Journal of Microencapsulation, May 2005; 22(3): 253–259

Effect of oil loading on microspheres produced by spray

drying
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L. H. TAN, L. W. CHAN, & P. W. S. HENG

National University of Singapore, Singapore

(Received 21 March 2004; accepted 10 September 2004)

Abstract
Oil-loaded microspheres were produced by spray drying emulsions consisting of fish oil and
modified starch suspensions with different oil loadings. The emulsion stability was assessed by oil
droplet size analysis. Microspheres were characterized in terms of size, morphology, yield and
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microencapsulation efficiency. It was found that an increase in oil loading resulted in emulsions
containing larger oil droplets. This corresponded with larger mean microsphere diameters and rounder
microspheres. However, high oil loadings produced lower yields and affected microencapsulation
efficiencies.

Keywords: Spray drying, fish oil, oil loading

Introduction
Spray drying is the transformation of feed from a fluid state into a dried particulate form
by spraying the feed into a hot drying medium (Masters 1991; Giunchedi and Conte
1995). It is a one-step, continuous drying process where the feed is atomized into a spray
of droplets followed by drying and particle formation. It is a widely used technique
in the pharmaceutical, chemical and food industries for microencapsulation purposes.
Oils are encapsulated for various reasons, including taste masking, improvement
of handling properties and protection of components of interest from evaporation or
oxidation.
Studies have found that !-3 polyunsaturated fatty acids (PUFAs) in fish oils, namely
eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have beneficial clinical
effects on the cardiovascular, central nervous and immunological systems in man
(Horrocks and Yeo 1999; Nordoy et al. 2001; Simpoulos 2002; Lemaitre et al. 2003).
However, fish oils are susceptible to oxidation due to their high levels of unsaturation,
leading to the production of off-flavours and toxic and rancid by-products (Wang et al.

Correspondence: P. W. S. Heng, Department of Pharmacy, Faculty of Science, National University of Singapore, 18 Science Drive
4, Singapore 117543, Republic of Singapore. Tel: 6874-2930. Fax: 6775-2265. E-mail: phapaulh@nus.edu.sg
ISSN 0265-2048 print/ISSN 1464-5246 online # 2005 Taylor & Francis Group Ltd
DOI: 10.1080/02652040500100329
 254 L. H. Tan et al.

1991). Thus, encapsulation of fish oils is of particular interest. Microencapsulation of fish

oils confers protection to the oil from oxidation by forming a physical and permeability bar-
rier to oxygen diffusion. In addition, the barrier masks its unpleasant taste and forms free
flowing ‘dry’ powders, improving patient acceptability and ease of handling.
The more common wall materials used for oil encapsulation by spray drying are carbohy-
drates (Shahidi and Han 1993; Mongenot et al. 2000), proteins (Sheu and Rosenberg 1995;
Moreau and Rosenberg 1999) and gums (McNamee et al. 1998; Bertolini et al. 2001).
Oil-to-wall ratios used ranged from 0.1:1–1:1, although ratios of 0.2:1–0.5:1 were more
common. Higher oil loadings were reported to result in poorer emulsion stability and oil
retention. However, preliminary work conducted in the laboratory has shown promise in
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spray drying emulsions with higher oil loadings. The aim of this study was to study the
effect of high oil loading on emulsion stability and its relationship to product particle size,
roundness, yield and microencapsulation efficiency (ME).

Materials and methods

Materials
ROPUFAÕ , a marine oil, was supplied by Roche Vitamins (UK). Modified food starch,
CapsulÕ , was obtained from National Starch and Chemical (USA). All other chemicals used
were of analytical grade.
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Emulsion preparation
CapsulÕ was added to distilled water to obtain a 15% w/w suspension and was allowed to
hydrate overnight. ROPUFAÕ was homogenized with the starch solution using a Silverson
L4RT homogenizer (Silverson, UK). Oil-to-starch weight ratios used were 0.5:1, 1:1 and
1.5:1. The homogenization conditions used were 4500 rpm for 3 min, followed by 5000 rpm
for 2 min.

Determination of emulsion stability

The size distribution of oil droplets (dispersed phase) in the emulsions was determined by a
laser diffraction analyser (LS 230, Coulter, USA). Emulsions were allowed to stand after
preparation and readings were taken at 10, 25 and 40 min. The experiments were carried out
in triplicate.

Preparation of spray-dried microspheres

The emulsions were spray-dried using a pilot-scale spray dryer (Mobile Minor, Niro,
Demark) equipped with a rotary atomizer. The operational conditions used were: air inlet
temperature 150
C, air outlet temperature 80
C and atomizer wheel speed 20 000 rpm. The
emulsions were subjected to gentle stirring to minimize oil droplet coalescence prior to
being fed into the spray dryer using a peristaltic pump. Starch suspensions that did not
contain oil were also spray-dried to produce blank microspheres for comparison. The
powders collected were sealed in plastic bags and kept refrigerated at 4
C while awaiting
further tests.
 Effect of oil loading on microspheres 255

Microsphere morphology
The morphology of the microspheres was evaluated using a scanning electron microscope
(JEOL, JSM-5200, Japan). Microspheres were gold-coated under an argon atmosphere
(Bio-Rad, SC 502, UK) and examined at 1500 magnification.

Size analysis of microspheres

Sizing of microspheres was carried out using a light microscope (BX61TRF, Olympus,
Japan) connected to an image analysis system (MicroImageTM, Olympus, Japan). At least
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250 microspheres were measured for each batch and the mean particle diameter reported.
The roundness of the microspheres was calculated using the following equation:

P2
Roundness ¼ ð1Þ
4:
:A
where P ¼ perimeter of microsphere and A ¼ cross-sectional area of microsphere

Yield determination
The yield of the process was calculated as follows:
Weight of product in collection bottle
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Yield ¼  100%: ð2Þ

Dry weight of feed

Each determination was carried out in triplicate. Any product adhering to the walls of the
drying chamber or other spray dryer components was not considered as part of the yield.

Determination of oil content of microsphere or ME

The amounts of surface and total oil were determined to calculate the ME. Surface oil
(or non-encapsulated oil) was determined by a modified method described by Varavinit et al.
(2001). Hexane (50 ml) was added to an accurately weighed amount (5 g) of powder
followed by stirring for 10 min. The suspension was then filtered and the residue rinsed
thrice by passing 20 ml of hexane through each time. The residual powder was then air dried
for 30 min and weighed. The amount of surface oil (OS) was calculated by the difference in
weights of the microspheres, before and after washing.
OS ¼ Original weight  Final weight of microspheres ð3Þ

The total oil (OT), which includes both the encapsulated oil (OE) and OS , was determined
using the Soxhlet extraction unit (B-811, Büchi Labortechnik AG, Switzerland). Accurately
weighed microspheres (5 g) were extracted using 180 ml hexane for 8 h to ensure complete
oil extraction. After extraction, the oil-exhausted powder was air-dried to constant weight.
The OT and the ME were calculated as follows:

OT ¼ Original weight  Weight of Soxhlet extracted microspheres ð4Þ

OE ¼ OT  OS ð5Þ
OE
ME ¼  100% ð6Þ
OT
 256 L. H. Tan et al.

Results and discussion

Emulsion stability
Studies have shown that emulsion stability influenced the amount of encapsulated oil,
microsphere quality and functionality (Lin et al. 1995; Sheu and Rosenberg 1995; Kim et al.
1996). Therefore, it is important to produce reasonably stable emulsions before they are
spray dried to obtain microspheres. Emulsion stability is affected by the rate at which the
emulsion undergoes creaming, flocculation or coalescence. These phenomena pre-dispose
the emulsion to instability and possibly phase separation. As the rate of coalescence can be
measured by determining the change in size of oil droplets in an emulsion (Huang et al.
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2001), this method was employed to assess emulsion stability. Besides the evaluation of
emulsion stability, it is important to know the size and size distribution of the oil droplets in
the emulsions as they also affect retention of oil in the dried powder and the amount of
surface oil present on the oil-loaded microspheres (Hogan et al. 2001). McNamee et al.
(1998) found that spray-dried emulsions with larger mean oil droplet diameters had larger
quantities of surface oil present, resulting in low MEs. The time taken to complete the spray
drying process for a typical batch was 30 min. Hence, the duration for which the emulsion
was required to be stable was arbitrarily set at 40 min. The results are shown in Table I.
As the oil:wall ratio was increased from 0.5:1 to 1.5:1, mean oil droplet diameter
increased significantly (>24%), with the change increasing by 24.8% at 10 min and
36.2% at 40 min. Modified starches contain functional groups that enable them to act as
emulsifying agents (Ré 1998). Hence, the larger oil droplets obtained with higher oil:wall
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ratio could be explained by the reduced emulsifying effect of a lower amount of starch
with respect to the oil. For the same oil:wall ratio, the mean oil droplet diameter was
found to increase with time. The change was minimal (2.8%) for an oil:wall ratio of 0.5:1
but significantly larger for higher oil:wall ratios (12–13%). However, the changes were
not expected to have a significant effect on emulsion stability as no phase separation was
observed.

Microsphere characterization
Scanning electron micrographs (SEM) showed that the spray-dried particles prepared from
the three formulations were generally spherical with surface indentations and no apparent
surface cracks (Figure 1). This was the typical appearance of spray-dried products. The
surface indentations were due to rapid particle shrinkage during the early stage of the drying
process (Kim and Morr 1996). These microspheres were likely to offer good protection of
the encapsulated oil as they had a continuous surface without cracks (McNamee et al.
1998). Further studies will be carried out to evaluate this.
The results of particle size, roundness, yield and ME determinations are shown in
Table II. Blank microspheres without oil were the smallest and least round, followed by

Table I. Change in mean oil droplet diameter with time for various emulsion formulations prepared.

Mean oil droplet diameter (mm) for oil:wall ratio of

Time (min) 0.5:1 1:1 1.5:1

10 3.606  0.013 3.698  0.074 4.500  0.138

25 3.711  0.051 4.126  0.275 4.910  0.036
40 3.709  0.015 4.185  0.168 5.046  0.042
 Effect of oil loading on microspheres 257
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Figure 1. SEM of spray-dried microspheres prepared at oil:wall ratio of 1:1 using modified starch as
wall material.

Table II. Mean particle size, roundness, yield and ME as a function of oil:wall ratio for spray-dried microspheres.

Mean particle Microsphere

Oil:wall ratio diameter (mm) roundness Yield (%) ME (%)

0:1 13.805  0.032 1.150  0.001 74.40  4.96 —

0.5:1 15.660  0.023 1.139  0.002 43.77  5.92 92.10  0.035
1:1 19.450  0.405 1.133  0.001 68.60  5.75 89.09  0.147
1.5:1 18.882  0.391 1.130  0.001 47.83  7.49 57.39  2.920

microspheres prepared with an oil:wall ratio of 0.5:1. This could possibly be due to the
absence of, or lower oil content, allowing more extensive microsphere shrinkage during
the early stage of the drying process (Mongenot et al. 2000; Bertolini et al. 2001). At a
higher oil:wall ratio, the oil occupied a larger volume of the microsphere. It also contributed
to the mechanical strength of the microsphere matrix. These effects reduced microsphere
shrinkage during drying. Oil:wall ratios of 1:1 and 1.5:1 gave similar mean diameters and
roundness ( p > 0.05), indicating that the aforementioned effects increased with higher oil
content before levelling off.
Low oil:wall ratio of 0.5:1 and high oil:wall ratio of 1.5:1 produced significantly lower
yields than that of oil:wall ratio of 1:1. This showed that an optimal amount of starch was
necessary for high yield. The low yield was due to microspheres sticking to the inner
walls of the spray dryer components. This problem was more predominant with small
microspheres produced at low oil:wall ratio or with tacky microspheres produced at high
oil:wall ratio. At a high oil:wall ratio, the amount of starch was insufficient to encapsulate
 258 L. H. Tan et al.

the oil, causing part of the oil to be unencapsulated and remain on the surface of the
microsphere. The shrinking could also force oil in the core to leach out as microspheres
shrank during drying, producing tacky microspheres. However, the yield prepared with an
oil:wall ratio of 1:1 was significantly higher ( p < 0.05). This could be due to its greater
size and more efficient encapsulation resulting in less surface oil and, consequently, less
sticking. Blank microspheres, although smaller than oil-containing microspheres, were not
tacky and, hence, adhered less readily to the spray dryer surfaces, resulting in higher yields.
ME values obtained in this study are comparable to those obtained by other researchers
using maltodextrins and modified starches for the encapsulation of meat flavour and liquid
cheese aroma (Mongenot et al. 2000; Jeon et al. 2003) and higher than the values obtained
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for the encapsulation of caraway essential oil using proteins and maltodextrin (Bylaitë et al.
2001). The difference in values is likely due to different core and wall materials used. It was
found that an increase in oil loading resulted in lower MEs, with less oil being encapsulated.
This could also be due to lower amount of wall material used to encapsulate the oil as the oil
load was increased (Fäldt and Bergenståhl 1995).

Conclusions
Higher oil loading increased the droplet size of emulsions formed. The droplets coalesced
over time but did not cause phase separation. The size, roundness, yield and ME of the
spray-dried microspheres were affected by the oil loading. The size and roundness of
the microspheres increased with the oil:wall ratio while the opposite effect was observed with
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ME. An optimum oil:wall ratio was necessary to obtain a high yield.

Acknowledgements
The authors would like to thank Roche Vitamins, UK, National Starch & Chemical, USA
and ISP Alginates Inc., USA for samples used during the study.

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