Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 62

(D'Jesús et al ., 2006) corn starch was used to evaluate the influence of potassium on the
gasification efficiency. The gasification yield increased with increasing potassium
concentration up to 500 ppm and did not increase further. Temperature was the most
important process variable and the gasification yield increased when the temperature was
increased from 823 to 973 K.
To summarize the significant research in this area, supercritical CO 2 has been used to
extract impurities from starch, and the gelatinization of starch in supercritical CO 2 has been
studied. Ethanol-containing critical fluids have been used to extract native lipids from
granular corn starch. Starch-based extruded foams were prepared using supercritical CO 2 as
the blowing agent in the foaming process. Under these processing conditions, foams with
good properties were prepared at temperatures less than 100 °C. At these lower temperatures,
starch degradation is reduced, extruder wear is minimized, and heat sensitive materials such
as flavorings can be dissolved in supercritical CO 2 and easily mixed into the cooked starch
prior to foam formation. Extrusion with supercritical CO 2 also improved the enzymatic
conversion of starch to ethanol. Supercritical CO 2 has been used as reaction medium for
preparing starch derivatives, such as esters and ethers, and as a solvent for preparing starch-
based porous matrices for tissue engineering. When supercritical water was used to treat
starch at 500 °C, hydrolysis of starch to low molecular sugars was observed due to the acidic
properties of supercritical water at this temperature. Supercritical water has also been used
at temperatures exceeding 650 °C to degrade polysaccharide-containing biomass into
gaseous products such as hydrogen, CO 2 , and low molecular weight hydrocarbons. This
process has been referred to as “gasification” and is receiving increased attention due to the
current interest in hydrogen as a fuel.

2.7 EXTRUSION PROCESSING

Extrusion equipment and its application to polymer and food processing in general have
been described in a number of topics and reviews (Guy, 2001; Kohlgruber, 2007; Rauwendaal,
2001; Riaz, 2000). Extruders generally consist of one or more steel screws powered by an
electric motor contained inside a closed barrel. Solids (such as starch) and liquids (such as
water) can be added at nearly any point in the extruder using appropriate metering feeders
or pumps. Extruder barrels consist of sections which are normally heated separately to vary
the temperature along the length of the extruder. Heat is also contributed by work supplied
by the motor and is termed specific mechanical energy (SME). SME is quantified by the
equation: SME = is rotation rate and m is mass flow rate. Extruder

screws can be designed in a wide variety of ways to generate forwarding motion, compression,
reverse flow, dispersive mixing and distributive mixing. Single-screw extruders are generally
used when high-pressure, stable flows are needed for sheet or foam extrusion, while twin-
screw extruders have excellent conveying and mixing capabilities.
Extrusion is a useful method for gelatinizing starch, blending starch with other
components or conducting reactions in highly-viscous, lower-moisture systems.
Additional functions of extrusion may include specific shape generation, sterilization,
reduction in moisture or other volatile components, encapsulation and flavor generation.
The food industry has long used extrusion to prepare ready-to-eat cereal snacks as well as
animal feeds (Guy, 2001 ; Harper, 1978 ; Mercier et al ., 1989 ; Riaz, 2000 ). Much of the
research on starch extrusion has been conducted to understand the structural changes and
properties of starch in such systems (Akdogan, 1999; Barsby et al ., 2002 ; Colonna et al .,

τω /m, where τ is torque, ω