Effects of acid concentration and extrusion variables on some physical characteristics and energy requirements of cassava starch

Effects of acid concentration and extrusion variables on some physical characteristics and energy requirements of cassava starch

Y.K.Chang; A.A.El-Dash

Faculdade de Engenharia de Alimentos, Departamento de Tecnologia de Alimentos, Universidade Estadual de Campinas, Cx. P. 6121, 13083 Phone:(+55) (19) 3788-4004, Fax:(+55) (19) 3289-3617, Campinas - SP, Brazil. E-mail [e-mail]

ABSTRACT

Some physical characteristics of cassava starch extruded using a single-screw extruder, varying acid concentration, feed moisture content and barrel temperature were evaluated using surface response methodology. The combined effects of different concentrations of sulphuric acid and extruder parameters provided unique physical functionality to the extruded cassava starch. Product hardness and torque value was highly significant for three variables. The addition of sulphuric acid (0.06N) during extrusion cooking gave the highest values for expansion and softness of the extruded samples. The highest torque values (60-70 Nm) were for the samples extruded at the lowest moisture content, while the lowest torque values (23.0-26.0 Nm) were for the samples extruded at the highest values of feed moisture and barrel temperature.

Keywords: Single-screw extruder, sulphuric acid ,hardness, cassava starch.

INTRODUCTION

Extrusion cooking of starch materials has been widely investigated for the last ten years. Conversion of starch in the extruder depends on a large number of variables in the machine and raw material control parameters. The independent process parameters include screw speed, screw configuration, product moisture content, temperature, total mass flow rate and die configuration. These independent parameters affect such system parameters as residence time distribution, energy requirement for the process, pressure profile along the barrel and pressure drop in the die (Meuser et al., 1987).

The relationship between operation variables and process variables (responses such as die pressure, motor torque, and product temperature) which can be measured on-line should be analysed. Experimental results of Akdogan and Rumsey (1996) showed that the step inputs in the screw speed and feed rate resulted in a range of dynamic responses of die pressure and motor torque. These authors concluded that die pressure and motor torque always responded in the same manner.

The initial moisture, high shear, mass temperature and pressures applied during extrusion make it possible to thermomechanically modify starch for a variety of end uses. Different ingredients, screw speed, the wear of screws and of the barrel, affected the viscous dissipation of the materials during extrusion, which ultimately affected extruder torque, specific energy, product temperature, expansion and shape of extrudates (Miller, 1984; Jin et al., 1994). Van Zuilichem et al. (1995) compared the engineering aspects of single and twin-screw extruders during the extrusion-cooking of biopolymers. In addition to extruder characteristics, the specific power consumption, throughput and extrudate properties were expressed as functions of parameters such as screw diameter, screw speed and feed moisture. In the field of extruded starchy products, some important progress has been made recently to explain product properties in terms of molecular transformations. However little has been done in the use of chemical reagents to modify the functional and physical characteristics of starch. Current product transformation is best correlated with the energy provided to the product. The extent of starch modification depends on diverse extruder parameters, raw material composition and the chemical ingredients used during the extrusion process, to give unique physical and chemical functionality to the extruded materials. The objective of this study was to evaluate the effect of acid concentration, moisture content and barrel temperature during extrusion process on some mechanical and physical characteristics of cassava starch.

MATERIALS AND METHODS

Samples

Commercial raw cassava starch obtained from Lorenz National Ind. Ltd., Cianorte, PR, Brazil, was used in all the experiments.

Extrusion Conditions

The starch samples were extruded in an EMBRAPA-Brazil laboratory scale cooker-extruder. Starch samples were fed by forced feeding of variable-speed, and maintained constant at 65.0 g dry matter/min. The screw speed was 100 rpm, 380 mm barrel length and 19 mm barrel diameter, the 3 zone barrel and die were heated by electric heaters. The compression screw ratio was 3:1 and die diameter 4 mm. The barrel temperature was fixed in zones 1 and 2 at 80 and 100ºC respectively. The temperature in zone 3 and the die zone, acid concentration and feed moisture content varied according to the experimental design.

Experimental Design for Extrusion Variables

Analyses of the treatments were carried out using a central composite surface response design, with the overall ranges and selected variables shown in Table 1 and 2. The data obtained were analysed using the SAS program (1987).

Expansion Ratio

The expansion ratio was calculated by dividing the average cross-sectional area of the extrudate by the cross-sectional area of the extruder die-nozzle orifice (Chinnaswamy and Hanna, 1988).

Hardness

The product hardness was determined using the Ottawa texture measuring system.

Torque

Torque was measured directly during the extrusion processing. In this work the screw speed (2p x rpm), and the feed flow rate (m) were fixed. The specific mechanical energy (SME) was calculated as:

Where:

T = torque (kNm)

m = mass flow rate (g/min.)

(2 p x RPM)= screw speed

Thermal energy was measured by the amperage consumption. The electrical current (I) as a function of temperature showed constant values of 6.5 amperes. Considering the extruder characteristics, the electrical resistance and potential difference were unchanged and these experimental values confirmed Ohm's law. Thus, the thermal energy requirements were a function of the heating system.

RESULTS AND DISCUSSION

Effects of Acid Concentration, Barrel Temperature and Feed Moisture on Some Properties of Extruded Cassava Starch

The significance of independent variables such as acid concentration, barrel temperature and feed moisture on some properties of extruded cassava starch are shown in Table 3. Barrel temperature significantly affected (p< 0.05) all the dependent variables. Except for expansion, acid concentration significantly influenced all the dependent variables (p< 0.05). Moisture content significantly affected (p< 0.01), expansion ratio, hardness and torque.

Effects of Independent Variables on Expansion Ratio

For the expansion ratio of the product, the significant factors were found to be feed moisture content followed by barrel temperature. Fig. 1a shows the effects of moisture content and barrel temperature on the expansion ratio of extruded cassava starch. In general, values lower than 23.0% of moisture content with increasing barrel temperature slightly increased the expansion, while a continuous increase in temperature reduced expansion. On the other hand, moisture contents higher than 23.0% together with increasing barrel temperatures, decreased the expansion of the extrudates. The highest values for expansion were at the lowest feed moisture contents (< 18%) and barrel temperatures (125-155° C), whereas the lowest expansion values were for the highest moisture contents and barrel temperatures. The expansion ratio of starch (or cereals) depends mainly on its degree of gelatinisation (Chinnaswamy and Bhattacharya, 1983) which in turn is determined by temperature, shear rate and moisture content of the feed material (Chiang and Johnson, 1977; Bhattacharya and Hanna, 1987). Low moisture content of the starch may restrict the material flow inside the extruder barrel, increasing the shear rate and residence time, which would perhaps increase the degree of starch gelatinisation, and thus, expansion. However, when the moisture content of the starch is very low (below 14% d. b.), it may create very high shear rates and longer residence times, and thus increase the product temperature. Such conditions are known to cause starch degradation and dextrinisation (Colonna et al., 1984). Several authors have cited that lower feed moisture contents and barrel temperature favoured the expansion of materials such as corn grits, corn starch (Gomez and Aguilera, 1984; Mercier and Feillet, 1975), potato starch (Mercier, 1977) and corn germ flour (Peri et al., 1983). Expansion generally decreases rapidly when moisture content increases (Seiler et al., 1980; Faubion and Hoseney, 1982; Antila et al., 1983; Guy and Horne, 1988). The effects of sulphuric acid, moisture contents and barrel temperature on the expansion of extruded cassava starch are shown in Fig. 3b-3d. The expansion ratio increases as the acid concentration and barrel temperature increase up to a barrel temperature of 140ºC and sulphuric acid concentration 0.051N. However, barrel temperatures and acid concentrations above these values, decrease expansion.

Davidson et al. (1984,1984a) explained the expansion ratio from a structural viewpoint (macromolecular degradation), that is, when the extent of macromolecular degradation of starch increases, extruded expansion decreased. However, these explanations were not quantifiable. On the other hand, and in accordance with the results found in this work. Tang and Ding (1992) concluded that the expansion ratios depended on the extrusion parameters, but a simple linear relationship between expansion ratios and the extent of macromolecular degradation may not exist. It appeared that the degree of starch hydrolysis played an important role in the expansion, because with more starch hydrolysis, the starch mass offered less resistance to the puffing of extrudates. The effect of pH affected the expansion ratio of the extrudates. When sodium bicarbonate was added to the wheat starch (Lai and Sarkanen, 1969) and corn starch (Chinnaswamy and Hanna, 1988) before extrusion, the expansion ratio decreased with increasing concentration. Chinnaswamy and Hanna (1988) speculated that sodium bicarbonate degraded the starch molecules, and starch molecular degradation during extrusion is known to reduce expansion. Martinez-Bustos et al. (1998) reported that the addition of calcium hydroxide during the extrusion of corn meal, slightly decreased expansion. These results indicated that the addition of some alkali and bicarbonate decreased expansion. However, the use of acid on cassava starch showed a different behaviour. Probably the expansion index depends on the type of starch, and the addition of alkali or acid (type and concentration) during extrusion cooking. The maximum expansion of cassava starch (4.089) was attained with the addition of 0.06N sulphuric acid, 16% moisture content and 120° C barrel temperature, whereas the maximum expansion without acid was 2.89 for extruded starch with 14.3% moisture content and 160° C barrel temperature. The extruded products were crispier after puffing. The viscosity of the plasticized mass is function of the extrusion parameters, as are its solubility and expansion, which are themselves functionally dependent on the energy input (Van Lengerich and Meuser, 1989).

Effects of Independent Variables on the Hardness of the Extruded Product

Product hardness was highly significant for the independent variables used. The effects of moisture content and barrel temperature on the hardness of cassava starch extruded are shown in Fig. 2a. The hardness is influenced by the expansion index and the starch layers alignment for the formation of the final structure. For all the barrel temperatures tested, high moisture contents inversely affected the firmness of the extruded products. The softest products were obtained with the samples extruded at the highest barrel temperatures (200-217° C) and the lowest moisture contents (< 20%). Mercier and Feillet (1975) cited that when the extrusion temperature was increased, breaking strength decreased. Extruded acidified doughs were described as a potential means of altering texture and physical properties (Kervinen et al. 1984).

The effects of acid concentration on the hardness of cassava starch extruded at different moisture contents and barrel temperatures, are shown in Fig. 2b-2d. The addition of acid with increasing barrel temperature, reduces the hardness of the products, showing a different behaviour with high acid concentrations. On the other hand, with increasing acid concentration and low moisture content (16%) the hardness increased, while a converse effect was observed at high moisture contents. Softer extruded products were obtained at the highest acid concentration and high moisture contents (24%). The hardness was associated with the expansion property. Extruded products with the greatest expansion were softer and crispy. Politz et al. (1994) cited that formulations containing high amylopectin levels were most prone to fragmentation. Die temperature, significantly affected such textural properties as cohesiveness, springiness, gumminess, and cohesiveness of the extruded flours.

Hsieh et al. (1990) reported similar results for the extrusion of corn meal with the addition of salt. These authors also cited that it seemed that a thinner cell wall was formed in extrudates with a greater radial expansion resulting in a lower breaking strength. Barret and Peleg (1992) reported that the use of citric acid during extrusion cooking reduced density and average cell size, and slightly decreased the plateau stress of the extruded products. Ryu and Walker (1995) found that the breaking strength of the extruded wheat flour significantly decreased with an increase in process temperature from 120 to 160ºC. Also these authors cited that the materials appeared to be fully cooked and to have more plasticity at higher process temperatures. Small air cells with thin cell walls may be distributed.

Effects of Independent Variables on Torque

Torque value was highly significant for the three variables used. According to the results obtained in this work, increasing the barrel temperature decreased the torque. This effect was more pronounced at high moisture content. On the other hand, torque was reduced with an increase in moisture content at high barrel temperature (Fig. 3a). The highest torque values (60-70 Nm) were for the lowest moisture contents for all the evaluated ranges of barrel temperature, while the lowest values (23.0-26.0 Nm) were for the highest values of moisture and temperature. The effect of process variables on torque confirms the effect of these variables on product transformation (expansion and hardness). The effects of acid concentration, moisture content and barrel temperature on the torque of extrudates is shown in Fig. 3b-3d. The highest torque values were for the sample extruded with 0.06N acid concentration 16% moisture content, and a barrel temperature of 120° C, and the lowest for the extrudates with 0.04N acid concentration and 26.7% moisture content, at 160° C. Higher process temperatures increased the degree of cooking and expansion. Increased mechanical energy input elevated the extruder torque and induced radial expansion (Guy and Horne, 1988).

Some workers developed models to simulate the die pressure and motor torque responses as a function of feed rate (Akdogan and Rumsey, 1996; Lu et al. 1993; Singh and Mulvaney, 1994; Moreira et al. 1990; Kulshreshintha and Zaror, 1992). Increasing feed rate increases both motor torque and die pressure. Korn (1982) cited that the energy requirement during corn extrusion for alcohol production increases with increase in processing temperature and decrease in feed rate. Grossmann et al. (1988) reported that the torque was affected by barrel temperature, screw speed, die diameter and moisture content the most important being screw speed, which was identified as having an intense effect on how fast the die pressure and motor torque responses reach steady state (Akdogan and Rumsey, 1996).

CONCLUSIONS

Feed moisture content and barrel temperature were shown to have a significant effect on the expansion of cassava starch extrudates. The maximum expansion of cassava starch (4.1) was obtained with 0.06N sulphuric acid, 16% moisture content and barrel temperature of 120° C, whereas the maximum expansion without acid was 2.89 for extruded starch with 14.3% moisture content and barrel temperature of 160° C. The extruded products were softer and crispier after puffing. Greater expansion of the extruded products resulted from the addition of acid. The highest torque values were for the lowest moisture contents and the lowest values were for the highest values of moisture and temperature. Variation of the operating parameters during single-screw extrusion processing, enables extrudates to be used for various industrial applications due to specific technological properties. Moreover, surface response analysis can be seen to provide a quite effective means for the study and the optimisation of operating conditions in extrusion technology.

Received: March 28, 2000

Accepted: September 18, 2002

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Publication Dates

• Publication in this collection
  25 June 2003
• Date of issue
  June 2003

History

• Accepted
  18 Sept 2002
• Received
  28 Mar 2000