Environmental Engineering Reference
In-Depth Information
complexes reduce the solubility of starch in water, alter the rheological properties of pastes, decrease
swelling capacity, increase gelatinization temperature, reduce gel rigidity, retard retrogradation,
and reduce the susceptibility to enzyme hydrolysis (Holm et al. 1983; Karkalas and Raphaelides
1986; Billaderis and Seneviratne 1990; Nierle and El Bayâ 1990; Guraya et al. 1997; Crowe et al.
2000; Kaur and Singh 2000; Oczan and Jackson 2002; Tufvesson et al. 2003a, b). According to
Stevneb et al. (2006), barley cultivars with low levels of amylose have a higher degree of starch
hydrolysis than normal and high amylose content for all time intervals (
P
< 0.05). Their study also
showed that the degree of starch hydrolysis is higher for B-granules than for A-granules despite an
increased amylose content and higher amount of amylose-lipid complexes in B-granules; hence,
granule size has a profound effect on the degree of starch hydrolysis, consistent with the low level
of crystallinity and a higher surface area in B-granules compared with A-granules. Studies have
also showed that the hydrolysis rate of maize starches is proportional to the surface area of the
granules, which may be closely related to the adsorption of enzyme onto the granule surface (Li
et al. 2004). External chains of amylopectin that construct the crystalline structures of starch
granules likely affect the rate of hydrolysis. Srichuwong et al. (2005) showed that starch hydrolysis
by α-amylase is positively correlated with the proportions of unit chains with a DP of 8-12 and
negatively correlated with a DP of 16-26 (
n
= 15,
P
< 0.01). Longer chains would make long
helices and strengthen hydrogen bonds between chains, spanning the entire crystalline region.
On the other hand, the existence of shorter chains form short or weak double helices, producing
inferior crystalline structures (Jane et al. 1999).
Crystalline polymorphic form, fraction of crystalline structures in starch, molecular associations
between starch components, amylose content, granule size, granule shape, and surface pores are
all factors that have been mentioned in enzyme digestibility. Of these factors, granular structure
is considered to be the most important in defining the rate and extent of enzymatic hydrolysis
(Zhang and Oates 1999). The natural variability in amylose and amylopectin molecules is due to the
complexity of starch biosynthesis (Copeland et al. 2009).
31.2.2 S
tarch
B
ioSynthESiS
The biosynthetic pathway of starch synthesis involves several types of enzymes. An overview of
starch biosynthesis as shown in Figure 31.2.
7
Glc
Glc-6-P
6
1
+H
2
O
ADP TP
5
Glc-1-P
+ Pi
ATP
2
PPi
ADP-Glc
α-1,4 Glc
n
Starch
α-1,4 Glc
n+1
4
3
FIGure 31.2
General scheme for starch biosynthesis. (1) phosphoglucomutase; (2) ADP-glucose pyrophos-
phorylase; (3) GBSS and soluble SS; (4) branching enzymes; (5) starch phosphorylase; (6) amylases, debranch-
ing enzymes, and maltases; (7) hexokinase.
