Apple Juice Dispersions - Science Direct

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Tunay Dik, Sinan Katnas and Mustafa ¨Ozilgen*. T. Dik: Vocational School of Higher Education, Department of Food Technology, Middle East Technical.
Lebensm.-Wiss. u.-Technol., 29, 673–676 (1996)

Research Note

Effects of Bentonite Combinations and Gelatin on the Rheological Behaviour of Bentonite – Apple Juice Dispersions ¨ Tunay Dik, Sinan Katna¸s and Mustafa Ozilgen* T. Dik: Vocational School of Higher Education, Department of Food Technology, Middle East Technical University, 06531 Ankara (Turkey) S Katna¸s: Department of Chemical Engineering, Gazi University, Maltepe 06450 Ankara (Turkey) ¨ M Ozilgen: Food Engineering Department, Middle East Technical University, 06531 Ankara (Turkey) (Received January 10, 1995; accepted April 18, 1995)

Rheological behaviour of bentonite – apple juice suspensions were experimentally determined at 25 °C, within the shear rate range of 4.3 to 43.1 s–1 with 0.90, 1.33 and 2.66 g/L bentonite mixtures and 0 to 0.25 g/L (on dry basis) gelatin concentrations. The mixtures consisted of 100%, 80%, 60%, 40% 20% and 0% Ca-bentonite and balance Na-bentonite combinations. The experimental data were described with the power law as τ = Kγ.1.56. The consistency index was related to the calcium, sodium and gelatin concentrations as K = K0 + K1CαCa + K2CßNa + K3CεG. ©1996 Academic Press Limited

τ = Kγ˙ n

Introduction Bentonites are industrial clays of montmorillonite, A12O3 4SiO2 H2O, with minor components of Fe2O3, TiO2, MgO, CaO, Na2O and K2O. They have a layered crystalline structure made up of two outer silica sheets, and a central sheet of aluminum ions. Water and the exchangeable sodium and calcium cations are located within the sheets. Bentonites make colloidal dispersions in water. Sodium-bentonites exhibit much more swelling capacity than the calcium variety (1). Bentonites are used to clarify apple juice (2–5). The major clarification mechanism is adsorption on the surface, which removes proteins, heavy metal ions and pesticides (6). Adsorption of proteins and a number of other soluble cationic constituents by bentonites is due primarly to the cation exchange capacity of these clays (7). Clarification capability of bentonites increases with their swelling capability in water. Bentonites are generally used together with gelatin in clarification processes, but there are also examples in the literature where bentonites are used alone with no gelatin addition (3,7). Rheological behaviour of the bentonite–apple juice dispersions is one of the major factors affecting energy consumption for pumping and agitation. Rheological behaviour of bentonite–apple juice dispersions were ¨ studied by Dik and Ozilgen (8) with different temperature, pH and bentonite concentrations, and the power law model was suggested: *To whom correspondence should be addressed.

0023-6438/96/070673 + 04$25.00/0

Eqn [1]

Only sodium-bentonite, and no gelatin, was used in this study and a constant power law index, n = 1.56, was found (8). An empirical power or exponential type of expression may be used to relate the consistency index K with the solids contents of the foods (9): K = K0 cα

Eqn [2]

K = K0 exp(αc)

Eqn [3]

where K0 and α are constants and c is the solids content. In the present study effects of the different Cabentonite/Na-bentonite combinations and gelatin on the rheological behaviour of bentonite–apple juice dispersions will be discussed.

Materials and Methods A local variety of fresh green apples was purchased in the local market in the Summer of 1994. The juice was prepared with a typical home juicer (Ar¸celik, Robopress model ARK-71RP, Turkey). The raw juice was treated with 0.1% pectinase (Pectinex 3XL, Novo, Denmark) for 2 h at 50 °C. Total soluble solids content and pH of the enzyme-treated juice was 14.5 ± 0.1 °Brix and 4.0, respectively. Sodium and calcium bentonites (Karakaya Inc., Turkey) were hydrated in deionized water at room temperature (20 °C) over night. Gelatin (Difco, U.S.A.) solutions were prepared after wetting with deionized water at 50 °C for 15 min, then ©1996 Academic Press Limited

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dissolving at 60–70 °C. Hydrated bentonite was dispersed in the juice to obtain suspensions with 0.90, 1.33 and 2.66 g bentonite/L (on dry basis) for 100%, 80%, 60%, 40%, 20% and 0% Ca-bentonite and balance Nabentonite combinations. Hydrated gelatin was added to these preparations to obtain 0, 0.5, 0.1, 0.15, 0.2, 0.25 g/L (on dry basis) gelatin concentrations. The shear stress of the apple juice–sodium bentonite dispersions was measured with a viscometer (Brookfield, synchrolectic, model RVT) with the spindle number 1 after removing the guard legs at 10, 20, 50 and 100 rpm at 25 °C. A typical constant temperature bath was used to adjust the temperature and there were no temperature fluctuations in the experiments. The revolution rates correspond to 0.275 to 2.75 m/s of spindle tip speed and covers the average linear velocity range of the dispersion during pumping. Shear stress values were calculated from the dial readings (10). Experiments were done in a 600 mL sample holder. The rheological measurements were free of any stationary surface effects. There was no settling during the experiments. The average experimental error in replicate runs was 5%.

Results and Discussion Experimental data were analysed with the same proce¨ dure as described by Dik and Ozilgen (8). Rheological behaviour of the bentonite–apple juice suspensions agreed with the power law model described in Eqn [1]. Although a different variety of apples were used, the power law index (n = 1.56) was the same as in our previous study (8) and the dilatant fluid behaviour of the bentonite–apple juice suspensions was reconfirmed. An improved form of Eqn [2] is suggested to express the consistency index K as a function of the additive concentrations: K = K0 + K1CαCa + K2CβNa + K3CεG

Eqn [4]

where CCa, and CG are concentrations of Ca-bentonite, Na-Bentonite and gelatin, respectively. Parameters K0, K1, K2, K3, α, β, and ε are constants. Nonlinear regression analysis was carried out with the Marquardt–Levenberg method to find the constants of the model (Table 1). The Marquardt–Levenberg method seeks the values of the parameters to minimize the sum

of the squared difference between the observed and the predicted values of the dependent variable. The norm of the regression was calculated as: Norm =



m

∑ [wi (Ki,exp – Ki,model]

where m = number of the data points, wi = weight factor, Ki,exp and Ki,model are the experimentally determined and the model consistency index values, respectively. The norm was 6.86 3 10–19 and implied an almost perfect correlation between Eqn [4] and the data. Comparison of Eqn [4] with the experimentally evaluated values of the consistency index are shown with arbitrarily chosen examples in Fig. 1. Sample values of the consistency index with arbitrarily chosen Ca-Bentonite, Na-Bentonite and gelatin concentrations are depicted in Table 2. Detailed results of the regression analysis are presented in Table 1, which shows that α and β are the most important independent parameters. The next two important parameters are K1 and K2. The regression analysis may imply complex interaction between K0, K3 and ε, or dependence of these parameters on the remaining parameters K1, α, K2 and β. Equation [4] is an empirical model, and valid within the range of the experiments only. It implies that each additive, i.e. Na-bentonite, Ca-bentonite and gelatin, have additive effects on the consistency index. Coefficients (K1, K2 and K3) and the powers (α, β and ε) of each term show the effect of individual additive concentrations on the overall consistency index. Although the coefficients of Ca-bentonite and Nabentonite concentrations, i.e. α and β, are almost the same; that of gelatin, i.e. ε, is one order of magnitude greater than the others, implying that gelatin is the major compound contributing to the consistency index when the bentonite concentrations are about 1 g/L and the gelatin concentration is about 0.5 g/L. Parameters α and ε are close, but parameter β was almost twice as large (Table 1). This observation may be attributed to the swelling ability of Na-bentonite, which contributes to higher values of the consistency index at high Nabentonite concentrations.

Conclusions In our previous study (8) a constant power law index, n,

Table 1 Detailed results of the regression analysis Parameter K0 (N K1 (N K2 (N K3 (N α β ε

Value

s1.56/m2) s1.56/m2) s1.56/m2) s1.56/m2)

aDependence

(g/L)–0.779 (g/L)–1.319 (g/L)–0.666

Standard error

1.08889 × 10–3 0.0340 × 10–3 0.0367 × 10–3 0.4891 × 10–3 0.7791 1.3191 0.6066

=1–

Eqn [5]

i=1

3.03 × 2.28 × 2.09 × 1.01 × 5.39 × 5.39 × 1.35 ×

10–20 10–20 10–20 10–19 10–19 10–19 10–19

variance of the parameter with other parameters constant variance of the parameter with other parameters changing

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Dependencea 0.95 0.91 0.93 0.95 0.69 0.87 0.96

lwt/vol. 29 (1996) No. 7

Table 2 Values of the consistency index K under various experimental conditions K × 103 N s1.56/m2

Concentration g/L

2.0 1.7

CNa

CCa

CG = 0.0 g/L

CG = 0.10 g/L

CG = 0.25 g/L

2.129 2.660 1.064 1.596 0.0 0.532 1.064 1.330 0.532 0.798 0.0 0.266 0.0 0.900 0.360 0.180 0.720 0.540

0.532 0.0 1.596 1.064 2.660 2.128 0.266 0.0 0.798 0.532 1.330 1.064 0.900 0.0 0.540 0.270 0.180 0.360

1.140 1.180 1.180 1.150 1.180 1.250 1.030 1.150 1.170 1.060 1.170 1.100 1.223 1.151 1.171 1.105 1.072 1.163

1.400 1.330 1.280 1.190 1.280 1.270 1.360 1.330 1.280 1.210 1.300 1.150 1.268 1.227 1.163 1.199 1.242 1.314

1.600 — 1.430 1.310 1.420 1.340 1.450 1.320 1.380 1.280 1.280 1.370 1.479 1.264 1.240 1.250 1.252 1.309

was found over a wide range of temperature, pH and Na-bentonite concentrations. In the present study the same constant power law index was found with a different variety of apples, bentonite combinations and gelatin addition. We have also suggested an expression for the consistency index as a function of the bentonite and gelatin concentrations.

(a)

1.4 1.1 0.8 0.5 0.0 2.0 1.7

0.1

0.2

0.3

Acknowledgements

(b)

We appreciate helpful discussions with Mr Ali Karakaya. He also supplied the bentonites.

K × 103 (N s1.56/m2)

1.4 1.1 0.8 0.5 0.0 2.0 1.7

0.1

0.2

0.3

References (c)

1.4 1.1 0.8 0.5 0.0 2.0 1.7

0.1

0.2

0.3

(d)

1.4 1.1 0.8 0.5 0.0

0.1 0.2 Gelatin concentration (g/L)

0.3

Fig. 1 Sample plots for the comparison of Eqn [4] with the experimentally evaluated values of the consistency index. Sodium-bentonite, calcium-bentonite and gelatin concentrations were: (a), CCa = 0.0 g/L, CNa = 2.66 g/L; (b), CCa = 0.18 g/L, CNa = 0.72 g/L; (c), CCa = 0.72 g/L, CNa = 0.18 g/L; (d), CCa = 2.66 g/L, CNa = 0 g/L

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