Expansion characteristics of the East African Highland Banana (Matooke) Flour
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A deeper understanding of the complex expansion mechanism, which leads to the pore structure, is crucial to control expansion product properties. Expansion is caused by flash vaporization of water, due to the high pressure drop at the die exit, and subsequent formation and growth of vapor bubbles (Kokini et al. 1992). Bananas are harvested according to the desired purpose and this brings the maturity period into play. 2. Bananas for food are harvested between 17 to 21 weeks because they are fully grown in this range while those for juice will be harvested from 21 weeks and above as they ripen on wards (Muranga, 2009). 3. Bananas for extrusion purposes are harvested between 9 to 15 weeks because in this maturity range, the quantity and quality of starch is highest. (Muranga, 2009). 4. Depending on the conditions the East African Highland Banana is exposed to, like temperature, humidity, physical damage of the skin among others, the ripening process commences. This is an irreversible process that involves several chemical and physical changes on the plant. Of interest to this study is the physical change of the starch levels along the maturity curve. (Muranga, 2009).
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Expansion characteristics of the East African Highland Banana (Matooke) Flour
- 2. THE EAST AFRICAN HIGHLAND BANANA PRESENTED BY ENG KIBUUKA RONALD SSEMPEBWA PhD GUEST RESEARCHER KARLSRUHE INSTITUTE OF TECHNOLOGY,GERMANY
- 3. A deeper understanding of the complex expansion mechanism, which leads to the pore structure, is crucial to control expansion product properties. Expansion is caused by flash vaporization of water, due to the high pressure drop at the die exit, and subsequent formation and growth of vapor bubbles (Kokini et al. 1992). ABSTRACT
- 4. MATERIALS AND METHODS Raw Material East African banana flour (Matooke) was produced by peeling fresh fruit and slicing into thin slices, which were then placed in a 0.2% solution of sodium meta-bisulphite for about 5 minutes to prevent browning. The slices were then dried in ITDG (batch) fuel energy drier before extruding using a co-rotating twin screw extruder.
- 5. Nandigobe (AAA-AE) Bukumu (AAA-AE) Embururu (AAA-AE) Moisture Content 9.6 8.1 8.8 Starch 81.8 82.5 82.9 Protein 4.71 5.1 4.01 Fat 0.87 ND 0.56 Crude Fiber 1.25 ND 1.33 Ash 4.34 3.58 4.1 Calcium (Ca) 0.0058 0.0044 0.0052 Potassium (K) 1.9 1.82 1.84 Magnesium (Mg) 0.09 0.09 0.01 Tannin (Abs at 500nm) 0.111 0.181 0.012 CHEMICAL COMPOSITION OF BANANA (MATOOKE ) FLOUR The original moisture content of the banana flour (Matooke) was 8% wet weight basis. The composition of the Raw material was 100% East African banana Flour with the chemical composition as stated in the table above.[Muranga,2007]
- 6. EXTRUSION Extrusion trials were conducted on a co-rotating twin screw extruder (Coperion Werner & Pfleiderer ZSK 26Mc) with a screw diameter of 25.5 mm. The extruder barrel has an overall length of 769 mm (barrel length- to-diameter ratio is 29) and is divided into seven sections. East African Banana flour(Matooke) and water were fed into the first barrel by a gravimetrically controlled feeder (Hess,Brabender DDW-DDSR 40) and a water feed pump (TrueDos, Alldos Eichler GmbH, Pfinztal, Germany), respectively.
- 7. VARIED EXTRUSION PARAMETERS The mass flow rate was set to 10 kg/ h. The moisture content was varied between 6 and 17 % wet weight basis (w.b.). The screw speed was varied between 300 and 700 rpm. The barrel temperature were 40,60,100,100,100,and 100o C, screw speed and feed water content were suggested by extrusion process engineers according to the extruder capabilities.
- 8. EXTRUDER OPERATION PROCEDURE A typical extrusion run involved a calibration step, an initial warm-up period, steady state conditions, and a warm-down period. The calibration step consisted of the standardization of the solid feeder and of the water pump and in bringing the barrel temperatures of zones 3.4 and 5 to, or close to, the final working temperatures. In the warm-up period, starch was fed into the extruder; initially in small amounts and gradually increasing the feed rate until the target feed rate setting was reached. Simultaneously, the water feed was started with high flow rates and then was decreased gradually to the desired setting. Moreover, in this period the barrel temperatures were gradually brought to the working temperature.
- 9. SAMPLE TAKING AND RECORDING DURING EXTRUSION Once the extrusion response parameters, such as screw torque, die temperature and die pressure, were constant for at least 15 min, Die and barrel pressures, torque, barrel temperatures and moisture contents measurements were made and extrudate samples collected with respect to the set screw speed at every change in the Process Variables. The picked samples are dried for ten minutes in the dryer before storage, to stabilize the water activity hence increasing the shelf stability.
- 10. EXTRUDATE SAMPLE ANALYSIS The extrudates were crushed using a laboratory mill to particles with a diameter less than 0.5mm. The resultant banana (Matooke) extrudate flour was subjected to all the lab tests below: MOISTURE CONTENT WATER SOLUBILITY AND ABSOPTION INDEX LONGITUDINAL AND SECTIONAL EXPANSION INDEX SPECIFIC MECHANICAL ENERGY ANALYSIS EXPANSION RATIO BULK DENSITY
- 11. RESULTS AND DISCUSSION The results are shown from the measurement of local and final SEI and LEI, respectively, and moisture content, specific mechanical energy as well as temperature of the expanding extrudate. The results are evaluated and the contributions of the parameters to the mechanisms of expansion are discussed.
- 12. TABLE A. EXTRUSION PROCESS PARAMETERS SAMPLE NUMBER SCREW SPEED (rpm) MOISTURE CONTENT (Kg/h) WATER CONT % THROUGH PUT (Kg/h) BARREL TEMPERATURE (°C) 1 300 1.3 15.8 16.3 40,60,100,100,100,100 2 300 1.4 16.4 16.4 43,60,100,100,100,100 3 300 1.6 17.9 16.6 44,60,100,100,97,99 4 400 1.2 15.4 16.3 42,60,100,101,100,100 5 400 1.4 16.5 16.4 42,60,100,100,100,100 6 400 1.6 17.9 16.6 42,100,100,100,100 7 500 1.6 17.9 16.6 42,60,100,100,100,100, 8 500 1.4 16.7 16.4 43,60,100,100,101,101 9 500 1.2 15.5 16.2 43,60,100,101,101,100
- 13. MAIN PROCESS CONDITIONS OBTAINED DURING THE EXTRUSION PROCESS SAMPLE NUMBER PRODUCT TEMPERATURE (°C) TORQUE % PRESSURE (bar) SME ( Wh/Kg) 1 146 42 52 135.1 2 145 42 50 142.3 3 145 40 51 132.0 4 136 32 36 137.2 5 143 37 39 163.1 6 143 36 39 155.4 7 147 32 30 166.1 8 149 31 31 161.4 9 154 39 29 218.1
- 14. EXTRUSION PRODUCT RESULTS OBTAINED FROM EXTRUDATE ANALYSIS SAMPLE NUMBER LEI LEI STD SEI SEI STD VEI WSI WSI STD WAI WAI STD 1 16.7 0.8 11.36 0.02 190.2 48.507 1.6032 2.1 0.13 2 16.3 0.9 10.33 0.06 168.2 73.4 0.38 1.8 0.04 3 15.3 1 9.61 0.15 146.8 67.4 0.79 2.2 0.01 4 17.3 0.8 7.09 0.02 122.5 96 1.07 1.3 0.04 5 18.4 1.6 8.14 0.05 149.4 95.7 1.04 1.2 0.01 6 17.3 1 7.4 0.05 127.7 95 0.72 1.5 0.03 7 21.3 0.8 5.7 0.08 121.3 70.1 1.85 1.8 0.13 8 22.5 2.2 7.16 0.04 161.2 77.7 0.23 1.4 0.05 9 31.8 2.3 6.47 0.04 205.5 78.587 1.1982 1.6 0.14
- 15. SME INCREASES WITH INCREASE IN SCREW SPEED CONSTANT •Barrel Temp •Feed Matrix
- 16. INCREASE IN SCREW SPEED REDUCES DIE PRESSSURE CONSTANT •Barrel Temp •Feed Matrix
- 17. INCREASE IN SCREW SPEED INCREASES PRODUCT TEMPERATURE CONSTAN T •Barrel Temp •Feed Matrix
- 18. INCREASE IN SCREW SPEED INCREASES LONGITUDINAL EXPANSION OF EXTRUDATES CONSTANT •Barrel Temp •Feed Matrix
- 19. WSI HAS A TENDENCY TO INCREASE WITH INCREASE IN SME AT REGULATED MOISTURE CONTENTS CONSTANT •Barrel Temp •Feed Matrix
- 20. WAI HAS A TENDENCY TO DECREASE WITH INCREASE IN SCREW SPEED CONSTANT •Barrel Temp •Feed Matrix
- 21. SEI DECREASES WITH INCREASE IN SME AT REGULATED MOISTURE CONTENTS CONSTANT •Barrel Temp •Feed Matrix
- 22. LEI INCREASES WITH INCREASE IN SCREW SPEED AT REGULATED MOISTURE CONTENTS CONSTANT •Barrel Temp •Feed Matrix
- 23. SEI DECREASES WITH INCREASE IN PRODUCT TEMPERATURE AT REGULATED MOISTURE CONTENTS CONSTANT •Barrel Temp •Feed Matrix
- 24. RESULTS ANALYSIS The extrudates were crushed using a laboratory mill to particles with a diameter less than 0.5mm. The resultant banana (Matooke) extrudate flour was subjected to all the lab tests and Table B above displays the major results that are varied with respect to the extrusion parameters to attain the Graphic interpretations below.
- 25. CONCLUSION In twin-screw extruders SME decreases when the screw speed decreases (Meuser et al., 1982, Fletcher et al., 1985, Della Valle et al., 1989). Tsao et al. (1978) and Della Valle (1989) observed that SME increased at higher screw speeds. SME is usually directly related to the expansion of expanded extrudates, in that case high expanded extrudates have high SME, since the energy spent during the extrusion process creates highly expanded extrudates which in turn is responsible for the puffed texture that is typical of expanded extrudates.
- 26. As a result, it was reasonable that the extruded flour in this study should be highly dispersed in water giving a high WSI. The WAI at 30°C of extruded flour was also higher than that of raw flour due to the swelling of highly degraded starch [Whalen 1999] Water absorption index values of all extruded samples were significantly higher compared to non-extruded corn grits. Extrusion resulted in decrease of peak, hot and cold viscosity of all samples. Starch damage significantly increased and resistant starch [RS] content decreased after extrusion.
Editor's Notes
- Experimental studies have shown that an increase in screw speed causes an increase specific mechanical energy (SME) and a decrease in viscosity. With the increase in shear energy, exerted by the rotating screw due to increase in speed, the specific mechanical energy increase but as seen from the graph, SME increases with increase in moisture content upto 16% but reduces with further increase in moisture content. This is because more water continues to reduce the viscosity of the melt hence reducing the shear effect responsible for heat generation. The melt viscosity is seriously affected by the nature of molecular distribution of the material. During extrusion, starch structures are disrupted and crystalline regions melt. After this melting process, high shear and high temperature conditions resulted in molecular fragmentation.
- As seen from the graph above, pressure reduces with increase in screw speed. Kokini & lai (1992) Suggest that High temperature leads to excessive softening and potential structural degradation of the starch melt which becomes un able to with stand the high vapour pressure and therefore collapses. This to is attributed to the fact that at high temperatures the melt viscosity is low hence early bubble growth that expand with a thin wall that is easily ruptured by vapor pressure. With low melt viscosity, pressure build up is low. High temperature is attributed to increase in screw speed which increases SME that increase product temperature.
- Increase in Screw speed as seen from above increase the Product temperature at regulated moisture contents. The increase in Screw speed increases the shear energy by subjecting the product to mechanical stresses by the successive sections of restrictive screw elements (smaller flights) resulting into the macromolecular degradation of Starch there by increasing the SME and the resultant heat is feed into the product there by increasing the product temperature. SME is the mechanical energy per unit mass. This energy is primarily converted into heat energy in the extruder. SME is put into the material through viscous dissipation which is converted primarily into heat in the extruder. The viscosity of the material is the rheological property governing the viscous dissipation generated by the shear stresses. However it should be noted that further increase in moisture content beyond 16% will cause a reduction in the product temperature due to the reduction of the melt viscosity hence reduction in shear energy.
- As seen from the graph above, screw speed increases LEI but only at regulated moisture contents. LEI is the consequence of several events such as biopolymer structural transition and phase transitions, nucleation, extrudate swelling, bubble growth and bubble collapse, with bubble dynamics dominatory contributing to the expansion. However it should be noted that the earlier nucleation starts in the die, the more the product will have increased Longitudinal Expansion. When the pressure at the die is low, nucleation starts early, die pressure is a function of mass flow rate, die geometry, and shear rate. Vapour Pressure is a function of Temperature. As seen from the graph of Screw speed Vs Pressure, it can be seen that increase in screw speed reduces die pressure hence increasing earlier nucleation resulting in the increase of longitudinal expansion. (The difference between the water vapor pressure inside the formed bubbles and the pressure of the surrounding atmosphere is the driving force of bubble growth )
- During extrusion, starch structures are disrupted and crystalline regions melt. After this melting process, high shear and high temperature conditions resulted in molecular fragmentation . The impact of macromolecular degradation on the properties of the end product is improved product solubility. High shear and high product temperature are attained by increase in specific mechanical Energy but at a regulated moisture content. During the extrusion process, high shear forces and temperature disrupted the molecular bonds of linear amylose chain and branched chain of amylopectin to produce lower molecular weight starch component that has higher solubility. As reported by Lai and Kokini (1991) that the main and secondary valence bonds and hydrogen bonds between neighboring starch polymers in starch structure can be broken during extrusion at high shear force and temperature, as a result the lower molecular weight starch component can be produced. The rise in solubility is due primarily to shortening of the chain lengths of the starch with a corresponding weakening of the hydrogen bonds holding the granule together. This allows some parts of granule to be dispersed in cold water, and later the entire granule becomes cold water soluble.
- As seen from the graph, water absorption reduces with increase in screw speed. At very high screw speeds, but under regulated moisture conditions, high starch gelatinization occurs, producing dextrins from partial hydrolysis of starch that show lower water absorption .(Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch). At low moisture and high shear extrusion, dextrinization appears as a predominant mechanism of starch degradation. The decrease of WAI may be attributed to an increase in the formation of fragmented granules due to the degradation of starch granules because of the combined effect of shearing and heating during the extrusion process. The fragmented granules may lose their water binding capacity. This agrees with Gomez and Aguilera (1983) who stated that the water binding activity depends on the availability of hydrophilic group and the gel formation capacity of the macromolecules.
- As seen from the graph above, SEI is seen to decrease with increase in SME. During expansion, increase in the moisture content is expected to negatively affect the expansion. Kokini & lai (1992) Suggest that sectional expansion decreases with increase in Temperature most likely due to excessive softening and potential structural degradation of the starch melt which becomes un able to with stand the high vapour pressure and therefore collapses. Therefore SEI will decrease with Increase in SME because SME directly increase temperature which has a negative impact on SEI
- As seen from the graph, LEI is seen to increase with increase in Temperature but at regulated moisture contents. At high temperature we have low melt viscosity which facilitates low die pressure leading to early nucleation that favors Longitudinal expansion. LEI is extensively favored by low melt viscosity at high temperatures and high moisture levels (Yulian et.al,2006) At High temperatures, this relationship can be attributed to the rapid cooling of extrudate surface that inhibits bubble growth resulting into decreased radial expansion but Increased Longitudinal Expansion.
- As seen from the graph above, Increase in product temperature causes a decrease in radial expansion. High temperature causes faster bubble collapse after the initial expansion at the die. At high temperature melt viscosity decreases thus facilitating faster bubble growth but due to the bubble walls being thin due to greater expansion, at the lower melt viscosity, they cannot with stand the vapour pressure inside resulting into wall fracture and rapid pressure loss that allows the extrudate to collapse. Kokini & lai (1992) Suggest that sectional expansion decreases with increase in Temperature most likely due to excessive softening and potential structural degradation of the starch melt which becomes un able to with stand the high vapour pressure and therefore collapses. Launay & Lish (1983) state that the expansion phenomena is basically dependent on the viscous and elastic properties of the melt dough. Therefore the elastic loss with increase in temperature is the reason for the decrease in SEI