Desorption and adsorption equilibrium moisture isotherms of
Some useful phytochemicals that include flavonoids, lipids, terpenes, alkaloids, steroids, saponins, and carbohydrates have been isolated from the plant [
Moreover, aqueous extract of various parts of the plant have showed analgesic, sedative, antinociceptive, anti-inflammatory, and antimicrobial effects [
People from Algerian Sahara use different parts from
Since vegetables are highly perishable products, the quality is affected by postharvest handling, transportation, storage, and marketing. The improper handling, storage, and transportation may result in decay and production of microorganisms [
Water activity has long been considered as one of the most important quality factors especially for long-term storage;it affects the shelf-life, safety, texture, flavor, and smell of foods. Water activity is defined as the ratio of the partial pressure of water over the wet solid system to the equilibrium vapor pressure of water at the same temperature [
The knowledge and understanding of sorption isotherms for foods are of particular importance especially in the determination of a drying end point which ensures economic viability and microbiological safety.
Several empirical and semiempirical equations have been proposed for the correlation of the equilibrium moisture content with the water activity of food products. Among them the GAB equation has been applied successfully to various foods [
The object of this research is to obtain experimental equilibrium moisture isotherms for
The equilibrium moisture content of
Six saturated salt solutions (KOH, MgCl2, K2CO3, NaNO3, KCl, and BaCl2) were prepared by dissolving an appropriate quantity of salt in distilled water at a higher temperature than equilibration to insure that they remain saturated when cooled.
The experimental apparatus utilized consists of six glass jars of 1 L each with insulated lid. Every glass jar is filled to one-quarter depth with a prepared saturated salt solution. A layer of solid salts was maintained during the whole period of equilibration to confirm that the solutions always remain saturated. A tripod was also put in each jar to place
Selected salts used for preparing saturated salt solutions and their corresponding water activities.
Salt | Water activity | ||
---|---|---|---|
30°C | 40°C | 50°C | |
KOH | 0.0738 | 0.0626 | 0.0572 |
MgCl2 | 0.3238 | 0.3159 | 0.3054 |
K2CO3 | 0.4317 | 0.423 | 0.4091 |
NaNO3 | 0.7275 | 0.71 | 0.6904 |
KCl | 0.8362 | 0.8232 | 0.812 |
BaCl2 | 0.898 | 0.891 | 0.8823 |
The
Eight mathematical equations were used for describing desorption and adsorption isotherms of
Mathematical models used to describe desorption and adsorption isotherms of
Models names | Models equations | References |
---|---|---|
GAB |
|
Iglesias and Chirife (1995) [ |
BET |
|
BET (1938) [ |
Henderson-Thompson |
|
Henderson (1952) [ |
Modified Chung and Pfost |
|
Chung and Pfost (1967) [ |
Halsey |
|
Halsey (1948) [ |
Oswin |
|
Chen (1990) [ |
Peleg |
|
Peleg (1993) [ |
Adam and Shove |
|
Chirife and Iglesias (1978) [ |
Where
Nonlinear regression analysis was used to estimate the constants of the models from the experimental results of sorption isotherms for
The net isosteric heat of sorption can be determined from moisture sorption data by using the following equation, which is derived from the Clausius-Clapeyron equation [
The initial moisture content of
Desorption and adsorption isotherms of
Desorption and adsorption isotherms of
Desorption and adsorption isotherms of
The figures also show the effect of hysteresis between adsorption and desorption over almost the entire range of water activity at the three temperatures, in which water content on the desorption isotherm is higher than that on the adsorption side at the same water activity. One of the reasons for differences in moisture content between the two closure points is that, during drying (desorption), some solutes may supersaturate below their crystallization water activity and thus hold more water as water activity is lowered especially for products with high sugar content (Table
There is also a decrease in the equilibrium moisture content with increasing temperature, at a constant water activity; 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 [
Tables
Results of fitting of the desorption isotherms of
Models names |
|
Parameters |
|
|
|
|||
---|---|---|---|---|---|---|---|---|
|
|
|
| |||||
GAB | 30 | 0.0125 | 0.684 | 3.171 | — | 0.98 | 0.028 | 14.74 |
40 | 0.0125 | 0.487 | 4.324 | — | 0.97 | 0.030 | 18.40 | |
50 | −0.707 | −0.438 | 0.678 | — | 0.96 | 0.037 | 22.76 | |
|
||||||||
BET | 30 | 0.0184 | 15.422 | — | — | 0.98 | 0.027 | 14.44 |
40 | 0.0192 | 13.474 | — | — | 0.97 | 0.028 | 18.19 | |
50 | 0.0963 | 17.708 | — | — | 0.99 | 0.017 | 17.30 | |
|
||||||||
Henderson | 30 | 11.054 | 17.086 | −29.39 | — | 0.99 | 0.019 | 10.97 |
40 | 34.723 | 1.838 | −38.65 | — | 0.99 | 0.017 | 8.90 | |
50 | 10.513 | 1.195 | −49.05 | — | 0.98 | 0.020 | 17.99 | |
|
||||||||
Chung and pfost | 30 | 7.683 | −29.09 | 15.607 | — | 0.97 | 0.107 | 57.06 |
40 | 8.256 | −39.074 | 17.287 | — | 0.97 | 0.024 | 11.34 | |
50 | 8.180 | −48.768 | 18.330 | — | 0.93 | 0.040 | 28.41 | |
|
||||||||
Halsey | 30 | −6.516 | 0.063 | 2.092 | — | 0.98 | 0.028 | 9.08 |
40 | −6.646 | 0.058 | 2.140 | — | 0.98 | 0.019 | 6.98 | |
50 | −9.262 | 0.102 | 1.558 | — | 0.99 | 0.009 | 6.99 | |
|
||||||||
Oswin | 30 | 0.142 | 0.356 | — | — | 0.99 | 0.017 | 9.17 |
40 | 0.132 | 0.341 | — | — | 0.99 | 0.010 | 4.91 | |
50 | 0.095 | 0.500 | — | — | 0.99 | 0.009 | 8.38 | |
|
||||||||
Peleg | 30 | 0.108 | 0.216 | 0.251 | 2.544 | 0.99 | 0.010 | 5.51 |
40 | 0.200 | 0.544 | 0.262 | 9.730 | 0.99 | 0.012 | 6.15 | |
50 | 0.343 | 6.529 | 0.120 | 0.474 | 0.99 | 0.004 | 3.16 | |
|
||||||||
Adam and Shove | 30 | 0.023 | 0.178 | −1.128 | 1.261 | 0.98 | 0.018 | 19.80 |
40 | 0.019 | 0.209 | −1.163 | 1.226 | 0.98 | 0.013 | 15.34 | |
50 | 0.0165 | 0.044 | −1.076 | 1.3137 | 0.97 | 0.013 | 21.09 |
Results of fitting of the adsorption isotherms of
Models names |
|
Parameters |
|
|
|
|||
---|---|---|---|---|---|---|---|---|
|
|
|
| |||||
GAB | 30 | 1.084 | 0.318 | 0.633 | — | 0.98 | 0.026 | 16.59 |
40 | 0.091 | 0.323 | 8.037 | — | 0.97 | 0.025 | 18.20 | |
50 | 0.047 | 1.061 | 3.329 | — | 0.99 | 0.006 | 9.51 | |
|
||||||||
BET | 30 | 0.03 | 7.88 | — | — | 0.98 | 0.023 | 15.47 |
40 | 0.007 | 31.976 | — | — | 0.97 | 0.025 | 18.41 | |
50 | 0.1143 | 1.3399 | — | — | 0.99 | 0.009 | 13.32 | |
|
||||||||
Henderson | 30 | 11.129 | 1.517 | −29.24 | — | 0.99 | 0.019 | 12.15 |
40 | 17.986 | 1.989 | −39.59 | — | 0.99 | 0.011 | 7.82 | |
50 | 10.988 | 1.086 | −48.90 | — | 0.99 | 0.014 | 16.17 | |
|
||||||||
Chung and pfost | 30 | 7.529 | −29.00 | 15.97 | — | 0.96 | 0.035 | 20.63 |
40 | 8.487 | −39.11 | 20.47 | — | 0.98 | 0.015 | 8.98 | |
50 | 8.081 | −48.66 | 19.019 | — | 0.92 | 0.041 | 36.65 | |
|
||||||||
Halsey | 30 | −6.44 | 0.065 | 1.928 | — | 0.98 | 0.027 | 10.74 |
40 | −6.91 | 0.049 | 2.259 | — | 0.98 | 0.017 | 6.90 | |
50 | −6.23 | 0.072 | 1.46 | — | 0.99 | 0.009 | 10.07 | |
|
||||||||
Oswin | 30 | 0.127 | 0.395 | — | — | 0.99 | 0.017 | 7.86 |
40 | 0.1178 | 0.317 | — | — | 0.99 | 0.006 | 3.32 | |
50 | 0.084 | 0.542 | — | — | 0.99 | 0.005 | 4.23 | |
|
||||||||
Peleg | 30 | 0.143 | 0.420 | 24.601 | 4.147 | 0.99 | 0.014 | 7.02 |
40 | 0.157 | 0.446 | 0.157 | 5.892 | 0.99 | 0.007 | 4.25 | |
50 | 0.155 | 0.832 | 0.374 | 9.498 | 0.99 | 0.006 | 8.32 | |
|
||||||||
Adam and Shove | 30 | 0.018 | 0.142 | −1.111 | 1.278 | 0.98 | 0.021 | 8.74 |
40 | 0.017 | 0.205 | −1.184 | 1.205 | 0.98 | 0.020 | 7.40 | |
50 | 0.011 | 0.026 | −1.069 | 1.3203 | 0.98 | 0.013 | 10.59 |
These results indicate that all the models are acceptable for predicting the equilibrium moisture content. However, the Peleg and Oswin models gave the best fitting of adsorption and desorption isotherms for the three temperatures, with lowest standard error and the highest coefficient of correlation.
Peleg and Oswin equations were found to be satisfactory for many other plant species [
Comparisons were done between experimental and calculated (Peleg and Oswin models) data of desorption and adsorption isotherms obtained for
Comparison between experimental and calculated (Peleg and Oswin models) data of desorption isotherms of
Comparison between experimental and calculated (Peleg and Oswin models) data of adsorption isotherms of
The net isosteric heats of sorption of
The variations of the heats of desorption and adsorption of
Variations of calculated and experimental values of desorption and adsorption isosteric heats of
For the most part of the curves, the heat of desorption has been observed to present a higher magnitude than the corresponding heat of adsorption. Iglesias and Chirife [
The figure shows also that the net isosteric heat of sorption decreased with an increase in moisture content. As shown in the curves, a steep slope of the curves is observed; this is indicative of intermolecular attraction forces between sorptive sites and water vapour.
At low moisture contents, the isosteric heat of sorption is high and then decreased at high moisture contents. According to Tsami et al. [
An exponential function was used to describe the relationship between the isosteric heat of sorption and the equilibrium moisture content:
The best statistical parameters show that the exponential function can be used to calculate the heat of sorption of
The moisture desorption and adsorption isotherms of
Both Peleg and modified Oswin equations were the best models for prediction of desorption and adsorption phenomena among eight commonly used models investigated. The desorption and adsorption isotherms show the occurrence of moisture sorption hysteresis.
The net isosteric heat of desorption and adsorption were calculated using the Clausius-Clapeyron equation.
The net isosteric heats of sorption of
The authors declare that there is no conflict of interests regarding the publication of this paper.