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Physicochemical properties of konjac glucomannan extracted from konjac flour by a simple centrifugation process
LWT - Food Science and Technology 44 (2011) 2059e2063
Contents lists available at ScienceDirect
LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Physicochemical properties of konjac glucomannan extracted from konjac flour by a simple centrifugation process Orawan Tatirat, Sanguansri Charoenrein* Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, Bangkok 10900, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 November 2010 Received in revised form 13 July 2011 Accepted 14 July 2011
Konjac flour (KF) contains main polysaccharide, konjac glucomannan (KGM) that is applied in various applications; however, extraction of KGM is complicated. A simple centrifugation process was used to extract KGM from KF and the influence of extraction temperatures on its physicochemical properties was determined. The centrifugation process was effective for the extraction of KGM and the results revealed two effective temperatures (35 C and 75 C). The extracted KGM was easy to grind. Ash and protein contents of the extracted KGM were clearly reduced, however the ash and protein contents of the KGM extracted at the high temperature (75 C) were significantly lower than that extracted at the low temperature (35 C), and thus proofs the extraction at the high temperature to be more effective. Furthermore, improved purities in both extracted glucomannan samples were attained in comparison to commercial KGM. While the yield percentages of the samples differed, no significant disparity in morphology and particle size was determined. Particles of both extracted KGM were comparable in shape and size. Moreover, the transparency of both extracted KGM solutions was higher than commercial KGM solution. These results suggested that the extraction temperature at 75 C is effective in extraction KGM from KF. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Konjac glucomannan Extraction Centrifugation Transparency Konjac flour
1. Introduction Konjac or Amorphousphallus is a perennial plant and a member of the family of Araceae. Konjac glucomannan is found abundantly in the konjac tuber (Amorphophallus Konjac). The tuber is usually grown in Asian countries such as China, Japan, and Thailand (Fang & Wu, 2004; Takigami, 2000). Konjac glucomannan is a neutral polysaccharide that composed of b-1,4 linked D-mannose, and Dglucose. The ratio of mannose and glucose is about 1.6:1, and there are some branching points at the C-3 position of the mannoses. The chain of konjac glucomannan has b-1,4 linked D-mannose, and Dglucose joined though the C-3 of D-mannose and D-glucose, with an approximate degree of branching of 8%. Furthermore, the chain has a 5e10% acetyl group substitute (Takigami, 2000). Konjac glucomannan is a type of amorphous polymer and the range of its molecular weight (Mw) is 105e106 (Li & Xie, 2003). Dispersions of konjac glucomannan (0.5 g/100 g) had the highest viscosity amongst 12 polysaccharides tested, and exhibits shear thinning behavior (Yaseen, Herald, Aramouni, & Alavi, 2005). The viscosity of konjac glucomannan solution (1.0 g/100 g) was * Corresponding author. E-mail address: [email protected] (S. Charoenrein). 0023-6438/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2011.07.019
w30,000 cps (Takigami, 2000). Moreover, konjac glucomannan has very high water absorbency, absorbing as much as 100 g of water per g of sample, and the water absorbency of konjac glucomannan was decreased as increasing degree of acetylation on its chains (Koroskenyi & McCarthy, 2001). Gelation behavior of konjac glucomannan is a phenomenon that is related to the limited presence of the acetyl group in the chain. Alkali solution is required for gelation of KGM at a low solid content (w1.0e3.0 g/100 g) (Gao & Nishinari, 2004); however, KGM can also form gel in the absence of alkali with its high solid content (Dave, Sheth, McCarthy, Ratto, & Kaplan, 1998). In addition, much research has proven the health benefits of konjac glucomannan (Chen, Fan, Chen, & Chan, 2005; Martino, Martino, Carnevali, Forcone, & Niglio, 2005; Wood et al., 2007). Consequently, konjac glucomannan is used in many applications such as food, pharmaceutical, biotechnology, and fine chemical areas (Zhang, Xie, & Gan, 2005). As for the extraction of konjac glucomannan, the konjac tuber is washed, sliced, dried and ground; and the konjac flour (KF) of refined powder is subsequently separated by wind shifting. KF contains numerous impurities, mostly insoluble starch and cellulose, inclusive a few proteins and lipids, many of which are impurities derived from the “sac” which encapsulates the flour in the tuber. Thus, the solution and gel of the KF have highly turbidity, and
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O. Tatirat, S. Charoenrein / LWT - Food Science and Technology 44 (2011) 2059e2063
cloudy (Ohashi, Shelso, Moirano, & Drinkwater, 2000; Takigami, 2000). Content of konjac glucomannan varies between 8 and 10 g/100 g in raw tuber (Takigami, 2000). The refined powder or KF contains konjac glucomannan in the range of 50e70 g/100 g, whereas purified powder or extracted konjac glucomannan from tuber has konjac glucomannan content in excess of 90 g/100 g (Fang & Wu, 2004; Ohashi et al., 2000; Takigami, 2000). To extract konjac glucomannan from KF, the flour is washed with ethanol solution to remove the impurities trapped in the konjac particles (Ohashi et al., 2000; Takigami, 2000). Several techniques to extract glucomannan from KF can be applied, for instance enzyme treatment, dialysis, washing with alcohol, or a combination of the aforementioned methods; however, most of these processes are complex and information on the influences of the extraction temperature on physicochemical properties is scarce. In this study, a simple centrifugation process at diverse temperatures was developed for the extraction of konjac glucomannan from KF. All extracted glucomannan, KF and commercially available konjac glucomannan (CK) were investigated for compositions, morphology, particle size, color and transparency as to determine such influences. 2. Materials and methods 2.1. KF and CK KF was supplied by Sahachol Food Supplies Co., Ltd. (Chonburi, Thailand), and the color of the flour is white to gray. CK of yellowish color was purchased from DKSH (Bangkok, Thailand) Limited. 2.2. Extraction process of konjac glucomannan from KF Konjac glucomannan was extracted from KF by means of a simple centrifugation process by the adaptation of the Ohashi method (Ohashi et al., 2000). Firstly, aluminum sulfate was dissolved in distilled water (0.3 g/100 mL), and KF was poured into the aluminum solution (3.0 g/100 mL). Then the mixture was subsequently stirred for 15 min in a water bath with varied temperatures (35 C, 75 C, 85 C, and 95 C). The mixture was diluted three folds; prior to centrifugation (1,500 g for 15 min at control temperature of 25 C). Ninety five percent of ethanol solution (supernatant:ethanol ¼ 1:1) was added to the supernatant as to precipitate the glucomannan then the precipitate was collected with filtrating the mixture through cheesecloth, and this step was repeated three times. The coagulated glucomannan was dried at 45 C overnight. The dried sample was ground with mortar and sieved with a screen (250 mm) then the sample left on the screen was pulverized with mortar before sieving with the screen again. All samples were analyzed for components, morphology, particle size, color values and transparency. In this study, extracted konjac glucomannan at 35 C, 75 C, 85 C, and 95 C were labeled as K35, K75, K85 and K95 respectively.
Welwyn Garden City, Hertfordshire, UK). The accelerating voltage and the magnification were displayed on the micrographs. 2.3.3. Particle size The particle size was determined by the application of Mastersizer 2000 with Scirocco 2000 (Malvern Instrument Ltd., Malvern, Worcestershire, UK). Each dried sample (3e5 g) was blown into a chamber; to gauge the size of the particles. The results were reported both the average particle size and the particle size distribution. 2.3.4. Color values Lightness of all samples (L*) was assessed with a Minolta spectrophotometer (Minolta, CM 3500d, Konica Minolta Sensing Inc., Ramsey, NJ, USA) Dried samples were put into a acrylic cylinder and the values were consequently measured. 2.3.5. Transparency Transparency was evaluated by visual observation. Dried samples were dissolved in distilled water (1.0 g/100 g) for 30 min with a magnetic stirrer; and subsequently photographed. 2.4. Statistical analysis A completely randomized design was used in this study. Determination of components, particles and color were performed in triplicate. All the experiments were conducted in triplicate and the average results were reported. Statistical analysis was applied by means of analysis of variance (ANOVA) and Duncan’s multiple range test with the application of SPSS version 10.0.0 (IBM SPSS, Chicago, IL, USA). Results were considered significant for p < 0.05. 3. Results and discussions Appearance of extracted konjac glucomannan (K35, K75, K85, and K95) samples was observed, and the clear film formation was founded in K85 and K95 samples (Fig. 1 (c) and (d)). This result
2.3. Determination of physicochemical properties 2.3.1. Components To elucidate effectiveness of extraction temperatures, protein, ash and fat contents were determined. The samples were measured for protein, ash and fat contents according to AOAC Method 981.10, 920.153 and 948.15 (A.O.A.C., 2005). 2.3.2. Morphology Morphology was determined with a Scanning Electron Microscope (SEM). Dried samples were placed on a stub and coated with gold, and observed with a JSM-5600LV microscope (JEOL,
Fig. 1. Appearance of extracted konjac glucomannan samples at different temperatures: (a) K35, (b) K75, (c) K85 and (d) K95.
O. Tatirat, S. Charoenrein / LWT - Food Science and Technology 44 (2011) 2059e2063
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Table 1 Protein, ash contents and extraction yield (g/100 g) of konjac flour (KF), konjac glucomannan extracted at 35 C (K35), 75 C (K75) and commercial konjac glucomannan (CK). Samples
Content (g/100 g) Protein
KF K35 K75 CK
0.98a 0.17c 0.09d 0.72b
Extraction yield (g/100 g) Ash
0.01 0.01 0.00 0.01
3.96a 0.42c 0.39c 1.03b
0.03 0.01 0.00 0.01
e 32.52a 2.53 35.41b 1.71 e
Mean values in each column with different superscripts significantly differ (p 0.05). Extraction yield was the average of three replicates.
indicated that these temperatures (85 C and 95 C) were higher than the exothermic transition temperature of konjac glucomannan (74e80 C) which is attributed with the disordereorder transition in a molecular chain (Karim et al., 2005). However, moisture content of konjac glucomannan dispersion in their work was slightly differed from the content in this work: the moisture contents in their work and this work were 99.0 g/100 g and 97.0 g/ 100 g. Moreover, the high temperatures (85 C and 95 C) should not be a cause of starch film formation although KF normally composed with starch. The gelatinization temperature of starch from most tubers such as tapioca, potato and sweet potato was normally 60e70 C (Chaisawang & Suphantharika, 2006; Li & Yeh, 2001), and the film formation was not still presented in the extracted konjac glucomannan sample at 75 C. Thus, the reason for the film formation should be the cause of konjac glucomannan disordereorder transition rather than starch interaction. On the other hand, K35 and K75 (Fig. 1 (a) and (b)) samples were easily grounded and sieved. Thus, K85 and K95 samples were excluded from further investigation. The result inferred that extracted konjac glucomannan had a good film forming ability; however, its ability to form film was not intended to study in this research. To elucidate influence of extraction temperature on purity of the samples, the contents of protein, ash and fat were determined as shown in Table 1. Protein and ash contents of both K35 and K75 were clearly below KF. The results indicated that the applied extraction process is effective as to yield glucomannan from KF. Protein contents of K75 were significantly lower than those of K35 (p 0.05): percentages of protein content in K35 and K75 samples
Fig. 3. Particle size distribution of samples: (a) KF, (b) K35, (c) K75 and (d) CK by Mastersizer.
were 0.17 0.01 and 0.09 0.00. This result revealed that extraction at 75 C was more effective than at 35 C, and the purities of both extracted samples were higher than that of CK. No fat content was detected in KF, fat was consequently absent both in K35 and K75 as well as CK. Furthermore, the yield percentages of K35 and K75 valued 32.52 2.53 and 35.41 1.71 respectively (Table 1): those values were significantly different (p 0.05). This result also emphasized that temperature affected extraction of konjac glucomannan form KF. Morphology and particles size distribution and average particles size of all samples are given in Fig. 2, Fig. 3 and Table 2. Table 2 Average particle size and L* value of konjac flour (KF), konjac glucomannan extracted at 35 C (K35), 75 C (K75) and commercial konjac glucomannan (CK).
Fig. 2. SEM photographs showing morphology of samples: (a) KF, (b) K35, (c) K75 and (d) CK (200, Bar ¼ 100 mm). The samples were prepared by scattering the dried powder on stab, and coating with gold before morphology observation.
Samples
Average granule sizes (mm)
L*
KF K35 K75 CK
285.31a 194.93b 183.24c 112.83d
73.99c 90.78a 88.48b 88.03b
0.70 1.67 2.69 0.70
0.18 0.16 0.03 0.10
Mean values in each column with different superscripts significantly differ (p 0.05). Average granule size, and L* values were the average of three replicates.
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Fig. 4. Transparency of 1.0 g/100 g of samples in distilled water: (a) KF, (b) K35, (c) K75, (d) CK and (e) distilled water. Black dot was a mark on the paper placed under the solution container.
The average particle sizes of KF, K35, K75 and CK were w280, 190, 190 and 110 mm respectively. Granules of KF were round in shape with smooth surfaces, and the particle size distribution ranged from w100 to 1000 mm, which corresponds to previous reports (Takigami, 2000). In contrast, granules of K35 and K75 were of irregular shapes. Additionally, the average particle size was considerably smaller than that of KF. A significantly increased surface roughness was detected in comparison with the smoother surface of KF granules, because the granular structures were broken by mechanical forces during extraction process. Furthermore, morphology of both extracted samples did not clearly differ from commercial samples and the morphology of both extracted konjac glucomannan samples was also similar to CK of previous study (Cheng, Karim, & Seow, 2007). However, the average particle size of both extracted konjac glucomannan were slightly larger than that of CK, and the distribution of the particles extracted samples (w10e600 mm) were also wider than that of CK (w30e400 mm), as the different methods for grinding the samples. In this study, a mortar was used for grinding konjac glucomannan while the commercial sample in the industry was normally ground with high powerful blender. Thus, the particle sizes of CK were more homogenous distribution than extracted samples in this work. L* values (lightness) of KF, K35, K75 and CK are shown in Table 2. Both K35 and K75 were determined to be lighter than KF. The result corresponded to the components of these samples in Table 1. These results indicate that both K35 and K75 were lighter and more purified subsequent to extraction. An extraction temperature at 35 C yielded slightly lighter samples than extraction at 75 C; moreover, L* of K35 was higher than that of CK. However, the distributions of samples in this work were not similar (Table 2). Normally, the lightness value of powder depended on the particle size, and lightness of small particle size was normally higher than that of large particle size (Galiba, Waniska, Rooney, & Miller, 1998). In this case, the lightness values of K35 and CK were not significantly different but the average particle size of K35 was larger than CK. This result implied that K35 should show more lightness value than CK in case of the same particle sizes. In addition, the water holding capacity of konjac glucomannan is an important property that is related to its application, and this capacity of konjac glucomannan is very high, absorbing as much as 100 g of water per g of sample (Koroskenyi & McCarthy, 2001). However, only extracted konjac glucomannan samples (K35 and K75) were determined for water absorption index (WAI) which indicates a capacity to hold water of konjac sample. WAI values of K35 and K75 did not significantly differ: the values of K35 and K75 were 88.53 7.68 and 86.24 7.55 (Detail of measurement was not described). This result indicated that an increase in extracted temperature from 35 C to 75 C did not influence water holding capacity of extracted konjac glucomannan. However, an increase in extracted temperature to > 75 C might decrease its water holding capacity because the high extracted temperature could encourage
the disordereorder transition in its molecular chain (Karim et al., 2005). Thus, the rearrangement of the chains at high extracted temperature (>75 C) might inhibit the interaction of hydrogen bonding between water and konjac glucomannan, resulting a reduction of water holding capacity. Moreover, all samples were dissolved in distilled water (1.0 g/ 100 g dry basis) and the transparency was observed. Samples dissolved in distilled water were found to be of comparable transparency (Fig. 4), although the lightness value of the K75 powder was lower than that of the K35 powder. This was probably caused by the application of aluminum sulfate as a flocculating agent in the extraction process, so the impurities such as protein content in both K35 and K75 were lower than those of CK (Table 1). Consequently, this extraction method is one of alternative ways to produce konjac glucomannan for industry. 4. Conclusions A simple process of extracting konjac glucomannan from KF by centrifugation was successful. The temperature influenced appearance, components, yield percentage, color and transparency of extracted konjac glucomannan. During extraction of konjac glucomannan, the temperature should not exceed 75 C as to prevent film formation. Extraction of konjac glucomannan at 75 C has been recommended in this study as to achieve good yield and physicochemical properties of the products. Acknowledgments We gratefully acknowledged financial supports from Thailand Research Fund though the Royal Golden Jubilee - PhD Program (Grant No. PHD/0127/2550). The authors wished to thank Prajongwate Satmalee and researchers at Institute of Food Research and Product Development of Kasetsart University (Thailand) for supporting this research. References A.O.A.C. (2005). Official method of analysis (AOAC Official Method 981.10, 920.153, 948.15). Arlington: Association of Official Analytical Chemists. Chaisawang, M., & Suphantharika, M. (2006). Pasting and rheological properties of native and anionic tapioca starch as modified by guar gum and xanthan gum. Food Hydrocolloids, 20, 641e649. Chen, H. L., Fan, Y. H., Chen, M. E., & Chan, Y. (2005). Unhydrolysed and hydrolysed konjac glucomannans modulated caecal and faecal microflora in Balb/c mice. Nutrition, 21, 1059e1064. Cheng, L. H., Karim, A. A., & Seow, C. C. (2007). Effects of acid modification on physical properties of konjac glucomannan (KGM) films. Food Chemistry, 103, 994e1002. Dave, V., Sheth, M., McCarthy, S. P., Ratto, J. A., & Kaplan, D. L. (1998). Liquid crystalline, rheological and thermal properties of konjac glucomannan. Polymer, 39(5), 1139e1148. Fang, W., & Wu, P. (2004). Variation of Konjac glucomannan (KGM) from Amorphophallus konjac and its refined powder in Chaina. Food Hydrocolloids, 18, 167e170.
O. Tatirat, S. Charoenrein / LWT - Food Science and Technology 44 (2011) 2059e2063 Galiba, M., Waniska, R. D., Rooney, L. W., & Miller, F. R. (1998). Couscous quality of sorghum with different kernel characteristics. Journal of Cereal Science, 7, 183e193. Gao, S., & Nishinari, K. (2004). Effect of degree of acetylation on gelation of konjac glucomannan. Biomacromolecules, 5, 175e185. Karim, A. A., Chang, Y. P., Norziah, M. H., Ariffin, F., Nadiha, M. Z., & Seow, C. C. (2005). Exothermic events on heating of semi-dilute konjac glucomannanwater systems. Carbohydrate Polymers, 61, 368e373. Koroskenyi, B., & McCarthy, S. P. (2001). Synthesis of acetylated konjac glucomannan and effect of degree of acetylation on water absorbency. Biomacromolecules, 2, 824e826. Li, B., & Xie, B. (2003). Study on gel formation mechanism of konjac glucomannan. Agricultural Sciences in China, 2(4), 424e428. Li, J.-Y., & Yeh, A.-I. (2001). Relationships between thermal, rheological characteristics and swelling power for various starch. Journal of Food Engineering, 50, 141e148. Martino, F., Martino, E., Carnevali, F., Forcone, R., & Niglio, T. (2005). Effect of dietary supplementation with glucomannan on plasma total cholesterol and low
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density lipoprotein cholesterol in hypercholesterolemic children. Nutrition, Metabolism and Cardiovascular Disceases, 15, 74e80. Ohashi, S., Shelso, G. J., Moirano, A. L., & Drinkwater W. L. (2000). Clarified konjac glucomannan. United States patent, number 6,162,906. Takigami, S. (2000). Konjac mannan. In G. O. Philips, & P. A. Williams (Eds.), Handbook of hydrocolloids (pp. 413e424). New York: Woodhead Publishing Limited. Wood, R. J., Fernandez, M. L., Sharman, M. J., Silvestre, R., Greene, C. M., Zern, T. L., et al. (2007). Effects of a carbohydrate-restricted diet with and without supplemental soluble fiber on plasma low-density lipoprotein cholesterol and other clinical markers of cardiovascular risk. Metabolism Clinical and Experimenrimental, 56, 58e67. Yaseen, E. I., Herald, T. J., Aramouni, F. M., & Alavi, S. (2005). Rheological properties of selected gum solutions. Food Research International, 38, 111e119. Zhang, Y. Q., Xie, B. J., & Gan, X. (2005). Advance in the applications of konjac glucomannan and its derivatives. Carbohydrate Polymers, 60, 27e31.
Contents lists available at ScienceDirect
LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Physicochemical properties of konjac glucomannan extracted from konjac flour by a simple centrifugation process Orawan Tatirat, Sanguansri Charoenrein* Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, Bangkok 10900, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 November 2010 Received in revised form 13 July 2011 Accepted 14 July 2011
Konjac flour (KF) contains main polysaccharide, konjac glucomannan (KGM) that is applied in various applications; however, extraction of KGM is complicated. A simple centrifugation process was used to extract KGM from KF and the influence of extraction temperatures on its physicochemical properties was determined. The centrifugation process was effective for the extraction of KGM and the results revealed two effective temperatures (35 C and 75 C). The extracted KGM was easy to grind. Ash and protein contents of the extracted KGM were clearly reduced, however the ash and protein contents of the KGM extracted at the high temperature (75 C) were significantly lower than that extracted at the low temperature (35 C), and thus proofs the extraction at the high temperature to be more effective. Furthermore, improved purities in both extracted glucomannan samples were attained in comparison to commercial KGM. While the yield percentages of the samples differed, no significant disparity in morphology and particle size was determined. Particles of both extracted KGM were comparable in shape and size. Moreover, the transparency of both extracted KGM solutions was higher than commercial KGM solution. These results suggested that the extraction temperature at 75 C is effective in extraction KGM from KF. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Konjac glucomannan Extraction Centrifugation Transparency Konjac flour
1. Introduction Konjac or Amorphousphallus is a perennial plant and a member of the family of Araceae. Konjac glucomannan is found abundantly in the konjac tuber (Amorphophallus Konjac). The tuber is usually grown in Asian countries such as China, Japan, and Thailand (Fang & Wu, 2004; Takigami, 2000). Konjac glucomannan is a neutral polysaccharide that composed of b-1,4 linked D-mannose, and Dglucose. The ratio of mannose and glucose is about 1.6:1, and there are some branching points at the C-3 position of the mannoses. The chain of konjac glucomannan has b-1,4 linked D-mannose, and Dglucose joined though the C-3 of D-mannose and D-glucose, with an approximate degree of branching of 8%. Furthermore, the chain has a 5e10% acetyl group substitute (Takigami, 2000). Konjac glucomannan is a type of amorphous polymer and the range of its molecular weight (Mw) is 105e106 (Li & Xie, 2003). Dispersions of konjac glucomannan (0.5 g/100 g) had the highest viscosity amongst 12 polysaccharides tested, and exhibits shear thinning behavior (Yaseen, Herald, Aramouni, & Alavi, 2005). The viscosity of konjac glucomannan solution (1.0 g/100 g) was * Corresponding author. E-mail address: [email protected] (S. Charoenrein). 0023-6438/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2011.07.019
w30,000 cps (Takigami, 2000). Moreover, konjac glucomannan has very high water absorbency, absorbing as much as 100 g of water per g of sample, and the water absorbency of konjac glucomannan was decreased as increasing degree of acetylation on its chains (Koroskenyi & McCarthy, 2001). Gelation behavior of konjac glucomannan is a phenomenon that is related to the limited presence of the acetyl group in the chain. Alkali solution is required for gelation of KGM at a low solid content (w1.0e3.0 g/100 g) (Gao & Nishinari, 2004); however, KGM can also form gel in the absence of alkali with its high solid content (Dave, Sheth, McCarthy, Ratto, & Kaplan, 1998). In addition, much research has proven the health benefits of konjac glucomannan (Chen, Fan, Chen, & Chan, 2005; Martino, Martino, Carnevali, Forcone, & Niglio, 2005; Wood et al., 2007). Consequently, konjac glucomannan is used in many applications such as food, pharmaceutical, biotechnology, and fine chemical areas (Zhang, Xie, & Gan, 2005). As for the extraction of konjac glucomannan, the konjac tuber is washed, sliced, dried and ground; and the konjac flour (KF) of refined powder is subsequently separated by wind shifting. KF contains numerous impurities, mostly insoluble starch and cellulose, inclusive a few proteins and lipids, many of which are impurities derived from the “sac” which encapsulates the flour in the tuber. Thus, the solution and gel of the KF have highly turbidity, and
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O. Tatirat, S. Charoenrein / LWT - Food Science and Technology 44 (2011) 2059e2063
cloudy (Ohashi, Shelso, Moirano, & Drinkwater, 2000; Takigami, 2000). Content of konjac glucomannan varies between 8 and 10 g/100 g in raw tuber (Takigami, 2000). The refined powder or KF contains konjac glucomannan in the range of 50e70 g/100 g, whereas purified powder or extracted konjac glucomannan from tuber has konjac glucomannan content in excess of 90 g/100 g (Fang & Wu, 2004; Ohashi et al., 2000; Takigami, 2000). To extract konjac glucomannan from KF, the flour is washed with ethanol solution to remove the impurities trapped in the konjac particles (Ohashi et al., 2000; Takigami, 2000). Several techniques to extract glucomannan from KF can be applied, for instance enzyme treatment, dialysis, washing with alcohol, or a combination of the aforementioned methods; however, most of these processes are complex and information on the influences of the extraction temperature on physicochemical properties is scarce. In this study, a simple centrifugation process at diverse temperatures was developed for the extraction of konjac glucomannan from KF. All extracted glucomannan, KF and commercially available konjac glucomannan (CK) were investigated for compositions, morphology, particle size, color and transparency as to determine such influences. 2. Materials and methods 2.1. KF and CK KF was supplied by Sahachol Food Supplies Co., Ltd. (Chonburi, Thailand), and the color of the flour is white to gray. CK of yellowish color was purchased from DKSH (Bangkok, Thailand) Limited. 2.2. Extraction process of konjac glucomannan from KF Konjac glucomannan was extracted from KF by means of a simple centrifugation process by the adaptation of the Ohashi method (Ohashi et al., 2000). Firstly, aluminum sulfate was dissolved in distilled water (0.3 g/100 mL), and KF was poured into the aluminum solution (3.0 g/100 mL). Then the mixture was subsequently stirred for 15 min in a water bath with varied temperatures (35 C, 75 C, 85 C, and 95 C). The mixture was diluted three folds; prior to centrifugation (1,500 g for 15 min at control temperature of 25 C). Ninety five percent of ethanol solution (supernatant:ethanol ¼ 1:1) was added to the supernatant as to precipitate the glucomannan then the precipitate was collected with filtrating the mixture through cheesecloth, and this step was repeated three times. The coagulated glucomannan was dried at 45 C overnight. The dried sample was ground with mortar and sieved with a screen (250 mm) then the sample left on the screen was pulverized with mortar before sieving with the screen again. All samples were analyzed for components, morphology, particle size, color values and transparency. In this study, extracted konjac glucomannan at 35 C, 75 C, 85 C, and 95 C were labeled as K35, K75, K85 and K95 respectively.
Welwyn Garden City, Hertfordshire, UK). The accelerating voltage and the magnification were displayed on the micrographs. 2.3.3. Particle size The particle size was determined by the application of Mastersizer 2000 with Scirocco 2000 (Malvern Instrument Ltd., Malvern, Worcestershire, UK). Each dried sample (3e5 g) was blown into a chamber; to gauge the size of the particles. The results were reported both the average particle size and the particle size distribution. 2.3.4. Color values Lightness of all samples (L*) was assessed with a Minolta spectrophotometer (Minolta, CM 3500d, Konica Minolta Sensing Inc., Ramsey, NJ, USA) Dried samples were put into a acrylic cylinder and the values were consequently measured. 2.3.5. Transparency Transparency was evaluated by visual observation. Dried samples were dissolved in distilled water (1.0 g/100 g) for 30 min with a magnetic stirrer; and subsequently photographed. 2.4. Statistical analysis A completely randomized design was used in this study. Determination of components, particles and color were performed in triplicate. All the experiments were conducted in triplicate and the average results were reported. Statistical analysis was applied by means of analysis of variance (ANOVA) and Duncan’s multiple range test with the application of SPSS version 10.0.0 (IBM SPSS, Chicago, IL, USA). Results were considered significant for p < 0.05. 3. Results and discussions Appearance of extracted konjac glucomannan (K35, K75, K85, and K95) samples was observed, and the clear film formation was founded in K85 and K95 samples (Fig. 1 (c) and (d)). This result
2.3. Determination of physicochemical properties 2.3.1. Components To elucidate effectiveness of extraction temperatures, protein, ash and fat contents were determined. The samples were measured for protein, ash and fat contents according to AOAC Method 981.10, 920.153 and 948.15 (A.O.A.C., 2005). 2.3.2. Morphology Morphology was determined with a Scanning Electron Microscope (SEM). Dried samples were placed on a stub and coated with gold, and observed with a JSM-5600LV microscope (JEOL,
Fig. 1. Appearance of extracted konjac glucomannan samples at different temperatures: (a) K35, (b) K75, (c) K85 and (d) K95.
O. Tatirat, S. Charoenrein / LWT - Food Science and Technology 44 (2011) 2059e2063
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Table 1 Protein, ash contents and extraction yield (g/100 g) of konjac flour (KF), konjac glucomannan extracted at 35 C (K35), 75 C (K75) and commercial konjac glucomannan (CK). Samples
Content (g/100 g) Protein
KF K35 K75 CK
0.98a 0.17c 0.09d 0.72b
Extraction yield (g/100 g) Ash
0.01 0.01 0.00 0.01
3.96a 0.42c 0.39c 1.03b
0.03 0.01 0.00 0.01
e 32.52a 2.53 35.41b 1.71 e
Mean values in each column with different superscripts significantly differ (p 0.05). Extraction yield was the average of three replicates.
indicated that these temperatures (85 C and 95 C) were higher than the exothermic transition temperature of konjac glucomannan (74e80 C) which is attributed with the disordereorder transition in a molecular chain (Karim et al., 2005). However, moisture content of konjac glucomannan dispersion in their work was slightly differed from the content in this work: the moisture contents in their work and this work were 99.0 g/100 g and 97.0 g/ 100 g. Moreover, the high temperatures (85 C and 95 C) should not be a cause of starch film formation although KF normally composed with starch. The gelatinization temperature of starch from most tubers such as tapioca, potato and sweet potato was normally 60e70 C (Chaisawang & Suphantharika, 2006; Li & Yeh, 2001), and the film formation was not still presented in the extracted konjac glucomannan sample at 75 C. Thus, the reason for the film formation should be the cause of konjac glucomannan disordereorder transition rather than starch interaction. On the other hand, K35 and K75 (Fig. 1 (a) and (b)) samples were easily grounded and sieved. Thus, K85 and K95 samples were excluded from further investigation. The result inferred that extracted konjac glucomannan had a good film forming ability; however, its ability to form film was not intended to study in this research. To elucidate influence of extraction temperature on purity of the samples, the contents of protein, ash and fat were determined as shown in Table 1. Protein and ash contents of both K35 and K75 were clearly below KF. The results indicated that the applied extraction process is effective as to yield glucomannan from KF. Protein contents of K75 were significantly lower than those of K35 (p 0.05): percentages of protein content in K35 and K75 samples
Fig. 3. Particle size distribution of samples: (a) KF, (b) K35, (c) K75 and (d) CK by Mastersizer.
were 0.17 0.01 and 0.09 0.00. This result revealed that extraction at 75 C was more effective than at 35 C, and the purities of both extracted samples were higher than that of CK. No fat content was detected in KF, fat was consequently absent both in K35 and K75 as well as CK. Furthermore, the yield percentages of K35 and K75 valued 32.52 2.53 and 35.41 1.71 respectively (Table 1): those values were significantly different (p 0.05). This result also emphasized that temperature affected extraction of konjac glucomannan form KF. Morphology and particles size distribution and average particles size of all samples are given in Fig. 2, Fig. 3 and Table 2. Table 2 Average particle size and L* value of konjac flour (KF), konjac glucomannan extracted at 35 C (K35), 75 C (K75) and commercial konjac glucomannan (CK).
Fig. 2. SEM photographs showing morphology of samples: (a) KF, (b) K35, (c) K75 and (d) CK (200, Bar ¼ 100 mm). The samples were prepared by scattering the dried powder on stab, and coating with gold before morphology observation.
Samples
Average granule sizes (mm)
L*
KF K35 K75 CK
285.31a 194.93b 183.24c 112.83d
73.99c 90.78a 88.48b 88.03b
0.70 1.67 2.69 0.70
0.18 0.16 0.03 0.10
Mean values in each column with different superscripts significantly differ (p 0.05). Average granule size, and L* values were the average of three replicates.
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O. Tatirat, S. Charoenrein / LWT - Food Science and Technology 44 (2011) 2059e2063
Fig. 4. Transparency of 1.0 g/100 g of samples in distilled water: (a) KF, (b) K35, (c) K75, (d) CK and (e) distilled water. Black dot was a mark on the paper placed under the solution container.
The average particle sizes of KF, K35, K75 and CK were w280, 190, 190 and 110 mm respectively. Granules of KF were round in shape with smooth surfaces, and the particle size distribution ranged from w100 to 1000 mm, which corresponds to previous reports (Takigami, 2000). In contrast, granules of K35 and K75 were of irregular shapes. Additionally, the average particle size was considerably smaller than that of KF. A significantly increased surface roughness was detected in comparison with the smoother surface of KF granules, because the granular structures were broken by mechanical forces during extraction process. Furthermore, morphology of both extracted samples did not clearly differ from commercial samples and the morphology of both extracted konjac glucomannan samples was also similar to CK of previous study (Cheng, Karim, & Seow, 2007). However, the average particle size of both extracted konjac glucomannan were slightly larger than that of CK, and the distribution of the particles extracted samples (w10e600 mm) were also wider than that of CK (w30e400 mm), as the different methods for grinding the samples. In this study, a mortar was used for grinding konjac glucomannan while the commercial sample in the industry was normally ground with high powerful blender. Thus, the particle sizes of CK were more homogenous distribution than extracted samples in this work. L* values (lightness) of KF, K35, K75 and CK are shown in Table 2. Both K35 and K75 were determined to be lighter than KF. The result corresponded to the components of these samples in Table 1. These results indicate that both K35 and K75 were lighter and more purified subsequent to extraction. An extraction temperature at 35 C yielded slightly lighter samples than extraction at 75 C; moreover, L* of K35 was higher than that of CK. However, the distributions of samples in this work were not similar (Table 2). Normally, the lightness value of powder depended on the particle size, and lightness of small particle size was normally higher than that of large particle size (Galiba, Waniska, Rooney, & Miller, 1998). In this case, the lightness values of K35 and CK were not significantly different but the average particle size of K35 was larger than CK. This result implied that K35 should show more lightness value than CK in case of the same particle sizes. In addition, the water holding capacity of konjac glucomannan is an important property that is related to its application, and this capacity of konjac glucomannan is very high, absorbing as much as 100 g of water per g of sample (Koroskenyi & McCarthy, 2001). However, only extracted konjac glucomannan samples (K35 and K75) were determined for water absorption index (WAI) which indicates a capacity to hold water of konjac sample. WAI values of K35 and K75 did not significantly differ: the values of K35 and K75 were 88.53 7.68 and 86.24 7.55 (Detail of measurement was not described). This result indicated that an increase in extracted temperature from 35 C to 75 C did not influence water holding capacity of extracted konjac glucomannan. However, an increase in extracted temperature to > 75 C might decrease its water holding capacity because the high extracted temperature could encourage
the disordereorder transition in its molecular chain (Karim et al., 2005). Thus, the rearrangement of the chains at high extracted temperature (>75 C) might inhibit the interaction of hydrogen bonding between water and konjac glucomannan, resulting a reduction of water holding capacity. Moreover, all samples were dissolved in distilled water (1.0 g/ 100 g dry basis) and the transparency was observed. Samples dissolved in distilled water were found to be of comparable transparency (Fig. 4), although the lightness value of the K75 powder was lower than that of the K35 powder. This was probably caused by the application of aluminum sulfate as a flocculating agent in the extraction process, so the impurities such as protein content in both K35 and K75 were lower than those of CK (Table 1). Consequently, this extraction method is one of alternative ways to produce konjac glucomannan for industry. 4. Conclusions A simple process of extracting konjac glucomannan from KF by centrifugation was successful. The temperature influenced appearance, components, yield percentage, color and transparency of extracted konjac glucomannan. During extraction of konjac glucomannan, the temperature should not exceed 75 C as to prevent film formation. Extraction of konjac glucomannan at 75 C has been recommended in this study as to achieve good yield and physicochemical properties of the products. Acknowledgments We gratefully acknowledged financial supports from Thailand Research Fund though the Royal Golden Jubilee - PhD Program (Grant No. PHD/0127/2550). The authors wished to thank Prajongwate Satmalee and researchers at Institute of Food Research and Product Development of Kasetsart University (Thailand) for supporting this research. References A.O.A.C. (2005). Official method of analysis (AOAC Official Method 981.10, 920.153, 948.15). Arlington: Association of Official Analytical Chemists. Chaisawang, M., & Suphantharika, M. (2006). Pasting and rheological properties of native and anionic tapioca starch as modified by guar gum and xanthan gum. Food Hydrocolloids, 20, 641e649. Chen, H. L., Fan, Y. H., Chen, M. E., & Chan, Y. (2005). Unhydrolysed and hydrolysed konjac glucomannans modulated caecal and faecal microflora in Balb/c mice. Nutrition, 21, 1059e1064. Cheng, L. H., Karim, A. A., & Seow, C. C. (2007). Effects of acid modification on physical properties of konjac glucomannan (KGM) films. Food Chemistry, 103, 994e1002. Dave, V., Sheth, M., McCarthy, S. P., Ratto, J. A., & Kaplan, D. L. (1998). Liquid crystalline, rheological and thermal properties of konjac glucomannan. Polymer, 39(5), 1139e1148. Fang, W., & Wu, P. (2004). Variation of Konjac glucomannan (KGM) from Amorphophallus konjac and its refined powder in Chaina. Food Hydrocolloids, 18, 167e170.
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