Biomedical Engineering Reference
In-Depth Information
1987 ; Lai and Kokini, 1991 ; Liu et al ., 2009 ; Wolf, 2010 ; Xie et al ., 2006a ). More recently,
interest has focused on rates of digestibility due to relationships to nutrition and disease
(Björck and Asp, 1983; Sajilata et al ., 2006 ; Singh et al ., 2010 ; Svihus et al ., 2005 ). Since
chemically-modified starches are used in foods, paper coatings and a myriad of other
areas, many studies have been conducted on the chemical modification of starch in by
extrusion (Carr, 1994 b; Narkrugsa et al ., 1992 ; Wilpiszewska and Spychaj, 2008 ; Xie
et al ., 2006b). More recently, there has been considerable interest in the extrusion of
starch alone and with other polymers for use as biodegradable plastics (Averous, 2004;
Dubois and Narayan, 2003 ; Janssen and Moscicki, 2009 ; Kalambur and Rizvi, 2006 ;
Shogren et al ., 1993 ; Yu et al ., 2006). This section briefly describes some fundamentals
related to starch extrusion, including the authors' work as well as some more recent
studies conducted in the past 5-10 years.
Gelatinization or melting of starch during extrusion is dependent on several factors,
including moisture content, temperature, pressure and shear or SME. These areas have been
the subject of reviews (Lai and Kokini, 1991, Liu et al ., 2009 ; Xie et al ., 2006 a). Amylopectin,
amylose and amylose-lipid complexes form separate phases during heating, so these will
have different melting temperature ranges (Shogren, 1992, 1994). Amylopectins from
different sources have different branch lengths, and hence different types of native crystalline
structures (A, B, C), and these can have different melting temperatures. In the absence of
shear, thermal transitions are conveniently characterized by DSC. Such data on the effect of
water or other diluent on the melting temperature of starch are often modeled by the lattice
theory of Flory (1953):
2
1/T
=
1/T
+
RV / H V (
Δ
ν − χν
)
m
m
2
1
1
1
where T m o is the melting temperature for dry starch, V 2 and V 1 are the molar volumes of
starch and water,
ν
1 is the volume fraction of water and
χ
is the starch-water interaction
parameter. Typical values of T m o ,
for the melting of the amylopectin phase of corn
starch in water are 286 °C, 18 kJ/mol and 0.64, respectively (Shogren, 2000). Thus, melting
temperatures for starch decrease as water content increases; this allows preparation of
starches having different kinds and amounts of crystalline phases depending on extrusion
temperature and moisture. At high extrusion temperatures, starch becomes completely
amorphous, although amylose-lipid V-type complexes often form on cooling, since these
complexes crystallize rather rapidly. Increasing screw speed, and hence shear (or SME), has
also been found to increase the extent of starch gelatinization or melting (Xie et al ., 2006a ).
Increasing pressure has been found to decrease the melting temperature of starch in water at
a rate of 0.075 °C/MPa (Douzals et al ., 2001). Thus, the effect of pressure is expected to be
rather small at typical extrusion pressures of ~10 MPa. Efforts have been made to
computationally simulate the melting, mixing, shearing and heating processes occurring
during the extrusion of starch (Edi-Soetaredjo, 2010).
Other solutes or solvents have been used to partially or completely replace water to process
and gelatinize starch by extrusion (Shogren, 1993). Some of the most common of these are
glycerol, sorbitol, and urea (Shogren et al ., 1992). Since starch is a weak acid (pKa ~ 12.8), it
is expected that mild bases, such as amines and amides, would tend to interact with starch and
promote melting. Tomka (1990) showed that solvents having a solubility parameter of
15-25 cal 1/2 /cm 3/2 were effective additives for the thermoplastic processing of starch. Recently,
several ionic liquids have been shown to be good solvents for starch, but no extrusion work
has yet been published in this area (Biswas et al ., 2006 b).
Δ
H, and
χ
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