296
13 Starch
faster; however, degraded rate does not seem to be affected by granule size. Retrogression decreases the starch degradation rate. This is due to retrograded starch having
a more ordered structure which inhibits degradation (Li et al. 2015). These findings
suggest that the rate at which starch is broken down varies for different types of
starch since as discussed in the earlier section, the molecular weight, granule size
and crystallinity vary for starch from different sources. This is important in the design
of products such as bioplastic films where the rate of degradation needs to be slow
enough to retain product stability during storage and usage life but fast enough to
allow safe degradation in the environment without accumulation.
Starch is broken down by amylase enzymes which can cleave alpha 1–4 and alpha
1–6 bonds in amylose and amylopectin. This can occur in nature either by microorganisms or within the digestive systems of animals and humans which metabolize the
required enzymes. Organisms which carry out degradation of starch in natural environment include fungi such as penicillium Appressoria and chlamydospores (LopezLlorca and Valiente 1993), Trichoderma viride I (Schellart et al. 1976) and bacillus
bacteria (Wang et al. 2019). These organisms produce amylase and glucoamylase
enzymes which catalyze starch degradation and therefore serve as sources of enzymes
for commercial processes involving starch degradation such as hydrolysis of starch
to fermentable sugars for bioethanol production. Degradation of starch also occurs
through acid hydrolysis and heat treatment. This can either be desirable or undesirable. The acid hydrolysis of starch is, for example, desirable in the production of
bioethanol where the starch is hydrolyzed into fermentable sugars. In the processing
of starch into plastic films, undesirable thermal degradation of starch occurs during the process of extrusion (Liu et al. 2010). This is considered undesirable and is
prevented by careful control of the process to ensure proper temperature distribution.
13.5 Floridean Starch
While starch is generally defined as consisting of amylose and amylopectin in different ratios, a form of starch exists where the amylose content is zero. This is known
as floridean starch and found in red algae as a highly branched phosphorylated amylopectin chain; floridean starch is a short-chain starch with a degree of polymerization
of 18 and a branching of 4.8 (Yu et al. 2002). The highly branched structure and low
molecular weight make it less stable than the long-chain starch with linear structure.
Ordinarily starch is found in land plants and green algae within the plastids. A
different form of starch, floridean starch is found in the cytosol of red algae and glaucophytes, cryptophytes, dinoflagellates and apicomplexa parasites. These organisms
use a different pathway to synthesize and store starch in a form called floridean starch
(Dauvillee et al. 2009). The storage of floridean starch within the cytosol in red algae
is similar to the manner in which fungi and animals have their storage carbohydrate (glycogen) stored in the cytosol. Accumulation of starch within the plastid in
photosynthesizing plants is more conventional since this is the part associated with
manufacture and storage. Although it was recently thought that floridean starch also
13 Starch
faster; however, degraded rate does not seem to be affected by granule size. Retrogression decreases the starch degradation rate. This is due to retrograded starch having
a more ordered structure which inhibits degradation (Li et al. 2015). These findings
suggest that the rate at which starch is broken down varies for different types of
starch since as discussed in the earlier section, the molecular weight, granule size
and crystallinity vary for starch from different sources. This is important in the design
of products such as bioplastic films where the rate of degradation needs to be slow
enough to retain product stability during storage and usage life but fast enough to
allow safe degradation in the environment without accumulation.
Starch is broken down by amylase enzymes which can cleave alpha 1–4 and alpha
1–6 bonds in amylose and amylopectin. This can occur in nature either by microorganisms or within the digestive systems of animals and humans which metabolize the
required enzymes. Organisms which carry out degradation of starch in natural environment include fungi such as penicillium Appressoria and chlamydospores (LopezLlorca and Valiente 1993), Trichoderma viride I (Schellart et al. 1976) and bacillus
bacteria (Wang et al. 2019). These organisms produce amylase and glucoamylase
enzymes which catalyze starch degradation and therefore serve as sources of enzymes
for commercial processes involving starch degradation such as hydrolysis of starch
to fermentable sugars for bioethanol production. Degradation of starch also occurs
through acid hydrolysis and heat treatment. This can either be desirable or undesirable. The acid hydrolysis of starch is, for example, desirable in the production of
bioethanol where the starch is hydrolyzed into fermentable sugars. In the processing
of starch into plastic films, undesirable thermal degradation of starch occurs during the process of extrusion (Liu et al. 2010). This is considered undesirable and is
prevented by careful control of the process to ensure proper temperature distribution.
13.5 Floridean Starch
While starch is generally defined as consisting of amylose and amylopectin in different ratios, a form of starch exists where the amylose content is zero. This is known
as floridean starch and found in red algae as a highly branched phosphorylated amylopectin chain; floridean starch is a short-chain starch with a degree of polymerization
of 18 and a branching of 4.8 (Yu et al. 2002). The highly branched structure and low
molecular weight make it less stable than the long-chain starch with linear structure.
Ordinarily starch is found in land plants and green algae within the plastids. A
different form of starch, floridean starch is found in the cytosol of red algae and glaucophytes, cryptophytes, dinoflagellates and apicomplexa parasites. These organisms
use a different pathway to synthesize and store starch in a form called floridean starch
(Dauvillee et al. 2009). The storage of floridean starch within the cytosol in red algae
is similar to the manner in which fungi and animals have their storage carbohydrate (glycogen) stored in the cytosol. Accumulation of starch within the plastid in
photosynthesizing plants is more conventional since this is the part associated with
manufacture and storage. Although it was recently thought that floridean starch also