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Carbohydrate content and composition of product from subcritical water treatment of coconut meal
Journal of Industrial and Engineering Chemistry 18 (2012) 225–229
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Carbohydrate content and composition of product from subcritical water treatment of coconut meal Pramote Khuwijitjaru a,*, Kumutakan Watsanit a, Shuji Adachi b a b
Department of Food Technology, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom 73000, Thailand Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
A R T I C L E I N F O
Article history: Received 22 November 2010 Accepted 10 March 2011 Available online 4 November 2011 Keywords: Copra Superheated water Waste utilization CCD Neutral sugar composition
A B S T R A C T
Coconut meal, a by-product from coconut milk production, was treated with subcritical water at 100– 200 8C for 30–240 min in a batch-type reactor. The analysis focused on the content and constituent neutral sugar of the soluble carbohydrate in the liquid products. The carbohydrate is composed of both monosaccharides and oligosaccharides. Treatments at 100–150 8C gave a small amount of a carbohydrate (3.5–5.1 g/100 g dry coconut meal). At 175 8C, the carbohydrate content increased from 4.9 to 9.6 g/100 g dry coconut meal (p < 0.05) for 30–240 min of treatment, but the value decreased from 10.6 to 6.1 g/ 100 g dry coconut meal for 30–240 min of treatment at 200 8C. The soluble carbohydrate contained mannose, glucose, galactose and arabinose. A response surface methodology study indicated that 13.9 g/ 100 g dry coconut meal of mannose in the soluble carbohydrate could be produced at 227 8C in 3 min. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Coconut (Cocos nucifera L.) is an important plant in several Asian and South American countries with an annual production of more than 58 million tons around the world in 2007 [1]. Coconut copra (kernel meat) represents about 12% of the total fruit weight [2]. Besides other uses, fresh and fully mature copra is used for coconut milk production which is an aqueous extraction process of protein, fat and carbohydrate. Coconut milk is used for food and confectioneries preparation in Southeast Asia. During the extraction process using a coconut copra–water weight ratio of 1, about 10 g of dried solid residue is obtained for every 100 g of coconut milk [3]. Because the annual production of coconut milk is about 36,000 tons in Thailand [4], it can be estimated that around 3600 tons of coconut meal was produced each year. Coconut meal is normally sold by the coconut milk factory for feed production, but finding a new way to utilize coconut meal is important for adding value to this by-product. Coconut meal contains high amount of carbohydrates, i.e., about 43–45% of which is mainly in the form of mannose polysaccharide (61%) [5]. Saittagaroon et al. [6] demonstrated the use of copra meal for the preparation of mannitol by borohydride reduction. Other work has been done using coconut meal as a substrate for the enzymatic hydrolysis by mannanase or acid hydrolysis to produce mannose and mannooligosaccharides
[7–9]. These mannooligosaccharides show their potential use as a prebiotic substance which is a health promoting material for both humans and animals. Subcritical water, i.e., hot liquid water at 100–374 8C under high pressure, has been recognized as a promising mean for converting agricultural by-products into more value-added products [10,11]. Several studies had been done on converting protein-containing wastes into peptides and amino acids [12]. The treatment of lignocellulosic materials, such as wood, corn stalk and rice straw, with sub- and supercritical water resulted in glucose- and/or xyloseoligomers and monomers which are hydrolyzed from cellulose and/ or hemicellulose in the raw material [13–16]. However, these studies focused on using subcritical water as a pretreatment of material for further enzymatic hydrolysis processes. The treatment of coconut meal with subcritical water has not been reported in the literature. Producing mannose and its oligosaccharides from coconut meal by treatment in subcritical water seems to be possible. In this context, the objective of this study was to investigate the effect of temperature and time on the carbohydrate content and the sugar composition of the obtained product after the subcritical water treatment of coconut meal. 2. Materials and methods 2.1. Samples and reagents
* Corresponding author. Tel.: +66 034 219361; fax: +66 034 272194. E-mail address: [email protected] (P. Khuwijitjaru).
Coconut meal used as a raw material was a by-product from a local coconut milk factory (Vara Food and Drink, Co., Ltd., Nakhon
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.010
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Pathom, Thailand). Standard sugars (arabinose, xylose, mannose, galactose and glucose) were purchased from Sigma Aldrich (Germany), and myo-inositol was purchased from Fluka (Japan). All other reagents were of analytical grade.
extraction using petroleum ether as the extractant. The ash content was determined by the dry ashing method.
2.2. Preparation of coconut meal
The residue from the filtration was dried at 130 8C for 1 h. The solid loss (% w/w) was calculated from:
The received coconut meal was dried at 60 8C for 4 h in a hot-air oven and sieved to obtain a coconut meal with particle size range of 0.3–2 mm. The dried sample was kept in an aluminum foil bag at room temperature for further studies. 2.3. Subcritical water treatment The subcritical water treatment was done in a batch-type reactor using a high pressure-resistant vessel made from stainless steel with a net volume of 100 mL (Taiatsu Techno Corporation, Tokyo, Japan). The temperature inside the vessel was controlled and monitored using an aluminum block heater (Applied Scientific Instruments, Bangkok, Thailand) equipped with a temperature controller and a type-K thermocouple. The pressure inside the vessel varied corresponding to the saturated vapor pressure of water at specific treatment temperature. A sample of dried coconut meal (8 g) was mixed with water (80 g) in the vessel. The vessel was tightly closed and heated at the desired temperatures. The heat-up time for any temperature was about 12 min. After reaching the desired treatment time, the vessel was immediately cooled with water to stop the reaction. The content in the vessel was vacuum filtered through a Whatman no. 1 filter paper. The filtrate was centrifuged at 10,000 g for 10 min at 4 8C, and the liquid portion was collected and adjusted to 100 mL with distilled water before further analyses. 2.4. Experimental design 2.4.1. Effects of treatment temperature and time The subcritical water treatment was performed at 100, 125, 150, 175 and 200 8C for 30, 60, 120 and 240 min using a factorial in a completely randomized design (CRD) with 3 replications. 2.4.2. Optimization of treatment temperature and time for mannose recovery After the first part of study (Section 2.4.1), further experiments were conducted at 200, 225 and 250 8C for 5, 10, 20 and 30 min without replication to determine the possible optimum condition for maximizing the amount of mannose in the soluble carbohydrate. The optimization was then performed using a response surface methodology with a central composite design (CCD) (Table 1). In total, 13 experiments including 5 replications at the center point were carried out. 2.5. Proximate analysis of coconut meal A proximate analysis of the coconut meal was performed using the AOAC method [17]. Briefly, the moisture content was determined using an oven drying method at 130 8C. The protein content was determined by the Kjeldalh method with a conversion factor of 6.25. The crude fat content was measured by Soxhlet Table 1 Levels of independent variables according to the central composite design (CCD). Variables
Temperature (8C) Time (min)
Levels Low
Center
High
Axial (a)
Axial (+a)
215 3
225 5
235 7
211 2.17
239 7.83
2.6. Solid loss, pH and UV-absorption
1
weight of dry residue 100% weight of dry coconut meal sample
(1)
Measurement of the pH value of the liquid product after centrifugation was done using a pH meter (PHM 210, Metro Lab, France). The absorption spectra of the liquid product were measured from 200 to 500 nm using a UV-Vis spectrometer (Genesys 20uv, Thermo Spectronic, USA) after the appropriate dilution. 2.7. Total carbohydrate content The total carbohydrate content of the product was determined using the phenol–sulfuric method [18]. Mannose was used for preparing a standard curve. Each sample was evaluated in duplicate. 2.8. Sugar composition The neutral sugar composition of the carbohydrate in the coconut meal sample and the liquid product was analyzed by a GCFID method. The meal or liquid product was hydrolyzed with sulfuric acid to release the monosaccharides using the protocol developed by NREL [19]. For measuring the amount of monosaccharides by the GC-FID method, an alditol acetate derivative was prepared according to the method of Ozga et al. [20] using myo-inositol as the internal standard. One microliter of a sample was injected (splitless) into a Shimadzu GC-8A (Kyoto, Japan) equipped with a DB-225 capillary column (30 m, Agilent Technologies, USA). The injector and detector temperatures were set at 240 8C and the column temperature was set at 200 8C. A quantitative analysis was done by preparing the standard curves of five sugars, i.e., mannose, glucose, galactose, xylose and arabinose. A GC-FID analysis was performed in duplicate for each sample. 2.9. Statistical analysis An analysis of variance (ANOVA) was performed to determine the significance of the treatment temperature and time for each analysis. The least significant difference (LSD, a = 0.05) was used for comparing the mean values when a significance was observed from the ANOVA test. Statistical tests were performed using IBM SPSS Statistics 18 (IBM, USA). In the optimization experiment, the response surface methodology was planned and analyzed using Design Expert 6.0.5 (Stat-Ease, USA). 3. Results and discussion 3.1. Proximate analysis and sugar composition of coconut meal The coconut meal sample used in this study had been extracted for coconut milk using water. After drying and grinding, the coconut meal sample contained a considerable amount of carbohydrate and fat, while most of the protein was extracted (Table 2). The carbohydrate composition analysis showed that the carbohydrate in the coconut meal contained mannose, glucose, galactose and a small amount of arabinose which agreed with that
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12
Table 2 Proximate analysis and sugar composition of coconut meal. Composition % wb 3.44 0.03 0.85 0.06 25.59 0.82 1.13 0.03 68.99 0.79 % of total carbohydrate 79.77 1.10 12.80 0.63 6.12 0.16 1.31 0.02
Total carbohydrate [g/100 g dry coconut meal]
10
Proximate analysis Moisture content Protein Fat Ash Total carbohydrate (by difference) Carbohydrate composition Mannose Glucose Galactose Arabinose
8
6
4
2
Data are expressed as mean standard deviation (n = 3).
0 0
60
120
180
240
300
Time [min] previously reported [5,7]. These results suggested that coconut meal is a good resource for producing a variety of carbohydrate products, especially mannose-containing oligosaccharides or mannose monosaccharide. Because the coconut meal contained about 25% fat, the recovery of fat and fatty acids should also be considered for another use of this by-product. 3.2. Effects of temperature and time 3.2.1. Solid loss and pH Dried coconut meal is white and the color changed to yellow, brown or black after treatment at a higher temperature and longer time. Because the subcritical water treatment promotes the hydrolysis reaction, converting the solid biomass into soluble substances was expected. In general, the solid content of coconut meal significantly decreased with the treatment temperature (p < 0.05). Treatments at 175 and 200 8C resulted in a rapid loss of the solid content. The effect of the treatment time was also more obvious at the higher temperature. About 40% of the coconut meal was solubilized after treatment for 240 min at 200 8C (Fig. 1). Measuring the pH values of the liquid product revealed that the formation of acidic components would decrease the pH from about 6 to 3 after an increase in temperature from 100 to 200 8C. Lu¨ and Saka [21] reported that acetic, glycolic, lactic, and formic acids could be detected after the subcritical water treatment of Japanese beech wood at 170–290 8C. Formation of the acids would help in promoting the hydrolysis reaction of coconut meal.
3.2.2. Carbohydrate content Since carbohydrate is the main component in coconut meal, we focused on determining the carbohydrate content and its composition in the liquid product obtained after treatment with subcritical water. Fig. 2 shows the changes in the carbohydrate content of the liquid product as a function of time at different treatment temperatures. At 100–150 8C, about 3.5–5.1 g/100 g dry coconut meal carbohydrate was obtained and the treatment time only slightly affected the content. However, at 175 8C, the carbohydrate content significantly increased (p < 0.05) with time and rapidly increased to 9.6 g/100 g dry coconut meal after a 240min treatment. In contrast, at 200 8C, the carbohydrate content was the highest at 30 and 60 min, i.e., about 10.6 g/100 g dry coconut meal, but a longer treatment resulted in a gradual decrease in the value (p < 0.05). This result indicated that the carbohydrate would be further degraded to others products, such as glyceraldehyde, 5hydroxymethylfurfural, and acids as reported by other studies [16,21–23]. Fig. 3 shows the relationship between the carbohydrate content and solid loss during the treatment at different temperatures. Under the conditions of this study, the carbohydrate content increased with the solid loss and reached a maximum value at about 30% solid loss, but a further decomposition of the solid did not produce more carbohydrates, indicating that the further 12
7
50
10
5 4
pH
30
20 3 10
2
Total carbohydrate [g/100 g dry coconut meal]
6
40
Solid loss [% w/w]
Fig. 2. Changes in the total carbohydrate content of liquid product for subcritical water treatment of coconut meal. Symbols used are the same as in Fig. 1.
8
6
4
2 1
0 0
60
120
180
240
300
Time [min]
0 0
10
20
30
40
50
Solid loss [% w/w] Fig. 1. Changes in solid loss (solid line) and pH of liquid product (dot line) for subcritical water treatment of coconut meal at 100 (*), 125 (&), 150 (4), 175(^), and 200 () 8C.
Fig. 3. Relationship between total carbohydrate content and solid loss for subcritical water treatment of coconut meal. Symbols used are the same as in Fig. 1.
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15
0.5
Mannose [g/100 g dry coconut meal]
Mannose 175°C, 120 min
Absorbance
0.4
0.3
175°C, 120 min
0.2
0.1
100°C, 30 min
175°C, 30 min
100°C, 120 min
10
5
0.0 220
270
320
0
370
0
Wavelength [nm]
10
20
30
Time [min]
Fig. 4. UV absorption spectra of liquid products from subcritical water treatment of coconut meal and pure mannose.
Fig. 5. Mannose content of carbohydrate in liquid product from subcritical water treatment of coconut meal at 200 (^), 225 (*), and 250 (4) 8C.
degradation of the soluble carbohydrates occurred during the longer treatment at high temperature as already described. UV-absorption of the liquid product was determined at the wavelengths of 200–500 nm but only part of the spectra that exhibited an absorption peak is shown in Fig. 4. Mannose was also treated with subcritical water at 175 8C for 120 min and its spectrum is also shown in the figure for comparison. The maximum absorption peak of the liquid product appeared at about 280–285 nm, which might correspond to 5-hydroxy methyl furfural or furfural. The spectrum from mannose would suggest that the absorption peak at 280–285 nm for the liquid product is ascribed to additional degradation products of the monosaccharides liberated from the coconut meal.
temperatures (100–150 8C), glucose was the main sugar of the carbohydrate in the liquid, however, at 175–200 8C, the glucose content decreased and mannose became the major sugar of the carbohydrate in the liquid product. This result suggested that glucose was more easily released from the polymer structure than mannose at the lower treatment temperature. The change in the mannose content with temperature and time was similar to that of the total carbohydrate content, that is, at 175 8C, it increased with time, but at 200 8C, it decreased with the treatment time. The highest amount of mannose (8.98 g/100 g dry coconut meal) was obtained at 200 8C and 30 min which was about 10 times compared to the amount of glucose. Galactose and arabinose occurred in only small amounts from the liquid product. The structure of the saccharides in the liquid product was not totally characterized in this study. However, preliminary experiments were performed using an HPLC method with a cation exchange column (Rezex RNM-Carbohydrate Na+ (8%), Phenomenex, USA). The chromatogram (data not shown) indicated that the liquid products contained mono- and disaccharides as well as oligosaccharides although the structure, degree of polymerization and amount were not determined. These results suggested that we
3.2.3. Carbohydrate composition Neutral sugar profiles of the carbohydrate in the liquid product obtained under the various treatment conditions are shown in Table 3. The carbohydrate in the liquid product was mainly composed of mannose, glucose, galactose and arabinose. The polysaccharide in coconut meal was hydrolyzed to release soluble carbohydrate, such as the mono-, di-, and oligosaccharides. At low
Table 3 Neutral sugar of carbohydrate products obtained by subcritical water treatment of coconut meal. Sugars (g/100 g dry coconut meal)
Mannose
Glucose
Galactose
Arabinose
Temperature
Time (min)
(8C)
30
60
120
240
100 125 150 175 200 100 125 150 175 200 100 125 150 175 200 100 125 150 175 200
0.76 0.17 0.86 0.05 0.94 0.06 1.16 0.07 8.98 1.00 1.60 0.31 1.62 0.15 1.45 0.11 0.90 0.05 0.65 0.09 0.15 0.06 0.28 0.06 0.47 0.06 0.64 0.03 1.15 0.05 0.02 0.03 0.15 0.03 0.40 0.04 0.39 0.01 0.03 0.02
0.92 0.04 1.05 0.21 1.14 0.10 2.08 0.05 7.89 0.65 1.87 0.10 2.00 0.05 1.82 0.20 0.78 0.12 1.07 0.08 0.19 0.01 0.31 0.01 1.07 0.09 0.89 0.11 0.54 0.05 0.03 0.01 0.20 0.04 0.56 0.06 0.39 0.05 Nd
0.87 0.15 0.76 0.10 0.92 0.10 3.57 0.42 2.62 0.07 1.90 0.23 1.58 0.18 1.37 0.03 0.49 0.02 0.49 0.12 0.19 0.03 0.31 0.00 0.83 0.09 0.99 0.08 0.06 0.03 0.05 0.02 0.22 0.03 0.54 0.06 0.12 0.01 Nd
0.78 0.20 0.68 0.13 0.95 0.12 5.07 0.11 0.52 0.27 1.88 0.21 1.56 0.04 1.21 0.09 0.44 0.08 0.18 0.17 0.23 0.02 0.38 0.03 0.84 0.02 0.84 0.07 Nd 0.10 0.03 0.30 0.01 0.50 0.04 Nd Nd
Data are expressed as mean standard deviation (n = 3). Nd = not detected.
P. Khuwijitjaru et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 225–229
7
6
where X1 and X2 are the treatment temperature and time, respectively. The maximum mannose content predicted from the model was 13.9 g/100 g dry coconut meal obtained at 227 8C and 3 min. This value is comparable to that from the enzymatic hydrolysis reported by several patents, e.g., Yokomizo and Kadota [24] obtained the maximum mannose content of 15.1 g/100 g coconut meal by hydrolyzing coconut meal using b-galactomannanase and xylanase at 60 8C for 72 h.
9
Time [min]
10 10
5
11
12
12
11
13
4
4. Conclusions This study showed that the subcritical water treatment is a promising technology for the production of both mono- and oligosaccharides from coconut meal by-product. In this batch-type reactor, a high temperature (ca. 230 8C) and short (<3 min) treatment time tended to give a high amount of carbohydrate with a high mannose content. Further analysis for structure and content oligosaccharides and other degradation products formed should be also conducted.
9
3 215
220
225
230
235
Temperature [°C] Fig. 6. Effects of treatment temperature and time on the mannose content of carbohydrate in liquid product from subcritical water treatment of coconut meal. The numerical values in the figure represent the content in units of g/100 g dry coconut meal.
can produce mannose oligomers and/or monomer from coconut meal using the subcritical water treatment instead of enzymatic hydrolysis [7,9,24] which usually takes a longer time and needs expensive enzymes for the reaction. 3.3. Optimization of subcritical water treatment Based on the results of this study, it seemed that the higher treatment temperature and shorter time might produce a higher mannose content of carbohydrates in the liquid product. Therefore, preliminary experiments for the treatment at 200, 225 and 250 8C for 5, 10, 20 and 30 min were performed without replication and the mannose content in the liquid product is shown in Fig. 5. Treatment at 200 8C for a shorter time did not give a high amount of mannose as we expected, and the highest mannose content was obtained at 225 8C and 5 min. Treatment at 250 8C completely decomposed the carbohydrate in the liquid product even for the shortest time. These results indicated that degradation of mannose and mannose-containing di- and oligosaccharides were quite fast at temperatures above 225 8C and this probably caused by catalysis effect of degradation products, although a complete degradation of mannose was not observed in this temperature range during the hydrolysis of woods in previous studies [16,25]. The response surface methodology was conducted to find the possible maximum yield of mannose in the liquid product using the CCD with a center point at 225 8C and 5 min. Fig. 6 shows the response contour plot for the mannose content in the liquid product. The contour line was modeled by the secondary order polynomial equation: Mannose
g 100 g dry coconut meal
¼ 2264:4 þ 19:8X 1 þ 23:0X 2 0:04X12 0:05X22 0:1X 1 X 2
229
(2)
Acknowledgement This work was done under the project ‘‘Coconut product development and quality improvement’’ supported by a grant from the Silpakorn University Research and Development Institute, Thailand. References [1] FAO. 2007. Available from: http://faostat.fao.org. [2] B.E. Grimwood, F. Ashman, D.A.V. Dendy, C.G. Jarman, E.C.S. Little, Coconut Palm Products: Their Processing in Developing Countries, FAO, Rome, 1975, pp. 279. [3] S. Sringam, (in Thai), Proceeding of the 26th Kasetsart University Conference, Bangkok, Thailand, (1988), pp. 309–316. http://www.lib.ku.ac.th/KUCONF/ KC2605035.pdf. [4] Office of Industrial Economics. 2003. Available from: http://www.oie.go.th. [5] S. Saittagaroon, S. Kawakishi, M. Namiki, Journal of the Science of Food and Agriculture 34 (1983) 855. [6] S. Saittagaroon, S. Kawakishi, M. Namiki, Journal of Food Science 50 (1985) 757. [7] I. Kusakabe, M. Zama, G.G. Park, K. Tubaki, K. Murakami, Agricultural and Biological Chemistry 51 (1987) 2825. [8] G. Yoshikawa, T. Yano, Japan Patent 2000139490 (2000). [9] H. Kanatani, F. Yokomizo, US Patent 20090311411 (2009). [10] O. Pourali, F.S. Asghari, H. Yoshida, Food Chemistry 115 (2009) 1. [11] J. Wiboonsirikul, S. Adachi, Food Science and Technology Research 14 (2008) 319. [12] H. Cheng, X. Zhu, C. Zhu, J. Qian, N. Zhu, L. Zhao, J. Chen, Bioresource Technology 99 (2008) 3337. [13] H. Boussarsar, B. Roge´, M. Mathlouthi, Bioresource Technology 100 (2009) 6537. [14] C.E. Wyman, B.E. Dale, R.T. Elander, M. Holtzapple, M.R. Ladisch, Y.Y. Lee, Bioresource Technology 96 (2005) 2026. [15] P. Khuwijitjaru, C. Wanpen, T. Mala, M. Ariyakriangkrai, S. Adachi, KKU Research Journal 14 (2009) 1084 (in Thai). [16] K.-H. Kim, I.-Y. Eom, S.-M. Lee, S.-T. Cho, I.-G. Choi, J.W. Choi, Journal of Industrial Engineering Chemistry 16 (2010) 918–922. [17] A.O.A.C., Official Methods of Analysis of AOAC International, 17th ed., AOAC International, Maryland, 2000. [18] M. DuBois, K.A. Gilles, J.K. Hamilton, P.A. Rebers, F. Smith, Analytical Chemistry 28 (1956) 350. [19] R. Ruiz, T. Ehrman, Dilute acid hydrolysis procedure for determination of total sugars in the liquid fraction of process samples, NREL, Golden, CO, 1996. [20] J.A. Ozga, A. Saeed, W. Wismer, D.M. Reinecke, Journal of Agricultural and Food Chemistry 55 (2007) 10414. [21] X. Lu¨, S. Saka, Biomass and Bioenergy 34 (2010) 1089. [22] M. Sasaki, B. Kabyemela, R. Malaluan, S. Hirose, N. Takeda, T. Adschiri, K. Arai, Journal of Supercritical Fluids 13 (1998) 261. [23] S. Haghighat Khajavi, Y. Kimura, T. Oomori, R. Matsuno, S. Adachi, Journal of Food Engineering 68 (2005) 309. [24] F. Yokomizo, T. Kadota, US Patent 6797292B2. [25] M. Matsunaga, H. Matsui, Y. Otsuka, S. Yamamoto, The Journal of Supercritical Fluids 44 (2008) 364.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Carbohydrate content and composition of product from subcritical water treatment of coconut meal Pramote Khuwijitjaru a,*, Kumutakan Watsanit a, Shuji Adachi b a b
Department of Food Technology, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom 73000, Thailand Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
A R T I C L E I N F O
Article history: Received 22 November 2010 Accepted 10 March 2011 Available online 4 November 2011 Keywords: Copra Superheated water Waste utilization CCD Neutral sugar composition
A B S T R A C T
Coconut meal, a by-product from coconut milk production, was treated with subcritical water at 100– 200 8C for 30–240 min in a batch-type reactor. The analysis focused on the content and constituent neutral sugar of the soluble carbohydrate in the liquid products. The carbohydrate is composed of both monosaccharides and oligosaccharides. Treatments at 100–150 8C gave a small amount of a carbohydrate (3.5–5.1 g/100 g dry coconut meal). At 175 8C, the carbohydrate content increased from 4.9 to 9.6 g/100 g dry coconut meal (p < 0.05) for 30–240 min of treatment, but the value decreased from 10.6 to 6.1 g/ 100 g dry coconut meal for 30–240 min of treatment at 200 8C. The soluble carbohydrate contained mannose, glucose, galactose and arabinose. A response surface methodology study indicated that 13.9 g/ 100 g dry coconut meal of mannose in the soluble carbohydrate could be produced at 227 8C in 3 min. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Coconut (Cocos nucifera L.) is an important plant in several Asian and South American countries with an annual production of more than 58 million tons around the world in 2007 [1]. Coconut copra (kernel meat) represents about 12% of the total fruit weight [2]. Besides other uses, fresh and fully mature copra is used for coconut milk production which is an aqueous extraction process of protein, fat and carbohydrate. Coconut milk is used for food and confectioneries preparation in Southeast Asia. During the extraction process using a coconut copra–water weight ratio of 1, about 10 g of dried solid residue is obtained for every 100 g of coconut milk [3]. Because the annual production of coconut milk is about 36,000 tons in Thailand [4], it can be estimated that around 3600 tons of coconut meal was produced each year. Coconut meal is normally sold by the coconut milk factory for feed production, but finding a new way to utilize coconut meal is important for adding value to this by-product. Coconut meal contains high amount of carbohydrates, i.e., about 43–45% of which is mainly in the form of mannose polysaccharide (61%) [5]. Saittagaroon et al. [6] demonstrated the use of copra meal for the preparation of mannitol by borohydride reduction. Other work has been done using coconut meal as a substrate for the enzymatic hydrolysis by mannanase or acid hydrolysis to produce mannose and mannooligosaccharides
[7–9]. These mannooligosaccharides show their potential use as a prebiotic substance which is a health promoting material for both humans and animals. Subcritical water, i.e., hot liquid water at 100–374 8C under high pressure, has been recognized as a promising mean for converting agricultural by-products into more value-added products [10,11]. Several studies had been done on converting protein-containing wastes into peptides and amino acids [12]. The treatment of lignocellulosic materials, such as wood, corn stalk and rice straw, with sub- and supercritical water resulted in glucose- and/or xyloseoligomers and monomers which are hydrolyzed from cellulose and/ or hemicellulose in the raw material [13–16]. However, these studies focused on using subcritical water as a pretreatment of material for further enzymatic hydrolysis processes. The treatment of coconut meal with subcritical water has not been reported in the literature. Producing mannose and its oligosaccharides from coconut meal by treatment in subcritical water seems to be possible. In this context, the objective of this study was to investigate the effect of temperature and time on the carbohydrate content and the sugar composition of the obtained product after the subcritical water treatment of coconut meal. 2. Materials and methods 2.1. Samples and reagents
* Corresponding author. Tel.: +66 034 219361; fax: +66 034 272194. E-mail address: [email protected] (P. Khuwijitjaru).
Coconut meal used as a raw material was a by-product from a local coconut milk factory (Vara Food and Drink, Co., Ltd., Nakhon
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.010
P. Khuwijitjaru et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 225–229
226
Pathom, Thailand). Standard sugars (arabinose, xylose, mannose, galactose and glucose) were purchased from Sigma Aldrich (Germany), and myo-inositol was purchased from Fluka (Japan). All other reagents were of analytical grade.
extraction using petroleum ether as the extractant. The ash content was determined by the dry ashing method.
2.2. Preparation of coconut meal
The residue from the filtration was dried at 130 8C for 1 h. The solid loss (% w/w) was calculated from:
The received coconut meal was dried at 60 8C for 4 h in a hot-air oven and sieved to obtain a coconut meal with particle size range of 0.3–2 mm. The dried sample was kept in an aluminum foil bag at room temperature for further studies. 2.3. Subcritical water treatment The subcritical water treatment was done in a batch-type reactor using a high pressure-resistant vessel made from stainless steel with a net volume of 100 mL (Taiatsu Techno Corporation, Tokyo, Japan). The temperature inside the vessel was controlled and monitored using an aluminum block heater (Applied Scientific Instruments, Bangkok, Thailand) equipped with a temperature controller and a type-K thermocouple. The pressure inside the vessel varied corresponding to the saturated vapor pressure of water at specific treatment temperature. A sample of dried coconut meal (8 g) was mixed with water (80 g) in the vessel. The vessel was tightly closed and heated at the desired temperatures. The heat-up time for any temperature was about 12 min. After reaching the desired treatment time, the vessel was immediately cooled with water to stop the reaction. The content in the vessel was vacuum filtered through a Whatman no. 1 filter paper. The filtrate was centrifuged at 10,000 g for 10 min at 4 8C, and the liquid portion was collected and adjusted to 100 mL with distilled water before further analyses. 2.4. Experimental design 2.4.1. Effects of treatment temperature and time The subcritical water treatment was performed at 100, 125, 150, 175 and 200 8C for 30, 60, 120 and 240 min using a factorial in a completely randomized design (CRD) with 3 replications. 2.4.2. Optimization of treatment temperature and time for mannose recovery After the first part of study (Section 2.4.1), further experiments were conducted at 200, 225 and 250 8C for 5, 10, 20 and 30 min without replication to determine the possible optimum condition for maximizing the amount of mannose in the soluble carbohydrate. The optimization was then performed using a response surface methodology with a central composite design (CCD) (Table 1). In total, 13 experiments including 5 replications at the center point were carried out. 2.5. Proximate analysis of coconut meal A proximate analysis of the coconut meal was performed using the AOAC method [17]. Briefly, the moisture content was determined using an oven drying method at 130 8C. The protein content was determined by the Kjeldalh method with a conversion factor of 6.25. The crude fat content was measured by Soxhlet Table 1 Levels of independent variables according to the central composite design (CCD). Variables
Temperature (8C) Time (min)
Levels Low
Center
High
Axial (a)
Axial (+a)
215 3
225 5
235 7
211 2.17
239 7.83
2.6. Solid loss, pH and UV-absorption
1
weight of dry residue 100% weight of dry coconut meal sample
(1)
Measurement of the pH value of the liquid product after centrifugation was done using a pH meter (PHM 210, Metro Lab, France). The absorption spectra of the liquid product were measured from 200 to 500 nm using a UV-Vis spectrometer (Genesys 20uv, Thermo Spectronic, USA) after the appropriate dilution. 2.7. Total carbohydrate content The total carbohydrate content of the product was determined using the phenol–sulfuric method [18]. Mannose was used for preparing a standard curve. Each sample was evaluated in duplicate. 2.8. Sugar composition The neutral sugar composition of the carbohydrate in the coconut meal sample and the liquid product was analyzed by a GCFID method. The meal or liquid product was hydrolyzed with sulfuric acid to release the monosaccharides using the protocol developed by NREL [19]. For measuring the amount of monosaccharides by the GC-FID method, an alditol acetate derivative was prepared according to the method of Ozga et al. [20] using myo-inositol as the internal standard. One microliter of a sample was injected (splitless) into a Shimadzu GC-8A (Kyoto, Japan) equipped with a DB-225 capillary column (30 m, Agilent Technologies, USA). The injector and detector temperatures were set at 240 8C and the column temperature was set at 200 8C. A quantitative analysis was done by preparing the standard curves of five sugars, i.e., mannose, glucose, galactose, xylose and arabinose. A GC-FID analysis was performed in duplicate for each sample. 2.9. Statistical analysis An analysis of variance (ANOVA) was performed to determine the significance of the treatment temperature and time for each analysis. The least significant difference (LSD, a = 0.05) was used for comparing the mean values when a significance was observed from the ANOVA test. Statistical tests were performed using IBM SPSS Statistics 18 (IBM, USA). In the optimization experiment, the response surface methodology was planned and analyzed using Design Expert 6.0.5 (Stat-Ease, USA). 3. Results and discussion 3.1. Proximate analysis and sugar composition of coconut meal The coconut meal sample used in this study had been extracted for coconut milk using water. After drying and grinding, the coconut meal sample contained a considerable amount of carbohydrate and fat, while most of the protein was extracted (Table 2). The carbohydrate composition analysis showed that the carbohydrate in the coconut meal contained mannose, glucose, galactose and a small amount of arabinose which agreed with that
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Table 2 Proximate analysis and sugar composition of coconut meal. Composition % wb 3.44 0.03 0.85 0.06 25.59 0.82 1.13 0.03 68.99 0.79 % of total carbohydrate 79.77 1.10 12.80 0.63 6.12 0.16 1.31 0.02
Total carbohydrate [g/100 g dry coconut meal]
10
Proximate analysis Moisture content Protein Fat Ash Total carbohydrate (by difference) Carbohydrate composition Mannose Glucose Galactose Arabinose
8
6
4
2
Data are expressed as mean standard deviation (n = 3).
0 0
60
120
180
240
300
Time [min] previously reported [5,7]. These results suggested that coconut meal is a good resource for producing a variety of carbohydrate products, especially mannose-containing oligosaccharides or mannose monosaccharide. Because the coconut meal contained about 25% fat, the recovery of fat and fatty acids should also be considered for another use of this by-product. 3.2. Effects of temperature and time 3.2.1. Solid loss and pH Dried coconut meal is white and the color changed to yellow, brown or black after treatment at a higher temperature and longer time. Because the subcritical water treatment promotes the hydrolysis reaction, converting the solid biomass into soluble substances was expected. In general, the solid content of coconut meal significantly decreased with the treatment temperature (p < 0.05). Treatments at 175 and 200 8C resulted in a rapid loss of the solid content. The effect of the treatment time was also more obvious at the higher temperature. About 40% of the coconut meal was solubilized after treatment for 240 min at 200 8C (Fig. 1). Measuring the pH values of the liquid product revealed that the formation of acidic components would decrease the pH from about 6 to 3 after an increase in temperature from 100 to 200 8C. Lu¨ and Saka [21] reported that acetic, glycolic, lactic, and formic acids could be detected after the subcritical water treatment of Japanese beech wood at 170–290 8C. Formation of the acids would help in promoting the hydrolysis reaction of coconut meal.
3.2.2. Carbohydrate content Since carbohydrate is the main component in coconut meal, we focused on determining the carbohydrate content and its composition in the liquid product obtained after treatment with subcritical water. Fig. 2 shows the changes in the carbohydrate content of the liquid product as a function of time at different treatment temperatures. At 100–150 8C, about 3.5–5.1 g/100 g dry coconut meal carbohydrate was obtained and the treatment time only slightly affected the content. However, at 175 8C, the carbohydrate content significantly increased (p < 0.05) with time and rapidly increased to 9.6 g/100 g dry coconut meal after a 240min treatment. In contrast, at 200 8C, the carbohydrate content was the highest at 30 and 60 min, i.e., about 10.6 g/100 g dry coconut meal, but a longer treatment resulted in a gradual decrease in the value (p < 0.05). This result indicated that the carbohydrate would be further degraded to others products, such as glyceraldehyde, 5hydroxymethylfurfural, and acids as reported by other studies [16,21–23]. Fig. 3 shows the relationship between the carbohydrate content and solid loss during the treatment at different temperatures. Under the conditions of this study, the carbohydrate content increased with the solid loss and reached a maximum value at about 30% solid loss, but a further decomposition of the solid did not produce more carbohydrates, indicating that the further 12
7
50
10
5 4
pH
30
20 3 10
2
Total carbohydrate [g/100 g dry coconut meal]
6
40
Solid loss [% w/w]
Fig. 2. Changes in the total carbohydrate content of liquid product for subcritical water treatment of coconut meal. Symbols used are the same as in Fig. 1.
8
6
4
2 1
0 0
60
120
180
240
300
Time [min]
0 0
10
20
30
40
50
Solid loss [% w/w] Fig. 1. Changes in solid loss (solid line) and pH of liquid product (dot line) for subcritical water treatment of coconut meal at 100 (*), 125 (&), 150 (4), 175(^), and 200 () 8C.
Fig. 3. Relationship between total carbohydrate content and solid loss for subcritical water treatment of coconut meal. Symbols used are the same as in Fig. 1.
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15
0.5
Mannose [g/100 g dry coconut meal]
Mannose 175°C, 120 min
Absorbance
0.4
0.3
175°C, 120 min
0.2
0.1
100°C, 30 min
175°C, 30 min
100°C, 120 min
10
5
0.0 220
270
320
0
370
0
Wavelength [nm]
10
20
30
Time [min]
Fig. 4. UV absorption spectra of liquid products from subcritical water treatment of coconut meal and pure mannose.
Fig. 5. Mannose content of carbohydrate in liquid product from subcritical water treatment of coconut meal at 200 (^), 225 (*), and 250 (4) 8C.
degradation of the soluble carbohydrates occurred during the longer treatment at high temperature as already described. UV-absorption of the liquid product was determined at the wavelengths of 200–500 nm but only part of the spectra that exhibited an absorption peak is shown in Fig. 4. Mannose was also treated with subcritical water at 175 8C for 120 min and its spectrum is also shown in the figure for comparison. The maximum absorption peak of the liquid product appeared at about 280–285 nm, which might correspond to 5-hydroxy methyl furfural or furfural. The spectrum from mannose would suggest that the absorption peak at 280–285 nm for the liquid product is ascribed to additional degradation products of the monosaccharides liberated from the coconut meal.
temperatures (100–150 8C), glucose was the main sugar of the carbohydrate in the liquid, however, at 175–200 8C, the glucose content decreased and mannose became the major sugar of the carbohydrate in the liquid product. This result suggested that glucose was more easily released from the polymer structure than mannose at the lower treatment temperature. The change in the mannose content with temperature and time was similar to that of the total carbohydrate content, that is, at 175 8C, it increased with time, but at 200 8C, it decreased with the treatment time. The highest amount of mannose (8.98 g/100 g dry coconut meal) was obtained at 200 8C and 30 min which was about 10 times compared to the amount of glucose. Galactose and arabinose occurred in only small amounts from the liquid product. The structure of the saccharides in the liquid product was not totally characterized in this study. However, preliminary experiments were performed using an HPLC method with a cation exchange column (Rezex RNM-Carbohydrate Na+ (8%), Phenomenex, USA). The chromatogram (data not shown) indicated that the liquid products contained mono- and disaccharides as well as oligosaccharides although the structure, degree of polymerization and amount were not determined. These results suggested that we
3.2.3. Carbohydrate composition Neutral sugar profiles of the carbohydrate in the liquid product obtained under the various treatment conditions are shown in Table 3. The carbohydrate in the liquid product was mainly composed of mannose, glucose, galactose and arabinose. The polysaccharide in coconut meal was hydrolyzed to release soluble carbohydrate, such as the mono-, di-, and oligosaccharides. At low
Table 3 Neutral sugar of carbohydrate products obtained by subcritical water treatment of coconut meal. Sugars (g/100 g dry coconut meal)
Mannose
Glucose
Galactose
Arabinose
Temperature
Time (min)
(8C)
30
60
120
240
100 125 150 175 200 100 125 150 175 200 100 125 150 175 200 100 125 150 175 200
0.76 0.17 0.86 0.05 0.94 0.06 1.16 0.07 8.98 1.00 1.60 0.31 1.62 0.15 1.45 0.11 0.90 0.05 0.65 0.09 0.15 0.06 0.28 0.06 0.47 0.06 0.64 0.03 1.15 0.05 0.02 0.03 0.15 0.03 0.40 0.04 0.39 0.01 0.03 0.02
0.92 0.04 1.05 0.21 1.14 0.10 2.08 0.05 7.89 0.65 1.87 0.10 2.00 0.05 1.82 0.20 0.78 0.12 1.07 0.08 0.19 0.01 0.31 0.01 1.07 0.09 0.89 0.11 0.54 0.05 0.03 0.01 0.20 0.04 0.56 0.06 0.39 0.05 Nd
0.87 0.15 0.76 0.10 0.92 0.10 3.57 0.42 2.62 0.07 1.90 0.23 1.58 0.18 1.37 0.03 0.49 0.02 0.49 0.12 0.19 0.03 0.31 0.00 0.83 0.09 0.99 0.08 0.06 0.03 0.05 0.02 0.22 0.03 0.54 0.06 0.12 0.01 Nd
0.78 0.20 0.68 0.13 0.95 0.12 5.07 0.11 0.52 0.27 1.88 0.21 1.56 0.04 1.21 0.09 0.44 0.08 0.18 0.17 0.23 0.02 0.38 0.03 0.84 0.02 0.84 0.07 Nd 0.10 0.03 0.30 0.01 0.50 0.04 Nd Nd
Data are expressed as mean standard deviation (n = 3). Nd = not detected.
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7
6
where X1 and X2 are the treatment temperature and time, respectively. The maximum mannose content predicted from the model was 13.9 g/100 g dry coconut meal obtained at 227 8C and 3 min. This value is comparable to that from the enzymatic hydrolysis reported by several patents, e.g., Yokomizo and Kadota [24] obtained the maximum mannose content of 15.1 g/100 g coconut meal by hydrolyzing coconut meal using b-galactomannanase and xylanase at 60 8C for 72 h.
9
Time [min]
10 10
5
11
12
12
11
13
4
4. Conclusions This study showed that the subcritical water treatment is a promising technology for the production of both mono- and oligosaccharides from coconut meal by-product. In this batch-type reactor, a high temperature (ca. 230 8C) and short (<3 min) treatment time tended to give a high amount of carbohydrate with a high mannose content. Further analysis for structure and content oligosaccharides and other degradation products formed should be also conducted.
9
3 215
220
225
230
235
Temperature [°C] Fig. 6. Effects of treatment temperature and time on the mannose content of carbohydrate in liquid product from subcritical water treatment of coconut meal. The numerical values in the figure represent the content in units of g/100 g dry coconut meal.
can produce mannose oligomers and/or monomer from coconut meal using the subcritical water treatment instead of enzymatic hydrolysis [7,9,24] which usually takes a longer time and needs expensive enzymes for the reaction. 3.3. Optimization of subcritical water treatment Based on the results of this study, it seemed that the higher treatment temperature and shorter time might produce a higher mannose content of carbohydrates in the liquid product. Therefore, preliminary experiments for the treatment at 200, 225 and 250 8C for 5, 10, 20 and 30 min were performed without replication and the mannose content in the liquid product is shown in Fig. 5. Treatment at 200 8C for a shorter time did not give a high amount of mannose as we expected, and the highest mannose content was obtained at 225 8C and 5 min. Treatment at 250 8C completely decomposed the carbohydrate in the liquid product even for the shortest time. These results indicated that degradation of mannose and mannose-containing di- and oligosaccharides were quite fast at temperatures above 225 8C and this probably caused by catalysis effect of degradation products, although a complete degradation of mannose was not observed in this temperature range during the hydrolysis of woods in previous studies [16,25]. The response surface methodology was conducted to find the possible maximum yield of mannose in the liquid product using the CCD with a center point at 225 8C and 5 min. Fig. 6 shows the response contour plot for the mannose content in the liquid product. The contour line was modeled by the secondary order polynomial equation: Mannose
g 100 g dry coconut meal
¼ 2264:4 þ 19:8X 1 þ 23:0X 2 0:04X12 0:05X22 0:1X 1 X 2
229
(2)
Acknowledgement This work was done under the project ‘‘Coconut product development and quality improvement’’ supported by a grant from the Silpakorn University Research and Development Institute, Thailand. References [1] FAO. 2007. Available from: http://faostat.fao.org. [2] B.E. Grimwood, F. Ashman, D.A.V. Dendy, C.G. Jarman, E.C.S. Little, Coconut Palm Products: Their Processing in Developing Countries, FAO, Rome, 1975, pp. 279. [3] S. Sringam, (in Thai), Proceeding of the 26th Kasetsart University Conference, Bangkok, Thailand, (1988), pp. 309–316. http://www.lib.ku.ac.th/KUCONF/ KC2605035.pdf. [4] Office of Industrial Economics. 2003. Available from: http://www.oie.go.th. [5] S. Saittagaroon, S. Kawakishi, M. Namiki, Journal of the Science of Food and Agriculture 34 (1983) 855. [6] S. Saittagaroon, S. Kawakishi, M. Namiki, Journal of Food Science 50 (1985) 757. [7] I. Kusakabe, M. Zama, G.G. Park, K. Tubaki, K. Murakami, Agricultural and Biological Chemistry 51 (1987) 2825. [8] G. Yoshikawa, T. Yano, Japan Patent 2000139490 (2000). [9] H. Kanatani, F. Yokomizo, US Patent 20090311411 (2009). [10] O. Pourali, F.S. Asghari, H. Yoshida, Food Chemistry 115 (2009) 1. [11] J. Wiboonsirikul, S. Adachi, Food Science and Technology Research 14 (2008) 319. [12] H. Cheng, X. Zhu, C. Zhu, J. Qian, N. Zhu, L. Zhao, J. Chen, Bioresource Technology 99 (2008) 3337. [13] H. Boussarsar, B. Roge´, M. Mathlouthi, Bioresource Technology 100 (2009) 6537. [14] C.E. Wyman, B.E. Dale, R.T. Elander, M. Holtzapple, M.R. Ladisch, Y.Y. Lee, Bioresource Technology 96 (2005) 2026. [15] P. Khuwijitjaru, C. Wanpen, T. Mala, M. Ariyakriangkrai, S. Adachi, KKU Research Journal 14 (2009) 1084 (in Thai). [16] K.-H. Kim, I.-Y. Eom, S.-M. Lee, S.-T. Cho, I.-G. Choi, J.W. Choi, Journal of Industrial Engineering Chemistry 16 (2010) 918–922. [17] A.O.A.C., Official Methods of Analysis of AOAC International, 17th ed., AOAC International, Maryland, 2000. [18] M. DuBois, K.A. Gilles, J.K. Hamilton, P.A. Rebers, F. Smith, Analytical Chemistry 28 (1956) 350. [19] R. Ruiz, T. Ehrman, Dilute acid hydrolysis procedure for determination of total sugars in the liquid fraction of process samples, NREL, Golden, CO, 1996. [20] J.A. Ozga, A. Saeed, W. Wismer, D.M. Reinecke, Journal of Agricultural and Food Chemistry 55 (2007) 10414. [21] X. Lu¨, S. Saka, Biomass and Bioenergy 34 (2010) 1089. [22] M. Sasaki, B. Kabyemela, R. Malaluan, S. Hirose, N. Takeda, T. Adschiri, K. Arai, Journal of Supercritical Fluids 13 (1998) 261. [23] S. Haghighat Khajavi, Y. Kimura, T. Oomori, R. Matsuno, S. Adachi, Journal of Food Engineering 68 (2005) 309. [24] F. Yokomizo, T. Kadota, US Patent 6797292B2. [25] M. Matsunaga, H. Matsui, Y. Otsuka, S. Yamamoto, The Journal of Supercritical Fluids 44 (2008) 364.