Mild alkali treatment alters structure and properties of maize starch: The potential role of alkali in starch chemical modification

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International Journal of Biological Macromolecules 274 (2024) 133238

Available online 17 June 2024

Mild alkali treatment alters structure and properties of maize starch: The

potential role of alkali in starch chemical modication

Zekun Xu

, Xiaoning Liu

, Chuangchuang Zhang

, Mengting Ma

, Bilatu Agza Gebre

Solomon Abate Mekonnen

, Harold Corke

, Zhongquan Sui

Department of Food Science and Technology, School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai, 200240, China

Department of Food Science & Nutrition, Ethiopian Institute of Agricultural Research, Addis Ababa, Ethiopia

Department of Biotechnology and Food Engineering, Guangdong Technion-Israel Institute of Technology, Shantou 515063, China

Faculty of Biotechnology and Food Engineering, Technion-Israel Institute of Technology, Haifa, 3200003, Israel

ARTICLE INFO

Keywords:

Alkali treatment

Pasting properties

Starch modication

Maize starch

ABSTRACT

Normal and waxy maize starches were treated with mild alkali treatment (pH 8.5, 9.9, 11.3) in two temperature-

time combinations (25 ◦C for 1 h and 50 ◦C for 18 h) to investigate the effect on starch structure and properties.

Mild alkali treatment partly removed the starch granule-associated proteins and lipids of normal (from 0.31 % to

0.24 % and from 0.77 % to 0.55 %, respectively) and waxy maize starches (from 0.22 % to 0.18 % and from 0.24

% to 0.15 %, respectively). Gelatinization enthalpy of waxy maize starch increased with alkali treatment from

16.20 J⋅g

−1

to 21.95 J⋅g

−1

, indicating that amylopectin (AP) rearrangement and AP-AP double helices formation

might occur. But amylose could inhibit these effects by restricting mobility of amylopectin, and no such changes

occurred for normal maize starch. Alkali treatment decreased gelatinization temperature and increased peak and

nal viscosity. Alkali treatment decreased trough viscosity and increased setback of normal maize starch. The

hydrothermal treatment promoted the effect of alkali, attributed to the more rapid molecular motion at higher

temperature. Normal and waxy starches showed different changes after alkali treatment, indicating that amylose

played an important role in controlling the effect of alkali and hydrothermal treatment, primarily as an

obstructer of amylopectin rearrangement in mild alkali treatment.

1. Introduction

Starch is unique among carbohydrates, mainly derived from plant

seeds, tubers and roots. It supplies the major energy to human daily diet

[1]. It is also widely used in food and other industries because of its

advantages of abundant availability, easy degradation, renewability and

low price [2]. Due to some inherent technical limitations (e.g., high

thermal sensitivity, low shear resistance, high retrogradation, etc.), the

direct application of native starches in industry is limited [1]. Therefore,

native starch is commonly modied by physical, chemical or enzymatic

processes to achieve the desired characteristics for particular applica-

tions. Chemical modication signicantly changes starch properties by

introducing functional groups into starch. The ionization of hydroxyl

groups on starch can bind to a variety of chemical groups to produce

various chemical modication outcomes including esterication,

etherication, oxidation and cross-linking [3].

Chemical modications are the most widely used modication

methods, due to distinct advantages, e.g. stable product, substantial

alteration in properties, and technically simple large-scale production

systems [4]. However, several problems and challenges remain to be

addressed. Due to the heterogeneity of starch granules, the distribution

of substituents is uneven [5,6]. In addition, the properties of modied

starches and the reaction efciency depend on many factors, e.g. con-

centration, temperature, duration, chemical reagents [7]. Alkali has a

vital role in starch modication. Chemical modications are usually

carried out in a mild alkaline condition, sometimes combined with hy-

drothermal treatment. Reagents usually react with starch at pH 8–12

and 20–70 ◦C for 1–24 h [1,8,9]. Changes in pH signicantly affect the

modication efciency, the properties of modied starches, and the

distribution of substituents [10,11]. The order of addition of reagents

* Corresponding author at: Department of Biotechnology and Food Engineering, Guangdong Technion-Israel Institute of Technology, Shantou 515063, China.

** Corresponding author at: Department of Food Science and Technology, School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai, 200240,

China.

E-mail addresses: harold.corke@gtiit.edu.cn (H. Corke), zsui@sjtu.edu.cn (Z. Sui).

Contents lists available at ScienceDirect

International Journal of Biological Macromolecules

journal homepage: www.elsevier.com/locate/ijbiomac

https://doi.org/10.1016/j.ijbiomac.2024.133238

Received 4 April 2024; Received in revised form 14 June 2024; Accepted 16 June 2024

International Journal of Biological Macromolecules 274 (2024) 133238

and alkali also signicantly affects the modication efciency [9,12].

These indicates that the alkaline condition plays a signicant role in

starch chemical modication even at low concentration and short

duration. However, the alkali is generally believed to mainly act as an

activating reagent and eliminate reaction byproducts in chemical

modication. The effect of alkali itself on starch structure and properties

has been little studied.

To explain the reaction mechanism of starch chemical modication,

the process can be studied at starch granular and molecular level.

Granule swelling is a key factor affecting the access of chemical reagents

to starch molecules [13,14]. Granule swelling is necessary for any

chemical modication of starch, even for reaction at granule surfaces

[15]. Thus, starch hydration rstly gradually induces substantial

swelling of starch granules, and then effectively controls the extent and

the patterns of chemical reaction [13,14].

Starch granules comprise semi-crystalline and amorphous regions.

The molecules in amorphous regions change from a rigid glassy state

into more exible rubbery state as a function of hydration degree [16],

which increases reactivity of starch molecules. The hydration of starch

granules is a capillary phenomenon, where internal water shows a

different solvent behavior from free water [17]. In theory, alkali causes

ionization of hydroxyl groups in amorphous regions, which can disrupt

the hydrogen bonds and the granule swelling by the repulsion among

anions [11].

The starch granule-associated proteins (SGAPs) and lipids (SGALs)

play important roles in maintaining granule structure [18,19]. The

presence of alkali can remove or degrade the proteins and lipids. The

granule structure can be loosened and swell more easily with the

destruction of proteins and lipids, and starch molecules will become

more exible. Therefore, it was hypothesized that alkali could inuence

starch structure and granule hydration by ionization of hydroxyl groups

and by alteration of SGAPs and/or SGALs.

The study focused on the effect of alkaline environment during

chemical modication on starch properties. The information obtained

from this study was expected to increase understanding of the role of

alkali in starch chemical modication and hence aid in design of

improved modication processes.

2. Materials and methods

2.1. Materials

Normal maize starch (NMS) and waxy maize starch (WMS) were

provided by Lihua Starch Co., Ltd. (Qinhuangdao, China). The amylose

content of NMS and WMS were 25.6 % and 1.9 %, respectively. All

chemicals and reagents were of analytical grade and above.

2.2. Alkali treatment

Starch (35 g, d.b.) was mixed with 100 mL distilled water. The starch

suspension was magnetically stirred at two different temperature-time

combinations (25 ◦C for 1 h, and 50 ◦C for 18 h) under different alka-

line conditions (pH 8.5, 9.9, and 11.3). The pH was maintained by

automatic titrator with 0.1 M NaOH. After treatment, starch suspension

was adjusted to pH 6.5 with 1 M HCl. Then the suspension was centri-

fuged at 3000 ×g for 15 min. Supernatant was removed, and the residue

was resuspended with distilled water and centrifuged again. This step

was repeated three times to remove salt from the starch suspension.

Starch was ltered through a Büchner funnel and dried at 40 ◦C for 24 h

in a conventional oven. Dried starch was ground to pass through 100

mesh sieve. Native starch and starch only treated for 18 h at 50 ◦C

without alkali were set as control groups to reect the hydrothermal

effect. To better describe and analyze the results, samples were divided

into two groups: those treated at 25 ◦C for 1 h named as RTG (room

temperature group), and those at 50 ◦C for 18 h (this condition being

similar to annealing) named as HTG (hydrothermal group).

2.3. Protein content

Protein content of starch was determined using the method AACC

46–08 (2000).

2.4. Lipid content

Lipid content of starch was determined according to the method of

Baszczak et al. with minor modication [20]. Briey, starch (10 g, d.b.)

was continuously stirred in 120 mL 75 % n-propanol at 95 ◦C for 2 h to

extract lipids. The suspension was centrifuged at 3000 ×g for 15 min,

and the supernatant was collected. The sediment was resuspended with

120 mL 75 % n-propanol (v/v) and reextracted. This was repeated three

more times to ensure the lipids were thoroughly extracted. The extracts

were combined and evaporated to around 1 mL with a rotary evapo-

rator, then the lipids were transferred to an oven and dried at 130 ◦C

overnight.

2.5. X-ray diffraction

X-ray analysis was performed using an X-ray diffractometer (D/

MAX-2200/PC, Rigaku Corporation, Tokyo, Japan). The target voltage

was 40 kV and tube current was 100 mA. The angular range of 2θ was

from 5 to 40◦and scanning rate was 2◦/min with a step interval of 0.01.

The relative crystallinity was calculated following the method of Zhang

et al. [21].

2.6. Swelling power and solubility

Swelling power and solubility were determined as described by Sui

et al. with minor modication [12]. Starch sample (0.5 g, d.b.) was

added into 40 mL distilled water and mixed by vortex. Sample was

incubated at 85 ◦C for 30 min in a water bath, followed by cooling to

ambient temperature. After centrifugation at 1800 ×g for 15 min, the

supernatant was dried at 150 ◦C to constant weight. The SP was esti-

mated as the wet weight of precipitate divided by the dry mass of pre-

cipitate. The SOL was estimated as the dried weight of supernatant solid

divided by the dry weight of sample.

2.7. Thermal properties

Thermal parameters of starches were determined using a Differential

Scanning Calorimeter (DSC 2500, TA Instruments Inc., New Castle, DE,

USA). Starch samples (2 mg, d.b.) and 6 mL distilled water were mixed

and sealed in an aluminum DSC pan. The pan was equilibrated at room

temperature for 24 h. The temperature range was 30–120 ◦C with the

heating rate of 10 ◦C/min. Gelatinization enthalpy (ΔH), onset (T

peak (T

) and conclusion temperature (T

) were obtained.

2.8. Pasting properties

The pasting properties of starches were measured by a Rapid Visco

Analyser (RVA4500, Perten Instruments, H¨

agersten, Sweden) following

a previous method [22]. Sample (1.96 g, d.b.) was mixed with distilled

water to reach a total weight of 28 g in a RVA container. A program

(standard prole 1 for starch, 13 min) of heating and cooling cycle was

used. Briey, samples were held at 50 ◦C for 60 s, then heated to 95 ◦C in

222 s and held at 95 ◦C for 150 s. Then the samples were cooled to 50 ◦C

in 228 s and held at 50 ◦C for 120 s. Peak viscosity (PV), trough viscosity

(TV), breakdown (BD), nal viscosity (FV) and setback (SB) were

obtained.

2.9. Statistical analysis

Differences of the properties of starches with treatments were

compared using analysis of variance (ANOVA). Duncan's test was

Z. Xu et al.

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