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LWT - Food Science and Technology 44 (2011) 1946e1951

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LWT - Food Science and Technology

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Response surface optimization and characteristics of rambutan (Nepheliumlappaceum L.) kernel fat by hexane extraction

Wanrada Sirisompong, Wannee Jirapakkul, Utai Klinkesorn*

Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, 50 Ngam Wong Wan Road, Chatuchak, Bangkok 10900, Thailand

a r t i c l e i n f o

Article history:Received 12 October 2010Received in revised form7 April 2011Accepted 26 April 2011

Keywords:Rambutan kernelFat extractionResponse surface methodologyFat characteristics

* Corresponding author. Tel.: þ662 562 5031; fax:E-mail address: utai.k@ku.ac.th (U. Klinkesorn).

0023-6438/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.lwt.2011.04.011

a b s t r a c t

Response surface methodology (RSM) was used to study the effect of moisture content (1.59e18.41 g/100 g), extraction time (2.3e10.7 h) and particle size (0.09e2.11 mm) on the fat yield from rambutankernels using hexane extraction. The physical and chemical characteristics of rambutan fat were alsodetermined. The optimum conditions obtained from response surface analysis was 4.99 g/100 g mois-ture, 1.05 mm particle size and 9.2 h extraction time. Under these optimum conditions, the maximum fatyield was 37.35 g/100 g. The extracted fat was a white solid at room temperature. The physical andchemical characteristics of the extracted fat compared well with those of conventional fats. The highlevel of arachidic acid (w 34.3 g/100 g fat) and low iodine value in rambutan kernel fat permits the use ofthe fat, especially where oxidation may be a concern, without its being subjected to hydrogenation.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Rambutan (Nephelium lappaceum Linn.) is a seasonal fruit nativeto west Malaysia and Sumatra. It is cultivated widely in SoutheastAsian countries and Thailand has become the leading producer(0.6e0.7 million tons a year) and exporter (w 10e15million dollarsUS) of the fruit (Salakpetch, 2000). This fruit is generally consumedfresh, although in the main producing countries like Malaysia andThailand it is industrially processed to obtain juice, jams, jellies andmarmalades (Morton, 1987). In addition, rambutan fruits are alsoprocessed as rambutan stuffed with a chunk of pineapple andcanned in syrup. The rambutan fruits are deseeded during pro-cessing and these seeds (w 4e9 g/100 g) are a waste by-productof the canning industry (Tindall, 1994). Some studies have re-ported that rambutan seed possesses a relatively high amount of fatwith values between 14 g/100 g and 41 g/100 g (Augustin & Chua,1988; Kalayasiri, Jeyashoke, & Krisnangkura, 1996; Morton, 1987;Solís-Fuentes, Camey-Ortíz, Hernández-Medel, Pérez-Mendoza, &Durán-de-Bazúa, 2010; Winayanuwattikun et al., 2008), and theincreasing demand for oils and fats, whether for humanconsumption or for industrial purposes, necessitates the search fornew sources of novel oils and fats. Therefore, the extracted fat fromrambutan seed not only could be used for manufacturing candles,soaps, and fuels, it also has a potential to be a source of naturaledible fat with possible industrial use.

þ662 562 5021.

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The main process for the separation and recovery of oils and fatsfrom seeds is solvent extraction. The fat yield from the seedsdepends on the nature of the solvent, the extraction temperatureand time, seed particle size and pretreatment conditions (Becker,1978). Recently, a little information has become available onrambutan fat extraction (Azam, Waris, & Nahar, 2005; Solís-Fuentes, Camey-Ortíz, Hernández-Medel Mdel, Pérez-Mendoza, &Durán-de-Bazúa, 2010). However, no work has been reported onthe effect of moisture content, extraction time and particle size onthe extraction efficiency of fat from rambutan seed kernels. In orderto study the effects of independent variables on fat extraction,central composite design (CCD) with response surface method-ology (RSM) is often used (Mani, Jaya, & Vadivambal, 2007; Shao,Sun, & Ying, 2008; Tan, Jinap, Edikusnadi, & Hamid, 2008; Wei,Liao, Zhang, Liu, & Jiang, 2009). RSM has increasingly been usedfor optimizing purposes due to its efficiency and lower datarequirements. This experimental methodology consists of a groupof mathematical and statistical procedures that can be used tostudy the relationships between one or more responses anda number of independent variables. It also defines the effect of theindependent variables, alone or in combination, in the process.Beside analysis the effects of independent variables, RSM createsa mathematical model that accurately describes the overall process(Hu, 1999; Montgomery; 2001). The main objective of the presentstudy was to determine and explain the effects of moisture content,extraction time and particle size on the yield of rambutan kernel fatusing hexane extraction. A model equation that would predictand determine the optimum conditions for total fat yield was

W. Sirisompong et al. / LWT - Food Science and Technology 44 (2011) 1946e1951 1947

developed, and as well, the physical and chemical characteristics ofthe extracted fat were also determined.

2. Materials and methods

2.1. Materials

Rambutan seeds (N. lappaceum L.) were obtained fromUniversalFood Public Company Limited (Nakornpathom, Thailand). Theseeds were washed and the kernels were removed manually fromthe seeds. The kernels were then ground and dried using a traydryer (Kan Seng Lee Machinery (1960) Ltd. Part, Thailand) at 55 �Cfor 5 h (w 5 g/100 g moisture). The prepared kernels were storedat�5 �C until used for experiments. The selected extraction solventwas hexane which was purchased from Mallinckrodt Baker, Inc.(Phillipsburg, NJ). All other chemicals were reagent grade or higher.

2.2. Sample preparation and extraction process

Based on preliminary experimental results, the chosen inde-pendent variables were moisture content, extraction time andparticle size. The moisture content of ground kernels was adjustedby slowly adding sprayed water while the ground kernels werecontinuously mixed in an Imarflex mixer (IF 309, Thailand). Thesesamples were left sealed overnight at 4 �C to reach equilibrium(Sandoval & Barreiro, 2007). The moisture content of the rambutankernel samples was determined in triplicate according to the airovenmethod AOAC 931.04 (AOAC, 2000). The ground particles wereseparately by size using an AS 200 Control sieve shaker (Retsch,Germany) and different particle sizes ranging from 0.09 to 2.11 mmwere obtained. The particles sizes based on sieve numbers used foranalysis were: 10 (2.0e2.36 mm), 12 (1.7e2.0 mm), 18(1.0e1.8 mm), 35 (0.5e0.6 mm) and 170 (0.09e1.1 mm). A lab scaleSoxhlet apparatus was used to extract fat from the rambutankernels. About 20 g of ground and dried kernels was used for eachcombination of process parameters. The amount of solvent used forfat extraction was 150 ml for each sample. The extraction temper-ature was approximately 65 �C. The extracted fat was expressed asa percentage, which is defined as the weight of the extracted fatover the dry weight of the sample.

2.3. Characterization of rambutan kernel fat

2.3.1. Color, refractive index and crystal polymorphismThe color of the extracted fat was measured using a Miniscan XE

(Hunter Association Laboratory Inc., USA) according to themodifiedmethod of Cheikh-Rouhou et al. (2007). The refractive index ofkernel fat was determined using an instrument from Atago Co. Ltd.Series No.11506, Japan. The crystal polymorphism of the fat samplewas analyzed using an X-ray diffractometer (JEOL JDX-3530, Japan)and the modified method of Reshma, Saritha, Balachandran, andArumughan (2008).

2.3.2. Iodine value, sponification number and unsponificationmatter

The iodine value of fat samples was determined according toAOCS Cd 1d-92 (AOCS, 1997). The sponification number andunsponification matter of rambutan fat samples were measuredaccording to AOCS Cd 3e25 and AOCS Ca 6a-40, respectively.

2.4. Analysis of fatty acid composition

The fatty acids were hydrolyzed from fat and thenmethylated tofatty acid methyl esters (FAMEs). The fatty acid composition of fatwas investigated using gas chromatography (Model 6890N

G1530N, Agilent Technologies, Inc., USA). The gas chromatographyapparatus was equipped with a Supelco SP-2560 capillary column100m� 0.25mm id with the film thickness of 0.2 mm (Supelco Inc.,USA) and FID detector, and operated in a split mode with a splitratio of 100:1. The injector and detector temperatures were 250 �C.The column temperature was held at 140 �C for 5 min, and thenprogrammed to rise to 250 �C at 3 �C/min and then held for 17 min.The carrier gas used was helium set at a flow rate of 1.1 ml/min. Thearea percents were used to determine the relative amounts of eachfatty acid. The response factors for each fatty acid according toAOAC 996.06 method were used to correct area percents. The fattyacid content was reported as grams per 100 g of fat.

2.5. Analysis of phytosterol content

The phytosterol content in rambutan fat was determinedaccording to the method of Schwartz, Ollilainen, Piironen, andLampi (2008). Gas chromatography analysis was conducted usingan HP 6890 (Hewlett Packard Co., USA) instrument equipped withan HP-5 capillary column 30 m � 0.32 mm id with the film thick-ness of 0.25 mm and FID detector, and operated in a splitless mode.The injector and detector temperatures were 300 �C. The columntemperature was held at 245 �C for 1 min, and then programmed torise to 275 �C at 3 �C/min and then held for 20 min. The carrier gasused was helium set at a flow rate of 3 ml/min. The relative amountof each phytosterol was reported as milligrams per gram of fat.

2.6. Analysis of a-tocopherol content

The a-tocopherol in rambutan fat was measured using an Agi-lent 1100 Series reverse-phase high performance liquid chroma-tography apparatus with a diode array detector (AgilentTechnologies, Inc., USA). A Hypersil ODS column (250 � 4.0 mm,Alltech Associates, Inc., USA) was used as the stationary phase. Themobile phase with this column was 98 percents methanol and 2percents water and a flow rate of 1 ml/min was used. Columntemperature was maintained at 25 �C and the a-tocopherol wasdetected at a wavelength of 292 nm.

2.7. Thermal behavior of rambutan fat

The thermal behavior experiment was conducted using a differ-ential scanning calorimeter (Model DSC1, Mettler-Teledo Interna-tional Inc., USA) and a method modified from Saloua, Eddine, andHedi (2009) and Besbe, Blecker, Deroanne, Drira, and Attia (2004).Approximately 4e5mg of the unrefined fat samplewas placed in analuminum sample pan and an empty aluminum pan was placed onthe reference platform. A linear heating and cooling rate of 5 �C/minover a temperature range of�50 to 90 �Cwas used. The thermogrampeak was used to provide an estimate of enthalpy (DH) and thethermogram peak points were used to determine themelting point.

2.8. Experimental design and statistical analysis

The effect of three independent variables: moisture content (X1;1.59e18.41 g/100 g), extraction time (X2; 2.3e10.7 h) and particlesize (X3; 0.09e2.11 mm) on the response variable (Y, percent fatyield) was evaluated using a three factor central composite design(CCD). The five coded levels of moisture content, extraction timeand particle size were incorporated into the design and wereanalyzed in 20 combinations (Table 1). The central point of thedesign was repeated six times to calculate the reproducibility ofthe method (Montgomery; 2001). For each combination of theindependent variables in the experimental design, the dependentparameter percent for extracted fat was determined. The effect of

Table 1Uncoded and coded levels of independent variables in the experimental design andpercent extracted fat.

Treatmentruns

Moisturecontent(X1, g/100 g)

Extraction time(X2, hours)

Particle size(X3, mm)

Extractedfat (Y, g/100 g)a

Uncoded Coded Uncoded Coded Uncoded Coded

1 10 0 10.705 1.682 1.1 0 35.53 � 0.47a

2 5 �1 4 �1 0.5 �1 35.74 � 0.38a

3 10 0 6.5 0 1.1 0 34.89 � 0.40a

4 10 0 6.5 0 1.1 0 33.57 � 0.24ab

5 15 1 9 1 1.7 1 23.13 � 1.48d

6 10 0 6.5 0 2.109 1.682 23.58 � 0.63d

7 10 0 6.5 0 0.091 �1.682 34.52 � 0.55ab

8 5 �1 9 1 1.7 1 35.40 � 0.84a

9 10 0 6.5 0 1.1 0 33.63 � 0.66ab

10 15 1 4 �1 0.5 �1 29.83 � 0.73c

11 10 0 6.5 0 1.1 0 34.75 � 0.09a

12 5 �1 9 1 0.5 �1 35.39 � 0.62a

13 18.41 1.682 6.5 0 1.1 0 20.63 � 1.11e

14 10 0 2.295 �1.682 1.1 0 28.17 � 1.49c

15 15 1 9 1 0.5 �1 32.46 � 1.34b

16 10 0 6.5 0 1.1 0 33.88 � 0.68ab

17 1.59 �1.682 6.5 0 1.1 0 35.34 � 1.10a

18 10 0 6.5 0 1.1 0 34.48 � 1.09ab

19 5 �1 4 �1 1.7 1 30.07 � 0.61c

20 15 1 4 �1 1.7 1 18.86 � 1.63e

a Means � standard deviation having the same letters are not significantlydifferent at the 5% level.

Table 2Regression coefficients and p-value of the response surface models and statisticalanalysis.

Regression Term Extracted Fat (Y, g/100 g)

Coefficient Probability (p-value)a

Intercept (b0) 24.9732 0.000Moisture (b1) 1.3885 0.000Time (b2) 1.3091 0.004Particle size (b3) 5.5557 0.004Moisture2 (b11) �0.0836 0.000Time2 (b22) �0.1157 0.000Particle size2 (b33) �4.7579 0.000Moisture/Particle size (b13) �0.5284 0.000Time/Particle size (b23) 0.7722 0.000R2 0.9743 e

R2 (adj) 0.9673 e

Regression e 0.000Lack-of-fit e 0.122

a The p-value more than 0.05 is not significantly different at the 5% level.

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these independent variables X1, X2 and X3 on the response Y wasinvestigated using the second-order polynomial regression equa-tion with stepwise elimination. This equation, derived using RSMfor the evaluation of the response variables, is as follows:

Y ¼ b0 þ b1X1 þ b2X2 þ b3X3 þ b11X21 þ b22X

22 þ b33X

23

þ b12X1X2 þ b13X1X3 þ b23X2X3 (1)

where Y is the response variable (fat yield, g/100 g); bo, b1, b2, b3, b11,b22.. are regression coefficients and X1, X2 and X3 are uncodedvalues for moisture content, extraction time and particle size,respectively. An analysis of variance (ANOVA) was performed todetermine the lack of fit and the effect of linear, quadratic andinteraction terms on fat extraction. The analysis of data and theoptimizing process were generated using Minitab statistical soft-ware v.15 (Minitab Inc., USA).

3. Results and discussion

3.1. Percent extracted fat

Fat from rambutan kernels (N. lappaceum L.) was extracted withhexane using a Soxhelt apparatus. The effect of different levels ofthe independent variables namely moisture content, extractiontime and particle size on percent fat yield was determined and theresults are summarized in Table 1. The yields of extracted fat rangedfrom 18.86 to 35.74 g/100 g with a mean value of 31.19 g/100 g.Augustin and Chua (1988) analyzed the seed composition oframbutan grown in Malaysia and reported that the seed contained37.1e38.9 g/100 g crude fat (petroleum ether extracted), which ishigher than the value (33.4 g/100 g) reported by other researchersinMexico (Solís-Fuentes et al., 2010). In Thailand, it has been shownthat rambutan seed contains a wide range of fat, between 14 and41 g/100 g (Kalayasiri et al., 1996; Winayanuwattikun et al., 2008).Those differences in the fat content of the seedmay be attributed tothe variability of the studied cultivars, a diversity in the maturity ofthe seeds used and the agricultural conditions in the area cultivated(Augustin & Chua, 1988; Cheikh-Rouhou et al., 2007).

3.2. Response surface analysis

The estimated regression coefficient of the response surfacemodels for the extracted rambutan kernel fat along with the cor-responding coefficient of determination values (R2) and lack of fittest are shown in Table 2. The linear coefficient indicates that the fatyield is positively correlated with all independent variables. On theother hand, the quadratic coefficient of all independent variablesshows a negative correlation with extracted fat yield. The R2 andadjusted R2 were 0.974 and 0.967, respectively. These results implythat the response surface model can explain more than 96 percentsof the variation in the response variables studied. Therefore, the R2

values of the response models are sufficiently high, to indicate thatthe response surface model is workable and can be used for esti-mation of the mean response and the subsequent optimizationstages. The lack of fit, which measures the fitness of the model, wasfound to be non significant (p> 0.05), indicating that the number ofexperiments were sufficient for determining the effect of inde-pendent variables on percentage fat yield (Montgomery; 2001). Asshown in Table 2, the extracted fat yield was significantly (p< 0.05)influenced by the main effects of moisture content, extraction timeand particle size and the quadratic effects of all independent vari-ables, as well as the interaction effect between moisture contentand particle size and extraction time and particle size. Thefollowing response surface model (Eq. (2)) was fit to the threeindependent variables (X1, X2 and X3) of the response variables (Y):

Y ¼ 24:97þ 1:39X1 þ 1:31X2 þ 5:56X3 � 0:08X21 � 0:12X2

2

�4:76X23 � 0:53X1X3 þ 0:77X2X3 (2)

3.3. Effect of independent variables on percent extracted fat

The response surface plot of fat yield using hexane for variouscombinations of moisture content, extraction time and particle sizeis shown in Fig. 1. The percent fat yield was higher at lower mois-ture content and vice versa (Fig. 1a). Similarly, the fat yield washigher for smaller particle size and vice versa (Fig. 1b). The fat yielddecreased when the moisture content and particle size increased.Higher moisture content samples have more resistance to pene-tration by the solvent into the samples, which resulted in low fatyield. As the particle size decreased, the surface area increased andthis enhanced fat extraction, resulting in a higher fat yield (Shahidi& Wanasundara, 2002). The fat yield increased in particles up to1.1 mm and then gradually decreased as particle size increased. Asshown in Table 2, the fat yield was significantly (p < 0.05) influ-enced by the interaction effect of moisture content and particle

Fig. 1. Response surface plots showing the interaction effects of extraction variables onthe extracted fat yield; a ¼ extraction time-moisture effect, b ¼ particle size-moistureeffect, and c ¼ particle size-extraction time effect.

Table 3Physical and chemical characteristics of rambutan kernel fat.

Properties Valuea

Color (CIE Lab)Solid fat L* ¼ 86.87 � 0.97;

a* ¼ �2.06 � 0.48; b* ¼ 3.55 � 0.35Melted fat L* ¼ 66.34 � 0.18;

a* ¼ �2.31 � 0.46; b* ¼ 6.62 � 0.56Polymorphic forms (g/100 g)Beta (b) 84.7 � 1.2Beta-prime (b0) 15.3 � 1.2

Refractive index 1.469 � 0.001Iodine value (g/100 g oil) 41.6 � 1.2Saponification value (mg KOH/g oil) 166 � 3Unsaponifiable matter (g/100 g) 0.19 � 0.04Phytosterol (mg/g)Stigmasterol 0.32 � 0.03b-Sitosterol 0.61 � 0.06Campesterol NDb

a-Tocopherol (mg/100 g) 0.103 � 0.001

a Mean � standard deviation of duplicate analysis.b ND ¼ Not detected.

W. Sirisompong et al. / LWT - Food Science and Technology 44 (2011) 1946e1951 1949

size. From the results (Fig. 1b) it is apparent that with a smallerparticle size (less than 1.1 mm) the fat yield slightly increased withan increase in moisture content up tow 10 g/100 g and the fat yielddecreased when the moisture content was further increased. Onthe other hand, for the bigger particle sizes (more than 1.1 mm) thefat yield slightly decreased with increasing moisture content andwhen the moisture content was increased from 5 to 15 g/100 ga marked decrease in fat yield was observed. In Fig. 1c, the fat yieldwas increase with an increase in extraction time and did notincrease after 6.5 h for small particle size (w 0.5 mm). At inter-mediated particle size (w 1.1 mm), the fat yield increased as theextraction time was increased and reached a maximum at 9 h. Anyfurther increase in extraction time did not increase the fat yield.

3.4. Optimization and validation of regression model

Response optimizations were performed to measure theoptimum levels of independent variables required to achieve thedesired fat yield. For hexane extraction, the process providingthe maximal fat yield would involve samples with low moisture

content and with medium particle size but with a long extractiontime (Fig. 1). To determine the exact optimum points for all theindependent variables necessary to achieve the optimized condi-tion, a numerical optimization was utilized. The results showedthat the extraction using 1.05mmparticles at 4.99 g/100 gmoisturefor 9.2 h of extraction time provided the maximal fat yield.Under these optimum conditions, the predicted fat yield was37.35 g/100 g. The adequacy of the response surface equationswas demonstrated by a comparison between the experimentalvalues and predicted values based on a response regression(Mirhosseinia, Tan, Hamid, & Yusof, 2008). Under the recom-mended optimum conditions, the experimental value for fat yieldwas 37.20 � 0.69 g/100 g which was not significantly different(p > 0.05) from the predicted value (37.35 g/100 g). Good agree-ment must exist between the values calculated using the modelequations and the experimental values at the points of interest, andno significant (p > 0.05) difference was reported between theactual and the predicted values (Predicted value ¼ 1.0016 Experi-mental value). The high correlation coefficients (z0.978) alsoconfirmed that a close agreement between experimental data andpredicted values calculated using the models had been obtained.The closer the experimental and predicted results, the better theyexplain the adequacy of the regression equation (Rossa, de Sá,Burin, & Bordignon-Luiz, 2011).

3.5. Physical and chemical characteristics of rambutan kernel fat

3.5.1. ColorThe rambutan kernel fat is consistently a white solid at room

temperature (25 � 2 �C). The L*, a* and b* values of solid fat were86.87,�2.06 and 3.55, respectively (Table 3). When the solid fat washeated (w 60 �C), the melted fat had a golden yellow color with L*,a* and b* values of 66.34, �2.31 and 6.62, respectively. The L*, a*and b* values of other vegetable oils, such as palm, soybean,sunflower, olive, and corn range from 63.4 to 69.5, 3.8 to 4.4 and 9.2to 10.4, respectively (Hsu & Yu, 2002). This shows that the meltedfat has b* values lower than those of other vegetable oils. It maysuggest the presence of less yellow pigments, e.g. carotenoids, inrambutan kernel fat (Cheikh-Rouhou et al., 2007).

3.5.2. Crystal polymorphismIn regard to polymorphic forms of rambutan kernel fat as

determined by XRD, it can be seen that the extracted fat contains

Table 4Fatty acid composition of rambutan kernel fat.

Fatty acid composition Content (g/100 g)a

Saturated fatty acid 49.57 � 0.14Myristic acid (C14:0) 0.02 � 0.00Palmitic acid (C16:0) 4.69 � 0.15Stearic acid (C18:0) 7.03 � 0.08Arachidic acid (C20:0) 34.32 � 0.01Heneicosanoic acid (C21:0) 0.05 � 0.00Behenic acid (C22:0) 3.10 � 0.04Tricosanoic acid (C23:0) 0.03 � 0.01Lignoceric acid (C24:0) 0.33 � 0.06Monounsaturated fatty acid 37.97 � 0.22Palmitoleic (C16:1u7) 0.49 � 0.04Trans-9-Elaidic acid (C18:1u9t) 0.03 � 0.00Cis-9-Oleic acid (C18:1u9c) 36.79 � 0.16Erucic acid (C22:1u9) 0.66 � 0.03Polyunsaturated fatty acid 7.89 � 0.01Cis-9,12-Linoleic acid (C18:2u6) 1.37 � 0.02a-Linolenic acid (C18:3u3) 6.48 � 0.03Cis-11,14-Eicosadienoic acid (C20:2) 0.04 � 0.00

a Mean � standard deviation of duplicate analysis.

Fig. 2. Melting and crystallization curves of rambutan kernel fat (a) and commercialcocoa butter (b).

W. Sirisompong et al. / LWT - Food Science and Technology 44 (2011) 1946e19511950

mixtures of b (84.7 g/100 g) and b0 (15.3 g/100 g) polymorphicforms (Table 3). The structure, composition and polymorphic formsof fatty crystals are most important criteria for determining thefunctional properties of fat (Reddy & Jeyarani, 2001). From theresults, it appears that rambutan kernel fat tends to crystallize inb form which is also seen in cocoa butter. For cocoa butter,a representative confectionery fat, the most functional polymorph(form V) is a b type, since this form results in optimal density,melting behavior, and surface appearance (Sato, 1999). Therefore,rambutan kernel fat has the potential to be used for confectioneryproducts.

3.5.3. Refractive index, iodine value and saponification numberThe refractive index of rambutan kernel fat is 1.469 � 0.001

(Table 3). This value is consistent with the values of othervegetable oils (Padley, Gunstone, & Harwood, 1986) and theresults of Solís-Fuentes et al. (2010), who reported that rambutanfat had a refractive index of 1.468. The iodine value of the fat,41.6 � 1.2 g per 100 g fat (Table 3), places it in the non-dryinggroup of oils which also included palm oil, palm kernel oil andcoconut oil (Tan & Che Man, 2000). This value is similar to theprevious studies (Azam et al., 2005; Solís-Fuentes et al., 2010),which reported that rambutan fat had an iodine value of about44e47 g per 100 g fat. The saponification number, which is anindication of the average molecular weight of the fat, was166 � 3 mg KOH per gram fat for rambutan kernels (Table 3). Thelow saponification value suggests that rambutan fat containsa long chain fatty acid and a relatively high average molecularweight (Onyieke & Acheru, 2002).

3.5.4. Unsaponification matterThe estimated unsaponification matter in rambutan kernel fat

found to be 0.19 � 0.04 g/100 g, as compared to 0.2e0.6 g/100 g incocoa butter (Padley et al., 1986). In the present study, thephytosterol along with a-tocopherol content were also determined(Table 3). The results show that the rambutan kernel fat had0.61 � 0.06 and 0.32 � 0.03 mg per gram fat of b-sitosterol andstigmasterol, respectively. The fat from rambutan kernel has lowerlevel of total phytosterol than do commercial oils and fats (Padleyet al., 1986). The extracted fat also contained 0.103 mg per 100 gfat of a-tocopherol. The low content of a-tocopherol in rambutankernel fat was similar to that of cocoa butter, cod liver oil and beeffat (Swern, 1964). These results suggest that rambutan kernel fatmay not be a good source of the natural antioxidants, phytosteroland a-tocopherol.

3.6. Fatty acid composition

The fatty acid composition of rambutan kernel fat is described inTable 4. The total saturated and unsaturated fatty acids found inrambutan fat were 49.6 and 45.9 g/100 g, respectively. The mostabundant fatty acids in rambutan seed fat was arachidic acid(C20:0) for the saturated fatty acids (34.3 g/100 g), and oleic acid(C18:1) was the main unsaturated fatty acid (36.8 g/100 g). Thesetwo fatty acids comprised more than 70 g/100 g of the total fattyacids, which is in agreement with previously published data(Augustin & Chua, 1988; Azam et al., 2005; Solís-Fuentes et al.,2010). Likewise, measurable amounts of palmitic (C16:0), stearic(C18:0) and behenic (C22:0), for saturated fatty acids as well aspalmitoleic (C16:1), linoleic (C18:2n-6) and linolenic (C18:3n-3) forunsatutated fatty acids were detected (Table 4). The high level ofarachidic acid is the main reason for the low iodine value inrambutan seed fat (Table 3) would allow the fat to be used withoutbeing subjected to hydrogenation, especially where autoxidationmay be a concern.

3.7. Thermal properties of rambutan kernel fat

The thermal behavior, melting and crystallization of rambutankernel fat is shown in Fig. 2a. These results show three well-definedpeaks with shoulders that correspond to groups of triglycerideswith high, middle, and low temperatures of melting and crystalli-zation. These results illustrate the complex nature of triglyceridesin fat samples (Tan & Che Man, 2000). For the melting behavior, thefirst peak had the lowest melting temperatures, with the temper-ature peak at 4.2 �C. The results suggest that this region includesa group of triglycerides with a greater abundance of unsaturatedfatty acids. The second and third peaks represent intermediate andhigh melting temperatures, with a peak temperature at 37.4 and49.8 �C, respectively. These regionsmight be represented by groups

W. Sirisompong et al. / LWT - Food Science and Technology 44 (2011) 1946e1951 1951

of triglycerides withmore saturated fatty acids. Similar results wereobserved for crystallization behavior (Fig. 2a) where the peaktemperatures were 40.9, 19.1 and �16.1 �C for the first, second andthird peaks, respectively. The beginning and completed phasechanges were determined based on the onset and offset tempera-ture values of the peaks. The crystallization process for rambutanfat began with the first crystal formation at 37.8 �C and endedat �14.8 �C. The total crystallization enthalpy was 92.1 J/g. For themelting process, the beginning temperature was �4.5 �C and thephase change was completed at about 58.9 �C. The total value ofenthalpy for the melting process was 85.4 J/g. These values indicatea large range of melting points for the rambutan fat studied, whichis slight different from previously published data (Solís-Fuenteset al., 2010). It is often difficult to compare the thermal profile ofoils and fats from various sources because of the lack of uniformityof analytical techniques used in qualitative or quantitative analysis(Tan & Che Man, 2000). Based on the results of crystal poly-morphism analysis in the present study (Table 3) which reportedthat rambutan kernel fat, like cocoa butter, tends to crystallize inb form, the thermal behavior of commercial cocoa butter (SiamCocoa Products Co Ltd., Thailand) was investigated under the sameanalytical conditions used for extracted rambutan fat. Thecommercial cocoa butter showed only one peak and a limitedmelting range which indicates a homogeneity of triglycerides(Fig. 2b). Its melting and crystallization occurred at the peaktemperatures of 36.25 �C and 13.24 �C, respectively. Similarities inthe thermal behaviour were observed for the second region oframbutan fat and cocoa butter. These results suggest that thisfraction of rambutan kernel fat has the potential to be used asa cocoa butter replacer for confectionery products.

4. Conclusions

The seed kernel of rambutan fruit is a typical waste materialobtained annually in large amounts as a by-product of the canningindustry and hexane can be used successfully as a solvent forextraction of fat from rambutan seed kernels. A moisture content of4.99 g/100 g, particle size of 1.05 mm and extraction time of 9.2 hprovided themaximumfat yield of 37.35g/100g. The rambutan seedkernels give a considerable yield of fat and the high arachidic acidcontent makes the fat highly stable to oxidation. Because of thesephysical and chemical characteristics, the merit of using rambutankernel fat in the cosmetic and food industries may be justified.However, through the seeds are considered edible after roasting insome Asian countries, the safety of rambutan kernel fat must still beevaluated before it is used as an ingredient in the food industry.

Acknowledgment

Funds for this research were provided by the Kasetsart Univer-sity Research and Development Institute (KURDI) with the projectNo. V-T(CH) 2.6.51.

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