articulo 1 ingles

Food Bioprocess Technol (2012) 5:2132-2139

DOI 10.1007M1947-011-0649-9

ORIGINAL PAPER

Discoloration Kinetics of Clarified Apple Juice Treated with Lewatit® S 4528 Adsorbent Resin During Processing

Ibarz Albert • Falguera Víctor • Garza Salvador • Garvín

Alfonso

Received: 6 September 2010/Accepted: 29 June 2011 /Published online: 13 July 2011 © Springer Science+Business Media, LLC 2011

Abstract The present work studies the adsorption of colored compounds in apple juice with a Lewatit® resin S 4528. The sorption equilibrium through the adsorption isotherms for 20, 35, and 50 °C was studied. The absorbance at 420 nm was used to measure the concentration of colored compounds, which permits correlating the residual concentration with the adsorbed concentration, proving that the data matched reasonably well according to the Langmuir and Freundlich models. Also, the efficiency of the adsorption process was studied for different resin/ juice mass ratios at different temperatures, from which it was observed that there was an improvement in efficiency as the resin content increased, while the increase in temperature was not so important in the process. The adsorption kinetics at 35 °C for different resin/juice mass ratios was also studied. The kinetic model developed by Ibarz was used, concluding that the data matched this model reasonably well. The adsorption kinetic constant was always higher than the desorption kinetic constant, which indicates that the adsorption stage predominates the desorption stage. The adsorption kinetic constant shows a decreasing tendency with the raise in the resin/juice ratio, and the desorption kinetic constant shows an increasing tendency. The variation of the

CIELab color parameters (L*, a*, and b*) with the adsorption

time was studied, obtaining results that match the kinetic adsorption model proposed.

Keywords Adsorption . Kinetics model . Discoloration . Resin.

Apple juice. Browning

Notation

A Absorbance (dimensionless)

A0 Initial absorbance (dimensionless)

C Resin concentration in the juice (grams resin/gramjuice)

Z Adsorption efficiency (%)m Adsorbate concentration on resin surface (grams

melanoidins/gram resin) S Adsorbate concentration in the juice

(grams melanoidins/gram juice) m0 Maximum concentration of adsorbate retained in the

juice (grams melanoidins/gram resin) Kads Equilibrium adsorption constant (Langmuir model,

grams juice/gram resin) KF Equilibrium adsorption constant (Freundlich model,

grams juice/gram resin) n Constant of adsorption equilibrium (Freundlich

model, dimensionless) t Adsorption time (hours)ka Kinetic constant of the adsorption stage (grams

juice/gram resin second) kd Kinetic constant of the desorption stage (per second) K Equilibrium constant of dynamic adsorption-

desorption process (grams juice/gram resin) mm Adsorbate concentration on resin surface in equilibrium

conditions (grams melanoidins/gram resin) L* CIELab color parameter (dimensionless) a* CIELab color parameter (dimensionless)b* CIELab color parameter (dimensionless) L0 Initial value of CIELab L* parameter

(dimensionless) L Value of CIELab L* parameter when equilibrium is

reached (dimensionless) a Parameter associated to the discoloration speed

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I. Albert • F. Víctor • G. Salvador • G. Alfonso (*) Food Technology Department, Universitat de Lleida, Av. Rovira Roure 191, 25198 Lleida, Spain e-mail: [email protected]

I. Alberte-mail: [email protected]

F. Víctore-mail: [email protected]

G.Salvadore-mail: [email protected]

mailto:[email protected]





(per hour)

Introduction

Two of the most important stability problems during the processing of fruit juices are browning and turbid appearance. In the case of apple juices, the main reason for these changes is the oxidation of polyphenolic substances (Constenla and Lozano 1995;Fernández de Simón et al. 1992;Falguera et al. 2011).

Browning is one of the most important processes of deterioration that occurs in fruit juices during processing and storage (Toribio and Lozano 1984), and consists of the appearance of colored reaction products. There are two kinds of browning: enzymatic browning and non-enzymatic browning (Shahidi and Naczk 1995). Enzymatic browning is caused by the action of polyphenol oxidase over phenolic substrates, resulting in melanins. Non-enzymatic browning is caused by the reaction between the reducing sugars and the free amino acids that result in melanoidins. Both melanins and melanoidins are colored products that confer the juice a dark color.

Different methods are currently used to reduce browning and the possible turbidity. Turbidity treatment usually takes place prior to the pasteurization process, and its utility is not to avoid browning but to allow the reduction of colored compounds (melanins and/or melanoidins) once they have been undesirably produced during storage and/or thermal treatment. One of the methods used to remove the color of juices is flocculation and later sedimentation through the use of gelatine and bentonite (Gokmen et al. 2001). This method has the disadvantage of obstructing the pores of the membranes used in the clarification stage. A method also used in the clarification of apple juices by ultrafiltration is adsorption through polyvinylpolypyrrolidone (PVPP; Gokmen et al. 2001;Humset al. 1980), but PVPP has the inconvenience of being expensive and hard to recover. Another method that can be used is the photochemical destruction of these compounds (Ibarz et al. 2005), although it is expensive and the products of the reaction are not known and could be toxic. The adsorption method with cheaper and easier-to-recover adsorbents is an alternative to remove the colored compounds causing the turbidity that appears in juices (Carabasa et al. 1998; Fisher and Hofsommer 1992; Qiu et al. 2007). The adsorbent that meets these requirements is activated carbon, and it is widely used with good results. If its particle size is very small, it makes it hard to remove and able to contaminate the juice. If the size is bigger, the price is much higher. There are other adsorbent materials such as zeolites (natural and synthetic), clays, silica gel, exchange resins, activated alumina, and some synthetic polymers.

Lewatit® S 4528 (Lanxess Energizing Chemistry, Edition 2009-11-19; www.lewatit.com) is a food grade resin and weak basic macroporous anion exchange resin (tertiary and very low level of quaternary amino groups) based on polystyrene. According to the manufacturer's specifications, it is suitable in the free base form for the decolorization of organic products (colorants) such as fruit concentrates. The manufacturer proposed this research because this resin has not been studied before as an adsorption resin for decoloring.

Although the adsorption of dark-colored compounds in apple juice has been studied by several authors (i.e., Gokmen and

Serpen 2002; Ataf and Gokmen 2011), the aim of this research was to study the elimination of color compounds (melanoidins) when they have been undesirably produced during the browning reaction of apple juice with a new adsorbent Lewatit® S 4528. For that purpose, isotherms of the adsorption equilibrium at different temperatures were obtained and adjusted to the Langmuir and Freundlich models. The adsorption efficacy according to the resin concentration, the adsorption kinetics, and the evolution of color parameters (L*, a*, and b*) with time for different resin/juice ratios were also evaluated.

Material and Methods

The sample of concentrated apple juice used in this study was provided by an industry near Lleida (Spain) from the most prevalent apple varieties (Starking Delicious, Golden Delicious, and Granny Smith) processed in the Lleida region. This sample was obtained from apples after the processes of selection, cleaning, massing, pressing, depecti-nization, clarification, and concentration up to 65°Bx through evaporation in a Unipektin® industrial evaporator (Unipektin AG, Eschenz, Czech Republic). The soluble solid content was determined using a digital refractometer Atago RX-1000 (Atago Co. Ltd., Tokyo, Japan).

The adsorbent resin used was the Lewatit® S 4528 (Bayer Chemicals AG, Leverkusen, Germany). Its physical data are: beige opaque ball-shaped grains with a specific surface (BET) of 25 m2/g, bead size between 0.4 and 1.25 mm, pore volume of 0.35 cm3/g, pore diameter between 10 and 70 nm, bulk density of 0.62 g/mL, and grain density of 1.02 g/mL. The resin was dried by storing in a vacuum oven at 60 °C until constant weight prior to use.

First of all, the samples of the concentrated juice were thermally treated at 70 °C in a laboratory stove for 24 h. This thermal treatment produced non-enzymatic (Maillard)

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browning reactions (melanoidins) in the juice that, after being treated, was stored in a fridge at 4 °C for further usage. This thermal treatment also produced the conversion of polyphenols to colorless forms (Brouillard 1982; Beveridge et al. 1986). In this way, melanoidin concentration was directly proportional to the absorbance at 420 nm, as was prior studied by Martins and Van Boekel (2003). The melanoidin content was obtained as the absorbance variation, as Falguera et al. (2010) did with melanins.

Absorbance measures were taken in a Helios gamma spectrophotometer (Thermo Fisher Scientific Inc., Waltham,USA) at 420 nm.

Equilibrium Model

Adsorption consists of the separation of a substance from a phase through its accumulation or concentration on the surface of the adsorbing phase. This transference process takes place until the conditions of dynamic equilibrium are reached, being then when the solution concentration remains constant, although the adsorption and desorption with equal rates still take place. The adsorption isotherms describe the relations between the equilibrium concentrations of the adsorbate in the adsorbing phase (m) and in the fluid phase (S) at a fixed temperature. The Langmuir and Freundlich models are the most widely used to describe the relation between both concentrations.

If a single adsorbate is only retained on a molecular surface, the Langmuir model can be described as follows (Langmuir 1918):

OTQ 1 + KadsS

where m0 is the maximum concentration of adsorption retained by

the adsorbent, Kads is the constant of adsorption equilibrium, and

S is the concentration of the adsorbate in the fluid phase. Both

parameters depend on the temperature and the adsorbate-

adsorbent system; thus, for a specific application at a fixed

temperature, both remain constant.

The Freundlich model is empirical and can be described with

the following power equation (Freundlich 1906):

m = kFSn

where kF and n are adsorption parameters that depend on the temperature and the adsorbate-adsorbent system.

To evaluate the adsorption efficiency of the colored compounds contained in a browned juice through resins, the following equation can be used:

AQ - AAQ

where A0 is the initial absorbance value and A is the final absorbance value corresponding to the equilibrium conditions.

The colored compounds' retained concentration in the resin can be obtained from the following equation:

AQ - A C

where A0 is the initial absorbance value, A is the concentration measure of colored compounds (melanins and melanoidins) in the

juice, and C is the resin concentration per juice unit, defined as grams resin/gram juice.

From the equilibrium constant, thermodynamic parameters such as Gibbs free energy (AG), enthalpy change (AH), and entropy change (AS) for the adsorption process can be obtained using the following equations (Gokmen and Serpen 2002):

AG =-RT ln Kads

(Van't Hoff's equation)

where Kads is the adsorption equilibrium constant, T is the absolute temperature, and R is the universal gas

constant.From the concentrated and thermally treated juice, 12°Bx

samples were obtained by dilution with distilled water.The absorbance measure prior to the addition of resin provides

information on the initial content of melanoidins in the juice.A preliminary study was carried out to determine the necessary

time to reach the dynamic equilibrium between the fluid phase (juice) and the adsorbent (resin). For that purpose, the absorbance was measured during 24 h for each of the resin-juice relation studied.

A known quantity of resin in the range of 0.5-3.5 g was added to 50 g of juice. In this way, several samples were obtained with a resin concentration in the juice between 0.01 and 0.07 g resin/gram juice. The recipients containing the juice and resin were placed for 24 h in a Bunsen BTG 1620 thermostatic shaking bath (Bunsen, Madrid, Spain) with a fixed working temperature, with the aim of reaching the equilibrium conditions at that temperature. Once the equilibrium conditions were reached, the samples were filtered using porous cellulose nitrate membranes (Whatman International Ltd., Maidstone, England) with a pore diameter of 0.45 |um. Finally, the absorbance at 420 nm was measured to obtain the content of melanins and melanoidins in the juice (S). This process was carried out at temperatures of 20, 35, and 50 °C. All the experiments were done in triplicate.

Kinetic Model

To carry out a kinetic study of the adsorption process, it is

necessary to define a mechanism that allows obtaining a kinetic

equation that properly describes the adsorption process. One of

these models is based on considering the

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(1)

(2)

x 100 (3)

m (4)

(5)

AHln Kads (6)ASR

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process as two-staged, one of adsorption and another of desorption, both of them being first-order kinetic stages (Langmuir 1918). This model has the disadvantages that it is necessary to know the equilibrium constant in order to calculate the equation parameters and the desorption constant cannot be measured independently. To work out this inconvenience, an approximate geometric model has been developed (Kuan et al. 2000; Qiu et al. 2007)to calculate the two constants.

Ibarz et al. (2008) developed an equation to describe the variation with time of the adsorbate contents (colored solute) in the resin:

For each resin/juice ratio, seven samples were prepared, one for each time: 0.5, 1, 2, 3, 4, 5, and 6 h. After the corresponding time of adsorption, these samples were filtered through a 0.45-|j.m membrane filter prior to the measurement of absorbance values at 420 nm and

measuringthe CIELabcolor parameters (L*b*). In the CIELab color space, L* stands for luminance, a* is the red-green axis, and b* corresponds to the blue-yellow axis. In this way, small values of a* and b* mean that the sample is less colored. The color parameters L*, a*,and b* were obtained with a Macbeth Color Eye 3000 spectrophotometer (Macbeth-Kollmorgen Int. Corp.,

a60 50

-

a so

uC 20

0,200 0,300

Absorbante at 4Z0 nm

0,16

O50!C Û3S*C 20K

dm ~d7(7) S 0,06

,-ti-

where m is the adsorbate concentration on resin surface, t is the time, ka is the kinetic constant of the adsorption stage, and kd is the kinetic constant of the desorption stage.

This equation can be integrated with the boundary condition that the initial concentration of colored compounds in the resin is zero, obtaining the following equation:

m = K(1 - e-kdt) ( 8 )

For high values of t, in which the equilibrium has been reached:

1,5

3,5 3

If 2,5

©0'

OS0«C A35SC

X205C

15 20

....a::—-**

m = K = '-f-; kfl = mikd (9)0.5

whichshows thatthe rates ofthe two stages

ofcolorcompound liberation and retention are in dynamic

equilibrium.

A fixed quantity of resin (0.5, 1.0, or 1.5 g) was placed

in an empty recipient. Subsequently, 50 g of browned juice

was added and the recipient was placed in a thermostatic bath

with a constant temperature of 35 °C.

-4 -3,5 -3 -2,5 -2 -1,5 -1 -0,5 0

Fig. 2 a Colored compounds adsorption isotherms on adsorbent resin at different temperatures. b Adsorption isotherms compared with the

linearized Langmuir model. c Adsorption isotherms compared with the linearized Freundlich model

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Fig. 1 Evolution of adsorption efficiency (Z) with resin/juice ratios at different temperatures

and

O50SC *

35?C X

20'C

0,5 OC0.CO30.100 0,400

0,12

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Table 1 Parameters for Langmuir and Freundlich isotherms Langmuir model Freundlich model

m m0K ads S

l+KBdBS m -k?Sn

T (°C)mo R2 KF

R2

20 35 50 23.75±2.32

37.26±1.09

81.98±8.82

7.95±1.84

8.70±0.61

3.79±0.48

0.9834

0.9988

0.9836

27.65±3.93

46.49±5.04

86.25±10.01

0.43±0.08

0.46±0.06

0.61±0.15

0.9593

0.9826

0.9470

Neuburgh, NY, USA) and the Macbeth Optiview

software, which uses the curve corresponding to the

light source D-75 and observer of 10°. All

absorbance measurements for the adsorption process

were corrected with those of the control sample for

each corresponding time interval in order to exclude

thermal effect on the measured absorbances.

Color Evolution

A consequence of melanoidin adsorption is the

discoloration of the juice. Thus, to know the

adsorption kinetics, the evolution of the L*, a*,and

b* CIELab parameters can be studied. The following

equation is used for L* (Carabasa and Ibarz 2000;

Ibarz et al. 1999):

¿0 + (Ll - L0)(1-e-at)

where L0is the L* parameter at time 0, Lx is the L* parameter once the equilibrium is reached, and a is a parameter associated with the discoloration rate.

Statistical Analysis

The experimental results obtained were fitted to the mathematical models by using the Statgraphics (versionPlus 5.1, STCSC Inc. Rockville, Md, USA) software fordata processing. All the fittings and the estimates

were calculated at a 95% significant level. All

experiments and analysis were carried out in

triplicate.

a 24-h time frame; thus, the 24-h period is adequate

for this experiment.

Adsorption Efficiency

Figure 1 shows the variation in the adsorption efficiency (z) for resin concentrations (C) at different temperatures. It can be observed that the efficiency of the adsorption increases with temperature between 20 and 35 °C. When temperature rises, the motion of the pigments in the juice increases. This behavior favors the different adsorption stages (external transference and diffusion inside the resin), making

the adsorption of the colored compounds on the surface and inside the resin favored. It

was also observed that the adsorption efficiency asymptotically increases with the resin concentration, which is evident since a rise in the resin concentration entails a rise in the adsorption surface. Minimal temperature influence on adsorption efficiency was observed with higher resin concentration in the juice; thus, at a low resin concentration, the adsorption efficiency was temperature-dependent. These results are consistent with the studies on vinegar presented by Achaerandio et al. (2002), on clarified peach juice by Carabasa et al. (1998), on peach juice by Ibarz et al. (2008), and on detergent by Leyva-Ramos (1989). Between 35 and 50 °C, the adsorption efficiency is

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L (10)


Results and Discussion

The preliminary experiment conducted has shown that a dynamic equilibrium between phases can be obtained

with

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Fig. 3 Resin adsorption dynamic curves, at 35 °C, for three different ratios resin/juice

ads AG (kJ/mol)T (°C)

Table 2 Thermodynamic parameters

20 7.95 -5.05235 8.70 -5.54050 3.79 -3.578AS (kJ/(mol-K)) -0.046

AH (kJ/mol) -19.02

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^ = ka - kdm $ m = K(1 - e-kdt)

Ratio (g resin/ g juice

k (a.u. g juice/ g resin h) kd (h-1) mi = K = kd/kd (g juice/g resin)

R2

0.01 7.81±1.44 0.45±0.13 17.35±1.81 0.98680.02 7.37±1.22 0.58±0.13 12.71±0.74 0.98940.03 7.15±0.38 0.69±0.05 10.36±0.20 0.9987

temperature-independent, as was found by

Fischer and Hofsommer (1992)and Kimetal.

(1992).

The fact that temperature has little effect on

adsorption efficiency, along with the

inconveniences related to raising the

temperature and the resin concentration

(favoring the non-enzymatic browning, warming

over-cost, and higher resin consumption over-

cost), suggests working with the lower

temperature and the lower resin concentration

that allow obtaining a juice according to a

colorimetric level prior defined. If a low color

level was desired, the process should use

temperatures between 35 and50°C.

Adsorption Isotherms

Figure 2 shows that for a fixed absorbance value, the m variable rises with temperature. The reason is that, as it is known, the temperature rise causes a viscosity reduction (Augusto et al. 2011) and, therefore, a diffusion acceleration of the colored compounds in the juice is produced, improving the global adsorption process.

Equilibrium data were adjusted according to the Langmuir and Freundlich models, which are the most widely used in the case of mono-compound systems in which the solute is kept in an only molecular surface. Table 1 shows the parameters of the adjustments of the experimental data to the Langmuir and Freundlich models, and Fig. 2 shows the fits. Figure 2 and the coefficients of determination of the adjustments of the data with the two adsorption isotherms reveal that the two models permit properly describing the behavior of the adsorption isotherms.

In Table 1, it can be observed that the adsorption equilibrium constant of the Langmuir model, Kads, decreases with the temperature increase. A value of Kads higher than the unit shows that the process is a favorable type adsorption (McCabe et al. 1993). It can also be observed that the maximum concentration of the adsorbed solute per adsorbent weight unit, m0, increases with temperature. With regard to the Freundlich model, the KF constant increases with temperature; the n exponent also increases with temperature, being <1 in all cases. The main difference between the Langmuir and

Freundlich models is that the Langmuir model is obtained as a theoretical process with simultaneous adsorption and desorption stages, both of them with first-order kinetics, while the Freundlich model is totally empirical.

Thermodynamic Parameters

Table 2 shows the thermodynamic parameters obtained for the adsorption process. The negative values of AG for all the temperatures tested indicate the spontaneous nature of the adsorption process, as was obtained by

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Table 3 Kinetic constants values in an adsorption process for different resin/juice ratios at 35 °C

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Ratio resin/juice (g resin/g juice) L» = L'a + (Li" Li) (1-e-at) Li

a R2

0.01 89.69±0.42 94.24±0.60 0.48±0.18 0.99010.02 89.37±0.66 94.68±0.43 0.92±0.32 0.98600.03 90.35±0.76 95.14±0.43 1.12±0.48 0.9778

Gokmen and Serpen (2002). The increment in the AG values with temperature indicates that the adsorption process is not favored by an increase in temperature. The fact that the value of AH is negative indicates the exothermic nature of the adsorption process. The slightly negative value of AS shows the slightly decreased randomness at the solid/solution interface during the adsorption process.

Adsorption Kinetics

Figure 3 shows the evolution of the concentration of the colored compounds that are retained by the resin at a temperature of 35 °C for different resin/juice ratios. It can be observed that the pigment concentration in the solid phase (m) rises with resin/juice contact time, reaching an asymptote that corresponds to the equilibrium achievement, as occurs in mass transfer when the driving force depends on the difference between the concentration and the equilibrium concentration. After 6 h, the variation of the parameters was meaningless. The experimental data fitted Eq. 8. Table 3 shows the values obtained in the adjustments. In all cases, it can be observed that the value of the kinetic constant of the adsorption stage (ka) is higher than the kinetic constant of the desorption stage (kd). This fact allows the adsorption of the colored compounds in the solid structure of the resin.

Figure 4 shows the evolution of the L* parameter. It can be observed that this evolution follows asymptotic kinetics with adsorption time, which is consistent with the evolution of the m concentration in Fig. 3, in accordance

with the fact that the adsorption kinetics consists of two stages (adsorption and desorption).

Figure 5 shows the correlation between L* and m, showing that the greater the adsorption, the lighter the juice becomes.

Table 4 shows the adjustment parameters of the L* parameter with Eq. 10. The initial parameter, L0, is almost the same in all cases; the equilibrium value, Lm, is also very similar. Thus, the total discoloration does not depend on the resin/juice relation in the ratio of the studied concentrations. The value of the a* parameter increases with the resin/juice mass ratio. This can be caused by the fact that when raising the resin/juice ratio, the interphase surface also rises, and therefore the material transference rate behaves in the same way.

Figure 6 shows the evolution of the a* and b* parameters, proving a loss of color because of the decrease of both parameters. Initially, both values decrease, but from a certain time (lower as the resin concentration gets higher), the a* parameter almost does not change and only the b* parameter continues its decrease. A higher resin/ juice ratio entails a higher reduction of both parameters, especially for b*. The global process is a reduction of the reddish colors.

Conclusion

The adsorption efficacy does not clearly rise when temperature increases and rises when the amount of resin used increases. Considering the technical and economic disadvantages associated with these optimal conditions, the most reasonable option is to work with the

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Fig. 5 Correlation between lightness and adsorption (m)Fig. 4 Evolution of lightness with time of adsorptionTable 4 Adjustment parameters of L* evolution for different resin/juice ratios at 35 °C

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lowest parameters for both of them that allow obtaining the desired characteristics of juice coloring.

The adsorption process is spontaneous, exothermic, and not favored by an increase in temperature.

The values of the kinetic constant of the adsorption stage are higher than the ones of the

desorption stage. Thus, the concentrations of adsorbed colored compounds in the steady state are high.

The evolution of the adsorbed concentration values (m)and the L* parameter values follows an increasing exponential function, in accordance with the observed adsorption kinetics. The greater the adsorption, the lighter the juice becomes.

The evolution of the a* and b* parameters shows a color reduction, which continued for the b* parameter but limited for the a* parameter, which is globally appreciated as a reduction of the reddish colors.

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