Adsorption of dark compounds with bentonites in apple juice

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• 作者：Hü ; lya Koyuncu ; Ali R&#x131 ; za Kul ; Ayla Ç ; al&#x131 ; ml&#x131 ; Nuray Y&#x131 ; ld&#x131 ; z and Hasan Ceylan
• 关键词：Adsorption equilibrium ; Apple ; Bentonite ; Discolouration
• 刊名：Lebensmittel-Wissenschaft und -Technologie
• 出版年：2007
• 期刊代码：121_00236438
• 类别：abs
• 出版时间：April 2007
• 卷：40
• 期：3
• 页码：489-497
• 文件大小：242 K

摘要

The adsorption equilibrium of dark-coloured compounds from apple juice has been investigated as a function of several variables including activation conditions of bentonite (heat and acid treatments), clay concentrations (2×10⁻³–8×10⁻³ kg clay/dm³ apple juice) and temperature (range of 296–336 K). The adsorption efficiency with acid-activated bentonite was greater than that with heat-activated and native bentonite, depending on surface properties; specific surface areas were 95.31, 71.95 and 71.76 m²/g, respectively.

Absorbance data at 420 nm were fitted reasonably well with the Langmuir and Freundlich isotherms. The parameters K, Q₀, K_fr and n were determined for different temperatures. Thermodynamic parameters such as Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) change of adsorption were determined as about −3.125, 9.43 and 0.039 kJ mol⁻¹ K⁻¹, respectively, for acid-activated bentonite. These parameters were also determined for native and heat-activated bentonites. It was shown that the adsorption process was endothermic, spontaneous and controlled by physical mechanism. The adsorption and desorption rate constants (k_a and k_d) were obtained separately by applying a geometric approach to the first-order Langmuir model. k_a varied from 5.717×10⁻⁴ to 20.667×10⁻³ s⁻¹ and k_d from 1.425×10⁻⁴ to 7.473×10⁻³ s⁻¹. The results showed that acid-activated bentonite is more suitable for the adsorption of dark compounds from apple juice.