1 fuel is erratic in supply and unevenly

1 Biopolymers

Polymers find use in many
applications in almost everything produced industrially such as cars, houses,
textiles, food packaging, electronics and medicine1 .
Traditionally, the feedstock for these synthetic polymers is from fossil
resource. Fossil fuel is erratic in supply and unevenly distributed by nature
and is considerably expensive. Furthermore, polymers synthesized from fossil
fuels; although cheap, give rise to environmental concerns because they are
non-degradable the cost of recycling are high. Another shortfall of polymers
synthetized from fossil fuels is pollution2  with non-degradable polymeric products
occupying valuable landfill. These shortcomings with synthetic polymers has
driven the exploitation of biopolymers that are naturally occurring3  and degradable in the natural environment when
acted upon by microorganisms4 .
They are found as part of plants or animals; hence they are renewable. Majorly,
biopolymers are used as food sources but are also a major raw material in
industry and find application in paper making, medicine textile and most
recently, biodegradable food packages5 .

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Biopolymers are extracted
from various sources. Plant sources which include cellulose, hemicellulose,
lignin and polyphenols from wood, and starch. Animal sources; chitin from
crustaceans, proteins and cellulose whiskers from molluscs, and other bacterial
polymers3 .
The most widely used industrial biopolymers are starch and cellulose. Some
biopolymers are synthetized from bio monomers such as lactic acid; polymerized
to poly lactic acid and succinic acid, polymerized to polyesters and polyamides6 .

Cellulose is the most
abundant naturally occurring polymer. It is derived mostly from wood and
constitutes about 33 percent of vegetable matter. It is a straight chain
polymer comprising of many ? 1-4 D-glucose monomers
linked together by glyosidic bonds. The major applications of cellulose are the
production of paper and fibers for textiles. Cellulose also finds use in
medicine, production of films and fillers for consumables. The hydroxyl
functional group in cellulose acetate can be partially or wholly esterified or
nitrated to produce derivatives with desirable properties for other
applications. Figure 1 below shows the molecular structure of cellulose:

Fig 1: Molecular structure
of Cellulose7

Hemicellulose is a
branched polymer found in plant cell walls. It includes xyloglucans, xylans,
mannans, glucomannans and glucans8 . Industrially,
this polymer is used as a source of energy and fuels. However, its application
is yet to be fully exploited on a large scale.

Proteins are linear polymers
synthesized from amino acid monomer. Compared to other biopolymers, protein is
expensive, as a result, the commercialization of protein as a biopolymer is
limited9 .

Starch is a polysaccharide
synthesized by plants and found majorly in cereals, roots tubers,
fruits10  and legumes in the range of 25-90%8.  Starch is semi crystalline polymer comprised
of about 1,000-2,000,000 glucose monomers linked by glyosidic bonds. This
literature review will focus on starch and its modification as a feedstock for
bioplastics.

2 Starch

Starch is synthesized from
photosynthesis of carbon dioxide in plants10 .
Starch, majorly used as food also finds use in industrial applications11 .
The major industrial use of starch is as a composition in adhesives and binders12 . Researches
have been interested in the commercial application of starch as a biopolymer
compared to other biopolymers for bioplastics production in recent times
because of the many benefits that starch brings. Starch can be got from various
plant sources grown in both tropical and temperate regions; hence starch is widely
distributed globally, readily available and inexpensive. Starch source is
renewable and can be grown typically within a period of 1year. Asides from
distribution, starch finds industrial use because of its physical, chemical and
functional properties like water dissolution and retention, gelatinization,
viscosity when subjected to temperature and ease of modification to
optimize functional limitations of starch11 .
Additionally, starch can be processed by methods used for petrochemical
polymers like extrusion and injection molding. Unfortunately, the hydrophilic
nature of starch along with its brittleness, retrodegradation and thermal
degradation has limited its use for industrial applications requiring
mechanical integrity. Therefore, the functional group in starch should be
modified to obtain desirable properties to expend its industrial application.

Amongst other components,
native starch granule is made up of 2 major polymers. Amylose; the linear
glucose chains and amylopectin which is highly branched8 ,
detailed explanation of this follows in starch composition. Research has shown
that the amylose-amylopectin ratio affects the functionality and chemical
properties of starch11 .
This ratio varies based on starch source, climate where cultivated and soil
type13 .
Table 1 below shows various sources of starch and the amylose-amylopectin
ratio.

Source

Amylose (%)

Amylopectin (%)

Reference

Rice

20-30

80-70

3

Potato

23-31

77-69

13

Cassava

16-25

84-75

14

Waxy cassava

0

100

14

Wheat

30

70

3

Corn

28

72

15

Sorgum

24-27

76-73

16

Table 1: Sources of starch
showing amylose/amylopectin ratio

3 Structure, Composition and Morphology of Starch

The structure and
composition of starch is responsible for the physical and chemical properties
of starch. The structure varies due to location, climatic conditions and
botanical source. As a result, understanding the structure and components is
very important for structural modification for industrial application of
starches. It has A, B and C crystal structure which is a function of the starch
origin. Starch has very tiny granule size and comes in various shapes based on
source8 .
It is composed of glucose monomer units linked by ? 1,4 glycosidic bonds. It is made up of 2 major
polymers; amylose, a linear polymer with ? 1,4 glycosidic bonds linking the glucose monomers and
average molecular weight of 1 x 106 g/mol17,
amylose accounts for amorphous nature in the starch granule and amylopectin;
with high molecular weight of 1 x 108 g/mol17  linked by short ? 1,4 glycosidic bonds with high branching in the ? 1,6 positions, which accounts for the crystallinity
in starch 8 . The
branching of amylopectin polymer creates double helix of approximately 5nm
length18
in the starch granule that align in the crystalline region The crystalline
region is represented as double helix.

 According to research, X-ray diffraction of the
macroscopic view of starch under illuminated light showed a positive
bifrigerence indicated by a Maltese cross demonstrating an arrangement of the
macromolecular units represented by a helix in the starch morphology10 ,
which disappears upon disruption of starch granule. This interchanging
arrangement of amorphous and crystalline lamellae in the starch granule is
responsible for the semi-crystalline nature of starch with a crystallinity of
20-45%18 .

Fig 2: Structure and morphology of starch molecule

Other components found in
starch include phosphorus; a non-carbohydrate component.  Phosphorus exists as monoesters of phosphate
and phospholipids. Its presence in starch granule influences the gel strength,
lucidity and solubility depending on the macropolymer it bonds with 8 . Starch
also contains 0.1-0.7% protein by weight and lipids present as free fatty acids
and lysophospolipids in up to 1.5% by weight19 .

4 Physical Properties

4.1 Solubility in organic
solvents

Starch is hydrophilic in
nature and insoluble in cold water. When in contact with cold water, starch
disperses and dissolves with time. However, when starch dispersed in cold water
is heated, starch solubilizes forming a gel-like paste. The chemical process
responsible for solubility and gel-like paste formation of starch dispersed in
water with heat is the formation of hydrogen bonds between water molecules and
the hydroxyl groups on the separated amylose-amylopectin molecules. Although
starch is soluble in water with temperature, water is not a suitable solvent
for starch for industrial applications because it partakes in the reaction,
usually the solvent media should exhibit an inert behavior without influencing
the reaction.

Starch is soluble in polar
solvents with a solubility parameter typically between 19-48MPa1/2 and
mixed solvents within same range. Typical examples include dimethyl sulphoxide
(DMSO), formamide and its derivatives, C1-C18 alcohols, hydrazine and ammonia;
90/10 v/v mixed solvent of DMSO/water, DMSO with LiCl, DMF with LiCl and
alkaline solvents (usually accompanied by degradation). Generally, the more
polar the solvent, the higher the starch solubility . For this reason, with
DMSO being the most polar among of these solvents, it is the best solvent for
dissolving starch. However, the extent of contamination of starch is higher with
DMSO and care should be taken while utilizing it. Other solvents typically used
include DMF and acetone. However, the choice of solvent depends on the
application.

4.2 Gelatinization and
Retrogradation

Gelatinization is the irreversible
order disruption of the granular structure of the starch molecule leading to a
loss in bifrigerence20 .
This occurs when starch in heated between 60-700C 21 in
excess water leading to maximum granular swelling, followed by bursting of the
granule. It occurs in 2 stages. Firstly, amylose-amylopectin separation
resulting from the absorption and swelling of the starch granule and hence, a
loss in semi-crystallinity8  in starch. This separation occurs when the
hydrogen bonds are broken and loosening the double helices22 .
It begins in the amorphous areas because of the ease of weakening the hydrogen
bonds in those areas. Secondly, separation and loss of amylose from the granule
to solution, this is described by the term leaching. The amount of water
affects gelatinization, in low starch-water ratio, granular swelling is
incomplete leading to a partial loss in crystallinity23,
this is called melting. On the other hand, the crystallites will be separated
because of excessive swelling with no crystallites to be disordered in the case
of higher than required excess water. Additionally, the components and ratio of
components of the starch granule affects quality of paste.

Another method to achieve
gelatinization is by the application of high pressures23 . However,
separation of amylose-amylopectin molecules also occurs and solution leaching
of amylose is reduced with minimal granule swelling. Like thermal
gelatinization, amount of water and treatment time affects high pressure
gelatinisation23 .
Baks et al23  revealed that at constant temperature in
different samples, gelatinization was faster at higher pressures above 400MPa.  As gelatinization occurs and granular
disordering occurs, starch granules loses it bifrigerince which is a characteristic
of gelatinized starch.

When gelatinized starch is
cooled, the segregated amylose-amylopectin molecules realign themselves to a
crystalline structure. Retrogradation is usually accompanied by expelled water,
increased viscosity and gel formation. When retrodegradation occurs, amylose
links up with multiple glucose units forming a double helix. At the same time,
the short chains of amylopectin crystallize. Components present in starch
granule affects retrogradation. While the presence of protein has been shown to
slow down retrogradation during refrigeration, low temperatures below 00C
accelerates it8 . Retrodegradation
can also be influenced by starch origin, storage length and conditions and the
amount of water 22 .

The resulting product of
retrogradation is the formation of a gel. In native starches with high
amylopectin ration, the gel formed is soft. Whereas, in the case of starches
with high amylose ratio, the resulting gel is flexible and strong exhibiting
resistance to deformation22 . Due
to the formation of the soft amylopectin gels, the properties of its
derivatives are undesirable8  due to low molecular strength, hence for most
industrial applications, starches with high amylose contents are preferred. The
figure below summarizes the steps involved in the gelatinization and
retrogradation process of starch.

Figure 3: Processes that
occur during gelatinization and retrodragation. (a) undisrupted starch granule;
(b) absorption of water, swelling of granule, molecular segregation and loss of
amylose to solution; (c)Realignment of amylose molecules because of cooling (d)recrystallization
of amylopection molecules during storage18  .

4.3 Melt Processability

To make bioplastics from
polymers, the polymer should be heated above its melting point to enable
viscous flow. However, starch when heated does not melt; rather it degrades due
to its high melting temperature. Plasticizers are generally added to starch to
improve its thermal and melt processability18 .
There are several plasticizers employed for use with starch including water,
polyols, sorbitol, urea, formamide24  and citric acid with water being the most
common especially in food industry. Starch in its native form contains 9-10%
w/w25  bound water and water from external source is
required to plasticize starch. The higher the water concentration in starch,
the lower the melting temperature. Unfortunately, starch plasticized with water
has undesirable mechanical properties and suffers retrogradation; hence cannot
be used alone in plasticizing starches. Plasticized starch also referred to as
thermoplastics come about when starch melts in the presence of added
plasticizer, elevated temperatures and at constant shear24 . Gelatinous
starch is referred to as thermoplastic starch if it is stable and
retrodegradation doesn’t occur. Retrodegradation can be prevented by the
addition of other polymers or use of substituted starch that restrict
recrystallization by interrupting the hydrogen bonds in starch.

Glycerol, polyols, sugar
alcohols ionic and non-ionic surfactants amongst others are plasticizers
employed for TPS. Care should be taken however when using glycerol with a high
affinity for water that bonds with water molecules leaving insufficient
plasticizer to interrupt the hydrogen bonds in starch. This is known as
anti-plasticization which results in increased phase transition temperatures
and brittleness. Also, plasticizers are known to leach into polymer solutions
with age as they do not bond chemically with the starch molecules.

4.4 Rheological and
Thermal Properties

Starch molecules are
frozen at room temperature making them immobile. With increased temperature,
the molecules become mobile and move past one another26 .
This is the glass transition of starch. When starch is heated, it undergoes
several phase transitions including granule swelling, gelatinization,
decomposition, melting and crystallization. Methods employed to study starch
phase transitions can be divided into two major groups: (1) Transitions studied
in the absence of shear, including, X-ray diffraction studies, DSC, NMR and
hot-stage microscopy: and (2) transitions studied while shearing including
rheometry18 .
Shearing aids the mechanical segregation of the starch molecules, thereby enhancing
loss of crystalline structure. The paste obtained after gelatinization contains
viscous leached granules, swollen granules and the rest of the starch
components. The viscosity of the starch gel is impacted by temperature change,
amylose/amylopectin ratio, starch components and shear rate27 . The
rheology of starch which is a function of its viscosity can be measured using a
rapid viscoanalyser. The RVA measure the peak, trough and final viscosity; peak
time and pasting property. According to the study carried out on rice by Simi
et al 28 ,
fixed concentration of starch was heated from 50 to 950C and the
temperature kept constant at 950C for 2mins. The viscosity profile
recorded showed a high peak viscosity which is the maximum viscosity at which
the starch granules hold water before they break. When granules disruption
followed by amylose leaching begins, that point is the breakdown viscosity, the
viscosity decreases due to amylose lost to solution. When starch granules begin
to realign themselves during cooling and retrodegradation forming a gel, this
is accompanied by an increase in viscosity; this is the setback viscosity.

The dynamic rheometer also
gives a measure of starch viscosity with temperature. The property (G’) known
as the storage modulus is a measure of stored energy in the granule per cycle
and (G”) called the loss modulus is a measure of energy lost during the
destructuration cycle. Their ratios called the tangent or damping factor
reflects the structure of the material. These properties are represented as
frequency dependence on the curve. Low values indicating frequency independence
is solid-like; high values indicating frequency dependence behaves more like a
liquid. Like the viscoanalyzer, G’ increases with temperature(TG’) to peak G’
depicting granular swelling. Following the peak G’ and with further increase in
temperature, a drop in G’ is observed as the crystals melts. In terms of G’,
peak G’ at TG’ less minimum G’ at maximum temperature is the breakdown27 .
This breakdown varies for different starches and is impacted by starch
botanical source, granular strength, granular size and starch components.

Temperature is measured
using differential scanning calorimetry and the temperature curves and
represented by thermograms. It records the onset temperature, peak temperature,
final temperature, enthalpy and specific heat29 .
A wide range between onset and final depicts that more energy called the energy
of gelatinization is required to gelatinize the starch; this could indicate
different crystal stability while a low range depicts crystal homogeneity.
Enthalpy of gelatinization is impacted by components present in granule and
amylose-amylopectin ratio.

 

 

 

 
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