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Desorption isotherms for local variety of tomatoes (lycopersicon lycopersicum).


Tomato fruit is a perishable produce and versatile vegetable crop with high moisture content. Tomato plants are tolerant of wide range of climatic conditions, the crop invariably performs best under irrigation during the dry season when insect pests and disease incidence are at a minimum (Pearson, 1987). In recent times, a great many tomato cultivars have been selected or bred to suit different uses. In Nigeria, two varieties are known; the Roma and the local types. In northern states of Nigeria, Roma tomatoes have been selected for both the dry and wet seasons for different ecological zones. Local varieties (tiwantiwa) are commonly grown in western states of Nigeria during rainy season. It is a seasonal crop being plentiful at some times of the year and scarce at other times. The fruits are processed into soups, juices, sauces, purees and pastes in canning industry. The residual press cake is used as stock feed and as fertilizer. Tomato fruits are useful food which provide vitamins that protect the body from diseases and it is an important antioxidant which lower risk of heart diseases (Goerge et al., 2001). Because of lack of adequate storage techniques and facilities for tomatoes, most of the fruits ripen at about the same time leading to high post-harvest losses. Keeping tomatoes for excessive periods can cause loss of nutrient due to enzymatic actions. A fundamental characteristic of food materials which influences every aspect of the drying process and the storage stability of the dried product is its water desorption characteristics (Ajibola, 1986). The drying and desorption of tomatoes are useful in determining the behaviour of the product when it is presented in environments of different temperature and water activities. The knowledge of water desorption isotherm is also very important for engineering purposes related to concentration and packaging. For this reason, there have been many schorlarly contributions which deal with the experimental means to obtain sorption isotherms. A recent review of these models is given by Akanbi et al.(2005). The objective of this study was to identify the best model that will accurately predict the desorption equilibrium moisture content of tomatoes at different temperatures and relative humidities


2.1 Sample Preparation

Fresh, mature and ripe fruits used in this study were purchased from a local market in Ile Ife, Nigeria. The fruits were of the Yoruba (tiwantiwa) variety. The whole fresh fruits were sorted, washed and drained.

2.2 Equilibrium Moisture Content Determination

A static gravimetric method was used to determine equilibrium moisture content of the fruits. Whole fresh fruits were placed in desiccators. Saturated salts (lithium chloride, calcium chloride, magnesium chloride, magnesium nitrate, sodium chloride and potassium chloride) were used to maintain relative humidity ranging from 11 to 82 % in the desiccators as stated by Ajibola (1989). To ensure that salt solutions remained saturated throughout the course of the experiment, excess salt was added to the solutions. The desiccators were placed into temperature controlled ovens at 40, 50, 60, 70 and 80[degrees]C. The samples were weighed daily using Mettler PC 200g balance with an accuracy of 0.01. Equilibrium condition was considered to have been reached when three consecutive measurements gave the same readings. The dry matter of the samples was obtained by drying in an oven at about 70[degrees]C for 24 hours to constant weight without caking (AOAC, 1990). Each experiment was replicated. Non-linear least squares regression procedure was used to fit five desorption models (Table 1) to the experimental data.


Average values of equilibrium moisture content of whole tomato fruits obtained at different temperatures of different saturated salt solutions are presented in Table 2. The results obtained showed that the equilibrium moisture content increased with increase in relative humidity and decrease with increasing temperature. The values are expressed in percentage dry basis.

The data obtained from the experiments were used to generate desorption curves. The desorption isotherms of tomato fruits at five temperatures (40, 50, 60, 70 and 80[degrees]C) are shown Figure 1. The desorption isotherms of tomato fruits are in the form of sigmoid shape. This shape was previously observed by Ajibola (1986) and Kechaou and Maleeji (2000).Temperature is acknowledged to be a dominant factor with respect to moisture sorption phenomenon. This is demonstrated in Figure 1, where equilibrium moisture content at specific water activity reduced with an increase in temperature.


The estimated constant parameters from the non linear regression analysis of the models used are presented in Table 3. The percentage standard errors of estimate were highest for Oswin (13.60), followed by Henderson (9.08) and Hasley (9.05) models, while GAB and BET models had the lowest values of 2.52 and 3.53 respectively. In comparison with other models, the results obtained indicated that BET and GAB models had the highest coefficient of determination of 0.97 and 0.94 respectively. The results were in agreement with Akanbi et al. (2006) which stated that the regression coefficient ([r.sup.2] values) of desorption models for tomato slices were within the range of 0.66 and 0.95. Hence the BET model with the highest [r.sup.2] value was the best for the experimental data obtained for whole tomato fruits.


This study has shown that temperature has significant effects of whole tomato fruits. The desorption isotherm of tomato fruits gave the sigmoid shaped characteristics curves, which is typical of many sorption isotherms of products. The BET and GAB models was found to be the best fit out of the five models evaluated since both models had the highest coefficient of determination (regression value) and lowest standard error. It can be concluded that GAB and BET models are best for predicting the desorption isotherms of whole tomato fruits at 40, 50, 60, 70 and 80[degrees]C and within the relative humidity range studied.


[1] Ajibola, O.O. (1986). Equilibrium Moisture properties of winged Bean Seed. Journal of Agric. Engineering. Vol.29.

[2] Akanbi C.T., Adeyemi R.S., and Ojo. A. (2006). Drying characteristics and sorption of tomato slices. Journal of Food Engineering. Vol. 73 , pp 157-163.

[3] AOAC, Official Methods of Analysis. 15th edition. Association of Official Analytical Chemists, Arlington, VA. USA.

[4] George, J., Nuttal, S.L., and Kendal, M.J. (2001).Prostate Cancer and antioxidants. Journal of Pharmacy and Therapeutics, vol. 26, pg 231-233

[5] Pearson, L.C (1987) Principles of Agronomy, Reinhold Corporation, New York, page 434.

[6] Vazquez, g., Chenlo, F., and Moreira, R., (2001). Modeling of desorption isotherms of chestnut. Influence of temperatures and evaluation of isoteric heats. Drying technology, vol. 19, no. 6, pg 1189-1199

T.B. Onifade (1), F.B. Akande (1), D.O. Idowu (1) and I.A. Bello (2)

(1) Department of Agricultural Engineering and (2) Department of Applied Chemistry, Ladoke Akintola University of Technology, Ogbomoso, Nigeria.

Table 1: Equilibrium moisture content and equilibrium
relative humidity models

Authors Model (Equations)

Henderson 1 - RH = EXP(-aT[M.sup.b])
Oswin RH = a[(M/1-M).sup.b]
Hasley RH = [e.sup.(-a+bT)][M.sup.c]
GAB RH = {(c-1)kM/1+ckM + kM/1-Mk}
BET RH = (cM)(k[M.sup.k]+k[M.sup.k+1])/

a, b, c, k = parameters of the models

M = moisture content, % dry basis

RH = relative humidity, %

T = temperature, [degrees]C

Table 2: Equilibrium moisture content obtained from tomato
fruits at different temperatures and relative humidity (RH).

Saturated Salt (R.H Temperature [degrees]C
Solutions values)
 % 40 50 60 70 80

Lithium Chloride 11.20 19.00 18.50 17.00 11.50 10.50
Calcium Chloride 21.0 37.20 33.50 30.50 23.00 19.50
Magnesium Chloride 30.8 59.79 53.50 41.99 39.79 31.92
Magnesium Nitrate 47.5 65.00 63.83 54.39 53.55 45.73
Sodium Chloride 60.0 72.00 70.88 68.48 58.82 55.73
Sodium Nitrate 74.7 76.00 74.48 73.74 64.00 60.90
Potassium Chloride 84.5 87.25 83.31 81.64 79.51 72.15

Table 3: Estimated parameter values of the models fitted
to tomato desorption isotherm.

Models a b c

GAB - - 1.37
Henderson lx[lO.sup.5] 1.79 -
Oswin 69.30 2.8x[lO.sup.1] 3.38x[lO.sup.1]
Hasley 5 lx[lO.sup.6] 1.43
BET - - 2.36

Models % [r.sup.2]

GAB 2.52 0.94
Henderson 9.08 0.89
Oswin 13.60 0.90
Hasley 9.05 0.68
BET 3.53 0.97
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Author:Onifade, T.B.; Akande, F.B.; Idowu, D.O.; Bello, I.A.
Publication:International Journal of Biotechnology & Biochemistry
Date:Jul 1, 2012
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