Investigation of physicochemical and antibacterial properties of dill (Anethum graveolens L.) microencapsulated essential oil using fluidized bed method

Abstract

The present study delves into the encapsulation of dill essential oil utilizing the fluidized bed coating methodology. The investigation focused on the impact of essential oil concentration and the application of maltodextrin and arabic gum as the primary and secondary coating agents. The dominant compounds in the dill essential oil were identified as limonene (32.32%), carvone (35.43%), and cis-dihydrocarvone (5.43%). The antimicrobial potency of the dill essential oil was evaluated, demonstrating notable inhibition against Streptococcus mutans with inhibition zone diameters ranging from 5.4 mm to 16 mm for concentrations between 250 μg/mL and 2000 μg/mL. For Streptococcus sobrinus, the inhibition zones measured from 6.6 mm to 18 mm across the same concentration gradient. An increase in maltodextrin concentration was associated with a decrease in moisture content, bulk density, and tapped density, while it improved microencapsulation efficiency and loading capacity. In contrast, a higher concentration of arabic gum increased moisture content, loading capacity, and encapsulation efficiency, but reduced bulk density and tapped density. Elevating the essential oil concentration increased all physicochemical properties of the microcapsules, except for tapped density. The optimal conditions for microencapsulation involve using a 2000 ppm concentration of dill essential oil with 75% maltodextrin and 0.1% arabic gum as carrier agents. Scanning electron microscopy images indicated that the microcapsule particles were nearly spherical with a smooth, intact surface. The release rate of phenolic compounds in a simulated saliva environment reached its maximum at 98.32% after 20 min, showcasing an efficient release profile.

Highlights

• •The antibacterial efficacy of Dill Essential oil against Streptococcus sobrinus and Streptococcus Mutans was approved.
• •Release of phenolic compound in simulated saliva revealed a peak of 98.32%.
• •Scanning electron microscopy results exhibited spherical-shaped powder particles with a smooth surface.
• •Maltodextrin and arabic gum demonstrated potential for creating a coating.

1. Introduction

The rise of drug-resistant bacteria has increased interest in using natural antibacterial compounds to control pathogenic agents (Chahal et al., 2017). With growing concerns about the side effects of chemical compounds and drug resistance, novel approaches to incorporating herbal essential oils in food products have emerged. Additionally, there is a growing consumer demand for products with natural preservatives or no preservatives at all (Kaur et al., 2021). Essential oils, composed of volatile compounds produced by plants, are known for their antimicrobial properties. The antimicrobial effectiveness of essential oils can be attributed to a specific set of small terpenoids and phenolic compounds, namely thymol, carvacrol, eugenol, and several other compounds such as citral, cinnamaldehyde, menthol, rosmarinic acid, and linalool (Swamy et al., 2016). These compounds act on pathogenic bacteria through various mechanisms, including membrane disruption, inhibition of protein synthesis, and reduction of intracellular ATP (de Sousa et al., 2023; Saad et al., 2013).

Anethum graveolens L., or dill, is a notable source of essential oils, particularly rich in D-carvone and D-phellandrene. These compounds make up a significant portion of dill oil, however, since the aerial organs of the herb were used before the formation of the flower, carvone is not among the main compounds of essential oil (Huopalahti et al., 1985). Dill essential oil has diverse applications in the pharmaceutical, health, and food industries, recognized for its antioxidant, antimicrobial, and anti-inflammatory properties (Jeet & Baldi, 2021). However, the application of essential oils in food products is limited by their instability under environmental conditions such as high temperature and light, which can degrade these compounds (Bellumori et al., 2021).

Encapsulation involves coating or enclosing a wide range of compositions used in the food industry, such as essential oils, flavorings, microorganisms, minerals, and ingredients with inappropriate taste, inside a secondary substance (shell or wall) to prevent or delay the release of the beneficial substances until the specified time and place, or under appropriate circumstances (Lipin & Lipin, 2022). The fluidized bed method is an effective encapsulation technique, where solid particles are suspended in air and coated with a protective layer as the temperature decreases (Yun et al., 2021). The choice of wall materials is crucial, with polysaccharides being preferred over proteins in the fluidized bed encapsulation process due to their low viscosity, good solubility, emulsifying capability, and ability to effectively maintain volatile compounds (Hosseini et al., 2019). Wall materials such as maltodextrin and arabic gum are commonly used in this process due to their emulsifying abilities, low viscosity, and ability to protect volatile compounds (Hosseini et al., 2019). Maltodextrin, a hydrolyzed starch, offers high encapsulation efficiency and good oxidative stability, although it has a low emulsifying capacity, often necessitating its combination with other substances like arabic gum (Šturm et al., 2019). Arabic gum, derived from the Acacia tree, is a versatile biopolymer with excellent solubility, emulsifying properties, and the ability to retain volatile substances. Its hydrophilic carbohydrate chains prevent coagulation, while its hydrophobic protein segments enhance emulsification (Desplanques et al., 2012; Musa et al., 2018).

This study focuses on encapsulating dill seed essential oil using the fluidized bed method, aiming to improve the stability and controlled release of its active compounds. Additionally, the research will assess the antibacterial efficacy of these encapsulated oils against bacteria responsible for tooth decay. The findings are expected to advance the use of essential oil encapsulation in food and pharmaceutical products, particularly in applications like chewing gum, enhancing antimicrobial properties and health benefits.

2. Materials and methods

2.1. Materials

Pure dill essential oil was obtained from Zardband Pharmaceutical Company, Tehran, Iran, with a density of 0.898 g/cm³ at 25 °C, a refractive index of 1.487 at 20 °C, and a viscosity of 21.01 cp at 30 °C. Maltodextrin, arabic arabic gum, microcrystalline cellulose (MCC) (Avicel ph -20), reagents and standards, and chemical compounds and solvents were purchased from Sigma-Aldrich CO. United states. Moreover, the bacteria of the present study (Streptococcus mutans, PCTT No. 1683, and Streptococcus sobrinus, PCTT 1601) were provided in lyophilized form from the Industrial Microorganisms Collection Center of the Scientific and Iranian Research Organization for science and technology.

2.2. Methods

2.2.1. Identification of essential oil compositions

The essential oil compositions were identified by injecting 0.5 μL of essential oil, diluted in cyclohexane at a 1:4 ratio, into the gas chromatography device (Agilent 6890 A, California, United States) with HP-5 column (30 m length, 250 μm inner diameter, and 0.25 μm thickness of the stationary phase) connected to the mass spectrometer (Agilent 5975, California, United States). The column temperature program was set based on the described method. The initial temperature of the column was 40 °C and then increased to 200 °C at the rate of 5 deg./min. It remained at that temperature for 1 min and then increased at 10 deg./min rate until 250 °C. After 5 min, the temperature reached 300 °C at 25 deg./min rate. The carrier gas employed was helium, flowing at a rate of 1 mL/min, while the temperature of the injection chamber was maintained at 240 °C. A mass spectrometer with an ionization voltage of 70 eV was used in the present research. Identification of essential oil composition types was conducted by using the normal range of alkanes (C8-C24) and reaching their inhibition index (Kovats index) (KI), and comparison to it. The components of the essential oil were identified using NIST05 software, which compared the mass spectra of each component with those in the Wiley7n.1 GC/MS library (Izadi & Mirazi, 2020).

2.2.2. The microbial test of essential oil and microencapsulated essential oil

First, the bacteria were removed from the lyophilized status, cultivated to achieve a single colony, and dissolved in sterile distilled water, and its turbidity was compared to the 0.5 McFarland control. Then, some wells were made using the culture medium. The bottom of the wells was filled with 10 μL of culture medium to prevent any possible penetration of essential oils into the plate floor and cause any errors. After that, the bacteria were sampled by sterile swab and cultivated on the Muller Hilton Agar culture medium. Then, 50 μL of essential oils prepared at different dilutions were poured separately into the wells. One well was considered as the control well in each culture medium. Dimethyl sulfoxide (DMSO) solvent was applied to dilute the essential oil. The intended essential oil was diluted at 250, 500, 750, 1000, and 2000 ppm dilutions. The cultured petri dishes were placed in an incubator set at 38 °C for 24 h. Additionally, the diameters of the inhibition zones were recorded. The experimental procedures were conducted in triplicate for all tests (Afshari et al., 2023).

2.2.3. Microencapsulation of essential oil using the fluidized bed Method

2% of DMSO emulsifier was used for a better distribution of essential oil in the aquatic phase, more proper distribution on the MCC cores, and aiding the final release of the flavor in the mouth. The essential oil and emulsifier solution was homogenized for 2 min by Ultraturrax with 10,000 rpm. Maltodextrin was used as the first coat in 25%, 50%, and 75% concentrations. The neutral substance of MCC was used as the carrier. After the device temperature reached 50 °C, 10 g of MCC was poured into the coating fluidized bed device as the first step. After 5 min and as the second step, 10 mL of essential oil emulsion at a specified concentration (1000, 1500, and 2000 ppm) was injected into the device container. During the third step, after 15 min of drying, 10 mL of maltodextrin solution was added. After another 15 min, arabic gum was injected at 0.10%, 0.15%, and 0.20% concentrations as the second wall. The obtained powders from each test for each sample were collected and stored in a darkened and moisture-impermeable compartment at 4 °C (Zaghari et al., 2020).

2.2.4. Physicochemical tests of microcapsules

2.2.4.1. Measuring the moisture content

The initial moisture of microcapsules was determined by using the weight method (Bringas-Lantigua et al., 2011).

$$M\left(\%\right)=\frac{W_2-W_3}{W_2-W_1}\times 100$$ (1)

In which M represents Moisture Content (%), W₁ stands for the mass of the container when empty (g), W₂ signifies the combined mass of the powder and container (g), and W₃ denotes the complete mass of the desiccated powder and container post-exposure to the oven (g).

2.2.4.2. Measuring the bulk density

Into a 10 mL graduated cylinder, an amount of 2 g from the manufactured powder was deposited. The cylinder underwent a gentle shaking to achieve a leveled state of the powder within. (Mohammed et al., 2021).

$${\rho }_b=\frac{m}{v}$$ (2)

In which;

$m$- powder mass (g), $v$- sample volume (cm³).

2.2.4.3. Measuring the tapped density

To ascertain the tapped density, 2 g of powder were placed within a graduated cylinder with a volume of 10 mL. By subjecting the cylinder to consistent tapping (100 taps), the process continued until alterations in powder volume no longer occurred. Eventually, calculations were performed to establish the relationship between powder mass and volume, along with the determination of tapped density (Mohammed et al., 2021).

$${\rho }_t=\frac{m}{v}$$ (3)

In which;

$m$- powder mass (g), $v$- sample volume (cm³).

2.2.4.4. Encapsulation efficiency (EE)

The efficiency of essential oil encapsulation was determined based on the described method of Jafari et al. (2007) with some modifications. 1 g of powder with 20 mL ethanol was spilled into the falcon tube and stirred for 5 min by shaker at room temperature to determine the non-encapsulated surficial essential oil. Then, using Whatman no. 1 filter paper, the powder particles were separated from the solvent and used to determine the encapsulated essential oil. 20 mL distilled water was spilled into the falcon tube and stirred for 5 min by a particular shaker. Then, 10 mL ethanol was added to the sample and maintained in a steam bath at 45 °C for 20 min. During this time, stirring continued intermittently. Then, the falcon tube was cooled at room temperature. The falcon tube was centrifuged at 4000 rpm for 20 min to separate the aquatic phase. The efficiency was measured by the spectrophotometer device (A-160, Shimadzu, Japan).

$$\mathrm{EE}\left(\%\right)=\frac{C_t-C_e}{C_t}\times 100$$ (4)

In which;

$C_e$- encapsulated phenolic compounds, $C_t$- total phenolic compounds.

2.2.4.5. Loading capacity (LC)

The amount of loading phenolic compounds was calculated by eq. (5) (Afshari et al., 2023).

$$\mathit{LC}\left(\%\right)=\frac{C_e}{C_{\mathit{Weight\; of\; nanoparticle}}}\times 100$$ (5)

In which;

$\mathit{LC}$-loading capacity (%), $C_e$-the amount of capsulated phenolic compounds (%), $C_{\mathit{Weight\; of\; nanoparticle}}$- the weight of loaded particles (%).

2.2.4.6. The release rate (Rr)

In this study, a suspension of microcapsules containing the essential oil was poured into a dialysis bag and placed in a thermostatic bath at 37 °C on the magnetic stirrer with 100 rpm in simulated saliva solution at a pH of 6.8 to imitate the swallowing condition. The test was performed at 5, 10, 15, and 20 min. The Rr of essential oil at these times was measured by a spectrophotometer (A-160, Shimadzu, Japan) (Asprea et al., 2017).

$$\mathrm{Rr}\left(\%\right)=\frac{\mathit{RPC}}{\mathit{TPC}}\times 100$$ (6)

In which;

$\mathrm{Rr}$ - release rate (%), $\mathit{RPC}$-released phenolic compounds (%), $\mathit{TPC}$-total phenolic compounds (%).

2.2.4.7. Examining the morphology of microcapsules

Utilizing a scanning electron microscope (SEM), an examination of the external surfaces and surface morphology of the microencapsulated particles was conducted.

2.3. Statistical analysis

The physicochemical characteristics of microencapsulated essential oil powder were studied and optimized using the response surface methodology (RSM) and Design Expert 12 software.

In this regard, a central composite design with three independent variables, including maltodextrin concentration (in three levels), arabic gum concentration (in three levels), and dill essential oil concentration (in three levels), was applied to investigate their impact on dependent variables (response). In eq. (7), $Y$ is the predicted response, ${\mathrm{\beta }}_0$ is fixed coefficient, ${\beta }_1,{\beta }_2$, ${\beta }_3$, and ${\beta }_4$ linear effects, ${\beta }_{11}$، ${\beta }_{22}$، ${\beta }_{33}$ و ${\beta }_{44}$ are quadratic effects, and ${\beta }_{12},$ ${\beta }_{13}$, ${\beta }_{14}$, ${\beta }_{23}$, ${\beta }_{24}$, and ${\beta }_{34}$ are interaction effects.

$$\mathrm{Y}={\mathrm{\beta }}_0+{\beta }_1{\mathcal{x}}_1+{\beta }_2{\mathcal{x}}_2+{\beta }_3{\mathcal{x}}_3+{\beta }_4{\mathcal{x}}_4+{\mathrm{\beta }}_{11}{\mathcal{x}}_1^2+{\mathrm{\beta }}_{22}{\mathcal{x}}_2^2+{\mathrm{\beta }}_{33}{\mathcal{x}}_3^2+{\mathrm{\beta }}_{44}{\mathcal{x}}_4^2+{\mathrm{\beta }}_{12}{\mathcal{x}}_1{\mathcal{x}}_2+{\mathrm{\beta }}_{13}{\mathcal{x}}_1{\mathcal{x}}_3+{\mathrm{\beta }}_{14}{\mathcal{x}}_1{\mathcal{x}}_4+{\mathrm{\beta }}_{23}{\mathcal{x}}_2{\mathcal{x}}_3+{\mathrm{\beta }}_{24}{\mathcal{x}}_2{\mathcal{x}}_4+{\mathrm{\beta }}_{34}{\mathcal{x}}_3{\mathcal{x}}_4$$ (7)

In which;

Y- the predicted response, ${\mathrm{\beta }}_0$-fixed coefficient${\beta }_1$,${\beta }_2$, ${\beta }_3$,${\beta }_4$ - linear effects, ${\beta }_{11}$, ${\beta }_{22}$, ${\beta }_{33}$, ${\beta }_{44}$ - quadratic effects, ${\beta }_{12}$, ${\beta }_{13}$, ${\beta }_{14}$, ${\beta }_{23}$,${\beta }_{24}$, ${\beta }_{34}$- interaction effects.

3. Results and discussion

3.1. The results of identification of chemical compounds present in essential oil

As displayed in Table 1 and Fig. 1, 14 compounds, 87.71% in total, were identified in the Anethum graveolens L. essential oil. The primary compounds identified in the essential oil were limonene (32.32%), carvone (35.43%), and cis-dihydrocarvone (5.43%). These compounds together constitute over 73% of the essential oil's composition. According to the studies, the type of main compounds and their percentages in the studied essential oil differs from another conducted research. It could depend on factors such as cultivation type, weather, harvesting time, storage period, soil conditions, the organ from which the essential oil was obtained, and genetic differences (Tsao, 2010).

Table 1.

The percentage of dill essential oil compositions.

Type of composition	%	RT
α-Pinene	0.62	14.50
Sabinene	0.26	15.35
β-Myrcene	0.54	15.61
p-Cymenene	1.9	16.49
Limonene	32.32	16.80
Linalool	0.61	18.07
trans-p-2,8-Menthadien-1-ol	4.79	18.65
l-4-Terpineol	0.5	19.98
Cis-Dihydrocarvone	5.43	20.2
Trans-(+)-Carveol	0.68	20.99
Carvone	35.43	21.37
Carvone oxide	0.49	21.79
Limonene Glycol	0.50	23.1
1,2-Methylenedioxy-5,6-dimethoxy-4-allylbenzene	3.64	28.08

Fig. 1.

Chromatogram of Anethum graveolens L. leaves essential oil.

3.2. Evaluation of the antimicrobial effect of dill essential oil

The inhibition zone diameter for Streptococcus mutans in concentrations of 250, 500, 750, 1000, and 2000 μg/mLwas 5.4, 6.7, 7.3, 10, and 16 mm, respectively. This diameter for Streptococcus sobrinus in the same concentrations was 6.6, 7.7, 9.1, 11, and 18 mm, respectively. The measuring of the inhibition zone diameter for the mentioned bacteria in the presence of dill essential oil using the well method demonstrates the sensitivity of these bacteria to dill essential oil. As the results have shown, dill essential oil in all concentrations had more antibacterial impact on Streptococcus sobrinus than Streptococcus mutans. The antimicrobial influence of dill essential oil can be ascribed to the presence of phenolic compounds. Flavonoids, a substantial category of phenolic compounds, are generated by the herb as a reaction to microbial infections and exhibit activity against an extensive array of microorganisms. This feature of flavonoids is due to the formation of a complex with the outer membrane of bacteria and soluble proteins connected to the membrane. However, it could be possible that the synergistic effect of effective ingredients of essential oil is also effective in its antibacterial impact (Shahidi & Wanasundara, 1992). Notably, Streptococcus mutans and Streptococcus sobrinus bacteria are gram-positive. In gram-negative bacteria, cell walls are multi-layers, while gram-positive ones have single-layer cell walls, which makes them more vulnerable to antimicrobial compounds. In other words, gram-negative bacteria exhibit both an outer membrane and a periplasmic space, whereas gram-positive bacteria do not possess either of these constituents. This membrane prevents the penetration of hydrophiles into the bacteria. The periplasmic space contains enormous enzymes that can decompose external molecules (Elgayyar et al., 2001). showed that dill essential oil is potently effective on Staphylococcus aureus, Escherichia coli, Yersinia enterocolitica, Geotrichum candidum, and Rhodotorula glutinis with a 36–69 mm zone of inhibition, while the impact on Salmonella typhimurium is mediocre with a 26 mm zone of inhibition. However, its effect on Aspergillus niger was reported to be weak due to a 12 mm zone of inhibition. On the other hand, this essential oil showed no inhibitory effect on the growth of Lactiplantibacillus plantarum, Pseudomonas aeruginosa, and Listeria monocytogenes. In a study done by Derakhshan et al. (2017), the highest antibacterial activity of dill essential oil was observed against Staphylococcus aureus (with a 15 mm zone of inhibition of undiluted essential oil) and V. cholerae (with a 14 mm zone of inhibition of undiluted essential oil). However, the sensitivity of Escherichia coli and Pseudomonas aeruginosa strains to the essential oil was similar (both with a 12 mm zone of inhibition of undiluted essential oil). The control well (95% of ethanol) showed no inhibitory effect.

3.3. Evaluation of Physicochemical Properties of Microcapsules

Table 2 displays the analysis of estimated regression coefficients in the second-order polynomial model for the response variables.

Table 2.

Estimated regression coefficients in the second-order polynomial model for the response variables.

Regression coefficients	Moisture (%)	F-value	p-value	Bulk density (g/cm−3)	F-value	p-value	Tapped density (g/cm−3)	F-value	p-value
${\beta }_0$	+5.56076	20.73⁎⁎	0.0003	+0.456910	4.38⁎	0.0322	+1.65490	9.14⁎	0.0040
${\beta }_1$	+0.001850	19.58⁎⁎	0.0031	+0.000186	6.226⁎	0.0560	−0.000230	6.21⁎	0.0002
${\beta }_2$	−0.021967	8.57⁎	0.0221	−0.000155	8.236⁎	0.0507	−0.004174	8.74⁎	0.0074
${\beta }_3$	−20.39153	49.21⁎⁎	0.0002	−1.92540	9.10⁎	0.0298	−10.08917	7.68⁎	0.0367
${\beta }_{11}$	−8.85000E-07	22.20⁎⁎	0.0022	−1.0760E-07	0.304ns	0.0086	−6.9400E-08	2.32ns	0.1714
${\beta }_{22}$	+0.000227	11.12⁎⁎	0.0125	−5.8240E-06	0.238ns	0.0099	−0.000030	2.71ns	0.1436
${\beta }_{33}$	+68.79245	16.32⁎⁎	0.0049	−15.13600	0.235ns	0.0014	−0.400000	0.007ns	0.9325
${\beta }_{12}$	+6.07589E-06	7.18⁎	0.0316	−4.7540E-06	17.47⁎⁎	0.0041	−4.5980E-06	17.03⁎⁎	0.0329
${\beta }_{13}$	+0.004938	18.97⁎⁎	0.0033	+0.002529	19.79⁎⁎	0.0030	+0.003734	15.82⁎⁎	0.0053
${\beta }_{23}$	−0.084759	13.97⁎⁎	0.0073	+0.052004	20.92⁎⁎	0.0026	+0.087787	21.86⁎⁎	0.0023
Lack of Fit	–	2.37ns	0.211ns	–	0.3514ns	0.7917	–	1.41ns	0.3266
R2	0.974	–	–	0.982	–	–	0.956	–	–
adj-R2	0.8812	–	–	0.9045	–		0.8740	–	–

Regression coefficients	Encapsulation efficiency (%)	F-value	p-value	Loading efficiency (%)	F-value	p-value
${\beta }_0$	+4.46135	11.85⁎⁎	0.0018	+0.000836	7.55⁎	0.0071
${\beta }_1$	+0.043538	47.64⁎⁎	0.0002	−0.000757	0.046ns	0.8363
${\beta }_2$	−0.203054	16.36⁎⁎	0.0049	+0.021923	14.86⁎⁎	0.0048
${\beta }_3$	+76.17476	13.07⁎⁎	0.0232	+1.55036	32.01⁎⁎	0.0042
${\beta }_{11}$	−6.0800E-06	0.7212ns	0.5562	+4.35000E-07	0.7837ns	0.4054
${\beta }_{22}$	+0.003548	1.46ns	0.3532	−0.000199	1.25ns	0.3007
${\beta }_{33}$	+62.90566	0.2946ns	0.9457	+2.22642	0.002ns	0.9615
${\beta }_{12}$	+0.000101	10.381⁎⁎	0.0238	+6.23438E-06	1.10ns	0.3282
${\beta }_{13}$	−0.071665	12.988⁎⁎	0.0667	−0.000383	0.016ns	0.9009
${\beta }_{23}$	−0.644709	0.0050ns	0.6041	−0.038344	10.417⁎⁎	0.0386
Lack of Fit	–	4.08ns	0.0038	–	0.7245ns	0.5881
R2	0.9819	–	–	0.9863	–	–
adj-R2	0.9312	–	–	0.9145	–	–

${\beta }_1$= Essential oil concentration, ${\beta }_2$= Maltodextrin, and ${\beta }_3$= Arabic gum, ^⁎Significant at 0.05 level, ^⁎⁎Significant at 0.01 level, ns: Not significant, P-values <0.0500 indicate model terms are significant.

3.4. Evaluation of Moisture Content

The interaction effects of arabic gum, maltodextrin, and dill essential oil concentrations on the moisture content of microcapsules are presented in Fig. 2. According to Table 2, the effect of essential oil concentration (${\beta }_1$), arabic gum concentration (${\beta }_3$), the interaction effect of the second-order of essential oil concentration (${\beta }_{11}$), maltodextrin concentration (${\beta }_{22}$), arabic gum (${\beta }_{33}$), also the interaction effect of the essential oil and arabic gum concentration (${\beta }_{13}$), the maltodextrin and arabic gum concentration (${\beta }_{23}$) ($p\le 0.01$), and also maltodextrin concentration (${\beta }_2$), and the interaction effect of the essential oil and maltodextrin concentration (${\beta }_{12}$) ($p\le 0.05$) on the moisture content of microcapsules are significant. Furthermore, the F-value and p-value and a high coefficient of determination (R²) show that the proposed model has a good fitting to determine the moisture content of samples. The moisture content of the powder plays a critical role in dictating the physicochemical stability of the powder while in storage. To effectively extend the storage period, it is imperative to ensure that the moisture content of the samples remains below 5–4%. Since, in such conditions, oxidative decomposition and microbial activity decrease significantly. In addition, the lower moisture content of samples leads to a lower cohesiveness of powder particles. Since maltodextrin is a non-ionic hydrophile compound, and arabic gum has a dual hydrophobic and hydrophilic structure, combining these two could generate a novel wall composition with appropriate efficiency for essential oil microencapsulation. The results showed that the moisture content of the samples was decreased when the maltodextrin concentration was increased. The observed outcome may be ascribed to the elevation in solid substance content, the affinity of sugars involved in maltodextrin formation to attract moisture, and the decrease in available free moisture for evaporation. Notably, adding the arabic gum increased the moisture content of powder samples, possibly since arabic gum is a heteropolysaccharide complex with a branched structure that involves hydrophilic groups and, as a result, forms bonds with water molecules. Increasing the essential oil concentration increases the moisture of powders, which could be related to the reduction of coating solid substance and, consequently, increasing the free moisture available for evaporation (Nikjoo et al., 2021). Other researchers have attained similar results through their studies (Bazaria & Kumar, 2016).

Fig. 2.

The interaction effect of Arabic gum, maltodextrin, and dill essential oil concentrations on the moisture content of microcapsules.

3.5. Evaluation of bulk density

According to Table 2, the effect of essential oil concentration (${\beta }_1$), maltodextrin concentration (${\beta }_2$), and arabic gum concentration (${\beta }_3$) ($p\le 0.05$), and also, the interaction effect of the concentration of essential oil and arabic gum (${\beta }_{13}$), the maltodextrin and arabic gum concentration (${\beta }_{23}$), and the essential oil and maltodextrin concentration (${\beta }_{12}$) ($p\le 0.01$) are significant on the bulk density of microcapsules. The higher coefficient of determination (R²), along with higher F-value and p-value, demonstrate good fitting of the proposed model. The interaction effect of arabic gum, maltodextrin, and dill essential oil concentrations on the bulk density of microcapsules can be observed in Fig. 3. The bulk density is determined by particle shape and size, trapped air within the particle, and moisture. These factors are dependent on coating features, temperature and time of drying, and processing operation (Izadi & Mirazi, 2020). As the results state, bulk density decreased by increasing maltodextrin concentration because of the reduction of bulk mass due to the moisture reduction and, consequently, the lightening of particles. Using a maltodextrin coat leads to less cohesiveness of particles and, as a result, an increase in porosity. It could be a reason for bulk density reduction. Moreover, since maltodextrin and arabic gum form a film on the particles, decreasing density with increasing the concentrations of these compounds can be related to the trapping air inside the structure of these particles. In this regard, the bulk density increased by increasing essential oil concentration. As the main reasons for this, increasing the cohesion of the particles to each other, reduction of film formation, and reduction of porosity and air trapped inside the particles, can be mentioned (Bazaria & Kumar, 2016; Nikjoo et al., 2021).

Fig. 3.

The interaction effect of Arabic gum, maltodextrin, and dill essential oil concentrations on bulk density of microcapsules.

3.6. Evaluation of tapped density

According to Table 2, the effect of essential oil concentration (${\beta }_1$), maltodextrin concentration (${\beta }_2$), and arabic gum concentration (${\beta }_3$) ($p\le 0.05$), and also, the interaction effect of the concentration of essential oil and arabic gum (${\beta }_{13}$), the maltodextrin and arabic gum concentration (${\beta }_{23}$), and the essential oil and maltodextrin concentration (${\beta }_{12}$) ($p\le 0.01$) on the tapped density of microcapsule are significant. The amounts of coefficient of determination (R²), F-value, and p-value illustrate a good fitting of the proposed model. Tapped density holds pivotal importance in both packaging procedures and transportation, as it governs the necessary substance quantities for occupying a defined volume within packaging and storage containers. According to the information presented in Fig. 4, an increase in the concentrations of essential oil, maltodextrin, and arabic gum led to a reduction in the tapped density of the samples. This can be attributed to the specific characteristics of maltodextrin that lessen particle cohesion. However, as the number of coats and concentrations of essential oils are increased, it causes the formation of larger droplets and, ultimately, leads to the generation of bigger dried particles within the drying container. Consequently, the presence of a high percentage of large particles in the powder results in fewer changes caused by tapping. As a result, the tapped density experiences a decrease (Bazaria & Kumar, 2016).

Fig. 4.

The Interaction effect of Arabic gum, maltodextrin, and dill essential oil concentrations on tapped density of microcapsules.

3.7. Evaluation of microencapsulation efficiency

According to Table 2, the effect of essential oil concentration (${\beta }_1$), maltodextrin concentration (${\beta }_2$), and arabic gum concentration (${\beta }_3$), and the interaction effect of the concentration of essential oil and arabic gum (${\beta }_{13}$), and the interaction effect of the concentration of essential oil and maltodextrin (${\beta }_{12}$) on the efficiency of microencapsulation of microcapsules are significant ($p\le 0.01$). Based on Fig. 5, the total amount of phenolic compounds increased by enhancing the concentration of maltodextrin and arabic gum since by increasing the wall concentration, the atomizer took out particles in larger sizes. Subsequently, the size of the final microcapsules became larger. Frascareli et al. (2012), during the encapsulation of coffee oil with arabic gum, observed that by increasing the concentration of wall material, the size of microcapsule particles gets larger. However, by increasing the oil percentage, this size gets smaller. A study conducted by Alemzadeh (2013), demonstrated that increasing the arabic gum and gelatin concentration used in the wall of a microcapsule containing mint oil reduces the size of capsule particles and also enhances its efficiency. Gupta et al. (2015), reported that arabic gum is one of the most prevalent and appropriate substances to improve and increase microencapsulation efficiency due to its emulsifying property and in the encapsulation of iron with a mixture of arabic gum, maltodextrin, and modified starch, by increasing the arabic gum concentration from 1 to 4 g and by increasing the ratio of ethanol to the mixture from 6 to 10 g, an improve in the efficiency of microencapsulation and reduction of microcapsule size would occur. Premi and Sharma (2017), showed that using an arabic gum-maltodextrin mixture, compared to a combination of maltodextrin-whey protein concentrate, has a significant role in increasing the efficiency of microencapsulation. By increasing the ratio of arabic gum in the arabic gum-maltodextrin mixture, the efficiency increased significantly due to the physicochemical properties of arabic gum (Premi & Sharma, 2017).

Fig. 5.

The Interaction effect of (essential oil × Arabic gum) concentration and (essential oil × maltodextrin) concentration on the efficiency of microencapsulation.

3.8. Evaluation of loading capacity

As stated in Table 2, maltodextrin concentration (${\beta }_2$) and arabic gum concentration (${\beta }_3$), and also, the interaction effect of the concentration of maltodextrin and arabic gum (${\beta }_{23}$) on the loading capacity of microcapsules are significant ($p\le 0.01$). Loading capacity serves as an indicator of carriers' ability to retain and subsequently release active compounds. Various factors, including the method of capsule preparation, sample volume, temperature, and surface characteristics, impact this attribute. As depicted in Fig. 6, elevating the concentrations of maltodextrin and arabic gum resulted in an augmentation of the loading capacity. The effect of increasing the concentration of wall substance on loading capacity can be attributed to the selective diffusion theory (Thijssen, 1968). According to this theory, with the reduction of water concentration at the droplet surface, the diffusion coefficient of volatile compounds decreased multiple times more than water. Thus, water constantly goes out through the formed film at a specified rate during the drying. However, volatile compounds in the oily phase (the core) diffused very slowly since they are trapped in a mass of solid substances (wall materials). This solid coat, as a semipermeable membrane, allows water molecules to leave microcapsules while it reduces or holds the release of volatile compounds. Additionally, molecular dimensions of used coats have a significant role in releasing core compounds since they directly affect the molecular diffusion of microcapsules' compositions and their transportation to the surface. Increasing the concentration of wall composition is effective in increasing the speed of wall formation and, as a result, reduces the waste of core compounds. Also, the reduction of wall moisture causes an increase in solid substance content and, consequently, reduces the diffusion rate of essential oil into the surface and, subsequently, its evaporation (Jafari et al., 2007).

Fig. 6.

The interaction effect of (Arabic gum × maltodextrin) concentration on loading capacity of microcapsules.

3.9. Optimization

To determine the optimum condition of the process, the purpose of encapsulation for each studied variable was defined as the dependent variables to minimize moisture, bulk density, and tapped density, and maximize the loading capacity and encapsulation efficiency in the defined range for independent variables.

The optimal amounts for each independent variable, including essential oil, maltodextrin, and arabic gum concentrations are illustrated in Fig. 7. As shown, the best concentration for dill essential oil, maltodextrin, and arabic gum was determined as 2000 ppm, 75%, and 0.1%, respectively.

Fig. 7.

The optimal amounts for each independent variable, including essential oil, maltodextrin, and Arabic gum concentrations.

3.10. Evaluation of the antimicrobial effect of microencapsulated dill essential oil

The optimum treatment (2000 ppm essential oil, 75% maltodextrin, and 0.1 arabic gum concentrations) was utilized to analyze the antimicrobial effect of microencapsulated dill essential oil. The inhibition zone diameter for Streptococcus mutans was 19 mm at 2000 μg/mLconcentration. This diameter was 20 mm for Streptococcus sobrinus in the same concentration. The results show that these two bacteria are sensitive to dill essential oil. In the context of describing how essential oils influence bacteria, it can be affirmed that the constituents of the essential oil disturb the lipid portion of the bilayered membrane. Moreover, once these components cross the membrane, they interact with essential intracellular sites, thereby exerting antibacterial activity (Noor et al., 2022).

Dima et al. (2014), showed that microcapsules of coriander essential oil enclosed in β-cyclodextrin have an inhibitory effect on Bacillus subtilis, Rhodotorula glutinis, Bacillus cereus, Saccharomyces cerevisiae, Candida utilis, Penicillium glaucum, Aspergillus niger and Geotrichum candidum in different inhibition degrees. Moreover, this microcapsule showed high antifungal activity against A. niger MIUG M5 and P. glaucum MIUG M9, compared to S. cerevisiae MIUG D9. Yuan et al. (2019), studied the essential oil of lavender, free and capsulated in hydroxypropyl-β-cyclodextrin, which formed an inhibition zone against Escherichia coli, Staphylococcus aureus, and Candida albicans. According to the tests, the minimum inhibitory concentration (MIC) of lavender essential oil against mentioned microorganisms after enclosing in HPCD was enhanced approximately three times. These results showed that alteration in the composition of compounds significantly affects stability and antibacterial activity. Weisany et al. (2019) conducted a study examining the impact of thyme and dill essential oils at concentrations of 80, 240, 720, and 1000 ppm on the inhibition of C. nymphaeae growth over 3, 6, 9, and 12 days of incubation. The most significant inhibition of mycelium growth was observed after 9 days of incubation at concentrations of 720 and 1000 ppm. The encapsulation of essential oils yielded a significant alteration in the mycelium growth of C. nymphaeae, particularly when dill and thyme essential oils were integrated with copper nanoparticles within the concentration range of 90 to 600 ppm. The incorporation of essential oils encapsulated by copper nanoparticles effectively suppressed mycelium growth over the course of 3, 6, and 9 days of treatment.

3.11. Evaluation of release rate

Releasing of capsulated dill essential oil in a simulated saliva environment was studied in periods of 5, 10, 15, and 20 min. To perform this test, optimum capsules (2000 ppm of dill essential oil, 75% of maltodextrin, and 0.1% of arabic.

gum concentrations), which were the optimum samples and determined through physicochemical tests of microcapsules, were applied. The release rate reached its maximum (98.32%) after 20 min, according to Table 3 and Fig. 8. The obtained result is because of the reduction in stability and coherence of biopolymers applied as coating substances and increasing in release rate due to environmental tensions, such as pH, temperature, and moisture. The moisture absorbance causes inflation in the wall of the biopolymer and simultaneously reduces its glass transition temperature. Therefore, the coherence and entanglement of biopolymeric chains are decreased, and the textures of raw materials collapse. The result is releasing the essential oil from the microcapsules into the environment. In this regard, similar results have been stated in another study (Akbarbaglou et al., 2018).

Table 3.

Release of capsulated dill essential oil in a simulated saliva environment.

Time (min)	Absorbance	Release (%)
5	0.518	42.16
10	0.732	77.34
15	0.812	87.52
20	0.869	98.32

Fig. 8.

Releasing of capsulated dill essential oil in a simulated saliva environment.

3.12. Evaluation of microcapsules morphology

The effect of the optimum concentration of essential oil (2000 ppm), maltodextrin (75%), and arabic gum (0.1%) on the surficial structure and morphological properties of powder containing dill essential oil using the fluidized bed method is shown in Fig. 9. Different morphologies occur due to the difference in the ratio of substances fed into the dryer, the size of droplets, and the temperature of the process (Mahdi et al., 2020). Generated microcapsules have different geometric shapes, possibly due to the drying mechanism. Based on the results, the powder particles are approximately spherical with smooth surfaces and without any cracks, possibly due to low and appropriate drying temperature, viscoelastic properties, and forming film (Weisany et al., 2019). On the other hand, some agglomeration was observed in the generated powders. Seemingly, the agglomeration of the particles and their cohesive structure are associated with the glass transition of the amorphous matrices of carbohydrates and the high amount of surficial oil of particles (Norkaew et al., 2019).

Fig. 9.

SEM image of microcapsules containing dill essential oil in the optimal condition.

4. Conclusion

This study investigated the effect of fluidized bed encapsulation and different concentrations of maltodextrin and arabic gum on the physicochemical and antimicrobial properties and release rate of microcapsules containing dill essential oil. Considering the physicochemical properties of produced microcapsules, in addition to maintaining the antimicrobial properties of encapsulated essential oils and releasing rate of microcapsules, using maltodextrin (a hydrophile and non-ionic compound) and arabic gum (with hydrophilic and hydrophobic dual structure) composition in wall structure could provide an appropriate coating for increasing the stability of dill essential oil in environmental conditions.

Ethical approval

Ethics approval was not required for this research.

CRediT authorship contribution statement

Narjes Jannesar: Writing – original draft, Software, Methodology, Investigation, Conceptualization. Alireza Bassiri: Writing – review & editing, Supervision, Conceptualization. Mehrdad Ghavami: Writing – review & editing, Supervision, Project administration, Conceptualization. Hossein Ahmadi Chenarbon: Writing – review & editing, Methodology, Formal analysis, Data curation. Babak Ghiassi Tarzi: Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Research data are not shared.