A Rapid Method to Determine the Sorption Isotherms of Peanuts

J. agric. Engng Res. (2000) 75, 401}408doi.10.1006/jaer.1999.0522, available online at http://www.idealibrary.com on

A Rapid Method to determine the Sorption Isotherms of Peanuts

Chiachung Chen

Department of Agricultural Machinery Engineering, National ChungHsing University, Taichung 40227, Taiwan;e-mail: [email protected]

(Received 23 April 1999; accepted in revised form 2 December 1999)

A rapid method has been developed to determine the sorption isotherms of peanuts. The equilibrium relativehumidities ranging from 10 to 95% for peanut pods, kernels and hulls were measured at temperatures rangingfrom 5 to 453C. No signi"cant di!erences were found between the experimental results of the equilibriummoisture content and equilibrium relative humidity methods. The curve-"tting agreement of the adsorption anddesorption data for four equilibrium relative humidity models was evaluated. The best models and theirestimated parameters were shown in this study.

( 2000 Silsoe Research Institute

1. Introduction

Peanuts are an important oilseed crop in Taiwan. Thecultivar grown by farmers is a Spanish variety. Prevailingweather conditions limit the harvesting periods. Peanutsmust be harvested under high moisture conditions, thendried by hot air at 503C. In terms of designing the dryingand storing systems, accurately predicting the relation-ships between equilibrium moisture content (EMC),equilibrium relative humidity (ERH), and temperature isof relevant concern.

Peanuts are composed of a hull, kernels and some airenclosed between two components, thereby making theirstructure quite complex. Limited data on the sorptionproperties of peanuts has been reported in the literature.

Two conventional methods for measuring EMC/ERHproperties are the EMC and ERH methods. In the EMCmethod, samples are placed in an environment havinga constant relative humidity and temperature. Aftera long period, the moisture of samples are measured andadopted as the EMC value. In the ERH method, thesample with a known moisture content is placed in a lim-ited volume environment. The air conditions of temper-ature and relative humidity are then measured.

Relatively few investigations have provided EMC dataat "xed temperatures, among which include Ayerst(1965), Karon and Hillery (1949), and Pixton and War-burton (1971). Bealsey (1962) reported on the equilibriumisotherms for Virginia peanuts as determined by the

0021-8634/00/040401#08 $35.00/0 40

Notation

A, B, C constantsD mean relative percentage deviationdf

degree of freedom of regression modele standard error of the estimate value

M percent moisture content by dry basis(d.b.), %

N number of data pointsrh

equilibrium relative humidity, decimal¹ temperature, 3CY measured value by the model>@ predicted value by the model

EMC method for three temperatures. Their EMC valuesexceeded 20%.

Chen and Morey (1989b) re"ned and tested an ERHtechnique to collect rapidly and accurately the ERH dataof maize kernels. By precisely calibrating relativehumidity (RH) sensors, isotherms of grain and oil seeds atvaried temperatures can be measured within a short time.The technique is adopted for this paper.

Many EMC/ERH models have been proposed forcalculating the ERH/EMC values of agricultural prod-ucts and for successfully modelling crop processing. Inassessing the isotherm data for 18 cereal grains and seeds,

1 ( 2000 Silsoe Research Institute

402 C. CHEN

Chen and Morey (1989a) found that no universal equa-tion could be established to "t all isotherms. The modi-"ed Henderson equation (Thompson et al., 1968) andChung}Pfost equation (Chung & Pfost, 1967) aresatisfactory models for most starchy grains and "brousmaterials. The modi"ed Halsey equation (Iglesias& Chirief, 1976) is an adequate model for products hav-ing a high oil and protein content. The modi"ed Oswinequation (Chen, 1988) can function as a good model for&popcorn', maize cobs, peanut pods, and some varieties ofmaize and wheat.

Pfost et al. (1976) employed the EMC data reported byBealsey (1962) to study the model that expressed therelationships between the EMC, ERH and temperatureof peanuts. The modi"ed-Henderson and Chung}Pfostequations were selected as the best-"tting models. Alsothe parameters of the two equations were adopted as theASAE Standard (ASAE, 1996a). Colson and Young(1990) noted that the predicted value of the EMC by theEMC/ERH model signi"cantly in#uences the "tting abil-ity of the drying model for peanuts. Establishing anERH/EMC model is very important for the simulationpurposes.

The objectives for this study are: (1) to develop a rapidmethod to determine the EMC/ERH relationships forpeanut pods, hulls and kernels at temperatures between5 and 453C for relative humidities in the range of 10}95%RH; (2) to assess the "tting ability of four equations (i.e.the modi"ed Henderson, Chung}Pfost, modi"ed Halsey,and modi"ed Oswin equations) for describing accuratelythe ERH data for peanut pods, hulls and kernels; (3) todetermine the e!ectiveness of the experimental methodsin obtaining EMC/ERH data for peanuts; (4) to deter-mine the in#uence of peanut cultivar grown in Taiwan onthe EMC/ERH relationships; and (5) to compare thesorption isotherms of peanuts with previously publisheddata.

2. Theoretical analysis

The ERH step-by-step determination method canaccurately measure a large amount of EMC/ERH datawithin a short period. This technique, which can beextended to simultaneously collect the ERH data forpods, hulls and kernels, is described below.

The peanut pods are installed in closed containers. Theenvironment surrounding the peanuts within the con-tainers is an isolated and enclosed system. At the initialstage, the moisture content of pods, hulls and kernels arenot equal. The equilibrium relative humidity of the com-ponents di!ers from the RH value of ambient air withinthe containers. The temperature of peanuts is not thesame as the air. As the container is stored in a temper-

ature controlled chamber for a su$cient period, the sys-tem reaches the equilibrium states of heat and mass. Theequilibrium relative humidity and temperature of thepods, hulls and kernels have the same numerical valuewith the air in the container. The RH and temperature ofthe air in the closed system can be measured by thesensors. As the temperature of this control chamber isadjusted to the next setting, a new equilibrium state isreached. In addition, the ERH values of di!erent temper-atures for this system are recorded easily. The peanuts inthe container are then taken out and the moisture con-tents of hulls and kernels are individually measured. Themoisture content of peanut pods can be calculated fromthe moisture contents of the hulls and kernels and themass ratio of the two components. Notably, the moisturecontents are the EMC values for pods, hulls, and kernelsof peanuts with the same ERH values.

3. Experimental procedures

Three commercial varieties of peanuts, Tainan No.9,Tainan No. 11, and Tainan No.5, grown in Taichung,Taiwan, in 1990 were used in this study. Prior to theexperiment, portions of the peanut pods werestored under di!erent RH environments at 253C for threeweeks to reach the required moisture content values.Portions of the peanut pods were dried by heated air at503C. The adsorption samples, with pods at an initialmoisture content of about 2% d.b., were rewetted byadding water to a pre-determined moisture content. Fi-nally, all the samples were sealed in plastic bags andstored at 23C for 6 weeks to ensure the moisture equilib-rium state.

3.1. Equilibrium moisture content method

The EMC method, maintaining a constant RH envi-ronment by saturated salt solutions, was used to obtainthe adsorption EMC value at 253C. Table 1 lists thesaturated salt solutions and their standard RH values forthe measurement of EMC data. The moisture content ofeach sample was determined according to ASAE Stan-dard S410.1 DEC92 (ASAE, 1996b).

3.2. Equilibrium relative humidity method

The measuring technique employed in the ERHmethod resembled that used by Chen and Morey (1989b).Herein peanuts at a known moisture content wereplaced in a plastic container. The containers weresealed to ensure an airtight condition. As the experiment

Table 1The saturated salt solution and its standard relative humidityvalue for the measurement of equilibrium moisture content prop-

erties of peanuts at 253C

Standard relativeSalt solutions humidity, %

KOH 8)23LiCl 11)30CH

3COOK 22)51

MgCl2)6H

2O 32)78

K2CO

343)16

Mg(NO3)2)6H

2O 52)84

NaBr 57)57KI 68)86NaCl 75)29KBr 80)89KCl 83)34KNO

393)58

K2SO

497)30

Source: Greenspan (1977).

Table 2Experimental design

Experiment 1 Experiment 2

Purpose Determination Determination ofof sorption varietial e!ect onisotherm sorption isotherm

Sample Tainan No. 9 Tainan No. 9Tainan No. 11Tainan No. 5

Drying 25 50Temperature, 3C 50

Table 3Four models and criteria to analyse equilibrium moisture con-

tent/equilibrium relative humidity data of peanuts

Model or criteria Equation

1. Modi"ed}Henderson rh"1!exp(!A(¹#C)MB)

2. Chung}Pfostrh"exp C

!A

¹#Cexp(!BM)D

3. Modi"ed}Halsey rh"exp [exp[A#B¹]MC]

4. Modi"ed}Oswinrh"

1

(A#B¹/M)C#1

5. Mean relative percentagedeviation D

D"

100

N+

D>!>@D>

6. Standard error of theestimated value e

e"S+ (>!>@)2

df

A, B, C are constants; M is percent moisture content ona d.b. as a %; rh is equilibrium relative humidity as a decimal;¹ is temperature, 3C; > is the measured value; >@ is thevalue; predicted by the model; N is the number of datapoints; d

fis the degree of freedom of regression model.

SORPTION ISOTHERMS OF PEANUTS 403

commenced, those containers were placed in a temper-ature-controlled chamber that was maintained at 53C.When the thermal and mass system within the containersreached a state of equilibrium, the readings of RH andtemperature values for RH sensors were recorded. Thechamber temperature was then adjusted to the next tem-perature level.

The experimental design for this study is given inTable 2. All the ERH values were collected at "ve temper-atures (i.e. 5, 15, 25, 35, and 453C). Three replicates wereconducted for each test.

3.3. Relative humidity sensor

The Vaisala Humidity Transmitter (HMD 30 US) wasused in the tests. All sensors were calibrated in di!erentclosed containers with various types of salt solutions.These solutions were the same as listed in Table 1.Adequate calibration equations were established by thesame technique used by Chen et al. (1989c).

3.4. Data analysis

The EMC/ERH data for peanut pods, hulls andkernels were analysed using four equations (Table 3).A program &NONLIN' based on the least-squaresmethod with Hartley's (1961) modi"cation was written inQBASIC language to estimate the parameters and calcu-late the statistics. Two quantitative standards, mean rela-tive percentage deviation D and standard error of the

estimated value e, and the residual plots, were used toassess the curve-"tting agreement of four models.

4. Results and discussion

4.1. E+ect of experimental method

Figure 1 shows the adsorption EMC data for peanuthulls and kernels at 253C. Mould development was foundat a high relative humidity (RH'90%), emphasizing thelimitation of the EMC method in determining EMC datafor samples at a high moisture content.

Figure 1 displays the e!ect of experimental method onthe EMC/ERH data. According to the statistical F-test at

Fig. 1. Comparison of the isotherm data for two methods: , hullisotherm data by the equilibrium relative humidity (ERH) method;

, kernel isotherm data by the ERH method; , hull isotherm databy the equilibrium moisture content (EMC) method; , kernel

isotherm data by the EMC method

Fig. 2. Desorption isotherm data of peanut pods at three temper-atures: , 53C; , 253C; , 453C

404 C. CHEN

the 5% probability level, no signi"cant di!erence couldbe found between the two methods. The above resultscon"rm the accuracy of the ERH method.

At a lower humidity (RH(20%), the EMC values ofthe kernels were higher and the EMC of the hulls waslower than that of the pods (Fig. 1). As this "gure reveals,an increasing humidity caused higher EMC values of thehulls and a lower EMC value of the kernels than that ofthe pods. The results could be attributed to the composi-tion of samples. Similar results were found at all isothermtemperatures.

4.2. Desorption isotherm for peanuts

Figure 2 depicts the desorption data for peanut podsdried at 503C. This "gure indicates that temperaturesigni"cantly in#uenced the isotherm curve. The distribu-tion curve of data was nearly of the exponential type; itdid not have the S-shape as for the EMC/ERH propertiesof cereal grains. Table 4 lists the estimated parametersand comparative criteria for four models of desorptiondata. Only the modi"ed Oswin equation could functionas an adequate model for this product. The other threemodels had a high value for the coe$cient of determina-tion R2; however, the values for e exceeded 2)7% and theresidual plots all displayed a systematic pattern.

Figure 3 shows the desorption data for peanut kernels.This "gure also contains an exponential shape of curveand temperature e!ect. Table 4 presents a comparison ofthe curve-"tting agreement for the four models. Themodi"ed Henderson and Chung}Pfost equations had

fairly high values for the deviation and standard error.The residual plots were patterned, thereby making theequations inadequate. The modi"ed Halsey and modi"edOswin equations had uniformly distributed residualplots. The quantitative criteria of the modi"ed Halseyequation is preferable over that of the modi"ed Oswinequation, thereby making it the best equation for kernels.

The S-shaped curves were observed for desorptiondata of peanut hulls (Fig. 4). Table 4 lists the estimatedparameters and statistics for four models. The residualplots of the modi"ed Henderson and Chung}Pfost equa-tions had uniformly scattered points. In addition, theChung}Pfost equation provided smaller values for thedeviation and standard error.

Chen and Morey (1989a) analysed the desorption dataof peanuts obtained from the Bealsey's original data(1962) in terms of the curve-"tting agreement for fourERH models. According to the study, the adequate equa-tions for this product were as follows: the modi"ed Oswinequation for pods; the modi"ed Halsey and modi"edOswin equations for kernels; and the modi"ed Hender-son and Chung}Pfost equations for hulls. The studyagrees with the "nding of Chen and Morey (1989a).

4.3. Adsorption isotherm of peanuts

Table 5 presents a comparison of the four ERH modelsin terms of adsorption data for peanuts dried at 503C.The modi"ed Oswin equation was the only model thatcan be used for adsorption data of pods. Also, the modi-"ed Halsey equation can function as the "tting model forkernels. Three equations, i.e. the modi"ed Henderson,Chung}Pfost, and modi"ed Oswin, can accuratelydescribe the adsorption properties of the hulls.

Table 4Estimated parameters and comparison criteria for four equilibrium relative humidity models of desorption data for

peanuts

Modixed Chung} Modixed ModixedSample Parameters Henderson Pfost Oswin Halsey

Pods A 1)8433]10~4 648)395 6)8229 3)0005B 1)6636 0)27153 !1)7698]10~2 !5)9431]10~3C 141)512 135)55 2)5404 1)7809R2 0)985 0)989 0)995 0)989e 3)197 2)780 1)907 2)728D 5)958 4)783 3)520 6)33

Residual plot Systematic* Systematic Random Systematic

Kernels A 1)879]10~4 747)86 6)23297 3)23456B 1)7939 0)3246 !1)811]10~2 !5)8726]10~3C 125)4 133)3 2)7527 2)0032R2 0)972 0)979 0)991 0)995e 4)34 3)74 2)50 1)92D 9)10 7)10 6)31 3)62

Residual plot Systematic Systematic Random Random

Hulls A 1)3360]10~4 594)338 8)9856 3)3757B 1)5936 0)1904 !2)5211]10~2 !6)169]10~3C 142)51 140)96 2)3765 1)7257R2 0)992 0)994 0)985 0)968e 2)28 1)96 3)18 4)67D 4)57 3)67 6)70 10)3

Residual plot Random Random Systematic Systematic

*Systematic or random pattern; A, B, C, constants; R2, coe$cient of determination; D, mean relative percentage deviation;e, standard error of the estimated value.


4.4. Hysteresis e+ect

Figure 5 plots the hysteresis e!ect for peanut pods attwo temperatures according to the "tting models. As this"gure indicates, hysteresis existed over the entire RHrange. Major hysteresis occurred in the RH range of

Fig. 3. Desorption isotherm data of peanut kernels at three tem-peratures: , 53C; , 253C; , 453C

30}75%. The magnitude of the hysteresis decreased withan increasing isotherm temperature. Comparing thedesorption and adsorption data of kernels and hullsreveals that hysteresis existed over the full range of rela-tive humidity. Furthermore, the magnitude of the hyster-esis for isotherms at 53C exceeds that at 453C.

Fig. 4. Desorption isotherm data of peanut hulls at three temper-atures: , 53C; , 253C; , 453C

Table 5Estimated parameters and comparison criteria for four equilibrium relative humidity models of adsorption data for peanuts

Modixed Chung} Modixed ModixedSample Parameter Henderson Pfost Oswin Halsey

Pods A 2)651]10~4 531)70 6)6792 2)9420B 1)4503 0)239 !1)8208]10~2 !5)6004]10~3C 154)70 141)60 2)302 1)7725R2 0)964 0)966 0)986 0)992e 4)35 4)23 2)68 2)03D 7)84 7)68 5)24 4)00

Residual plot Systematic* Systematic Random Systematic

Kernels A 3)533]10~4 514)57 6)1971 2)8227B 1)408 0)258 !2)075]10~2 !5)4890]10~3C 143)26 141)42 2)2087 1)7998R2 0)948 0)949 0)907 0)989e 5)23 5)17 3)61 2)41D 9)88 9)71 7)00 4)62

Residual plot Systematic Systematic Systematic Random

Hulls A 1)8502]10~4 589)84 8)5250 3)3654B 1)5215 0)194 !2)3876]10~2 !5)687]10~3C 129)15 147)81 2)3947 1)769R2 0)984 0)988 0)991 0)984e 3)08 2)51 2)41 3)10D 4)96 4)40 3)16 5)02

Residual plot Random Random Random Systematic

*Systematic or random pattern; A, B, C, constants; R2, coe$cient of determination; D, mean relative percentage deviation;e, standard error of the estimated value.

406 C. CHEN

4.5. Comparison with published data

The EMC/ERH values of desorption isotherms at253C for pods from the present study dried at two di!er-ent temperatures (25 and 503C) are compared with data

Fig. 5. Hysteresis ewect for peanut pods at two temperatures:, desorption data at 53C; , adsorption data at 53C; , desorp-

tion data at 453C; , adsorption data at 453C

of Bealsey (1962) and of Karon and Hillery (1949). Dataobtained from Bealsey were close to those of Karon andHillery (Fig. 6). At the same RH values, the EMC of bothsets of data are higher than the isotherm for pods dried at503C, but lower than those of the present data dried at

Fig. 6. Comparison of desorption data of peanuts pods to pre-viously published data at 253C: , Bealsey+s data (1962); , pres-ent study dried at 253C; , present study dried at 503C; , Karon

and Hillery+s data (1949)

Fig. 7. Comparison of desorption data of peanut kernels to pre-viously published data at 253C: , Young+s data (1976); , Beal-sey+s data (1962); , Present study (dried by 253C); , Present

study (dried by 503C); , Karon and Hillery+s data (1949)

Fig. 9. Desorption isotherm data of peanut pods for threevarieties at 253C: , Tainan No. 9; , Tainan No. 11; , Tainan

No. 5


253C. The above results apparently suggest that varietyand drying temperature in#uence the EMC/ERH prop-erties of pods.

Figure 7 shows the desorption data of kernels driedwith two temperatures in this study and the data ofYoung (1976), of Bealsey (1962), and of Karon and Hill-ery (1949). At 60% RH, the deviation of moisture contentbetween this study sample dried at 253C and Bealsey'sdata is within 1)0%; however, large deviations existed inthe higher RH region.

Figure 8 presents the desorption data for hulls from"ve di!erent sources. Comparing the two sets of desorp-

Fig. 8. Comparison of desorption data of peanut hulls to previouslypublished data at 253C: , Young+s data (1976); , Beasley+s data(1962); , Present study (dried at 253C); , Present study (dried at

503C); , Karon and Hillery+s data (1949)

tion data for samples dried at 25 and 503C reveals thatdrying temperature signi"cantly in#uenced the sorptionproperties. Large deviations also arose between the dataof Young (1976), of Bealsey (1962), and of Karon andHillery (1949).

4.6. E+ect of species on equilibrium relative humidityproperties

Figure 9 depicts ERH data at 253C for three cultivarsof pods grown in Taiwan. The middle range of RHdenotes the major di!erences among three species. At the50% RH, the di!erence in moisture content betweenTainan No. 9 and Tainan No. 5 was nearly 2%. Figure 10

Fig. 10. Desorption isotherm data of peanut kernels for three varie-ties at 253C: , Tainan No. 9; , Tainan No. 11; , Tainan No. 5

408 C. CHEN

shows the desorption properties of kernels for three var-ieties at 253C. The largest deviations of moisture contentwere below 1% in the middle range of RH.

The discussion clearly implies that the grain variety,drying temperature and other factors in#uence theisotherms of peanuts. Whilst further experiments arerequired to investigate fully the ERH/EMC properties ofpeanuts, the present tests con"rm the suitability of theproposed technique for the rapid and accurate collectionof ERH data.

5. Conclusions

The equilibrium relative humidity and equilibriummoisture content methods for determining the sorptionisotherms are not signi"cantly di!erent. This study dem-onstrates that the equilibrium relative humidity modeland the estimated values of parameters accuratelydescribe the desorption and adsorption data for pods,kernels and hulls. The hysteresis e!ects for pods, hullsand kernels persist over the entire range. The magnitudeof hysteresis decreases with an increasing isotherm tem-perature. Various sources signi"cantly di!er in terms ofsorption data for pods and hulls. Kernels, with a high oilcontent, show less deviation among various sources. Spe-cies in#uence the sorption data for pods at the middlerange of relative humidity. The technique proposed herefacilitates the rapid and accurate collection of equilib-rium relative humidity data.

Acknowledgments

The study was "nancially supported by the NationalScience Council of the Republic of China under projectno. NSC79-0409-B055-12.

References

ASAE (1996a). Moisture relationships of plants-based agricul-tural products. ASAE Standards D245.5 OCT95. Agricul-tural Engineering Yearbook (43rd Edn.), pp 452}464. St.Joseph, MI

ASAE (1996b). Moisture measurement* peanuts. ASAE Stan-dards D410.1 DEC92. Agricultural Engineering Yearbook(43rd Edn.), pp 505}506. St. Joseph, MI

Ayerst G (1965). Determination of the water activity of some bygyroscopic food materials by a dew-point method. Journal ofthe Science of Food and Agriculture, 16(3), 71}78

Bealsey E O (1962). Moisture equilibrium of Viginia bunchpeanuts. MS thesis, North Carolina State University

Chen C (1988). A study of equilibrium relative humidityfor yellow-dent corn kernels. PhD thesis, The University ofMinnesota

Chen C; Morey R V (1989a). Comparison of four EMC/ERHequations. Transactions of the ASAE, 32(3), 983}990

Chen C; Morey R V (1989b). Equilibrium relative humidity(ERH) relationships for yellow-dent corn. Transactions of theASAE, 32(3), 999}1006

Chen C; Tsao C; Lai C (1989c). Statistical evaluation on theperformance of electric RH sensors for ERH measurements.ASAE paper. No. 89}6539

Chung D S; Pfost H B (1967). Adsorption and desorption ofwater vapor by cereal grains and their products. Part II.Development of the general isotherm equation. Transactionsof the ASAE, 10(4), 552}555

Colson K H; Young J H (1990). Two-component thin-layerdrying model for unshelled peanuts. Transactions of theASAE, 33(1), 241}246

Greenspan L (1977). Humidity "xed points of binarysaturated an aqueous solution. Journal of Research NationalBureau of Standard* A. Physical and Chemistry, 81(A),89}96

Hartley H O (1961). The modi"ed Gauss}Newton method forthe "tting on nonlinear regression functions by least squares.Technometrics, 3, 269}280

Iglesias H A; Chirief J (1976). Prediction of e!ect of temper-ature on water sorption isotherms of food materials. Journalof Food Technology, 11, 565}573

Karon M K; Hillery B E (1949). Hygroscopic equilibrium ofpeanuts. Journal of the American Oil Chemists' Society,16}19 January

Pfost H B; Mourer S G; Chung D S; Milliken G A (1976).Summarizing and reporting equilibrium moisture data forgrains. ASAE Paper No. 76}3520

Pixton S W; Warburton S J (1971). Moisture content/relativehumidity equilibrium at di!erent temperatures of some oil-seeds of economic importance. Journal of Stored ProductsResearch, 7, 261}269

Thompson T L; Peart P M; Forst G H (1968). Mathematicssimulation of corn drying * a new model. Transactions ofthe ASAE, 24(3), 582}586

Young J H (1976). Evaluation of models to describe sorptionand desorption equilibrium moisture content isotherms ofVirginia-type peanuts. Transactions of the ASAE, 19(1),146}150, 155

Documents

A Rapid Method to Determine the Sorption Isotherms of Peanuts