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Some aspects of developmental trends in chemical modification and transformation of starch: products preparation and potential applications.

Introduction

Starch is the most important reserve carbohydrate and one of the most abundant organic chemicals on earth (Jane et al, 1994). It could be found in the leaves of green plants in the plastids, more importantly, starch is synthesised in any plant and stored abundantly in seeds, grains, roots and tubers of many plants. The principal commercial sources of starch are roots and tubers (cassava, potato and arrow root), cereals (maize, rice and wheat), fruits (banana, plantain and breadfruit) and pith of plants (sago palm). Starch is not region-selective, as a result all countries are endowed with the possibility of abundant supply of starch provided there is vegetation. The abundance of starch spurs scientists to search for food related and non-food applications of starch. Unfortunately, starch in its native form has limited industrial applications whether in food and non-food industries. Interestingly, the properties of native starch can be altered using minor modification such that positive attributes can be greatly improved and/or negative characteristics diminished by slight and relative simple modifications (BeMiller, 1997). The primary reasons for starch modification have been tabulated by BeMiller (1997). Yaacob et al. (2011) reportedthat from 1969 to 2001, there have been more than 23,000, scientific articles in food science and technology. Therefore, it is vivid that a comprehensive review of chemical aspect of starch modification for food and non-food applications cannot be accomplished within the limited space of this article. However, it is important to make a list of some texts notably, Wurzburg (1986); Mentzer et al. (1984) and Rutenberg (1980) that could furnish pioneer technology details and lead references on starch modification. Beyond the content and period of the cited references, a lot of research progress has been reported in many articles. Therefore, it would be informative and educative to pool some of these articles into one piece in a review.

This study aimed at brief review of developments in chemical method of starch modification and transformation for both food and non-food applications, and some of the manipulations employed to enhance or conferred the product-value-addition. This will motivate readership to seek further knowledge on details of technological accomplishment for preparation of value added products and organic-compliant substitute from starch.

Chemical method of starch modification. Inherent properties of native starch could be changed for industrial compilation and the basic modification types practiced in the US is presented in Table 1. However, of all the methods listed, chemical process offers the highest number of modification opportunities, hence focussed herein.

The characteristics cooks of native starch, irrespective of the source are undesirable for many industrial applications. This is because, they are susceptible to retrogradation, syneresis, unwanted viscosity as a result of their inability to withstand the typical industrial processing conditions such as extreme temperature, pH, high shear rate, and freeze thaw variation encountered during manufacture of food and allied products. Some of the positive attributes conferred by modification of starch in foods are reported in a review by Daramola and Falade (2006). Since this article captions chemical method of starch modification, it is necessary to explain in brevity the chemical aspect of starch modification outlined in Table 1.

Acid thinning. Acid thinning involves hydrolysis of starch by applications of both organic and inorganic acids to break starch molecular chains namely: amylose and amylopectin. Some of the inorganic acids used are hydrochloric acid, sulphuric acid and phosphoric acid (Whistler et al., 1984), and organic acids anhydride used include succinic acid anhydride, acetic acid anhydride, (Wang eta/., 1997; Agboola et al., 1991).

Oxidation. This involves the use of reagents such as sodium hypochlorite, potassium permanganate and sodium chlorite. All can be used in both acidic and alkaline medium (Kweon et al., 2001).

Derivatisation. As listed in Table 1, derivatisation can be divided into 2 sub-groups:

(1) Starch monosubstition in nature of etherification and esterification as well as forms of polymer grafting. Any substitution of the hydroxyl moiety on the molecular chains of starch by alkoxide and alkanoate is referred to as etherification and esterification, respectively. Examples of this type of modification is acetylation of starch (Daramola and Adegoke, 2007; Lawal, 2004).

(2) Another example is starch citrates (Agboola et al., 1991) and propyl-etherified starch (Teramoto et al., 2003). In addition, starch ethers could be cationised or anionised, a described procedure has been reported by Kweon et al. (1997). Also, preparation of starch succinates by reactive extrusion has been reported by Wang et al. (1997). Starch can be modified to carry charges positive (cation) or negative (anion) or both. When cation and anion are combined on a single starch it is termed amphoteric starch. Amphoteric starches, could be etherified, esterified, or grafted starches which simultaneously, contain cationic groups and anionic groups. Generally, the cationic groups could be amino, ammonium, sulphonium or phosphonium. Presently, tertiary amino groups or quaternary ammonium groups are the most important industrial positive group (moiety). Anionic groups are usually carboxyl groups, phosphate groups or sulphonate groups (Yang et al., 2007). A cheap reagent such as alum (double salt) has been used to modify starch as reported in an earlier study by Daramola and Aina (2007).

Starch cross-linkage. Starch cross-linkage could be accomplished using bi-functional compounds, which are capable of reacting simultaneously, with two or more hydroxyl groups in starch forming cross-linked starch. Examples of such compounds are phosphorus oxychloride, epichlorohydrin and sodium trimetaphosphate used mostly for food grade cross-linked starches (Radley, 1976). Cross-linkage facilitates intra and intermolecular bonds at random locations in the starch granules for their stabilisation. As a result of the intra and intermolecular linkages, paste of cross linked starch are more resistant to shear, and acidic conditions (Majzoobi et al., 2009). All the starch derivatives have amazing technological applications as illustratively summarised in Table 2.

Fundamental factors affecting starch modification using chemical method. The factors affecting starch modification using chemical method can be divided into three classes:

(1) The primary factors which include processing variables such as temperature, solid solvent ratio, stirring rate, concentration of solvent, time and pressure.

(2) The secondary factors are type or chemical nature of solvent (e.g. acid, alkanol, alkaline, esterification and etherification reagents). Nature of chemical is majorly affected by the chemical or inherent functional groups. Added to these methods of processing are: use of reactive extrusion (Wang et al., 1997), microwave heating (Shogren and Biswas, 2006) and use of ionic liquids among others. Also in this group, the use of catalysts, initiator, promoters and modifiers.

(3) The tertiary factors involves: high technology, involving manipulation of active site in molecular chains and granules of starch and combination of chemical method for modification of starch. For example, it is possible to combine cross-linkage method with ionic method in one of the modification. Another one is chemical modification of starch in solvent free environment (Aburto et al., 2005).

Instrumentation in chemical method of starch modification. The commonest analytical instruments for characterisation of modified starch accomplished using chemical method are viscosity analyses (RVA). RVA profiles, the pasting characteristics such as peak viscosity, set back viscosity, final viscosity, pasting temperature and time of starch. Their technological interpretation signifies the extent of conferred modification (Meadow, 2002; Delcour et al., 2000; Deffenbaugh and Walker, 1990; 1989).

Similarly, the thermal characteristics could be studied using a differential scanning calorimeter (DSC). Some of the evaluated parameters are: onset temperature, peak temperature, completion temperature and gelatinisation enthalpy. With the understanding that the two molecular chains, amylose and amylopectin are folded into granule and the fact that there are no two starch granules that are exactly same in terms of size, shape and configuration. Therefore, starch morphological characteristics are studied using scanning electron microscopy (SEM) (Jane et al., 1994; Jing-ming and Sen-lin, 1990). Besides morphological characteristics, SEM gives insight to location and conformation of hilum that contain the cavity through which chemical moiety passes into the amylose and amylopectin chains. Other importance of SEM on starch granules involves revelation on integrity of starch granule to hydration and dehydration and vapour pressure. It also reveals relationship between starch granules disruption and physicochemical properties of starch.

Structural characteristics of starch could be evaluated using fourier transform-infrared (FTIR). FTIR is important in revealing degree of substitution and the substituent chemical moiety. Other analytical instruments include; x-ray diffraction analytics, and nuclear magnetic resonance. Convectional tests carried out on starch are paste clarity, swelling capacity, solubility, tensile strength, stress, elongation at break, and tear strength. Other assessed parameters are apparent viscosity, freeze thaw stability, gel strength and thermal stability analysis (Chung et al., 2004).

Preparation of starch noodle. Besides the additives application of starch in food preparations, where it could function as binding or adhesion, clouding and dusting agents. Other functions include flowing aid, antistaling and gelling agents. Starch is also used to glaze, mould, shape and thicken during food preparations. Examples of food in which starch performed the itemized functions have been tabulated by Ihekoronye and Ngoddy (1985), as reviewed by Daramola and Falade (2006).

Beyond the uses of speciality starch as additives as previously explained, food can be produced using whole starch. Noodles can be produced from starch. Although, starch is void of gluten, pregelatinised starch is used as binder mixed with ungelatinised starch to facilitate extrusion or sheeting to produce noodles. Basic protocol for the commercial production of bihon-type noodles can be found in literature (Tam et al., 2004).

Food noodles can be prepared from flour or starch, provided the product has noodles qualities that are defined by visual attributes, cooking and eating qualities, transparency and glossiness. In cooked starch noodles, the noodles should remain firm and not sticky on standing after cooking. These characteristics have positive correlation to noodles texture and mouth feel acceptance.

Starch transformation. Precursor of starch sweeteners and other organic chemicals. Sweetener can be produced from starch using varieties of acids (Daramola and Falade, 2006). Some of the obtained products are glucose, maltose, dextrin and maltose. These sweeteners are superior on the basis of dearth of physiological abnormalities associated with sweetener of synthetic origin.

Starch sweeteners have been produced from starch using acids such as hydrochloric acid, sulphuric acid and phosphoric acid. Each of the acids has its advantages and limitation. For instance, hydrochloric acid is an inorganic acid and it hydrolyse starch and the hydrolysis is complete in comparison to hydrolysis carried out by using phosphoric acid. Nevertheless, sweeteners obtained from hydrochloric acid hydrolysed starch require treatment and other purification processes. However, phosphoric acid hydrolysed starch can be used directly for biological production of ethanol without purification. The application or use of phosphoric acid rather than hydrochloric acid presents some advantages. Phosphoric acid is non-volatile and safer to handle in comparison to hydrochloric acid. Since, phosphoric acid is less severe in comparison to hydrochloric acid, it could be speculated that the application of phosphoric acid should be accompanied by lesser by-products. The hydrolysed starch or sweetener can be diluted on addition to foods or beverages, the phosphoric acid concentration falls to values as low as 0.01-0.05% resulting to a pleasant taste of acidity at pH values of about 3.0. More importantly, if the hydrolysate is to be used in fermentation, there is no need to eliminate the phosphoric acid. Interestingly, neutralisation with ammonia leads to ammonium phosphate, an important supplement for fermentation growth (Fontana et al., 2008). The products from starch hydrolysis, called hydrolysates are made of molecules of different length, consequently, in addition to neutra-lisation and purification, the hydrolysates are fractionated using chromatographic techniques. Three distinct products namely, glucose, maltose and oligosaccharides fractions are recovered. This is one of the starch processing methods patented (Masuda et al., 1995). Besides being used as sweetener, the hydrolysate is also an ingredient for formulation of tablet coats (Czarnecki and Belniak, 2009).

Aside, from application of acids to hydrolyse starch, alkaline such as sodium hydroxide, potassium hydroxide can be used to depolymerise starch at high temperature approximately 240[degrees]C for a period not less than 60 min to yield organic compounds such as glycol and oxalic acids (Verendel et al., 2011). It is important to state that some of the end-products of starch hydrolysis notably glucose and maltose could be further modified and transformed to organic chemicals such as ethanol, hydroxyl methyl furfural (Zhang et al., 2012), and fructose (Roman-Leshkov et al., 2010), lactic acid and other feedstock chemicals.

Some miscellaneous modified starch products: Modified starch has applications in many different industries (Table 3) that range from food, drug and medicine to non food industries such as mining, building and electronic industries. Some of these are illustrated hereunder: other than cooking, native starch can be treated or modified to become granular, cold water soluble. The preparation of this kind of speciality of starch using alcoholic-alkaline treatment had been reported by Chen and Jane (1994). Another procedure that used liquid ammonia and ethanol was also described by Jackowski et al. (2002). Other novel product include functional modification of starch by esterification of the hydroxyl group by introduction of a chromophoric moiety leading to development of a thermostable and photo-responsive system on starch. A procedure of this kind had been reported by Chandran et al. (2012). A starch benzyl ether of high degree of substitution has a unique property of being not gelatinised when cooked at 100[degrees]C. However, when prepared at super atmosphere in a steam injection cooker at 150[degrees]C, dispersion fluid sol results, which when cast gives resistant films.

Most chemical modification of starch is based on material science properties. Starch could also be modified for exhibition of certain nutritional functionality other than dietary fibre. Starch could be conjugated to non-starchy substances thereby, lending significant changes in starch nutritional functionality. This was demonstrated by the work of Hatton et al. (1995), that changed the functional properties of carboxymethyl potato starch conjugated with whey proteins. The covalently linked carboxymethyl potato starch-whey was characterised by improved thermal stability and reduced retrogradation in comparison to unmodified starch. Also the conjugated starch-whey product possessed retinol binding ability present in [beta]-lactoglobulin present in whey and a-lactoalbumin present in whey known to participate in lactose synthetase reaction. One of the reasons for turning to starch for synthesis of starch-materials is its biodegrability. This is expressed in the capability of microorganisms to decompose polymeric materials to low molecular compounds, such as carbondioxide and nitrogen, so that carbon returns to the ground/soil or atmosphere. Beyond this, today research is focussing on dissembling the blocks coupled within functionalised starch. This claim has been demonstrated in the research of Auzely-Velty and Rinaudo (2003), for the synthesised starch betaine derivatives using disopropylcarbodiimide and 4-dimethylaminopyridine as coupling agents in an aprotic polar solvent, which carry cleavable cationic groups derived from naturally occurring glycine-betanine. The improvement in this study, lies in lower toxicity and larger biodegradability in comparison to the traditional cationic surfactant compounds.

Some modification enhancement manipulations. Although BeMiller (1997) in an earlier review predicted that there could not be approval of new reagents for derivation of starch, for safety concerns. However, as knowledge expands, new reagents with special attributes are discovered and other process enhancement manipulation. Two examples shall be discussed in brevity.

Use of ionic liquids. For example, effort on the use of ionic liquids as solvent for reaction-medium for starch has started to receive attention. Ionic liquids are a new technology considered to be green chemistry because of their biodegradability, and low toxicity. Added to this, ionic liquids have a spectrum of compound characteristics through a broad selection of anions and cations combi-nations. Other advantages inherent to ionic liquids are non-flammability and thermal stabilities and enhancement of reaction rates with higher selectivity, an example of such study is an early work of Stevenson et al. (2007) on changes in structure and properties of starch of four botanical sources dispersed in ionic liquid. Besides, acting as solvent, ionic liquids have been reported to exhibit catalytic property. This was demonstrated by acylation of maltodextrin in ionic liquid (Biswas et al., 2009). This process obviates the use of catalyst during acylation of maltodextrin with its economic and technical advantages.

Application of enhanced microwave heating. Starch can be hydrolysed using microwave field. The advantage of this form of heating lies in dissipation of heat inside the medium, which raises the energy of the molecules rapidity. This heating mode is different from convectional heating because all the molecules in the heated system receive same amount and rate of heat readily energized and resulting to higher reaction rates and greater selectivity. The afore-described process could be enhanced by the addition of metallic halides such as sodium chloride, potassium chloride, iron III chloride to the microwave heating solvent or medium. The importance of the inorganic salts could be demonstrated by the amount of (88-109 wt %) glucose hydrolysed from starch using hydrochloric acid for a period of 240 sata temperature of 145[degrees]C, while the control sample without salt (all other conditions were same), yielded less glucose (78wt %) not earlier than a period of 600s as demonstrated in the study of Kunlan et al. (2001).

Future prospects of chemical method of starch modification. Since chemical modification offers the most versatile opportunities of tailoring starch granules to fit for a particular properties/purpose/application, the future appears bright regarding the relevance of chemical modification of starch for products or preparation of speciality starch for food and non-food applications than is known presently. Another important reason for good prospects for continuous progress or advancement in chemicals modification of starch is the peculiarities of starch as a bioresource as outlined in Table 4. Additional projection on the future of chemical method of starch modification and prospective application is as follows: The future of chemical method of starch modification would be addressed from the two broad domain of application namely: (1) food application and (2) non-food application. Regarding food application perspective, there are no indications that new chemicals would be approved for modification of starch to be used in food processing. This is in agreement with the prediction of BeMiller (1997). However, any new chemical to be used for modification of starch for food would undergo strict examination to have passed safety test for approval. Beyond this, chemical modification processes for starch could be enhanced by combination with use of novel solvents such as ionic liquid as medium of reaction and catalytic agent (Biswas et al., 2009). Also chemical modification could be combined with physical process such as irradiation. Regarding chemical modification of starch for application in peripheral food and non-food applications, there exists an open end opportunity. Chemical modification of starch in preparation of edible packaging materials used in fast foods and institutional catering and short-time, and self degradative packaging that could be used as packaging materials in bottling of table water and flocculating agents. Such products could be prepared using starch and long chain fatty acids of different unsaturation and alkylation. Polymeric materials have inexhaustive applications list in bioprocessing. The aim of this section is to articulate the need and the possibility of using starch to replace the other materials produced from non-starchy or petrochemical products. Most especially, that biodegradability is of priority in material science, and polymer technology in present day technology. In separation technology, molecularly imprinted polymers (MIPs) have been applied in adsorptive separation as selective chromatographic materials and in the field of solid phase extraction. MIP is a synthetic porous polymer with selective and specific recognition ability of the binding cavities to target molecules by non-covalent interactions such as hydrogen bonding, electrostatic interactions n-n and hydrophobic forces as reported by Seechamnanturakit and Suede (2012).

In addition, verse opportunities await application of starch modified to meet application as immobilised enzymes with respect to reversibly soluble-insoluble polymer. Presently, all the polymers used in enzyme immobilisation are synthetic (Dourado et al., 2002) consequently, it would be more friendly to use biodegradable polymer of starch origin. Finally, chemically modified starch would find place in other stimuli-responsive polymers, sometimes referred to as smart or intelligent polymers which are presently dominated by synthetic polymers. Stimuliresponsive polymers have a series of biological applications, which include tissue engineering, drug release systems, biological sensors, temperature and light responsive films or optical sensors, intelligent microfluidic switches and diagnostic devices (Milichovsky, 2010) consequently an important tool in automation of bioprocess.

Conclusion

The admirable potentials of chemical modification technology to produce speciality starch with both, food and non-food applications are reviewed. This by no means exhaust the modification and transformation of starch for both food and non-food industrial applications. It is anticipated that this review would motivate readership to seek detail and accurate technology knowledge on know-how-accomplishment to develop domestic starch for import substitution for technological and economic benefits.

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(received January 21, 2014; revised June 19, 2014; accepted August 28, 2014)

Bode Daramola

Department of Food Technology, Federal Polytechnic, Ado-Ekiti, Ekiti State, Nigeria

E-mail: daramola bode@yahoo.co.uk
Table 1. Modification methods and probable conferred
attributes

Types of modification                Probable attribute conferred

I. Chemical modification

  A. Derivatisation

  1. Monostarch substitution         Improved paste clarity
     (etherification and
     esterification, including
     polymer grafting)

  2. Cross linking                   Increase paste stability,
     (via distarch                   lower tendency to retrograde,
     esterification)                 increase water solubility,
                                     improved water solubility.
  B. Acid thinning/hydrolytic        Increase clarity/increase in
     depolymerisation                digestibility

  C. Dextrinisation                  Increase in digestibility
     (depolymerisation and
     trans-glycosylation

  D. Oxidation (bleaching            Increase in clarity
     and depolymerisation)

  E. Hydrolysis (malto-              Increase in digestibility
     dextrins, glucose syrup,
     glucose etc.--both acid
     and enzyme-catalysed.

II Physical modification

  A. Pre-gelatinised process         Cold water solubility

  B. Preparation of cold-water
     swelling starch

III Genetic

  A. Waxy starch                     Variable quality profile

  B. High-amylose starch

Source: BeMiller (1997).

Table 2. Starch derivatives and potential applications

Modified starch/     Modification type         Domain of application
description          Amphoteric                Medicine: dialysis,
Complexes of                                   ultrafiltration
oppositely charged                             anti-thromobogenic
ionic starches                                 agent
e.g.,
polyelectrolyte
complexes

Oxidized starch      Oxidation                 Paper industry: surface
                     [H.sub.2][O.sub.2]        sizing coating and
                     [Cu.sup.2+]               binding
                     (CuS[O.sub.4]) catalyst
                     [Fe.sup.2+] (FeS
                     [O.sub.4])

Carboxymethylation   Etherification alkaline   Biodegradable
of starch propyl-    Sodium-mono               polymer for coating
etherification of    chlorocetate 1-bromo      for food and
starch               propane                   pharmaceutical
                                               products

Amphotenic           Amphoteric starches:      Paper making
starches             Etherification,           additives, for
                     Esterification,           retention, and
                     Grafting                  strength properties

Cross-linked         Cross-linkage             Food additives:
starch               phosphoryl chloride       thickners, colloidal
                                               stabilizer, water
                                               retention, agents for
                                               food and Non-food
                                               waste [H.sub.2]O
                                               treatment

Cross-linked,        Esterification,           To bind heavy metals
cationised           Etherification            from waste water
anionised                                      waste treatment
amphoteric

Cross-linked         Graft co-polymeric        Surgical pad
hydrophilic          cation                    pharmaceutical,
polymers super                                 domestic, sanitary
absorbent                                      use, soil condition

Complexation of      Complexation with         Improvement of
modified starch      pigment                   optical and pant
with pigment                                   ability performance
                                               of water

Etherification and   Cationization graft-      Water treatment as
cross-linkage        copolymerization          flocculant in
                                               place of alum

Cross-linkage of     High performance
starch               elastomer
                     (Tyre-making)

Aerogels based       Microcellular starch-     Alterative material
microcellular        for production of         Xu et al., 2005
foams                synthetic foam

Modified starch/     Leading references
description          Willett, 1995
Complexes of
oppositely charged
ionic starches
e.g.,
polyelectrolyte
complexes

Oxidized starch      Parovuori etal., 1995

Carboxymethylation   Yaacob et al., 2011;
of starch propyl-    Teramoto et al., 2003
etherification of
starch

Amphotenic           Yang et al., 2007
starches

Cross-linked         Majzoobi et al., 2009;
starch               Wang et al., 2005

Cross-linked,        Carmona-Garcia, et al.,
cationised           2009
anionised
amphoteric

Cross-linked         Xiao et al., 2010;
hydrophilic          Liu etal., 1996
polymers super
absorbent

Complexation of      Kuuti et al., 2010
modified starch
with pigment

Etherification and   Yang et al.,2007;
cross-linkage        Sharma et al., 2006

Cross-linkage of     Song et al., 2010;
starch               Qing et al., 2006

Aerogels based       Xu et al., 2005;
microcellular        Glenn and Irving, 1995
foams

Table 3. Spectrum of modified starch utilising industries
with example of application

Industry                  Application

Food processing           Pie filling, padding, mixes, fruit
                          juice sources, bulking agent,
                          molds, fluidifying agent main-
                          tenance of viscosity in acid foods
                          e.g., fruitjuice, decrease syneresis
                          in ice-cream or refrigerated
                          foods. Hard candy gum drops

Paper/board/corrugating   Pigment retention on paper
                          (wet-end addition), as surface
                          sizing agent coating

Brewing                   as adjunct

Textile/warp sizing       Text finishing agents to add
                          weight, smoothness and stiffness

Pharmaceuticals           Cosmetics: tooth paste, body
                          cream or lotion
                          Pharmaceutical tablets; binder,
                          filler, disintegrant

                          Body powders

Building pots             Binder for foundries

Agriculture               Soil conditioners

Adhesives                 Corrugated boards
                          paper bag

                          Guming applications

Crude oil mining          Oil drilling mud

Briquetting               Domestic and industrial heating

Packaging                 Plastics

                          High temperature stable hybrid
                          nylon e.g., meat roasting nylon;

                          Edible films
                          Preparation of surgeon gloves

Table 4. Starch bioresource peculiarities

Starch sources is non-regional specific
Starch is abundant
Renewable/sustainability
Ecofriendly or non-toxic
Amenable to modification in comparison to cellulose
and chitin much available bioresource
Inexpensive
Biodegradable
Availabilities of many reaction sites
Modification is not high technology
Procedure is less labourious and operational hazard
Non-depleting
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Article Details
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Author:Daramola, Bode
Publication:Pakistan Journal of Scientific and Industrial Research Series B: Biological Sciences
Article Type:Technical report
Geographic Code:6NIGR
Date:Nov 1, 2014
Words:5457
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