Physical Modification of Starch

Chapter 5

Physical Modification of Starch James N. BeMiller Purdue University, West Lafayette, IN, United States

1. INTRODUCTION Usually considered to be physical modifications of starches are those modifications that produce changes in starch properties effected by physical treatments alone that do not introduce any chemical modification of the starch polysaccharide molecules (other than limited glycosidic bond cleavage (depolymerization) that results only in some decrease in average molecular weight). Physical treatments are generally divided into thermal and nonthermal treatments, although as will be pointed out, some treatments often categorized as nonthermal are not or may have a thermal component. The primary interest in physical modification seems to be that they are clean label starches, i.e., they can be classified as ingredients rather than additives and do not need to be identified on food labels as modified food starch or food starch modified, thus making them acceptable as natural products. (Major starch-producing companies offer clean label starches, sometimes referred to as label-friendly or functional native starches, made by proprietary processes.) Physical modifications appear to have been investigated for other reasons as well. Native starches do not provide desired functionalities in many food products, and physical modifications are the preferred method of modification to improve the performance of poor-quality starches and flours if proper functionalities can be realized; for physical modifications are usually easier to do and often less expensive than are chemical modifications and produce no effluents containing salts, reagents, or reagent by-products. Physical modification of locally produced, small-volume starches for which chemical modification is neither practical nor economically feasible has been investigated. Another interest seems to be that several treatments produce increased amounts of resistant (RS) starch and slowly digesting starch and some investigations of physical treatments of starches were done because the treatments had been investigated for nonthermal processing of foods, so it was of interest to determine what effect the treatment had on any starch or flour used as an ingredient. Starch in Food. http://dx.doi.org/10.1016/B978-0-08-100868-3.00005-6 Copyright © 2018 Elsevier Ltd. All rights reserved.

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In general, physical treatments destroy or produce changes in the packing arrangements of the starch polysaccharide molecules within granules, but such structural changes can alter the properties and functionalities of the starch, including the characteristics of its hot pastes and gels and their digestibility. Physical treatments can produce starches with properties somewhat like those obtained by chemical modifications (especially like those of lightly cross-linked starches, such as increased tolerance to low pH, high temperatures, and high shear), but the changes are not as dramatic and as thermostable as those produced by chemical modification. Different treatment conditions with the same starch and different starches subjected to the same treatment conditions give products with different characteristics, the latter because different starches are affected differently by physical treatments. For these reasons and because the conditions under which the starch is treated can vary considerably, contradictory results have often been reported. This chapter attempts to present the most commonly obtained results or the general consensus on changes obtained and to cover recent papers not included in the more detailed review on physical modifications of starches that has appeared (BeMiller and Huber, 2015). Physical modifications done on flours, done in combination with other physical modifications, done before or after chemical modifications, or done before treatment with an enzyme have been investigated, are not presented here, except for a few references to applications of physically modified flours. Only the effects of the specific physical treatment itself are discussed. Also not considered to be a physical modification is the preparation of the pyrodextrins known as British gums, which are made by heating an untreated or buffered starch to a high temperature for a relatively long time, because in the process, the starch polysaccharides become more highly branched via transglycosidation, i.e., the treatment changes their structure. Dry heating a starch with an added acidic or alkaline catalyst results in hydrolysis of glycosidic bonds in addition to transglycosidation. Also not included in this chapter are one application paper with the term “clean label starches” in the title and two application papers with the term “physically modified starch” in their titles, as the first refers to native starches; one of the other two refers to a cooked and retrograded product, and the other refers to a commercial product of unknown origin.

2. THERMAL TREATMENTS 2.1 Pregelatinized Starch Pregelatinized (instant) starches are not usually considered to be physically modified starches, but they should be (BeMiller and Huber, 2015). They are starches that have been cooked/pasted and dried under conditions that allow little or no molecular reassociation. Pregelatinized products hydrate rapidly

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and are described as being cold water soluble. Thus, they can be used without cooking, although many of the commercial products generate additional viscosity when a dispersion of them is heated. There are three basic types of equipment used to make pregelatinized starches. With either, the specific type of equipment used and operating parameters may vary from site to site and batch to batch. Those factors and different starch types used in the process means that different commercial products (even those produced from the same native starch) often have different characteristics. In the original procedure, an aqueous slurry of a starch is applied to a steam-heated drum or into the nip between counterrotating drums heated with pressurized (high-temperature) steam where the starch is rapidly gelatinized, pasted, and dried. The dry film is scraped from the drum and reduced to powders of various mesh sizes. Another process employs extrusion. Because the variables include the type of starch, starchewater ratio, temperature, screw configuration and speed, and residence time, there is probably more variability in products made by extrusion. The extrudate must be dried and ground to a powder. In a third process, starch is pregelatinized by treatment with superheated steam, first in a two-fluid nozzle, then in a spray-drying chamber. The degree of gelatinization (as determined by DSC) in a group of pregelatinized starches made using a hot drum or an extruder varied from 44% to 100% and the degree of crystallinity (determined by X-ray diffraction) varied from 14% to 18% (BeMiller and Huber, 2015), indicating the variability of products. Some depolymerization occurs during the pregelatinization processes, at least for those products made using hot drums or extruders with a greater amount being found in products exposed to the high shear produced in extruders. How much depolymerization can be attributed to thermal degradation and how much to milling (Section 3.2) is unknown. Anastasiades et al. (2002), using double-drum equipment, studied the effects of process variables in the pregelatinization process and verified that swollen particles contribute much more to the viscosity of resulting product dispersions than do solubilized starch polysaccharides. Results of recent research have confirmed that hot-drum pregelatinized starch products produce additional viscosity when dispersions of them are heated (Li et al., 2014b) and that average molecular weights of the starch polysaccharides are reduced during the overall process (Majzoobi et al., 2011). Results of two studies (Li et al., 2014b; Majzoobi et al., 2015a) confirmed that pregelatinized starch products are more soluble in room-temperature water than are granular cold-water-swelling (GCWS) starch products (Section 2.2). Majzoobi et al. (2015a, 2016) determined that, as the concentration of acetic acid in water increased, the room-temperature-water solubility of hot-drum pregelatinized starches increased, while the viscosity of their dispersions in room-temperature water and the turbidity of their gels decreased. They hypothesized that depolymerization occurred in the acetic acid solutions; but molecular weight

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data were not reported. This reviewer doubts that dilute acetic acid at 25 C for 15 min will effect any glycosidic bond hydrolysis and believes that there must be another explanation. Majzoobi et al. (2015b) conducted a similar experiment using a solution of L-ascorbic acid and again found that room-temperaturewater solubility increased and the viscosity of the solution decreased as the acid concentration and time increased, but it had been determined previously that ascorbic acid depolymerizes starch via oxidation (Valle`s-Pa`mies et al., 1997; Sriburi et al., 1999). Hedayati et al. (2016) studied solution and gel properties of hot-drum pregelatinized starch dispersed in water at room temperature with stirring, after which the pH was adjusted and found that (1) the small, flat particles fragmented at pH values of 3 and 5, but changed only slightly, if at all, at pH values of 7 and 9, (2) the viscosity of dispersions decreased over 17 min at pH values of 3 and 5 (at pH 3 more than at pH 5), but increased at pH 9 (with the value at pH 7 being the same as the control value), and (3) solubility increased in the order pH 3 > 5 > 9 (with solubility at pH 7 being the same as the control). They also hypothesized that molecular degradation took place at the acidic pH values, but that has not been established and is questionable, especially at pH 5. Both native and chemically modified starches can be pregelatinized. Pregelatinized products are used when heat cannot be applied to the product because of thermal lability of an ingredient, no processing step requires sufficient heat to cook the starch, no heat is available, or no heating equates to use convenience (especially in mixes to be used in the home). They are established articles of commerce and are used for product thickening, moisture control, and texture provision and control. Applications of pregelatinized starches include dry-mix products designed for home use, cake mixes, puddings, cream fillings, snack foods, frostings, and toppings. Pregelatinized modified starches retain much of the attributes contributed by the modification. Because they hydrate rapidly, powders of pregelatinized starches of small mesh size need to be handled rather like hydrocolloids when dispersing them in water, but products of larger mesh sizes, which are designed to impart some graininess or pulpiness desired in some applications, disperse easily.

2.2 Granular Cold-Water-Swelling Starch Another group of “instant” starches consists of products which contain gelatinized, intact granules, i.e., amorphous granules, that swell extensively (without cooking) when placed in an aqueous system at room temperature (BeMiller and Huber, 2015). They are a special kind of pregelatinized starch and are often called cold-water-soluble starches; but the classic pregelatinized starches are usually more room-temperature water soluble (without application of shear) than are these products, so the author prefers the term GCWS or simply cold-water-swelling (CWS) starches to describe them. GCWS starches produce viscosities and gel characteristics more like those of cook-up starches

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when added to room-temperature aqueous systems than do classic pregelatinized starches. They can be made by four general methods: (1) heating an amylose-containing starch in an aqueous solution of an alcohol (Eastman and Moore, 1984; Rajagopalan and Seib, 1991, 1992a,b; Sun et al., 2009), (2) rapidly heating a starch dispersion in a special spray-drying nozzle and drying the droplets in a spray drier (Pitchon et al., 1981), (3) treating the starch with an alkaline, aqueous solution of an alcohol at room temperature (Jane and Seib, 1991; Chen and Jane, 1994), (4) instantaneous controlled pressure drop (DIC) (BeMiller and Huber, 2015). In addition, Zhang et al. (2012) developed a new aqueous ethanol procedure for producing GCWS starches from both Aand B-type starches. Conversion of high-amylose starch into a GCWS product useful for the manufacture of confectionery, convenience, and other food products using a modified spray-drying system has been claimed (Berckmans et al., 2013). Products made from a single starch using different methods, by varying the method conditions using a single starch, and by using a single method with different starches vary in characteristics, but they have the common characteristic that their granules rapidly hydrate, swell, and lose their crystalline order without granule disintegration (even when little or no shear is applied) when placed in an aqueous system at room temperature. In the end, they produce the functionalities of the cook-up starches from which they are made without application of heat. A drawback is that, when GCWS starch granules of an amylose-containing starch are added to unheated water without shear, ensuing retrogradation can result in poor-quality dispersions. However, when those granules are dispersed in a sucrose or glucose syrup with rapid stirring, the resulting dispersion sets to a rigid gel that can be sliced to make gum candies, and the ability to swell in unheated aqueous systems can be useful for making desserts in the home and in muffin batters containing fruit or nut pieces that would settle to the bottom when the batter thins as a result of heating before the gelatinization temperature of the starch is reached and the batter thickens. CGWS starch can be used in instant pudding mixes (Jane, 1992) and as a fat mimetic in spoonable salad dressings (Bortnowska et al., 2014). When waxy maize starch alone is heated in aqueous ethanol, it loses its granular structure, but a product can be made in this manner from a mixture of waxy and normal maize starch. Method 3 is almost always used in the laboratory because special equipment is not required and at least six process variables can be studied. Both native and chemically modified amylosecontaining starches can be converted into GCWS products. Kaur et al. (2011) confirmed that, during conversion of sago starch into a GCWS starch using an alkaline aqueous ethanol treatment, the C-type crystallites were changed into V-type crystallites as a result of starch polysaccharide molecules complexing with ethanol molecules, and Dries et al. (2014) confirmed that, during conversion of maize starch into a GCWS starch

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using hot, aqueous ethanol, the A-type crystallites were also changed into V-type crystallites. However, Jivan et al. (2014) found that the alkaline, aqueous alcohol method reduced the crystallinity of a B-type starch without changing the crystalline polymorph. Both Lu et al. (2013) and Jivan et al. (2014) reported significant increases in solubility after conversion into GCWS products. Majzoobi et al. (2015a) found that the solubility of GCWS maize starch in a dilute acetic acid solution increased (as it does for the classic pregelatinized starch), and the dispersion viscosity decreased with increasing concentration of acetic acid. They attributed the increase in solubility to acid-catalyzed depolymerization, which for reasons presented earlier, this reviewer is skeptical of. They also reported that acetic acid increased the turbidity of GCWS gels and that its gels were not as soft and had reduced viscosity, consistency, and cohesiveness as compared to gels of the classic pregelatinized starch. Hedayati et al. (2016) reported that the physical properties of room-temperature water dispersions of GCWS maize starch did not change as much with changes in pH as did the classic pregelatinized starch.

2.3 Heat-Moisture Treatment In 1944, Sair and Fetzer reported that, as a result of heat-moisture treatment (HMT), the water vapor sorptive capacity and X-ray pattern of potato starch (a starch with a B-type X-ray diffraction pattern) became more like that of cereal starches (starches with an A-type diffraction pattern). Because at the time, it was not considered that such changes improved the economic value of the starch, the phenomenon was not further investigated, except by Sair (1967), who much later published a complete description of his results and first used the term HMT. In the 1967 paper, he reported that little or no chemical change in HMT starches occurred at or below 100 C, and that above 100 C, degradation was appreciable; the X-ray patterns of those starches with A-type patterns were unchanged, while those with B-type patterns were changed to (A þ B)-, i.e., C-type, patterns. Compared to the native starches, the capacities of the starches for sorption of water vapor and swelling decreased, gelatinization temperatures increased, and pastes of the starches had changed viscosities, were more opaque, and formed gels with changed properties. He also noted that “The moisture content of the starch is an important factor in effecting this physical change” and that “Water within the granule apparently permits starch molecules, or parts of them to rotate,” allowing molecular rearrangements within granules. Today, HMT is known as a potentially valuable hydrothermal process that consists of heating starch granules at a temperature above the starch’s glass transition temperature (at the moisture content employed) in a closed and sealed vessel. Moisture contents employed generally have been adjusted to from 10% to 40%. Processing temperatures generally range from 84 to 140 C,

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with most investigations using temperatures above 94 C. Treatment times vary from 1 to >24 h. This chapter focuses on changes effected by this traditional method. HMT is by far the most studied method of physical modification of starch. The large literature on HMT has been reviewed by Jacobs and Delcour (1998), Hoover (2010), Zavareze and Dias (2011), and BeMiller and Huber (2015), each of which have provided details on specific changes in specific starches under specific conditions and which, along with original papers, should be consulted for those details. Because of the several variables (type of starch, moisture content, temperature, time of heating) using the conventional method alone, there is for all practical purposes, no limit to the conditions of HMT. For that reason, as pointed out in the previous reviews, when a specific attribute of a treated starch is compared to that of the native starch, increases, decreases, and no change are likely to have been reported. Rather than re-report the specific observed changes, this section focuses on general changes that have been proposed to take place in granule structures and why. Before that, however, it can be generally stated that structural changes occur in both crystalline and amorphous regions of granules and that the contradictory results occur because effects are a function primarily of the type of starch, its moisture content, and the temperature to which it is heated, which can vary considerably. Changes that are generally agreed upon are that starches with A-type crystallinity undergo no change in the type of crystallinity upon HMT (although there may be an increase in X-ray diffraction intensities), that starches with B-type crystallinity have their crystallinity changed to a C-type (partial conversion) or completely to A-type crystallinity with an accompanying decrease in X-ray diffraction intensities (although not by all moistureetemperature combinations), and that starches with C-type crystallinity have their crystallinity changed (partially or completely) to a greater proportion of the A-type crystallinity, with the B- to A-type crystallinity conversion being favored by higher temperatures and moisture contents. There is also general agreement that, even at the relatively low-moisture contents of HMT processes, the relatively high temperatures allow increased mobility of both starch polysaccharide chain segments and helical structures in both amorphous and crystalline regions of granules. To explain the decrease in the degree of crystallinity often observed in B-type starches, Hoover and Vasanthan (1994) proposed that adjacent helices move apart owing to the rupture of water bridges, resulting in less perfect ordering. The proposed mechanism of Vermeylen et al. (2006) involves disturbances in lamellar structures wherein double helices move laterally within high-density lamellae and also along their helix axes during the B to A transition. Vermeylen et al. (2006) and others (Chung et al., 2009; Li et al., 2011; Jiranuntakul et al., 2012; Kim and Huber, 2013) observed thermal degradation of certain starches at temperatures of 120e130 C, while others

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(Varatharajan et al., 2011; Ambigaipalan et al., 2014) did not. Noting that the thermal cleavage of glycosidic linkages coincided with an increase in granule crystallinity, Vermeylen et al. (2006) suggested that such cleavage resulted in greater mobility of released segments, allowing them to align with other segments or structures to form larger and/or more perfect crystallites. To explain the increase in the degree of crystallinity often observed in A-type starches, it has been logically proposed that the thermal energy and the plasticizing effect of water molecules allows double helices to move within crystallites so as to form more ordered and closely packed structures; or to put it in another way, HMT disrupts the least-stable structures and allows the growth and/or more perfect alignment of the more stable native crystallites (Hoover and Vasanthan, 1994; Jacobs and Delcour, 1998). Possibly related to the increases in crystallinity are increases in the onset (To), peak, and conclusion (Tc) gelatinization temperatures and a broadened phase-transition temperature range (Tc  To) (owing to a greater increase in Tc than in To) often, but not always, observed. However, these effects have been attributed to changes in amorphous regions of granules, viz., increased amyloseeamylose and amyloseeamylopectin associations and amyloseelipid complexes, the latter in cereal starches (Hoover, 2010; Zavareze and Dias, 2011). Often found after HMT are decreases in swelling power and leaching of amylose from the swollen granules, with the reductions increasing with increasing moisture content and temperature during treatment. Reductions in granule swelling and amylose leaching have been attributed to increased disruption of crystallites, increases in crystallinity, polymorphic B / A transitions, formation of amyloseelipid complexes, and/or changes in amylosee amylose and/or amylopectineamylopectin chain interactions (Hoover, 2010). Varatharajan et al. (2011) concluded that structural reorganizations (changes in chain interactions) predominated at lower temperatures (80e100 C), while A / B polymorphic transitions occurred at higher temperatures. Almost universally, HMT starches have increased RVA pasting temperatures, decreased peak viscosities and breakdown, and increased hot-paste viscosities, giving the starch pasting characteristics similar to those of a lightly cross-linked starch (Hoover, 2010; BeMiller and Huber, 2015). Watcharatewinkul et al. (2010) pointed out that while HMT starches generally possess the improved heat and shear stabilities characteristic of cross-linked starches, the starches also exhibit reduced granule swelling and viscosity development, which may require greater use level; but the greater use level may result in increased body. Contents of slowly digestible (SDS) and RS starch and the effects of physical treatments on them are of considerable interest. HMT results in both disruption of some native structures and formation of new, more-ordered structures with relative amounts of these processes depending on the specific starch, its moisture content, and the temperature and duration of

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treatment. Both increases and decreases in SDS and RS have been reported. Assays to determine the relative digestibilities of HMT starches have used the method developed by Englyst et al. (1992) or a modification of it, but when the Englyst et al. method is applied to starch, it measures relative contents of SDS and RS in raw starch. Because the preparation of most food products for commercial sale and consumption involves at least one cooking step, heating under similar conditions should be used in the assay so that thermostable SDS and RS fractions are measured. In fact, Qi and Tester (2016) state that “when starches are annealed or heat-moisture treated, gelatinization should be avoided” because gelatinization “accelerates amylase driven hydrolysis.” They were referring to the processes themselves, but the same great increase in digestibility is true of gelatinization after a hydrothermal or other physical treatment. The majority of those who have studied this aspect reported that HMT starches contain slightly to moderately more thermostable SDS and/or RS contents (Kweon et al., 2000; Chung et al., 2009, 2010; Gu¨zel and Sayar, 2010; Sui et al., 2011; Lee et al., 2012). Kim and Huber (2013) found that uncooked, native potato starch had total (i.e., thermostable and nonthermostable) SDS and RS contents of 2.0% and 93%, respectively, and that the same starch heated 3 h at 120 C increased in SDS content as the moisture content of the starch increased, reaching a maximum content of 22%e23% at 20% and 25% moisture, while the maximum RS content (83%) was in the HMT starch treated at 15% moisture. However, when the same native and HMT potato starches were cooked, the control had SDS and RS contents of only 1.6% and 5.9%, respectively. The thermostable SDS content increased as the moisture content of the starch being heated increased, but reached a value of only 6.4% at 25% moisture, while the RS content was not significantly different when the starch had moisture contents of 15%, 20%, or 25% when treated, being in the 15.5%e16.9% range. These values for thermostable SDS and RS in the cooked HMT starch (about 27% and 20%, respectively, of those for uncooked HMT potato starch) are more realistic approximations of the values expected for a consumed food product. Using optimized conditions of heating time and moisture content, Hoyos-Leyva et al. (2015) found that HMT Morado banana starch achieved thermostable SDS and RS contents of 12% and 31%, respectively, while the native starch had heat-stable contents of 7.5% and 13%, respectively. Hung et al. (2016) changed the thermostable SDS and RS contents of high-amylose rice starch from 3.7% to 4.5% and from 6.3% to 22%, respectively, of normal rice starch from 14% to 11% and from 6.5% to 24%, respectively, and of waxy rice starch from 13% to 24% and from 10% to 19%, respectively, via HMT. Wang et al. (2016) determined that HMT normal maize starch reached a maximum thermostable SDS content of 19% when the starch was treated at 30% moisture and a maximum thermostable RS content of 14% when the starch was treated at 20% moisture (from respective contents of 6.2% and 2.1% in the native starch), while amylomaize V starch treated at 30% moisture reached maximum contents of

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thermostable SDS of 19% and thermostable RS of 30% (from respective contents of 7.4% and 6.6% in the native starch). They concluded that the amylomaize starch was more susceptible to HMT (because of its B-type crystallinity) than was the normal maize starch (A-type crystallinity). Not considered in the thermostable RS analysis is that the cooked starch in some products may undergo retrogradation rather rapidly, producing RS; but the assays used (employing different heating conditions (mostly different times)) are indications of relative increases in thermostable SDS and RS. Addition of lauric acid to maize starch before HMT resulted in the formation of amyloseelauric acid V-type complexes. The HMT product contained a maximum of 10% thermostable SDS and 18% thermostable RS when the moisture content of the treated starch was 50% (as compared to 5.0% and 6.6%, respectively, in the native starch) (Chang et al., 2014). When the moisture content of the treated starch increased from 10% to 50% the RVA pasting temperature increased. When the moisture content of the treated starch increased from 10% to 30%, the peak viscosities, breakdown, and setback decreaseddthe increase in pasting temperature and the decrease in peak viscosity being attributed to reduced granule swelling owing to the presence of amyloseelauric acid complexes. However, when the moisture content of the treated starch increased from 30% to 50%, peak viscosities, breakdown, setback, and final viscosities increased, with setback increasing dramatically. It was concluded that the dramatic increase in setback resulted from formation of new amyloseelauric acid complexes from noncomplexed lauric acid molecules, many of which arose from dissociation of the original complexes during the heating process. Based on earlier findings of Brumovsky and Thompson (2001) and Lin et al. (2011) that prehydrolysis increased the thermostable RS content, Kim and Huber (2013) and Hung et al. (2014, 2016) reported that when HMT was conducted under slightly acidic conditions, thermostable RS contents increased to 24% for potato starch, to 30%e39% for rice starch, to 40% for sweet potato starch, and to 46% for yam starch. The conclusions were that very mild acidic conditions during HMT promoted limited hydrolysis of amylopectin molecules (primarily at branch points) and facilitated realignment of starch chains to more thermostable arrangements. Sun et al. (2015) conducted HMT of maize starch at pH 4 (different moisture contents and temperatures) and determined the properties of the products but did not determine digestibilities. They reported decreases in swelling power, peak and hot-paste viscosities, and the phase transition temperature range and increases in solubility, gelatinization and pasting temperatures, and gel hardness. The large number of recent papers that reported changes in the properties of various starches effected by HMT offers no new insights as to granular transformations that might occur in the process and are not presented here, although some papers report results of investigations on the effects of different

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experimental conditions (e.g., Sui et al., 2015). Use of alditol solutions in place of water alone as plasticizers was investigated by Sun et al. (2014a,b) and Juansang et al. (2015). The traditional method for HMT, which has been used for most laboratory investigations, is a procedure that would be expensive and difficult to do on a large scale; so alternative methods have been investigated. These alternative methods include microwave heating (Niu et al., 2013) (see Section 3.5 for a discussion of), infrared heating (Ismailoglu and Basman, 2015, 2016), direct steam injection, heating in an aqueous alcohol, direct-vapor HMT, and reduced pressure HMT (BeMiller and Huber, 2015) (see also Section 2.6). HMT has been practiced before, after, and simultaneously with chemical or other modification of starches (BeMiller and Huber, 2015). Dual treatments have been carried out (Klein et al., 2013). Hoover (2010) concluded that HMT starches have good potential to be used as “unmodified” thickeners in processed food products owing to their having temperature, acid, and shear stabilities during cooking similar to those of lightly cross-linked starches. It was found that addition of HMT rice starch to a poor quality rice flour enabled the production of noodles of acceptable quality (Hormdok and Noomhorm, 2007), that a HMT of a poor quality rice flour enabled use of that flour to produce dried and semi-dried noodles with appropriate tensile strength and hardness (Cham and Suwannaporn, 2010), and that noodles could be made from HMT sweet potato starch (Pranoto et al., 2014). However, other results obtained from trial applications in products such as noodles, doughs, bakery products, and pie fillings were both positive and negative. HMT starches have been investigated as ingredients in other food products (BeMiller and Huber, 2015), and HMT has been applied to flours and whole grains.

2.4 Annealing Annealing, another hydrothermal process, consists of holding starch granules in an excess of water (generally >39% w/w) at a temperature that is above the starch’s glass transition temperature and below its gelatinization temperature. The annealing process has been conducted for periods of time ranging from minutes to days. As with HMT, there is a large literature on annealing that has been reviewed previously by Jacobs and Delcour (1998), Tester and Debon (2000), Jayakody and Hoover (2008), Zavareze and Dias (2011), and BeMiller and Huber (2015). Like HMT, the variables are the specific starch used, the temperature, and the duration of heating. (Moisture content is not a variable because the process is conducted in excess water.) Also like HMT, when a specific attribute of a treated starch is compared to that of the native starch, increases, decreases, and no change are likely to have been reported because of the large numbers of starches and conditions that may be employed. Several of the observed property changes are in the same direction as produced by HMT,

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while there are differences in others. The proposed mechanisms have some similar features. There seems to be less consensus (than with HMT) on the changes in starch properties imparted by annealing. One thing there seems to be general agreement about is that annealing increases the onset and peak gelatinization temperatures and decreases the phase-transition temperature range (Liu and Du, 2013; Wang et al., 2014; Wu and Du, 2014; BeMiller and Huber, 2015; Zhang et al., 2015a; Zeng et al., 2015; Liu et al., 2015a, 2016b; Bhattacharjya et al., 2015). The preferred explanation for this change (and others resulting from annealing) is that, because of the high degree of plasticization of the molecules in starch granules by water, the increased temperature increases the mobility of double-helical chain segments, allowing them to improve their alignment, the extent of which is determined by the temperature employed. In other words, the least stable structures within both amorphous and crystalline regions are disrupted, and crystallization, perfection of existing crystallites, and/or increased molecular ordering forms more stable and more homogeneous structures, without increasing the number of double helices (in normal or waxy starch granules) (BeMiller and Huber, 2015). However, annealing of high-amylose starches has been reported to produce new double helices and amyloseeamylose, amyloseeamylopectin, and amylopectineamylopectin associations (Tester and Debon, 2000; Lin et al., 2009; Gomand et al., 2012). Gomand et al. (2012) concluded that for normal and waxy potato starches, annealing resulted in increased crystallite surface stability as a result of relaxation of conformationally strained chain segments at crystallite borders and their migration into amorphous regions. They also concluded that for a high-amylose potato starch, annealing produced additional cocrystallization of amylose and amylopectin chains and crystal thickening (Kiseleva et al., 2005). In addition to annealing affecting the thermal properties of starches via changes in crystalline regions of granules, annealing has also been hypothesized to effect rearrangements of structures within amorphous regions, as indicated by increases in glass transition temperatures (Seow and Teo, 1993; Tester and Debon, 2000). Liu et al. (2009) concluded that annealing primarily increased the helical length of short amylopectin helices, particularly in the amylopectin of nonwaxy starches. Starches are often subjected to annealing conditions during isolation and/or chemical modification. For example, the wet-milling process for isolation of maize starch involves steeping of maize kernels (usually for at least 24 h) at a temperature of 50  2 C, and chemical modifications are usually accomplished in starch slurries of elevated pH values at a similar temperature in solutions of a swelling inhibiting starch to prevent gelatinization under the alkaline conditions. In fact, Krueger et al. (1987) presented evidence indicating that maize starch is indeed annealed during its isolation by the commercial wet-milling process, and Sui et al. (2011) found that the conditions used for hydroxypropylation and cross-linking with phosphoryl chloride

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significantly changed the pasting and paste properties of maize starch in the direction that annealing at neutral pH would. There is generally no change in gelatinization enthalpy (DH), although small increases and decreases have been reported (BeMiller and Huber, 2015), indicating little or no changes in the total amounts of glassy and crystalline structures. Likewise, annealed starches usually retain their crystalline packing arrangements, although a partial C / A transition (which means a partial B / A transition) and a partial C / B transition (which means a partial A / B transition) have been reported (BeMiller and Huber, 2015; Zhang et al., 2015a). Together, these findings suggest that molecular rearrangements occur within both amorphous and crystalline arrangements in granules and that the changes within the crystallites primarily result in their perfection (rather than in the amount of crystalline material or in the crystal type), although increases in relative crystallinities (Yu et al., 2015; Liu et al., 2015a, 2016b; Zeng et al., 2015) and in the size of crystallites have been proposed (Gomand et al., 2012; Vamadevan et al., 2013). Alvani et al. (2014) and Gomand et al. (2012) obtained evidence that in both single- and multistep annealing processes, the extent of change in starch properties is the greatest in the starches with the greatest amounts of structural organization disorder and the least in starches with the least organizational defects. Based on results such as the finding that annealing did not change the swelling power of waxy maize starch, but reduced the swelling powers of normal maize and amylomaize starches, Wang et al. (2014) concluded that “amylose molecules play an important role in the structural reorganization of starch granules during annealing” and proposed that annealing “enhances the long-range interaction of amylopectin clusters by the rearrangement of amylose molecules.” Vamadevan et al. (2013) hypothesized that changes in gelatinization behavior effected by annealing were related to the average distances between branch points within amylopectin clusters interblock chain lengths (IB-CL) (Bertoft et al., 2012) and found that they could classify the 16 starches examined into four categories: (1) a group of starches with unfavorably short IB-CL and presumably restricted movement and alignment of double helices that were least responsive to annealing; (2) a group of starches suggested to have almost ideal IB-CL and external chain lengths that give them crystalline lamellae with few defects so that the crystallites can undergo little improvement upon annealing; (3) a group of starches with low phosphate content and long IB-CL, which gives them greater flexibility and movement of double-helical segments, enabling them to improve their alignment during annealing; (4) a group of starches that also have long IB-CL, but with relatively high-phosphate ester contents, which disrupt the usual crystalline order, giving them the greatest change upon annealing (Muhrbeck and Svensson, 1996; Muhrbeck and Wischmann, 1998). (Groups 1 and 2 seem to contain mostly A-type starches; group 3 seems to contain A- and C-type starches, and group 4 seems to contain B- and C-type starches.)

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As a consequence of the increased alignment of double helices and perfection of crystallites, annealed amylose-containing starches usually have reduced swelling powers, solubilities, viscosities, and pasting temperatures (Liu and Du, 2013; Wang et al., 2014; Wu and Du, 2014; Falade and Ayetigbo, 2015; Zhang et al., 2015a; Liu et al., 2015a,b, 2016b; BeMiller and Huber, 2015), i.e., characteristics of lightly cross-linked starches. Increased water and decreased oil sorption capacities have also been reported (Liu et al., 2015b). The presence of swollen granules in a gel gives the gel increased hardness/ firmness (Chung et al., 2010; Jyothi et al., 2011; Yadav et al., 2013). As with HMT starches and any native or modified starch, the only important data on digestibility are the thermostable RS and SDS values. Only slight to no increases in thermostable SDS and RS contents following annealing, then cooking, have been found (Chung et al., 2009, 2010; Alvani et al., 2014; Liu et al., 2015a, 2016b). Annealing has been conducted as single- or multistep processes, before or following HMT, with starches before isolation (in situ), and before and after modification (BeMiller and Huber, 2015; Zeng et al., 2015). Use of annealed starches and flours in food products has been less studied than has use of HMT starches and flours, with again the main application being their use in preparation of various noodles (Bhattacharjya et al., 2015). It has been reported that annealing of a poor-quality rice flour gives fresh noodles the required soft texture (Cham and Suwannaporn, 2010).

2.5 Heating Dry Starch In a patented process, Chiu et al. (1998) heated starch with <15% moisture at temperatures between 100 C and the temperature at which thermal degradation occurred and produced products with acid, shear, and temperature tolerances similar to those of chemically cross-linked starches. A low-moisture content and alkalinity was found to facilitate the transformation. Use of a fluidized bed is a way to heat starch, but this process has only been investigated with a flour (amaranth) (Gonza´lez et al., 2007a,b). Microwaves are a form of electromagnetic radiation with wavelengths between 1 mm and 1 m and frequencies between 300 MHz and 300 GHz. Dielectric heating occurs when microwave energy is absorbed by water. The basis for microwave heating has been described briefly by BeMiller and Huber (2015). Only microwave irradiation of starches with insufficient water content to effect gelatinization and pasting are considered to result in a physical modification of starches. (The microwave heating that has been used in the preparation of HMT starches is not included in this section.) Thermal effects produced by microwave irradiation are determined by the moisture content of the starch, wattage, and frequency of the microwave source, and duration of the treatment; so a large number of possible treatment conditions is possible.

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Fourteen papers related to microwave radiation treatment of starches were reviewed by Bras¸oveanu and Nemtanu (2014), but only one paper included in the review treated starches with less than 15% moisture. The other papers would be considered to involve microwave heating under HMT conditions. A major difference between microwave and conventional heating processes is that the latter are done in closed containers, while microwave heating is seldom done that way; as a result, the moisture contents of microwave-heated samples drops during the treatment as indicated by the results of Szepes et al. (2005). They irradiated (450 W, 15 min) maize (6.84% moisture) and potato (9.66% moisture) starches and found that their moisture contents were reduced to 0.00% and 0.07%, respectively; so when microwave irradiation is applied to starches of higher moisture contents, it is not known for how long the starch was subjected to HMT conditions and for how long the moisture content was below that needed for HMT. According to the results of Szepes et al. (2005), for potato starch, the degree of crystallinity increased as a result of microwave irradiation from 55% in the native starch to 66% and the crystal packing arrangement changed from a B-type to an A-type. (As a result of oven heating, the crystallinity of the potato starch dropped to 43%.) For maize starch, the original degree of crystallinity (85%) decreased to 30% (microwave heating) and 33% (oven heating). Szepes et al. (2007) irradiated (900 W, 15 min) the same starches and found that, for potato starch, crystallinity decreased and swelling power and swelling capacity increased, while the changes in maize starch were insignificant. Microwave irradiation of cassava starch of <18% moisture resulted in a slight increase in peak viscosity after a 5-min treatment, and a considerable decrease after 10- and 15-min treatments (Colman et al., 2014).

2.6 “Osmotic Pressure Treatment” BeMiller and Huber (2015) analyzed the results of the three studies supposedly reporting changes in starch properties effected by osmotic pressure treatment (OPT) and concluded that the changes were the result of heating the starch in a kosmotropic solution which restricted granule swelling and gelatinization, so OPT is actually another type of hydrothermal treatment.

3. “NONTHERMAL” TREATMENTS BeMiller and Huber (2015) pointed out that, in the treatments categorized as being nonthermal because heat is not applied intentionally, the force applied may often generate intense heat in local areas that can heat the entire system.

3.1 Ultrasonic Treatment Treatment with ultrasound is a process that generates areas of intense local heating and high-shear stresses. (The mechanism behind generation of these

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areas has been described briefly by BeMiller and Huber (2015).) Interest in the effects of ultrasound on starches arises from the fact that low-frequency (16e100 kHz; power, high-intensity) ultrasound may be used in food processing (Ashokkumar, 2015). Ultrasonic treatment of starches is carried out in aqueous systems. Variables are the nature of the medium (specifically its vapor pressure and surface tension), natures and concentrations of dissolved gases (the atmosphere above the aqueous suspension being sonicated), temperature, treatment time, starch concentration, power, frequency, and amplitude of the ultrasound. Different treatment conditions applied to the same starch and different starches treated with the same conditions give different results, and the degree of change increases with treatment duration. When conditions are such that changes occur, they often include damage to the granule surface that may include erosion, pitting, and cracking, increases in swelling power, solubility, and gel clarity, hardness, and adhesiveness, and decreases in paste and gel viscosities and the consistency coefficient (k) of pastes (Majzoobi et al., 2014; Hu et al., 2014a,b; Amini et al., 2015; BeMiller and Huber, 2015; CarmonaGarcia et al., 2016), although opposite effects have been reported in some cases. (In particular, the results for swelling power and solubility depend on the temperature of measurement, since granules are damaged and weakened; thus, they may swell more at a low temperature but disintegrate more easily as the temperature is raised.) These changes are consistent with a general weakening of granule structures. Ultrasonic energy is generally insufficient to disrupt, or even to distort, granule crystallites, but it can effect changes in amorphous regions and perhaps disrupt and/or destroy double-helical order in both amorphous and crystalline regions. Depolymerization of starch polysaccharide molecules is also a possibility (see below). B-type starches may be more susceptible to ultrasonic treatment than are A-type starches but that has not been established (BeMiller and Huber, 2015). Carmona-Garcia et al. (2016) reported that the larger granules of plantain starch were more damaged by ultrasonication than were the smaller granules of taro starch. Ultrasonic treatment of starch pastes does not produce a physically modified starch as usually defined. However, a brief consideration of it gives insight into one change that may occur during ultrasonic treatment of granules, viz., depolymerization. Treatment of starch pastes with ultrasound effects depolymerization of the starch polysaccharide molecules and large and fairly rapid decreases in paste viscosity (BeMiller and Huber, 2015). Both generation of OH radicals and mechanical effects have been proposed as being responsible for the chain cleavages. Undoubtedly, the same processes, especially the generation of OH radicals, occurs in starch granules and is the cause of the reductions in paste viscosities and consistency coefficients. However, decreases in paste viscosity during ultrasonic treatment have also been attributed to disruption of swollen granules (Chung et al., 2002) and to disintegration of supermolecular aggregates (Seguchi et al., 1994).

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Ultrasound has also been used for reasons other than physical modification as defined here. Because ultrasonic treatment weakens the granule structure, ultrasound has been applied to facilitate both acid- and enzyme-catalyzed hydrolysis of starch and to produce porous starch granules using amylases (BeMiller and Huber, 2015). There has been much recent interest in the production of starch nanocrystals/nanoparticles via sonication.

3.2 Milling Use of mechanical force is another method to change the characteristics of starches, but mechanical force can produce relatively high temperatures at the point of impact, so as pointed out by BeMiller and Huber (2015), although milling is usually categorized as a nonthermal process, changes to the granular and/or molecular structures are likely caused by both thermal and mechanical energies. For example, the temperature in ceramic, rolling ball mills usually used in laboratory research increases with milling time and the temperature at the point of impact can be quite high. Starches have been milled with a variety of types of mills, including different types of ball mills. A review of the effects of milling has been published (Li et al., 2014a), but it mostly covers the milling of flours (not covered here). In recent years, milling of starch has often been referred to as micronization, probably because pharmaceutical micronizing mills have been used. However, micronization means reduction in particle size (comminution), and when starch granules are milled, cracks may appear on the granule surface, granules may become deformed (in addition to fracturing), and fragments may agglomerate. Changes in starch granules as a result of milling include granule damage, increases in water vapor sorption, granule swelling, solubility, and susceptibility to the action of amylases, and reductions in paste viscosity, gel elasticity, onset, peak, and conclusion gelatinization temperatures, change in enthalpy during gelatinization, double-helix content, crystallinity, crystallite perfection, and granule birefringence. Depolymerization, particularly of amylopectin has been observed and is undoubtedly a factor in the paste viscosity reduction. In terms of granule damage, depending on the type of starch, the type and duration of milling, and the moisture content of the starch, various ratios of undamaged granules, damaged granules, and fragments of damaged granules and various extents of loss of crystallinity may occur. When enough amorphous granules and granule fragments are produced, the starch has the characteristics of a cold-water-soluble starch (Arai et al., 1989; Wang et al., 2005; Huang et al., 2005; Huang et al., 2006a,b), undoubtedly owing to extensive granule damage and loss of crystallinity. Recent results confirm what was previously known about the effects of milling of starches, i.e., that it results in partial fragmentation of granules (time dependent) (Martı´nez-Bustos et al., 2007), that smaller particles so

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produced tend to agglomerate (Ren et al., 2010), that there is a gradual loss of crystallinity until amorphous particles are formed (Huang et al., 2008; Ren et al., 2010; Li et al., 2014a; Zhang et al., 2015b; Santosa et al., 2015), that the products have increased cold water solubility (Martı´nez-Bustos et al., 2007; Huang et al., 2008; Zhang et al., 2010; Li et al., 2014a) and produce gels of greater clarity (Huang et al., 2008; Li et al., 2014a) and with a larger loss tangent value, i.e., that are more liquidlike (Kochkina and Khokhlova, 2016), that the products produce gels with decreased viscosity (Martı´nez-Bustos et al., 2007; Zhang et al., 2010; Li et al., 2014a; Kochkina and Khokhlova, 2016) and storage and loss moduli (Kochkina and Khokhlova, 2016), and that the sizes of the starch polymer molecules, particularly of amylopectin, are reduced (Zhang et al., 2010). Martı´nez-Bustos et al. (2007) determined that the extent of loss of crystallinity was a function of the moisture content of the starch, and Zhang et al. (2010) found that there was an optimum moisture content for destruction of granular, crystalline, and molecular structures. Huang et al. (2008) confirmed that different starches were affected by milling to different degrees. Waxy sorghum (Craig and Stark, 1984) and waxy maize (Han et al., 2002) starches were more susceptible to damage by milling than were the amylosecontaining sorghum and maize starches. Zhang et al. (2015b) milled maize starch in anhydrous ethanol and found that the amount of starch in the vessel made a difference in the outcome. The average granule diameter increased (owing to granule flattening) when the load was low but decreased at high loads. The loss of crystallinity was greatest at low loads. All products produced less viscosity than did the native starch. Knowing the effects of milling on starches are undoubtedly more important for understanding the transformations that can occur during grain milling than for any potential manufacture of a physically modified starch. For example, fragments of amylopectin produced during milling have been related to stickiness in food products (Miklus and Hamaker, 2003), and flours for different bakery products are milled to different extents to provide the amount of granule damage desired for, or that can be tolerated in, the specific application.

3.3 High-Pressure Treatment There are two basic types of high-pressure treatments: (1) a static type called ultrahigh pressure (UHP; high-hydrostatic pressure (HHP)) treatment as might be used for food processing; (2) use of homogenizers (also used in food processing) that produce turbulence, high shear, and cavitation as a result of forcing a starch slurry through an orifice under high pressure. The two types of treatment bring about different changes in the starch.

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3.3.1 Ultrahigh Pressure UHP treatment consists of subjecting an aqueous slurry of starch granules to a pressure exceeding 400 MPa (BeMiller and Huber, 2015). Variables are the type of starch, its concentration, and the pressure, temperature, and duration of treatment. The universal effect is partial or complete gelatinization of the starch with maintenance of the granular form, in other words, formation of CWS starch to some degree. (Complete gelatinization is not covered here because the result is gel formation, rather than recovered starch.) Gelatinization of granules of a starch effected by UHP occurs over a range of pressure because susceptibility to gelatinization varies from granule to granule. The higher the temperature, the lower is the pressure required for gelatinization of a granule and vice versa, and under any set of conditions, the degree of gelatinization is time dependent. Finally, the degree of gelatinization at any combination of pressure, temperature, and time decreases as the slurry concentration increases. Therefore, it is possible to obtain different degrees of gelatinization by manipulating the combination of slurry concentration, treatment duration, temperature, and pressure (Douzals et al., 1996, 2001; Douzals et al., 1998; Rubens and Heremans, 2000; Kawai et al., 2007). There are several pieces of evidence that A-type starches are more susceptible to UHP treatment than are B-type starches, but BeMiller and Huber (2015) pointed out that there must be more to the story than the type of crystallite packing, for waxy maize and waxy rice starches were reported to be more susceptible to pressure than were the amylose-containing maize and rice starches (all four being A-type starches). Two explanations have been offered: (1) Susceptibility to pressure is a function of how perfect is the packing of the starch polysaccharide molecules in granules (Rubens and Heremans, 2000); (2) Gelatinization may be a two-stage process in which the first stage is a reversible hydration of the amorphous phase and the second stage is an irreversible melting of crystallites (Douzals et al., 1996, 1998; Rubens et al., 1999). Other possible explanations might be that (as pointed out in Section 2.4) amylose may play a role in the structural organization of amylopectin, or that (also pointed out in Section 2.4) different gelatinization behaviors may be related to different amylopectin interblock chain lengths, it being known that the amylopectin in waxy maize starch is structurally different from that in normal maize starch. There has been much recent activity in determining the effects of UHP/ HHP on starches, in part because UHP treatment, which can kill bacteria and inactivate enzymes, has been investigated as a nonthermal procedure for food processing. In the investigation of Yang et al. (2016), a 1:1 (w/w) starchewater slurry was treated with UHP and concurrently analyzed by small-angle X-ray scattering (SAXS). It was found that, as the pressure increased, the peak area broadened and the area of the SAXS peak corresponding to the lamellar phase decreased, indicating both gelatinization and the penetration of water into

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crystalline regions. Earlier, Yang et al. (2013), in a similar study, found that amylopectin in a UHP-treated waxy maize starch recrystallized when the pressure was released. Others (Li et al., 2015, 2016) found that UHPgelatinized starches undergo retrogradation when stored at room temperature, even after being oven- or freeze-dried. Reports of both increases and decreases in swelling power, viscosity, storage (G0 ) and loss (G00 ) modulii, and pasting temperature have been reported recently (not referenced). (There should be no gelatinization or pasting temperatures in completely gelatinized and not retrograded products.) Such discrepancies probably arise from the fact that UHP treatment can produce partially gelatinized products, i.e., products in which some granules are gelatinized (while remaining intact), while others are not, if sufficient pressure or time for complete gelatinization is not applied. It is the more easily gelatinized granules that lose their crystallinity first. The presence of nongelatinized granules in gels made by UHP treatment will increase their viscosity and gel strength, so there have been reports that G0 and G00 values first increase, then decrease as the pressure or treatment time is increased (Guo et al., 2014, 2015). Inorganic salts inhibit gelatinization by UHP (Yu et al., 2016), just as they inhibit thermal gelatinization. The loss of crystallinity (gelatinization) effected by UHP treatment (Guo et al., 2015; Liu et al., 2016a) makes the granules more susceptible to attack by amylases (Mu et al., 2015; Yu et al., 2016). Based on increased pick-up of batters, a higher moisture content, increased crispness, and reduced oil content of crusts, Zhang et al. (2014b) found that addition of UHP-treated mung bean starch to the batter before deep-fat frying improved the quality of deep-fried battered foods.

3.3.2 High-Pressure Homogenizers and Jets Variables for this treatment are, at least, type of starch, pressure, and the number of cycles/passes. Changes in the starch that have been reported are granule deformation and fragmentation, partial gelatinization, increases in the onset, peak, and conclusion temperatures of gelatinization, and loss of crystallinity. All changes increased with increasing pressure (BeMiller and Huber, 2015). The increases in gelatinization temperatures are probably the result of the more easily gelatinized granules being gelatinized first. Earlier investigations used high-pressure homogenizers and the microfluidizer. High-pressure homogenization has been used to prepare starchefatty acid complexes (Meng et al., 2014a,b, 2015). In the high-speed jet homogenizer, a starch suspension is forced through a small orifice, producing an ultrahigh-velocity jet that impinges a target (Fu et al., 2015a). When a dispersion of cassava/tapioca starch was subjected to high-speed jet treatment, the particle size increased (Xia et al., 2015), which was attributed to granule aggregation brought about by surface damage

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and gelatinization that results in the granules sticking to each other, as found when the same starch was subjected to microfluidization (Kasemwong et al., 2011). It has also been reported that high-speed jet treatment destroys both crystalline and granular structures, resulting in a partially gelatinized product, with the percent of amorphous material increasing as the pressure increases (Xia et al., 2015; Fu et al., 2016). The treatment also results in decreases in molecular weights of the starch polysaccharide molecules (primarily amylopectin) and their root mean square radii of gyration and a loss of viscosity (Fu et al., 2015b). Solubility increased and the value of the storage modulus of resulting gels decreased (Fu et al., 2015a). Use of water to which 0.24% ethanol had been added in high-speed jet homogenization treatment of maize starch resulted in greater loss of granule crystallization than when pure water was used, a result that was attributed to greater cavitation (Liu et al., 2013).

3.3.3 Other Another technique involving pressure that has been applied to starches is instantaneous controlled pressure drop (DIC) (BeMiller and Huber, 2015).

3.4 Pulsed Electric Field Pulsed electric field (PEF) technology as applied to food processing (Nafchi et al., 2012) has advantages of low processing temperatures, continuous processing, short treatment times, and uniform treatment intensity and has been successfully applied to nonthermal pasteurization of liquid foods (Zeng et al., 2016). Results of recent investigations have confirmed that the effects on starch granules are a function of electric field strength and time (at a constant field strength), with the field strength predominating (Han et al., 2012b) and that PEF treatment results in damage to granules and to their crystalline structures (Han et al., 2009a,b, 2012b; Zhang et al., 2011; Hu et al., 2012; Zeng et al., 2016), making the starch more susceptible to enzyme-catalyzed hydrolysis (Hu et al., 2012; Zeng et al., 2016). From a SAXS study, Zeng et al. (2016) found that the lamellar peak was reduced, an indication of destruction of crystallites. PEF treatment also reduces the molecular weight of the amylopectin molecules (Han et al., 2012a; Zeng et al., 2016). The mechanism of the action of PEF on starch granules is unknown, but the limited information available indicates that the changes are similar enough to those produced by ultrasound radiation and the high-speed jet (the first of which is known to produce cavitation and the second of which is assumed to involve cavitation, at least in part) to suggest that cavitation may be at least partly involved. PEF-assisted chemical modifications have been investigated.

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3.5 Freezing and Thawing and Freeze-Drying Szymo nska and Krok (2003) and Szymo nska and Wodnicka (2005) found that repeated freezing and thawing of potato starch granules of 13% moisture resulted in surface erosion and increased specific surface area, total micro- and mesopore volume, and mean pore diameter (the latter two from undetectable to detectable). Zhang et al. (2014a) reported that freeze-drying significantly increased the amylase susceptibility of potato starch granules (polymorphic B-type granules without surface pores and channels that would allow access of enzyme molecules to the granule interior) but did not increase the susceptibility of maize starch granules (polymorphic A-type granules with surface pores and channels that do allow access of enzyme molecules to the granule interior). They also observed that freeze-drying damaged the surfaces of potato starch granules, but not of maize starch granules, and disrupted and reduced the amounts of both short- and long-range molecular order of potato starch granular amylopectin more than that of maize starch granular amylopectin. Especially the latter observations indicate that there is a difference in the effect of freeze-drying on B-type and A-type granules, and they proposed that the low temperature of drying during freeze-drying resulted in greater chain rigidity (as compared to that found during oven drying) and structural reorganization (at both nanometer and micrometer length scales) during water removal from the B-type starch.

4. PHYSICAL TREATMENTS THAT PRODUCE CHEMICAL CHANGES These treatments are physical treatments and do not involve added reagents, but this author does not consider that they produce physically modified starch because the chemical structures of the starch polysaccharide molecules are changed (BeMiller and Huber, 2015). Gamma irradiation produces low-molecular-weight carbonyl compounds and organic acids via generation of free radicals, so the starch polysaccharides are definitely degraded. Ultraviolet irradiation effects cross-linking, oxidative photodegradation (in the presence of air), and depolymerization via generation of free radicals. Cold plasma (glow discharge plasma, low-temperature plasma) also generates free radicals on starch polysaccharide molecules and may produce some cross-linking.

REFERENCES Alvani, K., Tester, R.F., Lin, C.-L., Qi, X., 2014. Amylolysis of native and annealed potato starches following progressive gelatinization. Food Hydrocolloids 36, 273e277. Ambigaipalan, P., Hoover, R., Donner, E., Liu, Q., 2014. Starch chain interactions within the amorphous and crystalline domains of pulse starches during heat-moisture treatment at different temperatures and their impact on physicochemical properties. Food Chemistry 143, 175e184.

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