Influence of spray drying parameters on the physicochemical characteristics of microencapsulated pomelo (Citrus grandis (L.) Osbeck) essential oil

Abstract

This study aimed to evaluate the encapsulation of pomelo (Citrus grandis (L.) Osbeck) essential oils using the spray drying technique. The parameters of the process include concentration of maltodextrin (20–35% by wt%/wt%), concentration of essential oil (1–2.5% by wt%/wt%), inlet temperature of spray drying (120–180 °C), and feed flow rates (120–240 mL/h) were soundly examined. The utilization of suitable parameters as the concentration of maltodextrin at 30% (by wt%/wt%), the concentration of essential oil at 1.5% (by wt%/wt%), the inlet temperature of 140 °C, and feed flow rate of 120 mL/h showed the highest drying yields (90.05%), microencapsulation yield (75.59%), and microencapsulation efficiency (89.44%). TGA and DSC results verified higher stability of Citrus grandis essential oil after encapsulation. The encapsulation of pomelo essential oils maintained most of the major components in comparison with the non-encapsulated essential oils without any significant changing in powder-obtained quality.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-022-01161-5.

Introduction

Citrus grandis (L.) Osbeck, normally known as pomelo, is a species in the Citrus genus and enlarged cultivated in the MeKong river delta. It is widely used for application in cosmetics, food, and medicine (Ni et al., 2014). Pomelo essential oil (PEO) is the type of botanical secondary metabolite compound in pomelo peels, which can be extracted by solvent extraction, microwave-assisted extraction, hydrodistillation, and so on. The major component in the PEO includes α-pinene, sabinene, β-pinene, myrcene, α-terpinene, limonene, terpinolene, γ-terpinene, and linalool with total concentrations of 85.9–96.5% (Aghbashlo et al., 2013). Hence, many studies reported that PEO has a wide range of bioactivities such as anti-inflammatory, antioxidant, bacteriostatic, and antiviral properties. Thus PEO can be utilized as an ingredient and flavor in the food production, cosmetic and pharmaceutical industries (Islam Shishir et al., 2016). However, the presence of volatile can cause oxidation, chemical interaction, and degradation reactions. This phenomenon results in a decline in quality, restraining their applications in the food medicine and cosmetic sectors.

To minimize the effect of these negative processes, microencapsulation is one of widely used technique in food industry. Microencapsulation is defined as a technique by which core materials such as essential oil and active substances are packed into thin wall material (Kim et al., 2021). The size of the microcapsules is in a range from 1 to 800 µm, entrapped by single or double-layered. The microcapsules are created by combining different wall material types such as protein, polysaccharides, lipids, and polymeric with a variety of core materials such as essential oils, vegetable oils, and bioactivities substances. Over the past decades, the microencapsulation techniques including spray drying, precipitation, spray cooling, extrusion, dissolving, and coacervation have been developed (Lee et al., 2019). Among them, spray drying is the most efficient technology due to its low-cost and wide applicability. Microencapsulation yield (MEY) and microencapsulation efficiency (MEE) are the main indicators to evaluate powder-obtained quality (Lee et al., 2016). Besides, the effectiveness of microencapsulation by spray drying also relies greatly on operating parameters such as concentration of wall materials, concentration of core oils, inlet temperature, and feed flow rates (Jafari et al., 2008).

It is noticed that conducting encapsulation of PEO via spray drying has not been reported. Even, Aguiar et al. (2020) only evaluated the effect of parameters including drying air flow rate and inlet air temperature on indicators of encapsulated orange essential oil powder. The response surface methodology was used to optimize the factors including powder recovery, oil content, encapsulation efficiency of obtained powder. However, other process parameters, e.g., concentration of wall material, and essential oils as well as chemical composition of bare orange essential oil were not mentioned in this study. Moreover, Melo et al. (2020) presented the encapsulation of lemongrass essential oils in Coalho cheese, showing the optimum parameters involving an inlet temperature of 170 °C, a feed flow rate of 900 mL/h, a ratio between essential oil and wall material as 1/5 (by wt.%/wt.%). This study was also found that the wall material as gum Arabic could bring good thermal stability and accomplish a high yield of powder encapsulation. Similarly, Mehran et al. (2020) optimized microencapsulation conditions of Mentha spicata essential oil by spray drying. This study displayed optimal conditions as follows, a wall material concentration of 35%, an essential oil concentration of 4%, and an inlet temperature 110 °C, obtaining a maximum encapsulation yield of 91%.

In general, these prior studies have not fully mentioned the effect of process parameters such as concentration of wall material, concentration of essential oil, inlet temperature, and feed flow rates on powder characteristics. Additionally, the chemical composition of core material such as essential oil before and after encapsulation has not been clarified. To the best of our knowledge, the encapsulation of Citrus grandis (L.) essential oil using spray drying technique is scarcely reported. Furthermore, Gum Arabic is generally combined with maltodextrin to serve as wall material due to its film-forming ability. The use of maltodextrin on the other hand as single wall material is rarely discussed. Thus, the present study deals with such literature gaps in previous researches. Firstly, the effects of spray-drying parameters such as concentration of wall material, concentration of essential oil, inlet temperature, and feed flow rates on the encapsulation process of PEO were evaluated. Secondly, chemical compositions changes in PEO before and after being encapsulated was clearly investigated. The indexes were used to evaluate the quality of resulting powder, including moisture content, drying yield, microencapsulation yield, microencapsulation efficiency, powder morphology, thermal stability, and volatile profile of PEO. The present results pave the way for the further optimization of microencapsulation parameters by spray drying, and minimizing chemical composition alteration in essential oil preservation.

Materials and methods

Materials

The core material, pomelo (Citrus grandis (L.) Osbeck) essential oil used for the microencapsulation was supplied by AOTA International JSC located in Mekong Delta region of Vietnam. The wall materials, maltodextrin from maize (DE 12) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Tween 80 as an emulsifying agent was obtained from Merk (Germany). N-pentane (99%) produced by Xilong Scientific Co., Ltd (Guangdong, China) was employed for the determination of the MEE indicator.

Preparation of microencapsulation powder by spray drying technology

First, to prepare wall material solution, maltodextrin was completely dissolved in 500 mL of distilled water under magnetic stirring. Prepared solution was kept overnight at 15 °C to make stability of the polymer molecules. Then, the PEO and Tween 80 with an amount of equivalence to 5% of PEO weight were added into the maltodextrin solution and rotated at a speed of 6000 rpm for 20 min by a rotor–stator homogenizer (IKA T-25 digital ultra-turrax, IKA company, Staufen, Germany) to form an emulsion. Afterwards, the feed emulsion was pumped into a laboratory-scale spray dryer (YC-015 spray dryer, Pilotech, Shanghai, China), and pressure nozzle of 1.00 mm in diameter, compressed air pressure 3 bar, air blow 3 m³/min (Fig. 1). The contact between both phases (gas and liquid) is carried out in co-current flow. The spray-dried powder was put into airtight glass bottle and stored at room temperature for further analysis.

Fig. 1.

Diagram of spray drying system

Tables S1 and S2 shows the amount of each feed emulsion composition used in the first two experiments. For the first investigation: the concentration of maltodextrin were 20%, 25%, 30% and 35% (by wt%/wt%) and the concentration of PEO was 0.5% (by wt%/wt%) of the mass of the feed emulsion, respectively (Table S1).

For the second investigation: the concentration of maltodextrin was 30% (by wt%/wt%) and the concentrations of PEO were 1.0%, 1.5%, 2.0% and 2.5% (by wt%/wt%) of the mass of the feed emulsion, respectively. The spray drying conditions including the inlet temperature of 140 °C and the feed flow rate of 120 mL/h were applied for both first and second investigation (Table S2).

For the third investigation: the concentration of maltodextrin was 30% (by wt%/wt%), the concentration of PEO was 1.5% (by wt%/wt%) of the mass of the feed emulsion, respectively. The inlet temperatures were varied from 120 to 180 °C and the feed flow rate was 120 mL/h. And the fourth investigation: the concentration of maltodextrin and the concentration of PEO remained 30% and 1.5% (by wt%/wt%) of the mass of the feed emulsion, respectively. The inlet temperature was 140 °C and the feed flow rates were varied from 120 to 240 mL/h.

Characterization of PEO and microparticles

Determination of moisture content

Moisture content of the spray dried powder was determined by a moisture analyzer (Ohaus MB90, US). The sample (5 g) was dried at 105 °C and each measurement was repeated five times. The average moisture content value was calculated and presented on a percent wet weight basis.

Measurement of drying yield (DY)

The DY is an indicator that measures the quantity of powder recovery after a given spray drying process (Jafari et al., 2017). The DY formula is calculated on a dry basis by dividing the amount of the obtained spray-dried powder by the quantity of feed solution as follows (Eq. 1):

$$DY(\%)=\frac{m_2\times (1-y)}{m_1\times x}\times 100$$ 1

in which $m_1$ (g) is the amount of the feed emulsion, $m_2$ (g) is the amount of the obtained powder, $x$ (%) is the fraction of solids and $y$ (%) is the moisture content of the obtained powder.

Measurement of microencapsulation yields (MEY)

MEY index is a decisive factor for the oil retention capacity. The MEY formula is calculated on the dry basis as the ratio of the total amount of PEO retained in the obtained powder to the amount of PEO in the initial feed emulsion. MEY was determined following the analytical procedure in the report by Fernandes et al. (2013) with some amendments. Firstly, distilled water of 200 mL was added to 30 g of the obtained powder. The mixture was put under a magnetic stirrer without heating until it fully dissolved with no powder on the wall of the beaker. The total amount of PEO was extracted during 4 h by the hydrodistillation equipment as Clevenger-type. The PEO was evaporated by heating the mixture followed by the liquefaction of the vapors in a condenser. Afterward, two immiscible phases in the collector were observed. Then, the top essential oil phase was dehydration by using anhydrous sodium sulfate (Na₂SO₄) and the dehydrated essential oil was measured in terms of volume. MEY formula was determined below (Eq. 2):

$$MEY(\%)=\frac{Totaloil(\%)}{Initialoilload(\%)}\times 100$$ 2

Measurement of microencapsulation efficiency (MEE) and surface oil (SO)

MEE value was calculated on the dry basis following the formula in which the total amount of PEO presenting in the obtained powder was divided by the amount of microencapsulated PEO. The mass of encapsulated PEO was measured as the following procedure that well described in preceding research by Thuong Nhan et al. (2020) with some modifications. First, 30 g of the obtained microcapsulated powder was mixed with 150 mL of n-pentane. The mixture was magnetically stirred at room temperature for during 1 h. Then n-pentane solvent was removed from the well-stirred mixture through filtration process by using Whatman Grade 1 filter paper. The retained powder was dried at 60 °C in the drying oven with drying time of 5 h. The obtained dried product was completely dissolved in 200 mL of distilled water and then undergone hydrodistillation for 4 h using Clevenger-type apparatus system. The amount of obtained PEO was utilized to measure MEE value with respect to the following formula (Eq. 3):

$$MEE(\%)=\frac{MassofencapsulatedPEO,drybasis}{MassoftotalPEOinproduct,drybasis}\times 100$$ 3

The surface PEO content was calculated to the following (Eq. 4):

$$SO(\%)=(1-MEE)\times 100$$ 4

Determination of chemical compositions in PEO

Gas Chromatography ‒ Mass Spectrometry (GC–MS) method was used to examine the chemical profile of the PEO sample prior to and following microencapsulation (Carvalho et al., 2019). First, 1.0 mL of n-hexane was employed to dissolve 25 µL of the PEO sample (the concentration of injected extract as 0.0215 g/mL). The GC‒MS analysis was performed on Agilent 6890/5973 GC/MSD with HP5‒MS analytical columns. The pressure on the head of the column was reached to 9.3 psi. The operation conditions of GC–MS system were in the following: Helium was carrier gas with constant flow rate of 1.0 mL/min in a split ratio of 1/100; injection temperature at 250 °C; the volume injected of 1.0 µL. The oven temperature was initially held at 50 °C for 2 min, then was programmed at a rate of 2 °C/min to 80 °C, and risen to 150 °C at 5 °C/min, increased by 10 °C/min to 200 °C, finally programmed as 200 °C to 300 °C at 20 °C/min and held for 5 min. All samples were analyzed in thrice replicates, and the total length of the GC method was 45 min.

Micro structural characteristics

Scanning electron microscope (SEM) was employed on JEOL JSM 7401F (Peabody, MA, USA) to examine the morphology of microencapsulated powder. Powder was mounted rigidly on highly electrical carbon tape onto aluminum SEM stubs and surrounded by a thin coating gold ‒ palladium layer.

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)

Thermogravimetric analysis (TGA) of PEO, maltodextrin and encapsulated PEO powder were performed on the Q500 analyzer (TA US ltd., United States). 5 mg of each sample was weighed in an aluminum pan, which was sealed and pierced. The thermal analysis was conducted under nitrogen gas flow rate, 50 mL/min; heating rate, 10 °C/min; and temperature from 20 °C to 800 °C.

Differential scanning calorimetry (DSC) curves were analyzed by differential scanning calorimeter (Seiko, DSC 6220, Japan). 5 mg of each sample (PEO, maltodextrin and encapsulated PEO powder) was accurately weighed and put in an aluminum pan. A sealed empty pan was used as reference. The analysis was employed under the following conditions: the nitrogen gas flow rate, 50 mL/min; heating rate of 10 °C/min, and the temperature from 25 °C to 800 °C.

Statistical analysis

Each experiment was repeated in triplicate. The statistical software as Statgraphics Centurion Version 20 (Statpoint Technologies, Inc., Warrenton, VA, USA) was utilized to perform a number of statistical tests. Analysis of variance (ANOVA) and least significant difference (LSD) tests were conducted to compare means in an analysis of variance with a significance of 0.05.

Results and discussion

Chemical composition of the PEO

The chemical composition of PEO was first analyzed using GC–MS technique. The spectra, constituent of samples, and proposed fragmentation of compounds are recorded in Fig. 2a–c, Table 1, and Table S3, respectively.

Fig. 2.

(A) The spectrum GC of bare PEO. (B) The spectrum GC of encapsulated essential oil. (C) The spectrum GC of PEO versus encapsulated essential oil

Table 1.

The chemical composition of PEO versus encapsulated essential oil

No.	Compounds	Retention time (min)	PEO before encapsulation (%)	PEO after encapsulation (%)
	The monoterpene class
1	α-pinene	7.292	1.172 ± 0.229	0.987 ± 0.183
2	β-myrcene	9.959	1.622 ± 0.534	1.028 ± 0.546
3	β-phellandrene	10.513	0.571 ± 0.056	0.231 ± 0.241
4	p-cymene	11.632	0.576 ± 0.290	0.703 ± 0.406
5	Limonene	11.956	95.246 ± 1.375	96.042 ± 1.399
	Total		99.187 ± 0.469	98.991 ± 0.583

At the first sight, the greatest intensity peak was placed on 11.956 min (Fig. 2a). The weight of compound was confirmed by the MS spectrum with m/z = 136 (Table S3). As the Table 1, there were main five compositions presented, accounting for 99.187% of the total essential oil content. The main compositions were volatile components such as α-pinene (1.172%), β-myrcene (1.622%), β-phellandrene (0.571%), p-cymene (0.576%), and limonene (95.956%). The highest limonene content was in step with other prior studies (He et al., 2019; Tuan et al., 2019). Indeed, these authors showed that the limonene content in Citrus grandis (L.) Osbeck ranged from 55.92 to 89.87%. Besides, other compounds were found and isolated in PEO, including α-pinene, β-myrcene, β-phellandrene, p-cymene and so on (Phi et al., 2015). Difference in chemical composition could be assigned to various of extraction methods (hydrodistillation, microwave-assisted hydrodistillation), other growing conditions (soil conditions and climatic factors such as temperature and rainfall) (Chen et al., 2016). By comparing the limonene content in current species with the other Citrus family, similarities to those of Citrus maxima. Merr essential oils were declared, reaching to 95.4% (Thavanapong et al., 2010).

Effect of wall materials concentration

In the present study, the maltodextrin was used as wall material with changing concentration from 20 to 35% (by wt%/wt%). Table S4 describes the properties of powder in terms of photograph, moisture content (%), and drying yield (%). According to photograph, there is almost no change in the color and size of microparticles at maltodextrin concentrations. There was a marginal decrease in moisture content (from 4% to 2.64%) and drying yield (from 86.21% to 85.38%) when using maltodextrin as wall material at varying concentrations.

To determine the efficiency of encapsulation process, Fig. 3a–d presents the microparticle characteristics including three indicators as MEY, MEE, and SO. One-way ANOVA was a technique that can be used to compare means of two or more samples (using the F distribution). The effect of maltodextrin concentrations on MEY, MEE and SO was statistically significant if p < 0.05. Figure 3a presents the microparticle characteristics, including three indicators as MEY, MEE, and SO at different wall concentrations. At 30% concentrations, it was found that the MEY and MEE indexes obtained the highest values off 89.66% and 87.54%, respectively. By using the LSD multiple range test for three indexes, there were statistically significant differences among all indexes obtained at the four concentrations. However, these indexes obtained tend to reduce sharply in the maltodextrin concentrations of 35% (by wt%/wt%).

Fig. 3.

Effect of parameters process on various indexes of microparticles. *Figures with same letters indicate statistical indifference

These results can be explained by the influence of surface-active carbohydrate of maltodextrin. Since surface-active carbohydrate could be linked with volatile components, rising concentration of maltodextrin led to enhancing the ability to store essential oils in obtained—powder (Carneiro et al., 2013). This result was ascribed to the interaction between limonene as major component of oil and maltodextrin as a wall material that could be occurred by the hydrophobic effect. The range of wall concentration utilizing from 20 to 35% (by wt%/wt%) was well commensurate with Nunes and Mercadante (2007) (20% by wt%/wt%) and Liu et al., (2000) (25% by wt%/wt%). Therefore, maltodextrin as wall material with concentration of 30% (by wt%/wt%) was used to operate the microencapsulation process.

Effect of core material concentration

The PEO was used as a core material at varying concentrations (1.0–2.5% by wt%/wt%). The properties of powder obtained at different concentration from 1.0 to 2.5% (by wt%/wt%) are displayed in Table S5. Visually, the higher concentration of PEO results in increasing agglomeration and moisture content in obtained powder. The highest moisture content (4.26%) was achieved at the oil concentration of 2.5% (by wt%/wt%) and declined to 2.96% when decreasing the concentration to 1.0% (by wt%/wt%). The DY index accomplished the highest value of 90.05% at the PEO concentration of 1.5% (by wt%/wt%).

Figure 3b illustrates the effect of PEO concentration on indicator of microparticles, including MEY, MEE, and SO. One-way ANOVA showed that effect of core concentrations on MEY, MEE and SO was statistically significant (p < 0.05). Based on Fig. 3b, the MEY and MEE indexes showed a rising trend when the concentration of PEO changes from 1.0 to 1.5% (by wt%/wt%). The MEE value reached the highest point of 89.44% at 1.5% (by wt%/wt%) and decreased to 81.53% at 2.0% (by wt%/wt%). For MEY index, the downward was observed when rising concentration of essential oil from 1.0 to 2.5% (by wt%/wt%). The LSD multiple range test affirmed a clear difference between values (MEY and MEE) at a concentration of 1.5% (by wt%/wt%) with other values.

This subject can be explained by a linkage between maltodextrin and volatile component (as essential oil) by Van der Waals bond or dipole ‒ dipole interaction in spray drying process (Coimbra et al., 2021). At a concentration of maltodextrin and essential oil as 30% (by wt%/wt%) and 1.5% (by wt%/wt%), respectively, the number of bonds created by maltodextrin might possibly improve its linkage ability with PEO in solution. In other words, this opinion can be used to explain the fact that the highest MEE value at PEO concentration as 1.5% (by wt%/wt%). Additionally, Jafari et al. (2008) indicated the enhancement of the amount of essential oil in the original solution, leading to a loss of essential oil in spray drying process, causing by shorting the distance between the core of microparticles and hot air. This finding was consistent with the study by Turasan et al. (2015), which a ratio of maltodextrin/rosemary essential oil was changed from 1.25% to 10% (by wt%/wt%). To sum up, the concentration of PEO as 1.5% (by wt%/wt%) was satisfactory for spray drying process.

Effect of inlet temperature

The properties of powder product, setting at different inlet temperatures (120–180 °C), are displayed in Table S6. Visually, higher inlet temperature leads to less agglomeration of powder. In general, the moisture content of microencapsulated powder declined slightly at high inlet temperature in spray drying device. The lowest moisture content (2.43%) could be obtained at 180 °C, but this value tended to increase with reducing inlet temperature. The DY index (91.81%) achieved the highest value at inlet temperature of 120 °C.

The effect of inlet temperature on indicators of microparticle, including MEY, MEE, and SO are clarified in Fig. 3c. Statistical analysis exhibited that the inlet temperature was a significantly effective parameter on MEY, MEE, and SO. According to Fig. 3c, the MEY and MEE values were 75.59% and 89.44% with an increase in the inlet temperature from 120 to 140 °C, respectively. Conversely, the indicators (MEY and MEE) were clearly fallen to lowest values when growing inlet temperature up to 180 °C. LSD test confirmed the significant difference between values (MEY and MEE) at temperature of 140 °C with other values.

To be elaborate, it could be seen that the thermal-sensitive compounds in PEO could be decomposed when using excessively inlet temperature (Lavanya et al., 2020). Indeed, Santhalakshmy et al. (2015) presented that the temperature inside the droplet atomization process would be aggravated when rising excessive inlet temperature. This phenomenon was found to increase the amount of steam and essential oil which moved out of microparticles via surface defects. Similarly, Sun et al. (2020) reported that inlet temperature between 100 and 130 °C in their study was suitable to conduct the encapsulation of carvacrol oil with the retention efficiency changing from 89.69 to 48.78%. Lavanya et al. (2020) obtained encapsulation efficiency from 77.92 to 80% and 76.13 to 80%, for encapsulated chia oil and fish oil powders, respectively when using inlet temperature varying from 100 to 160 °C. To summary, the inlet temperature was proposed in this study as 140 °C.

Effect of feed flow rates

This investigation was conducted to evaluate the effect of feed flow rates on indicators of microparticle (MEY, MEE and SO). Table S7 shows the properties of powder obtained at varying feed flow rates levels (120–240 mL/h). Overall, increasing feed flow rate led to a higher moisture content and more agglomeration of powder. The moisture content (3.01%) attained the lowest point at a feed flow rate of 120 mL/h. The DY index reached to 90.05% at a feed flow rates of 120 mL/h, but slightly decreased to 84.23% at level 240 mL/h.

Figure 3d presents the effect of feed flow rates on indicators of spray drying process as MEY, MEE and SO. One-way ANOVA displayed that the feed flow rates exerted significant effects if p < 0.05. Further LSD test showed that there was a significant difference observed among MEE value at 120 mL/h and other values. For MEY indexes, the maximum value (82%) could be reached at a feed flow rate of 180 mL/h. At another flow rate of 120 mL/h, the MEE index was 89.44%. It should be noted that the MEE tended to reduce with an increasing feed flow rate from 180 to 240 mL/h.

This trend in MEE value was explained by the increasing size of droplets in the atomization process, resulting in enhancing diffusion ability of essential oil in water. This mean that this process was likely to enhance the loss of essential oil in obtained powder. Similar to present report, Tontul and Topuz (2017) also reasoned that a higher feed flow rate at a constant pressure led to larger droplet size in the atomization process. This induced a larger surface area during the drying process that negatively affects the heat and mass transfer, causing an unfavorable encapsulation efficiency. Our outcome was in line with most studies, suggesting that higher feed flow rates had an adverse effect on oil retention (Can Karaca et al., 2016; Fazaeli et al., 2016). Hence, it was considered that the feed flow rate of 120 mL/h was suitable to obtain the highest MEE index.

Chemical composition of encapsulated PEO

Figure 2c presents the GC spectra of PEO before and after encapsulation. Conspicuously, GC spectrum of encapsulated essential oil seemed not to have any shift of peak in comparison with bare PEO (Fig. 2b). However, the change of peak intensity was clearly observed, indicating that there were no chemical composition changes in encapsulated PEO. Table 1 lists the chemical compositions of PEO before and after encapsulation. Based on Table 1, evident changes in chemical composition that were exhibited during encapsulation via spray drying were the enhancement of limonene content (from 95.246% to 96.042%) and moderate reduction in α-pinene (from 1.172% to 0.987%), β-myrcene (from 1.622% to 1.028%), and β-phellandrene (from 0.571% to 0.231%) content. The decline in α-pinene, β-myrcene, and β-phellandrene content could be explained by the volatility of monoterpenes at high spray drying temperature. In detail, due to their low boiling point, monoterpenes were easily released from droplets during the atomization process at raising spray dying temperatures, leading to a lower monoterpenes concentration (Adamiec and Kalemba 2006). The lower contents of α-pinene, β-myrcene, and β-phellandrene could be also contributed to the rise in limonene, p-cymene concentration via rearrangement processes (Turek and Stintzing 2012). Addition, the boiling point of limonene (176 °C) has been clearly higher than the others in PEO. Therefore, encapsulation by spray drying could maintain and enhance the concentration of limonene without considerable essential oil quality.

Thermal properties

The thermal stability of encapsulated PEO can be assessed through TGA/DSC analysis. According to Fig. 4a, the TGA thermogram of PEO showed a poor stability since the weight loss sharply decreased from 60 °C. The occurrence suggested that PEO can be decomposed into volatile molecular at above 60 °C. The amount of PEO residual still remained 50% at 160 °C. The absolute loss of PEO was observed at between 180 °C and 250 °C. Meanwhile, maltodextrin and encapsulated PEO acquired a higher thermal stability. Indeed, only 8% of their mass was loosen at below 150 °C. The phenomenon can be due to the evaporation of free water and oil in the surface of the microparticles (Cortés-Camargo et al., 2017). At 320 °C, 50% of encapsulated PEO mass was fallen. A sudden decrease at above 320 °C can be explained by the fact that the decomposition of materials such as oil, maltodextrin, etc. To have more insight into endothermic and exothermic temperatures, DSC analysis was used. According to Fig. 4b, the DSC thermogram of PEO showed an endothermic peak at 160 °C, which related to the evaporation and decomposition of PEO. The PEO encapsulated by maltodextrin revealed the appearance of endothermic peak at 100 °C. This event was in accordance with the loss of free water. In a range of 100 °C and 270 °C, the endothermic peaks were observed, which was attributable to the hydrated water and phase transitions (melting) (Cortés-Camargo et al., 2017). An endothermic peak at 320 °C can be ascribed by the pyrolysis of polysaccharides through the cracking of glycoside bonds (Timilsena et al., 2016). This result was in good harmony with the TGA. Iturri et al. (2021) indicated the only use of maltodextrin as wall material can improve the thermal stability of the encapsulated products. The authors explained that maltodextrin not only increased a higher evaporation energy but also reduced the water evaporation temperature. Therefore, maltodextrin has a key role in protecting the bioactive compounds from the thermal decomposition by heating. In this study, the researchers used spray-drying technology to encapsulate Eugenia stipitata pulp oil with maltodextrin as wall materials at the temperature at 120 °C. This temperature is relatively close to inlet temperature in our present study.

Fig. 4.

TGA (A) and DSC (B) analysis of PEO, maltodextrin, encapsulated PEO (EPEO)

Powder morphology

Figure 5 displays the SEM image (Scanning electron micrographs) of microparticles were produced at 140 °C, with maltodextrin as 30% (by wt%/wt%), essential oil as 1.5% (by wt%/wt%), and feed flow rate as 120 mL/h. According to Fig. 5, it is clear that fissures and cracks were not observed on the surface of microparticles. In order words, the use of maltodextrin as wall material provided protective effect for PEO, improving thermal stability of PEO. In spray drying temperature as 140 °C, almost particles were obtained as spherical with outer surface being shrunk and concave. This temperature was considerably suitable for the drying process which attenuated the decomposition of core essential oil (Santiago-Adame et al., 2015).

Fig. 5.

SEM image of the particles containing PEO after encapsulation

The current study was similar to results conducted by Tonon et al. (2011), in the microencapsulation of flaxseed oil, using gum Arabic as wall material. To be specific, the author also showed that microparticles obtained at 170 °C had a spherical outer surface with a continuous wall and no surface defects. Furthermore, surface microparticles were concave and shrinked, which was featured the morphology of microparticles produced by spray drying. To summarize, an inlet temperature of 140 °C was chosen as the suitable parameter for the encapsulation of PEO using maltodextrin as wall material.

In summary, the suitable parameters for the encapsulation process of PEO were selected at concentration of wall material as 30% (by wt%/wt%), concentration of essential oil as 1.5% (by wt%/wt%), inlet temperature as 140 °C and feed flow rates as 120 mL/h. With these parameters, the optimal indicators including DY (90.05%), MEY (75.59%), and MEE (89.44%) were obtained. The PEO in the microparticles was compositionally characterized with the predominant presence of limonene (95.25%). The microparticles were found to have higher thermal stability than that of free PEO via TGA/DSC analysis result. The micro-structure of obtained powder showed a spherical morphology with outer surface being shrunk and concave. This study suggests the potential applications of spray drying technology in the encapsulation of flavour and natural compounds.

Supplementary Information

Below is the link to the electronic supplementary material.

Declarations

Conflict of interest

The authors declare no conflict of interest.