INTRODUCTION
An emulsion consists of two immiscible liquids (usually oil and water), with
one of the liquids dispersed as small spherical droplets in the other (a continuous
phase). Emulsions are thermodynamically unstable due to the unfavorable contact
between oil and water molecules (Friberg, 1997) and as
a consequence their physical structures will tend to change over time by various
mechanisms (e.g., creaming, flocculation and coalescence), eventually leading
to complete phase separation (McClements, 2005).
Coconut milk is the natural oil-in-water emulsion extracted from the endosperm
of mature coconut (Cocos nucifera L.) either with or without the addition
of water (Seow and Gwee, 1997). It contains fat, water,
carbohydrate, protein and ash with the major components being water and fat
(Tansakul and Chaisawang, 2005). The emulsion is known
to be naturally stabilized by coconut proteins: globulins and albumins and phospholipids
(Birosel et al., 1963). However, the coconut
milk emulsion is unstable and readily separates into two distinct phases-a heavy
aqueous phase and a lighter cream phase (Cancel, 1979;
Gonzalez, 1990). The reason for the instability is that
the protein content and quality in coconut milk is not sufficient to stabilize
the fat globules (Monera and del Rosario, 1982).
Gravity is mainly associated with the slow sedimentation process of an immiscible
mixture. A common way to accelerate this sedimentation is by the use of centrifugation,
where the high achievable rotation frequencies permit an effective acceleration;
highly superior to the simple gravitation case (Starobinets
et al., 1979). Sometimes, gravity separation may be too slow because
of closeness of the densities of the particles and the fluid, or because of
association forces holding the components together. Gravity separation takes
hours, while centrifugal separation is accomplished in minutes, (Geankoplis,
2003). The centrifuge works using the sedimentation principle, where the
centripetal acceleration is used to separate substance of greater and lesser
density. By using centrifuge, it is possible to break down emulsions and to
separate dispersions of fine liquid droplets, though in this case the suspended
phase is in the form of liquid droplets which will coalesce following separation
(Coulson and Richardson, 1991).
MATERIALS AND METHODS
Sample Preparation and Procedures
Fresh coconut milk without added water were bought from a local market and
passed through cloth filters before experiments. Four bottles filled with 150
mL of coconut milk each. The four bottles placed inside a high speed centrifuge
(Sorvall Evolution RC) with rotor used is SLA-1000 and centrifuged for different
speeds ranging from 6000 to 12000 rpm (the angular velocities ω of the
rotor ranging from 628.32 to 1256.64 rad sec-1) and different times
ranging from 30 to 105 min, all experiments were run at 30°C. Same coconut
milk used on each speed and centrifugation time. Centrifugation produced layers
of an aqueous phase (water) on the bottom, an emulsion phase (cream) in the
middle and an oil phase on top. The height of the separated layers was measured
and converted to relative percentage as follows: for the same emulsion:
Moisture Content and Rancimat Analysis
These analysis include determine of moisture content and rancidity of VCO
by referring standard procedure (AOCS ca 28-38). All analysis was done in triplicate.
Moisture balance from Mettler Toledo was used to determine moisture content.
Rancimate machine supplied by Metrohmn, Germany was used for rancidity determination.
Analysis of Fatty Acids
The Fatty Acid Methyl Esters (FAME) of the oil were produced by weighing
30 mg of oil in screw cap tubes to which 4 mL of methanolic HCl was added and
mixed. The mixture was incubated at 50°C for 10 h and cooled to room temperature.
The FAME then was extracted using hexane three times. The hexane extracts were
combined and passed through anhydrous Na2SO4 for drying.
Gas Chromatography (GC) Analysis of Fatty Acids
The fatty acid compositions of total lipids from VCO were analyzed by GC.
GC analysis was performed on a HP5890 Series II GC machine, under the following
conditions: column, glass column (length 1.83 m and 2 mm ID); injector temperature,
250°C; detector temperature (FID), 250°C. Separation was done on a 100/120
Chmosorb-WAW column containing 10% SP2330. Oven temperature increased programmed
from 80 to 180°C at a ramp rate of 8°C min-1 for 7.5 min
and nitrogen at a flow rate of 20 mL min-1. The FAME was identified
and quantified through comparison with standard FAME.
RESULTS AND DISCUSSION
These yields, based on the volume of the coconut milk used, which can produce
VCO from 10.75 to 29.50% (Table 1). The highest yield of VCO
was obtained at 12000 rpm and 105 min, where the VCO yield is 29.50%.
| Table 1: |
Experimental results of VCO yield under centrifugal force
at different time and speed |
 |
|
| Fig. 1: |
Phase separation of coconut milk emulsion under centrifugation
force at different speed and 105 min. Data are presented in average of four
replications. |
Figure 1 shows the phase separation of coconut milk emulsion
by using centrifugation force at 105 min, comprising a top layer (VCO), cream
layer and a bottom layer of an aqueous phase.
Effect of Centrifuge Speed
The oil yield of the coconut milk emulsion has been studied for four different
speeds, namely 6000, 8000, 10000 and 12000 rpm of centrifuge speed, at 30°C
for different times. The highest yield of VCO was obtained at 12000 rpm. The
oil droplets tend to move in the opposite direction of the centrifugal force
because of their lower density. A droplet will be separated from the aqueous
phase when it reaches the inner wall of the bottle. The higher centrifuge speed,
gives higher yield of VCO. This observation is explained by the fact that increasing
the centrifuge speed results in increasing the rate of sedimentation and consequently
increases the separation of two immiscible liquid of the emulsion.
It is clear that increasing the centrifuge speed results in an increase in
the VCO yield (Fig. 2). According to Stoke’s law and
replacing gravity acceleration by centrifugal acceleration, if water is the
continuous phase, the settling velocity of oil droplets through water is given
by:
where, vo is the settling velocity of oil, ρw
is the density of water, ρo is the density of oil, r is the
radius of rotation, ω is the angular velocity of centrifugation, D is the
diameter of the droplets and μw is the viscosity of continuous
phase (water).
|
| Fig. 2: |
VCO yield with effect of speed (rpm) |
In Eq. 1, settling velocity is proportional to the density difference, square of droplet diameter, centrifugal acceleration and reciprocal to the viscosity of water. The viscosity of oil and water are very sensitive to the temperature. Centrifuge generates heat by centrifugal rotation. When temperature increases, the viscosity will decrease much faster than density difference, (ρw – ρo) does, results the increase of the droplet size. Therefore, the centrifugal acceleration increases the velocity of the oil and accelerates the separation of the emulsion.
Centrifugal force causes molecular rotations, which may reduce zeta potential of emulsion. Zeta potential is a layer of electrical charges; suspend oil droplets in oil-in-water emulsion which prevents the movement and coalescence of oil droplets. Since, water molecule is polar, it rotates at a high frequency under centrifugal forces.
The molecular rotation raises the temperature through friction and also neutralizes the zeta potential. Without the support of zeta potential, oil droplets are moved downward by gravitational force. Furthermore, when droplets collide with each other in their downward motion, coalescence takes place and droplet diameter, D increases, resulting in the acceleration of separation of emulsions.
Effect of Centrifugation Time
Six different times of centrifugation process were used, namely 30, 45,
60, 75, 90 and 105 min at 30°C for different centrifuge speed. Figure
3 shows a plot of the VCO yield as a function of different centrifugation
time. The highest yield of VCO is found at 105 min. The longer of centrifugation
time, it gives higher yield of VCO. This is because the separation process will
have longer times that allow the droplets of oil separate from the emulsion.
The plot clearly demonstrates the fact that increasing the centrifugation time
results in better separation process (VCO yield).
Quality Characteristics
VCO produced by using the different speed and centrifugation time, had some
differences in quality properties, these differences may not be large enough
to significantly affect the overall quality of the VCO (Table
2).
|
| Fig. 3: |
VCO yield with effect of time (min) |
| Table 2: |
Quality characteristics of Virgin Coconut Oil (VCO) samples |
 |
Further, their levels are still within the Asian and Pacific Coconut Community
(APCC) Standards for VCO. The free fatty acid content of the samples as well
as their moisture content may eventually have a bearing on the qualities of
the VCO during storage.
Fatty Acid Composition
The fatty acid composition of VCO sample fall within the range of Asia Pacific
Coconut Community (APCC) Standards for VCO and commercial VCO. Lauric acid (C12)
is the major fatty acid in VCO, ranged from 47.16 to 47.64% (Table
2).
Moisture Content
Moisture Content (MC) is another parameter that will determine the quality
of the VCO samples and shown in Table 2. The MC of the commercial
VCO ranged from 0.10 to 0.42% with VCO sample of centrifugation for 30 min having
the highest MC of 0.16% (Table 2), fall within the range of
APCC standards for VCO.
The use of different speed and centrifugation time of preparing VCO did not
result much differences in moisture content. This may be explained by the more
efficient and effective separation of the oil from non-oil constituents for
the laboratory produced samples. In oils, moisture is one of the reactants in
fat hydrolysis, which can lead to the increasing of free fatty acids. Production
of free fatty acids causes hydrolytic rancidity. Moisture is one of the most
important parameters for oil quality because, together with FFA, it can cause
oxidation in the presence of light (Che Man et al.,
1992).
CONCLUSION
Based on the results of this study, it can be concluded that, centrifugal radiation can be an effective and alternative tool to break (destabilize) oil-in-water coconut milk emulsions. This method does not require chemical addition.
Centrifugation force induced molecular rotation and neutralizes the zeta potential of emulsified oil droplets. Centrifugal force provides two contribution; reduction of viscosity and neutralization of zeta potential.
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