RESIDENCE TIME DISTRIBUTION (RTD) AND GOODNESS OF MIXING (GM) DURING CO2-INJECTION IN TWIN-SCREW EXTRUSION PART I: RTD STUDIES

The residence-time distribution (RTD) studies are needed to characterize mixing conditions, flow patterns, and the extent of conversions and reactions of the biopolymers in any plasticating or cooking extruder. For the supercritical fluid extrusion process (SCFX), mixing of the fluid and starch-based biopolymers is an important step affecting cell nucleation and growth. RTD was studied in the mixing zone of the extruder, where CO₂ and gelatinized starch are brought together. Three levels of each of the process variables (screw speed and CO₂ injection pressure) were investigated, using specially designed apparatus for color-dye injection, and a die-restrictor to change the pressure profile and hold up volume in the extruder. Control RTD plots were for no CO₂ injection conditions at the same pressure, and the RTD of the full extruder section. Experimental RTDs were evaluated by: (1) the Wolf-White model, (2) a cascade of continuously stirred reactors (CSTR) model and (3) a model based on plug-flow in series with cascade of CSTRs. The last approach was most successful in capturing all aspects of the observed RTD curves including their tails. Injection of CO₂ at the highest pressure (110.3 bar) increased the RTD compared to no CO₂-injection conditions. There were significant differences in the plug flow component of the RTD as well as differences in average residence time as a function of both screw speed and CO₂ injection pressure conditions.     

Extrusion of mixtures of starch and d-limonene encapsulated with β-cyclodextrin: Flavour retention and physical properties

d-Limonene was encapsulated with β-cyclodextrin to improve its retention during pre-added flavour starch extrusion. The objective of this work was to determine the effect of processing condition on the flavour retention and extrudate properties. Corn starch containing five levels of β-cyclodextrin-d-limonene capsules (0–5%) were extruded at five different maximum barrel temperatures (133–167 °C) and screw speeds (158–242 rpm) using a twin screw extruder. The effect of these parameters on the flavour retention, expansion, texture, colour difference (ΔE), Water Absorption Index, Water Solubility Index, and residence time distribution (RTD) were investigated. Barrel temperature and capsule level predominantly influenced flavour retention and extrudate properties, while screw speed primarily affected extruder performances such as torque, die pressure, specific mechanical energy and RTD.     

Residence Time Distribution for Evaluating Flow Patterns and Mixing Actions of Rice Extruded with Thermostable <Emphasis Type="Italic">α</Emphasis>-Amylase

The effect of enzyme concentration, feed rate, screw speed, moisture content, and barrel temperature on residence time distributions (RTDs) of rice flour in a co-rotating twin-screw extruder had been broadly investigated. Feed rate and screw speed as the most important operating variables affecting the mean residence time (MRT) were further emphasized by using a second-order central composite design for achieving high-efficiency enzymatic extrusion with other variables constant. Increasing enzyme concentration to 1‰ (db of starch), the MRT increased probably due to the largely reduced viscosity of extrudate caused by rapid gelatinization and degradation of starch. Limited range of moisture content and barrel temperature had only slight impact on MRT and axial mixing considering the required high enzymatic activity relying on their levels sensitively. Flow pattern of extruded rice with thermostable α-amylase was close to perfect mixing flow. Seven flow models were compared to fit the RTD data, and Yeh-Jaw simplified model showed the best fitting results for enzymatic extrusion. Overall, the mechanism of introduced enzyme affecting MRT and mixing actions of extrusion process is of industrial implication for main ingredients reactions and nutrients to retain during enzymatic extrusion.     

Residence Time Distribution (RTD) in a Single Screw Extrusion of African Breadfruit Mixtures

Mixtures of African breadfruit (Treculia africana Decne), corn, and defatted soybean were extruded in a single-screw Brabender laboratory extruder at process variables derived from a second-order central composite design. The variables consisted of feed composition (0–100% breadfruit, 0–55% soybean, and 0 or 5% corn); fed moisture (15–27%), and screw speed (100–180 rpm). Effects of these variables on residence time distribution were investigated using Congo red as tracer. The extrudate spent longer time in the extruder as feed moisture or screw speed was decreased from 27% to 15% or 180 to 100 rpm, respectively, thereby increasing the residence time distribution characteristics. At 70% feed composition and screw speed of 140 rpm, mean residence time ([`(t)]\overline t ) increased from 40 to 50 s, whereas extrudate total collection time (t <sub>c</sub>) increased from 65 to 70 s. At screw speed of 100–180 rpm, mean residence time ([`(t)]\overline t ) decreased from 55 to 35 s, whereas extrudate total collection time (t _c) decreased from 75 to 65 s. At 100% African breadfruit composition, these time values decreased to 35 and 70 s, respectively, at the same screw speed, indicating the significant influence of feed composition and soybean addition to the mixture on residence time distribution. Residence time distribution curves indicated an early breakthrough time of 20 s at maximum screw speed (180 rpm), minimum tail of 65 s, and a plug flow pattern of the extrudates.     

EXTRUSION COOKING OF RICE: EFFECT OF AMYLOSE CONTENT AND BARREL TEMPERATURE ON PRODUCT PROFILE

The effect of amylose content (5.0–28.6%) of rice and barrel temperature (80–120C) on extrusion system parameters torque and net specific mechanical energy and extrudate characteristics extrudate bulk density (ED), water solubility index, expansion ratio (ER) and Warner–Bratzler shear stress were studied using a twin‐screw extruder. The feed rate (15 kgh^?1), moisture content (20.0% ± 0.2) of feed and the screw speed (400 rpm) were kept constant. ED and ER of the product suggested that a barrel temperature of 120C was desirable to generate an expanded extrudate rice product from low‐amylose rice cultivar. Experimental data on system parameters and extrudate characteristics fit to second‐degree polynomial regression equations (r ≥ 0.904, P ≤ 0.01) with the amylose content of rice and barrel temperature of the extruder.     

Investigation of Growth and Shrinkage Mechanisms in Vapor-Induced Expansion of Extrusion-Cooked Corn Grits

The development of expansion models with good predicting capabilities requires experimental data and a more fundamental understanding of the vapor-induced expansion behavior in extrusion cooking. In this study, we used a conventional and also a high-speed camera to observe the radial and axial expansion behavior of extrusion-cooked corn grits. Image processing was performed with an automated evaluation code. The experimental setup consisted of a novel high-speed extruder, which enabled the determination of the influence of high screw speeds of up to 1,200 rpm on extrudate growth and shrinkage. In addition to optical measurement of the expansion, moisture content and temperature development during growth and shrinkage were determined. Three distinct stages of expansion were observed: extrudate growth, short-term shrinkage, and long-term shrinkage. The initial extrudate growth rate fitted well with a linear regression. Especially the second stage of shrinkage occurring in a time range of 24 h after the experiment showed that shrinkage is often underestimated in existing models. Furthermore, total shrinkage increased at higher screw speeds. The data obtained deliver new insight into expansion phenomena and can support the development of predicting models.     

EFFECT OF EXTRUSION OPERATING PARAMETERS ON AIR BUBBLE ENTRAPMENT

The effect of different extrusion operating parameters on air bubble entrapment during twin‐screw extrusion of amylopectin half‐products was studied. A ZSK‐30 twin‐screw corotating extruder with a slit die attached to the discharge end of the extruder to cool the extrudate to prevent water vapor puffing was used. The extrusion parameters investigated were screw speed (150, 300 and 500 rpm), mass flow rate (1.67 and 3.33 g s^?1), native versus pregelatinized amylopectin, screw configuration (with or without kneading blocks), and moisture content (32 and 35%, w.b.). The parameters that exerted the greatest effect on air bubble entrapment during twin‐screw extrusion are the type of amylopectin used, mass flow rate and screw speed. On the other hand, in the ranges studied screw configuration and moisture content did not show any significant effect on air bubble entrapment. Explanations for these results are interpreted with the aid of the barrel fill length mechanism proposed by Cisneros and Kokini (2001).     

Barrel-Valve Assembly Affects Twin-Screw Extrusion Cooking of Corn Meal

The effects of a barrel-valve assembly on extrusion system dynamics and product properties with various screw speeds and specific feeding loads (SFL) were studied using an APV Baker twin-screw extruder. Turning the barrel-valve from the open to closed position resulted in a convex response of die pressure due to the axial offset of paired orifice plugs, with concomitant changes in system responses and extrudate properties. Variations in SFL by changing feed rate or screw speed caused more pronounced effects on product properties compared with barrel-valve control.     

Process dynamics of starch-based microcellular foams produced by supercritical fluid extrusion. I: model development

Supercritical fluid extrusion (SCFX) has been used successfully to produce biopolymeric foams with bubble size in the range of 50–200 microns, and bubble density to the order of 10⁶ per cm³. Final bubble size and expansion ratio of extrudates depend on process and material parameters like CO₂ injection rate, nozzle temperature, oven temperature, melt viscosity, melt yield stress, etc. The objective of this study was to describe SCFX process dynamics and post-extrusion drying mechanism by using a mathematical model for bubble growth at the microscopic level in conjunction with a macroscopic model for flow of starch melt through the extruder nozzle, bulk diffusion of CO₂ and water to the atmosphere and heat transfer in the extrudate. The model was written in Visual Basic. The model provided the basis for a good understanding of the mechanisms of bubble growth and collapse, post-extrusion drying and open cell formation during SCFX processing, and is the only such one developed so far for extrusion puffed products. Simulation results and comparison with experimental data are presented in Part II of this paper.     

Extrudate expansion model in a twin-screw extrusion cooking process considering melt rheological property

A new extrudate bulk density model framework was developed, which is taking into account the melt rheological effects based on mixing principle and non-Newtonian power law theory for a twin-screw extrusion cooking process. The main ingredients in the extrusion process investigation were wheat, fish meal and soybean protein. The average absolute deviation (AAD) of the model prediction for extrudate bulk density is 5.3 % for the investigated process. The prediction results demonstrate that the proposed equation can be used to model the extrudate bulk density in the twin-screw extruder extrusion cooking process.     

Physicochemical properties of extruded germinated wheat and barley as modified by CO2 injection and difference extrusion conditions

Germinated wheat and barley were extruded with a pilot‐scale corotating twin‐screw extruder under different barrel temperatures (100 and 120 °C), feed moisture content (30 and 35%) and CO₂ injection. The effect of germination, barrel temperature, feed moisture content and CO₂ injection on physical properties, proximate composition, γ‐aminobutyric acid, β‐glucan, phytic acid and protease activity was investigated. Results indicated that the barrel temperature, moisture content and CO₂ injection significantly affected water absorption index (WAI), water‐soluble solid index (WSI), specific length, expansion ratio, bulk density, mechanical properties and colour of all extruded products (P < 0.05). Extrusion process significantly decreased γ‐aminobutyric acid and protease activity in extruded germinated wheat (EGW) and barley (EGB) products (with and without CO₂ injection). However, extrusion process (with and without CO₂ injection) increased β‐glucan in EGB and decreased phytic acid in EGW. The content of proximate composition, γ‐aminobutyric acid, β‐glucan, phytic acid and protease activity was slightly affected by CO₂ injection.     

Response surface methodology in the control of thermoplastic extrusion of starch

Effect of extrusion temperature, screw speed and moisture content on the gelatinisation properties of extruded corn starch using a single-screw extruder were studied using response surface methodology. The degree of gelatinisation can be effectively controlled by controlling the moisture content of the raw material and the extrusion temperature; control by manipulation of screw speed is also possible when the raw material has a high moisture content. The extrudate produced from starch with a low moisture content exhibited a very low retrogradation capacity, which was independent of the screw speed. It was evident that variation of operating conditions permits the production of an extrudate with various technological characteristics to meet varying food and industrial applications, and the use of response surface analysis seems to be an effective means of studying and optimising these conditions for extrusion technology.     

Extrusion behaviour of grits from flint and sweet corn

Studies were conducted to investigate the effect of feed moisture, extrusion temperature and screw speed on the extrusion behaviour and product characteristics of flint and sweet corn grits. The extruder die pressure and extrudate properties, such as expansion and water solubility index (WSI), were analyzed. Second order polynomials were computed to describe the extruder response and product properties of grits from both corn types as a function of feed moisture, extrusion temperature and screw speed. Among feed moisture, extrusion temperature and screw speed, feed moisture showed the most pronounced effect on die pressure, expansion and WSI. Die pressure of the extruder was significantly greater for sweet corn than flint corn grits. The grits from both the corn types differ significantly with respect to extrusion behaviour and product characteristics under similar extrusion conditions. The particle size distribution revealed that flint corn grits had more fine and opaque particles and resulted in extrudates with lower WSI and expansion than those from sweet corn grits which had fewer fine particles.     

Evaluating Energy Consumption and Efficiency of a Twin-Screw Extruder

ABSTRACT: Using the results from twin-screw extrusion of corn meal, both energy consumption and extruder efficiency were found to be significantly correlated with screw speed and specific feeding load (SFL). An increase in the SFL decreased the total specific mechanical energy, but increased the extruder efficiency. SFL influenced the extruder efficiency more than the screw speed. Increasing the screw speed from 300 to 450 rpm at a constant SFL level increased the extruder efficiency by 6 to 11%, whereas an increase of SFL from 0.0026 to 0.0038 kg rev ¹ raised the extruder efficiency by 30%. Of the mechanical energy consumed per unit mass of extrudate, over 98% were used for shearing or viscous dissipation and less than 1.5% were for pumping during twin-screw extrusion of corn meal.     

Methodology to Determine Destruction of Bacterial Spores during Extrusion Cooking

A methodology was developed for determining destruction of bacterial spores during extrusion cooking in a single screw extruder. Procedures were developed for estimating processing time at different screw speeds and calculation of “D-” and “z-values.” The proposed methodology evaluated destruction of Bacillus globigii spores in 18% moisture corn/ soybean mixture (70/30%, w/w). “D-value” expressed on the basis of average time at mass temperature >95°C during extrusion was the most conservative estimate of spore sensitivity. Times ranged from 1.7 sec at 115°C to 6.6 sec at 100°C maximum mass temperatures. These “D-values” resulted in a “z-value” of 25.3°C., similar to that reported for the same strain with dry heat.     

Screw Configuration Effects on Corn Starch Expansion During Extrusion

Normal corn starch was extrusion cooked in a Brabender single-screw extruder. Three screws with no mixing, one mixing or two mixing elements were used to extrude the samples through a 3 mm cylindrical die nozzle at 140°C barrel temperature and 140 rpm screw speed. Dependent variables included overall and radial expansion ratios, bulk density and specific mechanical energy (SME). Extrudates were ground and re-extruded using the same three screws, and the same extrusion conditions. Significant differences (P < 0.05) in bulk density, SME and radial expansion ratio were found on re-extrusion. No changes occurred in overall expansion ratio (P < 0.05).     

EFFECT of SCREW CONFIGURATION and SPEED ON RTD and EXPANSION of RICE EXTRUDATE

The influence of screw configuration and screw speed on the residence time distribution and product expansion was determined for rice meal processed in a corotating twin screw extruder. Screw speed had strong effect on the E(t)- and F(t)-diagrams, with the mean residence time varying inversely with screw speed from 206 s to 256 s. the F-diagram was modeled by the combination of perfect mixing and plug flow. the P estimates, which express the fraction of material in plug flow, varied inversely with screw speed from 0.41 to 0.55 for the operating conditions in this study. Both screw configuration and screw speed were statistically significant to the expansion ratios of rice extrudate, with the expansion in the height ranging from 2.98 to 4.13.     

Effect of Shear Energy on Size Reduction of Starch Granules in Extrusion

The changes of size and size distribution of starch granules under extrusion conditions have been investigated by using a single screw extruder and an optical microscope coupled with an image analysis system. The mechanical energy input and material processing time were key factors influencing the degree of starch granular size reduction. In a high shear extrusion process (T=40°C), the weight average granular size of extrudate was reduced from ˜12.4μ (raw waxy corn starch) to 1˜2μ, whereas in a low shear extrusion (T=90°C), and granular size was reduced to ˜7μ. Fine particles (±0.5μ) were observed in both extrusion processes. They were 76% (w/w) under a high shear and long processing time conditions and 5% in a low shear and short time process. During the size reduction process, the peaks of size distribution of starch granules shifted from the large size region toward the medium size region and finally to a rather narrowed peak at the small size region when extensive size reduction measures were applied. Extrusion processes can be used to reduce starch granular size effectively.     

Residence time distributions in extrusion-cooking. Part IV the feed zone of a conical,counter-rotating,twin-screw extruder processing maize grits

The residence time distributions in the feed zone of a counter-rotating, twin-screw extruder processing maize grits, were measured by means of a ⁶⁴Cu tracer. The detection system was mounted above the transport zone of the extruder. Three different screw pairs were investigated. The residence time distribution measurements were analysed by means of an axial mixing model developed for a twin-screw extruder. The feed rate was found to be the most important variable affecting the average residence time. The average residence time was inversely proportional to the screw speed. Measurements could be simulated by assuming backmixing action over approximately 15% of the axial length of the screw. Geometrical devices in these screw pairs, such as kneading zones and so-called drossel zones (high-shear throttle valves), resulted in a significant increase in the average residence time. Their action can be simulated by means of an axial plug-flow model.     

Effects of extrusion conditions on secondary extrusion variables and physical properties of fish, rice-based snacks

A formulation containing rice flour, fish powder, menhaden oil and vitamin E was extruded at a feed rate of 10 kg/h using a co-rotating twin-screw extruder. Primary extrusion (independent) variables were temperature (125-145 °C), screw speed (150-300 rpm) and feed moisture (19-23 g/100 g db). Response surface methodology (RSM) was used to study the effects of extrusion conditions on secondary extrusion variables (product temperature, pressure at the die, motor torque, specific mechanical energy input and mean residence time) and physical properties of the extrudates. Second-order polynomial models were computed and used to generate contour plots. Increasing feed moisture and screw speed decreased pressure at the die. Increased screw speed increased product temperature at the die but increased feed moisture lowered it. Increased barrel temperature, feed moisture and screw speed decreased motor torque. Increased screw speed increased specific mechanical energy, while increased feed moisture reduced it. Longer mean residence times were observed at lower screw speeds. Product density increased as feed moisture increased, but decreased with screw speed. Increased feed moisture decreased radial expansion.