Effects of cassava crush size and variety on starch extraction efficiency

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1 Effects of cassava crush size and variety on starch extraction efficiency Ruenrom Lerdlattaporn 1, Montira Nopharatana 1 and Warinthorn Songkasiri 2* 1 Department of Food Engineering, King Mongkut s University of Technology Thonburi 126 Phacha U-tid Road, Bangmod, Tungkru, Bangkok 10140, Thailand 2 National Center for Genetic Engineering and Biotechnology (BIOTEC), Excellence Center of Waste Utilization and Management 83 Moo 8 Thakham, Bangkhuntien, Bangkok 10150, Thailand * Corresponding author warinthorn@biotec.or.th O-STARCH-2 Abstract In a typical cassava starch factory, the highest starch loss of 62% of the total loss occurs in the extraction unit. The experiments were conducted to study the two main mechanisms in the extraction process: centrifugation and filtration. The effects of relative centrifugal force (RCF: 490g, 650g and 810g), pressure drop across the resistance (20, 28 and 35 kpa), cassava crushed size (less than 300, , , and m), and variety of cassava (Rayong 9, Rayong 11 and Kasetsart 50) on the starch extraction efficiency were studied. Experimental results showed that the fiber contents of Rayong 9, Rayong 11 and Kasetsart 50 varieties were 3.44, 3.14 and 3.77%, respectively. Extraction of Rayong 11 variety had the highest extraction efficiency due to its low fiber content. Extraction efficiency decreased with increasing crushed cassava size. A small crushed cassava contained a higher free starch content than a larger one. RCF at 650g provided the highest extraction efficiency. The reason is that low RCF was insufficient to extract starch granules from fiber complexes, while too high RCF compressed the cake, resulting in an increase in cake resistance. Pressure drop across the resistance at 28 kpa yielded the highest starch extraction efficiency. A very high pressure drop provided lower cake thickness and cake porosity, resulting in a higher cake resistance. Keywords Cassava starch, filtration, relative centrifugal force, starch extraction, separation 1. Introduction Thailand has become the number one as the producer and exporter of cassava starch with approximately 35% of the total world export since 2009 (EXIM Thailand, 2010). Cassava starch production process generally has 8 steps: receiving raw materials from suppliers, screening, washing, rasping, extracting, separating, dewatering and drying steps. During the production process, the total starch loss is approximately 15-30% (Tipwong et al., 2006). The extraction unit, which is used to separate starch slurry from fiber and pulp, has the highest loss of 28% with pulp (Tipwong et al., 2006). In another words, the starch loss during the extraction step is approximately 62% of the overall loss (equivalent to approximately 105 million baht/year for a 200 ton starch factory 240 days of operation in a year and 15 baht/kg starch product). Conical-screen centrifuge is used as an extractor to separate starch granules from fiber by filtration and centrifugation mechanisms. Extractors can be divided into 2 types -coarse and fine extractors. Perforated stainless basket size of the coarse extractor is mesh or m, while a filter cloth of fine extraction is mesh or mesh (Sriroth and Piyachomkwan, 2007). Starch extraction efficiency depends on design, operating and feed 38

2 variables. The design variables are angle basket and screen size (Pongnopparut, 2001; Lorsubkong, 2007; Wisitrungraung, 2005; Thanomsaksri, 1999). The operating variables are feed flow rate and rotational speed of extractor, while the feed variables are crushed cassava size, L/S ratio and varieties of cassava (Bucic et al., 2007; Piyatamrongrat, 2005; Pongnopparut, 2001; Thanomsaksri, 1999; Sirichaikul, 1997; Lorsubkong, 2007). Few researches have studied the effects of these variables, such as relative centrifugal force, varieties and crushed size of cassava on starch extraction efficiency (Bucic et al., 2007; Piyatamrongrat, 2005; Thanomsaksri, 1999; Sriroth and Piyachomkwan, 2007). Therefore, this research aimed to study the effects of pressure drop across the resistance, relative centrifugal force, crushed size and varieties of cassava on starch extraction efficiency. 2. Materials and Method 2.1. Materials and Root preparation Three cassava varities, Kasetsart 50 (KU50), Rayong 9 and Rayong 11, were used as raw material in this experiment. Cassava roots were harvested after 10 months, and they were grown under similar environmental conditions. Fresh roots were washed to remove sand, dirt, and peel. Then, they were cut and crushed to reduce into smaller pieces. To decrease the moisture content to be less than 20% (dry basis) for storage purpose, the crushed cassava was dried at 50 o C for 5-7 hours by a hot air oven (Model # ULN 500, Memmert, Germany). The temperature of 50 o C was selected for drying because this temperature is lower than the starch gelatinization temperature at 63 o C (Sriroth and Piyachomkwan 2007). The dried crushed cassava was sieved to separate crushed size into 4 ranges of <300, , and µm by a sieving machine (Model # AS200 basic, Retsch, Germany). The mesh sizes were 50, 35, 25 and 18 meshes, respectively Study of centrifugation and filtration mechanism The slurry sample, with an L/S ratio of 6.1, was prepared by mixing 3 g of dried crushed cassava and 17 ml of fresh water in 30 ml centrifuge tube. Then, it was centrifuged at various speeds for 5 minutes (Model himac CR21, Hitachi, Japan). The samples were filtered through 180 µm aluminum screen. Another part studied filtration mechanism. The slurry sample, with an L/S of 6.1, was prepared by mixing 6 g of dried crushed cassava and 34 ml of fresh water.sample was filtered pass aluminum screen with size of 180µm before experimentation with pressure drop across resistance of 20, 28 and 35 kpa Study of moisture content The crushed cassava was dried at 50 o C in hot air oven. Starch moisture content was calculated using by equation (1) (1) where: m 1 is a mass of crushed cassava before drying (g) m 2 is the mass of crushed cassava after drying (g) 39

3 2.4. Determination of Starch Content Starch content of samples was measured by optical method based on starch-iodine reaction (AOAC, 1995). It was measured at 570 nm by Spectrophotometer (Model UV-2101PC, Shimadzu, Japan) Statistical analysis Experiments were replicated 3 times at each condition. Analysis of various (ANOVA) was conducted to determine the effects of main factors on cassava starch extraction efficiency. Duncan s test was used to determine the multiple comparison of mean value. Mean value were considered at 95% confidence level ( = 0.05). 3. Results and Discussion 3.1. Effects of relative centrifugal force on starch extraction efficiency Centrifugation is the solid-liquid separation process by using a spinning rotor to generate centrifuge force. Centrifuge force is the driving force for driving the feed slurry through the cake and the filter medium as shown in equation (2) (Wallace and Leung, 1998). F 2 m r (2) Figure 1 shows starch extraction efficiency of cassava varieties with different relative centrifugal forces (RCF). Starch extraction efficiency increased with decreasing crushed cassava size for all cassava varieties. Fiber content of small crushed cassava had less than large one as shown in table 1. The starch granules were trapped in the fiber complex. This complex must be broken so that starch granules can be liberated. Fiber complex of small crushed cassava size was destroyed more than that of the large ones. Free starch granules from the fiber complexes were easily extracted (Pongnopparat, 2001). Further, extraction surface area of small crushed cassava size was greater than that of the larger ones. Similar results were found by Bucic et al. (2007) and Piyatamrongrat (2005). Figure 1 Starch extraction efficiency of difference relative centrifugal force 40

4 Extraction of Rayong 11 variety had the highest extraction efficiency, since it had fiber content less than Rayong 9 and Kasetsart 50 varieties. The high fiber content probably plays a significant role in retarding starch extraction as the fibrous network holds starch granules together. Starch granules contained in this complex polymer matrix are difficult to liberate (Pongnopparat, 2001). Table 1 Fiber content of crushed cassava with various sizes (dry basis) Varieties Crushed cassava size (µm) < Rayong ac ± bb ± aab ± aa ± 0.07 Rayong abc ± ab ± ba ± ba ± 0.02 Kasetsart bd ± bc ± ab ± aa ± 0.06 Mean value ± SD ns is indicated that values in the same column are not significantly different (p 0.05). Same lower case letters in the same column indicate that values are not significantly different (p 0.05). Same upper case letters in the same row indicate that values are not significantly different (p 0.05). The influence of centrifugal force affected the cake structure. The decrease in cake thickness caused the decline of cake porosity. The specific cake resistance increased. Moreover, at a high pressure drop, cakes were compressed. The increase of compression on the cake and the compact structure of the cake under higher centrifugal force caused higher resistance to flow through the cake from the large centrifugation force. RCF at 650g provided the highest extraction efficiency. A low RCF was insufficient to extract starch from fiber complexes, while too high RCF compressed the cake and increased the resistance. The influence of RCF affected the cake structure. Similar results were observed by Thanomsaksri (1999) Effects of Pressure Drop across the Resistance on Starch Extraction Efficiency Filtration is the process of using a filter medium to separate a solid-liquid mixture. The fluid and small particles are able to pass through the filter medium, while the large particles are retained. Water facilitates the starch particles to pass through the filter medium. From Darcy s law, the basic filtration equation relating the flow rate Q of a filtrate of viscosity through a bed of thickness L and face area A to driving pressure p. The reasons were as the following equation. The filtration mechanism was described by equation (3). Q A P A P ( R R ) ( R w ) (3) The physical parameter which characterizes cake formation is the specific cake resistance. Cake properties such as cake thickness, cake porosity, cake permeability and cake density affect the specific cake resistance. Due to the effects of the cake thickness and the compact structure of cake, the maximum level of pressure drop was reached. These cake properties depend upon the particle size of crushed cassava starch going to the extraction process. Large crushed size was retained on the filter medium, while the small crushed size cake was compressed and decreased cake porosity. In this experiment, varieties of cassava, crushed c 41

5 cassava size and pressure drop across the resistance were varied in order to study their effects on efficiency of starch extraction. 42 Figure 2 Starch extraction efficiency of difference pressure drop across the resistance Starch extraction efficiency of cassava varieties with difference pressure drop across the resistance was shown in figure 2. Pressure drop across the resistance at 28 kpa yielded the highest starch extraction efficiency. Similar results were found by Silva et al., 1999, Endo, et al., 1998 and Chen et al., The experimental results showed that the cake filter was influenced by the cake thickness given higher pressure cakes were compressed so cake porosity and specific cake resistance increased. An increase in pressure drop could result in a decrease in cake porosity and an increase in collection efficiency. Cake porosity decreases and specific cake resistance increases with the increases in the cake compression stress. 4. Conclusion This research studied the effects of varieties, crushed size of cassava, relative centrifugal force and pressure drop across the resistance on starch extraction efficiency. There are two main mechanisms which are centrifugation and filtration in starch extraction process. The cake thickness and cassava starch extraction efficiency of the filter system were measured and analyzed using Darcy s law. The crushed cassava size affected the cake formation. Small crushed size provided compressed cake, while large crushed size was retained on the medium. Starch granules contained in fiber complex and difficult to liberate. Fiber complex must be broken. Starch extraction efficiency depended on fiber content and level of destroyed fiber complexes. The best starch extraction efficiency was provided by the condition of Rayong 11, RCF of 650g and crushed size of <300 µm. It meant that starch can be efficiently separated from solid. Pressure drop across the resistance at 28 kpa yielded the highest starch extraction efficiency. A higher pressure drop lowered the cake thickness and cake porosity, resulting in a higher cake resistance. Acknowledgements The authors are grateful to the Food Engineering Department, King Mongkut s University of Technology Thonburi, and the Starch Engineering and Process Optimization (SEPO) program for financial support.

6 References Bucic, A., Planinic, M., Tomas S., Bilic, M. and Velic, D., 2007, Study of solid liquid extraction kinetics of total polyphenols from grape seeds, Journal of Food Engineering, Volume 81, p. Endo, Y., Chen D., and Pui, D.Y.H., 1998, Effects of particle polydispersity and shape factor during dust cake loading on air filters, Powder Technology, Volume 98, Chen, Y. and Hsiau, S., 2009, Influence of filtration superficial velocity on cake compression and cake formation, Chemical Engineering and Processing, Volume 48, Lorsubkong, K., 2007, Cassava Starch Extraction Using Two Basket with Different Angles, Master of Engineering Thesis, Chemical Engineering Department, Faculty of Engineering, King Mongkut s University of Technology Thonburi, 64 p. Piyatamrongrat, N., 2005, Effect of Particle Size of Crushed Cassava Root on Water Usage and Starch Extraction Efficiency in Extractor Unit, Master of Science Thesis, Natural Resource Management Department, School of Bioresources and Technology, King Mongkut s University of Technology Thonburi, 81 p. Pongnopparat, T., 2001, Cassava Starch Filtration, Master of Engineering Thesis, Chemical Engineering Department, Faculty of Engineering, King Mongkut s University of Technology Thonburi, 104 p. Silva, C.R.N., Negrini, V.S., Aguiar, M.L. and Coury, J.R., 1999, Influence of gas velocity on cake formation and detachment, Powder Technology, Volume 101, Sriroth, K., Lamchaiyaphum, B., and Piyachomkwan, K., 2007, Present Situation and Future Potential of Cassava in Thailand, Cassava and Starch Technology Research Unit (CSTRU), Kasetsart University, Bangkok, Thailand, 22 p. Thanomsaksri, P., Tawornpongpun, P., and Thanyaratsakul, P., 1999, Performance Tapioca Starch Extractor Effect of Revolution Speed, Special Research Problem, Chemical Engineering Department, Faculty of Engineering, King Mongkut s University of Technology Thonburi, 59 p. Tipwong, S., Wisitrungruang, W. and Sasadoor, W., 2006, Research Project in Food Industry I, Starch Engineering and Process Optimization, Food Engineering Department, Faculty of Engineering, King Mongkut s University of Technology Thonburi, pp Wisitrungruang, W., 2005, The Effect of Angle and Velocity of Water Injection During Starch Extraction on Starch Extraction Efficiency, Master of Engineering Thesis, Food Engineering Department, Faculty of Engineering, King Mongkut s University of Technology Thonburi, pp