Thermodynamics of moisture sorption in melon seed and cassava
Introduction
Melon seed (colocynthis citrullus L.) is a common Nigerian oil seed, which contains approximately 50% by weight of oil, 28.4% protein (60% of defatted flour), 2.7% fiber, 3.6% ash and 8.2% carbohydrate (Oyenuga & Fetuga, 1975). The seed is also a good source of essential amino acids.
Cassava is an important root crop, which is consumed as food in the tropics in various forms and used as animal feed. The tuber flesh is composed of about 62% water, 35% carbohydrate, 1–2% protein, 0.3% fat, 1–2% fiber and 1% minerals (Onwueme, 1978). Drying is one of the operations employed in the post harvest processing of melon seed and during the processing of cassava into gari, cassava flour or cassava chips. The design of effective drying and storage systems for both products requires knowledge of their energy requirements and the state and mode of moisture sorption within them.
Thermodynamics has been reported as one of the three approaches used to understand the properties of water and calculate the energy requirements of heat and mass transfer in biological systems (Fasina, Ajibola, & Tyler, 1999; Fasina, Sokhansanj, & Tyler, 1997; Rizvi & Benado, 1984). Others used a structural approach, which describes the mechanism of hydrogen bonding and molecular positioning obtained using spectroscopic techniques, or a dynamic approach, which involves the analysis of motion of water molecules and their contribution to the hydrodynamic properties of the system. The use of structural and dynamic approaches are limited in biological systems application due to limited information on the theory of the behavior of water associated with solid biological materials (Rizvi & Benado, 1984). Thermodynamic properties of food relate the concentration of water in food to its partial pressure, which is crucial in the analysis of heat and mass transport phenomena during dehydration. They determine the end point to which food must be dehydrated in order to achieve a stable product with optimal moisture content and yield a figure for the theoretical minimum amount of energy required to remove a given amount of water from food. The properties also provide insight into the microstructure associated with the food–water interface (Rizvi, 1986). Thermodynamic functions are readily calculated from sorption isotherms and this enables the thermodynamic approach to allow the interpretation of experimental results in accordance with the statement of the theory (Iglesias, Chirife, & Viollaz, 1976). The functions include free energy, differential heat of sorption, integral enthalpy and integral entropy. Enthalpy represents the total energy available to do work, while entropy at any temperature provides lost work and gives a measure of the energy that is not available to perform work. Thus, the energy that is available to do work is the difference between these two quantities. This is known as Gibbs free energy. This free energy, also known as heat of vaporization, is therefore, the total energy required to transfer water molecules from the vapor state to the solid surface and vice versa. It is a measure of work done by the system to accomplish an adsorption or desorption process, and can be used as an indicator of the state of adsorbed water by solid particles (Fasina et al., 1999). The state of adsorbed water is a measure of the physical, chemical and microbial stability of biological materials under storage (Labuza, 1968). Heat of vaporization is also useful in evaluating drying models, determining energy consumption during the drying and wetting of food products, designing the drying equipment and describing the heat and mass transfer related processes (Bloom & Shove, 1971; Sokhansanj, Lang, & Lischynski, 1991; Yang & Cenkowski, 1993). In a multi-component system such as food, some considerations have to be given to the composition, and Gibbs free energy will depend not only on temperature and pressure, but also on the amount of each component present in the system (Rizvi, 1986).
Fish (1958) studied the thermodynamics of water in potato starch gel and noted that starch at very high moisture content is thermodynamically similar to pure water. Iglesias et al. (1976) studied the thermodynamics of water vapor sorption by sugar beet root and advanced hypotheses, which were mainly concerned with configurational modifications of the adsorbents during the course of sorption, to explain the values and trends observed. Viollaz and Rovedo (1999) determined the equilibrium moisture sorption isotherms of starch and gluten and proposed a new method of calculating the isosteric heat (differential heat of sorption) at different temperatures. Fasina and Sokhansanj (1993), Fasina et al. (1997) and Fasina et al. (1999) calculated the thermodynamic functions of moisture sorption in alfalfa pellets, winged bean seed and gari, respectively from their moisture sorption isotherms.
In two previous papers (Ajibola, 1989; Ajibola & Adams, 1986), the modified Halsey equation was found to fit the experimental data on sorption isotherms of melon seed and cassava.
This paper uses the fitted equation to estimate the thermodynamic parameters related to the sorption of moisture by melon seed and cassava.
Section snippets
Heat of vaporization (L)
The heat of vaporization of moisture in agricultural product can be estimated by applying the Clausius–Clapeyron equation to the sorption isotherms. The Clausius–Clapeyron equation (Kapsalis, 1987; Rizvi, 1986) after some mathematical manipulations (Aviara & Ajibola, 2000) yieldswhere C is a constant.
The saturation vapor pressure (Ps) at different temperatures can be obtained from Rogers and Mayhew (1981) or calculated using the psychrometric relationships in ASAE (1996), and
Heat of vaporization
The plot of Ln(P0) versus Ln(Ps) for melon seed and cassava at various moisture contents are presented in Fig. 1(a) and (b). The effect of moisture content on L/hfg for melon seed, cassava and gari is shown in Fig. 2. Data on the L/hfg of gari, which is a dried form of gelatinized cassava mash, was obtained from Fasina et al. (1999). From Fig. 2, it can be seen that heat of vaporization decreased with increase in moisture content. This confirms the fact that at higher moisture levels, the
Conclusions
This study determined the thermodynamic functions associated with moisture sorption by melon seed and cassava. The heat of vaporization of melon seed was lower than that of cassava indicating that cassava has a higher affinity for moisture than melon seed. Cassava has higher heat of vaporization than gari indicating that gelatinization of starch lowers the binding energy of the moisture sorbed by the product. The monolayer moisture content of melon seed at any temperature is lower than that of
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