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Journal of Food Engineering 80 (2007) 300–305
www.elsevier.com/locate/jfoodeng
Eﬀect of pretreatment with gelatin and bentonite on permeate ﬂux
and fouling layer resistance during apple juice ultraﬁltration
¨ zge C
Vural Go¨kmen *, O
¸ etinkaya
Food Engineering Department, Hacettepe University, 06532 Beytepe, Ankara, Turkey
Received 21 March 2005; received in revised form 10 April 2006; accepted 18 April 2006
Available online 3 July 2006
Abstract
The eﬀects of pretreatment with gelatin and bentonite, pressure (DP) and molecular weight cut-oﬀ (MWCO) on ﬂux performance of
apple juice during ultraﬁltration were studied. Filtration data (volume versus time) were satisfactorily ﬁtted to De La Garza and Boulton’s
exponential model to ﬁnd the exponential fouling coeﬃcient (k) and the membrane resistance (Rm). Increasing the amounts of gelatin and
bentonite used for pretreatment of apple juice decreased k values during ultraﬁltration which was a clear evidence of ﬂux improvement by
means of delaying the membrane fouling. The molecules in apple juice which are responsible for membrane fouling could be successfully
retained by the aggregate formed by gelatin and bentonite. Both k and Rm values increased as DP increased during ultraﬁltration of apple
juice. A decrease in Rm was observed as MWCO of membrane increased. However, the membrane having MWCO of 100 kDa had the
highest k value which indicated the increased tendency of this membrane to foul.
2006 Elsevier Ltd. All rights reserved.
Keywords: Ultraﬁltration; Exponential model; Apple juice; Pretreatment; Gelatin; Bentonite
1. Introduction
Ultraﬁltration as a clariﬁcation process in fruit juice processing is of interest to the food industry for over 20 years
(Alvarez, Andres, Riera, & Alvarez, 1996; Girard & Fukumoto, 1999; Mondor, Girard, & Moresoli, 2000; Padilla &
McLellan, 1989; Riedl, Girard, & Lencki, 1998; Sheu,
Wiley, & Schlimme, 1987; Wu, Zall, & Tzeng, 1990). The
advantages of ultraﬁltration for clariﬁcation of apple juice
are based on avoidance of ﬁltering aids which are costly
and give disposal problems, and shorter process time than
traditional ﬁltration processes (Ben Amar, Gupta, & Jaﬀrin,
1990; Rao, Acree, Cooley, & Enis, 1987; Schneider & Czech,
1994). Successful development of membranes with increased
service life, separation capacity and chemical resistance has
been a reason for increasing use of membrane separation
processes (Tzeng & Zall, 1990). A common phenomenon
*
Corresponding author. Fax: +90 312 2992 123.
E-mail address: [email protected] (V. Go¨kmen).
0260-8774/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2006.04.060
in membrane processes is the declining permeate ﬂux with
time. This decrease is caused by the accumulation of feed
components in the membrane pores as well as on the membrane surface. In some instances, ﬂux decline may be so
important as to make membrane processes unattractive
for clariﬁcation of apple juice. Because of the highly heterogeneous nature of apple juice colloidal material, a wide variety of interactions should be responsible for fouling layer
formation. Pectic substances, phenolic compounds, proteins
and ﬁbers which are present in both soluble and suspended
solid fractions, can contribute to fouling layer resistance
during the membrane process (Mondor et al., 2000; Riedl
et al., 1998). Among all the compounds in apple juice, pectic
substances are most often identiﬁed as the major hindrance
to ﬁltration performance (Wucherpfennig, Dietrich,
Kanzler, & Will, 1987). Therefore, an enzymatic treatment
of the raw juice before ultraﬁltration is usually carried out
in order to degrade pectic substances and other polysaccharides, with enzymes such as pectinases and amylases. Pectinases hydrolyze pectin and cause pectin–protein complexes to
¨ . C¸etinkaya / Journal of Food Engineering 80 (2007) 300–305
V. Go¨kmen, O
ﬂocculate, thus improve the ﬂux behavior (Alvarez, Alvarez,
Riera, & Coca, 1998; Padila-Zakour & McLellan, 1993).
Alternatively, mechanical methods, such as backwashing
and pulsating inlet ﬂow techniques have also been reported
to improve ﬂux behavior (Ben Amar et al., 1990; Su, Liu, &
Wiley, 1993).
Model studies on ﬁltration in food industry are extremely limited. The Sperry model has formed the basis of
the ﬁltration theory and found useful for ﬁltrations in which
the solids being removed are rigid, or, when the cake being
formed with a ﬁlter aid is rigid (De La Garza & Boulton,
1984; Orr, 1977; Sahin & Bayindirli, 1991a). The Sperry
model assumes constant speciﬁc cake resistance and usually
provides a good ﬁt to experimental ﬁltration data for short
ﬁltration times. However, ﬁlter cake undergoes compaction,
or solid particles migrate within the pores, blocking ﬂow
and increasing the resistance as ﬁltration proceeds and the
model overestimates the ﬁltrate ﬂow (Toledo, 1991). Some
researchers attempted to use the Sperry model to estimate
the fouling layer resistance during apple juice microﬁltration (Lencki & Riedl, 1999; Riedl et al., 1998). The linear
regression of a time/volume versus volume plot was used
to determine the fouling layer speciﬁc resistance. However,
a negative value for the y-intercept involving the medium
resistance term, Rm, is apparent in the analysis of these data,
lacking the applicability of the Sperry model. A number of
semiempirical and empirical models were proposed to present ﬂux behavior during membrane ﬁltration process. These
models are able to represent ﬂux as a function of either time
or volume concentration ratio. According to these models,
change of ﬂux as a function of volume concentration ratio
or time is examined in two or three periods characterized
by an initial rapid ﬂux decline followed by a less severe
decrease of ﬂux until a steady-state is reached. However,
the boundaries in each period are arbitrarily selected and
are speciﬁc to the system under investigation (Constenla
& Lozano, 1997; Lahoussine-Turcaud, Wiesner, & Bottero,
1990).
Gelatin and bentonite are the most preferred ﬁltering
aids which are used during traditional ﬁltration and clariﬁcation of apple juice. The eﬀects of gelatin and bentonite
are towards the adsorption of phenolic compounds and
proteins, respectively. There is no need to use these ﬁltering
aids during clariﬁcation of apple juice by ultraﬁltration,
that the main reason why ultraﬁltration process is preferred
(Go¨kmen, Borneman, & Nijhuis, 1998). But on the other
hand, the membrane fouling that causes decline in permeate ﬂux with time lacks the system performance and this
limits the usage of ultraﬁltration process. It is informed
that, in practical applications, the fouling of the membrane
can be delayed and the performance of ultraﬁltration system can be improved by the pretreatment of apple juice
with gelatin and bentonite of lesser amounts prior to ultraﬁltration, by forming large aggregates that retain the foulants (i.e. phenolic compounds) in feed.
In this study, the eﬀects of pretreatment with gelatin and
bentonite, DP and MWCO of membranes on ﬂux perfor-
301
mance of apple juice were investigated during ultraﬁltration. Filtration data was analyzed using De La Garza &
Boulton’s exponential model to ﬁnd the model parameters
Rm and k from the plot of volume versus time using nonlinear regression analysis.
2. Materials and methods
2.1. Preparation of apple juice for ultraﬁltration
The apples (Golden Delicious) used in this study to produce apple juice, gelatin (Type-A) and bentonite (Na-bentonite) were kindly supplied by a local large-scale apple
juice processor. Apples were mashed in a Waring blender
and manually pressed using double layer cheesecloth to
obtain raw apple juice. Raw juice was treated with 1 ml/l
pectolytic enzyme preparation (Pectinol, Rohm and Haas
Company, Philadelphia, USA) and 0.2 ml/l amylase (Amylase, Gist-Brocades Food Ingredients Inc., Pennsylvania,
USA) at 50 C for 2 h to degrade pectin and starch. The
complete degradations of pectin and starch were conﬁrmed
by alcohol and iodine tests, respectively (Acar, Go¨kmen, &
Alper, 1999). After the separation of sedimented coarse particles, juice was divided into lots of 200 ml and stored at
18 C prior to ultraﬁltration experiments.
2.2. Pretreatment of apple juice with gelatin and bentonite
In order to determine the eﬀect of pretreatment on the
permeate ﬂux performance, gelatin and bentonite were
added to depectinized apple juice in varying amounts. Their
dosages required for conventional clariﬁcation of apple
juice were 300 mg/l and 1500 mg/l, respectively, as reported
by us elsewhere (Go¨kmen, Artık, Acar, Kahraman, &
Poyrazog˘lu, 2001). In this study, the ratio of gelatin and
bentonite was kept constant (1:5) to maintain an optimum
aggregation, and so, to increase adsorption of foulants.
The amounts of gelatin and bentonite added to depectinized
apple juice prior to ultraﬁltration were; 300 mg/l gelatin–
1500 mg/l bentonite (1:1 G–B), 150 mg/l gelatin–750 mg/l
bentonite (1:2 G–B), 60 mg/l gelatin–300 mg/l bentonite
(1:5 G–B), 30 mg/l gelatin–150 mg/l bentonite (1:10 G–B)
and no gelatin–bentonite (0 G–B). Aqueous suspensions
of 1% gelatin and 5% bentonite were used for pretreatment
of apple juice. Apple juice was kept at 50 C for 30 min for
ﬂocculation following the addition of gelatin and bentonite
prior to ultraﬁltration.
2.3. Ultraﬁltration system
Experiment setup consisted of three major components:
a pressure supply of compressed nitrogen gas, an ultraﬁltration unit and a reservoir in which permeate was collected.
Amicon 8200 Model ultraﬁltration cell (diameter 64 mm,
volume 180 ml) operating in dead-end mode with stirring
was used. Flux experiments were performed using membranes having MWCO of 10, 30, 50 and 100 kDa. The
¨ . C¸etinkaya / Journal of Food Engineering 80 (2007) 300–305
V. Go¨kmen, O
302
membranes having 30, 50 and 100 kDa MWCO (Millipore
Corp, Bedford, USA) were made of polyethersulfone (PES)
while the one having 10 kDa MWCO (Millipore Corp, Bedford, USA) was made of cellulose acetate (CA). New membrane was used for each set of experiments. Ultraﬁltration
experiments were performed at room temperature applying
DP of 1 · 105, 2 · 105, 3 · 105 and 4 · 105 Pa. Permeate
stream was collected in a graduated cylinder. The time
required to collect each 10 ml portion of permeate was
recorded up to a total volume of 130 ml for an initial feed
volume of 170 ml. Ultraﬁltration trials were replicated
twice.
Relative viscosity of apple juice (12 Brix) was measured
using capillary tube viscometer at room temperature. Since
times required to pass 100 ml of water and apple juice were
found to be same, the viscosity of apple juice was assumed
to be equal to that of water at 25 C (0.001 Pa s).
2.4. Analysis of V versus t data
According to De La Garza & Boulton’s exponential
model, the total resistance to ﬁltrate ﬂow is empirically
related to the ﬁltrate volume as follows:
Rtot ¼ Rm expðkV =AÞ
ð1Þ
where; Rtot is the total resistance (m1), Rm is the membrane resistance (m1), k is exponential fouling coeﬃcient
(m1), V is the ﬁltrate volume (m3) and A is the ﬁltration
area (m2).
The general ﬁltration equation is;
J¼
1 dV
DP
¼
A dt
l Rtot
3
ð2Þ
3. Results and discussion
3.1. Water ﬂux
Fig. 1 shows the water ﬂuxes of UF membranes used in
this study at diﬀerent DP. The water ﬂux generally represents the highest ﬂux that can be obtained with the membrane. Water ﬂux is also used as an indicator of cleaning
eﬃciency. If the cleaning process has been eﬀective and
foulants have been removed from the membrane, the membrane system will provide the same performance again in
the next process cycle (Cheryan, 1998). In our study, a
new membrane was used for each set of experiments. It is
clearly seen from Fig. 1 that increasing both the MWCO
and DP also increased the water ﬂuxes for the membranes
made of PES. The membrane made of CA having 10 kDa
MWCO had slightly higher water ﬂuxes at all DP than the
membrane made of PES having 30 kDa MWCO. The eﬀect
of DP was more signiﬁcant for the membranes having
MWCO of 50 and 100 kDa in which the changes of water
ﬂuxes were not linear. The non-linearity of the water ﬂux
versus pressure data suggests that the membrane material
compaction occurs at high pressures for these membranes.
Therefore, Rm for synthetic ultraﬁltration membranes
should be considered as a function of pressure, not constant. Although the idealized water ﬂux versus pressure
curve shows a linear change for a membrane possessing a
uniform pore size, synthetic microﬁltration and ultraﬁltration membranes do not generally possess a uniform pore
size hence idealized curve will not be observed (Mulder,
1991).
3.2. Eﬀect of pretreatment on ultraﬁltration performance
2
where, J is ﬂux (m /m s), DP is the pressure (Pa) and l is
the viscosity (Pa s).
Substituting Eq. (1) into Eq. (2) yields,
1 dV
DP
¼
A dt
lRm expðkV =AÞ
Fig. 2 shows the plot of V versus t as aﬀected by pretreatment gelatin and bentonite (G–B). Increasing the amount of
G–B improved the ﬂux performance by delaying the
ð3Þ
Rearranging and integrating Eq. (3) for the boundary conditions of V = 0 at t = 0, and V = V at t = t
Z t
Z
lRm V
dt ¼
expðkV =AÞdV
ð4Þ
DPA 0
0
2
40
lRm
ð6Þ
DPk
k
ð7Þ
b¼
A
The ﬁltration parameters k and Rm values were then calculated from the model parameters a and b.
30
5
ð5Þ
Raw data (V versus t) was ﬁtted to Eq. (5) using Curve Expert version 1.3 to ﬁnd a (s) and b (m3), where,
a¼
30 kDa
50 kDa
J(x10 )(m3/m .s)
lRm
½expðkV =AÞ 1
DPk
10 kDa
50
100 kDa
yields Eq. (5) that can be used to determine k and Rm.
t¼
60
20
10
0
0
1
2
P(x10-5) (Pa)
3
4
Fig. 1. Water ﬂuxes of UF membranes at diﬀerent DP’s.
¨ . C¸etinkaya / Journal of Food Engineering 80 (2007) 300–305
V. Go¨kmen, O
4500
4000
01:01
3500
01:02
01:05
3000
t (s)
01:10
2500
00:00
2000
1500
1000
500
0
0
2
4
6
8
10
12
14
V (x10 5 ) (m3)
Fig. 2. V versus t plots for ultraﬁltration of apple juice pretreated with
diﬀerent G–B. Operation conditions: DP 2 · 105 Pa, membrane 10 kDa
MWCO.
membrane fouling during ultraﬁltration of apple juice. The
time required to collect 130 ml of permeate was 4086 s
without pretreatment, but it was decreased to 2819 s with
pretreatment of apple juice with 1:1 G–B prior to ultraﬁltration. Gelatin and bentonite are well known as the clariﬁcation aids during conventional processing of apple juice.
Gelatin is capable of adsorbing some phenolic compounds
present in apple juice, and stabilize clarity during storage.
When gelatin is added, negatively charged colloids in apple
juice begin to ﬂocculate. In addition to this electrostatic
eﬀect, it also has a chemical eﬀect that accelerates the ﬂocculation. The main eﬀect of bentonite on clariﬁcation
depends on its adsorption capacity. (Acar & Go¨kmen,
2000; Cemerog˘lu & Karadeniz, 2001). Here, pretreatment
of apple juice with gelatin and bentonite prior to ultraﬁltration resulted in an increase in ﬂux performance. These
results suggest that the aggregate formed by gelatin and
bentonite retains some molecules which are capable of fouling of the membrane. As a general rule, a reduced particle
size in feed resulted in lower overall ﬂux levels. As the average particle size in feed decreases, a decrease in average ﬂux
is seen and higher permeate ﬂuxes are determined with the
feeds containing particles of greater sizes (Tarleton & Wakeman, 1993). In this study, foulants responsible for the fouling of the membrane and decrease in the permeate ﬂux, such
as phenolic substances and proteins, are retained in feed as
large aggregates by the electrostatic and adsorptive eﬀects
of gelatin and bentonite. It is well known that polysaccha-
303
rides, particularly pectin have also adverse eﬀects on the
ﬂux behavior of apple juice during ultraﬁltration. However,
its eﬀect was limited in this study by degrading pectin before
ultraﬁltration. Riedl et al. (1998) also reported a direct correlation between colloidal ﬂocculation behavior and fouling
layer resistance.
V versus t data were analyzed by non-linear regression
ﬁtting the data to Eq. (5) by using Curve Expert. The correlation coeﬃcients indicated that ultraﬁltration data of apple
juice were well ﬁtted to the exponential model. The model
parameters (a and b) and the calculated Rm and k values
are listed in Table 1. Bayindirli, Ozilgen, and Ungan
(1989) have also reported that the exponential model simulated the cake ﬁltration of apple juice satisfactorily. Exponential fouling coeﬃcient, k is an empirical constant
deﬁned by De La Garza and Boulton (1984). It depends
on many factors such as feed properties, operation conditions and membrane properties. The higher k denotes higher
total resistance, so increased lack of ﬂux performance especially at further stages of ultraﬁltration. Here, k values
decreased as the amount of added gelatin and bentonite
increased, conﬁrming the improvement of ﬂux performance
of apple juice during ultraﬁltration. The membrane resistance, Rm is dependent on membrane properties and DP,
but, independent of feed properties. As clearly seen from
the results, Rm of the membrane having MWCO of
10 kDa was found to be 1.1 · 1013 m1 at 2 · 105 Pa,
regardless of feed properties, for all amounts of gelatin
and bentonite used for pretreatment prior to ultraﬁltration
(Table 1).
3.3. Eﬀect of P on ultraﬁltration performance
Fig. 3 shows the plot of V versus t at diﬀerent DP for
apple juice ultraﬁltration using the membrane having
MWCO of 10 kDa. As clearly seen in Table 2, Rm increased
as the applied pressure increased. Sahin and Bayindirli
(1991b) deﬁned an optimum pressure where the ﬁltration
resistance was minimal for sour cherry juice ﬁltration and
suggested that increasing the pressure above this critical
point caused an increase in Rm, but a decrease in k. They
reported that the eﬀect of Rm on total resistance was less
than the eﬀect of k, and because of this, no signiﬁcant
change was observed in terms of ﬂow rates. On the other
hand, we observed that both Rm and k values increased with
the increase in DP. However, an increase in ﬂux was
Table 1
Model parameters and Rm and k values for diﬀerent G–B
G–B
a (s)
b (m3)
Rm (m1) · 1013
k (m1)
r
1:1
1:2
1:5
1:10
0
4639.61 ± 480.20
3542.40 ± 176.77
2540.70 ± 192.59
2230.24 ± 205.18
2042.05 ± 226.05
3671.94 ± 380.05
4860.96 ± 242.56
6662.97 ± 505.05
7724.09 ± 710.62
8425.43 ± 932.70
1.10 ± 0.11
1.11 ± 0.06
1.09 ± 0.08
1.11 ± 0.10
1.11 ± 0.12
11.81 ± 1.22
15.63 ± 0.78
21.42 ± 1.62
24.83 ± 2.28
27.09 ± 3.00
0.99
0.99
0.99
0.99
0.99
Operation conditions: DP 2 · 105 Pa, membrane 10 kDa MWCO.
¨ . C¸etinkaya / Journal of Food Engineering 80 (2007) 300–305
V. Go¨kmen, O
304
5000
12000
4500
10000
30 kDa
50 kDa
1x10^5 Pa
3500
2x10^5 Pa
3000
3x10^5 Pa
4x10^5 Pa
2500
100 kDa
8000
t (s)
t (s)
4000
2000
6000
4000
1500
2000
1000
500
0
0
0
2
4
6
8
5
10
12
0
2
4
14
3
V (x10 ) (m )
Fig. 3. V versus t plots for ultraﬁltration of apple juice at diﬀerent DP.
Operation conditions: G–B 1:5, membrane 10 kDa MWCO.
observed with the increase in DP since the eﬀect of DP on
the ﬂux was greater than that of total resistance. One possible explanation for the increase in fouling potential with
increasing DP was that, a denser fouling layer might be
formed at higher pressures; such that the same amount of
foulants would result in a higher rate of decline in permeate
ﬂux. De Bruijn, Venegas, and Bor´quez (2002) reported that
high fouling and total resistance were observed at high pressures and low crossﬂow velocity. They suggested that, the
rate at which membranes foul by deposition of colloidal
material is essentially controlled by the rate of deposition
and the rate of removal of deposited material. At high pressures, the rate of deposition would be high and the high
pressure would compress the rejected solutes into a thicker
and denser fouling layer with a high fouling resistance.
Padila-Zakour and McLellan (1993) also reported a
6
8
5
V (x10 ) (m3)
10
12
14
Fig. 4. V versus t plots for ultraﬁltration of apple juice using membranes
having diﬀerent MWCO. Operation conditions: G–B 1:5, DP 2 · 105 Pa.
decrease in resistance with lower pressures. Rao et al.
(1987) suggested that, the ﬂux of apple juice increased with
pressure until it reached a maximum value and then
decreased with a further increase in pressure.
3.4. Eﬀect of MWCO on ultraﬁltration performance
Fig. 4 shows the plot of V versus t for the membranes
made of PES. As seen in Table 3, Rm of the membrane having MWCO of 30 kDa was found to be highest. It
decreased as the MWCO of membranes increased. Increasing the MWCO of membrane caused lodging of small particles in the pores and as a result, a rapid decline in the ﬂux
was observed. The highest value of k for the membrane
having MWCO of 100 kDa indicated an increased tendency of membrane to fouling during ultraﬁltration of
apple juice.
Table 2
Model parameters and Rm and k values for diﬀerent DP
DP (Pa) · 105
a (s)
b (m3)
Rm (m1) · 1013
k (m1)
r
1
2
3
4
6032.66 ± 406.60
3228.93 ± 213.43
1883.64 ± 107.93
1118.20 ± 3.02
4276.24 ± 288.22
5401.27 ± 357.02
6727.52 ± 385.49
8625.33 ± 23.29
0.83 ± 0.06
1.12 ± 0.07
1.22 ± 0.07
1.24 ± 0.00
13.75 ± 2.95
17.27 ± 1.14
21.63 ± 1.24
27.73 ± 0.07
0.99
0.99
0.99
0.99
Operation conditions: G–B 1:5, membrane 10 kDa MWCO.
Table 3
Model parameters and Rm and k values for diﬀerent MWCO values
MWCO (kDa)
a (s)
b (m3)
Rm (m1) · 1013
k (m1)
r
30
50
100
6690.07 ± 268.94
3387.18 ± 73.84
1073.09 ± 31.66
7080.62 ± 284.64
6154.76 ± 134.17
9454.63 ± 278.91
3.04 ± 0.12
1.34 ± 0.03
0.65 ± 0.02
22.76 ± 0.91
19.79 ± 0.43
30.40 ± 0.90
0.99
0.99
0.99
Operation conditions: G–B 1:5, DP 2 · 105 Pa.
¨ . C¸etinkaya / Journal of Food Engineering 80 (2007) 300–305
V. Go¨kmen, O
4. Conclusions
The eﬀects of pretreatment with gelatin and bentonite,
pressure and MWCO of membrane on the ﬂux behavior
during ultraﬁltration of apple juice were studied. The exponential model can be successfully applied to analyze membrane fouling behaviour during ultraﬁltration of apple
juice as aﬀected by various parameters. The results revealed
that the adverse eﬀects of foulants on the ﬂux performance
can be limited during ultraﬁltration by pretreatment of
apple juice with gelatin and bentonite.
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