Twin-screw Extrusion Cooking Research Articles

The effects of twin-screw extrusion cooking on cereal enzymes

The effects of extrusion cooking on the residual activities of six enzymes in wheat, rye and oats and several milling products were investigated. In general,enzyme inactivation increased as extrusion temperature increased, α-Amylase activity in oats, however, was higher in some extrusion products than in the original grain. The thermostable enzymes (lipolytic acyl hydrolase, α-amylase, and peroxidase) differed consistently from the thermostable enzymes (α-benzoyl-L-arginine-p-nitroanilide-hydrolase, catalase and lipoxygenase). Wheat bran, broken and unbroken oat kernels were extruded at two moisture levels (18 and 30%). Whereas lipolytic aryl hydrolase activity was lower in material extruded at 30% moisture, residual α-amylase activities were higher at this moisture level. Peroxidase activity in oats, unlike that in wheat bran, was inactivated to a greater extent at the 30% than at the 18 % moisture level.

Influence of drum-drying and twin-screw extrusion cooking on wheat carbohydrates, II, effect of lipids on physical properties, degradation and complex formation of starch in wheat flour

Wheat flour was drum-dried or extruded with and without addition of 2% soya soil or 1% linoleic acid, otherwise maintaining similar processing conditions. The starches in the six resulting products were characterised in terms of their water solubilities, pasting properties, molecular weight distributions and in vitro digestibilities. Starchlipid interactions were investigated by measuring the efficiency of fat extraction, the amount of complexed starch calculated from iodine-binding capacities and the thermal behaviour of the flour in differential scanning calorimetry. All drum-dried flours exhibited less starch breakdown and higher initial Brabender viscosities than extruded flours. Lipid addition decreased viscosity and water solubility. The extent of starch breakdown was unaffected by linoleic acid but, surprisingly, increased upon addition of soya oil. Starches from all samples were equally available to glucoamylase, but pancreatic α-amylase digested drum-dried, and especially extruded, flours more slowly than an autoclaved control. Linoleic acid, but not soya oil, further decreased α-amylolysis rates. An additional autoclaving increased digestion rates and abolished differences between the two processes. A close relationship existed between the degree of α-amylolysis and the amount of complexed starch.

Influence of drum-drying and twin-screw extrusion cooking on wheat carbohydrates. I. A comparison between wheat starch and flours of different extraction

Wheat starch and two wheat flours of different extraction rates were processed under identical conditions by drum-drying and twin-screw extrusion cooking. Extrusion cooking of flours decreased the amounts of 80% ethanol-soluble carbohydrates. Both processes increased the amounts of 40% ethanol-soluble materials very slightly and did not produce malto-oligosaccharides smaller than about 15 glucose units. Neither process affected dietary fibre and starch contents, but the amount of starch measured depended on sample preparation. Extrusion cooking increased the water solubility more than did drum-drying, but in both cases solubility decreased in the order starch > 80% extraction wheat flour > 98 % extraction flour. Extrusion cooking led to slight starch degradation, as shown by gel-filtration chromatography. The extents of starch breakdown in flours and pure starch were the same, showing that the proteinaceous and fibrous matrix did not protect starch from degradation. Starch digestion rates assessed in vitro with four different amylases, were similar in all samples. Thus, water solubility, breakdown and in vitro digestion rate of starch were not related.

AQUEOUS DISPERSION RHEOLOGY OF EXTRUSION COOKED MAIZE

The effects of twin screw extrusion cooking of maize grits at different high moisture contents are described. Starch granules survive in the extrudate at high extrusion moisture or with a low shear screw configuration. The aqueous dispersion rheology of the milled extrudate and its dependence on shear rate and heat treatment is explained in terms of the starch swelling behaviour and solubility. These data resemble that of raw maize most closely for the low shear screw configuration. For the high shear configuration under similar conditions of moisture, temperature and mechanical energy dissipation, starch granules are not seen and the dispersion rheology is markedly different from that of raw maize.

The extrusion of soya with alginate using a twin-screw cooking extruder

The twin-screw extrusion cooking of soya grits with the addition of sodium alginate is described. These studies showed that the extruder torque and die temperature were reduced with 1% additive, as observed in independent studies with a laboratory scale single-screw extruder. While the mechanical power consumption of the extruder was reduced, the consumption of thermal energy to the barrel heaters increased to maintain the set temperature profiles on adding the alginate. The total power consumption of the extruder is therefore only reduced to 94% of the value for soya alone. The die pressure has been shown to scale with the mechanical power consumption whereas the die temperature increases linearly with the combined mechanical and thermal power consumption. The results indicate the addition of alginate confers a temperature insensitivity on the aqueous dispersion rheology of soya.

An experimental study of twin-screw extrusion-cooking of maize grits

The use of a twin-screw extrusion-cooker to process maize grits is described. The expansion, bulk density, colour and aqueous dispersion rheology have been related to the extrusion cooking variables: screw speed, feed rate, moisture content and barrel zone temperature. A pseudo-viscosity quantity has been defined to describe the extrudate viscosity at the die which was also related to the extrusion-cooking variables. The pressure and temperature profiles in the extruder were recorded together with the mechanical and thermal energy consumption as a function of the extrusion-cooking variables. These experiments with a highly instrumented cooking-extruder indicate the condition of the material in the barrel prior to formation of the product.

Twin-screw extrusion cooking of starches: Flow behaviour of starch pastes, expansion and mechanical properties of extrudates

Several starches, corn and durum wheat semolina, and wheat flour were extruded with a twin-screw, pilot-scale machine (Creusot-Loire BC 45). After grinding, pastes of the extruded samples were made and flow curves were obtained at 60°C with a coaxial-cylinder viscometer: a power law can be used to describe them from about 20 to 2600 s −1. Apparent Young's moduli and rupture strengths of extrudates were evaluated with an Instron, and their diametral and longitudinal expansion measured. The following variables were studied: corn semolina water content, barrel temperature and the corn starch amylose/amylopectin ratio. When water content and barrel temperature vary within certain limits, the values of the constants in the power law equation describe a characteristic line which depends on the amylose and lipid contents of the starch. The formation of an amylose-lipid complex during extrusion is suggested as a key factor in the flow properties of pastes and probably also in the rupture strength of extrudates. There is generally a negative correlation between longitudinal and diametral expansion. It is considered that volume expansion phenomena are mainly dependent on viscous and elastic properties of melted starch.

Engineering aspects of single- and twin-screw extrusion-cooking of biopolymers

A survey is given of the properties of single- and twin-screw extruders. The influence on the design of the different leakage gaps existing in corotating, counter-rotating, self-wiping, twin-screw extruders and single-screw equipment is discussed. The mixing effects in single- and twin-screw equipment and the shear distribution and shear levels that can be generated in the equipment are discussed. The overheating effect possible in single-screw extruders is related to the type of flow in the extruder channel. Finally, the properties and power consumption of Cincinnati conical, twin-screw extruders are discussed.