Journal of Food Engineering 81 (2007) 243–249 www.elsevier.com/locate/jfoodeng
Study upon kinetic process of apple juice adsorption de-coloration by using adsorbent resin Nongxue Qiu a,*, Shanguang Guo b, Yuhua Chang a b
a Department of Food Engineering, Shaanxi Normal University, Xi’an 710062, China College of Light Industry and Food Science, South China University of Technology, Guuangzhou 510640, China
Received 5 August 2005; received in revised form 12 September 2006; accepted 9 October 2006 Available online 19 December 2006
Abstract This paper deals with the effects of adsorbent resin upon kinetic process of apple juice adsorption de-coloration in the case of different temperatures (25–70 °C) and different resin concentrations (1, 2, 8 g/L). The testing results indicate that the resin adsorption equilibrium curve is found to be in coincidence with Langmuir and Freundlich models and model parameter values (Kad, Q0, Kf, and n) are obtained under the conditions of different temperatures. The variation in adsorption enthalpy values (DH) is 4.16 kJ/mol, indicating that this process is a heat adsorption process. Free energy (DG) appears to have the declined tendency with an increase in temperature, whereby showing that this process is a spontaneous process. The kinetic parameters of apple juice adsorbent de-coloration under the different resin concentrations and different temperatures are further calculated with testing data. With an increase in temperature, the adsorption ability is increased, but there is a reduction in the initial balancing time. Within the range of 2–4 g/L resin concentration, the adsorption effect is rather apparent; and at the temperature of 55 °C, the initial coverage rate (he) which resin reaches to initial equilibrium can be higher, on the basis of which the determination of optimal temperature and resin concentration ranges of the effect of resin upon apple juice adsorption de-coloration can be 55 °C and 2–4 g/L. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Apple juice; Adsorbent resin; Adsorption; De-coloration
1. Introduction Browning and post-turbidity have long been worldwide problems associated with fruit juices during thermal processing and storage. The main factor to cause these is small molecular polyphenol substances in apple juice (Brigida, Javier, Teresa, Carmen, & Isabel, 1992; Constenla & Lozano, 1995). These intractable problems are obstacle to produce high quality clear apple juice. At present, there have been many kinds of methods to reduce the random rate of apple juice in brown changes and post-turbidity. Gelatin and bentonite used to carry out the flocculating settlings of apple juice is the tradi-
*
Corresponding author. Tel./fax: +86 29 85307756. E-mail address: [email protected] (N. Qiu).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.10.030
tional method to maintain its stability (Vural & Nevzat, 2001). Since the 1980s in the 20th century, polyvinyl polypyrrolidone (PVPP) has been used as the adsorbent to treat apple juice via ultrafiltration, being served as the final key technology for maintaining apple juice stability (Hums, Krug, & Heess, 1980). However, the former is easy to cause the blocking on the ultrafiltration membrane, seriously threatening the service life of the membrane, while the latter has the high operation cost, with PVPP difficult to be recovered. Selecting and using optimal resin to adsorb apple juice is able to remove the compositions to cause browning and post-turbidity from apple juice, and thereby the transparency and stability of apple juice can be improved. Accordingly, using adsorbent resin to treat apple juice has attracted the extensive attention from apple juice industries so that the adsorbent resin is being widely used.
244
N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
The adsorbent resin used for the test is a kind of macroporous resin, which has been widely used in producing clear apple juice especially in China. However, since there has been no much report on the kinetic study of adsorbent resin to adsorb the pigment substances from apple juice, the de-coloration technology is assessed only by depending on A420 (absorbance at 420 nm) and/or the color value (transparence at 440 nm) of de-colored juice effluent from resin volume, without taking into account the effect of the resin adsorption traits, such as temperature and flow rate of juice upon adsorption so that the maximum de-coloration ability of resin can not be brought into full play and the resin has had a short working cycle with the frequent regeneration resulting in manpower and material resources wasted. For this reason, this paper deals with the adsorption kinetic behavior of apple juice de-coloration by this resin and determines the adsorption isotherms in the case of different temperatures. Further more, the parameter values of adsorption thermokinetics of Langmuir and Freundlich models have been obtained to determine that this resin to adsorb apple juice is a physical process or not. Therefore, the geometric approximation method is used to measure the kinetic parameters in the process of apple juice de-coloration. It is hoped that a theoretical basis can be provided for apple juice de-coloration and optimal adsorption technology parameters. 2. Model 2.1. Equilibrium model When the adsorption reaches equilibrium, the solute concentration in liquid phase is C; the solute concentration in solid phase is Q. What the adsorption isotherm describes is the relationship between the solute concentration Q (l/g resin) in the equilibrium adsorbent and the solute concentration C (as far as the definition of apple juice adsorption is concerned, it is the ratio value of absorbance A and A0 when adsorption terminates and begins, non-dimensional parameters) in the case of certain temperature. The quantitative explanation for adsorption experimental data can be expressed using different mathematical models, of which Langmuir model and Freundlich model are widely considered as the base for studying adsorption process. Langmuir model can be described as follows: Q=Q0 ¼ K ad C=ð1 þ K ad CÞ
ð1Þ
where Q0 – the maximum adsorption concentration on the adsorption surface, l/g; Kad – adsorption equilibrium constant, (absorbance units)1; C – equilibrium concentration of solute, non-dimensional parameter. In the certain temperature and fixed adsorbent–solute system, Q0 and Kad are constant. Eq. (1) linear form can be expressed as follows: 1=Q ¼ 1=Q0 þ 1=ðK ad Q0 CÞ
ð2Þ
Freundlich model can be described as follows: Q ¼ K f Cn
ð3Þ
where Kf – adsorption constant, 1/g; n – adsorption index As in the same way, Kf and n can be determined by temperature and adsorbent–solute system. 2.2. Kinetic model Adsorption and de-adsorption reaction can be mostly described as follows: Ka
AþB ¢ AB Kd
ð4Þ
where Ka – adsorption rate constant, min1; Kd – desorption rate constant, min1; A – adsorbent; B – adsorbate; A B – adsorbate adsorbed on the adsorbent. Langmuir described the kinetic formula of adsorption and desorption as follows (Langmuir, 1918): ra ¼ K a Cð1 hÞ rd ¼ K d h
ð5Þ ð6Þ
where h(Q/Q0) – coverage coefficient (0 6 h 6 1); C – equilibrium concentration of solute, non-dimensional parameters; ra – adsorption rate, min1; rd – desorption rate, min1. Because the calculation dynamic parameters need measuring adsorption equilibrium constant, while adsorption and desorption rate constant can not be measured independently, the geometric approximation method often adopted by Kuan et al. is employed to determine adsorption and desorption constants (Kuan, Lo, Chang, & Wang, 2000). The plot of h against time (t) can be employed to describe adsorption rate, being divided into the initial reaction stage and the later sluggish stage. The slope K0 (min1) yielded by the regression analysis curve at the initial stage can be used as the adsorption rate at the initial stage; the intercept at the late sluggish stage can be the equilibrium coverage coefficient (he). h ¼ K 0t h ¼ he
ð7Þ ð8Þ
Letting simultaneous equations (7) and (8), we can derive the following equation: tie ¼ he =K 0
ð9Þ
where tie – the initial equilibrium time, min. Eq. (7) is used to replace h in Eqs. (5) and (6), we can obtain the following equation: ra ¼ K 0 Cð1 K 0t Þ
ð10Þ
rd ¼ K d K 0t
ð11Þ
When in the case of equilibrium state, ra equals to rd, namely, K a Cð1 K 0t Þ ¼ K d K ot
ð12Þ
N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
The coverage coefficient (hc) in equilibrium can be expressed by the following: Z tie ðra rd Þ dt ð13Þ he ¼
245
of de-coloration apple juice should be corrected with the samples in the control under the same conditions so as to eliminate the effect of temperature upon absorbency values. The procedures were done in triplicate.
0
0
Integrating the above equations sorted with Eq. (12), we can obtain he following: he ¼ ð1=2ÞCK a tie
ð15Þ 2
he ¼ ð1=2ÞK d K 0 ðtie Þ =ð1 K 0 tie Þ
ð16Þ
From Eqs. (9), Eqs. (15) and (16) can be sorted as the following: K a ¼ 2K 0 =C
ð17Þ
K d ¼ 2ð1 K 0 tie Þ=tie
ð18Þ
Up to now, the constants of adsorption and desorption rates can be derived. 3. Materials and methods The soluble solids content of apple juice used in this test is 70.0 °Brix. The adsorbent resin, commercial LSA-800B obtained from Lanshen Technology Ltd., Xi’an, China, used in the test is brown ball-shaped grains; specific surface area: 800–1000 m2/g; Grain diameter: 0.3–1.2 mm; wet real density: 1.05–1.15 g/mL. Distilled water is used to dilute the concentrated apple juice to 10.0 °Brix, determined with a refractometer (Abbe 60 series), filtered via the filter paper (pore size 40 lm). The absorption values are measured using the Unico 2000 Type of visible spectrophotometer. The initial absorbance A0 (420 nm) can be regulated and controlled at 1.2. Taking 1 L of apple juice with above absorbance and putting it into five conical flasks, of which there are four conical flasks to which 1, 2, 4, 8 g of resin should be added respectively, and then resin concentrations of 1, 2, 4, 8 g/L apple juice are obtained respectively, while apple juice without resin added to serves as the control. In terms of requirements by apple juice processing technology and under the prerequisite of ensuring the fluidity, the temperatures are expected to be as low as possible in such a manner that the experiments are chosen to be carried out at the temperatures of 25, 40, 55, and 70 °C. Accordingly, all the conical flasks are put into water bath with fixed temperatures and shaken once at an interval of 30 min. Based on the preexperiment, resin can reach the adsorption equilibrium state within 24 h. In the adsorption process, absorbance value A420 at 420 nm can be measured at the time intervals of 0, 15, 30, 45, 60, 180, 360, 540, 720, 1260, and 1440 min. All the samples, when measured, must be filtered via Syringe Filter with 0.45 lm membrane. Absorbance values
4. Results and analysis 4.1. Adsorption efficiency The resin effect upon adsorption efficiency of apple juice can be calculated by the following equation: Adsorption efficiency ð%Þ ¼ ½ðA0 AÞ=A0 100
ð19Þ
where A0 – the initial absorbance value of apple juice at 420 nm, %; A – absorbance values of apple juice at 420 nm via adsorption after 1440 min, %. Fig. 1 indicates the variations in adsorption efficiency in the case of different resin concentrations and different temperatures. It can be seen that adsorption efficiency tends to rise with an increase in temperature. This is because raising temperature can decrease the viscosity of apple juice and accelerate the thermal motion of pigments in juice, whereby promoting the pigments to be adsorbed on the surface and inner of resin. However, it can also be seen from Fig. 1 that there is no much apparent difference in adsorption efficiency between 40 °C and 55 °C, and that when resin concentration is over 4 g/L, the two efficiency curves is basically overlapped. This is likely that because of being within the temperature range, within the adsorption system the viscosity of apple juice and the thermal motion of adsorbed matter molecules are in the state of relative equilibrium. With a further increase in temperature, this dynamic state is broken down; and adsorption efficiency is raised to a certain extent. In addition, adsorption efficiency is raised with an increase in resin concentration. This is because the expansion of resin concentration means an increase in adsorption area, but there has been some
70 70˚C 55˚C 40˚C 25˚C
60
Adsorption efficiency (%)
Eqs. (10) and (11) are used to replace ra and rd, respectively, we can obtain the following : Z tie ½k a Cð1 k 0t Þ k d k 0t dt ð14Þ he ¼
50 40 30 20 10 0
0
2
4
6
8
10
M g/L Fig. 1. The effect of adsorbent resin concentration (M) and temperature upon adsorption efficiency (%).
N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
4.2. Adsorption isotherm In the case of temperature is certain, adsorption amount is related with the adsorbent concentration. The plot of function relationship between adsorption amount and adsorbent concentration is called as the adsorption isothermal. Adsorption isotherm indicates the equilibrium adsorption amount at a given temperature, and can be used to deduce adsorption heat and the rest of other physical and chemical properties. At present, there is no maturity theory that can be used to estimate the fluid–solid adsorption equilibrium. Accordingly, it is necessary to measure the equilibrium data of the specified system via the experiments and to plot the loading adsorption amount on the adsorbent and the adsorption isotherm of the relationship among the adsorbance concentration in liquid phase. For apple juice adsorption de-coloration, the adsorption isotherm is closely related to the pigment substance concentration Q in adsorbent resin and the residual pigment substance concentration C in apple juice when adsorption reaches the equilibrium. They can be determined via measuring A420 in apple juice. It is here that C represents (A/A0), Q stands for [(A0 A)/A0]/m, where, m is the quality of resin, g. Fig. 2 is the result charted by Q over C. Based on the classification of adsorption isotherms, this upward convex curve belongs to the typical isotherm curve, being the favorable adsorption. This Fig. indicates that the adsorption resin can adsorb and concentrate high solute loading with lower resin concentration in apple juice.
0. 14 0. 12 70˚C 55˚C 40˚C 25˚C
0. 1
Q (1/g)
decrease in adsorption efficiency of mass resin per unit. Carabasa et al once reported that they conducted the similar experiment on de-coloration using active carbon in peach juice (Carabasa, Ibarz, Garza, & Barbosa-Ca´novad, 1998). Wang et al. had the similar conclusions in research on Vitamin B12 adsorbed using phenolic resin adsorbents (Wang & Shi, 2003). However, some scholars hold that raising temperature and accelerating molecular thermal motion can render the adsorbed matters easy to be desorbed, thereby lowering resin adsorption ability (Fischer & Hofsommer, 1992), whose main cause can be explained using mass transfer process for resin to adsorb pigment from apple juice: in the adsorption process, two kinds of counteraction exist at the same time, but under a certain temperature, one action is always in the dominate and advantage position, leading to the raising or decreasing of adsorption ability. In this test, adsorption efficiency increases with a raise in temperature, for this reason, it can be deduced that when the temperature goes up, the thermal motion effect of pigment substances in apple juice is larger than that of pigment substances adsorbed by LSA800B resin, resulting in the simultaneous acceleration of external and internal diffusion velocities in such a way that pigment substances are easy to reach and adsorbed on the surface of adsorbents.
0. 08 0. 06 0. 04 0. 02 0
0
0. 2
0. 4 0. 6
0. 8
1
C Fig. 2. Adsorption isotherms of pigments on adsorbent resin at different temperatures.
4.3. Adsorption models Based on the analysis of adsorption data of Langmuir model and Freundich model, the correlations of adsorption data with two models are investigated. The results indicate that adsorption data are in agreement with two models. Based on the linear description of Eq. (2) of Langmuir model, Fig. 3 is plotted by 1/Q(g) over 1/C (the dimensionless parameters). The interception of straight line on 1/Q axial is 1/Q0, and Q0 shows the resin maximum adsorption ability, whose slope is 1/KadQ0, and the adsorption equilibrium constant Kad and coefficient of determination r2 of the fitting equation are given by Table 1. When a = 0.05, n = 4, the critical value of correlation coefficientris 0.95. In Table 1, the coefficient of determination r2 of the fitting equation is larger than 0.95, indicating that the linear correlative relation is significant, on the basis of which it can be seen that the adsorption process is in agreement with Langmuir model. Q0 increases as temperature goes up, thereby indicating that this process is the endothermic reaction. Equilibrium constant Kad raises as temperature goes up in the similar way as to prove that adsorption ability improves with an increase in 18
70˚C
16
1/Q (g)
246
55˚C 40˚C
14
25˚C
12 10 8 6
0
1
2
3
1/C Fig. 3. Langmuir isotherms.
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N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249 Table 1 Langmuir model parameters
247
Table 2 Frenudlich model parameters
Temperature/°C
Q0/g1
Kada
R2
Temperature/°C
Kf/g1
n
r2
25 40 55 70
0.147 0.194 0.233 0.235
1.274 1.472 1.526 1.678
0.993 0.962 0.999 0.994
25 40 55 70
11.349 7.429 8.039 6.873
0.496 0.592 0.511 0.538
0.985 0.960 0.991 0.980
a
Units: (absorbance units)1.
temperature. Though the antagonistic effects exist at the same time in the test, the adsorption process prevails. Consequently, the result of combining Q0, and Kad is enhanced with temperature. However, since the adsorption operation procedures are carried out after the ultrafiltration, whose temperature is constrained by the ultrafiltration, the temperature can not be over 55 °C as a whole. Kad > 1 can prove that this process is a better adsorption process. Testing data are also in agreement with Freundlich model equation: Ln Q ¼ Ln K f þ n Ln C
ð20Þ
Based on Ln C to portray Ln Q, Freundkich isotherms can be obtained, as shown in Fig. 4. The straight line interception Kf, slope n and coefficient of determination r2 by the fitting equation are shown in Table 2. The above results can clearly show that data of apple juice adsorption de-coloration by LSA-800B macroporous resin are well in agreement with Langmuir model and Freundlich model. 4.4. Thermodynamic parameters Table 3 gives thermodynamic parameters such as Gibbs free energy DG (kJ/mol), enthalpy change DH (kJ/mol) and entropy change DS (kJ/mol K) in the tests of apple juice adsorption de-coloration by using the resin. DG can be calculated by the following equation: DG ¼ RT Ln K ad
ð21Þ
LnQ (1/g)
where Kad – adsorption equilibrium constant (derived from Langmuir model); T – absolute temperature, K; R – gas constant 8.3144 103, kJ/(mol K).
- 1. 5
70˚C
-2
55˚C 40˚C 25˚C
- 2. 5
-3 - 1. 5
-1
- 0. 5
LnC Fig. 4. Freundlich isotherms.
0
The relations among Kad and thermodynamic parameters DH as well as DS can be described using Van’t Hoff formula. Ln K ad ¼ DS=R DH =RT
ð22Þ
In this way, DH and DS can be obtained from the slope and interception of Van’t Hoff straight line shown in Fig. 5 respectively. DH is the positive values, whereby proving that the process of resin adsorbing pigments in apple juice is the endothermic reaction. In addition, it can be known from calculated value DH = 4.1557 kJ/mol that the above process is physical adsorption rather than chemical adsorption. Accordingly, it can be considered that this adsorption process is in agreement with the CAC (Codex Alimentarius Commission) provisions or specifications concerning the process of fruit juice production in which only the physical methods can be adopted(http://www.codexalimentarius.net/web/index_en.jsp). DG is the negative values with the general trend appearing to be the negative growth, whereby proving that the above process is the spontaneous process; and DS is the positive values thus, showing that this process is the entropy increasing process. 4.5. Adsorption dynamic curves The 16 groups of adsorption dynamic curves are obtained through the experiments in the case of setting temperatures (15, 40, 55 and 70 °C) and resin concentrations (1, 2, 4, and 8 g/L). In considering that apple juice after being ultrafiltration is adsorbed, the temperature of ultrafiltration apple juice is not over 55 °C in general. If the adsorption temperature still needs to be higher, it is necessary to add heating equipment. Since the length of this paper is restricted, what Fig. 6 only gives is the curve with the coverage rate (h) changing with time (t) when adsorbent de-coloration is carried out at the temperature of 55 °C and the resin concentration of 4 g/L. Based on the statistic software (ORIGN 6.1), geometric approximation treatment is made of data (h and t) so that the results are found to be in agreement with the growth model h = a(b ect), and it is here that a, b, and c are the constants. The derivative in the case of the curve at t = 0 can be the initial adsorption rate (K0). The initial equilibrium time (tic) can be calculated in terms of Eq. (9). The equilibrium coverage rate (he) can be calculated in terms of the intercept of approximation straight line in the late adsorption. Based on the curve approximation method as well as Eqs. (18) and (19), the different resin concentrations and
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N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
Table 3 Thermodynamic parameters for adsorption process of adsorbent resin de-coloration Kad
DG/kJ mol1
25 40 55 70
1.472 1.472 1.526 1.678
0.959 0.631 1.413 1.206
Ln Kad
Temperature/°C
DH/kJ mol1
DS/kJ mol1 K1
r2
4.156
0.016
0.939
0. 6
Table 4 Dynamic parameters for resin adsorption de-coloration (resin concentration is 4 g/L)
0. 5
Temperature/ °C
he
k0/ min1
tie/ min
ka/ min1
kd/ min1
r2
25 40 55 70
0.480 0.389 0.499 0.405
0.0008 0.0007 0.0010 0.0010
592.59 525.68 499.00 408.08
0.0023 0.0024 0.0033 0.0035
0.0018 0.0023 0.0020 0.0029
0.975 0.989 0.991 0.996
0. 4
0. 3
0. 2 0. 0028
0. 003
0. 0032
0. 0034
0. 0036
1/T (1/K) Fig. 5. Van’t Hoff straight line.
Fig. 6. Resin adsorption dynamic curves (55 °C, 4 g/L).
the values of K0, tiehe, and Kd under the different temperature can be calculated respectively; and the coefficient of determination r2 can be determined by the fitting equation. Table 4 only gives the dynamic parameters values in the case of resin concentrations being 4 g/L at the different adsorption temperatures, for the rest of other dynamic parameters in the case of resin concentrations are no longer described. It can be seen from the integration of Fig. 6 and Table 4 that the resin adsorption rate decreases with the prolonging
of time, while in the practical production, the constant flow rate through the adsorption columns is mostly adopted to carry out the apple juice de-coloration adsorption, being unfavorable for improving the resin adsorption de-coloration efficiency. Accordingly, it is felt essential to consider that the gradual changing flow rate from the high to the low can be adopted in one adsorption–regeneration cycle. It can be seen from Ka varying trend that the resin adsorption ability keeps a high level within a wide temperature range of 25–70 °C, rendering the adoption of gradual changing flow rate to carry out de-coloration more feasible. As described above, the adsorption ability can be improved as the temperature goes up, but the initial equilibrium time (tie) is shortened. This means that resin regeneration cycle becomes shorter, which is not expected in fruit juice production. It is again felt necessary to reconsider that apple juice through the ultrafiltration can be adsorbed so that the temperature of apple juice is not over 55 °C in general. For this reason, 55 °C is the suitable adsorption temperature, which makes it possible for the apple juice from ultrafiltration to have direct de-coloration without being reheated. 5. Conclusions Research findings indicate: Apple juice adsorption decoloration by resin is a heat adsorption process, showing the physical adsorption; the adsorption and desorption rate constants for apple juice adsorption de-coloration by resin are derived via the geometric approximation calculations respectively; the adsorption ability is raised as the temperature goes up or rises; the adsorption initial equilibrium time for different resin concentrations at 25 °C is 9.37 h on average, but with an increase in testing temperature, the initial equilibrium time is reduced to a certain extent; Within the range of resin concentration of 2–4 g/L, adsorption effect is apparently significant, but being over this range, resin adsorption efficiency per unit mass will have
N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
some drops; At 70 °C and 55 °C, the coverage rate (h) for resin reaching the initial equilibrium is higher, however, the temperature at 70 °C can just go against apple juice processing technology. For this reason, in terms of requirements by the rational technology in apple juice production, and the real temperature for apple juice via ultrafiltration, the suitable temperature and resin concentration for using adsorption resin to carry out apple juice adsorption de-coloration should be 55 °C and 2–4 g/L, respectively. Acknowledgement This research is supported by the National High Technology Research and Development Program of China (863 Program) under Grant No. 2002AA245091. References Brigida, F. S., Javier, P. I., Teresa, H., Carmen, C. C., & Isabel, E. (1992). Importance of phenolic compounds for the characterization of fruit juices. Journal of Agriculture and Food Chemistry, 40(9), 1531–1535.
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Carabasa, M., Ibarz, A., Garza, S., & Barbosa-Ca´novad, G. V. (1998). Removal of dark compounds from clarified fruit juices by adsorption processes. Journal of Food Engineering, 37, 25–41. Constenla, D. T., & Lozano, J. E. (1995). Effect of ultrafiltration on concentrated apple juice colour and turbidity. International Journal of Food Science and Technology, 3, 23–30. Fischer, K. P., & Hofsommer, H. J. (1992). Application of the adsorption technique in the fruit juice industry. Confructa Studien, 36(3–4), 10–107, http://www.codexalimentarius.net/web/index_en.jsp. Hums, N., Krug, K., & Heess, E. (1980). Die stabilizierung von apfelsaft mit polyvinylpolypyrrolidone (PVPP) im recycling. Flu¨ssiges Obst, 47, 283–287. Kuan, W. H., Lo, S. L., Chang, C. M., & Wang, M. K. (2000). A geometric approach to determine adsorption and desorption kinetic constants. Chemosphere, 41, 1741–1747. Langmuir, I. (1918). The adsorption of gases on plane surfaces of glass, mica and platinum. Journal of the American Chemical Society, 40, 1361–1403. Vural, G., & Nevzat, A. (2001). Effect of various clarification treatments on patulin, phenolic compound and organic acid composition of apple juice. European Food Research and Technology, 213, 194–199. Wang, Z., & Shi, Z. Q. (2003). Thermodynamic study of the adsorption of VB12 on phenolic resin adsorbents. Journal of Functional Polymers, 16(1), 1–5.
Study upon kinetic process of apple juice adsorption de-coloration by using adsorbent resin Nongxue Qiu a,*, Shanguang Guo b, Yuhua Chang a b
a Department of Food Engineering, Shaanxi Normal University, Xi’an 710062, China College of Light Industry and Food Science, South China University of Technology, Guuangzhou 510640, China
Received 5 August 2005; received in revised form 12 September 2006; accepted 9 October 2006 Available online 19 December 2006
Abstract This paper deals with the effects of adsorbent resin upon kinetic process of apple juice adsorption de-coloration in the case of different temperatures (25–70 °C) and different resin concentrations (1, 2, 8 g/L). The testing results indicate that the resin adsorption equilibrium curve is found to be in coincidence with Langmuir and Freundlich models and model parameter values (Kad, Q0, Kf, and n) are obtained under the conditions of different temperatures. The variation in adsorption enthalpy values (DH) is 4.16 kJ/mol, indicating that this process is a heat adsorption process. Free energy (DG) appears to have the declined tendency with an increase in temperature, whereby showing that this process is a spontaneous process. The kinetic parameters of apple juice adsorbent de-coloration under the different resin concentrations and different temperatures are further calculated with testing data. With an increase in temperature, the adsorption ability is increased, but there is a reduction in the initial balancing time. Within the range of 2–4 g/L resin concentration, the adsorption effect is rather apparent; and at the temperature of 55 °C, the initial coverage rate (he) which resin reaches to initial equilibrium can be higher, on the basis of which the determination of optimal temperature and resin concentration ranges of the effect of resin upon apple juice adsorption de-coloration can be 55 °C and 2–4 g/L. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Apple juice; Adsorbent resin; Adsorption; De-coloration
1. Introduction Browning and post-turbidity have long been worldwide problems associated with fruit juices during thermal processing and storage. The main factor to cause these is small molecular polyphenol substances in apple juice (Brigida, Javier, Teresa, Carmen, & Isabel, 1992; Constenla & Lozano, 1995). These intractable problems are obstacle to produce high quality clear apple juice. At present, there have been many kinds of methods to reduce the random rate of apple juice in brown changes and post-turbidity. Gelatin and bentonite used to carry out the flocculating settlings of apple juice is the tradi-
*
Corresponding author. Tel./fax: +86 29 85307756. E-mail address: [email protected] (N. Qiu).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.10.030
tional method to maintain its stability (Vural & Nevzat, 2001). Since the 1980s in the 20th century, polyvinyl polypyrrolidone (PVPP) has been used as the adsorbent to treat apple juice via ultrafiltration, being served as the final key technology for maintaining apple juice stability (Hums, Krug, & Heess, 1980). However, the former is easy to cause the blocking on the ultrafiltration membrane, seriously threatening the service life of the membrane, while the latter has the high operation cost, with PVPP difficult to be recovered. Selecting and using optimal resin to adsorb apple juice is able to remove the compositions to cause browning and post-turbidity from apple juice, and thereby the transparency and stability of apple juice can be improved. Accordingly, using adsorbent resin to treat apple juice has attracted the extensive attention from apple juice industries so that the adsorbent resin is being widely used.
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N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
The adsorbent resin used for the test is a kind of macroporous resin, which has been widely used in producing clear apple juice especially in China. However, since there has been no much report on the kinetic study of adsorbent resin to adsorb the pigment substances from apple juice, the de-coloration technology is assessed only by depending on A420 (absorbance at 420 nm) and/or the color value (transparence at 440 nm) of de-colored juice effluent from resin volume, without taking into account the effect of the resin adsorption traits, such as temperature and flow rate of juice upon adsorption so that the maximum de-coloration ability of resin can not be brought into full play and the resin has had a short working cycle with the frequent regeneration resulting in manpower and material resources wasted. For this reason, this paper deals with the adsorption kinetic behavior of apple juice de-coloration by this resin and determines the adsorption isotherms in the case of different temperatures. Further more, the parameter values of adsorption thermokinetics of Langmuir and Freundlich models have been obtained to determine that this resin to adsorb apple juice is a physical process or not. Therefore, the geometric approximation method is used to measure the kinetic parameters in the process of apple juice de-coloration. It is hoped that a theoretical basis can be provided for apple juice de-coloration and optimal adsorption technology parameters. 2. Model 2.1. Equilibrium model When the adsorption reaches equilibrium, the solute concentration in liquid phase is C; the solute concentration in solid phase is Q. What the adsorption isotherm describes is the relationship between the solute concentration Q (l/g resin) in the equilibrium adsorbent and the solute concentration C (as far as the definition of apple juice adsorption is concerned, it is the ratio value of absorbance A and A0 when adsorption terminates and begins, non-dimensional parameters) in the case of certain temperature. The quantitative explanation for adsorption experimental data can be expressed using different mathematical models, of which Langmuir model and Freundlich model are widely considered as the base for studying adsorption process. Langmuir model can be described as follows: Q=Q0 ¼ K ad C=ð1 þ K ad CÞ
ð1Þ
where Q0 – the maximum adsorption concentration on the adsorption surface, l/g; Kad – adsorption equilibrium constant, (absorbance units)1; C – equilibrium concentration of solute, non-dimensional parameter. In the certain temperature and fixed adsorbent–solute system, Q0 and Kad are constant. Eq. (1) linear form can be expressed as follows: 1=Q ¼ 1=Q0 þ 1=ðK ad Q0 CÞ
ð2Þ
Freundlich model can be described as follows: Q ¼ K f Cn
ð3Þ
where Kf – adsorption constant, 1/g; n – adsorption index As in the same way, Kf and n can be determined by temperature and adsorbent–solute system. 2.2. Kinetic model Adsorption and de-adsorption reaction can be mostly described as follows: Ka
AþB ¢ AB Kd
ð4Þ
where Ka – adsorption rate constant, min1; Kd – desorption rate constant, min1; A – adsorbent; B – adsorbate; A B – adsorbate adsorbed on the adsorbent. Langmuir described the kinetic formula of adsorption and desorption as follows (Langmuir, 1918): ra ¼ K a Cð1 hÞ rd ¼ K d h
ð5Þ ð6Þ
where h(Q/Q0) – coverage coefficient (0 6 h 6 1); C – equilibrium concentration of solute, non-dimensional parameters; ra – adsorption rate, min1; rd – desorption rate, min1. Because the calculation dynamic parameters need measuring adsorption equilibrium constant, while adsorption and desorption rate constant can not be measured independently, the geometric approximation method often adopted by Kuan et al. is employed to determine adsorption and desorption constants (Kuan, Lo, Chang, & Wang, 2000). The plot of h against time (t) can be employed to describe adsorption rate, being divided into the initial reaction stage and the later sluggish stage. The slope K0 (min1) yielded by the regression analysis curve at the initial stage can be used as the adsorption rate at the initial stage; the intercept at the late sluggish stage can be the equilibrium coverage coefficient (he). h ¼ K 0t h ¼ he
ð7Þ ð8Þ
Letting simultaneous equations (7) and (8), we can derive the following equation: tie ¼ he =K 0
ð9Þ
where tie – the initial equilibrium time, min. Eq. (7) is used to replace h in Eqs. (5) and (6), we can obtain the following equation: ra ¼ K 0 Cð1 K 0t Þ
ð10Þ
rd ¼ K d K 0t
ð11Þ
When in the case of equilibrium state, ra equals to rd, namely, K a Cð1 K 0t Þ ¼ K d K ot
ð12Þ
N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
The coverage coefficient (hc) in equilibrium can be expressed by the following: Z tie ðra rd Þ dt ð13Þ he ¼
245
of de-coloration apple juice should be corrected with the samples in the control under the same conditions so as to eliminate the effect of temperature upon absorbency values. The procedures were done in triplicate.
0
0
Integrating the above equations sorted with Eq. (12), we can obtain he following: he ¼ ð1=2ÞCK a tie
ð15Þ 2
he ¼ ð1=2ÞK d K 0 ðtie Þ =ð1 K 0 tie Þ
ð16Þ
From Eqs. (9), Eqs. (15) and (16) can be sorted as the following: K a ¼ 2K 0 =C
ð17Þ
K d ¼ 2ð1 K 0 tie Þ=tie
ð18Þ
Up to now, the constants of adsorption and desorption rates can be derived. 3. Materials and methods The soluble solids content of apple juice used in this test is 70.0 °Brix. The adsorbent resin, commercial LSA-800B obtained from Lanshen Technology Ltd., Xi’an, China, used in the test is brown ball-shaped grains; specific surface area: 800–1000 m2/g; Grain diameter: 0.3–1.2 mm; wet real density: 1.05–1.15 g/mL. Distilled water is used to dilute the concentrated apple juice to 10.0 °Brix, determined with a refractometer (Abbe 60 series), filtered via the filter paper (pore size 40 lm). The absorption values are measured using the Unico 2000 Type of visible spectrophotometer. The initial absorbance A0 (420 nm) can be regulated and controlled at 1.2. Taking 1 L of apple juice with above absorbance and putting it into five conical flasks, of which there are four conical flasks to which 1, 2, 4, 8 g of resin should be added respectively, and then resin concentrations of 1, 2, 4, 8 g/L apple juice are obtained respectively, while apple juice without resin added to serves as the control. In terms of requirements by apple juice processing technology and under the prerequisite of ensuring the fluidity, the temperatures are expected to be as low as possible in such a manner that the experiments are chosen to be carried out at the temperatures of 25, 40, 55, and 70 °C. Accordingly, all the conical flasks are put into water bath with fixed temperatures and shaken once at an interval of 30 min. Based on the preexperiment, resin can reach the adsorption equilibrium state within 24 h. In the adsorption process, absorbance value A420 at 420 nm can be measured at the time intervals of 0, 15, 30, 45, 60, 180, 360, 540, 720, 1260, and 1440 min. All the samples, when measured, must be filtered via Syringe Filter with 0.45 lm membrane. Absorbance values
4. Results and analysis 4.1. Adsorption efficiency The resin effect upon adsorption efficiency of apple juice can be calculated by the following equation: Adsorption efficiency ð%Þ ¼ ½ðA0 AÞ=A0 100
ð19Þ
where A0 – the initial absorbance value of apple juice at 420 nm, %; A – absorbance values of apple juice at 420 nm via adsorption after 1440 min, %. Fig. 1 indicates the variations in adsorption efficiency in the case of different resin concentrations and different temperatures. It can be seen that adsorption efficiency tends to rise with an increase in temperature. This is because raising temperature can decrease the viscosity of apple juice and accelerate the thermal motion of pigments in juice, whereby promoting the pigments to be adsorbed on the surface and inner of resin. However, it can also be seen from Fig. 1 that there is no much apparent difference in adsorption efficiency between 40 °C and 55 °C, and that when resin concentration is over 4 g/L, the two efficiency curves is basically overlapped. This is likely that because of being within the temperature range, within the adsorption system the viscosity of apple juice and the thermal motion of adsorbed matter molecules are in the state of relative equilibrium. With a further increase in temperature, this dynamic state is broken down; and adsorption efficiency is raised to a certain extent. In addition, adsorption efficiency is raised with an increase in resin concentration. This is because the expansion of resin concentration means an increase in adsorption area, but there has been some
70 70˚C 55˚C 40˚C 25˚C
60
Adsorption efficiency (%)
Eqs. (10) and (11) are used to replace ra and rd, respectively, we can obtain the following : Z tie ½k a Cð1 k 0t Þ k d k 0t dt ð14Þ he ¼
50 40 30 20 10 0
0
2
4
6
8
10
M g/L Fig. 1. The effect of adsorbent resin concentration (M) and temperature upon adsorption efficiency (%).
N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
4.2. Adsorption isotherm In the case of temperature is certain, adsorption amount is related with the adsorbent concentration. The plot of function relationship between adsorption amount and adsorbent concentration is called as the adsorption isothermal. Adsorption isotherm indicates the equilibrium adsorption amount at a given temperature, and can be used to deduce adsorption heat and the rest of other physical and chemical properties. At present, there is no maturity theory that can be used to estimate the fluid–solid adsorption equilibrium. Accordingly, it is necessary to measure the equilibrium data of the specified system via the experiments and to plot the loading adsorption amount on the adsorbent and the adsorption isotherm of the relationship among the adsorbance concentration in liquid phase. For apple juice adsorption de-coloration, the adsorption isotherm is closely related to the pigment substance concentration Q in adsorbent resin and the residual pigment substance concentration C in apple juice when adsorption reaches the equilibrium. They can be determined via measuring A420 in apple juice. It is here that C represents (A/A0), Q stands for [(A0 A)/A0]/m, where, m is the quality of resin, g. Fig. 2 is the result charted by Q over C. Based on the classification of adsorption isotherms, this upward convex curve belongs to the typical isotherm curve, being the favorable adsorption. This Fig. indicates that the adsorption resin can adsorb and concentrate high solute loading with lower resin concentration in apple juice.
0. 14 0. 12 70˚C 55˚C 40˚C 25˚C
0. 1
Q (1/g)
decrease in adsorption efficiency of mass resin per unit. Carabasa et al once reported that they conducted the similar experiment on de-coloration using active carbon in peach juice (Carabasa, Ibarz, Garza, & Barbosa-Ca´novad, 1998). Wang et al. had the similar conclusions in research on Vitamin B12 adsorbed using phenolic resin adsorbents (Wang & Shi, 2003). However, some scholars hold that raising temperature and accelerating molecular thermal motion can render the adsorbed matters easy to be desorbed, thereby lowering resin adsorption ability (Fischer & Hofsommer, 1992), whose main cause can be explained using mass transfer process for resin to adsorb pigment from apple juice: in the adsorption process, two kinds of counteraction exist at the same time, but under a certain temperature, one action is always in the dominate and advantage position, leading to the raising or decreasing of adsorption ability. In this test, adsorption efficiency increases with a raise in temperature, for this reason, it can be deduced that when the temperature goes up, the thermal motion effect of pigment substances in apple juice is larger than that of pigment substances adsorbed by LSA800B resin, resulting in the simultaneous acceleration of external and internal diffusion velocities in such a way that pigment substances are easy to reach and adsorbed on the surface of adsorbents.
0. 08 0. 06 0. 04 0. 02 0
0
0. 2
0. 4 0. 6
0. 8
1
C Fig. 2. Adsorption isotherms of pigments on adsorbent resin at different temperatures.
4.3. Adsorption models Based on the analysis of adsorption data of Langmuir model and Freundich model, the correlations of adsorption data with two models are investigated. The results indicate that adsorption data are in agreement with two models. Based on the linear description of Eq. (2) of Langmuir model, Fig. 3 is plotted by 1/Q(g) over 1/C (the dimensionless parameters). The interception of straight line on 1/Q axial is 1/Q0, and Q0 shows the resin maximum adsorption ability, whose slope is 1/KadQ0, and the adsorption equilibrium constant Kad and coefficient of determination r2 of the fitting equation are given by Table 1. When a = 0.05, n = 4, the critical value of correlation coefficientris 0.95. In Table 1, the coefficient of determination r2 of the fitting equation is larger than 0.95, indicating that the linear correlative relation is significant, on the basis of which it can be seen that the adsorption process is in agreement with Langmuir model. Q0 increases as temperature goes up, thereby indicating that this process is the endothermic reaction. Equilibrium constant Kad raises as temperature goes up in the similar way as to prove that adsorption ability improves with an increase in 18
70˚C
16
1/Q (g)
246
55˚C 40˚C
14
25˚C
12 10 8 6
0
1
2
3
1/C Fig. 3. Langmuir isotherms.
4
N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249 Table 1 Langmuir model parameters
247
Table 2 Frenudlich model parameters
Temperature/°C
Q0/g1
Kada
R2
Temperature/°C
Kf/g1
n
r2
25 40 55 70
0.147 0.194 0.233 0.235
1.274 1.472 1.526 1.678
0.993 0.962 0.999 0.994
25 40 55 70
11.349 7.429 8.039 6.873
0.496 0.592 0.511 0.538
0.985 0.960 0.991 0.980
a
Units: (absorbance units)1.
temperature. Though the antagonistic effects exist at the same time in the test, the adsorption process prevails. Consequently, the result of combining Q0, and Kad is enhanced with temperature. However, since the adsorption operation procedures are carried out after the ultrafiltration, whose temperature is constrained by the ultrafiltration, the temperature can not be over 55 °C as a whole. Kad > 1 can prove that this process is a better adsorption process. Testing data are also in agreement with Freundlich model equation: Ln Q ¼ Ln K f þ n Ln C
ð20Þ
Based on Ln C to portray Ln Q, Freundkich isotherms can be obtained, as shown in Fig. 4. The straight line interception Kf, slope n and coefficient of determination r2 by the fitting equation are shown in Table 2. The above results can clearly show that data of apple juice adsorption de-coloration by LSA-800B macroporous resin are well in agreement with Langmuir model and Freundlich model. 4.4. Thermodynamic parameters Table 3 gives thermodynamic parameters such as Gibbs free energy DG (kJ/mol), enthalpy change DH (kJ/mol) and entropy change DS (kJ/mol K) in the tests of apple juice adsorption de-coloration by using the resin. DG can be calculated by the following equation: DG ¼ RT Ln K ad
ð21Þ
LnQ (1/g)
where Kad – adsorption equilibrium constant (derived from Langmuir model); T – absolute temperature, K; R – gas constant 8.3144 103, kJ/(mol K).
- 1. 5
70˚C
-2
55˚C 40˚C 25˚C
- 2. 5
-3 - 1. 5
-1
- 0. 5
LnC Fig. 4. Freundlich isotherms.
0
The relations among Kad and thermodynamic parameters DH as well as DS can be described using Van’t Hoff formula. Ln K ad ¼ DS=R DH =RT
ð22Þ
In this way, DH and DS can be obtained from the slope and interception of Van’t Hoff straight line shown in Fig. 5 respectively. DH is the positive values, whereby proving that the process of resin adsorbing pigments in apple juice is the endothermic reaction. In addition, it can be known from calculated value DH = 4.1557 kJ/mol that the above process is physical adsorption rather than chemical adsorption. Accordingly, it can be considered that this adsorption process is in agreement with the CAC (Codex Alimentarius Commission) provisions or specifications concerning the process of fruit juice production in which only the physical methods can be adopted(http://www.codexalimentarius.net/web/index_en.jsp). DG is the negative values with the general trend appearing to be the negative growth, whereby proving that the above process is the spontaneous process; and DS is the positive values thus, showing that this process is the entropy increasing process. 4.5. Adsorption dynamic curves The 16 groups of adsorption dynamic curves are obtained through the experiments in the case of setting temperatures (15, 40, 55 and 70 °C) and resin concentrations (1, 2, 4, and 8 g/L). In considering that apple juice after being ultrafiltration is adsorbed, the temperature of ultrafiltration apple juice is not over 55 °C in general. If the adsorption temperature still needs to be higher, it is necessary to add heating equipment. Since the length of this paper is restricted, what Fig. 6 only gives is the curve with the coverage rate (h) changing with time (t) when adsorbent de-coloration is carried out at the temperature of 55 °C and the resin concentration of 4 g/L. Based on the statistic software (ORIGN 6.1), geometric approximation treatment is made of data (h and t) so that the results are found to be in agreement with the growth model h = a(b ect), and it is here that a, b, and c are the constants. The derivative in the case of the curve at t = 0 can be the initial adsorption rate (K0). The initial equilibrium time (tic) can be calculated in terms of Eq. (9). The equilibrium coverage rate (he) can be calculated in terms of the intercept of approximation straight line in the late adsorption. Based on the curve approximation method as well as Eqs. (18) and (19), the different resin concentrations and
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N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
Table 3 Thermodynamic parameters for adsorption process of adsorbent resin de-coloration Kad
DG/kJ mol1
25 40 55 70
1.472 1.472 1.526 1.678
0.959 0.631 1.413 1.206
Ln Kad
Temperature/°C
DH/kJ mol1
DS/kJ mol1 K1
r2
4.156
0.016
0.939
0. 6
Table 4 Dynamic parameters for resin adsorption de-coloration (resin concentration is 4 g/L)
0. 5
Temperature/ °C
he
k0/ min1
tie/ min
ka/ min1
kd/ min1
r2
25 40 55 70
0.480 0.389 0.499 0.405
0.0008 0.0007 0.0010 0.0010
592.59 525.68 499.00 408.08
0.0023 0.0024 0.0033 0.0035
0.0018 0.0023 0.0020 0.0029
0.975 0.989 0.991 0.996
0. 4
0. 3
0. 2 0. 0028
0. 003
0. 0032
0. 0034
0. 0036
1/T (1/K) Fig. 5. Van’t Hoff straight line.
Fig. 6. Resin adsorption dynamic curves (55 °C, 4 g/L).
the values of K0, tiehe, and Kd under the different temperature can be calculated respectively; and the coefficient of determination r2 can be determined by the fitting equation. Table 4 only gives the dynamic parameters values in the case of resin concentrations being 4 g/L at the different adsorption temperatures, for the rest of other dynamic parameters in the case of resin concentrations are no longer described. It can be seen from the integration of Fig. 6 and Table 4 that the resin adsorption rate decreases with the prolonging
of time, while in the practical production, the constant flow rate through the adsorption columns is mostly adopted to carry out the apple juice de-coloration adsorption, being unfavorable for improving the resin adsorption de-coloration efficiency. Accordingly, it is felt essential to consider that the gradual changing flow rate from the high to the low can be adopted in one adsorption–regeneration cycle. It can be seen from Ka varying trend that the resin adsorption ability keeps a high level within a wide temperature range of 25–70 °C, rendering the adoption of gradual changing flow rate to carry out de-coloration more feasible. As described above, the adsorption ability can be improved as the temperature goes up, but the initial equilibrium time (tie) is shortened. This means that resin regeneration cycle becomes shorter, which is not expected in fruit juice production. It is again felt necessary to reconsider that apple juice through the ultrafiltration can be adsorbed so that the temperature of apple juice is not over 55 °C in general. For this reason, 55 °C is the suitable adsorption temperature, which makes it possible for the apple juice from ultrafiltration to have direct de-coloration without being reheated. 5. Conclusions Research findings indicate: Apple juice adsorption decoloration by resin is a heat adsorption process, showing the physical adsorption; the adsorption and desorption rate constants for apple juice adsorption de-coloration by resin are derived via the geometric approximation calculations respectively; the adsorption ability is raised as the temperature goes up or rises; the adsorption initial equilibrium time for different resin concentrations at 25 °C is 9.37 h on average, but with an increase in testing temperature, the initial equilibrium time is reduced to a certain extent; Within the range of resin concentration of 2–4 g/L, adsorption effect is apparently significant, but being over this range, resin adsorption efficiency per unit mass will have
N. Qiu et al. / Journal of Food Engineering 81 (2007) 243–249
some drops; At 70 °C and 55 °C, the coverage rate (h) for resin reaching the initial equilibrium is higher, however, the temperature at 70 °C can just go against apple juice processing technology. For this reason, in terms of requirements by the rational technology in apple juice production, and the real temperature for apple juice via ultrafiltration, the suitable temperature and resin concentration for using adsorption resin to carry out apple juice adsorption de-coloration should be 55 °C and 2–4 g/L, respectively. Acknowledgement This research is supported by the National High Technology Research and Development Program of China (863 Program) under Grant No. 2002AA245091. References Brigida, F. S., Javier, P. I., Teresa, H., Carmen, C. C., & Isabel, E. (1992). Importance of phenolic compounds for the characterization of fruit juices. Journal of Agriculture and Food Chemistry, 40(9), 1531–1535.
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