vật liệu siêu hấp thụ

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Chemical modification of starch and its application as an adsorbent
material

Muhammad Saleem, Rizwan Ullah Khan, Raja Summe Ullah, Qing Chen, Jialiang
Wu
State Key Laboratory of Chemical Engineering, College of Chemical and Biological
Engineering, Zhejiang University, Hangzhou 310027, P.R. China
Abstract
Starch is a biopolymer of the plant origin which is cheap abundant and has many
applications in food and non-food industries. However, in the native form, its
applications are limited due to shortcomings, such as loss of viscosity and thickening
power upon cooking and storage, retrogradation characteristics and absence of certain
groups responsible for a particular function, etc. So, in order to reduce its limitations
and improve its applications, modification of starch is necessary. It can be modified
by several ways like chemical modification, physical modification and genetic
modification but the most important one is the chemical modification. In this review,
we selected the published data related to the chemical modification like grafting,
cross-linking, esterification, etherification and dual modification of starch and
application of modified starch for the adsorption of organic dyes and heavy metals
from water.
Keywords: Starch, chemical modification, heavy metals, dyes, adsorption.

1

∗

Introduction

Correspondence to Prof. Li Wang, E-mail: and Haojie Yu, E-mail:

Tel: +86-571-8795-3200; Fax: +86-571-8795-1612.

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Muhammad Haroon, Li Wang∗, Haojie Yu*, Nasir M. Abbasi, Zain-ul-Abdin,


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A naturally occurring biopolymer, starch is a cheap, biodegradable, renewable and
abundantly available polysaccharide molecule, which is obtained from plants.1 Starch
granules are made of mainly two kinds of alpha-glucan, amylose and amylopectin,

comparatively linear α-glucan which has 1 % α(1-6) and 99 % α(1-4) linkages while
amylopectin has extremely branched structure having about 5 % α(1-6) and 95 % α(14) linkages. Small amount of proteins and lipids are also present in starch. 2, 3

Naturally occurring starch has limited industrial applications due to insolubility in
water at room temperature, easy retrogradation and instability of its pastes and gels.
The functionality of starch can be modified by a several ways like chemical
modification, genetic modification and physical modification.1 In this review, we
have focused on the chemical modification of starch. The most important use of the
chemically modified starch is its use as an adsorbent for the removal of dyes and
heavy metals. The major contaminating sources of heavy metals are metallurgy,
electroplating industries, industrial sewage and household sewage.4 These metals
cause renal tubular damage, cancer, hyperkeratosis, anxiety and depression, irritation
and damage to the nervous system in human beings5 and cardiovascular, hematologic,
reproductive, metabolic and endocrine disturbances, necrosis, restricted growth, skin
lesions, and hypocalcaemia in fish.6
Similarly in our modern industrial society, many industries use dyes to color their
products.7 These dyes in an effluent, even in a small amount can have harmful effects,
not only on the environment, but also on living organisms. In addition, some dyes and
their degradation products are carcinogenic and toxic. These dyes are important
sources of water pollution and their treatment becomes a major problem for
environmental managers.8

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which are about 98-99 % of the total net weight of the starch. Amylose is a


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Usually, heavy metals and dyes are removed from wastewater by flotation, chemical
precipitation, electrochemical deposition, ion exchange and adsorption. However, the
processes other than adsorption have certain limitations like waste of chemicals,

best techniques used for the removal of heavy metals and other wastes from
wastewater. The process for removing waste on sorbents requires three main steps. In
first step, the adsorbate particles migrate from solution to the surface of sorbent. In
second step these particles get adsorbed on the surface and in third step further
movement of these particles within the sorbent particles occurs.9 Activated carbon is
considered as a good adsorbent because of its large surface area and outstanding
adsorption property, but its use is limited due to its high cost, non-selective adsorption
and regeneration problems.10-12 Mostly, the synthetic polymers used for the removal
of heavy metals are non-biodegradable and non-renewable and may act as secondary
pollutants. So, these synthetic polymers are not environmental friendly adsorbents.
Starch, a plant biopolymer is considered to be the excellent substitute comparing with
activated carbon and other synthetic polymer adsorbents because it is biodegradable
and environmentally safe. However, native starch can’t be used directly as an
adsorbent due to its no adsorption ability for heavy metals and most of the dyes. In
order to make starch as good adsorbent for heavy metals and dyes, there is a need to
modify native starch by the introduction of active groups like xanthate, carboxylate,
acrylate, amine phosphate and many other groups, which have chelating ability.13
Dithiocarbamate starch (DTCS),14 porous starch citrate (PSC), porous starch xanthate
(PSX)15 and etherified corn starch containing maleic acid and itaconic acid16 have
been used for the adsorption of heavy metals from water. These modified starches are
supposed to form chelation and ionic interactions with heavy metals causing the

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sludge production, poor settlement and non-selectiveness. So, adsorption is one of the


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removal of these metals. A novel amphoteric starch having quaternary ammonium and
phosphate groups has been effectively utilized for cationic and anionic contaminants
treatment.17 Similarly, magnetic nanocomposite hydrogel (m-CVP) beads, prepared

(CMS-g-PVI), poly(vinyl alcohol) (PVA) and Fe3O4 with glutaraldehyde (GA) in
boric acid, have been utilized for the removal of congo red (CR) and crystal violet
(CV) dyes and some transition metal ions like Cu+2, Pb+2 and Cd+2.18 Cross-linked
amphoteric starch having quaternary ammonium and carboxymethyl groups has been
used for the removal of acid and basic dyes. Acid dyes were removed by ammonium
group, while basic dyes were removed by carboxymethyl group.19
The purpose of the modification of starch is to enhance the useful properties (like
adsorption) of starch and to reduce its unwanted properties.20 Although, some review
papers have been published which describe the modification and applications of

starch,1,

21

but some aspects of chemical modification and

applications of the

modified starch are still not described in detail. In this review, we have focused our
discussion on the chemical modification of starch and its application as an adsorbent
material for the removal of different chemical dyes and heavy metals from
wastewater.

2

Chemical modification

The introduction of new functionality in the starch is called chemical modification of
starch. The new functionality may be carboxyl, acetyl, hydroxypropyl, amine, amide
or any other functional group which gives specific properties to the starch. The
presence of a large number of hydroxyl groups on starch provides more reactive sites
for the chemical modification of starch. Studies related to the chemical modification
of starch have been started in early 1940s. There are various methods of chemical

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by cross-linking the mixture of carboxymethyl starch-graft-polyvinyl imidazole


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modification of starch, but some important methods are grafting, cross-linking,
etherification, esterification and dual modification.21

2.1 Grafting

further polymerized on this chain. The time of grafting is variable and it may takes
minutes, hours and sometime days to complete.22 Like other biopolymer, starch is also
graftified for various applications in different fields like drug delivery, tissue
engineering and wastewater treatment. Generally three approaches grafting onto,
grafting from and grafting through are used for synthesis of graft co-polymers.
Grafting onto approach is related to the reaction between functional groups of two
different polymers. Grafting from approach is referred to the grafting in which a
polymer with specific functional group triggers the polymerization of vinyl
monomers.

Grafting

through

approach

involves

copolymerization

of

macromonomers.23 Among these approaches, grafting from approach is the most
frequently used technique, because of its high grafting yield, which is due to easy
access of the reactive groups to the chain ends of the growing polymers.24 The
different types of grafting are shown in the flowing sheet diagram (Fig. 1).22,

25

Basically grafting follows three reaction paths, free-radical path, ionic path and living
polymerization path.

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In this process, monomers are covalently bonded to the main polymer chain and then


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Fig. 1 Flow sheet diagram of grafting.21, 24
2.1.1 Free-radical grafting
Free-radical grafting (FRG) is the most important and most commonly used method
of grafting.21 It is the easiest and economical method for modification of biopolymers
for different applications like wastewater treatment, tissue engineering, drug delivery
and food additives. On the basis of initiators required to start FRG, it is further
divided into following three types.
2.1.1.1 Grafting induced by chemical initiators
In this type of grafting, usually, vinyl monomers are grafted onto biopolymers
initiated by chemical initiators. The different chemical initiators used are ceric
ammonium nitrate (CAN), cerium sulphate (Ce2(SO4)), ceric ammonium sulfate
(CAS), Fenton’s reagent (Fe+2 + H2O2), Co (II) potassium monopersulfate, Co (III)
acetylacetonate complex salts, azobisisobutyronitrile (AIBN), potassium persulfate
(KPS) ammonium persulfate (APS) and benzoyl peroxide (BPO).22, 26 Among redox
initiators, CAN is the most commonly used initiator because it results in the product
with high grafting efficiency and low amount of homopolymer formation. The general
synthetic rout of grafting of vinyl monomer on starch is given in Scheme 1. Nair et al.
prepared cassava starch-graft-polymethacrylamide (St-g-PMAM) using CAN as a free

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radical initiator. The maximum grafting percentage (79.9 %) was obtained, when
0.878 g/L CAN was used for grafting 20 g of methacrylamide (MAM) on 10 g of
starch and the reaction was carried out for 2 h at 55 ˚C.27 Lele grafted potato starch

starch grafted with acrylonitrile using two initiators system ((Ce(SO4)2 and CAN)
showed three times higher grafting percentage than using single initiator (CAN).29
Mishra et al. prepared starch-graft-polyacrylamide (St-g-PAM) by using microwave
radiations combined with CAN as radical initiators. This method resulted in
qualitative product with better grafting yield than the methods in which only chemical
initiators were used.30

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with acrylic acid (AA) using CAN as an initiator.28 Apopei et al. found that potato


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Scheme 1 General synthetic route for the grafting of vinyl monomer on starch
induced by Ce+4.31
This grafted polymer acted as superabsorbent for the removal of heavy metals.32 The
comparative mechanism of grafting of acrylamide on starch by CAN with and without
microwave assistance is given in Scheme 2. Witono et al. carried out grafting of
cassava starch with AA using Fe+2/H2O2 redox system as a radical initiator. Grafting
efficiency was found to depend on concentration of starch, temperature and starch to

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monomer ratio.33 Grafting of acrylic acid on starch using Fenton’s reagent is given in

Scheme 2 Grafting of acrylamide on starch by CAN: (a) with microwave irradiation
and (b) without microwave irradiation.34

Scheme 3 Grafting of starch with acrylic acid using Fenton’s reagent: (a) grafted
product and (b) homopolymer (a side product).33
Mohammed et al. synthesized a superabsorbent grafted polymer of potato starch by
grafting acryloylated starch with AA in the presence of same radical initiating system
(Fe+2+H2O2). The product synthesized by this method had lower homopolymer
concentration and higher adhesive and film forming properties than the copolymer
formed by direct grafting of AA on starch.35 Synthesis of acryloylated starch-graftpoly (acrylic acid) is shown in Scheme 4. Guo et al. used KMnO4, HIO4, and H2SO4
for grafting AM on starch. With this system, grafting yield and grafting efficiency
were increased and the homopolymer content was decreased in comparison with
KMnO4 alone.36 Djordjevic et al. found that when AA was grafted onto the
hydrolyzed potato starch in the presence of three different type of initiators i.e. AIBN,
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Scheme 3.


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KPS and BPO, KSP resulted into the higher grafting yield than the other two.37

Scheme 4 Synthesis of acryloylated starch-graft-poly acrylic acid (ASt-g-PAA).35

They also grafted AM on potato starch using the same three initiators and found that
the maximum grafting yield, grafting percentage and graft efficiency was obtained
with BPO.38

Scheme 5 Grafting of acrylic acid on hydrolyzed starch.37
Hydrogel based on grafting of L-aspartic acid on wheat starch was synthesized by
Vakili et al. using two types of initiators, CAN and AIBN. The maximum value of
grafting percentage for CAN and AIBN was 59.94 % and 80.25 %, respectively. So
AIBN was found as better initiator than CAN in this case.39 Grafting of L-aspartic
acid on starch is given in Scheme 6.

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Grafting of acrylic acid on hydrolyzed starch is given Scheme 5.


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Scheme 6 Grafting of L-aspartic acid on starch.39
Wang et al. prepared starch-graft-poly(2-methacryloyloxyethyl) trimethyl ammonium
chloride (St-g-PDMC) by grafting (2-methacryloyloxyethyl) trimethyl ammonium
chloride (DMC) on starch using KPS as a radical initiator. A graft copolymer was
used for wastewater treatment.40 Tali et al. also used KPS to graft AM and AA on

sorghum starch.41 Fakhru et al. grafted MMA on starch using CAN and KPS
separately. The grafting percentage with CAN was 246 % and with KSP as the
initiator was 90 %. So CAN was found better initiator than KPS. The resultant
product may have application as a biodegradable plastic.31 Another important radical
initiator is ammonium persulfate [(NH4)2S2O8]. Song used this initiator for grafting
AM and acrylacyloxyethyltrimethyl ammonium chloride (AAC) on corn starch along
with urea as a co-initiator. The products were found useful for waste water treatment
and gave better results than cationic polyacrylamide.42 Grafting of various monomers
on starch with different initiators is summarized in Table 1. Various grafting
parameters such as grafting percentage and grafting efficiency are also given in this
table.

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Table 1 Summary of grafting of various monomers on starch with different chemical initiators.
Grafting

Grafting

efficiency

Percentage

CAN

46.3

79.9

27

CAN

30.63

61.25

28

S/No.

Starch

Monomer(s)

Initiator(s)

1

Cassava

Methacrylate

2

Potato

Acrylic acid

+2

Reference(s)

3

Potato/Cassava

Acrylic acid

Fe /H2O2

44.1

—

33, 35

4

Potato

Acrylonitrile

CAN/Ce(SO4)2

—

218.38

29

5

Potato

Methyl acrylate

CAN

—

—

43

6

Potato

Acrylamide

KPS

69.85

30.25

38

7

Potato

Acrylamide

AIBN

78.09

30.31

38

8

Potato

Acrylamide

BPO

93.37

36.96

38

9

Corn

Acrylamide

KMnO4/HIO4/H2SO4

93

90

36

10

Cassava

Acrylamide

CAN

—

174.8

32

11

Maize

Acrylamide

CAN/microwave

—

907

30

12

Sago

Methyl methacrylate

KPS

—

90

31

13

Sago

Methyl methacrylate

CAN

—

246

31

14

Potato

Phenyl methacrylate

KPS

76

43.2

44

15

Corn

Acrylamide/acrylacyloxyethyltrimethyl

CO[(NH2)2]/[(NH4)S2O8]

—

215

42

ammonium chloride
16

Wheat

L-aspartic acid

CAN

43.32

54.94

39

17

Wheat

L-aspartic acid

AIBN

62.57

80.25

39

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2.1.1.2 Grafting induced by radiations

Fanta et al. grafted vinyl acetate onto granular corn starch initiated by cobalt-60
irradiation. The grafting efficiency of the grafted product was about 34 %. The

of AM, MA and methacrylic acid (MMA) as comonomers the grafting efficiency was
increased and reached to 70 % when the concentration of MMA in monomers mixture
was increased to 10 %. The grafting efficiency of 90 % was achieved when the
reaction was carried out near 0˚C.45 Sheikh et al. grafted polystyrene (PST) on wheat
starch using gamma rays as radical initiators. Maximum grafting yield (252.9 %) was
obtained when starch/styrene weight ratio was 1/3 and the applied dose was 10 kGy.46
Zhang et al. synthesized St-g-PAM cross-linked with N, N-methyl bisacrylamide
(MBA) with 10 MeV electron beam irradiation at room temperature. The optimum
dose was found to be 8 kGy, the optimum ratio of AM to AGU was 4.5 and the
optimum ratio of MBA to AM was 0.4. The resultant product showed excellent
absorbance and was categorized as superabsorbent polymer.47 Similarly, El-Mohdy et

al. synthesized starch-graft-poly(ethylene glycol)-co-poly(methacrylic acid) (St-gPEG-co-PMAA) hydrogel from water soluble starch, ethylene glycol (EG) and
methacrylic acid (MAA) using γ initiations as radical initiators.48 The synthetic route
is given in Scheme 7.

Scheme 7 Synthesis of St-g-PEG-co-PMAA hydrogel.48

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optimum radiation dose was found to be 1.0 Mrad. By the addition of smaller amount

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Grafting induced by radiations is useful, because it requires less time than grafting
induced by chemicals thus prevents the waste of time.

Chemical method of grafting has certain disadvantages such as difficulty in
controlling the reaction, impact of chemicals as a secondary pollutant and degradation
of starch.
Thus, enzymatic grafting was found to be environmental friendly alternative for
classic chemical grafting. Keeping in mind the importance of enzymatic grafting,
Wang et al. used horseradish peroxidase (HRP) for grafting of poly(methyl acrylate)
(PMA) onto the soluble starch in the presence of hydrogen peroxide (H2O2) and acetyl
acetone (Acac) as co-catalyst. The grafting percentage and grafting efficiency under
optimal conditions were reached to 30.21 % and 45.13 %, respectively.49 The grafting
of PMA on starch is given in Scheme 8.

Scheme 8 Enzyme catalyzed synthesis of St-g-PMA.49

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2.1.1.3 Grafting induced by enzymes

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With the same enzyme (horseradish peroxidase) Shogren et al. grafted polyacrylamide
on maize starch in water using H2O2/2,4-pentanedione as co-catalyst.50 Similarly, Lv

et al. grafted p-hydroxybenzoic acid (PHA) on corn starch with this system and the

properties. The starch was first degraded with α-amylase and then treated further.51
The degradation of starch followed by grafting of PHA on degraded starch is given in
Scheme 9. Using the same initiating system (horseradish peroxidase/H2O2), a new
cationic starch has been prepared by grafting poly(dimethyldiallylammonium
chloride) (PDMDAAC) on starch. The resultant product was used as a sludge
dewatering agent resulting in reduction of sludge water content to 50.6 % from 97.85
%.52 Grafting of PDMDAAC on starch is given in Scheme 10.

Scheme 9 (a) Degradation of starch with α-amylase and (b) grafting of phydroxybenzoic acid on starch with horseradish peroxidase.51

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resultant graft copolymer was found to have excellent tanning and retanning


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Scheme 10 Grafting of poly (dimethyl diallylammonium chloride) on starch.52
2.1.2 Grafting through living polymerization
In recent years, methods of ‘living polymerization’ have been developed to provide a
potential for grafting reactions. The definition of living polymer is ‘that retains their
ability to propagate for a long time and grows to a desired maximum size while their
degree of termination or chain transfer is still negligible’.53 Conventional free-radical
polymerization requires continuous initiation, with termination of the growing chain
radicals in coupling or disproportionation reactions, and as a result leads to unreactive
“dead” polymers and essentially time invariant degree of polymerization and broad
molecular weight distribution.
In case of living polymerization, it provides living polymers with regulated molecular
weights and low polydispersities. This method has got much interest because of its
well control over copolymer architecture. Controlled free-radical polymerization may
be effective through atom transfer radical polymerization (ATRP).22 Wang et al.
synthesized

starch

macro-initiator

in

1-allyl-3-methylimidazolium

chloride

([AMIM]Cl) by homogeneous esterification of starch with 2-bromoisobutyryl
bromide (BIBB) and then grafted PST and poly(methyl methacrylate) (PMMA) on
this macro-initiator through ATRP using CuBr/N,N,N′,N′,N′-pentamethyldiethylamine
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(PMDETA)

and

CuBr/2,2-dipyridyl

(BPY)

as

catalysts.

Compared

with

heterogeneous grafting using traditional free radical initiators, grafting ratio was
greatly improved.54 Synthesis of macro-initiator and grafting of PST and PMMA on

on expended starch using the same method as used by Wang et al.55 Liu et al.
synthesized starch-graft- poly(n-butyl acrylate) St-g-PBA by surface initiated atom
transfer radical polymerization(SI-ATRP) of n-butyl acrylate (BA) with starch bromoacetic ester macro-initiator in the presence of 1,10-phenanthroline and Cu(I)Br as
catalyst in toluene. The product was supposed to be used in preparation of the
biodegradable plastics.56 Synthesis of St-g-PBA is given in Scheme 12.

Scheme 11 Synthesis of the corn starch-based ATRP macroinitiator and starch graft
copolymers.54

Scheme 12 Preparation route of St-g-PBA.56

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this macro-initiator is given in Scheme 11. Bansal et al. also grafted PST and PMMA

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Similarly, Wang et al. prepared starch-graft-poly(N-isopropylacrylamide) (St-gPNIPAM) hydrogel by single electron transfer living radical polymerization (SETLRP) using starch-Br as a macro-initiator. The resultant product was thermosensitive

thermosensitivity was concluded from H1NMR. The intensities of signals a, b, c and d,
which were the characteristic peaks of PNIPAM side chains, decreased obviously
during temperature change from 25 °C to 35 °C, indicating that St-g-PNIPAM have a
good response to temperature (Fig. 2). The hydrogel was found to have good swelling
and rapid shrinking rate showing its application for drug delivery.57

Fig. 2 1H NMR spectra of St-g-PNIPAM (a) at 25 ˚C and (b) at 35 ˚C in D2O
(Adopted with permission from reference 57).
2.1.3 Ionic grafting
There are several examples in which starch has been grafted with acrylonitrile,
methacrylonitrile, acrylic and methacrylic esters and several other vinyl monomers in
the form of metal starch alkoxide. Liquid ammonia, tetrahydrofuran, N,Ndimethylformamide and dimethyl

sulfoxide have been used as different solvent

systems.58 Tahan et al. prepared starch-graft-poly(ethylene oxide) (St-g-PEO) in

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with LCST range 31.5 ˚C to 23 ˚C varying with length of PNIPAM chains. The


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DMSO. 31.9 % to 63.9 % of the hydroxyl groups of starch were deprotonated forming
starch alkoxide ions. However, the grafting yield was found to increase with the
increase in monomer concentration but independent of the alkoxide percentage.59

process. In first step, hydroxyl groups were activated with alkyl aluminum derivatives
(AlEt3). In the second step, ε-caprolactone (CL) was grafted onto starch with ring
opening polymerization (ROP). The two step procedure of grafting of PCL on starch
is given in Scheme 13.

Scheme 13 Two-step procedure for the in situ polymerization of ε-caprolactone from
the starch granule.60
They also grafted poly(δ-valerolactone) on starch in similar way as PCL.60 Phenyl
glycidyl ether has also been grafted on starch in DMSO.61 Cohen et al. prepared
starch grafted with poly(lauryl methacrylate) in DMSO using potassium alkoxides
derivative of starch. It was found that the grafting yield increased with the increase in
alkoxide concentration. However with increase in monomer concentration and rise in

temperature the homopolymerization increased.62 Under the same conditions the
anionic grafting of methyl methacrylate with starch alkoxide was also studied and it
was found that grafting yield was directly related with alkoxide concentration and
inversely related with temperature and monomer concentration.63

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Similarly, Houzé grafted poly(ε-caprolactone) (PCL) on granular starch in two-step


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2.2

Cross-linking

Cross-link is the chemical bond that links one polymer chain to another and this
phenomenon of making cross-link between two polymer chains is called cross-

in cross-linking of polysaccharides. Jyothi et al. cross-linked cassava starch by EPI in
three different media including water with phase transfer catalyst (PTC), water

without

phase

transfer

catalyst

and

N,N-dimethylformamide

(DMF).

Tetrabutylammonium bromide (TBAB) was used as PTC. The highest degree of
cross-linking was obtained, when the reaction was carried out in DMF. The modified
starch had higher water-binding capacities (WBC) and their α-amylase digestibility
was found to decrease with the increase in their degree of cross-linking.64 Guo
prepared cross-linked porous starch (CPS) by cross-linking corn starch with EPI and
then hydrolyzing it with α-amylase. This CPS was found to be biodegradable and safe
adsorbent having higher adsorption capability than native starch. This porous starch
was

applied

to

remove

methylene

blue

(MB)

from

water.65

N,N′-

methylenebis(acrylamide) (MBAA) is another excellent cross-linking agent. Hu et al.
enzymolysed waxy corn starch and then cross-linked this starch with MBAA, in the
presence of CAN for chromium (VI) adsorption. The resultant CPS also had excellent
adsorption capacity for the other heavy metal ions like cadmium (II) ion and lead (II)
ion.66 POCl3 is another interesting cross linking agent. Singh et al. cross-linked sago
starch with POCl3 which resulted in a highly substituted cross-linked starch phosphate
having higher thermal stability and swelling behavior.67 Kim et al. used POCl3 for
cross-linking corn starch at different pressure from 0.1 MPa to 400 MPa in order to
determine the effect of ultra-high pressure on the extent of cross-linking. The increase
in cross-linking with pressure was revealed by the decrease in swelling and

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linking. Epichlorohydrin (EPI) is the most familiar cross-linking agent which is used

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gelatinization.68 Sodium trimetaphosphate (STMP) is another well-known crosslinking agent. Hong et al. cross-linked granular maize starch with STMP in four
different reaction media (deionized water, aqueous sodium sulfate solution, aqueous

the degree of cross-linking. The degree of cross-linking of starch in aqueous ethanol
and aqueous acetone was higher than in deionized water and aqueous sodium sulfate
solution.69 Carbinatto et al. used STMP for cross-linking pectin–high amylose starch
mixtures with different ratio (1:4, 1:1 and 4:1). Cross-linked samples were found to
have higher thermal stabilities. The sample having higher amylose content showed
higher cross-linking.70 Wongsagonsup et al. cross-linked tapioca starch using mixture
of STMP and sodium tripolyphosphate (STPP) (99:1 (w/w) ratio). When STMP was
used as cross-linking agent , phosphorous content was about 0.04 %, and when
STMP and STPP were used as cross-linking agents, the content of phosphorous was
increased by 10 times.71 This shows that the mixture of STMP and STPP is better
cross-linking system than STMP. Citric acid is another important cross-linking agent.
Reddy et al. cross-linked starch films with citric acid to improve their tensile strength
and thermal stability and to decrease their dissolution in water and formic acid. The
resultant cross-linked films had 150 % higher tensile strength than normal films.72
The whole process of cross-linking is summarized in Table 2.
Table 2 Cross-linking of different starches with different cross-linkers.
S/No.

1

2

Starch
Cassava
starch, porous
corn starch

Cross-linker

Product

Reference(s)

64, 65

Waxy corn
starch

66

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ethanol and aqueous acetone) in order to check the effect of various reaction media on

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3

Sago starch,
Corn starch

67, 68

4

Granular
maize starch,
pectin-high
amylose starch
mixture,
tapioca starch

69-71

5

Corn starch

72

2.3

Esterification of starch

Esterification of starch can be carried out with acids and their derivatives due to the
presence of large number of hydroxyl groups in starch. To obtain product with high
degree of substitution, the reaction should be carried out in organic solvent.21 Fang
esterified four different types of starches, each having different ratio of amylose and
amylopectin with acid chlorides of different chain length. However, esterification
occurred with acid chlorides which contained 6-8 carbon atoms but not with acid
chlorides which had carbon atoms less than 6 or more than 8 because in such cases
hydrolysis (reverse reaction) was dominant over esterification. The maximum degree
of substitution obtained was almost 3.73 Similarly, Chi et al. acetylated corn starch
with acetic anhydride as acetylating agent. Different degrees of substitution (DS)
(0.85, 1.78 and 2.89) were obtained under different temperature conditions (50 ˚C, 65
˚C and 75 ˚C, respectively).74 Mei et al. found that when cassava starch was esterified
with citric acid, degree of substitution was increased from 0.058 to 0.178 with the
increase in citric acid concentration from 10 % to 30 %. However on further increase
in concentration of citric acid to 40 % the DS value was decreased to 0.129. The

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starch citrate was found to have lower swelling power and solubility than native
starch showing the increase in resistant starch content in the starch sample with
esterification.75

Potato

starch

oleate

ester

was

synthesized

in

1-butyl-3-

at different time and temperature. The maximum DS (0.22) was obtained when
reaction was carried out at 60 °C for 4 hours. The product could be used for
biodegradable packaging and as carrier for bioactive agents.76 Corn starch was
esterified with malic anhydride and its composite with polylactic acid (PLA) was
prepared. It was found that the tensile strength and bending strength of esterified

starch (ES)/PLA composite were higher than those of the native starch (NS)/PLA
composite.77 Lipase-coupling esterification of waxy corn starch was carried out with
octenyl succinic anhydride and it was found that the DS value of 0.0195 and the
reaction efficiency of 84.05 ± 2.07 % could be obtained in 30 minutes. The reduction
in reaction time was found to be useful for producing the product on large scale in
industry.78 Starch betainate, a cationic starch derivative, was prepared by
esterification of starch with betainyl chloride (BC). BC was first prepared from
anhydrous betaine and thionyl chloride and then the esterification process was carried
out. The product was found to greatly increase the strength of paper.79 The schematic
summarization of esterification is given in Scheme 14.

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methylimidazolium chloride reaction medium using immobilized lipase as a catalyst


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Scheme 14 Schematic summary of esterification.73-79
2.4

Etherification of starch

Etherification usually results in four types of the modified products, non-ionic,
cationic, anionic and amphoteric products. On the basis of

product obtained,

etherification is divided into four types.21
2.4.1

Non-ionic etherification

Huijbrechts et al. etherified waxy maize starch and high amylose maize starch with
allyl glycidyl ether to give 1-allyloxy-2-hydroxy-propyl starches with DS of 0.19 ± 1
and 0.20 ± 0.01, respectively. The reaction was regioselective and occurred mostly at
carbon 6 of anhydroglucose unit of starch.80 Azo and anthraquinone dyes, which are
very toxic and cancer causing,81 can be removed with tertiary amine starch ether, (
2,4-bis(dimethyl amino)-[1,3,5]-triazine-6-yl ) starch (BDATS). This modified starch
was synthesized by shi et al. by etherification of normal starch with 2,4-bis(dimethyl

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amino)-6-chloro-[1,3,5] triazine (BDAT).82 The flocculation behavior of this starch

Scheme 15 The flocculating behavior BDATS.82
In order to improve compatibility of polylactide(PLA)/starch composite, Wokadala et

al. etherified waxy and amylose-enriched starches with 1,2-epoxybutane which
resulted in the products with DS of 2.0 and 2.1, respectively. The PLA/butyletherified waxy and high amylose starch composite films were found to be more
flexible and had higher elongation at break compared to PLA/non-butyl-etherified
composite films.83
Misman et al. etherified sago starch with benzyl chloride in water and in 70 % ethanol
and found that solvent (ethanol) based etherification resulted in product with the
higher DS, higher thermal stability and better flow ability.84 Simillarly when high
amylose corn starch (HACS) was etherified with 1-bromopropane, the etherified
HACS was found to have higher decomposition temperature than unmodified HACS

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with dye is given in Scheme 15.