Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 64

Properties of starch extrudates, such as strength, crispiness, and brittleness, depend
to a large extent on molecular weight, which is influenced to a great extent by extrusion
conditions (Liu et al ., 2010 ; Van Den Einde et al ., 2003 ; Willett et al ., 1997 ). Increasing
SME leads to decreased molecular weights and viscosities of the amylopectin
component, while amylose is rather insensitive to shear. The high molecular weight
fractions of amylopectin, in particular, tend to be reduced the most in molecular weight
during extrusion.
Several applications of extruded starch in the food and pharmaceutical areas have been
described in the recent literature. The preparation of starch-encapsulated drugs by extrusion
for controlled-release applications has been described (Repka et al ., 2007 ; Trivedi et al .,
2007). Starch capsules which can replace those comprised of gelatin were prepared by
extrusion and injection molding (Stepto and Tomka, 1987). Microencapsulated flavors,
omega fatty acids and other food ingredients requiring protection and stabilization have
been prepared by extrusion (Gray et al ., 2007 ; Qi and Xu, 1999 ; Shahidi and Han, 1993 ).
Instant food powders have been prepared by extrusion with the benefit of reduced water and
energy consumption over traditional drum drying processes, which operate at higher water
contents (Schuchmann and Danner, 2000). Starches that are resistant to digestion with
amylases are termed resistant starches, and these starches have been prepared by extrusion.
The resistance or decrease in rate of digestion by amylases is due to the presence of residual,
ungelatinized native crystalline amylopectin or to the formation of retrograded, crystalline
amylose during subsequent storage of the starches in moist food (Sajilata et al ., 2006 ; Singh
et al ., 2010). Thus, high amylose starches are often used to prepare resistant starches
(Shrestha et al ., 2010). Extruded, acid-modified starches tend to yield more resistant starch
after retrogradation, since lower molecular weight starch can more easily reorganize into
crystalline structures (Hasjim and Jane, 2009). Health benefits of resistant starches include
slow glucose absorption for improved metabolic control in diabetes, and the functionality of
these starches as probiotics for colon cancer prevention and for reduction in cholesterol for
the prevention of heart disease (Sajilata et al ., 2006 ).
Chemically modified starches are used in a myriad of applications, including paper
coating, cardboard adhesives, foods, textile sizing, and various binders (BeMiller, 1997;
Tharanathan, 2005 ; Tomasik and Schilling, 2004 ; Wurzburg, 1986 ). Chemical modifications
serve to slow recrystallization, prevent syneresis, confer viscosity stability via cross-
linking or add anionic or cationic groups. Typical commercially-available modified starches
are low DS (<0.05) and include hydroxyethyl or hydroxpropyl starches, oxidized starches,
cationic starches, starch acetates, starch phosphates, starch alkenylsuccinates and starch
graft copolymers. These are usually prepared in aqueous, granular suspension with >60%
water in batch reactors, although some reactions such as phosphorylation can be conducted
in a dry state. The potential advantages of using reactive extrusion to conduct starch
reactions is the ability of extruders to handle viscous, low-water states and thus enhance
reaction efficiency and decrease by-products. Thermoplastic starch, formed during
extrusion, tends to react more quickly than granular starch; and the distribution of the
modifying groups is generally different. Extruders also offer continuous, rapid reactions in
minutes at high temperatures and under pressurized conditions, when it is necessary to
prevent the evaporation of volatiles. A disadvantage of extrusion processing is that it is
generally more difficult to extract by-products from gelatinized starch than from granular
starch suspensions.
Preparation of starch derivatives by reactive extrusion has been reviewed (Wilpiszewska
and Spychaj, 2008 ; Xie et al ., 2006b). Reaction efficiencies up to 92% for the cationization