Physically Modified Starch

Abstract

The demand for starch is never-ending and is increasing at an alarming rate worldwide for its food and nonfood applications. However, native starch has some limitations, such as low gel strength, viscosity, water binding capacity, and thickening properties. This leads the researchers to find various ways of modifying the starch to improve its beneficial qualities and correct the drawbacks of natural starches. The starch modification industry is changing continuously, providing various physical, chemical, and enzymatic methods and sometimes their hybrids. Among these, the physical modification methods possess higher safety, environmental friendliness, and cost-effectiveness without leading to the production of chemical wastes. The physical modification methods can be divided into two categories: thermal and nonthermal. The former category includes heat-moisture treatments, annealing, microwave heating, and osmotic pressure treatment, while the latter includes ultrasonication, ultrahigh-pressure treatments, and pulsed electric fields. This chapter covers recent advancements in thermal and nonthermal physical modification of starch, their principles, procedures, and their effect on starch functionalities.

1 Introduction

Starch is an abundant organic compound found in nature after cellulose. It is mainly present in seeds, roots, tubers, leaves, fruits, and vegetables. Starch granules are made up of amylose and amylopectin fractions [1]. Both these components differ significantly in their properties and, thereby, in functionalities [2]. The fraction of these components in starch also determines its nutritional properties, health benefits, and, thus, its end-use. High amylose starch is known to have more resistant starch (RS) and lesser rapid (RDS) and slow digestibility starch (SDS), resulting in a food with a low glycemic index (GI) [3].

However, the industrial exploitation of starch is restricted owing to the lack of properties such as low water solubility, lower swelling power, weak pasting behaviors, and syneresis, leading to weak and unstable gel. This leads to the need to modify the starch to achieve the required characteristics [1, 4]. Modification plays an important role in enhancing its positive attributes by eliminating its shortcomings and unlocking new paths for starch applications (Table 1). It leads to decreased retrogradation, lower syneresis, and improved paste clarity, gel texture, and strength, making it suitable for various applications [4, 5].

Table 1 Effect of starch modification on various properties, related advantages, and applications

The various ways to modify the starch can be categorized into physical, chemical, enzymatic, addition of natural and synthetic crosslinkers, and sometimes their combinations (Fig. 1). Among these, the physical method is simple, inexpensive, and nontoxic. The industry of starch modification is always changing and offers many opportunities to produce unique starches with new functionality and value-added qualities [5]. Therefore, feasible solutions for large-scale starch modification without causing negative impacts on the environment are required. Thus, these requirements lead to the acceptance of physical modification methods. In physical modification, native starch granules are treated using pressure, shear, and irradiation, as well as various temperature and moisture combinations. Since they do not entail any chemical treatment that could be dangerous for human usage, physical modification techniques are typically preferred [9,10,11].

Fig. 1

Methods for the starch modification

Depending on the use of heat treatment, physical modification techniques can be divided into two categories, namely, thermal and nonthermal [12, 13]. Thermal physical modification methods, such as annealing (ANN), heat moisture treatment (HMT), extrusion, osmotic heating, and microwave heating (MWH), employ the use of heat to modify the starches. Nonthermal physical modification methods, such as ultrasounds, high hydrostatic pressure (HHP), and pulse electric field (PEF), use high temperatures [14]. This chapter discusses the frequently used modification techniques for starch, particularly focusing on the physical methods, their principles, procedures, and their effect on starch properties.

2 Materials

The physical modification methods do not employ the use of any kind of chemical. The most important requirement is water to prepare starch suspension or for starch conditioning. All the suspensions should be prepared using deionized water, free from impurities (see Note 1). In the osmotic pressure method, the solution typically contains solutes such as sugars or salts. NaCl, sucrose, and sodium sulfate are most widely used to prepare osmotic solutions and should be of analytical grade.

3 Methods

This section focuses on various physical modification methods, their principles, procedures, and the associated effects. The different treatment methods have different impacts on the various properties of the starches (Table 2). The first step for every method is the selection of starch source. Choose the appropriate type of starch based on the desired modification and the specific application (see Note 16).

Table 2 Effect of various physical modification treatments on starch properties and structure

3.1 Heat–Moisture Treatment (HMT)

This process involves subjecting starch to controlled heat and moisture conditions. The method includes agitating the low-moisture starch granules at temperatures of about 80 to 140 °C [21]. The process can be divided into three stages: moistening, heat treatment, and equilibration-cooling.

3.1.1 Principle

The basic principle of the HMT method involves subjecting starch to controlled heating in the presence of moisture. The moisture content is typically maintained between 10% and 40%, and the processing temperature varies from 84 to 140 °C [16]. The key principles of this method include moisture absorption, gelatinization, and molecular rearrangement. Starch granules have a natural ability to absorb moisture from their surroundings. Structural variation of native starch upon heat moisture treatment is shown in Fig. 2. During gelatinization, heat and moisture cause molecular rearrangement within the starch granules. This rearrangement involves the breakdown and reformation of starch molecular bonds, leading to changes in the organization and structure of amylose and amylopectin molecules, and therefore alters the functional, pasting, textural, and structural properties and the functionalities of the modified starch [22].

Fig. 2

Structure of native and heat-moisture–treated starch chains. (Reprinted with permission from Elsevier [23])

3.1.2 Procedure

• 1. Condition the starch by adjusting its moisture content to a specific level. For this, the dry starch is sprayed with a fine mist of water to achieve the desired moisture content. Usually, the moisture content is adjusted within 10–40% depending upon the objective of the study [16]. The moisture content is critical for gelatinization and allows for the penetration of heat during subsequent processing steps (see Notes 1, 2, 3, and 4).

• 2. The conditioned starch is then heated to a specific temperature. The temperature can range from 80 to 150 °C depending upon desired modification [22].

• 3. Cool the gelatinized and moisture-adjusted starch to stop the gelatinization process. Cooling can be done by air-drying or using a cooling system.

• 4. After cooling, dry the modified starch to reduce its moisture content to a stable level, usually below 10%. Drying can be accomplished through various methods, such as hot air drying at 40 °C or freeze-drying (see Notes 5, 6, and 7).

3.1.3 Effect on Starch Properties

HMT can lead to the modification of starch properties and, thus, their related applications. The specific properties associated with HMT depend on various factors, including starch type, processing conditions, and intended application. The effects of HMT on some of the starch properties are as follows:

• 1. Pasting properties: The treatment has been reported to show a strong effect on the pasting properties of the rice starch maintained at 20, 25, and 30% moisture content and heated at 100 °C [24] shown in Fig. 3. Peak viscosity decreased and paste temperature rose as the moisture content rose. The starch needs greater heat for structural disintegration as the forces among the intra-granule linkages are strengthened. Thus, a high paste temperature suggests that the starch granules contain more forces and cross-links [22].

• 2. Improved stability: Modified starches obtained through HMT tend to exhibit improved stability, especially against factors like high temperature, shear, and pH variations. This enhanced stability makes them suitable for applications requiring heat resistance or prolonged shelf life.

• 3. Enhanced solubility: Modified starches obtained through HMT often exhibit improved solubility compared to native starch. This improved solubility enables their use in applications that require quick dispersion, such as instant food products, beverages, or powdered mixes.

• 4. Swelling and absorption: HMT can influence the swelling and water absorption properties of starch. The impact of HMT on the swelling power of potato, cassava, rice, sorghum, and maize starches has been investigated by researchers [22, 25,26,27,28]. The swelling power of HMT starches was reduced in each of these studies, according to the authors. An increase in the interactions between amylose and amylopectin molecules strengthened the intramolecular bonds and decreased hydration and swelling power leading to the changes in the configuration of the crystalline regions of starch [29].

  Fig. 3

  

  Effect of HMT on the pasting properties of rice starch [24]

3.2 Annealing (ANN)

The annealing process employs heating and cooling of starch under controlled conditions of moisture content and treatment temperature to induce molecular reorganization [30]. The molecular chains of the starch fractions can be effectively reorganized using the ANN process. The ANN process involves the following steps: dispersion, heating, gelatinization, holding, cooling, and drying.

3.2.1 Principle

The basic principle of the ANN method involves subjecting starch to controlled heating generally at a temperature between 25 °C and 80 °C and cooling in the presence of moisture, usually at 40–90% [22, 31]. The heating phase allows for the complete gelatinization of starch granules, while the cooling phase controls retrogradation and stabilizes the modified starch. The heating and cooling cycles during ANN induce structural changes in starch granules, disrupting the semicrystalline structure, rearrangement of amylose-amylopectin molecules, and modifications in molecular organization [22].

3.2.2 Procedure

• 1. Prepare a starch suspension: Measure the required amount of starch powder based on the desired moisture content. Place the starch in a clean and dry container. Gradually add the required amount of water to the starch, usually at 40–90% moisture content (see Notes 1, 2, 3, and 4). Continue stirring or mixing the starch suspension until it becomes homogenous, with a consistent texture and appearance.

• 2. Heat the starch dispersion gradually while stirring to ensure uniform heating. The time duration can vary from a few hours to days [31]. For instance, banana starch was annealed at 65% for 24 h [32], corn starch at 45 and 50 °C for 72 h [33], oat starch at 45 °C for 3 and 24 h [34], and wheat starch at 30, 40, and 50 °C for 24 h [35].

• 3. After the required heating duration, cool the starch dispersion to stop gelatinization. This can be achieved by reducing the heat source and/or using a cooling system. The cooling rate should be controlled to minimize retrogradation.

• 4. Once the ANN process is complete, dry the modified starch to remove excess moisture (see Notes 5, 6, and 7).

3.2.3 Effect on Starch Properties

The ANN treatment can lead to various properties and characteristics in the modified starch. The specific properties associated with the ANN treatment of starch modification depend on factors such as temperature, duration, and starch type. The effects of ANN on common properties are as follows:

• 1. Thermal properties: ANN treatment can result in modified starches with an altered gelatinization temperature compared to native starch. The onset gelatinization temperature of wheat starch dramatically increased when the annealing temperature rose from 30 to 50 °C. Peak (T_p) and conclusion (T_c) temperatures, on the other hand, increased considerably after annealing at 50 °C as opposed to remaining virtually unchanged at 30 and 40 °C. The enthalpy change of wheat starch was not considerably altered by annealing at 30 and 40 °C, but it was significantly reduced by 50 °C, going from 10.3 to 5.6 J/g. These findings showed that wheat starch’s ordered structures were only slightly impacted by annealing at 30 and 40 °C but significantly lowered by annealing at 50 °C [35].

• 2. Enhanced paste clarity: ANN treatment can lead to modified starches with improved paste clarity. The process helps to reduce the retrogradation of starch, resulting in a more translucent or clear paste.

• 3. Enhanced viscosity stability: Annealed starches often exhibit improved viscosity stability during prolonged heating or processing. The modified starches maintain their thickening properties and viscosity over an extended period, making them suitable for applications where prolonged cooking or processing is required.

• 4. Altered textural properties: ANN treatment can modify the textural properties of starch. ANN results in a rearrangement of starch molecules, reducing swelling power and solubility. This decrease in gel volume encourages a rise in gel hardness [36]. Rice starch that had been exposed to ANN showed an increase in gel hardness [27].

• 5. Morphological properties: The effect of annealing temperatures (30, 40, and 50 °C) was observed on the wheat starch [35]. Grooves and depressions were observed on the surface of the native starch. No significant difference was observed for the starch treated at 30 and 40 °C; however, starch annealed at 50 °C showed evidence of destruction and fusion of starch granules (Fig. 4).

Fig. 4

Effect of annealing temperatures 30 °C (W-30), 40 °C (W-40), and 50 °C (W-50) on the morphological properties of wheat starch. (Reprinted with permission from Elsevier [35])

3.3 Osmotic Pressure Treatment (OPT)

In OPT, starch modification is done by subjecting starch to an osmotic solution, typically containing solutes (sugars or salts). It can induce structural and functional changes in starch by altering the osmotic environment surrounding the starch granules [37].

3.3.1 Principle

The key principle of the OPT for starch modification is osmosis. In the context of starch modification, starch granules act as the semipermeable membrane, and solutes in the osmotic solution create an osmotic pressure that affects the water movement in and out of the starch granules. This leads to swelling or shrinkage of the granules, changes in granule morphology, and alterations in the molecular arrangement of starch molecules [37].

3.3.2 Procedure

• 1. Weigh the exact amount of starch to be treated (see Notes 1 and 4).

• 2. Prepare an osmotic solution by dissolving specific solutes. Most commonly, sodium sulfate (Na₂SO₄) is used. Fasuan and Akanbi [38] prepared a Na₂SO₄ solution by dissolving 120 g Na₂SO₄ in 200 mL distilled water (see Note 8).

• 3. Immerse the starch into the osmotic solution (see Note 9). The starch:osmotic solution ratio can be varied to observe their effects. For example, in a study, 100 g starch was suspended in 200 mL saturated sodium sulfate solution [37], while in another, amaranth starch varying from 50 to 150 g was dispersed in 150 to 250 mL saturated Na₂SO₄ solution [38].

• 4. Allow the starch to remain in the osmotic solution for a specific duration, mainly for 10–20 min. The treatment time can vary depending on the desired modification and the specific starch being used (see Note 10). The time–temperature combinations can be varied, and usually, the temperature varying from 105 to 120 °C corresponding to the calculated osmotic pressures of 328 and 341 atm, respectively, has been reported [37].

• 5. After the desired treatment duration, remove the starch from the osmotic solution. Rinse the starch with water or a suitable solvent to remove excess osmotic solution.

• 6. After OPT, filter, wash with water, and dry the modified starch to remove excess water or solvents (see Notes 5, 6, and 7).

3.3.3 Effect on Starch Properties

The osmotic pressure causes changes in the starch structure, which in turn causes a significant effect on the starch functionality. The effects of osmotic pressure-assisted techniques for starch modification are as follows:

• 1. Moisture content control: OPT can be used to control the moisture content of starch by subjecting it to a hypertonic or hypotonic solution. This process can result in modified starches with controlled water content, impacting their functionality and storage stability.

• 2. Improved texture and structure: OPT can influence the texture and structure of starch [38]. For example, osmotic dehydration can result in modified starches with reduced water content, leading to enhanced crispness or texture in certain applications, such as snacks or baked goods.

• 3. Modified chemical composition: Depending on the osmotic solution used and treatment conditions, osmotic pressure-assisted techniques may result in the exchange or uptake of solutes by the starch. This can lead to modified starches with altered chemical compositions, potentially impacting their functional properties and interactions in food systems [37, 38].

3.4 Microwave Heating (MWH)

This method is used to modify starch, where starch is exposed to microwave radiation to induce heating and bring about structural and functional changes. It provides benefits such as homogenous operation throughout the entire sample volume, rapid processing, deep penetration, and improved product quality [39].

3.4.1 Principle

The basic principle of the MWH method for the physical modification of starch involves subjecting starch to electromagnetic wave within the frequency range of 300 GHz and 300 MHz [40]. The treatment has been reported for the modification of corn, potato, chestnut, Bambara groundnut starches, and many more [39,40,41,42]. Starch, being a polar molecule, is capable of absorbing microwave energy and interacting with the alternating electromagnetic field generated by microwaves. Exposure of the sample to the microwave causes the alignment of the polarized or charged atoms with the electric field, causing the rapid change in the direction of the microwaves (Fig. 5). The mechanism of starch modification by this method can be classified into four stages [46]: (1) dielectric relaxation phenomenon of water molecule causing initial starch heating, (2) rapid surge in temperature which is accompanied by loss of moisture from the starch granule interior, (3) creation of high pressure inside the granules causes expansion which occurs from the center, and (4) starch granule degradation. This rapid and selective heating leads to structural and functional changes in starch, providing opportunities for tailored modifications and improved functionality.

Fig. 5

Mechanism of microwave heat treatment (a and b) and traditional heating (c) [43]

3.4.2 Procedure

• 1. Prepare a starch suspension or dispersion by dispersing the starch in the water to maintain a moisture content of 20–30% (see Notes 1, 2, 3, and 4) [44, 45].

• 2. Equilibrate the moistened starch at least for 2 h, before heating [40].

• 3. Transfer the starch suspension into a suitable container that is microwave-safe and allows for efficient microwave energy absorption. Oyeyinka et al. [45] reported the use of perforated polyethylene foil specially designed for microwave ovens.

• 4. Ensure that the starch suspension is spread evenly in the container to facilitate uniform heating (see Note 11).

• 5. Place the starch suspension container in a microwave oven and initiate the heating process (see Notes 10 and 12). Oyeyinka et al. [45] treated the Bambara groundnut starch at 700 W output and a frequency of 2450 MHz. The time duration for the treatment varied from 10 to 60 s.

• 6. Intermittently pause the heating process to stir or agitate the starch suspension to ensure even heating and avoid localized hotspots.

• 7. After the desired heating time, remove the starch suspension from the microwave oven and allow it to cool.

• 8. After cooling, the modified starch suspension may require further processing steps. These can include steps such as filtration, drying, or other treatments to remove excess water or solvents and obtain the modified starch in the desired form (see Notes 5, 6, and 7).

3.4.3 Effect on Starch Properties

The MWH treatment involves subjecting starch to microwave radiation, which generates heat and leads to various changes in the starch structure and properties such as:

• 1. Enhanced gelatinization: MWH can induce efficient and uniform gelatinization of starch. The heat generated by microwaves rapidly raises the temperature of the starch, leading to its swelling.

• 2. Gelatinization temperature: The gelatinization temperatures of Bambara groundnut starch were dramatically raised by microwaving. With longer microwaving times, the gelatinization temperatures rose while the enthalpy dropped. An increase in gelatinization temperature following microwaving may be attributed to reorganization in the amorphous and crystalline areas, which may have reinforced contacts along the starch chains between amylose–amylose and amylose–amylopectin [45].

• 3. Improved digestibility: MWH has been shown to enhance the digestibility of starch by modifying its structure. It can lead to starch with increased susceptibility to enzymatic hydrolysis which can be beneficial for applications requiring increased starch digestibility.

• 4. Altered granule morphology: MWH can cause changes in the morphology and structure of starch granules. These changes may include swelling, partial melting, or even fragmentation, leading to modified starches with unique morphological properties [46].

3.5 Extrusion

Amorphous or semi-amorphous starch particles can be produced fast from a starch suspension using extrusion technology. It involves the application of heat, shear, and pressure to starch as it passes through a narrow barrel under controlled conditions. The combination of these forces causes gelatinization, molecular rearrangement, and textural [47].

3.5.1 Principle

The principle behind the extrusion method of starch modification is the application of mechanical and thermal forces, which lead to physical and chemical changes in the starch structure [13]. The process typically involves high temperatures ranging from 100 to 200 °C. These elevated temperatures cause the starch granules to swell and undergo gelatinization. During extrusion, starch is forced through a narrow orifice under high pressure. The shear forces involved promote molecular alignment, breakdown of starch molecules, and disruption of the starch granule structure.

3.5.2 Procedure

• 1. Precondition the starch by adding water or steam to adjust the moisture content (see Notes 1, 2, 3, and 4). This step helps to facilitate the gelatinization process during extrusion. Gonzalez and Perez [48] conditioned the lentil starch to a moisture content of 25%.

• 2. Blend the preconditioned starch with other ingredients, such as additives, flavors, or colorants, if desired. This step ensures uniform distribution of ingredients throughout the starch matrix.

• 3. Feed the starch mixture into the extruder using a hopper or feeder. The extruder consists of a barrel with a screw that conveys the material along its length.

• 4. Subject the starch mixture to heat and mechanical shear in the barrel. For the modification of lentil starch, the temperature was maintained at 150 °C, and the screw speed was 90 rpm [48].

• 5. After the cooking zone, the modified starch is rapidly cooled to stop gelatinization.

• 6. The starch can be shaped into its final form using dye or shaping equipment attached to the extruder.

• 7. Once the modified starch has been shaped, cut it into desired lengths, such as pellets or flakes.

• 8. Dry the extrudate at a temperature of 45 °C to reduce the moisture content and stabilize the modified starch (see Notes 5, 6, and 7).

3.5.3 Effect on Starch Properties

The extrusion treatment can affect the properties and characteristics of the modified starch as discussed below:

• 1. Enhanced gelatinization: The high temperature and shear forces in the extrusion process promote the gelatinization of starch. This leads to modified starches with improved solubility, viscosity, and thickening properties.

• 2. Improved stability: The extrusion treatment can enhance the stability of starch by modifying its structure. The process can result in modified starches with increased resistance to retrogradation, which is the tendency of starch gels to undergo syneresis or become firm upon cooling.

• 3. Increased digestibility: The extrusion process can partially hydrolyze starch molecules, resulting in modified starches with increased digestibility. The treatment can break down the starch into smaller fragments, making it more susceptible to enzymatic digestion.

• 4. Altered granule morphology: The extrusion treatment can cause changes in the morphology and structure of starch granules. These changes may include partial disruption, fragmentation, or reshaping of the granules.

3.6 Ultrasonication

The use of ultrasounds, known as ultrasonication, is a green technology and can be used to modify starch by exposing the starch to different frequencies, intensities, water, temperature, and time combinations [14]. In food processing, ultrasounds are used in the frequency range of 20 kHz to 10 MHz. They can be created using piezoelectric or magnetostrictive transducers, which are projected or transported directly or indirectly over the fluid [49].

3.6.1 Principle

As sinusoidal ultrasonic vibrations move through the aqueous medium, it undergoes a continuous wave-like motion as shown in Fig. 6. During the compression cycle, bubbles contract, but not all of them entirely disappear into the liquid. The bubbles expand over numerous cycles until the oscillation of the bubble wall approaches the applied frequency of the sound waves, at which point the bubbles explode during one compression cycle. Due to the compression and rarefaction process, the system suffers cavitation, causing the bubbles to collapse [50]. The bubble rupture alters the physical and chemical properties and also produces powerful shearing forces, huge energetic waves, and locally substantially increased turbulence and temperature [51]. The mechanical forces generated by cavitation can disrupt the starch granule structure, causing the release of starch molecules.

Fig. 6

Ultrasound system. (Reprinted with permission from Elsevier [52])

3.6.2 Procedure

• 1. Prepare a starch suspension or dispersion by dispersing the starch in water or a suitable solvent (see Notes 1, 2, 3, and 4). Zhu et al. [53] prepared 10% potato starch suspension, Hu et al. [54] reported 5% corn starch suspension, while Sujka [55] and Sujka and Jamroz [56] prepared 30% starch suspension of rice, corn, potato, and wheat starches.

• 2. Set up an ultrasonic bath or reactor, which consists of a container filled with the starch suspension. The container is equipped with ultrasonic transducers that generate high-frequency sound waves as shown in Fig. 6.

• 3. Place the starch suspension into the ultrasonic bath or reactor and activate the ultrasonic transducers. Zhu et al. [53] treated the potato starch suspension for 20 min with different power levels of 60, 105, and 155 W and at a frequency of 20 kHz.

• 4. After the ultrasound treatment, the modified starch suspension may require further processing steps. These can include steps such as centrifugation, filtration, and drying to remove excess water or solvents and obtain the modified starch in the desired form (see Notes 5, 6, and 7).

3.6.3 Effect on Starch Properties

While the use of ultrasound for starch modification is still an area of active research, it has shown potential in altering the properties of starch, such as:

• 1. Enhanced enzymatic digestibility: Ultrasound treatment can disrupt the starch granule structure and increase the surface area available for enzymatic attack. This can lead to modified starches with enhanced enzymatic digestibility.

• 2. Increased solubility and dispersion: Ultrasound treatment can improve the solubility and dispersibility of starch in water or other solvents. The mechanical forces generated by ultrasound waves can break down the starch granule structure, facilitating the release of starch molecules into the surrounding medium. This results in modified starches with improved solubility and dispersibility.

• 3. Altered rheological properties: When starch pastes are subjected to ultrasound treatment, the molecules of the starch polysaccharide are depolymerized, and the paste’s viscosity is significantly and quickly reduced [16]. Chain cleavages have been attributed to mechanical forces as well as the formation of OH radicals. The reductions in paste viscosities and consistency coefficients are unquestionably the result of the same processes, particularly the production of OH radicals, which take place in starch granules.

• 4. Altered granule morphology: Ultrasound treatment can cause changes in the morphology and structure of starch granules. The mechanical forces generated by ultrasound waves can induce granule swelling, partial disruption, or reshaping [16]. These changes can result in modified starches with unique morphological properties, which may impact their functional properties and applications.

3.7 High Hydrostatic Pressure (HHP)

In food-processing industries, HHP at room temperature is used to acquire modification and sterilization by exposing the food product to a pressure of 100–1000 MPa. The process is known to destroy the noncovalent bonds, resulting in structural changes and starch gelatinization [57]. The schematic view of the high hydrostatic pressure treatment is shown in Fig. 7.

Fig. 7

Representation of high hydrostatic pressure processing [59]

3.7.1 Principle

The basic principle of HHP on starch modification involves the disruption of the starch granule structure. The hydration, as well as pressure, distorts the crystalline region of starch. During this, the secondary and tertiary structures are broken, whereas the covalent bonds remain intact. The manner in which the starch polymer remains in the granule structure decides the behavior of starch under the treatment [58]. When starch is exposed to HHP, the pressure forces water molecules into the granules, increasing the internal pressure. This internal pressure can cause swelling of the granules, leading to the disruption of the crystalline regions within the starch.

3.7.2 Procedure

• 1. Prepare the starch suspension by adding the required amount of water (see Notes 1, 2, 3, and 4). For example, the effect of HHP on 20% (w/w) mung bean starch suspension [60] and the effect on starch suspension (w/v) prepared in the ratio of 1:3 were studied [57].

• 2. Transfer the starch suspension into a suitable container that can withstand high pressure, such as flexible pouches or metal cylinders (see Note 13).

• 3. Place the packaged starch samples into a high-pressure vessel, which is specialized equipment designed to withstand high pressures (see Notes 14 and 15). Li et al. [60] carried out HHP treatment of mung bean starch at varying pressures of 120–600 MPa at an interval of 120 MPa for 30 min at room temperature. A variation of pressure from 100 to 600 MPa and maintained for 30 min at a controlled temperature of 30 °C was reported [57].

• 4. Control the pressure level and treatment duration based on the desired modification and the specific starch being used.

• 5. After the desired treatment duration, slowly release the pressure from the vessel to atmospheric pressure (see Note 16).

• 6. After the pressure release, the modified starch samples may require further processing steps such as filtration, drying, or other treatments to remove excess water or solvents and obtain the modified starch in the desired form (see Notes 5, 6, and 7).

3.7.3 Effect on Starch Properties

The high hydrostatic pressure (HHP) treatment involves subjecting starch to elevated pressures. Some potential properties associated with HHP treatment for starch modification are:

• 1. Enhanced gelatinization: HHP treatment can promote the gelatinization of starch. The applied pressure disrupts the starch granule structure, leading to increased water absorption, swelling, and release of starch molecules. This can result in modified starches with improved solubility, viscosity, and thickening properties.

• 2. Pasting properties: In comparison to native mung bean starch, the Rapid Visco-Analyzer (RVA) viscograms for the HHP-treated starch showed a substantial increase in the peak viscosity, trough viscosity, final viscosity, and setback and a decrease in the breakdown viscosity and pasting temperature [60]. The sample subjected to a pressure of 600 MPa, however, shows the greatest values for peaking time and pasting temperature and the lowest value for peak viscosity. These variations in granular structures throughout the transformation of the crystalline structure led to changes in viscosity characteristics, pasting temperature, and peak time.

• 3. Altered rheological properties: HHP treatment can influence the rheological properties of starch, such as viscosity, shear-thinning behavior, and gelation. The treatment can induce changes in the interactions between starch molecules, resulting in modified starches with different thickening, gelling, or textural properties.

• 4. Altered granule morphology: HHP treatment can cause changes in the morphology and structure of starch granules. The native mung bean granules are reported to possess kidney and ellipse shapes with smooth surfaces without pores. However, exposing them to high pressure of 600 MPa for 30 min resulted in the collapse of starch shapes. Further, the morphologies of starch granules changed from being regular to having deeper grooves as the pressure level rose from 120 to 600 MPa. This meant that the greater pressure caused a more severe loss of granule integrity [60].

3.8 Pulse Electric Field (PEF)

Pulse electric field technology is another nonthermal technique that has been widely employed for food preservation. The technique kills and inactivates the pathogenic microorganisms and enzymes, resulting in minimal destruction of the nutrients.

3.8.1 Principle

The basic principle of the PEF method involves subjecting starch to short, intense electric pulses [14]. The general system of the treatment is shown in Fig. 8. This process can induce structural and functional changes in starch by applying high-voltage pulses to disrupt the starch granules and modify their properties [14]. When starch is exposed to intense electric pulses, the electric field causes the formation of temporary pores or openings in the cell membrane of the starch granules. This phenomenon is known as electroporation. The electroporation of starch granules during PEF treatment results in the rearrangement of molecular structures within the granules. The disruption of the starch granule matrix can alter the organization of amylose and amylopectin molecules, potentially leading to changes in the crystallinity and molecular arrangement of the starch [61].

Fig. 8

Process of pulse electric field system

3.8.2 Procedure

• 1. Prepare a starch suspension or dispersion by dispersing the starch in water (see Notes 1, 2, 3, and 4). The water-to-starch ratio may vary. For instance, 8% (w/w), 10% (w/w), and 40% (w/w) for corn starch, waxy rice starch, japonica rice, wheat, potato, and pea starch have been reported in the literature [61,62,63,64].

• 2. Transfer the starch suspension into a suitable treatment chamber or container that allows for the application of electric pulses (see Note 17).

• 3. Place two electrodes in the treatment chamber, ensuring proper positioning and alignment with respect to the starch suspension.

• 4. Apply short, intense electric pulses to the starch suspension using a high-voltage pulse generator.

  • (a) Control the pulse duration, intensity, and frequency based on the desired modification and the specific starch being used. The effect of the electric field of intensities 30, 40, and 50 kV/cm on the tapioca and corn starch has been reported [20, 61].

  • (b) Monitor and control the temperature of the starch suspension during the PEF treatment. The temperature is usually kept below 50 °C to avoid starch swelling and gelatinization.

• 5. After the PEF treatment, subject the modified starch suspension to filtration and drying (see Notes 5, 6, and 7).

3.8.3 Effect on Starch Properties

While PEF treatment is not as extensively studied for starch modification compared to other methods, it has shown the potential to alter the properties of starch as follows:

• 1. Enzymatic digestibility: PEF treatment can disrupt the starch granule structure, leading to increased surface area and affecting the accessibility of enzymes to starch molecules. PEF treatment on esterified potato starch was used to assess the impact on its digestibility [65]. Granules with greater surface roughness and improved glycemic digestibility were produced by esterified starch treated with PEF.

• 2. Morphological properties: Native tapioca starch granules had a smooth, round, and atypically formed surface morphology. The surface of the tapioca starch granules exhibited some roughness or damage following PEF treatment at 30 kV/cm. As a result of the treatment at 40 kV/cm, some pits and smaller starch particles grew larger, demonstrating that PEF had changed the native starch‘s granule structure. The tapioca starch appears to lose its granular shape after PEF treatment at 50 kV/cm and is replaced by some grouped pieces with gel-like properties [20].

• 3. Increased solubility: PEF treatment can improve the solubility of starch in water or other solvents. The electric pulses can disrupt the starch granule structure and facilitate the release of starch molecules into the surrounding medium, leading to modified starches with improved solubility and dispersibility.

• 4. Improved swelling and hydration: PEF treatment can enhance the swelling and hydration properties of starch. The electric pulses can disrupt the starch granule structure, allowing for increased absorption of water and improved swelling capacity.

4 Notes

• 1. Use only deionized water for the starch modification, wherever required. The presence of impurities in the water may lead to a change in the starch structure and bonding, which affects the functional, pasting, textural, and morphological properties.

• 2. The equipment should be properly cleaned before use. Wipe using acetone. There should be no chance of contamination.

• 3. The glassware used in the starch isolation and modification must also be washed and sanitized properly.

• 4. Weighing of the starch and water for preparing the starch suspension and conditioning purposes should be carried out accurately. Slight differences in the weight can lead to significant changes in the starch properties.

• 5. Modified starch should be packaged in suitable containers or bags to protect it from moisture and other environmental factors. 

• 6. Proper storage conditions, such as cool and dry environments, are essential to maintain the quality and stability of the modified starch.

• 7. Dry the sample at a temperature of 45 °C or lower; otherwise, it would lead to gelatinization.

• 8. The fresh solution, for example, the osmotic solution, must be used each time to avoid discrepancies in the result.

• 9. Ensure that the starch is fully submerged and that the osmotic solution covers the starch particles uniformly.

• 10. The heating process should be carried out under controlled conditions to avoid overheating or scorching. Ensure that the samples are properly arranged and spaced within the HHP.

• 11. It is important to avoid overcrowding the container, as it can affect the efficiency of MWH.

• 12. Control the microwave power level and the heating time based on the desired modification and the specific starch being used.

• 13. The container should be sealed to prevent leakage during the HHP treatment and are mostly vacuum packed.

• 14. Ensure that the samples are properly arranged and spaced within the HHP vessel to allow for efficient pressure transmission.

• 15. The specific parameters, such as temperature, moisture content, and processing time, can vary depending on the type of starch, the desired modifications, and the end-use applications of the modified starch.

• 16. The release of pressure should be done gradually to prevent sudden decompression, which can affect the integrity of the starch samples.

• 17. The PEF chamber should be designed to withstand high voltage and ensure proper alignment of the electrodes.

5 Conclusion

Physical methods of starch modification offer a versatile and efficient approach to tailoring the properties of starch for various food and industrial applications. These methods involve minimal use of chemical additives or enzymes, preserving the natural composition and clean-label characteristics of starch. The procedures for physical starch modification vary depending on the specific method employed. The step-by-step procedures for the various physical modification methods are discussed in the chapter, along with the mechanism of the method and their effect on the starch properties. The important precautions to be followed by modifying the starch are also given in the “Note” section. The effect of physical modification on starch properties is significant and diverse. As the demand for modified starches with specific functionalities continues to grow, physical methods are expected to play an increasingly important role in the development of innovative starch-based ingredients for diverse applications across the food and industrial sectors. By understanding the effects of physical modification on starch properties, food scientists and manufacturers can develop innovative starch-based ingredients and products with enhanced quality, stability, and functionality. These modified starches play a critical role in meeting the diverse needs and preferences of consumers, contributing to the continuous advancement of the food and industrial sectors.