CIGR Ejournal Style and Format Guidelines ARID ZONE JOURNAL OF ENGINEERING, TECHNOLOGY & ENVIRONMENT AZOJETE - CIGR Section VI Special Issue: Innovation & Technologies for Sustainable Agricultural Production & Food Sufficiency AZOJETE, December, 2018. Vol. 14(SP.i4): 62-73 Published by the Faculty of Engineering, University of Maidiguri, Maidiguri, Nigeria. Print ISSN: 1596-2490, Electronic ISSN: 2545-5818 www.azojete.com.ng *Corresponding Author’s Email: funkeeze@gmail.com 62 ORIGINAL RESEARCH ARTICLE MOISTURE SORPTION CHARACTERISTICS OF DEHYDRATED IN-SHELL AFRICAN WALNUT (TETRACARPIDIUM CONOPHORUM) O. O. Ezekiel*1, T. O. Olurin2 and A. O. Akomolafe1 1Department of Food Technology, University of Ibadan, Ibadan, Nigeria; 2Department of Food Science and Nutrition, College of Natural and Applied Science, Bells University of Technology, Ota, Nigeria ARTICLE INFORMATION Received: October, 2018 Accepted: December, 2018 Keywords: African walnut Drying Sorption isotherm Sorption models Equilibrium moisture content Nonlinear regression ABSTRACT Moisture sorption isotherms are useful thermodynamic tools for determining water interactions within food systems and providing information that can assist in optimizing food processing operations such as drying, mixing, packaging and storage, as well as to maximize retention of quality parameters such as colour, aroma, texture, and nutrient. Moisture sorption isotherm characteristics of African walnut were evaluated at three different temperatures (28, 33 and 38°C) and relative humidity range of 11.20 - 97.00 % using gravimetric method; five mathematical models (GAB, BET, Peleg, Smith and Ferro Fontan) were fitted into the experimental data. Sorption isotherms of the dehydrated walnut gave type II (S-shaped) isotherms according to BET classification. Temperature had significant effect on the equilibrium moisture content (EMC). A nonlinear regression analysis method was used to evaluate the constants of sorption models. The models were evaluated statistically by calculating coefficient of determination (R2), the mean relative percentage error (P) and the reduced chi-square (λ2). The BET model gave the best fit for the obtained data among the tested models with R2 value of 0.9892. Calculated monolayer moisture (Mo) content from BET ranged from 5.018 to 7.922% db for adsorption and 9.842 to 10.143% db for desorption respectively. © 2018 Faculty of Engineering, University of Maiduguri, Nigeria. All rights reserved. 1.0 Introduction African walnuts (Tretracarpidium conophorum) can be described as rounded, stone fruits of the walnut tree with single seed. The fruit of the walnut is enclosed in a hull which is green, leathery, and fleshy. After harvest, this hull is usually removed to reveal the wrinkly walnut shell that encloses the kernel, and the shell is then removed to obtain the kernel (Atungulu et al., 2013). Primarily, walnut plant is cultivated in Nigeria for the nuts, which is traditionally eaten after boiling (Akpuaka and Nwankwor, 2000; Ndie et al., 2010). Biochemically, walnut is composed of polyunsaturated fatty acids, especially linoleic and oleic acid, it is also high in protein content (Savage et al., 2001), which makes it of significant economic value and medicinal importance for human health; due to these facts, there has been an increasing interest in its consumption and utilisation. Some of the other beneficial components it contains include plant protein (for example, arginine, leucine), carbohydrates (for example, dietary fibre), http://www.azojete.com.ng mailto:funkeeze@gmail.com Arid Zone Journal of Engineering, Technology and Environment, December, 2018; Vol. 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng 63 vitamins (for example, vitamin A and E), pectic substances, minerals (magnesium, potassium, phosphorus, sulphur, copper and iron), plant sterols and phytochemicals (Savage et al., 2001; Colaric et al., 2006; Ogunmoyole et al., 2011). Although African walnuts have high socio-economic potentials, the product is still under-utilised in Nigeria, especially at industrial level. This can mainly be attributed to the fact that there is lack of storage facilities for the walnut market which has been hampering the production on a full scale and exploration of its inherent potentials (Ekwe and Ihemeje, 2013). Thus, in order to overcome the challenge of spoilage and inconsistent seasonal availability, there is need for development and provision of appropriate storage facilities for the African walnut (Babalola, 2011) as well as appropriate preservation methods. Deterioration of food occurs during handling or due to mechanical, physical, chemical or microbial damage (Rahman, 1999; Mujumdar, 2004); microbial growth depends on the storage conditions and the moisture level in the product. Among the commonly employed methods for preserving food such as freezing, vacuum packing, canning, preserving in syrup, food irradiation, addition of preservatives, the most popular is dehydration (Jangam et al., 2010). 1.1 Research Justification Dehydration which removes water from food materials will result in its stability and as well reduce storage and transportation cost. To a great extent, most dried foods quality depends upon their physical, chemical and microbiological stability, which is mainly a consequence of the relationship between the equilibrium moisture content (EMC) of the food material, and its correspondence water activity (aw) or relative humidity at a given temperature (Guilan et al., 2007). Moisture sorption isotherms (MSI) is the terminology usually used to express the relationship between equilibrium moisture content and equilibrium relative humidity (ERH) or water activity of materials being studied. It is worthy of note to know that this relationship is complex and unique for each product due to different interactions which may be colligative, capillary or surface effects between the water and the solid components of the product at different moisture contents. According to Al-Mahasneh et al., 2011, MSI are useful thermodynamic tools for determining water interactions within food systems and providing information that can assist in optimizing food processing operations such as drying, mixing, packaging and storage, as well as to maximize retention of quality parameters such as colour, aroma, texture, and nutrient. Several authors (Johnson and Brennan, 2000; Chowdhury et al., 2005; Akanbi et al., 2006; Samapundo et al., 2007; Oyelade et al., 2008) have reported some of the importance of knowing the sorption characteristics of products as being essential in respect to storage stability and food product acceptability, drying process modelling, design and optimization of drying equipment, aeration, calculation of moisture changes which may occur during storage and for selecting appropriate packaging materials for dehydrated products. Isosteric heat of sorption, often referred to as differential heat of sorption, is used as an indicator of the state of water adsorbed by the solid particles (Fasina et al., 1997; Togrul and Arslan, 2007). Water sorption properties of foods can be predicted using water sorption isotherm equations. Guilan et al., 2007 noted that many empirical and semi-empirical equations describing the sorption characteristics of foods have been proposed in the literature; these equations are suitable for some food products only, or for selected ranges of aw, and part of these equations clearly show the effect ../../../user/Downloads/azojete143/www.azojete.com.ng Ezekiel et al: Moisture sorption characteristics of dehydrated in-shell African walnut (Tetracarpidium conophorum), AZOJETE, 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng 64 of temperature on sorption isotherms. According to Togrul and Arslan (2007), among the models which were reviewed, 23 common equations for fitting sorption isotherms to different food materials were reported, some of the models took into consideration the effect of temperature; these include modified Chung-Pfost (Chung and Pfost, 1967), modified Henderson (Henderson, 1952), modified Halsey (Halsey, 1948) and modified Oswin (Oswin, 1946) models. Brunauer– Emmett–Teller (BET) and Guggenheim–Anderson–de Boer (GAB) equations were reported to be the most popular food isotherm equations (Cadden, 1988; Togrul and Arslan, 2007). While the sorption characteristics of other exotic walnut of the Juglandaceae family (Juglans regia, microcarpa, hindsii cultivars) have been studied extensively, there is dearth of information on the sorption characteristics of African walnut (Tetracarpidium conophorum) specie available in Nigeria and other neighbouring African countries. The study of sorption isotherm properties of African walnut will provide information that can be used directly to know the storability, solve process design problems (e.g. dryer design and operations), predict energy requirements and determine its proper storage conditions (Ekwe and Ihemeje, 2013). 1.3 Objectives This study therefore experimentally determined the sorption characteristics of dehydrated in-shell African walnut kernels at three different temperatures of 28, 33, and 38 °C, evaluated the suitability of five moisture isotherm model equations and investigated the nature of the moisture sorption hysteresis. 2.0 Materials and Methods 2.1 Materials Raw unshelled walnuts were obtained from Ogbese market, Ondo State, Nigeria. All other reagents used were of analytical grade. 2.1.1 Sample Treatment The bulk quantity of walnuts was cleaned and divided into two portions. One portion of the walnuts used for adsorption isotherm was dried in an air-draught oven at 120 °C for 10 hours to obtain a moisture content of about of 4 % (dry basis). The other portion used for desorption process was first dampened by placing in known amount of water so as to allow them pick up moisture for 4 days This enabled the moisture content to be raised to a stable and uniform level. 2.2 Methods 2.2.1. Experimental sorption isotherm Water sorption isotherms (adsorption and desorption) were determined by gravimetric method which involved exposure of samples to different salts (LiCl, CH3COOK, MgCl2.6H2O, K2CO3, Mg (NO3)2.6H2O, NaBr, SrCl2.6H2O, NaCl, (NH4)2SO4, BaCl2.2H2O) at aw of 0.112 to 0.970 as shown in Table 1. The salts were employed as saturated solutions to give constant water activity environment. The samples were placed on perforated lid in different desiccators containing the salts. After which the desiccators were placed in a temperature-controlled chamber at different temperatures of 28, 33 and 38°C covering the main range of possible storage condition (to investigate the effect of different temperatures). http://www.azojete.com.ng Arid Zone Journal of Engineering, Technology and Environment, December, 2018; Vol. 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng 65 Samples were weighed consistently daily until a constant weight was achieved. Moisture content was determined by AOAC vacuum oven method (AOAC, 1990) at 60°C. An analytical balance (model) with a sensitivity of 0.1mg was used to measure water uptake. This procedure was performed in triplicate. Table 1: Amount of water and salt used to prepare the different saturated salts and expected water activity No. Name Salt (g) Distilled water (g) aw (30oC) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. LiCl CH3COOK MgCl2.6H2O K2CO3 Mg(NO3)2.6H2O NaBr SrCl2.6H2O NaCl (NH4)2SO4 BaCl2.2H2O K2SO4 112 126 300 300 225 300 300 300 300 375 20 63 79 100 135 34 120 75 90 120 105 100 0.112 0.226 0.327 0.431 0.528 0.577 0.708 0.752 0.800 0.903 0.970 2.2.2 Determination of moisture content Moisture content was determined by gravimetric method (AOAC, 2000). Five gram (5g) of the sample was pre-weighed (W1) in a petri dish and placed in an oven at 105 °C for 3 hour. The sample was removed from the oven, cooled in a desiccator, and re-weighed. It was taken back to oven for another 1 hour, cooled and weighed, this was repeated until a constant weight was achieved and recorded as (W2). All analyses were carried out in triplicates. Moisture percentage was calculated as shown in Equation 1: (1) 2.2.3 Data Analysis: Mathematical Modelling of Moisture Sorption Isotherm Five mathematical equations namely; GAB, BET, Peleg, Smith and Ferro Fontan were used for describing desorption and adsorption isotherms of in-shell walnut in the range of temperature 28, 33 and 380C. The expressions and the parameters of the five models used to fit the data are presented in Table 2. The goodness of fit of the data to the equations was evaluated by the criteria of correlation coefficient (R2), the mean relative percentage error (P) and the reduced chi-square (X2) presented as in equations 2, 3 and 4 respectively. The higher the R2 value and the lower the X2 and P values, the better is the goodness of the fit.                              n i iipre n i iprei n i n i iipreiprei MMMM MMMM R 1 2 exp,, 2 1 ,exp, 1 1 exp,,,exp, 2 (2) ../../../user/Downloads/azojete143/www.azojete.com.ng Ezekiel et al: Moisture sorption characteristics of dehydrated in-shell African walnut (Tetracarpidium conophorum), AZOJETE, 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng 66     N i i iprei M MM N P 1 exp, ,exp,100 (3) (4) where, Mexp,I = ith experimentally observed equilibrium moisture content, Mpre,I = ith predicted equilibrium moisture content, N = number of observations and n = number of constants (Kaya and Kahyaoglu, 2007). Where A, B, C and D are parameters of the equations, T is temperature (oC), Xe is equilibrium moisture content (kg/kg d.b.), and aw is the water activity (Kaya and Kahyaoglu, 2007). Table 2: Mathematical models used to describe moisture adsorption and desorption isotherm behaviours of the dehydrated African walnut Model name Equilibrium Moisture Content Model equation Smith  we aBAX  1ln GAB Ferro-Fontan                        r w e a X 1 ln11   Peleg BET 3.0 Results and Discussions 3.1 Moisture Sorption Isotherms of Dehydrated Walnut: Effect of Temperature on Sorption Isotherms The isotherms shown in figures 1 and 2 were obtained by plotting the corresponding mean EMC of three replicates against aw at each temperature for adsorption and desorption processes respectively. The effect of temperature on the moisture sorption isotherm reveals that there was an initial increase in the equilibrium moisture content as temperature increased to 33°C and then the equilibrium moisture content decreased as temperature further increased to 38°C at constant aw (0.226 – 0.800), but there was decrease in the equilibrium moisture content with increasing temperature, at a constant aw (0.903 – 0.970); the decrease in EMC with increasing temperature signifies that the product became less hygroscopic with increasing temperature which means it can absorb more moisture at lower temperatures than at higher temperatures at constant relative humidity environment (Ariahu et al., 2006). According to Ronald et al., (2005), this can be explained by the change in the excess enthalpy of water binding, dissociation of water, or increase in solubility of solute in water as temperature increases. Also, this trend may be due to a reduction in the total number of active sites for water binding as a result of physical and/or chemical changes in the product induced by temperature (Mazza and LeMaguer, 1980; Moreira et al., 2008; Saad et al., 2014). The effect of temperature on the sorption isotherm is of great importance (Falade and Awoyele, 2005), this is due to the fact that foods are exposed to a range of temperatures during storage and processing, and also aw changes with temperature. Temperature affects the mobility of the water molecules and the dynamic equilibrium between the vapour and adsorbed phases (Al-Muhtaseb et al., 2004). http://www.azojete.com.ng Arid Zone Journal of Engineering, Technology and Environment, December, 2018; Vol. 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng 67 Figure 1: Effect of temperature on the adsorption isotherm of dehydrated African walnut Figure 2: Effect of temperature on the desorption isotherm of dehydrated African walnut The monolayer moisture content (Mo) for each temperature, calculated using the BET and GAB models, is presented in Table 3. In general, the Mo value calculated using the BET model was lower than that calculated using the GAB equation this is in agreement with the reports of other researchers (Erbas et al., 2005; McMinn et al., 2007; Lee and Lee, 2008). For adsorption, Mo decreased as temperature increased while it decreased for desorption isotherm. The behaviour of Mo decreasing with increase in temperature been ascribed to a reduction in the number of active sites due to physical and chemical changes induced by temperature (Moreira et al., 2008). The Mo is the minimum moisture content covering hydrophilic sites on the material surface, and it is a necessary data for achieving storage with minimum quality loss for long time. Furthermore, at this condition, the rates of spoilage reactions, except for oxidation of unsaturated fats, are minimal. Therefore, at a given temperature, the safest aw level is that corresponding to Mo or lower (Moreira et al., 2008) Table 3: Estimated parameters and fitting criteria of the models applied to experimental sorption data of dehydrated African walnut Model names Constants Adsorption Desorption 28°C 33°C 38°C 28°C 33°C 38°C Smith A 9.3806 9.4023 9.6824 15.1562 14.5851 11.8149 B -8.1272 -9.5799 -10.5799 -11.3994 -12.6404 -17.7076 R2 0.9992 0.8758 0.8758 0.7906 0.8929 0.9499 P 0.0000 0.0211 1.164E-10 1.707E-15 0.0000 8.574E-10 X2 1.69E-17 1.34E-02 4.00E-19 8.08E-29 0.00E+00 1.25E-17 BET C -6.02E+00 -8.93E+00 -9.02E+00 -1.13E+01 -1.12E+01 -1.62E+01 Mo 5.018 5.856 7.922 10.143 9.913 9.842 R2 0.9027 0.9392 0.9892 0.987 0.9771 0.9771 P -2.36E-08 0.0008222 0.0001921 0.0027622 1.63E-12 1.63E-12 X2 8.79E-14 1.90E-09 8E-05 3E-05 8.13E-14 8.13E-14 GAB C -3.79E-07 -2.38E-07 -2.38E-07 -8.35E-07 -1.12E-07 -1.12E-07 K 0.8219 0.73913 0.73913 0.7688 0.1622 0.1622 Mo 8.642 9.2241 14.8728 20.3271 17.9138 17.9138 R2 0.943 0.869 0.962 0.922 0.9731 0.9731 P -8.36E-10 3.59E-08 4.921E-08 1.67E-15 1.13E-15 1.13E-15 X2 1.53E-13 0.001 1E-05 8.01E-23 3.03E-23 3.03E-23 Ferro-Fontan Α 4.4426 5.3367 6.0169 11.0965 7.03742 4.1174 ../../../user/Downloads/azojete143/www.azojete.com.ng Ezekiel et al: Moisture sorption characteristics of dehydrated in-shell African walnut (Tetracarpidium conophorum), AZOJETE, 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng 68 Y 2.5528 2.7230 2.9101 4.8628 5.0022 7.9389 R 0.216736 0.2451 0.2699 6.05E-02 9.58E-02 9.87E-02 R2 0.8375 0.8892 0.8892 0.9856 0.9697 0.9812 P 3.36E-10 0.05979 1.192E-06 -0.003498 0.0098708 0.0330662 X2 1.17E-03 1.07E-01 2.81E-01 3.39E-04 1.83E-03 1.86E-02 Peleg K1 3.9892 6.9166 7.6596 8.1121 7.62 6.18 K2 10.84 11.89 13.901 12.3797 14.8789 16.1923 n1 1.8529 2.50449 2.82829 2.4099 2.2645 2.2917 n2 0.00079 0.00085 0.00091 0.00081 0.00099 0.0010 R2 0.9003 0.8540 0.8540 0.9895 0.9616 0.9715 P -8.36E-05 0.0214 0.0369261 0.0008715 0.0277175 0.0277175 X2 0.0000 0.0137 0.0000 0.0000 0.0144 0.0144 3.2 Modeling of Sorption Isotherms Table 3 showed the result of the nonlinear regression analysis of adsorption and desorption isotherms of in-shell walnut obtained at 28°C, 33°C, and 38°C. The values of constants of the five models, that is, GAB, BET, Smith, Ferro-Fontan, Peleg, fitted to the desorption and adsorption data along with their standard error (P), the correlation coefficient (R2), and the reduced chi-square (X2) for the studied temperatures are given. Examination of the results revealed that some of the models are acceptable for predicting the EMC. In considering the models with lowest standard error and the highest coefficient of correlation, the BET gave the best fitting for adsorption at 33°C and 38°C and Smith model was best fit at 28°C, while Peleg models gave the best fitting of desorption isotherms at 28°C, BET gave the best fitting at 33°C and Ferro Fontan gave the best fitting at 38°C. The R2 of the models that fit the isotherms ranged from 0.903 to 0.999. Lomauro et al., (1985) pointed out that when percentage average relative deviation is less than 5, the fit is considered to be excellent. It is well-known that the fit becomes better as the coefficient of determination ‘R2’ approaches 1. Also, the smaller the standard errors of models the better is the fit. Considering all these three criteria for the isotherms of the walnut at the three temperatures, the sorption data were in good agreement with GAB and other tested models. However, BET model seems to be excellent to represent the experimental sorption data. Comparison of the experimental and predicted isotherms for dehydrated walnut over the range of temperature and aw commonly encountered in the tropics in food storage structures are shown in figures 3-5 for the adsorption isotherms and figures 6-8 for the desorption isotherms. 3.3 Moisture Sorption Hysteresis of Dehydrated African Walnuts The experimental adsorption and desorption isotherms (change in EMC with aw) obtained at 28, 33, and 38°C for African walnut are shown in Figures 9, 10, and 11. The EMC at each aw represents the mean value of three replications. The temperature chosen is suggestive of average ambient temperature at which most food products are stored in tropics. The results indicated that the adsorption isotherms of the dehydrated walnut exhibit about type II curve. The adsorption isotherms at the temperatures lay below their desorption counterparts and both enclosed hysteresis loop. Hysteresis has been related to the nature and state of the components in a food, reflecting their potential for structural and conformational rearrangements (Yan et al. 2008). The sorption http://www.azojete.com.ng Arid Zone Journal of Engineering, Technology and Environment, December, 2018; Vol. 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng 69 isotherms have the sigmoidal-shaped profile according to the BET classification. These curves are typical of plant products as reported by Kouhila et al., (2002), Ait-Mohammed et al., (2004) and Idlimam et al., (2008). Hysteresis between adsorption and desorption existed over almost the entire range of water activity at 28°C. Similar behaviour of adsorption and desorption isotherms was observed for 33°C and 38°C. The hysteresis effect was more pronounced at lower temperatures in which water content on the desorption isotherm is higher than that on the adsorption side at the same water activity, this is one of the reasons for difference in moisture content between the two closures. Figure 3: Comparison of experimental and GAB predictive adsorption isotherms of dehydrated African walnut at 28°C Figure 4: Comparison of experimental and GAB predictive adsorption isotherms of dehydrated African walnut at 33°C Figure 5: Comparison of experimental and GAB predictive adsorption isotherms of dehydrated African walnut at 38°C Figure 6: Comparison of experimental and GAB predictive desorption isotherms of dehydrated African walnut at 28°C Figure 7: Comparison of experimental and GAB predictive desorption isotherms of dehydrated African walnut at 33°C Figure 8: Comparison of experimental and GAB predictive desorption isotherms of dehydrated African walnut at 38°C ../../../user/Downloads/azojete143/www.azojete.com.ng Ezekiel et al: Moisture sorption characteristics of dehydrated in-shell African walnut (Tetracarpidium conophorum), AZOJETE, 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng 70 Figure 9: Adsorption and desorption isotherm of dehydrated African walnut at 28oC Figure 10: Adsorption and desorption isotherm of dehydrated African walnut at 33oC Figure 11: Adsorption and desorption isotherm of dehydrated African walnut at 38oC points is that, during drying (desorption), some solutes may supersaturate below their crystallization water activity and thus hold more water as aw is lowered (Barbosa-Carnovas et al., 2007; Saad et al., 2014). . The swelling of polymeric materials during moisture adsorption can also lead to hysteresis (Raji and Adeniran, 2011). Iglesias and Chirife (1976) recognized that it is not possible to give a single explanation of the hysteresis phenomena in foods due to food being a complex biological material. The figures revealed that the equilibrium moisture content increased with water activity at constant temperature. This may be due to the fact that vapor pressure of water present in foods increases with that of surroundings (Shivhare et al., 2004). In the first segment (with low water activity) of the S-shaped sorption isotherm curves, walnut kernels sorbed relatively lower amounts of moisture. However, larger amount of moisture was absorbed at higher aw. Similar behaviour has been reported by other authors for different foods (Sanni et al., 1997; Lee and Lee, 2008). Two bending regions are noted, one around 0.1 to 0.3 and another at 0.5 to 0.6. The isotherm is therefore divided into three zones. According to Aguilera and Stanley (1999), in zone I (aw between 0.05 and 0.2) minimal water is contained in the product, and the water molecules present are tightly bound to active sites (e.g., polar groups in molecules) mainly by hydrogen bonding. In zone II (aw between 0.2 and 0.5) the water is more loosely bound, initially as multilayer above the monolayer; later, as moisture content increases, this water successively fills micro-pores and macro-pores in the system. In this region, chemical and biochemical reactions requiring solvent water start to take place because of the increased mobility of solutes. In zone III (aw between 0.6 and 0.9), excess water is http://www.azojete.com.ng Arid Zone Journal of Engineering, Technology and Environment, December, 2018; Vol. 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng 71 present in macro-capillaries, exhibiting nearly all the properties of bulk water. Microbial growth becomes a major deteriorative reaction in this region (Saad et al., 2014). According to Vanden Berg and Bruin (1981), a general sigmoid sorption isotherm can be divided into three different parts; ranges I (aw 0–0.22), II (aw 0.22–0.73) and III (aw 0.73–1.0). In ranges II and III, water molecules penetrate newly created pores of the already swollen structure and are mechanically entrapped in the void spaces. Therefore, water uptake particularly at higher aw would be markedly influenced by the stability of the micro-porous structure. 4.0 Conclusions Moisture sorption isotherms were determined for dehydrated African walnut at three different temperatures of 28, 33 and 38°C. There was a significant effect of temperature on the equilibrium moisture content (EMC) in the range of temperatures studied. The EMCs were found to decrease with increasing temperature at constant aw; they were also found to increase with increasing aw at constant temperature. The isotherms obtained are sigmoidal in shape and showed evident effect of hysteresis. In general, BET model provided a better fit to the experimental data than other tested models, thus it was found to be the most appropriate equation for representing the sorption isotherms. Mo increased with temperature for adsorption, while it decreased with increasing temperature for desorption. References Ait-Mohammed, L., Kouhila, M., Jamali, A., Lahsasni, S. and Mahrouz, M. 2004 “Experimental Study of Adsorption-Desorption Isotherms of Bitter Orange Leaves,” in Proceeding of the 14th International Drying Symposium, Sao Paulo, Brazil. pp. 1404–1410. Akanbi, CT., Adeyemi, RS. and Ojo, A., 2006. Drying Characteristics and Sorption Isotherm of Tomato Slices. Journal of Food Engineering 73, 157–163. Akpuaka, MU. and Nwankwor, E. 2000. Extraction, Analysis and Utilization of a Drying-Oil from Tetracarpidium Conophorum. Bioresources Technology 73: 195-196. Al-Mahasneh, MA., Bani-Amer, MM. and Rababah. 2011. Modeling Moisture Sorption Isotherms in Roasted Green Wheat using Least Square Regression and Neural-Fuzzy Techniques. Food Bioproducts Processing doi:10.1016/j.fbp.2011.02.007 Al-Muhtaseb, AH., McMinn, WAM. and Magee, TRA. 2004. Water Sorption Isotherms of Starch Powders. Part 1: Mathematical Description of Experimental Data. Journal of Food Engineering 61: 297–307. AOAC 1990. Official Methods of Analysis, 15th Edition. Association of Official Analytical Chemists, Arlington, VA. AOAC 2000. Official Methods of Analysis, 17th Edition. Association of Official Analytical Chemists, Washington. Ariahu, CC., Kaze, SA. and Achem, CD. 2006. Moisture sorption characteristics of tropical fresh water crayfish (Procambarus clarkia). Journal of Food Engineering 75: 355–363. Atungulu, HE., Wang, T., Fu R., Wang, X., Khir, R. and Pan, Z. 2013. Infrared Pre-Drying and Dry- Dehulling of Walnuts for Improved Processing Efficiency and Product Quality. American Society of Agricultural and Biological Engineers 29(6): 961-971 Babalola, FD. 2011. Marketing of African Walnut [Tetracarpidium conophorum Mull. (Arg)] in Southwest Nigeria: Production Issues and Contributions to Stakeholders. Journal of Agricultural Science and Technology 1: 523-531 ../../../user/Downloads/azojete143/www.azojete.com.ng Ezekiel et al: Moisture sorption characteristics of dehydrated in-shell African walnut (Tetracarpidium conophorum), AZOJETE, 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng 72 Barbosa-Canovas, GV., Fontana, AJ., Schmidt, SJ. and Labuza, TP. 2007. Water Activity in Foods, Blackwell Publishing Company, Ames, Iowa, USA. Cadden, AN. 1988. Moisture Sorption Characteristics of Several Food fibers. Journal of Food Science. 53: 1150–1155. Chowdhury, MMI., Huda MD., Hossain, MA. and Hassan, MS. 2005. Moisture Sorption Isotherms for Mungbean (Vigna radiata L). Journal of Food Engineering. 74, 462–467. Chung, DS. and Pfost, HB. 1967. Adsorption and Desorption of Water Vapor by Cereal Grains and their Products. Part II. Development of the general Isotherm equation. Transactions of the American Society of Agricultural Engineers 10: 552–555. Colaric, MF., Stampar, M., Hudina, A. 2006. Sensory Evaluation of Different Walnut Cultivars (Juglans regia L.). Acta Agric. Slovenica 87: 403-413. Ekwe, CC. and Ihemeje, A. 2013. Evaluation of Physicochemical Properties and Preservation of African Walnut (Tetracarpidium conophorum). Academic Research International 4 (6). Erbas, M., Ertugay, MF. and Certel, M. 2005. Moisture adsorption behavior of semolina and farina. Journal of Food Engineering 69: 191–198. Falade, KO. and Awoyele, OO. 2005. Adsorption isotherms and heat of sorption of fresh and preosmosed oven-dried bananas. Journal of Food, Agriculture and Environment 3(1): 97-102 Fasina, O., Sokhansanj, S. and Tyler, RT. 1997. Thermodynamics of Moisture Sorption in Alfalfa Pellets. Drying Technology. 15: 1553–1570. Guilan, P., Xiaoguang, C., Wenfu, W. and Xiujuan, J. 2007. Modeling of Water Sorption Isotherm for Corn Starch. Journal of Food Engineering. 80: 562–567 Halsey, G. 1948. Physical Adsorption on Non-Uniform Surfaces. Journal of Chemical Physics. 16: 931–937. Henderson, SM. 1952. A Basic Concept of Equilibrium Moisture. Agricultural Engineering 33: 29–32. Idlimam, A., Lamharrar, A. and Abdenouri, N. 2008. Thermodynamic properties and moisture sorption isotherms of Argania spinosa and Zygophyllum gaetulum. Journal of Agronomy. 7 (1): 1–14. Iglesias, HA. and Chirife, J. 1976. A Model for Describing the Water Sorption Behavior of Foods. Journal of Food Sciences. 41(5): 984-992. Jangam, SV., Law, CL. and Mujumdar, AS. 2010. Drying of Foods, Vegetables and Fruits. Volume 1, pp 1-30 Johnson, NT. and Brennan, JG. 2000. Moisture Sorption Isotherm Characteristics of Plantain (Musa, AAB). Journal of Food Engineering 44:79-84 Kaya, S. and Kahyaoglu, T. 2007. Moisture sorption and thermodynamic properties of safflower petals and tarragon. Journal of Food Engineering 78, 413-421. Kaya, S. and Kahyaoglu, T. 2007. Moisture sorption and thermodynamic properties of safflower petals and tarragon. Journal of Food Engineering 78, 413-421. Kouhila, M., Kechaou, N., Otmani, M., Fliyou, M., and Lahsasni, S. 2002. Experimental Study of Sorption Isotherms and Drying Kinetics of Moroccan Eucalyptus Globules. Drying Technology. 20 (10): 2027–2039. Lee, JH. and Lee, MJ. 2008. Effect of drying method on the moisture sorption isotherms for Inonotus obliquus mushroom. LWT 41: 1478–1484 Lomauro, CJ., Bakshi, AS. and Labuza, TP. 1985. Evaluation of Food Moisture Sorption Isotherms Equations. Part II: Milk, Coffee, Tea, Nuts, Oilseeds, Spices and Starchy Foods. Lebensmittel- Wissenschaft andTechnologies. 18: 118–124. http://www.azojete.com.ng Arid Zone Journal of Engineering, Technology and Environment, December, 2018; Vol. 14(sp.i4): 62-73. ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng 73 Mazza, G. and LeMaguer, M. 1980. Dehydration of onion: some theoretical and practical considerations. Journal of Food Technology 15: 181–194. McMinn, WAM., McKee, DJ. and Magee, TRA. 2007. Moisture adsorption behaviour of oatmeal biscuit and oat flakes. Journal of Food Engineering 79, 481–493. Moreira R., Chenlo, F., Torres, MD. and Vallejo, N. 2008. Thermodynamic analysis of experimental sorption isotherms of loquat and quince fruits. Journal of Food Engineering 88: 514–521 Mujumdar AS. 2004. Dehydration of Products of Biological Origin, Science Publishers, UK. Ndie, EC., Nnamani, CV. and Oselebe, HO. 2010. Some Physicochemical Characteristics of Defatted Flours Derived from African Walnut (Tetracarpidium conophorum): An Underutilized Legume. Pakistan Journal of Nutrition 9 (9): 909-911 Ogunmoyole, T., Kade IJ. and Korodele, B. 2011. In Vitro Antioxidant Properties of Aqueous and Ethanolic Extracts of Walnut (Juglans regia). Journal of Medicinal Plants Research. 5(31): 6839- 6848. Oswin, CR. 1946. The Kinetics of Package Life. III. The Isotherm. Journal of Chemical Industry 65: 419–421. Oyelade, OJ., Tunde-Akintunde, TY., Igbeka, JC., Oke, MO. and Raji, OY. 2008. Modelling Moisture Sorption Isotherms for Maize flour. Journal of Stored Products Research 44: 179–185 Rahman, MS. 1999. Handbook of Food Preservation, Marcel Dekker, Inc. New York Raji, AO. and Ojediran, JO. 2011. Moisture sorption isotherms of two varieties of millet. Food and Bioproducts Processing 89, 178–184 Ronald, EW., Eric, AD., Steven, JS. and Peter, S. 2005. Handbook of Food Analytical Chemistry, Water, Proteins, Enzymes, Lipids, and Carbohydrates, John Wiley & Sons, Hoboken, NJ, USA. Saad, A., Touati, B., Draoui, B., Tabti, B., Abdenebi, A. and Benaceur, S. 2014. Mathematical Modeling of Moisture Sorption Isotherms and Determination of Isosteric Heats of Sorption of Ziziphus Leaves. Hindawi Publishing Corporation, Modelling and Simulation in Engineering Volume 2014, Article ID 427842, 8 pages.http://dx.doi.org/10.1155/2014/427842 Samapundo, S., Devlieghere, F., Meulenaer, B., Atukwase, A., Lamboni, Y. and Debevere, JM. 2007. Sorption Isotherms and Isosteric Heats of Sorption of Whole Yellow Dent Corn. Journal of Food Engineering. 79: 168–175. Sanni, LO., Atere, A. and Kuye, A. 1997. Moisture sorption isotherms of fufu and tapioca at different temperatures. Journal of Food Engineering 34, 203–212. Savage, GP. 2001. Chemical Composition of Walnuts (Juglans regia L.) Grown in New Zealand. Plant Foods for Human Nutrition. 56(1): 75-82. Shivhare, US., Arora, S., Ahmed, J. and Raghavan, GSV. 2004. Moisture adsorption isotherms for mushroom. LWT-Food Science and Technology 37, 133–137. Togrul, H. and Arslan, N. 2007. Moisture Sorption Isotherms and Thermodynamic Properties of Walnut Kernels. Journal of Stored Products Research. 43: 252–264 Van den Berg, C. and Bruin, S. 1981. Water Activity and its Estimation in Food Systems. In: Water Activity, Influence on Food Quality (Eds L.B. Rockland and G.F. Stewart). Academic Press, New York, 1-61. Yan, Z., Sousa-Gallagher, MJ. and Oliveira, FAR. 2008. Sorption isotherms and moisture sorption hysteresis of intermediate moisture content banana. Journal of Food Engineering, 86: 342–348. ../../../user/Downloads/azojete143/www.azojete.com.ng ARTICLE INFORMATION 1.0Introduction 2.0Materials and Methods 3.0Results and Discussions