Physical modification of starch by heat-moisture treatment and annealing and their applications: A review

Heat-moisture treatment (HMT) and annealing are hydrothermal starch modifications. HMT is performed using high temperature and low moisture content range, whereas annealing uses excess of water, a long period of time, and temperature above the glass transition and below the gelatinization temperature.

Available online 11 September 2021

0144-8617/© 2021 Elsevier Ltd This article is made available under the Elsevier license (http://www.elsevier.com/open-access/userlicense/1.0/)

Review

Physical modification of starch by heat-moisture treatment and annealing

and their applications: A review

Laura Martins Fonseca*, Shanise Lisie Mello El Halal , Alvaro Renato Guerra Dias , Elessandra da

Rosa Zavareze

Department of Agroindustrial Science and Technology, Federal University of Pelotas, Pelotas, RS 96010-900, Brazil

A R T I C L E I N F O

Keywords:

Annealed-starch

Combined modifications

Food

HMT-starch

Physical modification

A B S T R A C T Heat-moisture treatment (HMT) and annealing are hydrothermal starch modifications HMT is performed using high temperature and low moisture content range, whereas annealing uses excess of water, a long period of time, and temperature above the glass transition and below the gelatinization temperature This review focuses on: research advances; the effect of HMT and annealing on starch structure and most important properties; combined modifications; and HMT-starch and annealed-starch applications Annealing and HMT can be performed together

or combined with other modifications These combinations contribute to new applications in different areas The annealed and HMT-starches can be used for pasta, candy, bakery products, films, nanocrystals, and nano-particles HMT has been studied on starch digestibility and promising data have been reported, due to increased content of slowly digestible and resistant starches The starch industry is in constant expansion, and modification processes increase its versatility, adapting it for different purposes in food industries

1 Introduction

Starch is a natural, biodegradable, abundant, biocompatible, low-

cost, and nontoxic polymer highly used in several industries as textile,

food, paper, packages, biomedical, and pharmaceutical In food

in-dustries, starches are used for the production of instant foods, noodles,

baked goods, and food packages, among others Starch presents inherent

limitations that can be overcome by its modification through methods as

chemical, enzymatic, physical, and a combination of them (Lacerda

et al., 2015; Molavi et al., 2018) There are several studies reporting the

effect of these modifications in starch properties to suit specific

appli-cations, such as: Majzoobi et al (2015), who modified rice flour and rice

starch by HMT for application in biodegradable films; Chandla et al

(2017), who used HMT-amaranth starch for the production of noodles;

and Choi and Koh (2017), who used annealed-starch from rice for the

same purpose

Physical methods have gained widespread acceptance for their low

cost, safety, and effective characteristics, being a green (not requiring

chemical reagents) alternative to improve starch applicability, by achieving specific enhanced properties for specific types of applications (Punia, 2020) There are various physical modifications such as pre- gelatinization, a thermal process; heat-moisture treatment (HMT) and annealing, two hydrothermal processes; and the non-thermal modifi-cation includes high pressure processing, micronization, ultra-sonication, and pulse electric field (Alc´azar-Alay & Meireles, 2015; Punia, 2020) Annealing and HMT are the most commonly used methods effective in altering starch properties, such as relative crystallinity, water absorption capacity, and pasting properties, maintaining the molecular integrity of starch These hydrothermal treatments also in-fluence directly starch digestibility: this is extremely important to ach-ieve health benefits for the consumer, by slowly digestible starch (SDS) and resistant starch (RS) formation and reduction of rapidly digestible starch (RDS) Thus, the application in food products can focus in in-dividuals with chronic diseases as diabetes, among other serious health issues (Pratiwi et al., 2018; Sudheesh et al., 2020) These changes in starch digestibility are mainly promoted by the disruption in starch

Abbreviations: AFM, Atomic Force Microscopy; DIC, D´etente Instantan´ee Contrˆol´ee; DSC, Differential Scanning Calorimetry; DV-HMT, Direct vapor-heat-moisture

treatment; GRAS, Generally recognized as safe; HMT, Heat moisture treatment; NC-AFM, Non-contact Atomic Force Microscopy; RHMT, Repeated heat moisture treatment; RP-HMT, Reduced-pressurized heat-moisture treatment; RS, Resistant starch; RDS, Rapidly digestible starch; RVA, Rapid Visco Analyzer; SEM, Scanning Electron Microscopy; SDS, Slowly digestible starch; XRD, X-ray diffraction

* Corresponding author

E-mail address: laura_mfonseca@hotmail.com (L.M Fonseca)

Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

https://doi.org/10.1016/j.carbpol.2021.118665

Received 6 June 2021; Received in revised form 8 September 2021; Accepted 9 September 2021

structure, increasing accessibility of starch molecular chains to the

amylolytic enzymes, changing its crystalline structure, and thus

enhancing starch digestibility (Wang et al., 2016)

The most important structural changes achieved by HMT and

annealing are promoted by the extension of double helical lengths,

reduction of double helix content, strengthening of interactions between

amylose and amylopectin branching, and more heterogeneous semi-

crystalline lamellae Thus, more mobile chains are available due to the

greater proportion of very long chains/branches The starch structure

becomes more organized The modification mechanism involves

increasing the interactions of starch chains, which starts by disruption of

the crystalline structure, followed by dissociation of double helix

structures and then reassociation of the disrupted crystals The different

starch sources present distinct granule organization and behaviors when

undergoing hydrothermal processes: the high amylose starches have

potential to form thermostable molecular orders (Li et al., 2020; Pratiwi

et al., 2018)

Various food applications are cited in the literature for HMT and

annealed starches, as in baked goods, noodles, doughs, pie fillings,

flours, and whole grains or kernels In processed foods, these modified

starches can be used as unmodified thickeners, to improve their stability

to temperature, shear, and acid conditions, also reducing retrogradation

Other applications are in thermoplastic materials, resins, films,

nano-particles, and nanocrystals (BeMiller, 2018; BeMiller & Huber, 2015) In

addition, the HMT and annealed starches present granules more

sus-ceptible to chemical and enzymatic modifications and to acid hydrolysis

(Alc´azar-Alay & Meireles, 2015) The use of HMT and annealing

pro-motes thermal stability for starches, being promising for the

develop-ment of products that are exposed to high temperatures during

production In addition, they are safe and innovative methods that

improve the functional and technological properties of starch for

in-dustrial proposes (Molavi et al., 2018)

In this study, we present an updated review (2011− 2021) focused on

HMT and annealing, examining their effect on starch structure and the

properties of different types of starches, the combination with other

modifications, and recent trends in the application of modified starch,

based on reports from literature of the last 10 years

2 Starch composition, structure and properties

Starch is a polysaccharide of plant origin formed by two

macro-molecules with glucose units linked together through glycosidic bonds

Amylose is a long and linear macromolecule containing α-(1–4) linkages

with helical structure, with hydrogen atoms inside the helix and

hy-droxyl groups outside the helix Amylopectin is branched, containing

α-(1–4) and α-(1–6) linkages arranged radially in the granule, double

helix as crystalline areas and branching points as amorphous areas

(Rocha et al., 2012; Sudheesh et al., 2020) The amorphous areas are

more susceptible to hydrolyses by enzymes and water absorption due to

the influence of amylose in the packing of amylopectin crystallites in the

crystalline lamellae, which influences swelling and gelatinization

(Zavareze & Dias, 2011) Starches can be found with different amylose

contents depending on the botanical source: in corn, for example, the

normal amylose content for starch is 20–30%; a high amylose content is

50–80%; and a low amylose content is 0–8% (waxy starch) (Obadi & Xu,

2021)

The functional properties of starch are related to its botanical source,

amylose/amylopectin ratio, molecular weight, and organization of the

granule chains (Sudheesh et al., 2020) The X-ray diffraction pattern can

exhibit A-type structure with medium length helices showing reduced

crystallinity, mostly reported for starches from cereal grains, and B-type

with higher length and crystallinity, mostly found for starch from tubers

(Moran, 2019) The crystallinity of starch is affected when it is heated in

excess of water It undergoes a phenomenon named gelatinization, in

which amylose is leached, the granule is swollen and the disappearance

of the double-helical crystalline structure occurs along with the loss of

birefringence These changes influence starch behavior in terms of swelling power and solubility (Singla et al., 2020) Gelatinized starch is used in industries as a thickener, gelling agent, stabilizer, and fat sub-stitute in foods (Oliveira et al., 2018)

Thermal properties of starch can be measured by differential scan-ning calorimetry It provides data of heat flow associated with gelati-nization, showing changes in enthalpy related to the transition of ordered and disordered crystals from low order crystalline regions of the granule Through this analysis, the gelatinization temperature is also obtained It defines the proportion of required energy for cooking (Zavareze & Dias, 2011)

Another important property of starch is digestibility Starch is clas-sified according to its nutritional aspects after ingestion, by RDS, SDS, and RS RDS and SDS are digested and absorbed after passage through the gastrointestinal tract and arrival in the small intestine RS is not digested, being fermented in the large intestine SDS and RS are well known for preventing diseases, showing health benefits Therefore, their application in food industries is growing over the years (Maior et al., 2020; Yan et al., 2019)

Starches can be applied in food and non-food industries For this, specific characteristics are required, which are not often achieved by native starches Therefore, starch modifications are alternatives to enhance starch properties They broaden its industrial end-use by providing products with higher thermal and shelf stability, as well as mechanical, texture, and pasting properties, for instance (Falade & Ayetigbo, 2015)

3 Physical modification

Physical modifications are low-cost, safer, and green alternatives when compared to chemical and enzymatic processes Their study has been showing promising outcomes on the improvement of starch prop-erties (Oliveira et al., 2018) Among the physical modifications are pre- gelatinization, ultrasonication, heat-moisture treatment, mechanical milling, and annealing They present variations in parameters as tem-perature, pH, and pressure, acting by modifying the starch molecular structure, packing arrangements, and crystallinity, for example (BeMiller & Huber, 2015; Singla et al., 2020)

Annealing and HMT are hydrothermal processes highlighted by achieving unique starch properties, unaltering its molecular integrity (Obadi & Xu, 2021) In the next sections, we are going to discuss the reported effects of HMT, annealing, and combined methods on starch properties and starch applications

3.1 HMT

Hydrothermal processes, such as HMT, are performed by heating the starch granules at temperatures above gelatinization temperature, with moisture content insufficient to gelatinize starch, and submitted to a specific period of time, maintaining granular integrity The parameters used for this starch modification are: high temperature, ranging from 84

to 140 ◦C; low moisture content, ranging from 10 to 35%; and time of exposure ranging from 1 to 16 h (Adawiyah et al., 2017; BeMiller, 2018) The effectiveness of the treatment depends on the process parameters (moisture content, temperature, and heating time) and the starch characteristics, such as botanical source, structure, and amylose/ amylopectin ratio and organization (Adawiyah et al., 2017; Alc´azar- Alay & Meireles, 2015)

HMT influences and improves several starch properties, which are discussed in the following sections Changes in starch caused by HMT include effects on morphology, swelling capacity, crystallinity, gelati-nization, thermal stability, retrogradation, digestibility, and pasting properties (Punia, 2020), broadening the range of starch applications in the food industry Table 1 summarizes several studies explored in this review in the following sections, regarding different types of starch modified by HMT and the treatment parameters used

3.1.1 HMT influence on morphological properties

Microscopy analysis is used to characterize starch granules, showing

its different sizes and shapes (Schafranski et al., 2021) BeMiller and

Huber (2015) reviewed various studies regarding starches modified by

HMT, and found the following results: direct influence of treatment

conditions (e.g., high temperature and low moisture) on starch

proper-ties when compared with their native counterparts, showing increased

mobility of starch chains and helical structures and resulting in major

structural changes; molecular degradation of starch chains; and

morphological changes to granules as size, surface cracking, hollowing

at granule centers, decreased birefringence, and partial gelatinization or

agglomeration of granules However, other studies (Andrade et al.,

2014; Costa et al., 2019; Lacerda et al., 2015; Pranoto et al., 2014) have

not found changes in the morphology of HMT-starches from different

sources as organic cassava, green Prata banana, avocado, and sweet

potato

Oliveira et al (2018) evaluated the morphology of potato and sweet

potato starches treated with moisture content of 10, 15, and 20% at

121 ◦C for 1 h, by Non-contact Atomic Force Microscopy (NC-AFM) The

authors reported an agglutination of the granules and changes in size

and roughness, when applying HMT to the starches The shape of the

granules was not modified by HMT: while native potato starch showed

an ellipsoidal shape, the native sweet potato starch presented a

polyg-onal shape HMT promoted reduction in the size of the granules of

po-tato and sweet popo-tato starches This was accentuated by the higher

moisture content of treatment, with reductions of 17% for sweet potato

starch and 63% for potato starch, both when applied the highest mois-ture content in HMT The authors associated this behavior to the evap-oration of water molecules during the physical modification The roughness increased for sweet potato starch and decreased for potato starch after the treatment, being the greatest changes for the HMT using 20% moisture content, increasing the roughness by 46% for sweet po-tato starch and decreasing it by 40% for popo-tato starch This study pro-vides quantitative information about changes in size and roughness of starch granules from different botanical sources, showing that the in-fluence of HMT depends on moisture content

Lacerda et al (2015) studied avocado starch modified by HMT at

120 ◦C for 1 h, with moisture contents of 10, 20, and 30% The morphology was analyzed by Atomic Force Microscopy (AFM), and native and modified avocado starches showed an oval shape The native starch presented protruded, flat, and smooth regions, not showing an appropriate order after HMT, but showing a smooth region on the sur-face The authors attributed this change to the melting of the protruded regions by partial gelatinization, in which the water molecules evapo-rate, causing the amylopectin double helix chains to organize into a denser packing structure In the study of Lacerda et al (2015), the moisture content in HMT was higher than in Oliveira et al (2018) However, the effect of HMT on starch morphology was less pronounced, thus showing that the botanical source of starch plays an important role

in the application of HMT

Bartz et al (2017)) reported the intense effect of HMT on physico-chemical properties, morphology, swelling power, and amylose content,

by subjecting potato starch to HMT at 12, 15, 18, 21, and 24% of moisture content for 1 h at 110 ◦C The effect on these properties has been accentuated along with the increase in moisture content The morphology was evaluated by Scanning Electron Microscopy (SEM), and the native potato starch granules varied in size, showing ellipsoidal shape with some small spherical granules HMT-starches modified with

12, 15, 21, and 24% of moisture content showed similar granule morphology However, the treatment using 18% of moisture presented discrete agglomeration, as well as grooves In their study, the differences

in moisture did not promote changes in the effect of HMT

In another study, Wattananapakasem et al (2021) examined the morphology of HMT-geminated-black rice by SEM, with moisture con-tents of 25 and 30%, treated at 90 ◦C for 1 and 2 h The authors reported the presence of agglomerations and pores in the granules after applying HMT The increase in time of exposure to HMT from 1 to 2 h did not change the morphology of the rice starch granules In our point of view, this may have happened because, at these treatment conditions, the changes in the granule occurred in full during the shorter time of exposure (1 h)

According to the studies presented in this review, the treatment time, moisture, and starch source are determining factors for changes in the morphology of granules promoted by HMT Depending on the starch gelatinization temperature, high moisture contents in the HMT can promote partial gelatinization and agglomeration in the starch granules

3.1.2 Influence of HMT on solubility and swelling power

The starch properties of swelling power and solubility elucidate in-teractions between the starch chains of crystalline and amorphous structures (Thakur et al., 2021) Repeated heat-moisture treatment (RHMT) has been used by researchers in order to improve the effect of HMT within the starch granules (Niu et al., 2020; Zhao et al., 2020) According to Zhao et al (2020), HMT provides starch with limited de-gree of modification Thus, repeated processing allows a redistribution

of moisture in the granules during the cooling process, providing a new

equilibrium from the new starting point (e.g., previously HMT-modified

starches) The authors have modified mung bean starch by HMT at

120 ◦C for 2, 4, 6, 8, 10, and 12 h, and using RHMT at 120 ◦C for 2, 3, 4,

5, and 6 h They evaluated solubility at various temperatures ranging from 50 to 90 ◦C, reporting increase for all treatment times at all tem-peratures evaluated, when compared to native starch The swelling

Table 1

Heat-moisture treatment (HMT) parameters, source of starch, and references

Temperature

( ◦ C) Time (min) Moisture content

(%) Sago and

arenga 120 60, 90 20 Adawiyah et al (2017)

Grain paddy

rice and rice

starch

60 13 Arns et al (2015)

21, 24 Bartz et al (2017)) Mango kernels 110 180 20, 25, 30 Bharti et al (2019)

(2020)

Germinated

brown rice

grain

100 60 30 Chung et al (2012)

Green Prata

Rice, cassava,

and pinh˜ao

starches

100, 120 120 22 Klein et al (2013)

35 Liu et al (2016) Potato and

sweet

potato

121 60 10, 15, 20 Oliveira et al (2018)

240,

300

25 Pranoto et al (2014) )

Geminated-

black rice 90 60, 120 25, 30 Wattananapakasem et al (2021)

240,

360,

480,

600,

720

30 Zhao et al (2020)

power presented the same trend of increased values for the analysis at

50 ◦C and decreased values as the temperature increased from 60 to

90 ◦C This may have occurred by the reorganization of amylose and

amylopectin molecules, reducing the absorption of water during

heat-ing The solubility of native starch reported by the authors was not high,

ranging from 1 to 8%, with increase in temperature from 50 to 90 ◦C

However, values of about 20% solubility were found for some samples

modified and analyzed using 90 ◦C These variations in solubility and

swelling power achieved by RHMT allow more possibilities of starch

modification by HMT, opening up a range of applications

Klein et al (2013) also evaluated the effect of RHMT on swelling

power and solubility, in rice, cassava, and pinh˜ao starches (22% of

moisture content, at 100 and 120 ◦C for 2 h, and then for another 1 h at

100 and 120 ◦C) Both parameters were decreased by HMT when

compared to their native counterparts The swelling power of all

starches was reduced with the increase in temperature from 100 to

120 ◦C When comparing HMT-starch and RHMT-starch at the same

treatment temperature, only rice starch showed increased swelling

power Thus, the rice starch was more susceptible to a new arrangement

caused by RHMT than cassava and pinh˜ao starches The modifications of

HMT and RHMT present distinct influence according to the different

types of starches, thus providing different outcomes to starches

regarding its swelling power and solubility Both properties are very

important for starch application, as in bakery and pasta production, in

which the starch interacts with water and other ingredients

Pranoto et al (2014)) studied sweet potato starches of Indonesian

varieties submitted to HMT, for future application in noodle production

The modification was performed using moisture content of 25%, at

110 ◦C for 3, 4, and 5 h The data showed differences for swelling power

and solubility among the different potato starch varieties HMT-starches

reduced swelling power and increased solubility when compared to their

respective native starches This behavior was explained by an expansion

in the starch granule, an increase of molecular bond interaction, and the

loss of double helix formation of the starch chains when heated in water

The treatment time did not affect swelling power or solubility in the

sweet potato starches Costa et al (2019) evaluated the effects of HMT

on the swelling power of green Prata banana starch, with moisture

contents of 15, 20, and 25%, treated at 102 ◦C for 1 h When increasing

the temperature of the analysis (70 to 90 ◦C), there was an increase in

swelling power for native and low-HMT-starches HMT decreased

swelling power by the molecular arrangement of starch, which reduced

the hydration of the granules This can be related to a reduction in

crystallinity, the formation of amylose-lipid complex, and the limit

temperature of gelatinization in the starch suspension that breaks the

hydrogen bonds and releases the water molecules bound from the

hy-droxyl groups These changes in swelling power and solubility are

important for applications in baked goods and other food products

Properties as swelling power and amylose leaching are generally

reduced by HMT, thus presenting advantages to the production of

starch-based food products This reduction in swelling power is

pro-moted by the restructuring of starch chains and repositioning of bonds in

them In addition, HMT makes the granule more rigid and resistant to

heating, conferring a more hydrophobic granule (Mathobo et al., 2021)

The starches from different botanical sources of the studies presented in

this review showed distinct behavior regarding the effect of HMT on

solubility and swelling power Increased values of these properties were

found for mung bean starch, while decreased values were found for rice,

cassava, pinh˜ao, potato, and green Prata banana starches

3.1.3 Influence of HMT on pasting properties

The pasting properties of starches are most commonly measured by

Rapid Visco Analyzer (RVA), which obtains the following parameters of

starch pastes: pasting temperature (temperature that starts the increase

in viscosity); peak viscosity (maximum viscosity); breakdown viscosity

(difference between peak and minimum viscosity); setback (tendency to

retrograde, difference between final and minimum viscosity); and final

viscosity (viscosity at the end of cooling) (Schafranski et al., 2021) The influence of HMT on pasting properties was studied by Costa

et al (2019), Wattananapakasem et al (2021), Thakur et al (2021), and Dhull et al (2021), in starches from green Prata banana, geminated- black rice, rice bean, and black rice, respectively Costa et al (2019) treated green Prata banana starch at 102 ◦C for 1 h, with moisture contents of 15, 20, and 25%, finding increase in pasting temperature and reduction in breakdown viscosity, tendency to retrogradation, and peak and final viscosities The changes in pasting temperature indicate that HMT provided a swelling initiating at higher temperatures (compared to native starch), which is related to the lower swelling power that the HMT-starch presented (data shown in Section 3.1.2) The reduction in breakdown indicates improvement in shear stability and tendency to retrogradation: this is related to the amylopectin chains that may inhibit amylopectin retrogradation due to the lower formation of double heli-ces Wattananapakasem et al (2021) modified the geminated-black rice starch using moisture contents of 25 and 30%, at 90 ◦C for 1 and 2 h The authors reported improved pasting properties promoted by HMT, as well

as higher pasting temperature when compared to native starch They also attributed the improvements in starch pasting properties to the molecular rearrangement in the starch granule The viscosity and pasting temperature were higher for the HMT-starch when compared to native starch The authors did not report changes in pasting properties regarding the time of exposure to HMT, which varied at 1 and 2 h The reorganization of molecular structure is promoted by HMT without breaking the amylose and amylopectin chains, but promotes intense changes in starches, significantly altering the paste profile

Thakur et al (2021) modified the rice bean using 25% of moisture content at 110 ◦C for 16 h, and reported that HMT reduced the peak, breakdown, and final viscosities, and improved the mechanical and thermal stabilities of starch This modified starch did not present setback viscosity, which was associated to reduction in amylose leaching, resulting in a decrease for this parameter Dhull et al (2021) modified black rice starch by HMT, using 30% of moisture content at 100 ◦C for

16 h, and reported that HMT reduced the peak and final viscosities and increased the breakdown viscosity and pasting temperature These au-thors attributed the changes on the pasting properties to a protective shell promoted around the exterior of partially gelatinized starch gran-ules after HMT, acting as a barrier to the penetration of water and inhibiting gelatinization and pasting In the studies of Thakur et al (2021) and Dhull et al (2021), the time of HMT applied (16 h) was higher than in other studies: Costa et al (2019) used 1 h and Wattana-napakasem et al (2021) used 1 and 2 h of treatment exposure This can also explain the different effect on pasting properties

This physical modification can be performed directly in grains: Chung et al (2012) modified germinated brown rice grains, whereas Arns et al (2015) modified paddy rice grains and rice starch Chung

et al (2012) found increase in pasting viscosity after HMT This was due

to improvement of starch chain interactions, promoting a granular ri-gidity that is attributed to the increased volume occupied by the swollen granules in the continuous phase Arns et al (2015) used HMT at 120 ◦C, with 13% of moisture content and 10, 30, and 60 min of treatment, showing reduced pasting temperature, breakdown and setback, and increased peak viscosity, when comparing the grain and starch in their native and modified forms In our point of view, these changes in pasting properties are related to the effect of HMT on the following starch granule characteristics: the strengthening of bonds between adjacent amylopectin chains; increased crystalline lamella; enhanced thermal and mechanical stability; and greater resistance to swelling due to a rearrangement of the internal forces The exterior layers of the grains protected the starch structure; however, HMT had effect even when applied directly in the grains Also, it can be a cost-effective option, since

no starch extraction is performed and HMT provides changes and im-provements to the granule properties

3.1.4 Influence of HMT on crystallinity

The different types of crystallinities (diffraction pattern) of starch

granules are characterized by X-ray diffraction (XRD) analysis The

diffraction patterns are produced by the packages of hexagonal chains of

amylopectin (Schafranski et al., 2021) Basically, the diffraction patterns

found in starches are A-, B-, and C-types The A- and B-types differ in

their compactness and both show a double helical structure The C-type

is considered a mixture of A- and B-types XRD measures the relative

crystallinity of starches, which present a semi-crystalline structure with

crystalline and amorphous lamellae in its granule Overall, this

param-eter is reduced by HMT (Khatun et al., 2019)

Changes in crystallinity and X-ray pattern promoted by HMT were

found in potato and sweet potato starches treated with moisture

con-tents of 10, 15, and 20% (Oliveira et al., 2018) The native sweet potato

starch showed A-type diffraction pattern (with diffraction peaks at (2θ)

15, 17, and 23◦) that did not change after HMT However, the diffraction

pattern for potato starch changed in intensity from B-type (diffraction

peaks at (2θ) 5.6, 17, 22, and 24◦) to A-type (diffraction peaks at (2θ) 15

and 23◦) These authors also reported changes in the crystalline region

of the granules after HMT A decrease in relative crystallinity for both

starches was found with gradual increase in the moisture of the HMT

applied The most pronounced effect in crystallinity was for the highest

moisture contents used in HMT for both starches, decreasing 11% for

potato starch and 36% for sweet potato starch, when compared to the

native starches HMT promotes dehydration and movement of double

helices into the central channel that can induce changes in the

diffrac-tion pattern and relative crystallinity of starches This movement

occurred during the HMT, being likely to disrupt starch crystallites and/

or change the crystalline orientation

Other studies also found changes in X-ray pattern when applying

HMT in potato starch Brahma and Sit (2020) modified potato starch

using HMT at different treatment conditions, applying moisture contents

of 30 and 35% for 24 h, then heating at 100 and 120 ◦C for 2 h They also

found changes in X-ray pattern that changed from B-type (native) to A +

B type (HMT-starch) The native starch showed peaks at 17.3, 22, and

24◦(2θ) Changes were found in the intensity of the peak at 17◦, as well

as a merge obtained for the peaks at 22◦and 24◦ The relative

crystal-linity was lower for the HMT-starches than for their native counterpart

Bartz et al (2017)) treated potato starch using HMT with various

moisture contents (12, 15, 18, 21, and 24%) The X-ray pattern

pre-sented changes varying along with the moisture content of the

treat-ment The native potato starch showed a B-type pattern (5.6, 15, 17, 20,

22, and 24◦), then changing to a mixture of A- and B-types For the

starches modified with 12, 15, 18, and 21% of moisture contents, the

diffractograms showed changes in the intensity of the pattern (mostly in

peaks of 5.6, 17, 22, and 24◦) On the other hand, the starch modified

using the highest moisture content (24%) presented A-type (15, 20, and

23◦) pattern The changes in intensity by using HMT occur due to the

disappearance of the double helices among the chains within the starch

crystals, resulting in a matrix that is more orderly than in native starch

Relative crystallinity decreased with moisture contents of 12 and 15%,

and it increased with 21% and 24% contents The changes found in X-ray

pattern and crystallinity can be attributed to a partial gelatinization of

starch granules, which had influence on other parameters as reported

here

Depending on the starch source, the diffraction pattern may not

change by HMT, as reported by Lacerda et al (2015) in avocado starch

The increase in moisture content of HMT from 10 to 20 and then to 30%

influenced crystallinity with a reduction of 15% in relative crystallinity

for the modified samples (not differing statistically among different

moisture content applied) when compared to untreated starch This may

be due to the breaking of starch granules that was proportional to the

extent in moisture content which promoted partial gelatinization at high

moisture content The authors related this behavior to the increase in

amorphous area in the semi-crystalline lamella, reducing crystallinity

with the excessive heat during HMT Bharti et al (2019) examined

mango kernel starches from Indian cultivars modified by HMT, using 25,

30, and 35% of moisture content, at 110 ◦C for 3 h The different cul-tivars presented A-type diffraction pattern with diffraction peaks at (2θ)

15 and 23◦, which remained the same after HMT For all mango culti-vars, relative crystallinity decreased However, the authors did not report the values of crystallinity for all samples, which makes difficult the evaluation of the influence of HMT at different moisture content exposure

Andrade et al (2014) modified organic cassava starch using HMT in

an autoclave, at 120 ◦C for 60 min, with 10, 20, and 30% of moisture content The authors reported that the intensity of the treatment (different moisture content) reduced relative crystallinity by 11% for the starch modified using 10 and 20% of moisture content, and by 50% for the starch modified using 30% moisture content, when compared to native starch Thus, relative crystallinity data has been inversely pro-portional to the moisture content of the HMT

Starches modified by HMT are used for the production of nano-crystals This is explored in Section 5, which presents recent trends and applications of starch The changes in crystallinity of HMT starches promote improved properties of the nanocrystals, such as higher ther-mal stability than the nanocrystals produced with native starch Thus, the changes in crystallinity broaden the applications of HMT-starch

3.1.5 Influence of HMT on thermal properties

Differential Scanning Calorimetry (DSC) evaluates the gelatinization properties of starches when heated in excess of water This endothermic phenomenon can be evaluated by DSC through parameters as gelatini-zation, transition temperatures (onset, peak, gelatinigelatini-zation, and final temperatures), and gelatinization enthalpy (Schafranski et al., 2021) Overall, HMT promotes increase in the thermal stability of starches

by a shift in the onset, peak, and final gelatinization temperatures to higher values Oliveira et al (2018) analyzed potato and sweet potato starches modified by HMT, reporting an increase in gelatinization temperatures and a decrease in the gelatinization enthalpy This trend was more accentuated for the sweet potato starch than for the potato starch The increase in gelatinization temperatures was attributed to a strengthening of interactions between amylose and amylopectin branching Gelatinization enthalpy is related to the stability of the crystalline domains of starch Thus, we believe that the reduced enthalpy can be explained by the collapse in the crystal structure of starch granules, which were evaluated by XRD (reported in Section 3.1.4) The authors also evaluated HMT with different moisture contents (10, 15, and 20%) and found increase in thermal stability with the in-crease in moisture content, when compared to the native starch This is seen by the higher initial temperatures and lower gelatinization enthalpy parameters, which represent the thermal stability of starch granules This outcome could be due to the disruption of the amorphous regions of partially gelatinized amylose and amylopectin, and the structural changes induced

The changes in thermal properties promoted by HMT can be attrib-uted to the formation of a stable configuration This is due to a realignment of polymer chains with the non-crystalline regions of starch after HMT Reductions in gelatinization enthalpy can be attributed to the disruption of hydrogen bonding in the crystalline region of starch, throughout the exposure to high temperature (Adawiyah et al., 2017) Differences in thermal properties were reported by Lacerda et al (2015), who modified avocado starch by HMT (moisture contents of 10, 20, and 30%) There was an increase in gelatinization temperature and a decrease of approximately 50% for gelatinization enthalpy, only for the sample modified using 20% moisture content A large endothermic event was observed for the starch treated with moisture content of 30%, which made it impossible to measure thermal properties Adawiyah

et al (2017) used HMT to modify different starches at 120 ◦C and 20%

moisture content For sago (Metroxylon sago) starch, the HMT exposure time was 60 min and for arenga (Arenga pinnata), 90 min They reported

a shift in gelatinization temperature and reduced gelatinization

enthalpy, with 39% reduction for sago starch and 28% for arenga starch

The gelatinization temperatures as onset, conclusion and peak, all

shifted to higher values when comparing HMT and native starches The

increase in gelatinization temperatures indicates an increase in thermal

stability rendered by HMT However, the treatment had more influence

on some of the starches, as sago This can be explained by the different

chain arrangement, the amylose/amylopectin ratio, and other

charac-teristics that vary for each type of starch

Costa et al (2019) modified green Prata banana starch by HMT (at

102 ◦C for 1 h, with moisture contents of 15, 20, and 25%) The

treat-ment showed decrease in gelatinization enthalpy by 32% for the HMT-

starches when compared to their native counterparts, regardless of

moisture content The physical modification also influenced the

gelati-nization temperatures, decreasing the onset temperature and increasing

peak and final temperatures, thus indicating improved thermostability

for the starch modified by HMT The effect of HMT on thermal properties

can be explained by the perfection of the crystalline areas after

in-teractions of amylose–amylose and amylose–amylopectin The effect of

HMT on gelatinization temperatures is in agreement with the changes in

pasting properties found by the authors, shown in Section 3.1.3

3.1.6 Influence of HMT on enzymatic digestibility

HMT-starch has potential for the prevention of chronic diseases, due

to changes in native starch digestibility Digestion of starch is an

enzy-matic hydrolysis in which starch is broken down into glucose, which is

converted to energy for the human body (Khatun et al., 2019) Studies

reporting in vitro methods for measuring starch digestibility are

per-formed by simulating in vivo conditions (Iuga & Mironeasa, 2020)

HMT is widely reported for changes in starch digestibility, enhancing

the nutritional value of starch-based products by increasing the SDS and

RS contents (Iuga & Mironeasa, 2020) There are specific factors directly

related to the effect of HMT in digestibility, such as the starch botanical

source, starch properties as crystallinity, granule size, amylose, and

amylopectin content, interactions and organization between those two

macromolecules, and the process parameters (Pratiwi et al., 2018)

According to Khatun et al (2019)), the alteration of starch digestibility

by HMT is very important for consumers, since starch digestion is linked

with population health, especially for individuals with diabetes These

authors reviewed the digestibility of HMT-rice starches and state that

the rice starch morphology presents very small, medium, and large

granules (comparing to other starch sources), being the last related to

the reduced in vitro rice starch digestibility Other starch botanical

sources were cited for HMT modification, providing effect in α-amylase

susceptibility that was increased for rice, wheat, and potato starches

Also, Khatun et al (2019) reported decrease in RDS and increase in SDS

and RS in corn, pea, and lentil starches The RS content relies on the

processing and storage conditions of food, such as temperature It is also

related to several starch properties as gelatinization and retrogradation,

crystalline structure, and amylose and amylopectin ratio These factors

are crucial for the enzymatic susceptibility of starch (Liu et al., 2016)

Normally, reduced digestibility is reported in the literature,

confirmed by decreased content of RDS and increased content of SDS

and RS This occurs due to structural changes in starch granules, which

become more rigid and make it difficult for enzymes to attack during

digestion (Chung et al., 2012) Brahma and Sit (2020) performed HMT

in potato starched at 100 and 110 ◦C for 2 h using 30 and 35% of

moisture content HMT promoted an increase in SDS and RS, and

reduction in the RDS of potato starches When comparing the HMT-

starches, the RS values were higher with 35% moisture content,

increasing by 95% in the treatment at 100 ◦C and by 88% in the

treat-ment at 120 ◦C This outcome suggests the formation of a rigid structure

that reduced the accessibility of the enzymes in the analysis to disrupt

starch molecules In the highest moisture content (35%) exposure and

the lowest temperature (100 ◦C), a greater effect on RDS and SDS was

obtained The different data reported by the authors regarding distinct

parameters elucidate that the influence of temperature and moisture

content provides different outcomes to starch, which could enable various applications

Liu et al (2016) modified corn starch using HMT, with moisture contents of 20, 25, 30, and 35%, at 110 ◦C for 16 h The modification efficiently increased SDS and RS contents and decreased RDS At the highest moisture content (35%) used in HMT, the RDS presented its lowest value, while SDS and RS had their highest value The degree of hydrolysis decreased with the increase in the moisture content used in HMT We believe that the outcome of their study could suggest the effective application of this physically modified starch in the prevention

of chorionic disease Chung et al (2012) used HMT to modify germi-nated brown rice grains (moisture content of 30%, at 100 ◦C for 1 h), in order to evaluate its effect on digestibility The contents of RDS and SDS decreased and RS increased, which is explained by the extended perfection of the crystalline structure of the granule and the retrogra-dation of amylose In contrast, Zhao et al (2020) reported reduction in

RS and enhanced SDS and RDS of mung bean starch after HMT (30% moisture content, at 120 ◦C for 2, 4, 6, 8, 10, and 12 h) or RHMT (30% moisture content, at 120 ◦C for 2, 3, 4, 5, and 6 h) In general, HMT increases RS; thus, the authors explain that the different trend found in their study is due to RS being possibly transformed into SDS by enhanced enzyme susceptibility, and this would be caused by the disruption of starch granules and changes in crystalline structures when the associa-tions between starch molecule chains are weakened The SDS and RDS values were higher when comparing the RHMT and HMT modifications, which could be related to the higher relative crystallinity of starches after RHMT Thus, RHMT promoted a starch with greater digestibility, which can be applied in foods with faster energy supply

Amylose content in starches can diversify the digestibility and the effect of HMT on this property Wang et al (2016) modified regular corn starch and high-amylose corn starch using HMT, with moisture contents

of 20, 25, or 30%, at 120 ◦C for 2 h The RDS values decreased with the increase in moisture content used in the modification The greater re-ductions were found using 30% of moisture content for both starches, with 24% reduction for regular corn starch and 43% for high-amylose corn starch, when compared to their native counterparts The SDS content increased and RS increased drastically when compared with native starches: RS increased by about 1000%, for high-amylose content modified at 30% moisture content Therefore, this study shows that HMT is a great alternative to modify high amylose content starch and produce resistant starch

The HMT parameters, such as temperature, moisture content, and the botanical source of starch, promote distinct effect on SDS, RDS, and

RS Thus, in order to efficiently apply HMT-starches in applications that require high levels of RS, for example, a thorough study must be performed

3.2 Annealing

Annealing is a hydrothermal process that subject starch with

exces-sive (~70%, v/v) or intermediate (~40%, v/v) water content to

tem-peratures above the glass transition temperature and below the gelatinization temperature of starch, under a period of time that varies from minutes to days The process parameters and starch characteristics define the effect of annealing treatment, as previously discussed for HMT However, in annealing the moisture content does not vary, since the treatment is held in excess of water (BeMiller, 2018; BeMiller & Huber, 2015; Punia, 2020)

Table 2 shows the studies here reported on annealing applied to different types of starch, as well as their treatment parameters The following sections explore these studies

The effects of annealing on starch properties include: increase in crystallinity, thermal stability, gelatinization temperature, starch gran-ular size, and molecgran-ular mobility, due to an increase in the mobility of the amorphous regions to a crystalline state It also promotes the reor-ganization of molecular chains (Alc´azar-Alay & Meireles, 2015) This

occurs due to the hydration of the granule improving the arrangement of

double helices, causing a reversible swelling of the granule and

rendering an ordered structure with higher granule stability (Rocha

et al., 2012)

3.2.1 Influence of annealing on morphological properties

Annealing has been used recently on the modification of different

botanical sources of starch, such as yam starch, in which Falade and

Ayetigbo (2015) used annealing at 50 ◦C for 24 h The authors modified

the starch of various yam cultivars (water, white, yellow, and bitter)

Different outcomes were obtained among the samples, which influenced

the functional properties of starch Regarding morphology, the

evalua-tion was performed in light microscope and for each cultivar, the

granules showed different shapes, but annealing did not affect the

shapes of any cultivar When compared to the morphology of native yam

starches, the differences were the following: the modification changed

the size of the water cultivar granules; for the yellow cultivar, annealing

promoted reduced mean, modal, and median dimensions of the

gran-ules; and bitter and white cultivar showed no change in size In this

study, the authors reported that not only the botanical source of starch,

but also the cultivar influenced the annealed starch properties, due to

amylose content, molecular rearrangement and size of amylose and

amylopectin chains, among other parameters

Puelles-Rom´an et al (2021) studied oca starch physically modified

by annealing at 42 and 50 ◦C for 24 h The structure of starch granules

was not affected by annealing, due to the low temperature used in the

process of modification, which is below gelatinization temperature The

shape of starch (evaluated by SEM analysis) was not affected either

However, the granule size of annealed starch was higher than the size of

native starch The same result was observed by Rocha et al (2012), who

modified normal and waxy corn starches by annealing in excess of

water, for 24 h at 63 and 62 ◦C, respectively The morphology was

evaluated by SEM, showing no changes in granule shape: all granules

were round and polyhedral The annealed normal and waxy starches

showed more pores and increased pore size in the granule surface, when

compared to their respective native starches These changes can be due

to a spread of proteins over the starch granule surface, along with

annealing application and the weakening in tissue structure under

heating, forming a compact shape

Again, annealing did not change morphology in the study of Shi et al

(2021), who assessed Castanopsis fruit starch treated in two steps, first

at 40 ◦C for 24 h and then at 55 ◦C for more 24 h, totalizing 48 h The authors reported that the granules presented rough surfaces with irregular, polygonal, and spherical shapes, unaltered by annealing, regardless of the number of steps in the treatment Another study re-ported no changes in morphology: Song et al (2013) performed annealing in potato and sweet potato starches (at 55 ◦C for 72 h) Both native and modified starches presented granules without pores and with different morphologies among the cultivars (irregular, polygonal, spherical, oval, round, and bell-shaped) Therefore, we can conclude that the shape of starch granules is not affected by annealing, regardless

of the starch botanical source or the annealing treatment parameters Annealing is well known for modifying several properties of starch Morphology is affected when evaluating the size of starch granules; in contrast, its shape and structure are not significantly affected This was confirmed by the studies showed in this review in which starches from different botanical sources were modified by annealing

3.2.2 Influence of annealing on solubility and swelling power

The swelling power and amylose leaching of starch granules are reduced by annealing, which improves the quality of starch paste and gel for application in food products (Mathobo et al., 2021) Shi et al (2021) modified Castanopsis fruit starch using a two-step annealing process (step one at 40 ◦C for 24 h, then step two at 55 ◦C for more 24 h) The authors reported substantial decrease in swelling power and solubility after modification; both parameters decreased along with the increase in annealing treatment from step one to two In addition, as temperature increased from 60 to 90 ◦C, swelling power also increased for native and annealed starches The decrease in swelling power after annealing can

be attributed to the increase in crystallinity (see Section 3.2.4) The reduction in solubility indicates that there was a strengthening of the bonds, with an increase in the interactions between amylose and amylopectin molecules, forming a more stable structure and reducing the leaching of amylose

Wang et al (2014) performed a comparative study of annealing in waxy, normal, and high-amylose corn starches, using parameters as water excess, at 45 ◦C for 24 and 72 h The swelling power increased prominently with the increase in amylose content of the starches When comparing native and annealed starches, the modification did not change the swelling power for waxy starch However, swelling power decreased for normal and high-amylose starches, by 7 and 18%, respectively The distinct changes in solubility and swelling power of the starches reported can be attributed to interactions between the starch chains and the amylose-lipid complexes

3.2.3 Influence of annealing on pasting properties

The effect of annealing on starch pasting properties depends on factors as amylose leaching, branch chain length distribution of amylopectin, granule swelling, and relative crystallinity This physical modification promotes more resistance of the granules to deformation

by strengthening its intragranular binding forces (Song et al., 2013) Hu

et al (2020) evaluated the effect of annealing at different time periods of treatment on sweet potato starch, at 50 ◦C for 1, 3, and 5 days The authors reported that according to the increase in annealing time, there was an increase in the pasting temperature and a decrease in break-down, peak, and final viscosities, as well as in setback (compared with native starch) As annealing time elapsed, these changes in pasting properties became more prominent From the data reported by the au-thors, we can see that annealing promoted more resistance to high temperatures and weakened retrogradation, thus improving the pasting properties of sweet potato starch We can attribute the changes in pasting properties of starches treated with annealing to the associations among the chains within the amorphous region of the granule and the changes in crystallinity promoted during this treatment

In general, annealing does not change the shape of viscoamylograph curves by RVA Its effect on pasting properties is widely reported, as

Table 2

Annealing parameters, types of starch, and references

Temperature ( ◦ C) Time (min) Germinated brown

Ayetigbo (2015)

4320 Hu et al (2020) Corn and wheat 55, 60, 65, 70, 80 30 Li et al (2020)

et al (2021)

(2012)

Waxy starches from

corn, potato, rice

and barley

10 ◦ C below the temperature of gelatinization

120, 180,

480, 960,

1440, 2880,

4320

Samarakoon

et al (2020)

Castanopsis fruit 40, 55 1440, 2880 Shi et al (2021)

Sweet potato and

(2014)

(2021)

these are strongly influenced by treatment parameters as exposure time

Song et al (2013) modified potato and sweet potato starches using

annealing (55 ◦C for 72 h) The annealed-starches presented reduction in

peak viscosity, breakdown, and setback, and increase in pasting

tem-perature In our point of view, the changes mentioned for pasting

properties can be related to improved crystallinity (data can be found in

Section 3.2.4) and to the enhancement of the packing arrangement of

starch through annealing The decrease in breakdown shows that the

starch is more stable during heating and continuous shear This opens up

new applications that require starch stability in high temperature

Werlang et al (2021) modified oat starch (50 ◦C for 24 h) using

annealing, and it had effect only on setback, which presented a lower

value when comparing annealed-starch to its native counterpart In this

study, the other pasting properties of oat starch gels remained the same

after the modification This outcome showed that in oat starch the effect

of annealing did not present the same influence on the pasting properties

as for other starches showed here, such as potato and sweet potato

starches This elucidates the distinction of treatment response to

different types of starch with various structures and characteristics

Amylose content is an important factor for obtaining the pasting

properties of starches by RVA High-amylose starches did not present a

viscoamylograph curve under the conditions used by RVA, due to their

low viscosity Therefore, the pasting parameters of high-amylose

starches cannot be measured by this analysis Wang et al (2014)

annealed corn starches containing different amylose contents (waxy,

normal, and high-amylose) at 45 ◦C for 24 and 72 h In the three

starches, annealing had no effect on pasting temperature, decreased

peak temperature and final viscosity, and increased final viscosity and

setback The time of annealing had effect on these properties, which

increased or decreased gradually, as annealing time elapsed The

breakdown viscosity decreased after 24 h of annealing and increased

after 72 h The studies of Werlang et al (2021), Hu et al (2020), Song

et al (2013), and Wang et al (2014) showed the effect of annealing on

pasting properties of starches from oat, sweet potato, potato and sweet

potato, and corn, respectively The data obtained did not show any

trends of increase or decrease in pasting parameters, and the effect of

annealing was different for each type of starch cited Nevertheless, when

comparing native and annealed starch, changes were observed for all

botanical sources evaluated, which can be attributed to the disruption of

intra and intermolecular hydrogen bonds of the starch grain in the

presence of heat and water

3.2.4 Influence of annealing on crystallinity

The diffraction pattern of starch is mostly not affected by annealing

In contrast, HMT-starches show shifts in the diffraction pattern, as

shown in Section 3.1.4 The different effect on starch properties among

the treatments is directly related to the parameters used in both

modi-fications, e.g., moisture content, temperature, and exposure time

Recently, starches from different botanical sources were evaluated

for their diffraction pattern, which did not present any changes (Li et al.,

2020; Samarakoon et al., 2020; Shi et al., 2021) Annealing was applied

by Shi et al (2021) to modify Castanopsis fruit starch (step one at 40 ◦C

for 24 h and step two at 55 ◦C for more 24 h) Native starch showed

diffraction peaks at (2θ) 5.7, 15.1, 17.2, 22.3, and 23.9◦ This represents

a C-type pattern, which was not changed by annealing, regardless of

treatment step Relative crystallinity decreased for starch treated with

two-step annealing (total of 48 h) when compared to native starch

However, the starch modified after only one step (24 h at 40 ◦C) kept the

same relative crystallinity Thus, the addition of a second step in

annealing using a higher temperature (55 ◦C) for additional 24 h

increased crystallinity by 19% We believe that this behavior can be

attributed to the greater effect of heat, which affects the structure of the

starch granule in the second step

Amylose content is known to influence the relative crystallinity of

starches Li et al (2020) modified three types of wheat starches with 37,

85, and 93% of amylose content, and two types of corn starches with

58% and 82% of amylose content, using annealing (at 55, 60, 65, 70, and

80 ◦C for 30 min) The normal amylose content of starches showed A- type diffraction pattern, while the starch with high amylose content presented B-type pattern These patterns were not changed after annealing, but the relative crystallinity decreased as the annealing temperature increased This study is important for showing that different types of botanical sources, as well as amylose content and modification parameters, can modify starch properties and structure with different outcomes

Samarakoon et al (2020) investigated the effect of annealing on the properties of waxy starches from different botanical sources (corn, po-tato, rice, and barley) It can be observed in this study that annealing affects the properties and the structure of starches from different botanical sources differently The diffraction patterns were not changed

by annealing Native waxy starches from corn, rice, and barley presented A-type pattern and potato starch showed B-type pattern Relative crys-tallinity was not changed for barley, rice and potato starch; however, for corn starch it increased by 9% after annealing Another study (Rocha

et al., 2012) reported the effect of annealing on normal and waxy corn starches (water excess for 24 h at 63 and 62 ◦C, respectively) Regarding the amylose content, annealing showed no influence The relative crystallinity of normal amylose content did not change; however, there was an increase of 7% for waxy starch The decreased relative crystal-linity found for some studies can be explained by partial gelatinization and helix changes, leading to the destruction and reorientation of starch crystallites For each type of starch and the different amylose content, the effect of annealing was distinct, which occurred for all the studies shown in this review This reinforces that the botanical source of starch

is able to change the effect of physical modification When a starch is chosen for application, this needs to be taken into consideration, since the modifications can promote specific outcomes for each starch

3.2.5 Influence of annealing on thermal properties

Overall, annealing led to increased onset, peak, and final tempera-tures and gelatinization temperature range, and decreased gelatiniza-tion enthalpy This improved the thermal properties of starches, broadening their application in food products that are exposed to high temperatures during production Different cultivars of potato and sweet potato starches were modified by Song et al (2013) The gelatinization enthalpy decreased for all cultivars, except for one that also showed increased relative crystallinity (see Section 3.2.4) The onset, peak, and final temperatures decreased, and gelatinization temperature increased upon annealing

Modification parameters, such as time of exposure to high temper-ature, provide greater influence on the thermal properties of starch According to the increase in days of treatment, annealing provided starches with greater onset, peak, and final temperatures and reduced gelatinization temperature, as reported by Hu et al (2020) The authors performed annealing in sweet potato starch (50 ◦C for 1, 3 and 5 days) Only in the 5-day treatment did the gelatinization enthalpy of starch increase In 1- and 3-day treatments, this property decreased slightly

We believe that this occurs probably due to partial gelatinization during annealing

Variations in annealing temperature are also reported to present greater effect Thermal properties improve with the increase in tem-perature, as reported by Puelles-Rom´an et al (2021), who modified oca starch using annealing at 42 and 50 ◦C for 24 h, using different water

ratios (1:2 w/v, being 35 g starch/70 ml distilled water, and 1:6 w/v,

being 15 g starch/90 ml distilled water) Annealing led to increase in onset, peak, and final temperatures and gelatinization temperature range, becoming more accentuated as annealing temperature increased The higher temperatures are related to a partial gelatinization of starch during annealing The changes in water ratio did not have effect on the thermal properties, not differing among the samples at the same treat-ment temperature Gelatinization enthalpy was not changed by any of the annealing conditions Therefore, we can state that oca starch is more

resistant than other types of starch, since the changes in thermal

prop-erties were achieved using the highest temperatures tested and, even so,

there were no changes in gelatinization enthalpy

The improvements in thermal properties indicate that annealing

enhances the crystal quality of starch by amylopectin aggregation

through the rearrangement of amylose, which needs less energy to break

the crystal structure due to the weakening of the network Oat starch

was annealed by Werlang et al (2021) (50 ◦C for 24 h) The onset, peak,

and final temperatures and gelatinization temperature range increased

for the annealed oat starch, when compared to its native counterpart

Gelatinization enthalpy reduced by 13% from native to annealed starch,

which can be associated to the destruction of the semi-crystalline

structure and the melting of imperfect amylopectin crystals

The reported studies about the use of annealing on potato, sweet

potato, oca, and oat starches showed increased thermal stability All

studies (Hu et al., 2020; Puelles-Rom´an et al., 2021; Song et al., 2013;

Werlang et al., 2021) showed reduced onset, peak, and final

tempera-tures, regardless of the botanical source of starch On the other hand, the

gelatinization temperature and enthalpy presented different trends with

the application of annealing, when compared to native counterparts

3.2.6 Influence of annealing on enzymatic digestibility

The degrading enzymes of enzymatic digestibility initially attack the

amorphous regions of starch molecules and then the crystalline regions

if they are exposed (Song et al., 2013) After annealing, RDS and SDS

contents increased, and RS content decreased when Song et al (2013)

modified potato and sweet potato starches The increase in RDS is

attributed to the easy and rapid attack of the enzymes of digestion on the

amorphous region and melted crystalline defects The loss of the

α-he-lical structure upon partial gelatinization of annealed starches can

explain the increase in SDS and the decrease in RS

Shi et al (2021) determined the digestibility of Castanopsis fruit

starch after a two-step annealing process (step one at 40 ◦C for 24 h and

step two at 55 ◦C for additional 24 h) After annealing, RDS content

decreased and SDS content increased Interestingly, RS was not changed

by annealing, even for the starch that showed increased crystallinity (see

Section 3.2.4) Following the same trend, annealing (at 50 ◦C for 24 h) in

corn starch increased SDS and RS contents and decreased RDS, when

compared to native starch (Liu et al., 2016) In contrast, Wang et al

(2014) annealed corn starches with different amylose contents (waxy,

normal, and high-amylose), but found no effect on digestibility

Annealing can be promising for the modification of grains such as

germinated brown rice, which was modified using HMT (moisture

content of 30%, at 100 ◦C for 1 h) (Chung et al., 2012) When comparing

germinated brown rice grains to their modified form, RDS content

decreased, RS increased, and SDS remained the same The higher RS

content can be attributed to improved interactions in starch chains

formed during annealing Therefore, this modification is an effective

alternative for controlling the digestibility of grains, which needs to be

the subject for future studies

Annealing does not gelatinize starch; thus, the increase in RS is due

to structural changes as its increased crystallinity and molecular

reor-ganization Throughout the studies reported to explore the effect of

annealing on digestibility (Chung et al., 2012; Liu et al., 2016; Shi et al.,

2021; Song et al., 2013; Wang et al., 2014), it could be seen that

annealing does not affect the digestibility of the starches Thus, for

ap-plications that require decreased digestibility, with higher SDS and RS

content, annealing must be applied correctly according to each type of

starch

4 Combined modifications

Annealing and HMT can also be performed together or combined

with other modifications, such as chemical, physical, and enzymatic

Combinations are an asset due to their improvement of properties,

opening up new applications in different fields In addition, there are

new perspectives for science and technologies of starch modification Table 3 summarizes the studies reported for combined modifications using HMT or annealing

Combined methods with HMT, such as infrared heating, can be performed in order to achieve the same improved properties in a shorter time and with less energy These combined methods for modification were performed by Ismailoglu and Basman (2015) in corn starch and by Ismailoglu and Basman (2016) in wheat starch In both studies, the authors used microwave with powers of 550 or 730 W, exposure times of

30, 60, or 90 min, and moisture content of 20 and 30% The two studies also had results regarding the botanical source of starch The authors found a retained A-type diffraction pattern; changes in thermal prop-erties as increased onset, conclusion and gelatinization temperatures were reported by the realignment of the polymer chain and formation of

a stable configuration; the pasting properties were affected by a decrease

in viscosity only for the samples treated using 550 W and 730 W and 30% of moisture content We believe that the changes in pasting tem-peratures are attributed to changes in crystallinity and the chains in the amorphous region association The authors showed a slight increase in relative crystallinity

The effect of heating methods with different impact of vacuum pressure on corn starch properties was investigated by Bahrani et al (2013) using DV-HMT (direct vapor-heat-moisture treatment), RP-HMT (reduced-pressurized heat-moisture treatment) and DIC (D´etente Instantan´ee Contrˆol´ee) An improvement in the swelling capacity of the granules was induced by the intensification of the hydrothermal process DIC promoted larger decrease of about 85% in viscosity when compared

to the other modifications, in which a decrease of 58% was found for DV- HMT and 53% for RP-HMT In this study, the intensification in hydro-thermal process provides significant changes in rheological properties, which can directly affect the functionality and application of starch The

Table 3

Combined modifications of HMT and/or annealing with other modifications in starches from different sources

Starches Modifications Important outcome References Corn DV-HMT/RP-HMT/

and DIC Decreased swelling capacity and viscosity Bahrani et al (2013)

Corn and potato Annealing/dry heating Decreased SDS and RS after treatment for corn

starch, increased for potato starch

Chi et al (2019)

Barley Annealing/

hydroxypropylation Improved freeze-thaw stability, final viscosity,

rheological properties, and solubility

Devi and Sit (2019)

Corn HMT/infrared heating Decreased viscosity,

thermal and pasting properties

Ismailoglu and Basman (2015)

Wheat HMT/infrared heating Decreased viscosity,

thermal and pasting properties

Ismailoglu and Basman (2016)

Corn HMT/organic acids Increased thermal

properties, SDS and RS Maior et al (2020)

Corn HMT/lactic acid Increased SDS, RS,

viscoelasticity and thermal stability

Reyes et al (2021)

Kithul HMT/annealing/cross-

linking Decreased swelling power and solubility, increased

RS

Sudheesh

et al (2020)

Corn HMT/extrusion Decreased swelling power

and solubility, increased SDS and RS

Yan et al (2019)

Potato HMT/Amylose‑sodium

stearate complexes Reduced retrogradation, improved thermal

properties and granule stability

Yassaroh

et al (2021)

HMT: Heat-moisture treatment; DV-HMT: direct vapor-heat-moisture treatment; RP-HMT: reduced-pressurized heat-moisture treatment; DIC: D´etente Instantan´ee Contrˆol´ee

modified starches showed the same size and shape as native starch with

some agglomerations, which is aligned with the reported studies using

only HMT: Bartz et al (2017)) and Wattananapakasem et al (2021),

who modified potato and rice starches, respectively These studies are

shown in Section 3.1.1

Chemical modification along with HMT was also reported to have

impact on the digestibility and thermal, structural, and pasting

prop-erties of corn starch, as evaluated by Maior et al (2020), who combined

organic acids (lactic, citric, and acetic) and HMT The authors found no

changes in diffraction pattern and a slight reduction in relative

crys-tallinity for all samples This trend was also found by Ismailoglu and

Basman (2015) and Ismailoglu and Basman (2016), who combined HMT

and microwave modifications The samples treated with HMT combined

with citric acid and lactic acid did not show gelatinization curves; the

sample treated with HMT and acetic acid showed a reduction of 15% in

gelatinization enthalpy; and the sample treated only with HMT showed

no difference for this property The surface of the granules showed in its

morphology an increase in pores and some irregularities Regarding

digestibility, there was increase in SDS and RS: we can suggest a

promising use for the production of low-carbohydrate foods for

con-sumers with chronic diseases such as diabetes, for example The changes

in SDS and RS can be attributed to changes in starch structure as the

rupture of the double helices, which is proven by the reduction reported

in gelatinization enthalpy Thus, we conclude that these modifications

performed together allow the possibility of applying modified starch in

functional food, producing higher levels of RS

The combined modification of HMT and lactic acid was also

evalu-ated by other authors (Reyes et al., 2021), presenting changes in corn

starch such as reduced relative crystallinity of about 62% for the starch

modified only with HMT and about 33% for the starch modified by HMT

and lactic acid combined The gelatinization enthalpy presented a

similar reduction for the starch modified by HMT and HMT combined

with lactic acid, reducing about 62% In addition, increased solubility,

viscoelasticity and thermal stability were found Regarding in vitro

di-gestibility, the modifications combined decreased RDS and increased

SDS and RS We can see that combining the modification promoted more

accentuated influence on the enzymatic hydrolysis of starch granules by

acting more effectively on the enzymatic fractionation of starch chains

The combined effects of extrusion and HMT on the physicochemical

properties and digestibility of corn starch were studied by Yan et al

(2019) The modifications showed decreased swelling power and

solu-bility, and changed X-ray diffraction pattern from V- to V + A-type The

pasting properties improved, with higher enthalpy values of about

133% SDS and RS increased, with a 12% rise for RS: this shows that

combined modifications make the granules less susceptible to enzymatic

hydrolysis, due to more crystal perfection that leads to a resistance to

enzyme digestibility In this study, starch structure, physicochemical

properties and digestibility were modified by combining HMT with

extrusion, allowing more applications of starch Sudheesh et al (2020)

combined different types of modification, performing annealing, HMT,

and cross-linking on underutilized kithul (Caryota urens) starch In all

combinations performed, no changes were reported for the A-type

dif-fractogram pattern The authors found a decrease in swelling power and

solubility, an increase in gelatinization enthalpy, and increase in

hard-ness of modified starch gel when compared to native starch The dual

modification of cross-linking and HMT promoted the highest relative

crystallinity among the other combinations, and these modifications

combined also showed higher RS content, with an 18% increase when

compared to native starch The RS was higher for all the combined

modifications when compared to the modifications performed alone

The RS content is generally increased when HMT is combined with other

modifications, showing promising applications Thus, changes in starch

elucidate improved granular stability and we believe that the

confec-tionery industry is one of the areas interested in good gel forming

capacity

The improvement in the modification of starch is related to the

possible untangling in the entanglements among starch chains, which induces strong movement of the chains, thus facilitating molecular rearrangement during annealing (Zhong et al., 2020) It should be noted that the effects of annealing or HMT on starch properties are enhanced when these modifications are combined with each other or with other types of modifications The effectiveness of annealing was improved when combined with microwave pretreatment, intensifying starch properties as gelatinization enthalpy, particle size, peak viscosity, and breakdown viscosity

Devi and Sit (2019) performed annealing in one and two steps fol-lowed by hydroxypropylation on barley starch Relative crystallinity and paste clarity increased, whereas swelling power, solubility, freeze- thaw stability, and paste viscosities decreased when performing annealing alone When the modifications were combined, they enhanced freeze-thaw stability, final viscosity, paste clarity and tem-perature, rheological properties, swelling power, and solubility The use

of both physical and chemical modifications improves particular starch properties that can be explored by food industries Dry heating and annealing combined to modify starch are a green alternative for improving starch properties: the synergistic effect of both methods alter starch lamellar thickness, increase double helical orders and improve digestibility (Ashogbon, 2020) These modifications synergistically modulate the starch structure, increasing thermostability and homoge-neity and presenting direct influence on digestibility This was reported

by Chi et al (2019), who modified corn and potato starches using annealing combined with dry heating, focusing on improvements in starch digestibility SDS and RS decreased after combined modification for corn starch and increased for potato starch This can be attributed to the different molecular structure of those two starches, shown in their diffraction patterns (A-type for corn and B-type for potato) The inter-esting data present by Chi et al (2019) showed an increase in efficiency

of annealing when combined with dry heating and a significant improvement in digestibility for potato starch It is worth mentioning that the two physical modifications used are green and low-cost For

targeting applications as in the prevention of chronic diseases, the in vitro enzymatic digestibility of starch must be further investigated in

order to efficiently control the effect of dry heating and annealing per-formed together

The physicochemical properties of amylose‑sodium stearate com-plexes (at concentrations of 2, 5, and 8%) in HMT-potato starch were evaluated by Yassaroh et al (2021) The amylose inclusion complexes were used with sodium stearate as guest molecules The addition of amylose‑sodium stearate complexes reduced starch retrogradation and improved thermal properties, which are promising results for cooking- related applications Throughout changes in X-ray pattern to V6- type amylose crystallite, the formation of amylose inclusion complexes was proven The combined modifications increased relative crystallinity, pasting temperature and granule stability, and decreased the swelling ability, which was more accentuated than in HMT performed alone The authors explored different applications for the modified potato starch as filler, emulsifier, and thermally stable thickener For future studies, it would be interesting that the digestibility of the starches modified by amylose‑sodium stearate complexes and HMT could be evaluated, since due to the properties obtained it is possible that SDS and RS contents are improved

The changes upon starch when using HMT or annealing alone can be insufficient depending on the applications requested Thus, using these physical modifications together or with other modifications promotes more efficient outcomes, as shown in this review by expressive changes

in thermal, pasting, and swelling properties, as well as in digestibility Therefore, performing combined modifications can enhance starch functionality, broadening its spectrum of applications

5 Recent trends and applications

Starch application has been expanding over the years and