| Research Article |
Open Access |
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| Experimental Study of Lactose Hydrolysis and Separation in Continuous
Stirred Tank -Ultrafiltration Membrane Reactor |
| M. Pakizeh* and M Namvar-Mahboub |
| Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, P.O. Box 91775-1111, Mashhad, Iran |
| *Corresponding author: |
Dr. M. Pakizeh
Department of Chemical Engineering
Faculty of Engineering, Ferdowsi University of Mashhad
P.O. Box 91775-
1111, Mashhad, Iran
Tel: +98-511-8815100
Fax:+98-511-8763301
E-mail:
pakizeh@um.ac.ir |
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| Received April 11, 2011; Accepted June 08, 2011; Published June 25, 2011 |
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| Citation: Pakizeh M, Namvar-Mahboub M (2011) Experimental Study of Lactose
Hydrolysis and Separation in Continuous Stirred Tank -Ultrafiltration Membrane
Reactor. J Chem Eng Process Technol 2:110. doi:10.4172/2157-7048.1000110 |
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| Copyright: © 2011 Pakizeh M, et al. This is an open-access article distributed
under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the
original author and source are credited. |
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| Abstract |
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| In this study, lactose was hydrolyzed using β-galactosidase enzyme in a continuous stirred tank-Ultrafiltration
(CSTR-UF) to produce galactose and glucose. The UF membranes of Molecular Weight Cut of 3 kDa of regenerated
cellulose material were used to separate enzyme from products. Experiments were performed with 0.139 molar
aqueous solution of lactose as feed. The effect of operating pressure ranging between 2 and 5 bar and time on
rejection and permeate flux were studied. Results showed that the UF membrane rejects the enzyme completely.
Also according to experimental data lactose concentration in permeate decrease with time due to concentration
polarization and hydrolysis. It was found that rejection factor of lactose increases from 33 to77%, with time from 5
to 85 min. |
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| Keywords |
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| Hydrolysis of lactose; Reactor membrane; Ultrafiltration |
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| Introduction |
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| Whey disposal is one of the most important economical and
environmental problems in diary industry. This liquid produced when
milk is changed in to cheese and approximately it consists of 5%
lactose, 0.7% protein and 93% water and salt. Lactose waste increases
biochemical and chemical oxygen demand (BOD and COD) which is
in contrast with legal standard for wastewater [1]. In addition, lactose
as a disaccharide is scarcely digestible for some people due to lack of
β-galactosidas enzyme in their body. Furthermore, high amount of
lactose in diary product like ice cream, condensed milk, etc., lead to
undesirable grainy texture. Thus it is necessary to remove lactose from
diary product and waste streams through methods like hydrolysis or
separation processes [1-3]. |
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| Lactose can be hydrolyzed by two principle method, using acid
treatment at high temperature (above 150°C) or enzymatic catalysis.
The second one is preferred because of its milder operating temperature
and pH. β-galactosidas is suitable enzyme to perform hydrolysis, that
change lactose to glucose and galactose monosaccharide [4-6]. There
are several mechanisms to describe this reaction in presence of water
such as those presented by Segal and Cha [7]. Some researches consider
in detail, using enzymatic lactose hydrolysis in batch CSTR reactor as
well as fixed bed reactor. In the comparison, superior performance of
CSTR reactor was clearly established [8-10]. |
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| A lot of researchers focus on methods, recover valuable components
from diary waste streams, like adsorption, NF, RO, UF membrane
processes and chromatography [11-16]. In terms of lactose removal
(342 Da) from diary product and waste streams, RO and NF processes
are more efficient, but need higher operating pressure and energy
consumption than UF. On the other hand, enzymatic hydrolysis has
some problem such as the effects of galactose formation on hydrolysis
completion, and the high enzyme cost. One possibility to ensuring
protection of purified enzyme is offered by using membrane processes
[17]. So, UF system in combination with hydrolysis reactor might be
preferred due to enzyme separation via membrane [18-19]. |
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| In present study, CSTR-UF system was used to hydrolyze
lactose and separate β-galactosidas enzyme (116 kDa) from glucose and galactose (180 Da). The UF membrane of MWCO of 3 kDa of
regenerated cellulose (RC) material was used to separate enzyme from
products. All experiments were carried out with 0.319 molar aqueous
solution of synthesized lactose as feed. The effect of operating pressure
ranging between 2 and 5 bar and time on rejection and permeate flux
were studied. |
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| Materials and Methods |
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| Chemicals |
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| β-galactosidase (activity of the enzyme at 37°C=3000 lau/cc) and
lactose were purchased from Sigma-Aldrich Company. In order to
analyze the concentration of lactose, sulfuric acid (98%) and phenol
(89%) were obtained from Merck Company. |
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| Membrane module and experimental set-up |
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| Experiments were performed in a dead-end filtration setup. A
schematic of CSTR-UF system is shown in Figure 1. The stirred cell
was fed from 3 L reservoir which was pressurized using nitrogen gas
cylinder. Feed pressure is monitored by pressure gauges and Permeate
side was connected to the atmosphere so its pressure was assumed
approximately equal to 0 bar (gauge pressure). |
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|
Figure 1: A schematic diagram of CSTR-UF setup. |
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| The RC ultrafiltration membrane of 3 kDa (MWCO) was placed in
the 150 mL stirred cell and supported with sintered stainless still disc. |
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| Determination of lactose concentrations |
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| Lactose concentration was determined by means of calibration plot of absorbance value. In order to plot standard absorbance curve, five
sample of lactose were diluted to prepared aqueous solution of 0.01,
0.02, 0.03, 0.04 and 0.05 gL-1. Then 2 mL of each sample was mixed
with 0.1 mL phenol solution and 6 mL sulfuric acid for 10 min at room
temperature. The absorbances of samples were measured via 490 nm
UV-Vis, spectrometer (Perkin-Elmer, USA). By using absorbance data
represented in Table 1, it is possible to determine lactose content in
every permeate sample. |
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Table 1: Calibration absorbance data. |
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| Governing equation |
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| The most frequent parameters used to characterize membranes are
founded on its performance such as flux and rejection. |
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| Rejection |
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| In order to evaluate membrane performance, rejection of lactose
should be obtained. The observed rejection, R, was calculated using
equation (1): |
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 (6) |
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| where Cp and Cf are the solute concentration in permeate and feed side,
respectively. |
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| Volumetric flux |
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| Permeate volumetric flux is expressed as volume of fluid permeated
in certain time step. To meet this value equation (2) was used: |
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 (2) |
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| where J, volumetric flux (L.m-2.h-1), A, effective membrane area (m2),
ΔV, permeate volume (L) and Δt is sampling time (h). |
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| Result and Discussion |
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| Influence of pressure on permeate flux |
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| In order to illustrate the effect of pressure on permeate flux, the operating pressure was controlled at different value in the range of 2
to 5 bar. Permeate flux verses pressure change is shown in Figure 2.
It can be observed from the results that permeate flux increased with
increasing operating pressure. Nevertheless, flux may not be increased
proportional with pressure at high level because of pore compressing.
It should also be noticed that Figure 2 didn't show any compaction due
to pressure effect [15,20]. |
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Figure 2: Influence of pressure on permeate flux. |
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| Influence of time on permeate concentration |
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| To study the effect of time on lactose concentration at certain
pressure, 2 bar, sample were collected from permeate at 5, 25, 45, 65
and 85 min. each sample were diluted thousand time and set for lactose
amount determination. Test results are represented in Figure 3 and are
shown that after 20 min lactose amount is decreased drastically from
initial value of 0.139 molL-1 to 0.078 molL-1, but further operation
declined the value to 0.071 molL-1. This phenomenon is caused because
of lactose consumption in hydrolysis reaction by time spending. On
the other hand, β-galactosidas as a high molecular weight protein are
accumulated near membrane surface which cause resistance against
lactose permeation. This behavior is known as the concentration
polarization which can lead to fouling and plugging of membrane pore [1,15]. Although, stirring minimized this effect by good mixing of bulk
feed solution and turbulent flow near membrane surface [20]. |
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Figure 3: Influence of time on lactose concentration. |
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| Based on equation 1, rejection is enhanced with time at constant
lactose concentration in feed. It should be mentioned that in all cases,
sample appearance reveal that the UF membrane rejects the enzyme
completely. |
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| Influence of pressure on rejection of lactose |
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| Effect of pressure rise between 2 and 5 bar on rejection of lactose is
presented in Figure 4. Determinations of lactose amount on permeate
side after 80 min showed that rejection is reduced from 0.77 to 0.57.
This result suggest that increasing in permeate flux with pressure cause
decrease of retention time of lactose in reaction zone. Thus lactose
concentration on permeate side might be increased and rejection might
be decreased. On the other hand, declining of rejection was slower at
high pressure due to presence of enzyme molecules near membrane
surface. |
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Figure 4: Influence of pressure on lactose rejection. |
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| Effect of feed concentration on permeate flux |
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| Figure 5 shows permeate flux verses pressure for both pure water
and 0.319 molar of lactose as feed. This figure indicates that as lactose
and enzyme added to feed, permeate flux decreased at all pressure,
considerably. Base on this result, one can conclude that lactose and
enzyme have resistance role on water permeation by deposition on top
surface and pore wall of membrane [20]. Although, flux reduction is
for the most part due to surface fouling of membrane by existence of
enzyme. |
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Figure 5: Permeate flux (L.m-2.h) vs. pressure (bar). |
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| Conclusion |
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| Lactose hydrolysis has been investigated using β-galactosidase
enzyme in a continuous stirred tank -Ultrafiltration (CSTR-UF) to
produce galactose and glucose. The major findings of the present study
are summarized as follows: |
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| • Increase in operation pressure as driving force, enhance
permeate volumetric flux. |
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| • Unchanged color of permeate samples reveal that no enzyme
present in permeate and the UF membrane rejects the enzyme
completely. |
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| • Lactose concentration in permeate decrease with time due to
concentration polarization and hydrolysis. |
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| • It was found that rejection factor of lactose increases from
33 to77%, with time from 5 to 85 min due to drop lactose
concentration. |
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| • Fouling effect observed due to presence of lactose and enzyme
in feed. |
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