Effect of ultrasound-microwave and microwave-ultrasound treatment on physicochemical properties of corn starch

Highlights

• •Natural corn starch was modified by using ultrasound and microwave-assisted methods.
• •The highest changes on starch morphology was obtained after high US treatment.
• •Microwave modification is an efficient method because of the shorter treatment time: 1, 2, 3 min.
• •Premodification of starch can decrease amount of energy and chemical reagent which will be used during main modification.

Abstract

Natural starch is an agricultural sourced biopolymer being low cost, biodegradable, high efficiently, renewable and easy available. Despite these advantages, phisochemical properties of native starch are limited for most industrial applications and must be modified. Ultrasound and microwave treatment have been widely applied separately for starch modification. Ultrasound treatment, with high efficiency and low cost, and microwave treatment, which produces homogeneous and high quality products, are short proceesing time technologies that can be used together to change the structure and properties of starches obtained from various plants. In this study the effects of ultrasound and microwave combined treatment on the physicochemical properties of natural corn starch were investigated. Corn starch was irritated using different combination of ultrasound-microwave and microwave-ultrasound treatment; using 90, 180, 360 and 600 W microwave power during 1, 2, 3 min, and using ultrasound at 35 °C constant temperature for 20, 30, 40 min. The structural changes of modified corn starches were determined by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analyses. Nowadays, many physical methods are used for starch modification, but limited studies were on ultrasound-microwave and microwave-ultrasound combined treatment method. As a result of this study, it was observed that ultrasound and microwave combination is an efficient, fast and environmentally friendly method for natural corn starch modification.

1. Introduction

Starch is a natural biopolymer and uses as an energy sources in many plants such as corn, sweet potato, potato, wheat, rice, etc. Starch is widely used in food, feed, chemical, petrochemical, adhesive, paints, paper, textile and pharmaceutical industry. Starches consist of two types of glucose polymers, the linear amylose with rare branches and the highly branched amylopectin components that are arranged in a complex semi-crystalline structure and assembled in granular form. But it has limited application in native form because of poor resistance to high temperature, to shear and to digestion, low emulsifying power, low solubility in cold water, high degree of retrogradation and instability in acids. Therefore, starches are modified to overcome these disadvantages and to increase their use for various industrial applications. Starch modification may be realized by enzymatic, genetic, chemical and physical methods or combination of these methods [1], [2], [3], [4], [5], [6]. Physical modification methods have been taken growing interest, being safe, cheap and environmentally friendly green chemistry processes. At the same time, they are more suitable for food applications [7], [8], [9]. Ultrasonication is the use of high frequencies acoustic waves (15–20 kHz) that can be mechanically transferred through solid, liquid, and gaseous systems, to support desired changes and modifications in the products [10]. Ultrasound cause physical corruption of starch granules but the shape and size don't changed. Ultrasound pretreatment increases the water solubility, decreases the gelatinization, pasting viscosity, swelling parameters and retrogradation of starchs. Sonication power and frequency, energy amount, treatment temperature and time, starch type and composition, starch concentration of the slurry are other factors affecting ultrasonification of starch [11], [12]. Ultrasound treatment, is an environment friendly starch modification method, having many advantages like higher selectivity and quality, reduced use of chemicals and processing time [13]. Microwaves are electromagnetic waves in the frequency range of 300 MHz to 300 GHz. Microwave radiation is a kind of non-ionizing radiant energy, which is used for starch modification, and which can effectively change the structure and functional characteristics of food materials. It is widely used in the food industry at the working frequency of 915 MHz or 2450 MHz. Nowadays, microwave treatment has been frequently used in modifying starch because of its advantages of high heating speed, high efficiency and environmental protection [1], [5], [6], [14].Various researchers have recorded that combining microwave heating with other methods have great effect on starch modification. Dual modified starch find application in food and non-food industries [15]. The objective of the present study was to examine the effect of heat treatment by different combination of ultrasound-microwave and microwave-ultrasound technologies on the physicochemical properties of corn starch. Microwave power was applied at 90, 180, 360 and 600 W during 1, 2, 3 min; ultrasound treatment, was applied at 35 °C constant temperature during 20, 30, 40 min. It is aimed to modify and change the morphology of natural corn starch, and to overcome the negative effects that occur during processing. The effects of modification parameters such as temperature and sonication time for ultrasonic assisted modification and, microwave power and irradiation time for microwave assisted modification, on the starch treatment were investigated. The structural changes of modified corn starches were determined by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analyses. Thus, the most efficient corn starch treatment technology was determined, and morphological structure was investigated. The outcomes of this work will enable, corn starch treatment with green methods, and decreased energy and chemical agents consumption, making a contribution to the national economy and environmental protection.

2. Materials and methods

2.1. Raw materials and sample preparation

Natural corn starch (11.11% moisture content) used in this study was purchased from Pendik Nisasta (Istanbul, Turkey). The chemical reagents NaOH and H₂SO₄ were purchased from Merck (Darmstadt, Germany). All chemicals and solvents used in this study were of analytical grade. The natural corn starch was dried in an oven at 60 °C for 24 h to remove any moisture, then the dried starches were milled, and the resulting peel powder was stored in low-density polyethylene bags at room temperature.

2.2. Ultrasound-microwave assisted modification

Aqueous natural corn starch suspension at a concentration of 30% (w/v) was mixed with appropriate distillated water using magnetic stirrer at 685 rpm for 30 min before ultrasonic treatment. The pH value of the solution was adjusted to 7.0 by adding 1 mol/L NaOH aqueous solution. To determine optimum conditions for ultrasonic modification, corn starch solution was treated by ultrasound irradiation at 35 °C constant temperature for 20-, 30- and 40-minutes using Bandelin Sonopuls HD 2070 (20 kHz) model ultrasonic homogenizer (Bandelin electronic GmbH & Co. KG, Berlin, Germany). The temperature was kept constant using an ice bed. Then the solution was centrifuged at 1500 rpm for 20 min and dried in an oven (MMM Ecocell 111, Munich, Germany) at 45 °C up to a constant weight was reached. After ultrasonic treatment, starch solution was mixed with 98% concentrated H₂SO₄ solution using magnetic stirrer to adjust pH around 11. It was washed and filtered with distilled water, and then dried at 45 °C for 24 h. The starch samples obtained were ground before microwave treatment. After grinding, they were placed in petri dishes and modified at different microwave power (90 W, 180 W, 360 W, 600 W) and different process time (1 min, 2 min, 3 min) [16], [17], [18]. The modification conditions were given in Table 1.

Table 1.

Ultrasound-microwave modified starch samples conditions.

Sample	Ultrasound modification	Microwave modification
	time (min)	power (W)	time (min)
S1	30	90	3
S2	40	180	3
S3	30	360	3
S4	40	600	1

2.3. Microwave-ultrasound assisted modification

Corn starch samples in granular form, at 11–12% moisture content of, were treated by microwave. To determine the optimum microwave power and irradiation time, microwave treatment was applied in (Bosch HMT72G420, Stuttgart, Germany) microwave oven at the following conditions: 90 W, 180 W, 360 W or 600 W microwave power and 1 min, 2 min or 3 min irradiation time [19]. After microwave treatment, microwaved corn starch was suspended in an appropriate volume of distilled water to obtain the concentration of 30% (w/v). The pH value of the corn starch solution was adjusted to 7.0 by adding 1 mol/L NaOH aqueous solution. To determine optimum conditions for ultrasonic modification, the corn starch solution was subjected to the second modification with some changes, by ultrasonic method applied by Cao and Gao [20], at a frequency of 20 kHz for 20, 30 and 40 min. The temperature was kept constant at 35 ⁰C using an ice bed. After that, solution obtained was continuously mixed with a magnetic stirrer using 1 M H₂SO₄ solution and the pH value was adjusted to 7. The solution, whose pH value was fixed, was removed from the water by purification process and dried in an oven at 45 ⁰C for 24 h. The modification conditions were given in Table 2.

Table 2.

Microwave-ultrasound modified starch samples conditions.

Sample	Microwave modification		Ultrasound modification
	power (W)	time (min)	time (min)
S5	90	3	20
S6	180	3	40
S7	360	3	20
S8	600		30

2.4. Fourier transform infrared (FTIR) spectroscopy

The chemical structure analysis of the modified starches was performed using FT-IR spectrophotometer (IRPrestige21, Shimadzu Corporation, Kyoto, Japan). Starches samples were mixed with KBr (1:100) and pressed into KBr pellets before FTIR analysis. The spectra were obtained in the frequency range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹.

2.5. Scanning electron microscopy analysis

The morphological properties of the modified starches were observed using a scanning electron microscope (SEM, Zeiss EVO LS10) with an accelerating voltage of 7 kV and a magnification of 500, 1000, 10,000 and 20,000 X.

3. Results and discussion

3.1. Chemical structure analysis by FT-IR

FT-IR spectra of the natural and modified corn starches are shown in Fig. 1. It was seen that the functional groups of natural corn starch were not changed under these treatments, which indicated that the microwave and ultrasonic modification were physical modification processes. Typical absorption bands seen in natural starches were also seen in modified starches, but the amplitudes of the peaks were different [21], [22]. The broadbands seen at around 3400–3600 cm⁻¹ represent the –OH peaks of amylose and amylopectin. The intensity of this bands was reducing due to the ultrasound treatment, mentioned that the starch microstructure’s capacity to hold bound water was disrupted [23]. The main absorption bands, typical for polysaccharides, observed at around 2887–2978 cm⁻¹ correspond to the vibration of C-H bonds. The peak around 1645–1730 cm⁻¹ is related to bending vibration of C-H and O-H from adsorbed water. Main absorption bands, appeared at 1650–1250 cm⁻¹ range due to vibrations of glycoside bonds C–O–C. The peak at 1373 cm⁻¹ of microwave modified corn starches represent bent modes of O-C-H, C-C-H and C-O-H. The bands at 1150 cm⁻¹ and 1040 cm⁻¹ are due to C-O stretching in glucose monomer. They are associated with the ordered structures of starch, and sensitive to the crystallinity changes. The band at around 950 cm⁻¹ was assigned to represent the amorphous state as well as vibration of C-O-H [24], [25], [26]. These results were consistent with the previous studies [27], [28].

Fig. 1.

The FTIR spectroscopy of the natural (S₀) and modified (S₁, S₂, S₃, S₄, S₅, S₆, S₇, S₈) starches.

3.2. Morphology analysis by SEM

Fig. 2 shows the SEM images of natural and modified corn starches. The surface of native corn starch samples was polygonal, smooth, round and without cracks. Clear changes were observed on the surface of the modified corn starches. After microwave and ultrasound assisted modification, starches completely lost its original structure. A rough and irregular shape was formed on the surface, and an amorphous granular structure emerged by connecting the starch granules to each other. Structural changes, such as grooves and notches, were observed on the surfaces of the starch granules after ultrasonic modification. These changes may result from the formation of localized hot spots through bubble collapse and cavitation, which leads to high pressure gradients in the surrounding space. The surface of the ultrasound treated starch is partially gelatinized due to an increase in temperature during ultrasound treatment. Pits and cavities were appeared on the corn starch granules after microwave treatment. Some granules were clustered together, resulting in a wider particle size distribution. Zhou et al. indicated that microwave heating changed morphology of starch, the granules deformed and broke with increasing microwave time and eventually lost their integrity [21], [23], [29], [30], [31]. These results were consistent with the previous studies [28], [32], [33].

Fig. 2.

The SEM images of the natural (S₀) and modified (S₁, S₂, S₃, S₄, S₅, S₆, S₇, S₈) starches.

4. Conclusions

This study investigated the effect of heat moisture treatments (ultrasound-microwave and microwave-ultrasound consecutively) on the physicochemical properties of corn starch. Corn starch was irritated using 90, 180, 360 and 600 W microwave power during 1, 2, 3 min, and using ultrasound at 35 °C constant temperature for 20, 30, 40 min. The effects of temperature and time for the ultrasound-assisted modification method, and microwave power and time for the microwave-assisted modification method on the morphology and structural properties of the product were investigated. It has been determined that increasing the time for ultrasound modification increases the modification efficiency. The greatest change in starch granules was observed under 360 W microwave power and 30–40 min ultrasonic conditions. It was concluded that ultrasound and microwave combination is an efficient, fast and environmentally friendly method for natural corn starch modification. Combining microwave modification with other modification methods can be investigated for corn and other starch types modification in future studies.

CRediT authorship contribution statement

Aslıhan Yılmaz: Data curation, Writing – original draft, Visualization, Investigation, Software, Validation. Nurcan Tugrul: Conceptualization, Methodology, Software, Supervision, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.