Microencapsulation of red palm oil as an oil-in-water emulsion with supercritical carbon dioxide solution-enhanced dispersion

Journal of Food Engineering 222 (2018) 100e109

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Microencapsulation of red palm oil as an oil-in-water emulsion with supercritical carbon dioxide solution-enhanced dispersion Wan Jun Lee a, Chin Ping Tan a, Rabiha Sulaiman a, Richard Lee Smith Jr. c, Gun Hean Chong a, b, * a b c

Department of Food Technology, Faculty of Food Science and Technology, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Supercritical Fluid Centre (SFC), Faculty of Food Science and Technology, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Research Center of Supercritical Fluid Technology, Tohoku University, Aramaki Aza Aoba, 6-6-11-403, Aoba-ku, Sendai, 980-8579, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 August 2017 Received in revised form 10 November 2017 Accepted 12 November 2017 Available online 15 November 2017

Feasibility of microencapsulating red palm oil (RPO) with solution enhanced-dispersion by supercritical carbon dioxide (SEDS) without using high-temperatures or organic solvents was assessed. RPO prepared as oil-in-water (o/w) emulsion (11.7% RPO, 69.9% water, 3.5% sodium caseinate, 14.0% maltodextrin, 1.0% soy lecithin) could be encapsulated at all conditions (100e150 bar, 40e60
C, feed injection flow rate 2.5 mL/min). Microcapsules produced with the SEDS method (125 bar, 50
C, CO2 feed 150 L/h) were flowable, spherical powders (d ¼ 5.8 mm, s ¼ 2.8 mm) containing 31.6% oil with 92.1% ME, 82.7% RE for carotenes and 94.3% RE for vitamin E, whereas those from spray drying were irregular-shaped particles (d ¼ 16.6 mm, s ¼ 8.6 mm) containing 39% oil, 79% ME, and having similar RE values. The SEDS method allows microencapsulation of food oils prepared as o/w emulsions without thermal stress or organic solvents. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Red palm oil scCO2 SEDS Particle formation Microcapsule Spray drying

1. Introduction Red palm oil (RPO) is regarded as being similar to crude palm oil (CPO) due to its characteristic red colour and high concentration of carotenoids. However, there are differences between the two oils. CPO is the fresh oil obtained from the mesocarp before refinement (Edem, 2002), whereas RPO is produced from CPO through a process of pre-treatment, de-acidification and deodorization using molecular distillation (Van Rooyen et al., 2008). RPO has high levels of micronutrients such that are of interest to the food and pharmaceutical industries. The micronutrients in RPO are associated with health benefits and are made up of palm carotenoids with concentrations of 500e700 ppm, vitamin E in the form of tocopherols and tocotrienols with concentrations of 500e1000 ppm and sterols and ubiquinones (Edem, 2002; Mba et al., 2015). The major palm carotenoids found in RPO are a- and b-carotenes, which are known to exhibit provitamin A activity. Palm carotenoids are able to enhance the immune system (Rao and

* Corresponding author. Department of Food Technology, Faculty of Food Science and Technology, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia. E-mail address: [email protected] (G.H. Chong). https://doi.org/10.1016/j.jfoodeng.2017.11.011 0260-8774/© 2017 Elsevier Ltd. All rights reserved.

Rao, 2007) and serve as cancer chemoprotection (Amorim-Carrilho et al., 2014). The four major types of vitamin E present in RPO are atocopherol and a-, d-, and g-tocotrienols. In particular, the tocotrienol group in RPO is well-recognized as an antioxidant that is able to prevent cancer as well as having cholesterol-lowering and neuroprotective properties (Colombo, 2010). Both carotene and vitamin E are natural antioxidants (Seppanen et al., 2010; Sharif et al., 2017), being able to reduce the oxidative stress and serve as a potent mechanism for preventing or delaying several chronic degenerative diseases (Rao and Rao, 2007). However, these active components (carotenes and vitamin E) are unstable and are sensitive to oxidation when being exposed to light or temperature. Degradation of the active components causes the nutritional quality of the RPO to be lost. Moreover, when oxidative degradation occurs, the RPO can become rancid. If RPO can be encapsulated without the use of heat, however, it may be possible to improve its stability and preserve its active components from deterioration during storage and processing. There are several different methods that are suitable for encapsulating oil-based materials such as fluidised bed agglomeration (Fuchs et al., 2006), co-extrusion technology (Wang et al., 2013), and spray drying (Calvo et al., 2012). In previous studies, it was reported that crude

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palm oil (Ferreira et al., 2016) and palm oil (Kelly et al., 2014) can be encapsulated with spray drying. The encapsulation of RPO with supercritical carbon dioxide (scCO2) has not been reported, although palm oil is commonly used in food products. The use of scCO2 in the food industry is recognized as a safe and compatible method for separations (Dima et al., 2014), food preservation (Marszałek et al., 2015) and particle formation (Chemat et al., 2017; Jung and Perrut, 2001; Mihalcea et al., 2017; Zhao and Temelli, 2017) and thus it is an attractive method to consider for encapsulating and preserving the characteristics of RPO. The solution-enhanced dispersion by scCO2 (SEDS) particle formation process has been performed on bioactive components (Aguiar et al., 2016; Boschetto et al., 2014, 2013; Machado et al., 2014; Zhao et al., 2015), however, most solutes in those studies were encapsulated or precipitated with the use of organic solvents (acetone, dichloromethane, dimethyl sulfoxide) that are incompatible with food products. There are not many literature studies on
e et al., 2000; making use of water in the SEDS process (Moshashae Zhang et al., 2012) as researchers generally modify scCO2 with organic solvents to increase the water expansion rate (Rodrigues et al., 2006). Virgin coconut oil in the form of o/w emulsion has been microencapsulated using scCO2 spray drying (Hee et al., 2017); nevertheless, the encapsulating conditions (pressure, temperature and feed flow rate) and emulsification process differ from this work. The role of scCO2 in encapsulating RPO and its micronutrients affect the characteristics of the oil microcapsules as discussed later in the text. In this work, the feasibility of microencapsulating RPO o/w emulsions without the addition of heat or the use of organic solvents was investigated. Particle formation that is based purely on water as the solvent and appropriate food-grade encapsulating materials with unmodified scCO2 as the anti-solvent has not been reported. Effect of conditions on the RPO microcapsules properties including microencapsulation efficiency of oil, retention efficiency of active components, particle size and distribution, morphology, fatty acid composition and moisture content were determined. The properties of RPO microcapsules produced with the SEDS method are compared with those produced with the spray drying method. 2. Materials and methods 2.1. Materials The RPO was supplied by Orifera™ (Selangor, Malaysia). Sodium caseinate (NaCas), maltodextrin (MD) DE 10 and soy lecithin were purchased from V.I.S. Food Tech Ingredient Supplies Sdn. Bhd. (Kuala Lumpur, Malaysia), Porrima (M) Sdn. Bhd. (Selangor, Malaysia) and SE Scientific Supplies (Selangor, Malaysia), respectively. Liquid CO2 (purity of 99.99%) was purchased from Mox-Linde Gases Sdn. Bhd. (Selangor, Malaysia). Standards of tocopherol (95.0%) and tocotrienols (99.5%) were purchased from Calbiochem (San Diego, CA, USA) and ChromaDex (Santa Ana, CA, USA). Type II synthetic b-carotene (95%), a-carotene (98.0%) and antioxidant 3,5-Di-tert-4-butylhydroxytoluene (BHT) were purchased from Sigma-Aldrich (St. Louis, MO). Solvents of AR grade petroleum ether, ethanol and HPLC grade methanol, acetonitrile, ethanol, tetrahydrofuran (THF) and chloroform were used. 2.2. Preparation of RPO emulsion Different combinations of materials and formulations were tested and the formulation that offered the most stable emulsion was selected. The final RPO emulsion formulation included 11.7% oil, 69.9% water, 3.5% NaCas, 14.0% MD and 1.00% soy lecithin, (40% RPO, 60% wall material). NaCas and MD were completely dissolved

101

in distilled water at 40
C before blending the mixture together with oil and soy lecithin for 3 min and then it was homogenized at 200 bar through a double-stage valve homogenizer (GEA Niro Soavi, PandaPLUS 2000; Germany) for four cycles.

2.3. Supercritical anti-solvent microencapsulation (SEDS process) An industrial scale aqueous solution and particle formation unit (FeyeCon, Weesp, The Netherlands) as in Fig. 1, was used for the microencapsulation process. The system consisted of a CO2 feed pump (NP16/14e210 C, Speck-Kolbenpumpenfabrik, Geretsried, Germany) for delivery of the CO2 and a syringe pump (Teledyne Isco model 260D, USA) for delivery of the emulsion into the spraying vessel. The spraying vessel was custom-made and had an internal volume of 25 L and was equipped with a polyester filter bag (Nedfilter, Lelystad, The Netherlands) to collect the microcapsules formed. The SEDS process was started by pressurizing the spraying vessel with continuous supply of scCO2 into the system at CO2 flow rate of 150 L/h at the operating pressures and temperatures. When the desired operating conditions had been reached, the RPO emulsion was injected into the spraying vessel. The scCO2 and the RPO emulsion were contacted and mixed internally in a single orifice set-up spray nozzle (full cone spray nozzle, PNR-Nozzles, Voghera, Italy) with diameter of 6.35 mm before being sprayed into the pressurized atmosphere. After the RPO emulsion feed was depleted, scCO2 was flowed through the spraying vessel for an additional 30 min to eliminate any remaining liquid before depressurizing the system for collection of the product particles. The challenge in this work is due to the form of the sample, which is an oil-in-water (o/w) emulsion, so that water needs to be removed by scCO2. Water has a low solubility in scCO2 (Tabasinejad et al., 2011) in comparison with organic solvents commonly used for SEDS particle formation, although supercritical fluid drying of bioactive components is common. When the solubility of water is low, it implies that the extraction efficiency of the water by scCO2 will be correspondingly low and the SEDS process will be less effective as water has to be removed from the emulsion system to form microcapsules. However, the limitations of using o/w emulsions as samples for SEDS microencapsulation can be overcome by modifying the process setup and the operating parameters such as the pressure, temperature and the ratio between the scCO2 and emulsion flow rate. An important factor in the SEDS method is the type of nozzle used. A single orifice full cone spray nozzle with cap was used in this study whereby internal mixing was created between the scCO2 and the RPO emulsion before injecting the emulsion into the spraying vessel. During the intense mixing, turbulence creates small emulsion droplets. The mass transfer between the scCO2 and the water occurs at a high rate due to the larger surface area formed by the small droplets and hence the water removal from the emulsion can be enhanced (Cheng et al., 2016). In this case, scCO2 acts not only as an anti-solvent, but also as a dispersion agent (Jung and Perrut, 2001; Yeo and Kiran, 2005). High turbulence created by the implementation of high scCO2 to emulsion injection flow ratios further reduces the size of the droplets. The water removal efficiency can also be raised by varying the operating conditions. Rodrigues et al. (2006) reported that extraction efficiency of water increases by 35% through increasing the pressure and by 16% when the temperature is raised. Thus, by changing the operating parameters, the water removal efficiency from the sample is affected. With all of the aforementioned factors, experiments were performed to study the microencapsulation of RPO with scCO2 as a dispersion and non-thermal drying agent.

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Fig. 1. Schematic diagram of the SEDS method showing the red palm oil (RPO) emulsion feed and particle formation unit (spraying vessel). Symbols: PI e pressure indicator; FIL e filter; CU e cooler unit; C-1/C-2 e coolers; MP e main pump for CO2 feed pump; H-1 e CO2 heater; DS e drying system; PIC e pressure indicator controller; F-1 e flow indicator; SN e spraying nozzle; SV e spraying vessel; TI e temperature indicator.

2.4. Preliminary experiments on feed flow rate One of the significant factors that will affect the microcapsules characteristics is the feed flow rate. The microencapsulation efficiency (ME) as defined later in Section 2.6 was chosen to be the main criteria on deciding the feed flow rate to be used. To determine the feed flow rate that will produce microcapsules with high ME, two flow rates of 2.5 mL/min and 5.0 mL/min were tested. The other parameters were fixed at 125 bar, 50
C and CO2 flow rate at 150 L/h. Values of 92% and 61% were recorded for ME at feed flow rates of 2.5 mL/min and 5.0 mL/min, respectively. Based on the preliminary results obtained, a feed flow rate of 2.5 mL/min was selected for study in all experiments.

In the total oil content analysis, 20 mL of aqueous ethanol (85 mL ethanol/100 mL) was poured into 2 g of sample followed by adding 50 mL of petroleum ether. The samples were then stirred with a magnetic stirrer for 30 min. Phase separation was observed after the stirring stopped and the top layer was extracted into a tared round bottom flask. Petroleum ether (5 mL) was used to reextract the remaining ethanol solution and the procedure was repeated until the yellowish RPO microcapsules turned white. The total oil content was calculated after solvent evaporation. The microencapsulation efficiency (ME) was calculated from eq. (1): ME (%) ¼ (TOC e SOC)/ TOC x 100%

(1)

2.5. Spray drying microencapsulation

where TOC is the total oil content and SOC is the surface oil content.

RPO microcapsules were prepared by spray drying as a control. RPO emulsion was fed into a spray dryer unit (GEA Niro A/S, model Mobile Minor 2000; Denmark) at constant flow rate of 15 mL/min. Inlet temperature of the spray dryer was set at 165 ± 2
C with recorded outlet temperature of 60 ± 2
C.

2.6.2. Concentration and retention efficiency of carotene and vitamin E The oil was extracted from the microcapsules using the total oil content method (section 2.6.1), followed by sample preparation for high performance liquid chromatography (HPLC) analysis. HPLC analyses of the oil were made using an Agilent Technologies 1200 chromatographic system. Sample preparation for carotene analyses was according to Craft and Craft (2001). The identification and quantification of the a- and b-carotene were determined by comparison with standards and using a Develosil RP-Aqueous C30 column (250  4.6 mm, 5 mm) (Hinode-cho Seto, Japan). The mobile phase (acetonitrile/chloroform - 8:2 v/v) was delivered isocratically at 1 mL/min and the wavelength of the UV detector was set to 450 nm. Vitamin E concentration was determined using the method of Chia et al., (2015). Vitamin E isomers were identified with a

2.6. Characterization of red palm oil microcapsules 2.6.1. Surface and total oil content The method to determine surface oil and total oil was adapted from Quispe-Condori et al. (2011). In the surface oil analysis, 2 g of sample was washed with 1 mL of petroleum ether through a layer of filter paper (Whatman No. 1) into a tared round bottom flask. The washing step was repeated for another four times. The surface oil content was determined after evaporating the solvent from the round bottom flask.

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Phenomenex Kinetex PFP column (250 mm  4.6 mm, 5 mm) (Madrid Avenue, Torrance, USA). The mobile phase (methanol and water) was delivered in gradient mode at 1 mL/min, and a fluorescence detector was used at a wavelength of Ex/Em: 295/330 nm. Retention efficiency (RE, %) for the active components (carotene and vitamin E) was calculated with eq. (2): RE (%) ¼ AC in microcapsules/ Initial AC in bulk RPO x 100%

(2)

where AC is the amount of active components. 2.6.3. Particle size and morphology Surface morphology and inner structure of the RPO microcapsules were viewed with a variable pressure scanning electron microscope (LEO, 1455 VPSEM, UK and Hitachi SU1510 VPSEM, USA). Microcapsules were prepared by spreading the microcapsules on a carbon tape attached to aluminium stubs and were then coated with gold-palladium with a sputter coater (Bal-Tec, SCD 005, Switzerland). To study the microcapsules inner structure, the method of Foerster et al. (2017) was used in which microcapsules were placed on a specimen holder and fractured randomly using a razor blade that was used to slice through the microcapsule layers before being coated with gold. The diameter of the microcapsules was measured with a Fiji (ImageJ v1.51j) image processing and image analysis software. Measurements were made based on at least three SEM images with approximately being used 1400 particles that were taken at different locations. 2.6.4. Fatty acid composition The RPO was esterified into methyl esters with the sodium methoxide method (Qian, 2010). A gas chromatography-flame ionization detector (GC-FID) (Agilent Technologies 6890N Series Network Gas Chromatograph) equipped with a BPX70 SGE capillary column (30 m  320 mm  0.25 mm) was used. The temperature of the oven was initially set to 100
C for 2 min and was heated to 230
C using a ramp of 6
C/min. The injection temperature and detector temperature were both set to 250
C. Samples were injected using split mode (20:1), using an injection volume of 1.0 mL. A single analysis required about 38 min. 2.6.5. Moisture content and flow properties Moisture content of the RPO microcapsules was analysed with a moisture analyzer (XM 120, Precisa, Switzerland). Samples (1 g) were weighed into an aluminium pan and were dried at 105
C. The method of Chinta et al., (2009) was used to determine the bulk (rB) and tapped density (rT) of the RPO microcapsules. Approximately 3 g of RPO microcapsules were poured into a graduated cylinder and the volume was recorded. The rB was calculated with eq. (3):

rB ¼ m0/ V0

(3)

where m0 is the mass of sample used and V0 is the volume recorded from the graduated cylinder. For rT, the graduated cylinder with sample was repeatedly tapped to a constant volume, Vn and then eq. (4) was applied.

r T ¼ m0 / Vn

(4)

Flow properties of the RPO microcapsules was evaluated by Carr's Index (CI) and Hausner Ratio (HR) (Turchiuli et al., 2005) given as: CI ¼ (rT e rB)/ rT x 100

(5)

HR ¼ rT/ rB

(6)

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Microcapsules with CI of 10e25 and HR of 1.00e1.34 are considered to have acceptable flow properties (Quispe-Condori et al., 2011; Turchiuli et al., 2005). 2.7. Statistical analysis All analyses were performed in triplicate and the results are presented as average values. Statistical analysis using analysis of variance (ANOVA) with Tukey's procedure at a confidence level of 95% was made with Minitab 16 statistical software (Minitab Inc., Pennsylvania). Results with p-values less than 0.05 were evaluated as being significantly different. 3. Results and discussion 3.1. Characterisation of RPO microcapsules 3.1.1. Encapsulation and retention efficiency of SEDS method As shown in Table 1, the SEDS method produced RPO particles with microencapsulation efficiencies (ME) of 65%e92%, with retention efficiencies (RE) of carotene and vitamin E ranging from 46% to 83% and 59%e95%, respectively. The operating conditions for the SEDS method that are considered to be optimal for producing microcapsules with high ME and RE are 125 bar, 50
C (run 5). The high surface oil content of (7e13) % at low-pressure conditions (100 bar, Runs 1, 4, 7) had low ME and RE values, showed that less oil was encapsulated and that there was lower entrapment of active components (carotene and vitamin E) compared with other conditions. The total oil content from microcapsules was (26e39) %, which was lower than the 40% of oil introduced into the emulsion formulation (section 2.2), showing that RPO was most likely removed from the emulsion by dissolution into the scCO2 phase during the SEDS process. The loss of RPO in the formation of the microcapsules is attributed to the RPO being more soluble in scCO2 at some conditions leading to less oil being available for encapsulation (Hu et al., 2012). Pressure effects on the total oil in the microcapsules was in accordance with other SEDS studies (Boschetto et al., 2013; Hu et al., 2012) and higher pressures tended to lower the total oil in the microcapsule for a given temperature. 3.1.2. Particle size and particle size distribution (PSD) The SEDS method produced RPO capsules which were micronized to have particle sizes of (5e9) mm as shown in Table 1. Smaller microcapsules were produced at high-pressures and lowtemperatures, with exceptions being runs 3 and 6, where large microcapsules were formed at high-pressures (ca. 150 bar). The particle size is reported to depend on the change in the pressure which € et al., 2002). corresponds to the change in scCO2 density (Rantakyla An example of the effect of pressure on the particle size and particle size distribution (PSD) with the SEDS method is shown in Fig. 2. The higher scCO2 density (higher pressures) probably created a better dispersion of RPO emulsion in the scCO2, promoting the formation of smaller RPO emulsion droplets and these small droplets had short average lifetimes and high mass transfer rates, leading to fast particle formation rates and small particles (Aguiar et al., 2016). At constant pressure, the effect of scCO2 density (higher temperatures) diminished with increasing temperature and large RPO emulsion droplets were created that lead to larger particle sizes and resulted in a wider PSD as shown in Fig. 3. The effect of pressure and temperature on the particle size are similar in trends to results reported for other types of samples (Aguiar et al., 2016; Li et al., 2016; Machado et al., 2014). For runs 3 and 6, it is likely that the diffusion coefficient of scCO2 is insensitive to minor changes in the scCO2 density when the pressure is increased from 125 bar to 150 bar, whereby the density increases from 731.2 kg/m3 to 780.2 kg/m3, and from 613.0 kg/m3 to 699.8 kg/

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Table 1 Properties of red palm oil microcapsules produced at different pressures and temperatures with supercritical carbon dioxide solution-enhanced dispersion technique (SEDS) at fixed feed flow rate of 2.5 mL/min. Run

T (
C)

P (bar)

CO2 density (kg/m3)

Total oil (%)

1 2 3 4 5 6 7 8 9

40

100 125 150 100 125 150 100 125 150 e

628.6 731.2 780.2 384.3 613.0 699.8 290.0 471.5 604.1 e

35.5 29.0 29.1 38.3 31.6 26.6 38.8 33.9 25.7 39.0

50

60

Spray drying

± ± ± ± ± ± ± ± ± ±

1.8 7.6 0.9 0.4 0.4 4.8 1.1 1.4 1.9 0.8

Surface oil (%)

MERPO (%)

7.2 ± 3.6 4.4 ± 0.3 4.7 ± 0.1 9.7 ± 0.7 2.5 ± 1.7 6.9 ± 2.8 13.6 ± 1.3 10.7 ± 1.3 6.5 ± 0.3 8.1 ± 0.6

79.6 84.8 80.6 74.0 92.1 74.6 64.8 68.3 74.6 79.3

± ± ± ± ± ± ± ± ± ±

REcarotene (%) 9.2 3.5 0.9 3.0 4.3 4.7 3.9 4.5 3.5 1.8

46.3 52.4 55.5 55.9 82.7 74.6 50.1 61.1 60.2 84.6

± ± ± ± ± ± ± ± ± ±

2.1 0.3 6.3 1.4 12.2 8.8 1.5 14.8 1.5 7.9

REvitamin (%) 78.4 77.9 78.7 59.0 94.3 95.3 70.6 64.9 71.7 93.2

± ± ± ± ± ± ± ± ± ±

E

6.3 10.3 3.4 2.5 8.0 14.7 3.6 10.8 3.8 1.9

Flowabilitya

PS (mm) 5.06 4.50 5.13 6.12 5.81 9.03 7.03 6.35 6.29 16.6

± ± ± ± ± ± ± ± ± ±

2.3 2.0 2.3 2.5 2.8 3.9 2.8 3.6 2.8 8.6

Unacceptable Unacceptable Acceptable Unacceptable Acceptable Acceptable Acceptable Acceptable Unacceptable Acceptable

MERPO: microencapsulation efficiency; RE: retention efficiency; PS: particle size; rB: bulk density; MC: moisture content. a Criteria according to Quispe-Condori et al. (2011) and Turchiuli et al. (2005); See also Table S1.

Fig. 2. Particle size distribution of red palm oil microcapsules produced at a constant temperature of 50
C with the SEDS method for various pressures ( ( ): 125 bar; ( ): 150 bar.

): 100 bar;

Fig. 3. Particle size and particle size distribution of red palm oil microcapsules produced at a constant pressure of 100 bar with the SEDS method for various temperatures ( ): 40
C; ( ): 50
C; ( ): 60
C.

m3, respectively, according to equations of state applied to the conditions (NIST Chemistry WebBook). Extended particle formation

times at these conditions can lead can lead to large microcapsules being produced (Chen et al., 2013).

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3.1.3. Morphology In general, spherical RPO microcapsules were produced with the SEDS method although different surface morphologies were observed according to the different operating conditions. The surface morphology and inner structure of the optimal RPO microcapsules (run 5) formed with the SEDS method is shown in Fig. 4 (a) and (b), respectively, where it can be seen that the microcapsules had smooth surfaces with porous structures. The encapsulation of RPO into the microcapsule was evident with the presence of oil droplet footprints throughout the inner structure of the microcapsules as shown in Fig. 4 (b). Voids or pores were also noticeable in the structure of the microcapsule. During the water extraction stage, the CO2 dissolved into the particles and was stabilised and dispersed in the water-continuous phase (Nalawade et al., 2006). The CO2 then patterned the polymerisation of the wall materials in the water phase and voids were created at the end of the process re et al., when CO2 was removed during depressurisation (Boye 2014). The sphericity of the RPO microcapsules was not affected by the pressure which is common in SEDS studies on other products (Boschetto et al., 2014; Priamo et al., 2010). However, the sintering of microcapsules was found to be pronounced at low-pressures as shown in Fig. 5. The sintering effect was visible for microcapsules

Fig. 5. Scanning electron microscope images of red palm oil microcapsules produced with the SEDS method at 40
C and at pressures: (a) 100 bar and at (b) 150 bar.

Fig. 4. Scanning electron microscope images of (a) surface morphology and (b) crosssectional view of red palm oil microcapsules produced with the SEDS method (50
C, 125 bar, CO2 flow rate of 150 L/h, emulsion feed flow rate of 2.5 mL/min). Symbols: WM, wall material; V, void; OD, oil droplet.

produced at ca. 100 bar (Fig. 5(a), Run 1), and was more pronounced than microcapsules produced at ca. 150 bar (Fig. 5(b), Run 3). At the lower pressures (ca. 100 bar), the anti-solvent effect of scCO2 is weaker and thus the solvent expansion rate of water in the microcapsule during formation is low. Therefore, at low-pressures, there will be a small quantity of water retained in the emulsion that interacts with the RPO microcapsules and induces sintering before completion of the drying process (Reverchon et al., 2008). Correspondingly, the water solvent expansion rate is greater at high pressures, so that less sintering seems to occur. The effect of temperature on the morphology was visible on the microcapsules produced at the highest temperatures (Run 9), where microcapsules with dimpled surfaces and reduced sphericity were formed (Fig. 6). It has been shown in another study that temperature affects the sphericity of particles formed and that high-temperatures tends to cause the morphology of the particles to be less spherical in shape (Zhao and Temelli, 2017). The cause of the reduced sphericity at high-temperatures for particles formed with the SEDS method and the emulsion in this work is postulated to be due to increased fluidity of the wall materials caused by heat, which lowers membrane stability, thus making it easier to form elongated particles.

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attributed to the different core and wall materials. Both the core materials and wall materials affect the ME and RE values obtained with encapsulation methods. The particle size, the particle size distribution (PSD) and powder flowability can be very important in final product design. All RPO microcapsule particle sizes produced with the SEDS method in this work (Table 1) were less than 10 mm and were smaller than those produced with the SD method. RPO microcapsules produced by the SEDS method had a much narrower particle size distribution (PSD) compared with that of the SD method (Fig. 7). However, not all of the RPO microcapsules produced with the SEDS method produced flowable powders (Table 1) due to agglomeration that is caused by sintering (Fig. 5 (a)). Microcapsules produced with the SEDS method were found to be more spherical in shape and to have smoother surfaces than those produced with the SD method (Figs. 4 (a) and 8 (a)). Supplementary Fig. S1 provides further comparisons between the morphologies of SEDS and SD microcapsules at different magnifications. The surface morphology of RPO microcapsules formed with the SD method (Fig. 8 (a)) is usual due to the shrinkage of the capsule wall upon cooling from high-temperatures (Gallardo et al., 2013). Examination of the inner structure of microcapsules produced by both SEDS and SD methods can provide some insight into the property differences. RPO microcapsules produced with the SEDS method contain many fine pores (Fig. 4 (b)) that may contain oil or CO2 gas, whereas those produced with the SD method have heavy walls (Fig. 8 (b)) and large voids that may contain oil and air. RPO microcapsule walls formed by the SEDS method (Fig. 4 (b)) are much thinner than those of the SD method (Fig. 8(b)). The porous structures of microcapsules produced with the SEDS method are due to CO2 exsolution, whereas the formation of the hollow structures in microcapsules formed by the SD method are due to rapid expansion in the last stage of spray drying (Carneiro et al., 2013; Kagami et al., 2003). RPO microcapsules produced with the SEDS method will have oil (and CO2) that is most likely uniformly distributed throughout the particle volume, whereas microcapsules produced with the SD method will have oil non-uniformly distributed on the inner cavity and inner cavity thick walls of the particles. The bulk and tapped densities were lower for all RPO microcapsules produced by the SEDS method than those produced by the SD method (Table S1) in support of this phenomenological description. RPO microcapsules formed with the SEDS method had higher moisture content than those with the SD method (Table S1) although the values are considered to be sufficient for storage stability (Klaypradit and Huang, 2008). The bulk densities of microcapsules produced with the SEDS method were comparable with other oil microcapsules produced by different techniques (Dima et al., 2015). Depending on the core oil and the conditions, it is possible that

Fig. 6. Scanning electron microscope image of red palm oil microcapsules produced with the SEDS method at 60
C and 150 bar.

3.2. Comparison of SEDS method with spray drying (SD) method The properties of RPO microcapsules obtained with the SEDS and SD methods are comparable, but have some important differences. As shown in Table 1, the total oil content in the microcapsules was lower for the SEDS method than the SD method and generally, the RE values of microcapsules produced by the SEDS method were lower than those of the SD method. However, the SEDS method produced smaller microcapsules than the SD method and at the best conditions determined in this study (Table 1, Run 5), the SEDS method gave higher ME values than the SD method. One reason for the lower RE values for most of the microcapsules produced by the SEDS method (Table 1, Runs 1e4, 6e9) compared with the SD method is related to processing conditions. In many of the runs with the SEDS method, it is likely that components are solubilised in the scCO2 (Davarnejad et al., 2010, 2009; Lik Nang Lau et al., 2008) such that retention efficiencies depend on the solubility of the bioactive compounds in scCO2 as noted in the literature (Hu et al., 2012). Nevertheless, as shown in Table 1 (Run 5), it is possible to adjust conditions with the SEDS method such that the microcapsules produced have higher microencapsulation efficiencies than the SD method and comparable retention efficiencies. Table 2 shows a comparison of microcapsule properties obtained with the SEDS method and with the SD method applied to different core materials and for different wall materials. The ME values of crude palm oil and palm oil particles produced with the SD methods of Ferreira et al. (2016) and Kelly et al. (2014) had higher values than those reported in this work and can be

Table 2 Comparison of microcapsules produced from red palm oil with the supercritical carbon dioxide solution-enhanced dispersion (SEDS) method and for microcapsules produced from red palm oil or crude palm oil with spray drying for several wall materials. Encapsulation method

Core materials

Wall materialsa

Microencapsulation efficiency Retention efficiency of carotene Particle (%) (%) size (mm)

SEDS Spray drying Spray drying

Red palm oil Red palm oil Crude palm oil Palm oil

A A B

64.8e92.1 79.3 92.8e97.9

46.3e82.7 84.6 98.7

4.43e9.58 3.1e4.5 16.61 2.2 12e32 0.7e2.8

C

90.7e93.6

e

26.6e157

Spray drying a

Moisture content (%)

1.8e2.3

Ref.

This work This work (Ferreira et al., 2016) (Kelly et al., 2014)

A: maltodextrin, sodium caseinate, emulsifier, water; B: cassava starch, gum acacia and whey protein concentrate; C: sodium caseinate and lactose.

W.J. Lee et al. / Journal of Food Engineering 222 (2018) 100e109

Fig. 7. Particle size distribution (PSD) for RPO microcapsules produced with the SEDS method ( method: ( ).

): 40
C, 125 bar; (

107

): 50
C, 150 bar and with the spray-drying

the composition of a fatty acid mixture becomes altered with the SEDS method (Priamo et al., 2011). Table S2 shows that even though the fatty acid profile slightly changed, the composition was still in accordance to the Codex fatty acid standards (Codex Alimentarius, 2013). Moreover, the differences in the fatty acid composition for microcapsules produced by the SEDS method or SD method were not significant. 4. Conclusions

Fig. 8. Scanning electron microscope images of (a) surface morphology and (b) crosssectional view of spray-dried red palm oil microcapsules (inlet temperature of 165
C, outlet temperature of 60
C and emulsion feed flow rate of 15 mL/min).

Red palm oil (RPO) prepared as oil-in-water emulsion could be successfully microencapsulated with the solution enhanceddispersion by supercritical carbon dioxide (SEDS) method using a single orifice full cone spray nozzle with cap. RPO microcapsules with the most favourable properties were obtained at 125 bar, 50
C for a CO2 feed of 150 L/h and were fine flowable powders with average particle sizes being 5.8 mm and having a narrow particle size distribution (s ¼ 2.8 mm) and containing 31.6% total oil. Microencapsulation efficiency (ME), carotene retention efficiency (REcarotene) and vitamin E retention efficiency (REvitamin E) of the RPO microcapsules obtained at the optimum conditions were 92.1%, 82.7% and 94.3%, respectively. Scanning electron microscopy of the RPO microcapsules produced at the optimum conditions with SEDS showed them to have spherical shapes with smooth surfaces and to have porous internal structures that probably are responsible for their extremely low bulk density (0.21 g/cm3) and low packed density (0.23 g/cm3). The spray drying (SD) method was compared with the SEDS method for producing microcapsules from the same oil-in-water emulsion. The SD method applied to the o/w emulsion gave RPO microcapsules that contained higher amounts of oil than the SEDS method, but lower ME values, similar RE values and larger particles that had irregular shapes and surfaces. The operating temperature and pressure of the SEDS method have a large effect on the properties of the RPO microcapsules produced, but do not have a large effect on the fatty acid compositions. In conclusion, the SEDS method applied to the above oil-in-water emulsion allows production of RPO microcapsules with neither thermal treatment nor organic solvents. The properties of RPO microcapsules produced with the SEDS method were found to be comparable with those produced by the SD method.

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