A Rapid Method to determine the Sorption Isotherms of Peanuts

J. agric. Engng Res. (2000) 75, 401}408 doi.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 relative humidities ranging from 10 to 95% for peanut pods, kernels and hulls were measured at temperatures ranging from 5 to 453C. No signi"cant di!erences were found between the experimental results of the equilibrium moisture content and equilibrium relative humidity methods. The curve-"tting agreement of the adsorption and desorption data for four equilibrium relative humidity models was evaluated. The best models and their estimated parameters were shown in this study.  2000 Silsoe Research Institute

1. Introduction Peanuts are an important oilseed crop in Taiwan. The cultivar grown by farmers is a Spanish variety. Prevailing weather conditions limit the harvesting periods. Peanuts must be harvested under high moisture conditions, then dried by hot air at 503C. In terms of designing the drying and storing systems, accurately predicting the relationships between equilibrium moisture content (EMC), equilibrium relative humidity (ERH), and temperature is of relevant concern. Peanuts are composed of a hull, kernels and some air enclosed between two components, thereby making their structure quite complex. Limited data on the sorption properties of peanuts has been reported in the literature. Two conventional methods for measuring EMC/ERH properties are the EMC and ERH methods. In the EMC method, samples are placed in an environment having a constant relative humidity and temperature. After a long period, the moisture of samples are measured and adopted as the EMC value. In the ERH method, the sample with a known moisture content is placed in a limited volume environment. The air conditions of temperature and relative humidity are then measured. Relatively few investigations have provided EMC data at "xed temperatures, among which include Ayerst (1965), Karon and Hillery (1949), and Pixton and Warburton (1971). Bealsey (1962) reported on the equilibrium isotherms for Virginia peanuts as determined by the 0021-8634/00/040401#08 $35.00/0

Notation A, B, C D d D e M

constants mean relative percentage deviation degree of freedom of regression model standard error of the estimate value percent moisture content by dry basis (d.b.), % N number of data points r equilibrium relative humidity, decimal F ¹ temperature, 3C Y measured value by the model > predicted value by the model

EMC method for three temperatures. Their EMC values exceeded 20%. Chen and Morey (1989b) re"ned and tested an ERH technique to collect rapidly and accurately the ERH data of maize kernels. By precisely calibrating relative humidity (RH) sensors, isotherms of grain and oil seeds at varied temperatures can be measured within a short time. The technique is adopted for this paper. Many EMC/ERH models have been proposed for calculating the ERH/EMC values of agricultural products and for successfully modelling crop processing. In assessing the isotherm data for 18 cereal grains and seeds,

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Chen and Morey (1989a) found that no universal equation could be established to "t all isotherms. The modi"ed Henderson equation (Thompson et al., 1968) and Chung}Pfost equation (Chung & Pfost, 1967) are satisfactory models for most starchy grains and "brous materials. The modi"ed Halsey equation (Iglesias & Chirief, 1976) is an adequate model for products having a high oil and protein content. The modi"ed Oswin equation (Chen, 1988) can function as a good model for &popcorn', maize cobs, peanut pods, and some varieties of maize and wheat. Pfost et al. (1976) employed the EMC data reported by Bealsey (1962) to study the model that expressed the relationships between the EMC, ERH and temperature of peanuts. The modi"ed-Henderson and Chung}Pfost equations were selected as the best-"tting models. Also the parameters of the two equations were adopted as the ASAE Standard (ASAE, 1996a). Colson and Young (1990) noted that the predicted value of the EMC by the EMC/ERH model signi"cantly in#uences the "tting ability of the drying model for peanuts. Establishing an ERH/EMC model is very important for the simulation purposes. The objectives for this study are: (1) to develop a rapid method to determine the EMC/ERH relationships for peanut pods, hulls and kernels at temperatures between 5 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 accurately the ERH data for peanut pods, hulls and kernels; (3) to determine the e!ectiveness of the experimental methods in obtaining EMC/ERH data for peanuts; (4) to determine the in#uence of peanut cultivar grown in Taiwan on the EMC/ERH relationships; and (5) to compare the sorption isotherms of peanuts with previously published data.

2. Theoretical analysis The ERH step-by-step determination method can accurately measure a large amount of EMC/ERH data within a short period. This technique, which can be extended to simultaneously collect the ERH data for pods, hulls and kernels, is described below. The peanut pods are installed in closed containers. The environment surrounding the peanuts within the containers is an isolated and enclosed system. At the initial stage, the moisture content of pods, hulls and kernels are not equal. The equilibrium relative humidity of the components di!ers from the RH value of ambient air within the containers. The temperature of peanuts is not the same as the air. As the container is stored in a temper-

ature controlled chamber for a su$cient period, the system reaches the equilibrium states of heat and mass. The equilibrium relative humidity and temperature of the pods, hulls and kernels have the same numerical value with the air in the container. The RH and temperature of the air in the closed system can be measured by the sensors. As the temperature of this control chamber is adjusted to the next setting, a new equilibrium state is reached. In addition, the ERH values of di!erent temperatures for this system are recorded easily. The peanuts in the container are then taken out and the moisture contents of hulls and kernels are individually measured. The moisture content of peanut pods can be calculated from the moisture contents of the hulls and kernels and the mass ratio of the two components. Notably, the moisture contents are the EMC values for pods, hulls, and kernels of 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 the experiment, portions of the peanut pods were stored under di!erent RH environments at 253C for three weeks to reach the required moisture content values. Portions of the peanut pods were dried by heated air at 503C. The adsorption samples, with pods at an initial moisture content of about 2% d.b., were rewetted by adding water to a pre-determined moisture content. Finally, all the samples were sealed in plastic bags and stored at 23C for 6 weeks to ensure the moisture equilibrium state.

3.1. Equilibrium moisture content method The EMC method, maintaining a constant RH environment by saturated salt solutions, was used to obtain the adsorption EMC value at 253C. Table 1 lists the saturated salt solutions and their standard RH values for the measurement of EMC data. The moisture content of each sample was determined according to ASAE Standard S410.1 DEC92 (ASAE, 1996b).

3.2. Equilibrium relative humidity method The measuring technique employed in the ERH method resembled that used by Chen and Morey (1989b). Herein peanuts at a known moisture content were placed in a plastic container. The containers were sealed to ensure an airtight condition. As the experiment

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Table 1 The saturated salt solution and its standard relative humidity value for the measurement of equilibrium moisture content properties of peanuts at 253C Salt solutions KOH LiCl CH COOK  )6H O MgCl  K CO   Mg(NO ) )6H O  NaBr   KI NaCl KBr KCl KNO K SO  

Table 2 Experimental design Experiment 1

Standard relative humidity, %

Purpose

8)23 11)30 22)51 32)78 43)16 52)84 57)57 68)86 75)29 80)89 83)34 93)58 97)30

Sample

Source: Greenspan (1977).

commenced, those containers were placed in a temperature-controlled chamber that was maintained at 53C. When the thermal and mass system within the containers reached a state of equilibrium, the readings of RH and temperature values for RH sensors were recorded. The chamber temperature was then adjusted to the next temperature level. The experimental design for this study is given in Table 2. All the ERH values were collected at "ve temperatures (i.e. 5, 15, 25, 35, and 453C). Three replicates were conducted for each test.

Experiment 2

Determination of sorption isotherm Tainan No. 9

Drying Temperature, 3C

Determination of varietial e!ect on sorption isotherm Tainan No. 9 Tainan No. 11 Tainan No. 5 50

25 50

estimated value e, and the residual plots, were used to assess 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 peanut hulls and kernels at 253C. Mould development was found at a high relative humidity (RH'90%), emphasizing the limitation of the EMC method in determining EMC data for samples at a high moisture content. Figure 1 displays the e!ect of experimental method on the EMC/ERH data. According to the statistical F-test at

Table 3 Four models and criteria to analyse equilibrium moisture content/equilibrium relative humidity data of peanuts Model or criteria

3.3. Relative humidity sensor

1. Modi"ed}Henderson

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

2. Chung}Pfost 3. Modi"ed}Halsey 4. Modi"ed}Oswin 5. Mean relative percentage deviation D

3.4. Data analysis The EMC/ERH data for peanut pods, hulls and kernels were analysed using four equations (Table 3). A program &NONLIN' based on the least-squares method with Hartley's (1961) modi"cation was written in QBASIC language to estimate the parameters and calculate the statistics. Two quantitative standards, mean relative percentage deviation D and standard error of the

6. Standard error of the estimated value e

Equation r "1!exp(!A(¹#C)M ) F !A r "exp exp(!BM) F ¹#C





r "exp [exp[A#B¹]M!] F 1 r" F (A#B¹/M)!#1 100 ">!>" D" N >



e"

(>!>) d D

A, B, C are constants; M is percent moisture content on a d.b. as a %; rh is equilibrium relative humidity as a decimal; ¹ is temperature, 3C; > is the measured value; > is the value; predicted by the model; N is the number of data points; d is the degree of freedom of regression model. D

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Fig. 1. Comparison of the isotherm data for two methods: , hull isotherm data by the equilibrium relative humidity (ERH) method; , kernel isotherm data by the ERH method; , hull isotherm data by the equilibrium moisture content (EMC) method; , kernel isotherm data by the EMC method

the 5% probability level, no signi"cant di!erence could be found between the two methods. The above results con"rm the accuracy of the ERH method. At a lower humidity (RH(20%), the EMC values of the kernels were higher and the EMC of the hulls was lower than that of the pods (Fig. 1). As this "gure reveals, an increasing humidity caused higher EMC values of the hulls and a lower EMC value of the kernels than that of the pods. The results could be attributed to the composition of samples. Similar results were found at all isotherm temperatures.

4.2. Desorption isotherm for peanuts Figure 2 depicts the desorption data for peanut pods dried at 503C. This "gure indicates that temperature signi"cantly in#uenced the isotherm curve. The distribution curve of data was nearly of the exponential type; it did not have the S-shape as for the EMC/ERH properties of cereal grains. Table 4 lists the estimated parameters and comparative criteria for four models of desorption data. Only the modi"ed Oswin equation could function as an adequate model for this product. The other three models had a high value for the coe$cient of determination R; however, the values for e exceeded 2)7% and the residual plots all displayed a systematic pattern. Figure 3 shows the desorption data for peanut kernels. This "gure also contains an exponential shape of curve and temperature e!ect. Table 4 presents a comparison of the curve-"tting agreement for the four models. The modi"ed Henderson and Chung}Pfost equations had

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

fairly high values for the deviation and standard error. The residual plots were patterned, thereby making the equations inadequate. The modi"ed Halsey and modi"ed Oswin equations had uniformly distributed residual plots. The quantitative criteria of the modi"ed Halsey equation is preferable over that of the modi"ed Oswin equation, thereby making it the best equation for kernels. The S-shaped curves were observed for desorption data of peanut hulls (Fig. 4). Table 4 lists the estimated parameters and statistics for four models. The residual plots of the modi"ed Henderson and Chung}Pfost equations had uniformly scattered points. In addition, the Chung}Pfost equation provided smaller values for the deviation and standard error. Chen and Morey (1989a) analysed the desorption data of peanuts obtained from the Bealsey's original data (1962) in terms of the curve-"tting agreement for four ERH models. According to the study, the adequate equations for this product were as follows: the modi"ed Oswin equation for pods; the modi"ed Halsey and modi"ed Oswin equations for kernels; and the modi"ed Henderson and Chung}Pfost equations for hulls. The study agrees with the "nding of Chen and Morey (1989a).

4.3. Adsorption isotherm of peanuts Table 5 presents a comparison of the four ERH models in terms of adsorption data for peanuts dried at 503C. The modi"ed Oswin equation was the only model that can be used for adsorption data of pods. Also, the modi"ed Halsey equation can function as the "tting model for kernels. Three equations, i.e. the modi"ed Henderson, Chung}Pfost, and modi"ed Oswin, can accurately describe the adsorption properties of the hulls.

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

Modixed Henderson

Chung} Pfost

Modixed Oswin

Modixed Halsey

Pods

A B C R e D Residual plot

1)8433;10\ 1)6636 141)512 0)985 3)197 5)958 Systematic*

648)395 0)27153 135)55 0)989 2)780 4)783 Systematic

6)8229 !1)7698;10\ 2)5404 0)995 1)907 3)520 Random

3)0005 !5)9431;10\ 1)7809 0)989 2)728 6)33 Systematic

Kernels

A B C R e D Residual plot

1)879;10\ 1)7939 125)4 0)972 4)34 9)10 Systematic

747)86 0)3246 133)3 0)979 3)74 7)10 Systematic

6)23297 !1)811;10\ 2)7527 0)991 2)50 6)31 Random

3)23456 !5)8726;10\ 2)0032 0)995 1)92 3)62 Random

Hulls

A B C R e D Residual plot

1)3360;10\ 1)5936 142)51 0)992 2)28 4)57 Random

594)338 0)1904 140)96 0)994 1)96 3)67 Random

8)9856 !2)5211;10\ 2)3765 0)985 3)18 6)70 Systematic

3)3757 !6)169;10\ 1)7257 0)968 4)67 10)3 Systematic

Sample

* Systematic or random pattern; A, B, C, constants; R, 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 at two temperatures according to the "tting models. As this "gure indicates, hysteresis existed over the entire RH range. Major hysteresis occurred in the RH range of

30}75%. The magnitude of the hysteresis decreased with an increasing isotherm temperature. Comparing the desorption and adsorption data of kernels and hulls reveals that hysteresis existed over the full range of relative humidity. Furthermore, the magnitude of the hysteresis for isotherms at 53C exceeds that at 453C.

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

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

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

Modixed Henderson

Chung} Pfost

Modixed Oswin

Modixed Halsey

Pods

A B C R e D Residual plot

2)651;10\ 1)4503 154)70 0)964 4)35 7)84 Systematic*

531)70 0)239 141)60 0)966 4)23 7)68 Systematic

6)6792 !1)8208;10\ 2)302 0)986 2)68 5)24 Random

2)9420 !5)6004;10\ 1)7725 0)992 2)03 4)00 Systematic

Kernels

A B C R e D Residual plot

3)533;10\ 1)408 143)26 0)948 5)23 9)88 Systematic

514)57 0)258 141)42 0)949 5)17 9)71 Systematic

6)1971 !2)075;10\ 2)2087 0)907 3)61 7)00 Systematic

2)8227 !5)4890;10\ 1)7998 0)989 2)41 4)62 Random

Hulls

A B C R e D Residual plot

1)8502;10\ 1)5215 129)15 0)984 3)08 4)96 Random

589)84 0)194 147)81 0)988 2)51 4)40 Random

8)5250 !2)3876;10\ 2)3947 0)991 2)41 3)16 Random

3)3654 !5)687;10\ 1)769 0)984 3)10 5)02 Systematic

Sample

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

4.5. Comparison with published data The EMC/ERH values of desorption isotherms at 253C for pods from the present study dried at two di!erent temperatures (25 and 503C) are compared with data

of Bealsey (1962) and of Karon and Hillery (1949). Data obtained from Bealsey were close to those of Karon and Hillery (Fig. 6). At the same RH values, the EMC of both sets of data are higher than the isotherm for pods dried at 503C, but lower than those of the present data dried at

Fig. 5. Hysteresis ewect for peanut pods at two temperatures: , desorption data at 53C; , adsorption data at 53C; , desorption data at 453C; , adsorption data at 453C

Fig. 6. Comparison of desorption data of peanuts pods to previously published data at 253C: , Bealsey+s data (1962); , present study dried at 253C; , present study dried at 503C; , Karon and Hillery+s data (1949)

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Fig. 7. Comparison of desorption data of peanut kernels to previously published data at 253C: , Young+s data (1976); , Bealsey+s data (1962); , Present study (dried by 253C); , Present study (dried by 503C); , Karon and Hillery+s data (1949)

253C. The above results apparently suggest that variety and drying temperature in#uence the EMC/ERH properties of pods. Figure 7 shows the desorption data of kernels dried with two temperatures in this study and the data of Young (1976), of Bealsey (1962), and of Karon and Hillery (1949). At 60% RH, the deviation of moisture content between this study sample dried at 253C and Bealsey's data is within 1)0%; however, large deviations existed in the 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 previously published 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)

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Fig. 9. Desorption isotherm data of peanut pods for three varieties at 253C: , Tainan No. 9; , Tainan No. 11; , Tainan No. 5

tion data for samples dried at 25 and 503C reveals that drying temperature signi"cantly in#uenced the sorption properties. Large deviations also arose between the data of Young (1976), of Bealsey (1962), and of Karon and Hillery (1949).

4.6. E+ect of species on equilibrium relative humidity properties Figure 9 depicts ERH data at 253C for three cultivars of pods grown in Taiwan. The middle range of RH denotes the major di!erences among three species. At the 50% RH, the di!erence in moisture content between Tainan No. 9 and Tainan No. 5 was nearly 2%. Figure 10

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

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shows the desorption properties of kernels for three varieties at 253C. The largest deviations of moisture content were below 1% in the middle range of RH. The discussion clearly implies that the grain variety, drying temperature and other factors in#uence the isotherms of peanuts. Whilst further experiments are required to investigate fully the ERH/EMC properties of peanuts, the present tests con"rm the suitability of the proposed technique for the rapid and accurate collection of ERH data. 5. Conclusions The equilibrium relative humidity and equilibrium moisture content methods for determining the sorption isotherms are not signi"cantly di!erent. This study demonstrates that the equilibrium relative humidity model and the estimated values of parameters accurately describe the desorption and adsorption data for pods, kernels and hulls. The hysteresis e!ects for pods, hulls and kernels persist over the entire range. The magnitude of hysteresis decreases with an increasing isotherm temperature. Various sources signi"cantly di!er in terms of sorption data for pods and hulls. Kernels, with a high oil content, show less deviation among various sources. Species in#uence the sorption data for pods at the middle range of relative humidity. The technique proposed here facilitates the rapid and accurate collection of equilibrium relative humidity data.

Acknowledgments The study was "nancially supported by the National Science Council of the Republic of China under project no. NSC79-0409-B055-12.

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ASAE (1996b). Moisture measurement * peanuts. ASAE Standards D410.1 DEC92. Agricultural Engineering Yearbook (43rd Edn.), pp 505}506. St. Joseph, MI Ayerst G (1965). Determination of the water activity of some by gyroscopic food materials by a dew-point method. Journal of the Science of Food and Agriculture, 16(3), 71}78 Bealsey E O (1962). Moisture equilibrium of Viginia bunch peanuts. MS thesis, North Carolina State University Chen C (1988). A study of equilibrium relative humidity for yellow-dent corn kernels. PhD thesis, The University of Minnesota Chen C; Morey R V (1989a). Comparison of four EMC/ERH equations. 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 the ASAE, 32(3), 999}1006 Chen C; Tsao C; Lai C (1989c). Statistical evaluation on the performance of electric RH sensors for ERH measurements. ASAE paper. No. 89}6539 Chung D S; Pfost H B (1967). Adsorption and desorption of water vapor by cereal grains and their products. Part II. Development of the general isotherm equation. Transactions of the ASAE, 10(4), 552}555 Colson K H; Young J H (1990). Two-component thin-layer drying model for unshelled peanuts. Transactions of the ASAE, 33(1), 241}246 Greenspan L (1977). Humidity "xed points of binary saturated an aqueous solution. Journal of Research National Bureau of Standard* A. Physical and Chemistry, 81(A), 89}96 Hartley H O (1961). The modi"ed Gauss}Newton method for the "tting on nonlinear regression functions by least squares. Technometrics, 3, 269}280 Iglesias H A; Chirief J (1976). Prediction of e!ect of temperature on water sorption isotherms of food materials. Journal of Food Technology, 11, 565}573 Karon M K; Hillery B E (1949). Hygroscopic equilibrium of peanuts. 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 for grains. ASAE Paper No. 76}3520 Pixton S W; Warburton S J (1971). Moisture content/relative humidity equilibrium at di!erent temperatures of some oilseeds of economic importance. Journal of Stored Products Research, 7, 261}269 Thompson T L; Peart P M; Forst G H (1968). Mathematics simulation of corn drying * a new model. Transactions of the ASAE, 24(3), 582}586 Young J H (1976). Evaluation of models to describe sorption and desorption equilibrium moisture content isotherms of Virginia-type peanuts. Transactions of the ASAE, 19(1), 146}150, 155