ORIGINAL ARTICLE
Stability of CoQ10-Loaded Oil-in-Water (O/W) Emulsion:
Effect of Carrier Oil and Emulsifier Type
Sook Wah Chan&Hamed Mirhosseini&
Farah Saleena Taip&Tau Chuan Ling&Chin Ping Tan
Received: 14 December 2012 / Accepted: 20 May 2013 / Published online: 29 June 2013
#Springer Science+Business Media New York 2013
Abstract Co-enzyme Q10 (CoQ10), a lipophilic compound that widely used in the food and pharmaceutical products was formulated in a k-carrageenan coated oil-in-water (O/W) emulsion. In this work, we examined the solubility of CoQ10 in different carrier oils and effects of emulsifier type on the formation and stability of CoQ10-loaded O/W emulsion. Nine vegetable oils and four types of emulsifiers were used. CoQ10 was found significantly (p<0.05) more soluble in medium chain oils (coconut oil and palm kernel oil) as compared to other vegetable oils. The O/W emulsions were then prepared with 10 % (w/w) coconut oil and palm kernel oil containing 200 g CoQ10/L oil stabilized by 1 % (w/v) emulsifiers (sucrose laurate (SEL), sodium stearoyl lactate (SSL), polyglycerol ester (PE), or Tween 80 (Tw 80)) in 1 % (w/v)k-carrageenan aqueous solution. Particle size distribution and physical stability of the emulsions were monitored. The droplet sizes (surface weighted mean diam- eter, D[3,2]) of fresh O/W emulsion in the range of 2.79 to 5.83μm were observed. Irrespective of the oil used, results indicated that complexes of SSL/k-carrageenan provided the most stable CoQ10-loaded O/W emulsion with smaller and narrower particle size distribution. Both macroscopic and microscopic observations showed that O/W emulsion stabilized by SSL/k-carrageenan is the only emulsion that
exhibited no sign of coalescence, flocculation, and phase separation throughout the storage period observed.
Keywords CoQ10 .k-carrageenan . O/W emulsion . HLB . Emulsion stability . Particle size
Introduction
Coenzyme Q10 (CoQ10), also known as ubiquinone, ubiquinol, and ubidecarenone, is a lipid-soluble compound that synthesized endogenously in the human body [1, 2].
Found in most cell membranes, CoQ10 functions as a co- factor in the mitochondrial electron transport chain and is essential for the production of ATP [3,4]. The important role of CoQ10 in various clinical aspects has been well established. Several studies have provided evidences of the beneficial effect of CoQ10 in the treatment of cardiovascular disorders [1,5]. Recently, CoQ10 has even been incorpo- rated in energy drinks as nutraceutical supplement and in skin care products as anti-wrinkle agent [6]. However, CoQ10 is practically insoluble in water and poorly absorbed in the gastrointestinal tract [3, 7]. The low absorption of CoQ10 (Tmax: 2–10 h) is mainly attributed to its high molecular weight (863.34 g/mol) and low water solubility (<0.0001 mg/L) [1,3,6,8]. Therefore, their bioavailability in the commercial formulation is questionable. Furthermore, CoQ10 is vulnerable to heat, light, and oxygen [3]. This limits its application in food fortification [3,6].
Oil-in-water (O/W) emulsions consisting of homoge- nized oil droplets dispersed in an aqueous continuous phase are proved to enhance the bioavailability of liposoluble bioactive compounds and protect them from destructive effects of the environment [9, 10]. In an earlier study, Cornacchia et al. [9] found that stable O/W system enhanced the protection of β-carotene from reactive species in the surrounding. Similarly, Neves et al. [10] demonstrated that S. W. Chan
:
H. Mirhosseini:
C. P. Tan (*)Department of Food Technology, Faculty of Food Science and Technology, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia
e-mail: [email protected] F. S. Taip
Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia
T. C. Ling
Institute of Biological Sciences, Faculty of Science, University of Malaya, Lembah Pantai, 50603, Kuala Lumpur, Malaysia
β-carotene and γ-oryzanol-loaded O/W emulsions were physically stable for at least 4 months during storage in darkness 5 °C. Conventionally, O/W emulsions are prepared by using homogenization technology. First, a lipid soluble compound is dissolved in an oil phase. Second, the oil phase was mixed together with an aqueous phase by homogeniza- tion in the presence of emulsifier [9]. Unfortunately, O/W emulsion is a thermodynamically and kinetically unstable system which tends to break down over time through a variety of physiochemical mechanisms, including coales- cence, flocculation, and gravitational separation. It is possi- ble to form emulsion that is thermodynamically stable over a reasonable period of time by incorporating emulsifier into the system [11,12].
Emulsifier is defined as surface-active molecules (surfactants) that adsorb to the surface of newly formed droplets, lowering the surface tension and preventing droplets against immediate recoalescence [11,13]. However, low molecular weight emulsi- fiers alone are effective in generating small droplets during homogenization but they may not be able to provide sufficient protection against long term destabilization processes [14]. Hy- drocolloids for instance, polysaccharide are often incorporated into the O/W emulsions to form thicker interfaces around the oil droplets and improve the emulsion stability. Bule et al. [3]
performed the microencapsulation of CoQ10 using blends of gum Arabic, maltodextrin and modified starches. A complex between CoQ10 and cyclodextrins was prepared by Takahashi et al. [15] to improve water solubility and absorption of CoQ10.
In this study,κ-carrageenan, an anionic polysaccharide extracted from marine red algae was used to coat and stabilize the CoQ10- loaded emulsion droplets [16, 17]. Recently, utilization of κ-carrageenan in food and pharmaceutical industries has received considerable attention due to their non- toxicity and biocompatibility [18, 19]. Moreover, k- carrageenan can either shrink or swell upon a change in pH and temperature. These characteristics make k- carrageenan one of the smart delivery carriers for the controlled release of bioactives in the gastrointestinal tract [19, 20]. To date, no research on the microencap- sulation of CoQ10 in o/w emulsion using k-carrageenan as coating material has been reported.
Therefore, the aim of this study was to produce a stable CoQ10-loaded O/W emulsion coated with emulsifier/κ-car- rageenan complexes using different oils and emulsifiers. In the initial stage, the solubility of CoQ10 in nine different vegetable oils was evaluated. Subsequently, oil with highest CoQ10 solubility was used for the preparation of CoQ10- loaded O/W emulsions. The effects of four food grade emulsifiers (SEL, SSL, PE and Tw 80) on the emulsion stability were examined and compared based on their parti- cle size distribution and physical changes such as coales- cence, flocculation, and phase separation. Storage stability of the resulting emulsions was also investigated.
Materials and Methods
Materials
CoQ10 API powder (purity >99 %) was procured from PharmaEssentia Corporation (Taipei, Taiwan). Food-grade κ-carrageenan sample used in this study was obtained as gift from Tacara Sdn. Bhd. (Tawau, Malaysia). Canola oil, co- conut oil, corn oil, grape seed oil, olive oil, palm oil, soybean oil, and sunflower oil were obtained from a local market. Palm kernel oil was donated by Sime Darby Berhad (KL, Malaysia). The emulsifier Tw 80 and hexane were purchased from Merck KGaA (Darmstadt, Germany). Other emulsifiers (SEL, SSL, and PE) were gift from Mitsubishi- Kagaku Foods Corporation (Tokyo, Japan), Rikevita Sdn.
Bhd. (JB, Malaysia), and Daniso A/S (Copenhagen K, Den- mark), respectively. All the solvents and chemicals used for the analyses were of analytical grade. Deionized water pu- rified by Sartorius Atrium®611D1 (Sartorius Stedim Bio- tech, Germany) was used for the preparation of all solutions and emulsions.
Evaluation of Solubility of Co-Q10 in Different Oils One milliliter of different oil was pipetted into 15 mL cen- trifuge tube, which wrapped with aluminium foil to prevent light exposure. CoQ10 (approximately 250 mg) was added to the tubes up to saturated concentration with stirring at 175 rpm in a water bath shaker (ProTech, Saintifik Maju, Malaysia) at 37 °C to facilitate the solubilization. Each CoQ10/oil mixture was prepared in duplicate. The mixture was then equilibrated for 24 h and was centrifuged at 3,000 rpm for 5 min to separate the insoluble CoQ10.
Aliquots of the supernatant (5μL) were diluted with hexane, and the total volume of the solutions was adjusted to 25 mL.
The absorbance was measured in a 1 cm quartz cuvette at 275 nm using UV spectrophotometer (UV mini 1240, Shimadzu, Japan). The oil-n-hexane solutions without CoQ10 were used as blanks. The concentration of CoQ10 was calculated from the regression equation of the standard curve (y=0.0154x; R2=0.9929) where y is the absorbance reading and x is the CoQ10 concentration (mg/L). Each sample was analyzed in triplicate. Oil with the highest CoQ10 solubility will be used as core oil in the next stage for the selection of emulsifier.
Gas Chromatography Analysis
Fatty acid composition of derivatized oil samples was ana- lyzed using GC (Model: HP 6890 series, Agilent Technol- ogies, Santa Clara, USA) fitted with column BPX70*
(length: 60 m; ID: 0.25 mm) (SGE Analytical Science Pty, Ltd., Australia) and flame ionization detector (FID). The
temperature of the column increased from 115 °C to 180 °C at the rate of 8 °C/min and was held for 10 min. The temperature was then allowed to reach up to 240 °C at the rate of 8 °C/min and was held for 10 min. Injected volume was 1μL. Helium was used as carrier gas and the flow rate was 1.6 mL/min. The results were analyzed with Chemostation software (Agilent Technologies, Santa Clara, USA) and FAMEs of oil samples were identified by comparing the retention time with the FAMEs standard mixtures. Each oil samples was analyzed in duplicate.
Examination of Stability of Emulsion with Different Emulsifiers
O/W emulsion system was developed in this part to evaluate the effects of different emulsifiers on the emulsion stability.
The O/W emulsion comprised of the oil phase (CoQ10 and oil) and the aqueous phase (emulsifier and k-carrageenan).
The aqueous phase was firstly prepared by dissolving 1.0 g of k-carrageenan in 100 mL deionized water heated to 80 °C.
Four emulsifiers (SEL, SSL, PE, and Tw 80) were then added individually tok-carrageenan solution at 1 % w/v. The mix- tures were warmed at 60 °C for 1 h to facilitate the dissolution of emulsifiers. 10 % w/w of selected oils was slowly dispersed into the aqueous phase and the emulsions were homogenized at 5,000 rpm for 10 min using a high speed rotor/stator mixer (model L4RT, Silverson, England). In each oil, CoQ10 was added at 200 g/L. The emulsions were stored under refriger- ation at approximately 4 °C and were sampled at certain time intervals (Day 0, Day 10, Day 20, and Day 30) for the determination of oil droplet size distributions.
Droplet Size Measurement
The oil droplet size distributions of emulsions were determined using a laser diffraction particle size analyzer (Mastersizer 2000,
Malvern, UK) connected to a flow sample unit cell. This machine can detect particle sizes ranging from 0.02 to 2,000 μm. Deionized water (18.2 MΩ·cm) which is ion-free was used as the dispersant. Refractive index of deionized water, coconut oil, and palm kernel oil was set as 1.33, 1.448, and 1.4569, respectively. Emulsions were added drop by drop to the circulating deionized water until laser obscuration fall between 4.5 % and 5.0 %. The samples were stirred continuously at constant speed (1,500 rpm) throughout the measurement to ensure the homogeneity of the samples. In this study, the particle size was reported as surface weighted mean D[3,2] (=∑ nidi
3/∑nidi
2, where ni is number of particles with diam- eter di) in which the mean diameter is based on the volume frequency. Each sample was analyzed in triplicate and the results were reported as an average.
Visual Observation
Emulsion samples were inspected visually to detect any physical instability, such as coalescence, flocculation, and phase separation. The appearances of emulsions were eval- uated for 30 days of storage.
Microscopic Observation
A drop of emulsion was placed in a microscope slide and then covered with cover slip. The microstructures of O/W emulsions were analyzed using a light microscope (Nikon Eclipse 80i, Kanagawa, Japan) equipped with a CCD camera (Nikon 5 megapixel, Kanagawa, Japan) connected to digital image processing software (NIS- Elements Basic Research, Nikon Instruments) at 400×
magnifying power. The samples were analyzed at room temperature.
Fig. 1 Solubility of CoQ10 in different oils. All the results were mean ± SD of triplicate determinations. Theerror bars represent the standard deviation. Means with the same letter are not significantly different atp<0.05
Statistical Analysis
The experimental results were analyzed using Minitab soft- ware (Minitab Version 14.1). All data were expressed as means ± standard deviations of triplicate measurements.
One-way analysis of variance (ANOVA) at 5 % significance level was used to determine the significant differences (p<
0.05) between the means.
Results and Discussion
Solubility of CoQ10 in Different Oils
High lipophilicity compounds such as CoQ10 have very limited solubility in the aqueous environment. It was proven that solubility of lipophilic compounds can be significantly enhanced by formulating it in an O/W emulsion system [21].
To prepare a stable O/W emulsion, it is necessary to disperse the CoQ10 crystals into carrier oil and heat it to a temper- ature where it melts. The hot oil phase was then homoge- nized with an aqueous phase containing hydrophilic emul- sifier to form an oil-in-water emulsion. The O/W emulsion is expected to increase the solubility of CoQ10 meanwhile enhance its oral bioavailability. Therefore, selection of a suitable oil medium becomes very important to maximize the dissolution of CoQ10 in the formulation.
In this study, the solubility of CoQ10 was examined in nine different vegetable oils at 37 °C. The oils were selected based on their availability in the market. Elevated tempera- ture can aid in the dissolution of CoQ10 in oil medium.
Figure 1 shows that Co-Q10 was significantly (p<0.05) more soluble in coconut oil (226.62 g/L) and palm kernel oil (210.71 g/L), followed by palm oil (149.68 g/L), olive oil (142.97 g/L), grape seed oil (140.04 g/L), sunflower oil (122.94 g/L), corn oil (121.32 g/L), canola oil (117.42 g/L), and soybean oil (117.10 g/L). The soybean oil that commonly used in the conventional CoQ10 formulation such as Super Bio-Quinone (Pharma Nord, Holland) and GNC preventive nutrition CoQ10 (General Nutritional Corporation, US) showed the lowest solubility of CoQ10 in the current finding.
Soybean oil might not be the best medium for the optimal dissolution of CoQ10.
The solubility difference was probably due to the type of fatty acids present in the oils. GC analysis tabulated in Table1 shows that coconut oil (49.97 %) and palm kernel oil (49.87 %) contain medium chain fatty acid (lauric acid) as the major component while canola oil, corn oil, grape seed oil, olive oil, palm oil, soybean oil, and sunflower oil are mainly made up of long chain fatty acids (palmitic acid, stearic acid, oleic acid, and linoleic acid). The same phe- nomenon was reported by Bule et al. [3] and Thanatuksorn
et al. [22] that Co-Q10 soluble the most in medium chain Table1FattyacidcompositionoftheoilsusedinCoQ10solubilitytest OilsFattyacids(%) CaprylicacidCapricacidLauricacidMyristicPalmiticacidPalmitoleicacidStearicacidOleicacidLinoleicacidLinolenicacidArachidicacid acid C8:0C10:0C12:0C14:0C16:0C16:1C18:0C18:1C18:2C18:3C20:0 Canola––––5.82±0.52–2.13±0.1761.66±3.1322.40±2.90–8.00±0.45 Coconut9.42±0.087.25±0.1349.97±0.6017.52±0.417.92±0.16–2.58±0.154.39±0.11––– Corn––––14.28±0.81–2.34±0.0135.50±0.5347.89±0.26–– Grapeseed––––9.94±0.06–4.07±0.0526.63±0.0559.37±0.06–– Olive13.28±0.010.76±0.042.63±0.0474.65±0.018.70±0.08–– Palm–––0.88±0.0941.24±0.60–4.18±0.0743.71±0.4710.00±0.16–– Palmkernel4.94±0.334.39±0.3049.87±1.2515.34±0.077.80±0.40–1.88±0.2113.73±1.201.65±0.17–– Soy––––12.37±0.00–5.32±0.0523.95±0.0353.72±0.09–4.64±0.07 Sunflower––––6.86±0.27–3.12±0.0839.35±0.2250.67±0.13––
fatty acid (coconut oil) at solubility of 215 g/L and 205 g/L, respectively. These results were slightly lower than the one reported in this work (226.62 g/L). Likewise, Kommuru et al. [7] who investigated the solubility of CoQ10 in various oils found that medium chain fatty acid (capric acid) pro- vides higher solubility for Co-Q10 than oils with long chain fatty acid (corn oil, peanut oil, and soybean oil). Addition- ally, coconut oil and palm kernel oil are both exist in solid form at room temperature whereas others appear in liquid form. A study done by Cornacchia and Roos [9] reported that solid lipid carrier (palm kernel oil) protected the bioac- tive compound better than liquid carrier (soybean oil) against oxidation. Therefore, coconut oil and palm kernel oil which are rich in medium chain fatty acid can be a potential carrier for lipid soluble active compounds in the supplement formulation. Coconut oil and palm kernel oil were thus selected for use in the determination of suitable emulsifier type.
Effect of Emulsifier Type on the Droplet Size Distribution and Emulsion Stability
Another crucial parameter for stable O/W emulsion is the selection of appropriate type of emulsifier. Selection of emulsifier is the key factor determining the shelf-life and physicochemical properties of the emulsion. In the present study, four emulsifiers (SEL, SSL, PE, and Tw 80) were selected based on their HLB value and commercial use in O/W emulsion [3, 5, 22]. The importance of emulsifier in stabilizing O/W emulsion is depicted in Fig. 2. CoQ10- loaded emulsion appeared to be opaque and creamy yellow when stabilized in emulsifier/κ-carrageenan system. With- out adding emulsifier, a layer of orange colour CoQ10- loaded oil was found floating on the top of emulsion due to phase separation. The colour of oil phase was darker than the aqueous phase which consisting of emulsifier, k- carrageenan, and deionized water. Polysaccharides can act
as both emulsifier and stabilizer in the O/W emulsion system but they must be surface-active to be effective as an emul- sifying agent [23]. High molecular weight and non-surface active polysaccharide like k-carrageenan has more charac- teristic roles as a colloid stabilizer and gelling agent. There- fore, k-carrageenan must work synergistically with low molecular weight emulsifier in order to produce a stable O/W emulsion. During homogenization, low molecular weight emulsifiers absorb at the oil–water interface, reduce
0.01 0.1 1 10 100 1000 3000
Particle Size (µm) 0
5 10 15
0.01 0.1 1 10 100 1000 3000
0 5 10 15 20
a
b
Volume (%)
10% coconut oil/sucrose laurate 10% coconut oil/sodium stearoyl lactate 10% coconut oil/polyglycerol ester 10% coconut oil/Tween 80
Volume (%)
Particle Size (µm)
10% palm kernel oil oil/sucrose laurate 10% palm kernel oil/sodium stearoyl lactate 10% palm kernel oil/polyglycerol ester 10% palm kernel oil/Tween 80
Fig. 3 Comparison of particle size distribution of CoQ10-loaded emulsions containing (a) 10 % (w/w) coconut oil and (b) 10 % (w/
w) palm kernel oil and various emulsifiers (SEL, SSL, PE, and Tw 80) at 0 day. All the results are mean ± SD of triplicate determinations Fig. 2 CoQ10-loaded coconut
oil emulsions with (a) no emulsifier; (b) SEL; (c) SSL;
(d) PE; and (e) Tw 80
oil–water interfacial tension to generate small oil droplets while κ-carrageenan hold the oil droplets into a gel-like network leading to long term emulsion stability against creaming, flocculation, and gravitational separation.
The effectiveness of four emulsifiers in stabilizing CoQ10-loaded O/W emulsions was compared in response to oil droplet size. Oil droplet size is known to be related to the physical changes of an emulsion, and its evolution can be taken as a measure for the long-term stability of the emulsion [24]. Figure 3a shows that unimodal emulsion with average particle size of 3.15 μm was obtained at 0 day when SSL was used in the emulsion containing CoQ10 in coconut oil. For palm kernel oil emulsion, SEL exhibited a more uniform and narrower particle size distri- bution at 0 day among all the emulsifiers (Fig.3b). The use of PE and Tw 80 resulted in wider particle size distribution and bimodal emulsions regardless of the oil type.
Furthermore, the droplet size of CoQ10-loaded emulsions with storage time was evaluated (Table2and Fig.4). With
Table2Thedropletsizeandstabilityofemulsionafterstorage OilEmulsifierAmountofoil,%(w/w)Day0Day10Day20Day30 D[3,2](μm)y Stabilityx D[3,2](μm)y Stabilityx D[3,2](μm)y Stabilityx D[3,2](μm)y Stabilityx CoconutoilSEL102.79±0.15b ++3.22±0.48ab −−3.17±0.15ab −−3.57±0.02a −− SSL3.15±0.09a ++3.03±0.19a ++3.13±0.16a ++3.34±0.35a ++ PE4.22±0.15a ++3.69±0.07b +−3.93±0.28ab +−4.35±0.11a +− Tw803.12±0.19b ++3.71±0.08b −−4.50±0.53a −−4.55±0.03a −− PalmkerneloilSEL103.90±1.11a −−4.27±0.91a −−4.19±0.82a −−6.74±1.56a −− SSL3.49±0.19a ++3.18±0.09a ++3.47±0.10a ++3.59±0.23a ++ PE5.83±0.15b +−5.88±0.46b +−6.26±0.69b +−7.76±0.68a +− Tw804.61±0.04b+−5.28±0.57ab+−5.92±0.32a+−5.59±0.36a+− x ++nophaseseparationandcoalescence,+−coalescencebutnophaseseparation,−−phaseseparationandcoalescence y Alltheresultsaremean±SDoftriplicatedeterminations.Meanvaluesinthesamerowfollowedbydifferentlettersaresignificantlydifferentatp<0.05
Fig. 4 Particle size of CoQ10-loaded emulsions containing (a) 10 % (w/w) coconut oil and (b) 10 % (w/w) palm kernel oil and various emulsifiers (SEL, SSL, PE, and Tw 80) after storage. All the results are mean ± SD of triplicate determinations. Theerror barsrepresent the standard deviation
SSL as emulsifier, the emulsions tended to be more stable with insignificant increment (p>0.05) in particle size, from 3.15 to 3.34 μm and 3.49 to 3.59 μm in coconut oil emulsion and palm kernel oil emulsion, respectively. These
emulsions exhibited no sign of phase separation through- out the 30 days of storage (Table 2). For coconut oil emulsion, SEL and Tw 80 stabilized systems showed significant (p<0.05) increase in particle size after 1 month
a b
10 µm 10 µm
Fig. 5 Microscopy images (x400) of CoQ10-loaded (a) coconut oil and (b) palm kernel oil emulsion prepared without addition of emulsifier.Scale bar in all images represents 10μm
Day 0 Day 30
SEL
SSL
PE
Tw 80
10 µm 10 µm
10 µm 10 µm
10 µm 10 µm
10 µm 10 µm
Fig. 6 Microscopy images (x400) of CoQ10-loaded coconut oil emulsions prepared using different emulsifiers at 0 day and after 1 month storage.
Scale barin all images represents 10μm
storage at 4 °C. SEL which maintained a homogenous oil droplet in freshly prepared emulsions started to phase separate after only half day (palm kernel oil emulsion) and 10 day (coconut oil) of storage depending on the oil type used (Table 2). Particularly, the particle size of CoQ10-loaded palm kernel oil based emulsion was found increased dramatically from 3.90 (Day 0) to 6.74 μm (Day 30). Emulsifier with medium HLB (SSL, HLB= 8.3) were observed to be more effective than those with low (PE, HLB= 7.0) and very high (Tw 80, HLB = 15.0; SEL, HLB = 16.0) HLB.
The results above can be well explained by relating the HLB value of the emulsifiers to required HLB (rHLB) value of the oil phase. Schmidts et al. [25] mentioned stable emulsion is best formulated with emulsifier which possesses HLB values close to the rHLB of the oil phase. Therefore,
the most stable O/W emulsions were obtained using SSL as SSL having HLB (8.3) closest to the rHLB (8.0) of coconut oil and palm kernel oil. CoQ10 with HLB value of 0 was not considered in the calculation of rHLB [26]. In contrast, the stability of coconut and palm kernel O/W emulsions were lowest when they were prepared at a HLB (SEL, HLB=
16.0; Tw 80, HLB=15.0) much higher than the best rHLB of coconut oil and palm kernel oil. On the contrary, Seo et al. [5] who studied on the CoQ10 self-emulsifying system reported that hydrophilic emulsifiers with HLB values ex- ceeding 10 are much superior at producing fine and uniform emulsion droplets. This difference might arise from the variation in the oil phase used and the processing methods.
In order to achieve greater emulsion stability, higher me- chanical energy such as high pressure homogenization and addition of more emulsifier can be employed.
Day 0 Day 30
SEL
SSL
PE
Tw 80
10 µm
10 µm
10 µm
10 µm 10 µm
10 µm 10 µm 10 µm
Fig. 7 Microscopy images (x400) of CoQ10-loaded palm kernel oil emulsions prepared using different emulsifiers at 0 day and after 1 month storage.
Scale barin all images represents 10μm
Emulsion Stability by Microscopy Observation
The microstructures of the eight samples of CoQ10-loaded coconut oil and palm kernel oil emulsions were captured under a light microscope (Figs.5,6and7). These images revealed the presence of spherical oil droplets which contained CoQ10 surrounded by a continuous aqueous phase. Oil-in-water emul- sion prepared with onlyk-carrageenan (without emulsifier) had large and highly polydisperse (>30μm) oil droplets (Fig.5). In addition, coalescence occurred in palm kernel oil emulsion without emulsifier where two or more oil droplets merge to- gether to form flocs or larger droplet (Fig.5b). Upon addition of emulsifier, emulsions exhibited a smaller and more uniform oil droplet size distribution. Emulsifier increased the stability of oil in the emulsion by reducing the interfacial tension between the oil phase and aqueous phase. Interfacial tension is the surface tension caused by intermolecular interactions at the surface separating two immiscible fluids [27]. After 1 month storage, oil droplets of SSL/k-carrageenan stabilized O/W emulsions remained clear and spherical while other emulsions showed coalescence and/or flocculation, which are the instability mech- anisms of emulsion. The existence of well-defined oil droplets is very unlikely in most of the unstable emulsions after 30 days.
These results indicated that stabilization of CoQ10-loaded O/W emulsion was predominantly affected by the oil droplet sizes.
Conclusions
In the present study, a stable O/W emulsion for the delivery of CoQ10 was developed using suitable oil and emulsifier. The O/W emulsions were prepared using conventional homogeni- zation method. This process enabled the formation of fresh O/W emulsion droplets ranging from 2.79 to 4.22 μm and 3.49 to 5.83μm for coconut oil emulsion and palm kernel oil emulsion, respectively. Results from the study highlighted two important pieces of evidence. First, the fatty acid composition of the carrier oil significantly (p<0.05) influenced the solubility of CoQ10. Particularly, coconut oil and palm kernel oil that are rich in medium fatty acids provided higher CoQ10 solubility than those with long chain fatty acids. Second, the nature of emulsifier played a role in determining the stability of O/W emulsion. SSL with HLB value (8.3) closest to the rHLB value (8.0) of carrier oils showed better efficacy in stabilizing O/W emulsions with non-significant increase in droplets size after 30 days of storage. In addition, no sign of coalescence, floccu- lation, and phase separation were observed in SSL/κ-carrageen- an stabilized O/W emulsions. The results reported in this study contribute to a better understanding of effect of carrier oil and emulsifier type on CoQ10 solubility, oil droplet size, and emul- sion physical properties, towards the goal of designing stable emulsions. Future studies on the effect of processing parameters
on the encapsulation efficiency of CoQ10 in κ-carrageenan coated O/W emulsions will be carried out to achieve optimal bioavailability.
Acknowledgments Financial support provided by a grant from Agro- Biotechnology Institute, Malaysia (ABI) R&D Initiative (08-05-ABI- PB033) for this study is gratefully acknowledged. Special thanks are given to Mitsubishi-Kagaku Foods Corporation (Tokyo, Japan), Rikevita Sdn. Bhd. (JB, Malaysia), and Daniso A/S (Copenhagen K, Denmark) for providing emulsifiers for this study.
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