Eur Food Res Technol (2006) 223: 849–852DOI 10.1007/s00217-006-0279-z

ORIGINAL PAPER

Sanjeev Mehta · Amarjit Singh

Adsorption isotherms for red chilli (Capsicum annum L.)

Received: 9 November 2005 / Revised: 11 January 2006 / Accepted: 20 January 2006 / Published online: 31 March 2006C© Springer-Verlag 2006

Abstract The adsorption equilibrium moisture contentsfor red chilli were determined experimentally in relativehumidity range of 11–97% at the temperatures of 20, 30,40, and 50◦C. Six equilibrium moisture content modelswere fitted to the experimental data. The modified Oswinmodel was the best fitted equation for relative humidityrange of 11–97% for the adsorption data of red chilli.

Keywords Chilli . Adsorption . Isotherm models .Equilibrium moisture content

Abbreviations A, B, C: Dimensionless coefficients ofthe models . EMC: Equilibrium moisture content .Me: Equilibrium moisture content (decimal dry basis) .n: Number of constants . N: Number of observations .P: Mean relative deviation modulus . rh: Relativehumidity (decimal) . r2: Coefficient of determination .RMSE: Root mean square error . T: Temperature (◦C) .χ2: Reduced chi-square

Introduction

Chilli (Capsicum annum L.) is an important condimentand cash crop of the world. It is an essential ingredient ofIndian curry that characterizes the curry by its temptingcolor and titillating pungency. Red ground pepper madeby drying and pulverizing the matured red chillies is usedas a spice and flavor ingredient in food industry. Bosland[1], Varghese and Nataranjan [2] reported its wide range ofmedicinal application from increasing appetite, relievingpain associated with arthritis to diuretic effects, swellingand inflammation. There is a great need to improve thedrying and storage system in order to improve the quality ofprocessed chilli. The properties such as structure, texture,

S. Mehta (�) · A. SinghDepartment of Processing and Food Engineering,Punjab Agricultural University,KVK, Langroya, NawanShahr, Punjab, Indiae-mail: kufisa007@yahoo.com

and storage life of a dried food depend on the sorptioncharacteristics [3].

Many investigators have developed mathematicalequations, theoretical, semi-theoretical, and empiricalto describe the sorption isotherms of food materials.A number of models have been previously suggestedfor the dependence between the equilibrium moisturecontent (EMC) and the relative humidity (rh) of the air[4–6]. Some of them have taken into account the effectof temperature on equilibrium moisture content. Themodified Chung-Pfost [7, 8], modified Henderson [9, 10],modified Halsey [11, 12], and modified Oswin [13, 14]equations have been adopted as standard equations by theAmerican Society of Agricultural Engineers for describingsorption isotherms [15]. Chirife and Iglesias [16] notedthat no single adsorption isotherm model is applicable forall type of food materials and over entire range of humidity.Menkov [17] reported that the modified Oswin modelwas the most suitable one for describing the relationshipsbetween equilibrium moisture content, relative humidityand temperature. Soysal and Oztekin [18] evaluated sevenequilibrium moisture content equations for their abilityto fit data for some medicinal and aromatic plants. Theyfound that the modified Halsey and the modified Oswinequations were the most versatile equations for medicinaland aromatic plants, the modified Henderson equation isgood for fennel and cinnamon. Kaleemullah and Kailappan[19] fitted 10 equilibrium moisture content equations andreported that the modified Halsey equation gave the bestfit for adsorption process of chillies.

Therefore, this study was undertaken to determine ex-perimentally adsorption isotherms for red chilli at differenttemperatures and to fit the experimental data to the selectedmodels describing the isotherms.

Materials and methods

Freshly harvested chillies (variety CH-1) were obtainedfrom the local farmer. The raw chillies were washed withtap water to remove dust and adhering impurities, and al-

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lowed to drain excess water for 2 h. The adsorption studywas carried out by using static desiccator’s technique.Incubator was used as temperature control chamber andthe desiccators were used as humidity control chambers.Equilibrium moisture content was determined at six levelsof relative humidities (0.11–0.97) and four temperatures(20, 30, 40, and 50◦C). Various supersaturated salt solu-tions of different relative humidities, viz., lithium chloride(0.111–0.114), magnesium chloride (0.312–0.330), sodiumdichromet (0.463–0.552), sodium chloride (0.745–0.755),potassium nitrate (0.850–0.932), and potassium sulphate(0.958–0.972) were used to provide constant relative hu-midities at 20, 30, 40, and 50◦C [20]. The prepared saturatedsalt solutions were transferred into the desiccators. About5 g of fresh sample was taken in each petri dish and placedon a platform inside the desiccators to avoid direct contactbetween chilli and salt solution. The desiccators were thentransferred to the incubator maintained at a predeterminedtemperature. The petri dishes were weighed after every 48 htill a constant weight was observed. The moisture contentsof the sample were determined by oven method at 105◦Cfor 24 h [21]. The equilibrium moisture contents (EMC)were determined by calculating the means of triplicatemeasurements.

Model fitting

Six isotherm models were selected for fitting the experi-mental data of adsorption isotherms of red chilli. The mod-els that have been used in this study are listed in Table 1.The parameters of the models were estimated by using theSPSS (Statistical Package for Social Science) 7.5 software.The suitability of the models was evaluated and comparedusing the coefficient of determination (r2), root mean squareerror (RMSE), reduced chi-square (χ2), and mean relativedeviation modulus (P), which are given as follows:

RMSE =√∑N

i=1 (EMCexp,i − EMCPred,i )2

N(1)

Table 1 Isotherm models for fitting the experimental data

Name of the equation Equation References

Modified Henderson Me =[

ln(1−rh)−A(T +B)

]1/c[10, 27]

Modified Halsey Me = [ exp(A−BT )− ln rh

]1/c[12, 18]

Modified BET Me = A(1−rh B) [26]

Modified exponential Me = (A − BT ) exp(C rh) [30]

Modified Chung-Pfost Me = [1

−A

]ln

[ (T +B) ln(rh)−C

][7, 27]

Modified Oswin Me = (A + BT )(

rh1−rh

)1/c[28]

χ2 =∑N

i=1 (EMCexp,i − EMCpred,i )2

N − n(2)

P = 100

n×

n∑i=1

|EMCexp,i − EMCpred,i |EMCexp,i

(3)

where EMCexp,i is the ith experimental value, EMCpred,ithe ith predicted value, N the number of observations, andn the number of constants. The value of RMSE representsthe fitting ability of a model in relation to the number ofdata points. Smaller the RMSE value, better the fittingability of a model.

Selection of best model

A model was considered as good when coefficient of de-termination (r2) is high, root mean square error (RMSE)is low [22, 23], reduced chi-square (χ2) and mean relativedeviation modulus (P) is low [14, 24, 25].

Results and discussion

The experimental adsorption isotherms for chilli at 20,30, 40, and 50◦C temperatures are shown in Fig. 1.Equilibrium moisture content increased with increasingrelative humidity for all temperatures. The equilibriummoisture content decreased with increase in temperatureat constant relative humidity. The isotherms obtainedwere similar to those of most of the biological products.Hossain and Bala [26], Phoungchandang and Woods [27],Menkov and Dinkov [28] and Soysal and Oztekin [18] alsoreported a similar pattern for red chillies, banana, tobaccoseeds and medicinal and aromatic plants, respectively. The

0

10

20

30

40

50

60

0 20 40 60 80 100

Relative humidity (%)

Eq

uili

bri

um

mo

istu

re c

on

ten

t (%

db

)

Temp 20˚C

Temp 30˚C

Temp 40˚C

Temp 50˚C

Fig. 1 Experimental adsorption isotherms for red chilli for differentrelative humidities at different temperatures

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Table 2 Estimated parametersand criteria for the comparisonof equilibrium moisture contentmodels

ModelsModifiedHenderson

ModifiedHalsey

ModifiedBET

ModifiedExpon

ModifiedChung-Pfost

ModifiedOswin

CoefficientsA 75873490.6 − 4.46338 0.069792 0.52877 10.97658 0.207717B 7640546.47 0.02727 0.875279 0.00047 − 2.152410 − 0.00174C 1.3210472 2.56308 – 2.51779 113.3140 3.046329

Comparison criteriar2 0.92340 0.96090 0.93290 0.97340 0.97520 0.98370

RMSE 0.03899 0.02783 0.03649 0.02297 0.02153 0.01745χ 2 0.00174 0.00088 0.00152 0.00060 0.00053 0.00003P 22.100 19.147 20.926 12.265 14.995 8.489

Fig. 2 Comparison ofexperimental data with valuespredicted by different models

models (Table 1) were fitted to the experimental data andthe estimated parameters of the models are listed in Table2. The coefficient of determination (r2), root mean squareerror (RMSE), reduced chi-square (χ2), and mean relativedeviation modulus (P) are presented in Table 2.

The low r2 and high RMSE, χ2 and P values for modi-fied Henderson, modified Halsey, modified BET, modifiedexponential and modified Chung-Pfost models lead to itsrejection whereas high r2 and low RMSE, χ2 and P val-ues for modified Oswin model lead to its acceptance. EMCvalues predicted by different models for four different tem-peratures are shown in Fig. 2. The experimental data wasbanded around the modified Oswin curve, which indicatedthe suitability of the modified Oswin model in describingthe adsorption behavior of red chillies. Mishra [21] fittedadsorption and desorption isotherms for Indian red chilli tothe Henderson equation and found reasonable agreementbetween the predicted and the experimental data. Hossainand Bala [26] fitted seven equilibrium moisture contentmodels for red chillies and found modified Smith equation

was the best-fitted equation. Kaleemullah and Kailappan[19] reported that the modified Halsey equation gave thebest fit for adsorption process and modified Oswin equationfor desorption process of chillies.

The modified Oswin model was found as the most ver-satile model to accurately describe the EMC–ERH rela-tionship for coriander, fennel, peppermint, ginger, and cin-namon [18], banana [27], pea seeds [29] tobacco seeds[28].

Conclusion

Six equilibrium moisture content models were used to fitthe experimental adsorption data in the relative humidityrange of 11–97% at temperatures of 20, 30, 40, and 50◦C.The modified Oswin model was found to describe accu-rately the adsorption isotherm for red chilli in the relativehumidity range of 11–97%. The modified Oswin model

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recommended for prediction of the adsorption isotherm forred chilies.

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