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Journal of Food Engineering 79 (2007) 828–833

Drying characteristics of unshelled kernels of candle nuts

Elieser Tarigan a,b, Gumpon Prateepchaikul b, Ram Yamsaengsung c,Anchalee Sirichote d, Perapong Tekasakul b,*

a The Joint Graduate School of Energy and Environment (JGSEE), King Mongkut’s University of Technology Thonburi, Bangkok, 10140, Thailandb Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand

c Department of Chemical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailandd Department of Food Technology, Faculty of Agro-Industry, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand

Received 24 August 2005; accepted 24 February 2006Available online 21 April 2006

Abstract

Candle nuts are cultivated in approximately 170000 ha. in Indonesia, and yield around 140000 tons of fruit per year. Prior to remov-ing the shells, drying is an important process in the post-harvesting practice of candle nuts to obtain kernels as the final products. Thispaper gives a brief description of the efforts to obtain the basic drying characteristics of candle nuts, which is a pre-requisite for dryerdesign. The measurements were taken for unshelled kernels of candle nuts, for both fresh and stored samples, at temperatures of 40, 50,and 60 �C and drying air velocity of 0.67 m/s and 1 m/s using a forced convection dryer. The total drying time was substantially reducedwith an increase in temperature, and the effect of drying air velocity was relatively small. Page’s model adequately describes the dryingbehavior for the range of temperatures measured. The dependence of the drying constant on air temperature can be described by theArrhenius model.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Candle nuts; Unshelled kernels; Moisture content; Drying rate

1. Introduction

Candle nut (Aleurites moluccana) is one of the importantmultipurpose trees of the world. It is indigenous to Indo-Malaysia, and today is widespread throughout the tropics(Elevitch & Manner, 2004). Candle nuts, locally calledkemiri in Indonesia, have been utilized for various pur-poses: the lipid-rich seeds provide material for illumina-tion, cooking, pharmaceuticals, and the trunks providetimber. Candle nuts are cultivated in approximately170000 ha. in Indonesia, and yielded around 140000 tonsof fruit in the year 2003. The production has been for bothlocal consumption and export. Additionally, candle nutstrees have been planted as a part of the rehabilitation offorest areas in some regions of Indonesia (Koji, 2002).

0260-8774/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2006.02.048

* Corresponding author.Tel.: +66 74 287035; fax: +66 74 212893.E-mail address: perapong.t@psu.ac.th (P. Tekasakul).

The physical features of candle nuts have been explainedin a previous study by Tarigan, Prateepchaikul, Yamsaeng-sung, Sirichote, and Tekasakul (2006). A candle nut con-sists of a kernel covered with a shell and an outer bulkskin. The outer bulk skin of a mature nut is pliable andcan be easily removed. In Indonesia, harvesting of candlenuts includes collecting the nuts which have dropped fromtrees and removing the outer bulk skin, thus leaving theunshelled kernels. Candle nuts can be stored either in theform of shelled or unshelled kernels. Drying is an impor-tant process prior to removing the shells to obtain kernelsas the final products. The removal of the shells can only bedone for the dry product.

Knowledge of the drying kinetics of biological materialsis essential to the design, optimization, and control of thedrying process (Mohamed et al., 2005). Several studieshave been conducted to investigate the drying behaviorof different kinds of nuts, such as peanuts (Chen, 2000),

Fig. 1. Schematic diagram of dryer.

E. Tarigan et al. / Journal of Food Engineering 79 (2007) 828–833 829

pistachio nuts (Yanniotis & Zarmboutis, 1996), Turkishhazelnuts (Ozilgen & Ozdemir, 2001), and Macadamia nuts(Palipane & Driscoll, 1994; Wang, Kanoh, Kaneko, Lu, &Duong, 2002). However, very little data on the dryingbehavior of candle nuts is available for the engineeringdesign of drying. In a previous work, Tarigan et al.(2006) reported the experimental results on sorption iso-therms of candle nuts for both unshelled and shelled ker-nels at temperatures of 30, 40, 50, and 60 �C. Thereported equilibrium moisture content of unshelled kernelsamples is higher than shelled kernel samples at any giventemperature and relative humidity. In both unshelled andshelled kernel samples, hysteresis was pronounced andwas reduced as the temperature increased, and the isostericheat of desorption was higher than the isosteric heat ofadsorption. The study did not involve the candle nuts dry-ing characteristics, such as drying time and effect of dryingair temperature and velocity.

The main objective of this work was to study dryingcharacteristics of candle nuts, so that post-harvest operat-ing conditions could be established. The measurementswere taken for unshelled kernels. Drying must be a partof the unshelled kernel of post-harvesting processes, andthis is a form in which the product can be stored. In thisstudy, temperatures of 40, 50, and 60 �C were chosen. Thistemperature range represents the preliminary stage of studyof candle nuts drying in a solar dryer working with an aux-iliary heating system.

1.1. Traditional and alternate drying methods

After the harvest, the unshelled fruits of candle nuts arecommonly first dried by sun drying for 4–5 days (for sunnydays), then the shells are cracked to obtain the kernels.Cracking of the shells is usually done by farmers using asmall ladle-like tool made of rattan or wood. A candlenut is placed on the tool and struck against a large stoneto crack the shell. The product is sold under the name can-dle nuts or kemiri is the shelled kernels. After the nuts areextracted, the remaining shells can be directly used as fuel.

During the sun drying process, the dry level is com-monly assessed by feeling with the hand. This method doesnot provide significant information about the moisturecontent of the fruit. The cracking process is very sensitiveto the dry level. If they are not dry enough when cracked,the kernels will be broken. On the other hand, some dete-rioration in the obtained kernels is commonly found inthe over-dried products. These is associated with poorquality of the products.

Sun drying is not considered fully appropriate for dryingcandle nuts, especially during high product marketdemand, and during poor weather conditions. Solar dryingwith a back-up system seems to be a good alternative forcandle nuts. To design any new dryer system, basic dryingparameters of the products to be dried, such as the dryingrate under different conditions must be known. Such basicinformation is not available for many kinds of products

such as candle nuts. This paper gives a detailed accountof efforts made to determine this information.

2. Materials and method

The experiments were conducted for both fresh andstored unshelled candle nuts. Fresh product refers to theproduct that is recently separated from its outer bulk skin.Stored product sample refers to the unshelled kernel sam-ple that was separated from its outer bulk skin, pre- sundried by sun drying for about 2 days, and stored at roomtemperature about for 1 month before the experiments.The measurements were taken using a forced convectiondryer. The measured parameters were drying time, temper-ature, drying air velocity, and mass of dried products. Mea-surements were taken at temperatures of 40, 50, and 60 �Cand drying air velocity of 0.67 m/s and 1 m/s. The dryerwas installed in an environment with a relative humidityof about 90–95%, and the ambient temperature of about27–40 �C.

The dryer consists of two fans, heater block, dryingchamber, and control and measurements unit. The sche-matic diagram of the dryer is shown in Fig. 1. The dryingchamber of 80 · 40 · 40 cm3 was made from galvanizedsheet metal of 1.5 mm thickness. The drying chamber hasa door at the front for insertion and removal of the sample.The drying chamber, heater block, and air tunneling werecovered with 30 mm rock wool and aluminum foil to min-imize heat during the experiments. Air supplied by the fanswas heated to the required temperature in the air heaterblock with a 3000 W electrical heater. The air velocityand temperature were controlled with an assembled electri-cal controller and measurements unit. The air velocity wasdirectly measured in the drying chamber using a hot-wireanemometer as a sensor with a measurement range of 0–5 m/s. The temperature and relative humidity in the dryingchamber were measured by a themocouple and Rense-HT733M06 relative humidity sensor, respectively.

In the drying chamber, the dried samples were placed ina cubic basket made from large-mesh metal in which the

830 E. Tarigan et al. / Journal of Food Engineering 79 (2007) 828–833

products can be stacked so they are evenly dried. Theexperiments were considered as thin layer drying. About300 g of candle nuts was tested for each experiment. Thedried samples weights were recorded on a scale/balancehaving an accuracy of 0.01 g. Weighing was made at leastonce every hour during the first 5 h of drying test, and atleast once every 3 h afterwards. Removing, weighing, andreplacing the sample took about 2 min. The moisture con-tents of the samples were measured by air-oven at a fixedtemperature of 100 �C for 72 h until the mass did notchange (Hall, 1980). At least three replicates of each exper-iment were made.

2.1. Data analysis

The arithmetic mean of product samples moisture con-tent from the experimental replicates was determined andplotted versus drying time. For each experimental point,the standard deviation (S) was determined by

S ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPNi¼1ðMi �MÞ

N � 1

s; ð1Þ

where Mi;M and N are moisture content at a certain point,mean value of moisture content from replicates, and num-ber of replicates, respectively. In addition to S, the relative% deviation (D) between the mean and individual valuewas determined by

D ¼ 100

N

XN

i¼1

Mi �M�� ��

M

!. ð2Þ

The description of the drying kinetics experimental datawas verified according to the following three models (Baba-lis, Papanicolaou, Kyriakis, & Belessiotis, 2006; Kaleemul-lah & Kailappan, 2006; Sacilik, Elicin, & Unal, 2006):

Page’s model

MR ¼ expð�k � tnÞ; ð3ÞLogarithmic model

MR ¼ a � expð�k � tÞ þ c; ð4ÞTwo-term exponential model

MR ¼ a � expð�k � tÞ þ b � expð�k0 � tÞ; ð5Þwhere MR is dimensionless moisture ratio; t is drying time(h); k and k0 are drying constants (1/h); a, b, and n aredimensionless constants. The moisture ratio is given as

MR ¼M �M eq

M0 �M eq

; ð6Þ

where M, M0, Meq are the moisture content at any time, theinitial moisture content, and equilibrium moisture contentin % d.b., respectively. The values of Meq were obtainedfrom a previous study by Tarigan et al. (2006).

The goodness of fit for fitting of the three models wasevaluated with the correlation coefficient (r2) and the meanrelative deviation (MRD), which is defined as

MRD ¼ 1

m

Xm

i¼1

MR;i � MR;i

�� ��MR; i

!; ð7Þ

where MR,i is the measured value data, MR;i is the esti-mated value through fitting of the model, and m is thenumber of experimental points.

The dependence of the drying constant on air tempera-ture, calculated from the best fit of the there models above,was analyzed using the Arrhenius model, which is given as(Sogi, Shivhare, Garg, & Bawa, 2003)

k ¼ A � expð�B=T Þ; ð8Þ

where T is the absolute temperature (K); A and B arecoefficients.

3. Results and discussion

From the experimental data, we observed a small vari-ance for measured moisture contents (replicates), wherethe average standard deviation (S) were equal to 0.16 and0.23 for stored and fresh samples, respectively. The averageof relative % deviation (D) were 1.5% and 2% for storedand fresh samples, respectively. For most cases, the higherdeviation was observed at lower moisture contents.

Measurement results indicated that the initial moisturecontent, in dry basis (d.b.), of fresh samples was 16.6%,and the initial moisture content of stored samples was8.3%. This implies that after the harvest, pre-sun drying,and one month storing at room temperature and relativehumidity, the candle nuts moisture content remain thesame as half of fresh condition. Another separate measure-ment for the samples that were stored without pre-sun dry-ing indicated that it almost reached equilibrium moisturecontent of 10% after one month of storing.

The most suitable of level of dryness for cracking pro-cess was 4–6%. The observation showed that the crackingprocess can only be done if the dryness level of the samplesis lower than 6%, otherwise the kernel broke or crackedduring the shells cracking. Broken or cracked nuts indi-cates bad quality of the final product. However, some dete-rioration in the obtained kernels was found in the sampleswith the level of dryness lower than 3%.

The moisture contents of samples as a function of dryingtime are presented in Figs. 2 and 3 for stored and freshsamples, respectively, at drying air velocity of 1 m/s. Thefigures also show the standard deviation bars for eachexperimental data. The periods of drying that can be iden-tified were mostly falling rate periods. This indicates thatdiffusion-controlled process in which the rate of moistureremoval is limited by diffusion of moisture from inside tothe surface of the product.

The increase of drying air velocity for both stored andfresh samples from 0.67 to 1 m/s has a relatively smallinfluence to decrease drying time. Fig. 4 shows the effectof drying air velocity between 0.67 m/s and 1 m/s to thedrying time at drying temperatures of 50 �C for the stored

Fig. 2. The drying experimental data with the standard deviation bars forthe stored candle nuts at drying air velocity of 1 m/s.

Fig. 3. The drying experimental data with the standard deviation bars forthe fresh candle nuts at drying air velocity of 1 m/s.

Fig. 4. The experimental data (with the standard deviation bars) on theeffect of drying air velocity on the drying time for stored and freshunshelled candle nuts at temperature of 50 �C.

Table 1Parameters of the three drying models for drying kinetics of storedunshelled candle nuts

Model Model parameters/goodness of fit

Drying temperature, �C

40 50 60

Page k 0.1461 0.1839 0.2786n 0.5956 0.7165 0.7172r2 0.9965 0.9989 0.9937MRD 0.01955 0.0092 0.0301

Logarithmic a 0.6344 0.7452 0.7443k 0.3020 0.2195 0.2538c 0.0927 0.1608 0.3349r2 0.9808 0.9916 0.9932MRD 0.0457 0.0322 0.0381

Two-term a 0.7173 1.6940 0.6685k 0.2764 �0.7708 0.3312b 0.0261 0.0907 0.3712k0 0.3835 0.0918 0.0252r2 0.9964 0.9756 0.9996MRD 0.0196 0.0571 0.0370

r2 = correlation coefficient; MRD = mean relative deviation.

E. Tarigan et al. / Journal of Food Engineering 79 (2007) 828–833 831

and fresh samples. The figure also shows the standard devi-ation bars for each experimental data.

The effect of drying temperature is significant to the dry-ing time for both stored and fresh samples, and total dryingtime was substantially reduced with an increase in temper-ature. When the drying temperature was 40 �C, more than25 h was required for stored samples to reach moisturecontent of 6%, and for fresh samples more than 33 h wasrequired. This agrees with sun drying of the nuts by thefarmers which usually takes approximately 3–4 days ofdrying before the cracking process. When the drying tem-perature was 50 �C, the drying time to reach a moisturecontent of 6% was about 6.5 h for stored samples, andfor fresh samples about 21 h. The drying time was signifi-cantly shortened when drying temperature was 60 �C, andit only took 4 h and 7.5 h for stored and fresh samples,

respectively, to reach a moisture content of 6%. For alltemperatures tested, the predominant direct effect of airtemperature on the drying rate becomes less importantafter reaching approximately 3% of moisture content.From the experiment, the maximum drying temperatureof 60 �C for candle nuts is recommended. When the sam-ples were dried at a temperature higher than 70 �C somedeterioration was observed in the final products.

The moisture ratio, MR, of candle nuts at drying airtemperatures of 40, 50, and 60 were fitted in three models:Page’s model (Eq. (3)), Logaritmic model (Eq. (4)), andTwo-term exponential model (Eq. (5)). The estimatedvalues of the parameters of the models are listed in Tables1 and 2 for stored and fresh samples, respectively. From

Table 2Parameters of the three drying models for drying kinetics of freshunshelled candle nuts

Model Model parameters/goodness of fit

Drying temperature, �C

40 50 60

Page k 0.0871 0.1206 0.2173n 0.7603 0.7437 0.9270r2 0.9959 0.9988 0.9981MRD 0.0262 0.0113 0.0264

Logarithmic a 0.7996 0.7056 0.9471c 0.1481 0.2671 0.0437k 0.0534 0.1108 0.2093r2 0.9916 0.9950 0.9973MRD 0.0291 0.0213 0.0301

Two-term a 0.8873 0.8267 15.010b 0.1121 0.1668 �43.990k 0.0356 0.0474 0.2395k0 0.8926 0.4683 0.2404r2 0.9980 0.9980 0.9911MRD 0.0265 0.0134 0.04731

r2 = correlation coefficient; MRD = mean relative deviation.

Fig. 6. Comparison of experimental (with the standard deviation bars)and predicted moisture ratios of fresh candle nuts by fitting of Page’smodel.

832 E. Tarigan et al. / Journal of Food Engineering 79 (2007) 828–833

the values of coefficient of determination (r2) and meanrelative deviation (MRD), calculated from Eq. (7), it isclear that the Page’s model has the best fit in predictingthe moisture ratio of both stored and fresh samples candlenuts during thin layer drying. The values of correlationcoefficient of fitting to Page’s model were always greaterthan 0.99, and in most cases, the mean relative deviationswere less than 0.03. Comparisons of the experimentaland predicted moisture ratios obtained using Page’s modelare shown in Figs. 5 and 6 for stored and fresh samples,respectively. The standard deviation bars for eachexperimental data for the moisture ratio are shown in thefigures.

Fig. 5. Comparison of experimental data (with the standard deviationbars) and predicted moisture ratios of stored candle nuts by fitting ofPage’s model.

For both stored and fresh samples, the drying constant,k, which is a measure of drying rate, increased with temper-ature. The dependence of the drying constant on air tem-perature can be described by the Arrhenius model (Eq.(8)). Relationships between the parameters of the modeland drying air temperature calculated based on Page’smodel were found to be for:

Stored samples:

k ¼ 1:106 � 106 � expð�5149=T Þ ðr2 ¼ 0:9760Þ; ð9Þ

Fresh samples:

k ¼ 1:125 � 104 � expð�3539=T Þ ðr2 ¼ 0:9800Þ ð10Þ

4. Conclusion

The most suitable of level of dryness for the crackingprocess for unshelled candle nuts was found to be about4–5%. The effect of drying temperature is significant onthe drying time. The maximum drying temperature for can-dle nuts is recommended to be 60 �C. When drying temper-ature was 60 �C, it took about 4 h for stored samples and7.5 h fresh samples to reach a moisture content of 6%(d.b.). The influence of drying air velocity was relativelysmall. Page’s model adequately describes the drying behav-ior for the range of temperatures measured. The dryingconstant increased with temperature and its dependencefollows the Arrhenius model.

Acknowledgements

This research has been financially supported by the JointGraduate School of Energy and Environment (JGSEE),King Mongkut’s University of Technology Thonburi,Bangkok, Thailand.

E. Tarigan et al. / Journal of Food Engineering 79 (2007) 828–833 833

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