Adsorption of two coffee aromas from synthetic aqueous solution onto granular activated carbon derived from coconut husks

Abstract

Soluble coffee production requires several processes that cause the loss and degradation of aromas, which reduces the quality of the final product. However, such aromas can be recovered into aqueous solution during the production process. Aromas must be separated from water, and pleasant compounds, such as benzaldehyde, must be separated from unpleasant compounds, such as acetic acid. In this study, the kinetic and thermodynamic parameters of benzaldehyde and acetic acid adsorption from synthetic single-solute aqueous systems onto commercial grade granular activated carbon derived from coconut husks were investigated. The contact time required to attain adsorption equilibrium of benzaldehyde was 6 h, while that of acetic acid was 10 h. Adsorption isotherm data revealed that the adsorption of benzaldehyde and acetic acid was exothermic and spontaneous in nature. Moreover, the isosteric heat of adsorption indicated that the adsorbent surface was energetically heterogeneous. The adsorption equilibrium was fitted according to Langmuir, Freundlich and Temkin isotherms.

Introduction

Aroma is one of the most important attributes of coffee. During the roasting process, several aroma compounds are formed by the pyrolysis of water-soluble components, such as sugars, amino acids and trigonelline. Volatile components of coffee are numerous and varied in their quality, potency and concentration; thus, each compound contributes to the overall aroma (Dart and Nursten, 1989). Some volatile components such as benzaldehyde have a pleasant odour, while others, such as acetic acid, have an unpleasant smell. Benzaldehyde (C₆H₅CHO) is an aromatic aldehyde naturally found in almonds and roasted coffee, and possesses the bitter odour of almond oil. Alternatively, the irritating odour of vinegar is attributed to acetic acid (C₂H₄O₂) (Flament, 2002).

Soluble coffee processes including extraction, evaporation and drying result in the loss and degradation of aromas, reducing the quality of the final product. During these processes, aromas can be collected into aqueous waste streams and separated from wastewater. To allow aromas to be reincorporated into the final product (Varnam and Sutherland, 1994, Smith, 1989), pleasant-smelling compounds such as benzaldehyde must be separated from unpleasant compounds such as acetic acid. The recovery and/or separation of volatile compounds are usually achieved by expensive thermal treatments, which can damage the final product (Diban et al., 2007). Consequently, there is considerable interest in the development of alternative separation processes. Adsorption is a promising operation that has been used for the recovery and/or separation of organic compounds such as benzaldehyde and pyridine in waste water (Rajoriya et al., 2007, Lataye et al., 2008), phenylethyl alcohol, ethanol and acetic acid in fermentation broth (Fabre et al., 1996, Bowen and Vane, 2006), methyl mercaptan, ethyl acetate and furfural in coffee aroma (Sakano et al., 1996, Sakano et al., 1999, Lucas et al., 2004) and the main pear aroma from aqueous fruit juice (Diban et al., 2007).

The recent increase in coconut production, processing and consumption in Brazil has generated a large quantity of coconut husks residues. The disposal of these materials is expensive and may lead to environmental problems. An alternative application for this raw material is the conversion of coconut husks into activated carbon (Gratuito et al., 2008), which can be used as an adsorbent in many applications (Sekar et al., 2004, Hameed et al., 2008).

The application of granular activated carbon from coconut husks to the recovery and/or separation of coffee aromas in soluble coffee production is relatively rare. Currently, approximately 1000 different coffee aromas have been detected, and experts suggest that dozens of compounds have yet to be identified; thus, the development of alternative aroma separation and recovery processes is important.

Previous studies on the adsorption of benzaldehyde and acetic acid onto granular activated carbon derived from coconut husks have not yet been reported. The aim of this study was to evaluate the adsorption of volatile compounds in aqueous solution onto commercial grade granular activated carbon made from coconut husks. For this purpose, equilibrium batch experiments were performed in synthetic single-solute aqueous systems. The physical properties of the adsorbent were evaluated to better understand the adsorption process. Moreover, Langmuir, Freundlich and Temkin models were used to describe and predict single system adsorption behaviour. The equilibrium parameters obtained from monocomponent adsorption studies may be helpful to describe the behaviour of adsorption from mixtures, since the multicomponent isotherm models require the constants derived from single-solute adsorption isotherms for the data fitting (Choy et al., 2000).