Microwave assisted acetylation of mung bean starch and the catalytic activity of potassium carbonate in free-solvent reaction

RESEARCH ARTICLE

Microwave assisted acetylation of mung bean starchand the catalytic activity of potassium carbonate infree-solvent reaction

Maisa Bushra, Xiao-Yun Xu and Si-Yi Pan

College of Food Science and Technology Huazhong Agricultural University, Wuhan, P. R. China

Mung bean starch acetate has been successfully produced with transesterification reaction of

the starch under microwave assistance using vinyl acetate (VAC) as esterifying agent and

potassium carbonate (K2CO3) as a catalyst. Various concentration of potassium carbonate

(0–0.117 mmoleq/AGU) was resulted in starch acetate with increased mood of the degree of

substitution (DS ¼ 0.003–0.27) in a short time. An increase in the degree of substitution,

resulting in increase of the peak intensity of ester carbonyl group (C––O) at 1742 cm�1, which

was further proved by FTIR. Obvious change in the starch granules morphology, increasing in

the hyrophilicity and decreasing in the temperature and enthalpy of gelatinization were

noticed accordingly to the degree of the starch modification. The X-ray pattern of native

starch was A type, with similar pattern in modified derivatives.

Received: April 19, 2012

Revised: July 28, 2012

Accepted: July 30, 2012

Keywords:

Acetylation / Mung bean starch / Microwave assistance / Potassium carbonate / Vinyl acetate

1 Introduction

Chemical modification of starch has been used to alter the

hydroxyl group in native starch by ester or ether group.

Acetylation reaction has been the subject of extensive

research for both food and non-food applications [1], but

it had restricted range and level of modification since it had

been required for use in foodstuffs. A maximum content of

2.5% acetyl groups was introduced to produce a maximum

degree of substitution of 0.1 [2]. The first starch acetates

were already described in literature in 1865 [3]. It signifi-

cantly improved physiochemical and functional properties

of the starch [4]. The acetylated starch was classified

depending on its degree of substitution (DS) that ranging

from low to high. Acetylated starches with a low DS of

0.01–0.2 were still of commercial interest [5], it possesses

a number of good properties based on improving food

texture, film forming, binding and texturing so it has been

applied in food industry for many years [6–8]. Starch

with high DS (>1.0) also has many good properties and

applications such as hydrophobicity and its use as thermo-

plastic cellulose acetate substitutes [4, 5]. The properties

of acetylated starch depend on the botanical starch

source, the DS, the amylose/amylopectin ratio, and how

the molecular structure of starch is modified [7].

Acetic anhydride, vinyl acetate, acid chlorides and ace-

tic acid is the more frequently esterifing agent which is

used for esterification of native starch [9–11]. Most starch

esters used commercially such as starch acetates ranging

from low to high DS, are commonly produced by aqueous

suspension of starch reacting with acetic anhydrides and

NaOH to maintain pH 7–9 for several hours [9, 10, 12].

Preparation of starch esters in non-aqueous systems

using organic solvent like heating in formic acid [13], acetic

acid [14], pyridine, and DMSO [10] had been extensively

Colour online: See the article online to view Figs. 1, 2, and 5 incolour.

Correspondence: Dr. Si-Yi Pan, College of Food Science andTechnology, Huazhong Agricultural University, Wuhan, P. R. ChinaE-mail: [email protected]: þ86-027-87288373

Abbreviations: AGU, anhydroglucose unit; DS, degree ofsubstitution; HCl, hydrogen chloride; KBr, potassium bromide;K2CO3, potassium carbonate; MBSA, mung bean starch acetate;NMBS, native mung bean starch; VAC, vinyl acetate

DOI 10.1002/star.201200081236 Starch/Starke 2013, 65, 236–243

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.starch-journal.com

studied. Such procedures and solvents, that is, water or

organic solvents, have many drawbacks such as time

consuming procedure, water hydrolysis of acetic anhy-

dride, expensive solvent for commercial use, reduction

of the reaction rate by dilution of the modifiers, chemicals

recovery after the end of the reaction is problematic owing

to the complicated separation of the solvent. In addition,

organic solvents are often harmful to humans and environ-

ment [15–18]. Therefore, a solvent-free reaction is pref-

erable to eliminate the use of organic solvents [18].

Microwave or dielectric heating is an accepted new

technique to run chemical conversion on lab scale as an

alternative heating principle. It offers an advantage over

conventional techniques by reducing both the required

amount of solvent and the reaction time, and the radiation

can easily reach particles inside [1]. In previous reports, a

number of different polysaccharide derivatives have been

synthesized with the aid of microwave radiation in solvent-

free or aqueous-based reaction systems [17–20].

Among all the acetylation reactions that achieved under

microwave assistance, acetic anhydride is more common

acetylating agent. Acetylation of banana starch, corn

starch, and cellulose using a microwave heating and iodine

as catalyst was studied [1, 17, 18, 20]. The disadvantages

of the acetic anhydride is the formation of acetic acid as

a by-product that must be removed from the modified

materials, otherwise it causes degradation of the polymeric

chain of the starch [8]. In addition to its high cost, the

potential use of acetic anhydride for production of heroin

causes the restriction by the government [21].

Acetylation of starch with vinyl esters is an alternative

approach that does not produce acid as by-product [8], also

it is cheaper than acetic anhydride and the acetaldehyde

as a by-product can be easily removed from the reaction

medium due to its low boiling point (bp ¼ 218C) [22].

Preparation of starch acetates with K2CO3 as activator

and different esterefying agent in different conditions were

studied [8, 23, 24]. It was reported that when comparing

the catalytic activity of K2CO3 with other catalyst, the

degree of substitution directly dependent upon the basicity

of the catalyst [24–26].

As is known, there have been no reports of potassium

carbonate used for activating mung bean starch acetyl-

ation with vinyl acetate under microwave assistance.

The aims of this research were to investigate the

effects of mung bean starch acetylation with vinyl acetate

as esterifying agent under microwave radiation assist-

ance and the catalytic activity of different concentration

of potassium carbonate on DS and the physiochemical

properties of mung bean starch acetate. The specific

objective is to prepare mung bean starch acetates by

solvent-free reaction and reduce energy inputs by care-

fully using the resources in an environmentally friendly

way.

2 Materials and methods

2.1 Materials

Mung beans were purchased from local market in Wuhan-

Hubei province, China. The seeds were screened man-

ually to remove the damaged ones and other impurities.

Mung bean starch was extracted in the laboratory which

had a moisture content of about (4% w/w) and amylose

content of 34.7%. A � 97% analytical grade vinyl acetate

(VAC) C4H6O2 (Sinopharma Chemical Reagent Co., Ltd.,

Shanghai, China), Potassium carbonate (the Chemical

Factory of Hubei University, Wuhan, China), and ethanol

(Sinopharma Chemical Reagent Co., Ltd., Shanghai,

China) were used as received.

2.1.1 Analytical equipment

Analytical equipments used in this study were microwave

Apex Atmospheric pressure (Microwave Synthesis/

Extraction System, Shanghai EU Microwave Chemistry

Technology Co., Ltd.), Differential Scanning Calorimeter

(DSC; NETZSCH 204F1; Germany), Fourier Transform-

Infra Red (FT-IR; Thermo Nicolet NEXUS 670 FTIR; USA),

Scanning Electron Microscope (SEM; JEOL, JSM-6390/

LV; Japan), and an X-ray diffractometer (Bruker AXS/D8

Advance; USA).

2.2 Methods

2.2.1 Starch extraction

Mung beans (5 kg) were soaked in water (1:3 v/v) at 308Cfor 18 h, and then manually peeled. The peeled seeds

were blended using 0.05 N NaOH (1:15 w/v) in a warring

blender (Philips, 750W, 220V, type A001238). The resulting

slurry was passed through a 200 mesh sieve and left

overnight at 48C for starch sedimentation. The starch

was mixed with water and centrifuged for 15 min at

1789g this washing step was repeated for five times then

dried overnight in the air drier at 408C. It was ground into

powder and stored in a tightly closed polyethylene bag for

further analysis.

2.2.2 Synthesis of starch acetate

Microwave equipped with reflux condenser and a fiber

optic temperature probe was used for carrying out the

reactions. Three-necked round bottom glass flask with a

mechanical stirrer as a reactor for mixture preparation was

used. VAC (28.7 mL, 333.3 mmol) was added to about

10 g (61.73 mmol) of the dried NMBS sample in the reac-

tor, and then the reactor was placed in the microwave

cavity, and mixed for 5 min by magnetic stirrer. The mixture

Starch/Starke 2013, 65, 236–243 237


was then heated to 728C and kept at this temperature

for 2 min while stirring, so that VAC can penetrate

the dried starch granules before adding potassium

carbonate [14, 23]. Afterwards, the reactor was removed

out of the cavity, and the K2CO3 (five concentrations

were used, (0.4 mmol; 0.006 mmoleq/AGU, 0.7 mmol;

0.011 mmoleq/AGU, 1.4 mmol, 0.022 mmoleq/AGU,

1.8 mmol; 0.029 mmoleq/AGU, 7.2 mmol; 0.117 mmoleq/

AGU) was immediately added. The mixture was returned

back to the microwave cavity and again heated to 958Cfor another 2 min while stirring, then removed and cooled

in cold water to room temperature. For precipitation of

the starch acetate, 20 mL of ethanol was added to

the mixture, to remove any unreacted compounds, and

filtered by filter paper. The filtrate was washed by centrifu-

gation at 1789g for 10 min using distilled water to remove

unreacted K2CO3. This was confirmed by checking the

water every time after washing using phenolphthalein as

an indicator. The starch acetate was then dried at 508Covernight. The different acetylated starches were then

ground to a fine powder and tightly closed and stored

under dry condition for further analysis.

2.2.3 Acetyl percentage and degree ofsubstitution

Following the method of Sodhi and Singh [27], the per-

centage of acetyl content (%) was titrimetrically deter-

mined by alkaline hydrolysis of acetyl group then

titration of the excess alkali (Eq. 1). The degree of substi-

tution was calculated following Eq. (2)

Acetyl contentð%Þ

¼ ðblank titre� sample titreÞ � normality of HCl� 0:043� 100

weight of sample ðgÞ(1)

DS ¼ 162� acetyl content ð%Þ4300� ð42� acetyl content ð%ÞÞ (2)

2.2.4 Fourier transform-infra red (FT-IR)analysis

Fourier Transform Infrared (FT-IR) analysis was used to

confirm starch acetylation which was enhanced by differ-

ent concentration of the K2CO3. The spectra of native and

acetylated starch samples were obtained in the wave

number range of 4000–400 cm�1. Using mortar and pes-

tle, about 1 mg of sample with 50 mg of dry KBr was

thoroughly ground together for pellets preparation. The

thin and transparent pellet was then tested. A number of

scans as a multiple of 32 scans were accumulated at a

resolution of 4 cm�1.

2.2.5 X-ray diffraction analysis

Crystallinity and type of NMBS and MBSA with different

DS were studied by an X-ray diffractometer. Data was

collected in the scanning region of the diffraction angle

(2Ø) from 5.008 to 35.008 [20].

2.2.6 Thermal properties

A differential scanning calorimeter (DSC) equipped with a

thermal analysis data station was used to study the gelati-

nization parameters of the native starch, and starch

acetates with different DS. About 3mg of each sample

was weighed into aluminum DSC pans and mixed with

11 mL distilled water using micro syringe. The aluminum

pans were hermetically sealed, and left for 24 h at room

temperature to ensure equilibration, and then tested at a

rate of 108C/min from 20 to 1208C in a nitrogen atmos-

phere. An empty aluminum pan was used as a reference.

The instrument was calibrated with indium.

2.2.7 Scanning electron microscopy (SEM)analysis

Electron micrographs of the NMBS and MBSA samples

granules were studied using a scanning electron micro-

scope. The starch samples were sprinkled on double-sided

tape, fixed to an aluminum stub, and then coated with gold.

The images were taken at an accelerating voltage of 10 kV.

Micrographs were recorded at 2500� magnification [28].

2.2.8 Water absorption and solubility

Water absorption and solubility of NMBS and MBSA with

different DS was determined by adding 30 mL of distilled

water to 1 g of NMBS (Ws) in clean, dried and pre-weighed

centrifuge tube (W1). The tube was then mixed for 1 h, in a

Figure 1. Effect of potassium carbonate concentration(mmoleq/AGU) on the DS of MBSA.

238 M. Bushra et al. Starch/Starke 2013, 65, 236–243


shaking water bath, at 308C, afterward it was centrifuged at

1789g for 15 min. The supernatant was carefully poured

into clean dried pre-weighed glass dish (W3), and dried for

8 h at 1008C, then placed in desiccator until cooled down

to room temperature, and then re-weighed again (W4). The

centrifuge tube with the remaining sediment was weighed

again (W2). This protocol was repeated for all MBSA with

different DS. Water absorption (g/g) and solubility (%) was

calculated according to the following equations.

Water absorptionðg=gÞ ¼W2 �W1

Ws(3)

Figure 2. FTIR spectra of NMBS and MBSA with different DS. [A1] – (A) NMBS (B) DS ¼ 0.003(C) DS ¼ 0.07, and [A2] –(D) DS ¼ 0.08, (E) DS ¼ 0.09, (F) DS ¼ 0.1, and (G) DS ¼ 0.27.

Starch/Starke 2013, 65, 236–243 239


Water solubilityð%Þ ¼W4 �W3

Ws(4)

2.2.9 Statistical analysis

The data were analyzed using the Statistical Package

for the Social Sciences (SPSS17) to perform Analysis

of variance (ANOVA) at a significance level of 5%

(a ¼ 0.05).

3 Results and discussion

3.1 Effect of potassium carbonateconcentration on the degree of substitution

Esterefication reaction was carried out with VAC and an

alkaline base salt (K2CO3) as the catalyst for acetylation of

NMBS under microwave assistance. Linear increment of

the DS of MBSA from 0.003 to 0.27 was observed following

the increase of the potassium carbonate concentration

from 0 to 7.2 mmol (Fig. 1). The K2CO3 content of

0.27 mmol resulted in degree of substitution that signifi-

cantly differed from what obtained by the beginning of the

content of K2CO3 (p < 0.05). This reaction was achieved

through base-catalyzed transesterification reaction in

which the deprotonation of hydroxyl group of starch was

activated by base catalyst; then the resultant intermediate

starch alkoxide was later reacted with ester to produce

starch acetate [5, 24, 26, 29–31].

3.2 Fourier transform infra red spectroscopy

The FTIR spectra of NMBS and MBSA with different DS

were displayed in Fig. 2 (A1, A2). The spectrum of NMBS

has peaks at wave numbers around 1018, 1080, and

1157 cm�1 indicating C–O bond stretching, the C–H

stretch vibration appeared at 2960 cm�1. Tightly bonded

water (H2O) and an extremely broad band, resulted from

vibration of hydrogen bonded hydroxyl groups (O–H),

which were represented by the peak at 1641 and

3450 cm�1 respectively. Strong absorption band at

1742 cm�1 is a sign of the stretching of the ester carbonyl

C––O group and indication to the acetylation of starch [4].

3.3 Thermal properties

The thermal properties (onset temperature (T0), gelatini-

zation temperature (Tp), conclusion temperature (Tc),

and change in enthalpy of gelatinization (DH)) of NMBS

and MBSA with different DS were determined by DSC

(Table 1). Acetylation resulted in inverse relation between

different DS and thermal properties of starch acetates.

This chemical reaction led to partial disorganization of

starch components, due to the introduction of the acetyl

group, which in turn resulted in weakening of associative

bonding forces within the starch molecules. Enthalpy of

gelatinization is mainly due to disruption of the double

helices, rather than the longer-range disruption of crystal-

linity [32]. This was appeared as higher enthalpies of

gelatinization combined with MBSA with different DS,

which is due to highly associated double helices, formed

by the outer branches of adjacent amylopectin chains and

melt during gelatinization.

Similar observation of acetylated legume starch was

observed [33, 34]. Acetylation was found to reduce

Gelatinization temperatures of starch from diverse bota-

nical sources [1, 33–36].

3.4 X-ray diffraction

Figure 3 shows the crystalline patterns of native starch and

starch acetate with (0.27) degree of substitution (data of

Table 1. Thermal properties of native mung bean starchand mung bean starch acetate with differentconcentration of potassium carbonate

Starch (DS)

K2CO3

(mmoleq/AGU) Tp (8C) DH (J/g)

NMBS 0 70.8 � 0.5a 19.6 � 0.5a

0.003 � 0.05a 0 70.3 � 0.6b 17.8 � 0.1b

0.07 � 0.05b 0.006 65.51 � 0.5c 17 � 0.1c

0.08 � 0.5b 0.011 64.3 � 0.05d 16.7 � 0.2d

0.09 � 0.5b 0.022 64.4 � 0.05e 14 � 0.1e

0.10 � 0.05c 0.029 63.7 � 0.1f 11 � 0.1f

0.27 � 0.1d 0.117 60.2 � 0.1g 13 � 0.05g

All values are means of triplicate determinations � SD.Means within columns with different letter are significantlydifferent at a significance level of 5% (a ¼ 0.05).

454035302520151050

500

1000

1500

2000

2500

heta(degree)2-T

B

A

Intensity(counts)

Figure 3. Wide angle XRD pattern of (A) NMBS and(B) AMBS (DS ¼ 0.27).



other degree of substitution was not shown). X-ray diffrac-

togram of native mung bean starch showed strong peak

at 158, 178 and 238 (2u), these peaks were characterized

starches with C-type diffraction pattern [37] which in turn is

a mixture of A and B-type crystalline structures [38, 39].

The different degree of substitutions of starch acetate

did not obtain substantial changes in the XRD patterns.

The acetylation probably took place at the amorphous

region when it achieved at low temperature [14], and

since the amylopectein is mostly responsible for the

crystallinity of the starch, it could be argued that the

starch crystallinity did not change. Similar observation

Figure 4. Scanning electron micrographs of (a) NMBS, (b) DS ¼ 0.003, (c) DS ¼ 0.07, (d) DS ¼ 0.08, (e) DS ¼ 0.09,(f) DS ¼ 0.1, and (g) DS ¼ 0.27.

Starch/Starke 2013, 65, 236–243 241


was reported by Lawal [34], Wang and Wang [36], and

Chen et al. [40].

3.5 Scanning electron microscopy

The scanning electron micrograph of the native and modi-

fied starch granules at 2500� magnification was shown in

Fig. 4. The NMBS granules appeared to have irregular

shapes, which varied from oval to round and bean shape

with smooth and free of fissures surfaces (Fig. 4A).

Modified starch with the increased DS showed interest-

ing modifications in comparison to the native counterparts.

Figure 4B shows no obvious change on the surface

and this is due to the very low DS (0.003). With the

increased DS, smooth surface became rough, and

then turned into fissures surface, ended up with partial

rupture of the granules with 0.27 DS (Fig. 4C–G). This

indicated that the K2CO3 could be used as an effective

catalyst in non-aqueous reaction mixture under microwave

assistance.

3.6 Water absorption and solubility

Figure 5 shows the water absorption of the native mung

bean starch and its derivatives. The introduction of the

acetyl functional groups and their electrostatic repulsion

enhanced the access of water within the starch matrices

which resulted in an increase of hydration [41].

With the increase in the DS of acetylated starch, water

absorption of the native starch was increased. This indi-

cated that the modified starch hydrophilic tendency was

improved after acetylation. The increased hydrophobicity

of the starch acetate was found when it has a DS greater

than one [14, 20].

The DSs did not affect the water solubility of the native

starch. This was due to relatively high amylose content of

the starch and the low degree of substitutions. Similar

observation was shown by Shogren and Biswas [14]

who studied the effect of different amylose content and

DS in solubility of the starch acetate.

4 Conclusions

It could be concluded that esterification catalyzed

by K2CO3 and esterified with VAC showed a promise as

a straightforward and simple method to modify the NMBS

under microwave assistance. The increment of the DS of

starch acetate indicated that the K2CO3 could be used as

an effective catalyst in solvent-free reaction under micro-

wave assistance in short time to produce starch acetate

with low degree of substitution for food applications. On the

other hand, VAC as acetylating agent for starch was readily

available and cheaper than acetic anhydride.

Further studies are needed to study the effect of differ-

ent time and temperature in order to investigate their

effects on the degree of substitution.

The first author is grateful to Professor Pan Si Yi and

Dr. Xu Xiao Yun for offering the financial support and

required facilities to conduct this research. Many thanks

to all the members of food analysis laboratory in Huazhong

Agricultural University-Wuhan-Hubei for their enthusiastic

help during the period of my study.

The authors have declared no conflict of interest.

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Microwave assisted acetylation of mung bean starch and the catalytic activity of potassium carbonate in free-solvent reaction