Higroscopicidad Del Maní 2609946 61923

INTRODUCTION

Peanut (Arachis hypogaea L.) is a dicotyledonous legu-minous plant originated from Central America andadapted to equatorial-tropical climate, presenting highcontents of proteins, vitamins, lipids, carbohydrates andmineral salts. Peanuts are composed of a hull, kernelsand some air enclosed between two components, therebymaking their structure quite complex. Limited data onthe sorption properties of peanuts has been reported inthe literature.

Peanut harvesting is many times conducted underadverse climatic conditions, especially during heavy

raining seasons, which may induce fungus developmentand toxin production due to traditional harvesting, dry-ing and storage practices.

The control of moisture content of foods during pro-cessing and storage is very important as water has manyroles in food reactions and food quality. In this respectthe moisture sorption isotherm is an extremely impor-tant tool in food science because it can be used to pre-dict changes in food stability and to select appropriatepackaging materials and ingredients (Ayranci andDuman, 2005).

All agricultural products have the capacity to loss orabsorb ambient water, thus maintaining a constant equi-librium relation between their moisture content and theambient air conditions. Equilibrium moisture content(EMC) is attained when partial water vapor pressure isequal to that of the air that surrounds it. The relationbetween the moisture content of a given product andthe equilibrium relative humidity (ERH) for a specifictemperature may be expressed by mathematical equa-tions, termed moisture sorption isotherms. In every foodproduct there is an inherent relationship between water

Sorption Isotherms and Isosteric Heat of Peanut Pods,Kernels and Hulls

P.C. Corrêa1, A.L.D. Goneli1, C. Jaren2*, D.M. Ribeiro1 and O. Resende1

1Department of Agricultural Engineering, University Federal of Viçosa, Campus UF,P.O. Box 270, 36570-000 – Viçosa-MG, Brazil

2Department of Proyect and Rural Engineering, Public University of Navarre, Campus Arrosadia,31006 Pamplona, Spain

This study was carried out to evaluate the sorption isotherms of peanut pods, kernels and hulls for severaltemperature and humidity conditions and to fit different mathematical models to the experimental data,selecting the one best fitting the phenomenon. The dynamic method was applied to obtain the hygroscopicequilibrium moisture content. The environmental conditions were provided by means of an atmosphericconditioning unit, in which removable perforated trays were placed to allow air to pass through peanutmass, each one containing 50 g of the product.The mathematical models frequently used for the represen-tation of hygroscopicity of agricultural products were fit to the experimental data. Based on those results,it was concluded that peanut pods, kernels and hulls presented differentiated hygroscopicity.The equilibriummoisture content for peanut pods, kernels and hulls increased with an increase in the relative humidity at anyparticular temperature and decreased with increase in temperature at constant relative humidity. At aconstant water activity, peanut hulls samples had higher equilibrium moisture content than the pods andkernels samples. Based on statistical parameters, the modified Henderson and Chung-Pfost models werefound to adequately describe the sorption characteristics of peanut pods, kernels and hulls. Isosteric heatof desorption were evaluated by applying the Clausius–Clapeyron equation to experimental isothermsand decreased with increasing moisture content. The peanut hulls had higher isosteric heat of sorptionthan that peanut pods and kernels.

Key Words: equilibrium moisture content, Arachis hypogaea, mathematical models, desorption, isosteric heat

Food Sci Tech Int 2007; 13(3): 231–238© SAGE Publications 2007Los Angeles, London, New Delhi and SingaporeISSN: 1082-0132DOI: 10.1177/10820132013207079601

*To whom correspondence should be sent(e-mail: [email protected]).Received: 19 October 2005; revised: 17 October 2006.

at Reprints Desk Inc PARENT on February 27, 2015fst.sagepub.comDownloaded from

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232 P.C. CORRÊA ET AL.

content and the relative humidity of the atmosphere inequilibrium with it, which is equivalent to water activity.

The chemical composition of a product has a directinfluence on the humidity sorption process. Accordingto Brooker et al. (1992), grains with high oil contentadsorb smaller amount of moisture from the environ-ment than grains with high starch content. Moreover,the variety, degree of maturity, and physical and sani-tary conditions as well as the method through whichequilibrium was obtained (adsorption or desorption)are also determining factors in the establishment of theequilibrium humidity of hygroscopic products (Chen,2000b; Fan et al., 2000).

Moisture sorption isotherms may be obtained exper-imentally by means of the dynamic and static methods.In the dynamic method, the pod is submitted to air flowunder controlled and fixed temperature and relativehumidity conditions until equilibrium is reached. In thestatic method, the hygroscopic equilibrium between theproduct and the environment, under controlled condi-tions, is reached without air movement (Wang andBrennan, 1991; Jayas and Mazza, 1991; Chen, 2000b).

Several researchers have studied the hygroscopicbehaviour of several agricultural products, describingdifferentiated models to express the EMC as a functionof the temperature and air relative humidity. However,for the establishment of isotherms that represent thisequilibrium relation, empirical mathematical modelsare used, since no theoretical model has been capableof precisely predicting the EMC for a wide range oftemperature and air relative humidity.

Over 200 equations are currently available in theliterature proposing to represent the hygroscopic equi-librium phenomenon of agricultural products. Suchmodels differ in their theoretical or empirical basis andnumber of parameters involved (Mulet et al., 2002).

The modified Henderson equation and Chung Pfostequation were the best models for many starchy grainsand fibrous materials.The modified Halsey equation wasthe best for high oil and protein products. The modifiedOswin model was good for popcorn, peanut pods andother varieties of corn and wheat (Chen, 2000b; Chenand Morey, 1989). The GAB model was very popular tobe adopted by some researchers (Ayranci and Duman,2005; Mulet et al., 2002; Pagano and Mascheroni, 2005;Van den Berg, 1984). However, this model was found tobe inadequate to describe the relationship betweenmoisture content and water activity for some agriculturalproducts (Chen, 2002, 2003; Chen and Jayas, 1998).

Moisture sorption isotherms constitute an essentialpart of the theory of drying and provide useful informa-tion in the design of drying equipment and in the studyof storage of dehydrated products. A thermodynamicparameter such as isosteric heat is frequently evaluatedfrom equilibrium data at different temperatures(Iglesias and Chirife, 1976).

The application of thermodynamic principles tosorption isotherm data has been used to obtain more

information about the dehydration process energyrequirement, the properties of water, food microstruc-ture and physical phenomena on the food surfaces, andsorption kinetic parameters. One method widely usedfor calculation of isosteric heat of sorption of manyfoods is based on the Clausius–Clapeyron equation,which assumes temperature-independent heat of sorp-tion and allows an easy calculation of the isosteric heatfrom the sorption isotherms (Aguerre et al., 1988).

In view of the importance of understanding thehygroscopicity of agricultural products, this work aimedto determine the sorption isotherms of peanut pods,kernels and hulls for various temperature and air relativehumidity conditions and to fit different mathematicalmodels to the experimental data, selecting the best-fittingmodel, and calculate the isosteric heat of sorption atdifferent moisture levels.

MATERIAL AND METHODS

The present work was carried out in the Laboratoryfor the Physical Properties and Quality Evaluation ofAgricultural Products of National Grain StorageTraining Center – CENTREINAR, Federal Universityof Viçosa, Viçosa, MG, Brazil.

The initial moisture content of peanut pods, kernelsand hulls were 31.0, 25.0 and 47.0% dry basis, respec-tively. Samples were stored in polythene bags kept in arefrigerator to attain moisture uniformity.When neededfor experiments, samples were allowed to equilibrate atambient condition for 6 h. The peanut pods, kernels andhulls moisture content was determined by applying thedrying in an oven at 105 � 1°C, for a 24 h period, in trip-licate according to the seeds analysis standard of Brazil(Ministério da Agricultura e Reforma Agrária, Brazil,1992).

The sorption method used was the dynamic tech-nique or gravimetric method, in which the material isbrought into equilibrium with air of fixed temperatureand relative humidity and the equilibrium moisture con-tent (EMC) of the material is measured. Thin-layer dry-ing was carried out at different controlled temperature(20, 35, 50 and 65°C) and air relative humidity of the dry-ing air (between 0.2 and 0.8) until the product reachedequilibrium humidity at the specified air condition.

The environmental conditions for the performance ofthe tests consisted of a temperature controlled chamber,manufactured by Aminco, model Aminco-Aire 150/300CFM. Removable perforated trays containing 50 g ofproduct were placed inside the equipment to allow air topass through the samples. Air flow was monitored withan anemometer with rotating blades and kept around10 m3/min/m2. Temperature and air relative humiditywere monitored with a psychrometer installed next tothe trays containing the samples.

The trays containing the product were periodicallyweighed during drying. Hygroscopic equilibrium was



Sorption Isotherms Peanut Pods, Kernels and Hulls 233

reached when the mass variation of the containersremained constant during three consecutive readings.

In this work, the relationship between the equilibriummoisture content data and the relative humidity andtemperature for peanuts pods, kernels and hulls wasevaluated according to the models of Chung–Pfost(Pfost et al., 1976), Copace (Corrêa et al., 1995), modi-fied Halsey (Iglesias and Chirife, 1976b), modifiedHenderson (Thompson, 1972), modified Oswin (Chenand Morey, 1989) and GAB (Anderson, 1946). Thesemodels are presented in Table 1 where Me is the equi-librium moisture content (% dry basis), t is the temper-ature (°C), rh is the relative humidity (decimal), A, Band C are parameters of models.

The experimental data were interpreted by means ofnon-linear regression analysis by the Quasi-Newtonmethod, using a computer program STATISTICA 6.0®.The models were selected based on the significance ofthe regression coefficients by the t-test, at 1% probabil-ity, the mean relative error (MRE), the standard errorof estimate (SEE), the determination coefficient (R2)and residual distributions plots were used to evaluatethe fitting quality.

The mean relative error value lower than 10 was one ofthe criteria for selecting the models, according toMohapatra and Rao (2005).The mean relative error MREand the standard error of estimate SEE are given by:

(6)

(7)SEEpre

f

=−( )

=∑ M M

Di

n

exp

2

1

MREn

=−⎛

⎝⎜⎜

⎞

⎠⎟⎟=

∑100

1n

M M

Mi

exp pre

exp

where, Mexp is the experimental value of EMC; Mpre isthe value predicted by the model; N is the number of datapoints; Df is the degrees of freedom of regression model.

The residuals were plotted against predicted values ofEMC. A model is considered acceptable if the residualvalues fell in horizontal band centered around zero, dis-playing no systematic tendencies (i.e., random in nature)towards a clear pattern. If the residual plot indicatesclear pattern, the model is not considered acceptable.

The net isosteric heat of sorption was determinedfrom moisture sorption data using the following equa-tion, which is derived from the Clausius–Clapeyronequation (Iglesias and Chirife, 1976a):

(8)

Integrating Eq. (12), assuming that the net isostericheat of sorption (qst) is temperature independent, givesthe following equation (Wang and Brennan, 1991):

(9)

where qst is the net isosteric heat of sorption (kJ/kg), Ris the universal gas constant (kJ/kg/K), T is absolutetemperature (K).

The sorption isosteric heat Qst can be calculated byadding the latent heat of vaporization for pure water Lto the net sorption isosteric heat qst (Sanchez et al.,1997):

(10)

where Qst is the sorption isosteric heat (kJ/kg), L is thelatent heat of vaporisation for pure water, kJ/kg forthe temperatures taken into account; a and b are constants.

RESULTS AND DISCUSSION

The mathematical models tested to determine theequilibrium moisture content of peanuts pods presentedsignificant regression coefficients at 1% probability andvalues of determination coefficient (R2) above 93%(Table 2). The modified Halsey model gave a relativemean error above 10%, showing to be inadequate todescribe the equilibrium moisture content (Özdemirand Derves, 1999). The residual plot of modified Halseymodel displayed a clear pattern. Thus, the model wasalso non acceptable. However, the Copace, Chung-Pfost, modified Henderson and modified Oswin modelspresented a better fitting to the experimental data ofequilibrium moisture content of peanut pods, showingrandom residual plot, smaller mean relative error(lower than 10%). Thus, the application of these fourmodels is recommended to estimate the equilibrium

Q q L a b M Lst st e

= + = − +.exp( . )

ln( )rhq

R TK=

⎛⎝⎜

⎞⎠⎟

⋅ +� st 1

∂∂

=⋅

ln( )rh

T

q

R Tst

2

Table 1. Mathematical models used to identify theEMC of products of vegetable origin.

Model Model Expression

Chung-Pfost

Copace

Modified Halsey

Modified Henderson

Modified Oswin

GAB MABC rh

B rh B rh BC rhe=

− − +( )

[ ( )][ ( ) ( )]1 1

M A Btrh

rh

C

e= +

−⎡

⎣⎢

⎤

⎦⎥( )

( )

/

1

1

Mrh

A t B

C

e=

−− +

⎡

⎣⎢

⎤

⎦⎥

ln( )

)

/

11

× (

MA Bt

rh

C

e=

−−

⎡

⎣⎢

⎤

⎦⎥

exp( )

ln( )

/1

M A Bt Crhe

= − +⎡⎣ ⎤⎦exp ( ) ( )

M A t C rhe

B= − − +⎡⎣ ⎤⎦ln ( ) ln( )




humidity of peanut pods. Chen (2000a) obtained also abetter fit to the experimental data of equilibrium mois-ture content of peanut pods with the Oswin model.

For the peanut kernels, the coefficients estimated forthe models showed significance at 1% probability bythe t-test and relative mean error below 10% (Table 3).In addition, all the models showed a coefficient ofsquared above 95%, except the modified Halsey model.However, only the modified Henderson and Chung-Pfost models presented a random residual plot. TheChung-Pfost model exhibited a higher determinationcoefficient and lower estimated and mean relative errorand standard error of estimate, being thus recommendedfor predicting the hygroscopic equilibrium of peanutkernels. This result disagreed with Chen (2000a) whorecommended the modified Halsey model, and withChen and Morey (1989) who used modified Halsey andmodified Oswin models to estimate the hygroscopicequilibrium of peanut kernels.

For the peanut hull, all the models presented values ofdetermination coefficient above 96% (Table 4). Besides,the models studied presented significant coefficients at1% probability by the t-test, and mean relative errorbelow 10%.As well as the peanut pods, only the modifiedHalsey model was inadequate to describe the equilibriummoisture content in peanut hulls because the residual plotindicated clear pattern. Chen and Morey (1989) observedthat the modified Henderson equation satisfactorily

represented the hygroscopic equilibrium experimentaldata, and Chen (2000a) recommended the Chung-Pfostmodel to predict equilibrium humidity of the peanut hull,being both in agreement with this work.

All residual plots of the GAB model for samplesdried at four temperatures had systematic patterns(Table 5). The results indicated that the GAB equationwas not an adequate model either for peanut pods, ker-nels or hulls. A similar systematic pattern of residualplots also were found for the GAB models of desorp-tion and adsorption data for potato slices (Wang andBrennan, 1991) and cassava (Sanni et al., 1997). Theseresults agreed with Chen and Jayas (1998) investiga-tions on adequacy of the GAB equation to describeEMC/ERH relation for agricultural products.

The desorption isotherms for peanut pods, kernelsand hulls obtained at 20, 35, 50 and 65°C (Figures 1–4,respectively) were adjusted by fitting the modifiedHenderson model, Eq. (4), to the experimental data.The equilibrium moisture content at each water activityrepresents the mean value of three replications. Asexpected, the equilibrium moisture contents increasedwith an increase in the relative humidity at any particu-lar temperature and decreased with increase in temper-ature at constant relative humidity.

At higher temperature water molecules reach higherenergy levels and this allows them break away fromtheir sorption sites, thus decreasing the equilibrium

Table 2. Estimated parameters and comparison criteria for the equilibrium moisture content modelsof peanut pods desorption data.

Model Parameters*

Models A B C R2 (%) SEE (% d.b.) MRE (%) Residual Plot

Chung-Pfost 23.8213 4.1930 49.3712 96.52 0.5542 7.1140 RandonCopace 1.1878 0.0079 1.9151 96.27 0.5733 9.2428 RandonModified Halsey 3.6073 0.0158 1.8467 95.06 0.6601 10.7800 PatternedModified Henderson 0.0005 51.6250 1.5244 96.72 0.5381 7.8789 RandonModified Oswin 8.3203 �0.0510 2.3683 96.30 0.5717 8.8750 Randon

*Significant at 1% probability by the t-test.

Table 3. Estimated parameters and comparison criteria for the equilibrium moisture content modelsof peanut kernels desorption data.

Model Parameters*


Chung-Pfost 19.8681 3.2246 75.5245 97.08 0.3730 6.1137 RandonCopace 1.0877 0.0051 1.6755 95.54 0.4607 8.6219 PatternedModified Halsey 3.6797 0.0120 2.0960 93.68 0.5484 9.9758 PatternedModified Henderson 0.0003 85.5428 1.7519 96.54 0.4059 7.2175 RandonModified Oswin 6.8118 �0.0302 2.7020 95.65 0.4549 8.2203 Patterned





moisture content (Palipane and Driscoll, 1992). As tem-perature changes, the excitation of molecules, as well asthe distance, and thus attraction between moleculesvaries. This causes the amount of sorbed water tochange with temperature at a given relative humidity(Mohsenin, 1986).

The temperature shifts observed have an importantpractical affect on the chemical and microbiologicalreactions which cause quality deterioration.An increasein temperature causes an increase in the water activity,at the same moisture content, which in turn causes anincrease in the reaction rates leading to quality deterio-ration (Van den Berg and Bruin, 1981).

The sorption isotherms of peanut pods, kernels andhulls samples showed type II behaviour according tothe BET classification. At a constant water activity,peanut hulls samples had higher equilibrium moisturecontent than the pods and kernels samples, indicating a

higher hygroscopicity of the product. This might be dueto the separation of hulls from peanut kernels, sincehulled process reduces the fibre content, which absorbsmore water. These results agreed with those found byKaya and Kahyaoglu (2006) who evaluated the influ-ence of dehulling and roasting process on the thermo-dynamics of moisture adsorption in sesame seed. In allrange of temperature used, the sorption isotherms ofpeanut pods and kernels had similar values.

The interaction between water vapour and the adsor-bent food material should be determined to defineeffect of temperature, isosteric heat of sorption (Qst).Atconstant moisture content, relative humidity of theequilibrium at each studied temperature were deter-mined using the modified Henderson model (since itfitted the experimental data satisfactorily). The isostericheat of sorption values was calculated by applyingEqs. (9) and (10), and represented with respect to

Table 4. Estimated parameters and comparison criteria for the equilibrium moisture content modelsof peanut hulls desorption data.

Model Parameters*


Chung-Pfost 37.2538 6.6636 37.7956 96.72 0.8787 7.2498 RandonCopace 1.7432 0.0089 1.8720 97.39 0.7831 7.0524 RandonModified Halsey 4.6864 0.0179 1.8827 96.55 0.9011 8.5662 PatternedModified Henderson 0.0002 38.4115 1.5619 97.66 0.7421 5.8444 RandonModified Oswin 14.0039 �0.0925 2.4131 97.35 0.7895 6.7912 Randon


Table 5. Estimated parameters of GAB model for peanut pods, kernels and hulls desorption data.

Model Parameters*

t(°C) A B C R2 (%) SEE (% d.b.) MRE (%) Residual Plot

Pods20 5.5786* 0.6337* 7.0288 ns 97.05 0.3211 3.2341 Patterned35 4.5593* 0.6868* 11.6252 ns 95.44 0.4156 4.3730 Patterned50 22.2841 ns 0.0097 ns 69.7799 ns 93.19 0.4151 7.3296 Patterned65 21.5076 ns 0.0035 ns 166.3946 ns 92.77 0.3268 7.0494 Patterned

Kernels20 5.7234* 0.7402* 7.4220 ns 91.94 0.7803 6.8384 Patterned35 4.9688* 0.7497* 8.5307 ns 96.36 0.4996 5.0275 Patterned50 14.4176 ns 0.2435 ns 4.1322 ns 93.29 0.4647 7.3610 Patterned65 32.0225 ns 0.0067 ns 58.6284 ns 90.88 0.4121 8.3395 Patterned

Hulls20 8.3337* 0.7945* 11.3414 ns 95.08 1.0325 5.4565 Patterned35 8.3536* 0.7249* 8.5374 ns 95.67 0.8516 5.8888 Patterned50 8.4497* 0.6869* 5.4861* 95.52 0.5815 6.3883 Patterned65 31.6009 ns 0.0154 ns 46.6346 ns 96.77 0.3541 3.9581 Patterned

*Significant at 1% probability by the t-test; ns: not significant.




moisture content values.These results agreed with thosefound by Martinez and Chiralt (1996), who reportedthat the hulled process decreased the heat of sorptionvalues of hazelnut, almond and peanut samples, likelydue to enhancement of lipid–lipid interaction thatincreases the hydrophobicity of cellular components ofseeds. The net isosteric heat of sorption with respect toequilibrium moisture content was adequately describedby the power law relation of the form:

equilibrium moisture content (Figure 5). At low mois-ture content the heat of sorption is high, indicating thehighest binding energy for removal of water. Increasingmoisture content decreased the heat of sorption due toreduced water interactions. As the moisture contentincreases further, the heat of sorption tends to that ofpure water an indication of the moisture existing in thefree form. The isosteric heat of sorption peanut hullswas lower than those of pods and kernels samples for all

Figure 1. Equilibrium moisture content values ofpeanut pods (�), kernels (�) and hulls (�) obtainedby desorption at 20°C, and their isotherms calculatedby the modified Henderson model.




Peanut pods: st

Q Me

= − +1691 86 0 24 2400 4. exp( . ) . 33 0 9999

1617 41

2( . )

. exp(

R

Q

=

=Peanut kernels:st

−− + =0 31 2400 43 0 99992. ) . . ) (

Peanut hulls:

M Re

QQ M Rest

= − + =2062 27 0 31 2400 43 0 99992. exp( . ) . ( . ))




The knowledge of the magnitude of the heat ofsorption, at a specific moisture content, provides anindication of the state of the sorbed water and hence, ameasure of the physical, chemical and microbiologicalstability of the food material under given storage condi-tions. In addition the variation in heat of sorption withmoisture content, and magnitude relative to the latentheat of vaporization of pure water, provides valuabledata for energy consumption calculations and subsequentdesign of drying equipment, and an understanding ofthe extent of the water–solid versus water–water inter-actions (McMinn and Magee, 2003).

CONCLUSION

The experimental results illustrated that the equilib-rium moisture content (EMC) increased with decreas-ing temperature, at constant equilibrium relativehumidity (ERH). Furthermore, at constant tempera-ture, the EMC increased with increasing ERH. Peanutpods, kernels and hulls have different hygroscopicity.The order in the magnitude of equilibrium moisturecontents at each water activity values was found aspeanut hulls � peanut pods � peanut kernels. Based onstatistical parameters, the models modified Hendersonand Chung-Pfost were the ones best representing thehygroscopicity phenomenon of peanut pods, kernelsand hulls, compared with the Copace, modified Halseyand modified Oswin models. The GAB equation wasnot an adequate model for either peanut pods, kernelsor hulls. The isosteric heat of desorption of all samples,calculated using the Clausius–Clapeyron equation,showed power relations with moisture content; wholepeanut hulls has higher isosteric heat of sorption thanthat peanut pods and kernels.

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