INTRODUCTION
The herbal industry in Malaysia was reported to have achieved an annual sale
of more than RM4.5 billion and the figure is expected to double by 2010. This
represents a vast opportunity for industries within Malaysia especially in view
of the fact that Malaysia has the 12th largest biodiversity in the world, with
1200 plants species recorded as having medicinal value (Aziz,
2003). Malaysia is endowed with rich rain forests which comprise thousands
of plants with potential medicinal values. One such plant is the tall shrub
tree from the Simaroubaceace family, Eurycoma longifolia known locally
as Tongkat Ali which is commonly found along the hilly jungle slopes of Malaysia.
This plant is wellknown in several regions of South East Asia for its high
medicinal values. It has been promoted as the Malaysian ginseng due to its aphrodisiac
effects (Shafiqul et al., 2006).
Traditionally, Tongkat Ali is valued for its aphrodisiac properties and treatment of diverse ailments ranging from cuts and wounds, skin infection, fever, malaria, highblood pressure, diabetes to increased energy and stamina. In Malaysia, research has confirmed that isolated constituents as well as whole plant extract show aphrodisiac, antimalarial, antitumor or cytotoxic properties, antiulcer and antiviral activity.
Research conducted at Universiti Sains Malaysia has shown that Tongkat Ali
has antimalarial property that is stronger than chloroquinine (Ang
et al., 2002).
There are two specific challenges in herbal and phytochemical processing. The first challenge is to increase product yield while maintaining overall process reasonable economics and secondly is to produce or achieve a standardized extract with the active ingredients available in the desired concentration and profile. Tongkat Ali requires a longer time to produce the secondary bioactive metabolites and their biological activities may be derived from more than one of the constituents. The phytochemical content in herbs is extremely low and losing a small fraction of the desired extract is significant in terms of extraction yield. If yield can be increased, it would increase profitability.
Current yield of Tongkat Ali achieved was in between 3 to 4%. However, an increase
of production of even 0.5% in weight could increase profits by up to 20%. The
current selling price of Tongkat Ali water extract spray dried powder is at
RM 2000 to RM 3000 per kg which converts to approximately RM 20,000 per kg in
capsule form. Therefore, there is a strong incentive to optimize production
yield (Kaur, 2003).
Tongkat Ali holds a vast potential and value for commercial production in health
and wellness industry based on these traditionally benefits and scientifically
proven properties. Therefore, it is necessary to study in depth the effects
of the processing parameter such as extraction temperature, solvent to raw material
ratio and agitation speed to the yield of production. In Eurycoma longifolia
production, the yield of bioactive constituents is extremely low. In this study,
a mathematical model of Eurycoma longifolia extraction was presented
that predicts the extraction of major phytochemical components. The model was
developed to improve the extraction process in order to increase the yield of
major phytochemical components. The parameters in the model were estimated using
previous experimental data. The effects of operating conditions such as temperature,
solvent to raw material ratio and agitation speed on the yield of effective
compounds were investigated through the model simulation.
MATERIALS AND METHODS
• 
The work was divided into two main phases 
• 
Development of mathematical model 
• 
Simulation and validation of the model 
Development of mathematical model: A mathematical model was developed
for extraction of major phytochemical components from Eurycoma longifolia.
Three major compounds from Eurycoma longifolia were considered in this
work which were eurycomanone, eurycomalactone and 14,15β hydroklaineanone.
Tongkat Ali root chips were boiled in water in a pressurized vessel between 100 to 120°C for approximately 2 h during the extraction process. This boiling operation was maintained for 2 h to provide sufficient time for the phytochemical components in the root chips to be leached into the extraction solvent (water). Then, the boiled extract was run through a filter system where the water extract was separated from the solid particles. Approximately 35% of the solvent (water) was absorbed in the discharged chips and this was taken as process losses.
Mass and energy balance were performed. The degree of freedom must be zero in order to solve the system. The extraction consists of two major unit operations which are extractor and filter system. Overall mass balance for the system can be expressed as:
where, F_{1} is the inlet flow rate to the extractor; F_{2} is the inlet flow rate for solvent; F_{4} is the outlet flow rate of liquid extract from filter and F_{5} is the outlet flow rate of solid waste from filter.
For the energy balance, the equation is:
where, Q_{acc} is the heat accumulation rate by the system, Q_{ag} is the heat generation due to mechanical agitation and Q_{sen} is the rate of sensible enthalpy gain by the flow system streams (exitinlet).
where, E represents the energy accumulated in the system. ρ is the density of solution, V is the solution volume, c_{p} is the heat capacity and ΔT is the temperature difference between the temperature in the system and the reference temperature.
Assuming that ρ, c_{p} and T_{ref} are constant with respect to time:
The extraction process from herb can be described in the following stages:
• 
The solvent must be transferred from the bulk solvent solution
to the surface of the solid 
• 
The solvent must penetrate or diffuse into the solid 
• 
The solute dissolves into the solvent 
• 
The solutes then diffuse through the solid solvent mixture to the surface
of the particle 
• 
The solute is transferred to the bulk solution 
The extraction of phytochemical compounds from the herb is in nature, a mass
transfer process of solute from the herb body to the solvent. Phytochemical
compounds exist in the cells of the herb which makes the herb leaching process
difficult and complex. The leaching rates of phytochemical compounds are dominated
by the diffusion rates. Therefore, the mathematical model for the solidliquid
extraction process focuses on the diffusion process of the phytochemical compounds,
which consists of diffusion to the surface of the herb from the inside particle
and then move to the bulk solution.
There are two fundamental concepts that define Eurycoma longifolia extraction
process which are the equilibrium and mass transfer rate (Cacace
and Mazza, 2003). The diffusion model inside herb for eurycomanone can be
expressed as follows:
where, M_{A1} is the concentration of eurycomanone transferred across the cell membrane; k_{1} is the overall mass transfer coefficient; C_{A0} and C_{A1} is the concentration of eurycomanone inside and outside the cell and A_{1} is the area across which diffusion occurs. Equation 5 is also used for compound B for eurycomalactone and compound C for 14,15β hydroklaineanone.
The overall mass transfer coefficient, k_{1} can be written as follows:
where, α_{1} is the mass transfer coefficient of phytochemical compounds across the liquid near the cell membrane and α_{2} is the mass transfer coefficient across the cell membrane.
For the diffusion model from the surface of the herb to the bulk solution as the following:
where, M_{A2} is the concentration of eurycomanone transferred from the surface of the herb to the bulk of the solution; k_{2} is the overall mass transfer coefficient; C_{A2} is the concentration of eurycomanone in the bulk solution and A_{2} is the interfacial area between the particle and the liquid which can be approximated by the area of the herb particle absorbing the solvent. Equation 7 was used for compound B and C by substituting compound A with compound B and C.
In contrast, when the solution was agitated, the mass transfer coefficient is related to the Reynolds number and the Schmidt number by the following Eq. 8:
where, Sh is the Sherwood number represented by Sh = k_{2}δ/D; Re and Sc are Reynolds number and Schmidt number and δ is the characteristic length.
The following general assumptions were made when developing the mathematical
model:
• 
Solid particles were considered as spheres with a radius R
and the effective constituents are initially uniformly distributed in the
spheres 
• 
The solvent was perfectly mixed. So the transfer resistance in the liquid
phase was negligible and the concentration of the effective constituents
in the solvent only depends on time 
• 
The transport of the effective constituents in the particles was a diffusion
phenomena and can be described by a diffusion coefficient (D) independent
of time 
• 
The diffusion of the effective constituents was carried out parallel and
there was no interaction between them 
• 
There was no adsorption of solute by the solid in the leaching process 
It was found that the rate of extraction increases with extraction temperature
until a certain temperature where increment in yield of extraction diminished.
Extraction yields declined due to the degradation of temperature sensitive compounds.
By increasing extraction temperature, the higher solubility of solute resulted
in the increase of diffusion rate and thus increases the diffusion coefficient.
This has reduced the extraction time. The effect of temperature was modeled
using the Arrhenius type of relationship as shown in Eq. 9:
where, D is the diffusivity of solute in the solid; E_{a} is the activation energy for diffusion; R is the Universal Gas Constant and T is the absolute temperature.
For the diffusion coefficient of solute in the liquid solvent, the effect of temperature used was the equation introduced by WilkeChang as shown in Eq. 10. Similarly, high temperature decreased the solvent viscosity and increased the diffusion coefficient.
where, M_{B} is the molecular weight of solvent; T is the absolute temperature; μ_{B} is the viscosity of solvent; V_{A} is the molar volume of solute at the normal boiling point (cm^{3} mol^{1}) and Φ is the association factor of solvent.
Simulation and validation of the model: Equation 5
and 7 which represented the mass transfer model for Eurycoma
longifolia extraction process involves some differential equations. The
model was simulated using Matlab programming environment. The differential equations
were solved using FourthOrder RungeKutta algorithm. The work by Athimulam
et al. (2006) was used as the benchmark for the simulation study
and the kinetic parameters were based on their work. The effect of extraction
temperature on the yields of phytochemical compounds was investigated by varying
the operating conditions individually. The effect of extraction ratio and agitation
speed was also investigated.
RESULTS AND DISCUSSION
Effect of temperature on the yield of extraction of Eurycoma longifolia: The results shown in Fig. 1 are the comparison between simulation and experimental data. The extraction temperature studied was in the range of 60 to 110°C. It can be seen from Fig. 1 that the yield of extraction increased by increasing the extraction temperature but the yield decreased when the extraction temperature reached 100°C which is the boiling point for water as the solvent for the extraction process.
Heat treatment was performed to accelerate the mechanism of the diffusional process when extracting from plants. The surface tension and viscosity of the solvent reduced and the solvent reached the active sites inside the matrix more easily at higher temperature. In addition, high temperature can decrease the cell barrier by weakening integrity of the cell wall and membrane. As a result, the solvent can easily get in contact with phytochemical compounds. The temperature effect on the extraction yield came from its influence on diffusion phenomena.
An increase in temperature from 60 to 110°C increased the yield of extraction and reduced the extraction time. However, further increased of temperature from 100 to 110°C resulted in lower extraction yield. This was due to degradation of phytochemical compounds. Increasing temperature favored extraction by increasing the diffusion coefficient which increased the extraction rate.
Effect of solvent to raw material ratio on the yield of extraction of Eurycoma longifolia: The final yield was plotted as the function of water to Tongkat Ali ratio as shown in Fig. 2. From Fig. 2, the increment of extraction yield becomes insignificant when ratio was increased beyond 5:1. When the ratio increases, extraction yield increases until it reaches ratio 5:1. The higher the ratio of water to Tongkat Ali, the higher the difference of concentration between the bulk solution and the solutes. Therefore, more phytochemical can diffuse out if a higher volume of water is used. The synergetic effect of the phytochemical components that comes out of the sample has longer contact time with water may cause the decrease of extraction yield for ratio 7:1.

Fig. 1: 
Effect of temperature on the yield of extraction of Eurycoma
longifolia 

Fig. 2: 
Effect of water to Tongkat Ali ratio on the yield of extraction
of Eurycoma longifolia 
Effect of agitation speed on the yield of extraction of Eurycoma longifolia:
From Fig. 35, the effect of agitation speed
was studied at 100, 200 and 400 rpm. The best agitation rate was found to be
at 400 rpm.

Fig. 3: 
Effect of agitation speed at 100 rpm on the yield of extraction
of Eurycoma longifolia 

Fig. 4: 
Effect of agitation speed at 200 rpm on the yield of extraction
of Eurycoma longifolia 
When the solution is agitated, mass is transported by the bulk motion of the
fluid which known as convective mass transfer. Convective mass transfer occurred
at the surface when a fluid is outside the solid. Therefore, when the agitation
speed increases, the Reynolds number is also increased. The Reynolds number
is related to the mass transfer coefficient. So, the higher the Reynolds number,
the higher the mass transfer coefficient.

Fig. 5: 
Effect of agitation speed at 400 rpm on the yield of extraction
of Eurycoma longifolia 
CONCLUSION
A good fitting between experimental data and the model was obtained. The work concluded that when extraction temperature was increased, the yield of extraction also increased and this has reduced the extraction time. The best agitation speed was found at 400 rpm. The yield of extraction will increase when the ratio of water to Tongkat Ali increases until 5:1 ratio. Further work will be carried out to determine the optimum operating conditions such as extraction time.
ACKNOWLEDGMENTS
This research is supported by the Chemical Engineering Pilot Plant (CEPP), Universiti Teknologi Malaysia research fund.