolic compounds extracted from date pit produced by the fluidized-bed method. It should be noted that using the fluidized-bed method for encapsulation and other ingredients, such as MCT oil, Alhagi maurorum gum, and maltodextrin in three layered coating on phenolic compounds, are innovative aspects of this study.

| Sample preparation
Date pits were cleaned and washed with distilled water. After washing, they were dried in a vacuum oven (Memmert VO400, Germany) at 70°C until the moisture content reached 5%, and by using an industrial milling machine (Model S-G5 500 Swantek, made in Germany) were milled. The powder was passed through a sieve with a diameter of 710 microns (mesh 25) and stored in polyethylene bags at 4 ± 2°C.

| Extraction of phenolic compounds
Water (100%) and ethanol (100%), as well as water-ethanol (50/50, 35/65, 25/75 v/v), were used as extraction solvents due to Moradi et al. (2022) method with some modifications. Ten gram of date pit powder was poured into Erlenmeyer containing 100 ml of the desired solvents and stirred for 5 h at 25°C in an incubator with a shaker (PIT053RS, Iran) at 280 rpm. Then, the suspension was filtered by Büchner funnel and Whatman filter paper (≠4). All samples were kept in the refrigerator at 4°C until the tests were performed.

| Measurement of total phenolic compounds
The total phenolic compounds were measured using the colorimetric method and the Folin-Ciocalteu reagent. One milliliter of the extract was mixed with 2.5 ml of 10% folic acid reagent diluted in water.
After 8 min, 5 ml of 7.5% sodium carbonate was added, and then the volume of the solution was increased to 50 ml with distilled water.
The resulting mixture was kept in the dark condition for 30 min, and finally, the absorbance of the samples was measured using spectrophotometry at 765 nm. The total amount of phenolic compounds was stated as mg/g dry weight of the extract using the line equation drawn for Gallic acid (Selahvarzi et al., 2022).
The DPPH radical scavenging activity was measured according to the methodology described by Kashfi et al. (2020). One milligram of date pit extract, as well as TBHQ synthetic antioxidant compounds, was prepared in pure ethanol solvent. Then, 0.3 ml of the prepared solution was mixed with 2.7 ml of pure ethanol solution containing DPPH reagent (concentration 0.1 mM). The stirred mixture was kept in the dark for 60 min. Subsequently, the adsorption was measured at 517 nm using a spectrophotometer (Bruker Optik, Ettlingen, Germany), and the percentage of the radical reduction in DPPH free radicals was calculated using Equation (1) (Kashfi et al., 2015).
where A o = sample absorption at time zero and A s = sample absorption after 60 min.
where M is the moisture content, W 1 is the weight of the empty container (g), W 2 is the total weight of the powder and the container (g), and W 3 is the total weight of the dried powder and the container (g) after putting in the oven.

| Solubility of powder
One gram of the produced powder was poured into 100 ml of distilled water to determine the solubility. The sample was put in a centrifuge (Sigma 2-16P, Germany) at 112 RCF for 10 min to separate the insoluble parts. The supernatant separated from the centrifuge tube was poured into a fixed-weight glass container and dried at 105°C until reaching a constant weight. The solubility percentage of the powder was determined from the difference between the weight of primary and secondary dry matter (Goula & Adamopoulos, 2010).

| Bulk density measurement
Five gram of the produced powder was poured into a 10 ml graduated cylinder, and the cylinder was shaken slightly to smooth the surface of the powder inside the cylinder. Then, according to Equation (3), the bulk density was obtained from the ratio of powder mass to the volume occupied in the cylinder (Goula & Adamopoulos, 2010).
where m is the mass of the powder (g) and V is the sample volume in ml.

| Tapped density measurement
After determining the bulk density, continuous taps were applied to the graduated cylinder until the powder volume changes stopped in the cylinder to obtain the tapped density. Finally, the ratio of powder mass to volume was calculated, and tapped density was obtained (Etzbach et al., 2020).
where HR is the Hausner ratio, ρ t is the density of the impact mass ( g cm 3 ), and ρ b is the density of the mass ( g cm 3 ).
CI is the compressibility index.

| Measurement of encapsulation efficiency (EE)
First, the calibration curve was plotted with different amounts of gallic acid in mg. Using an Amicon filter, the encapsulated date pit extract was separated from the free extract. One milliliter of the free extract was mixed with 1 ml of 2% sodium carbonate solution and 200 μl of Folin-Ciocalteu reagent and centrifuged for 5 min at 1200 rpm. After being at room temperature for 30 min, the absorbance of the samples was measured by a spectrophotometer at 765 nm.
Then, inserting the results in the calibration curve, the amount of total phenol in the free extract was determined in mg, gallic acid, and according to Equation (6) (Arulmozhi et al., 2013).
where C e is the amount of encapsulated phenolic compounds, and C t is the amount of total phenolic compounds.
where LC is the loading capacity (%), C e is the amount of encapsulated phenolic compounds (%), and C is the weight of the loaded particles.

| Measurement of the release rate (Rr) for 24 h
The release rate of finely microencapsulated phenolic compounds was measured based on the Folin-Ciocalteu method. For this purpose, at certain time intervals (1, 2, 3, 4, 6, 8, 10, and 24 h), 3 g of the capsule was mixed with 3 g of phosphate buffer (pH 7), and the resulting solution was centrifuged at 4500 rpm at room temperature for 90 min Then, the supernatant was collected. The total amount of phenolic compounds was measured and stated in terms of gallic acid according to Equation (8) (Esfanjani et al., 2015).
where Rr is the release rate (%), RPC is the percentage of released phenolic compounds, and TPC is the percentage of total phenolic compounds.

| Morphological study of microcapsules
Scanning electron microscopy (MIRA\\LMU, TESCAN Co, The Czech Republic) was used to examine the surface morphology (Carneiro et al., 2013).

| Phenolic compounds
The results of comparing the average data obtained from different solvents on the total phenolic compounds and their antioxidant activity are shown in Table 1.  (Antolovich et al., 2000).
Since applying water (alone) creates a thoroughly polar environment and some phenolic compounds with a low degree of polarity are less extracted, fewer amounts of phenolic compounds extracts are extracted by water (100% W). In addition, aqueous extracts contain impurities such as organic acids, proteins, and soluble sugars that can interfere with the detection and quantification of phenolic compounds (Chirinos et al., 2007). But ethanol solution with water has a more prominent ability to extract phenolic compounds since they extract both polar and nonpolar compounds. However, the higher amount of ethanol than water in the solution (25% W: 75% E) has a more significant effect on the extraction of phenolic compounds since increasing the amount of ethanol reduces the dielectric constant of the solution, which in turn reduces the energy required for separation of solvent molecules. Thus, solute molecules are more simply placed between solvent molecules and dissolved (Pompeu et al., 2009). Similar results have been reported in the extraction of phenolic compounds from blackberry (Cacace & Mazza, 2003) and pomegranate peel (Wissam et al., 2012).  extraction of antioxidant compounds increased. The scavenging power of different extracts depends on the number and position of hydroxyl groups and the molecular weight of phenolic compounds (Pompeu et al., 2009). In lower-molecular-weight phenolic compounds, hydroxyl groups are more readily available. In addition, after hydrogen donation, phenolic compounds are converted into phenoxyl free radicals. The stability degree of these radicals can affect the antioxidant capacity of phenolic compounds, as less stable phenoxy radicals compete with DPPH radicals for the adsorption of hydrogen atoms, and therefore the trapping percentage of DPPH radicals is reduced (Chirinos et al., 2007). According to the results, each compound with more phenolic compounds showed more scavenging power. Other studies have shown a direct relationship between phenolic content and antioxidant capacity (Jimoh et al., 2010). According to some other studies, since a number of phenolic compounds have antiradical power and, in some cases, other substances also have antiradical properties, the relationship between phenolic compounds and free radical scavenging power is not necessarily direct (Chandini et al., 2008).

| Evaluation of physicochemical properties of microcapsules
The levels of independent variables in the real and coded form are presented in Table 2. On the other hand, the analysis of the estimated regression coefficients in the second-order polynomial model for the response variables is presented in Table 3. 3.3.1 | Evaluating the moisture content According to Table 3, the effect of temperature (β 1 ), maltodextrin concentration (β 3 ), the second-order mutual effect of temperature (β 11 ) (p < .01), the mutual effect of temperature and maltodextrin concentration (β 13 ), and the second-order mutual effect of maltodextrin concentration (β 33 ) (p < .05) was significant on the moisture content of microcapsules. The high coefficient of determination (R 2 ) and the amounts of F-value and p-value indicate that the proposed model has an optimal fit to determine the moisture content of the samples.
The physicochemical stability of powders during storage depends on the moisture content of the powder. To increase the storage time, the moisture content of the samples should be <4%-5% (Sarabandi & Sadeghi Mahoonak, 2016). The low moisture content of the powder prevents the degradation of the microcapsule active compounds in the powder structure. As Figure 2 shows, the moisture content of the powders decreased by increasing the temperature of the dryer inlet air. The result is that increasing the temperature difference between atomized particles and the drying medium increases the simultaneous transfer rate of mass and energy, which results in more moisture released from the powder particles (Oberoi & Sogi, 2015;Santhalakshmy et al., 2015). Increasing the concentration of maltodextrin, the moisture content of the powder samples increased.

| Measurement of the solubility percentage
Based on Table 3, the effect of temperature (β 1 ), maltodextrin concentration (β 3 ), A. maurorum concentration (β 4 ), and the mutual effect of temperature with maltodextrin (β 13 ) as well as temperature with A. maurorum gum (β 14 ) on the solubility of microcapsules is significant (p < .01). The high coefficient of determination (R 2 ) and the amount of F-value and p-value indicate that the proposed model has an optimal fit to determine the solubility of the samples. The solubility of the powder is an important property that affects its behavior when dissolved in water. Factors such as size, shape, composition, surface properties, type and composition of raw material, type of feed (solid concentration), and drying conditions affect the solubility of powders. Powder rehydration in the water process is divided into four stages: wetting, immersion, dispersion, and dissolution (Kim et al., 2005). According to Figure 3a,b, with rising inlet air temperature in the dryer chamber, the solubility of powders increased due to the decrease in moisture content (Santhalakshmy et al., 2015). Walton (2000) showed that the particle size increased as the inlet air temperature increased. Consequently, the required time for re-dewatering was reduced. Increasing the inlet air temperature increases the apparent porosity and thus improves the susceptibility of the particles. However, a study by Quek et al. (2007) showed that increasing the dryer temperature reduces the solubil-

TA B L E 2 Independent variables and their values
Code and levels It is also possible to increase the solubility of powders by increasing the size and space between particles and subsequently facilitating the penetration of moisture into the structure of powders, which occurs due to increasing the concentration of maltodextrin (Sarabandi & Sadeghi Mahoonak, 2016

| Bulk density measurement
Based on Table 3, temperature (β1), maltodextrin concentration (β3), and the mutual effect of temperature with maltodextrin (β13) have a significant impact on the bulk density of microcapsules (p < .01). The high coefficient of determination (R 2 ) and the amount of F-value and p-value indicate the optimal fit of the proposed model. Bulk density depends on the size and shape of the particle, moisture, chemical composition, and the amount of air trapped inside the particle.
These factors depend on feed characteristics, inlet air volume, temperature and drying time, processing, and transportation operations (Goula & Adamopoulos, 2010). According to Figure 4, the density of the mass decreased with increasing inlet air temperature.
Because of increasing temperature, the rate of moisture evaporation increases. As a result, the size of the particles becomes larger, and their weight decreases. Also, more spherical particles are produced (Goula & Adamopoulos, 2010). Increasing maltodextrin concentration, the bulk density increased due to the higher bulk mass due to the moisture (Sarabandi & Sadeghi Mahoonak, 2016). The particles tend to stick together with increasing humidity. Thus, the space between the particles decreases, and a more significant amount of powder occupies a certain volume of space, which can also be a reason for increasing bulk density (Goula & Adamopoulos, 2010;Zendeboodi et al., 2018).

| Tapped density
According to Table 3

TA B L E 3 (Continued)
as a result, the particles become lighter, and the density decreases (Goula & Adamopoulos, 2010;Zendeboodi et al., 2018). The decrement of tapped density with increasing maltodextrin concentration is due to the particular properties of maltodextrin, which reduces the adhesion between particles (Peighambardoust & Sarabandi, 2017).
Increasing the concentration of maltodextrin and A. maurorum gum increases water absorption because the powder particles become larger and more spherical, and the space between the particles is slightly filled with air. This process is an effective factor in reducing tapped density. As mentioned, particle size is a factor influencing tapped density. If the percentage of coarse particles in the powder increases, the volume does not change much due to the tapping.
Thus, the tapped density decreases. Increasing the concentration of carriers and date pit extract leads to producing larger droplets and eventually larger dried particles in the drying chamber (Goula & Adamopoulos, 2010;Kha et al., 2010).

| Compressibility index
According to Table 3 values of coefficient of determination (R 2 ), F-value, and p-value indicate that the proposed model has an optimal fit. Compressibility is one of the most essential and influential properties of powder handling and final processing. It is a function of the powder's physical properties, such as particle size, shape, surface structure, particle density, bulk density, moisture content, temperature, pressure, and fat (Kim et al., 2005). According to Figure 6a,b, with increasing temperature and concentration of date pit extract, the compressibility index of the samples decreased because of increasing temperature, and the formation of liquid relations between the powder particles decreased.
Furthermore, increasing the concentration of the extract, the particle size and the empty spaces between them increase (Peighambardoust & Sarabandi, 2017). On the other hand, this index increased with increasing concentrations of maltodextrin and A. maurorum gum. The result is that increasing the moisture softens and plasticizes the powder components, especially water-soluble components, which causes deformation and provides a higher contact surface (Kim et al., 2005). 3.3.6 | Evaluating encapsulation efficiency According to Table 3 is significant on the encapsulation efficiency of microcapsules (p < .01). According to Figure 7b, the total phenolic compounds increased with increasing inlet air temperature to 42°C and then decreased; by increasing the temperature to 42°C, capsule formation was done well and more extracts were coated. But a further increase in temperature could destroy the structure of the phenolic compound due to enzymatic decomposition or thermal degradation (Akbarbaglu et al., 2018). The study of Goula and Adamopoulos (2010) showed that by increasing the air temperature entering the dryer, the amount of lycopene in tomato powder decreases. Similar results have been observed in the research of other researchers (Goula & Adamopoulos, 2010). Additionally, according to Figure 7a 3.3.7 | Evaluating loading capacity According to Table 3, temperature (β 1 ), the concentration of A.
maurorum gum (β 4 ), the mutual effect of temperature with the concentration of maltodextrin (β 13 ), the mutual effect of temperature with the concentration of A. maurorum gum (β 14 ), the mutual effect of the second order of temperature (β 11 ), and the mutual effect of the second order of the A. maurorum gum concentration (β 44 ) significantly affected the loading capacity of microcapsules (p < .01).
Loading capacity is the capacity of carriers to store and then release active compounds. This feature is affected by the capsule preparation process, sample volume, temperature, and surface properties of the sample. According to Figure 8a,b, the loading capacity increased with increasing inlet air temperature to 42°C and then decreased; by increasing the temperature to 42°C, capsule formation was well done, and more extracts were coated. But a further increase in temperature could destroy the structure of the phenolic compound due to enzymatic decomposition or thermal degradation (Akbarbaglu et al., 2018). Besides, the loading capacity increased with increasing concentrations of maltodextrin and A. maurorum gum (up to 20%) and then decreased. The reason for increasing phenolic compounds is the protective effect of these coatings at high temperatures, in contrast, the increase in nonphenolic carriers (coatings) mass is the reason for the phenolic compounds decrement (Moser et al., 2017;Tolun et al., 2016).

| Release rate
The release of phenolic compounds from date pit extract was investigated at intervals of 1, 2, 3, 4, 6, 8, 10, and 24 h. According to

| CON CLUS IONS
In this study, the effect of different solvents on the total phenolic compound extracted from date pit extract and the physicochemical properties of microcapsules produced at different temperatures and concentrations of maltodextrin and A. maurorum gum as a coating by fluidized-bed dryer were investigated. According to the results, the type of solvents and their polarity in the extraction of phenolic compounds significantly affected the extraction efficiency. In this regard, the highest and lowest amount of total phenolic compound was extracted in a water-ethanol mixed solvent (25% W: 75% E) and water (100% W). The ethanol and water mixture had a more impressive ability to extract phenolic compounds than any solvents alone and this effect was more significant in higher percentages of ethanol. On the other hand, a direct relationship between free radical inhibition and the amount of total phenolic compound was observed.
The highest value of IC 50 was observed in a water solvent (100% W), and the lowest value was observed in a water-ethanol mixed solvent (25% W: 75% E).
Notably, in solvents containing 75% ethanol, the IC 50 reached its lowest value, and the extraction of antioxidant compounds increased. Also, all the physicochemical properties of microcapsulesrelated tests, including moisture, bulk density, loading capacity, F I G U R E 1 0 SEM micrographs of date pit phenolic compounds microencapsulated powders produced by maltodextrin (20 % W/V) as the first wall, Alhagi maurorum gum (20 % W/V) for the second wall, and MCT oil (15% W/W) as the third wall.
impact density, compressibility, and functionality, such as solubility and microencapsulation efficiency, were significantly dependent on process conditions, such as carrier concentrations and process temperature. Excessive increase in carrier ratio and process temperature due to adverse effects on the drying process and increase in viscosity caused the quantitative and qualitative loss of microcapsules.
The optimal physicochemical properties of the microcapsules were obtained at 45°C and 20% of each carrier (maltodextrin and tangerine). The release of phenolic compounds increased after 24 h due to the decrease in stability and cohesion of the biopolymer compounds used as coating material and the effect of environmental stresses, such as temperature and heat. The surface structure of particles and the SEM results also showed the surface properties and uniformity of the particles under the influence of the concentration of maltodextrin and A. maurorum gum and the process temperature. Overall, the research results show that the use of maltodextrin and A. maurorum gum carriers and a fluid bed dryer can be effective as a promising method in increasing the stability of microencapsulated date pit extract against environmental conditions. Also, microencapsulated compounds can be used in a wide range of bakery products, oil, etc., to improve their quality characteristics and extend their shelf life.

ACK N OWLED G M ENTS
The authors thank the Iranian Research Organization for Science & Technology (IROST) for technical and laboratory supports.

FU N D I N G I N FO R M ATI O N
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

CO N FLI C T O F I NTE R E S T
The authors declare that they do not have any conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
Research data are not shared.