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On the Molecular Characteristics, CompositionalProperties, and Structural-Functional Mechanisms ofMaltodextrins: A ReviewIoannis S. Chronakis aa Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, Box124, S-221 00 Lund, SwedenVersion of record first published: 03 Jun 2010.

To cite this article: Ioannis S. Chronakis (1998): On the Molecular Characteristics, Compositional Properties, and Structural-Functional Mechanisms of Maltodextrins: A Review, Critical Reviews in Food Science and Nutrition, 38:7, 599-637

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Critical Reviews in Food Science, 38(7):599637 (1998)

On the Molecular Characteristics,Compositional Properties, and Structural-Functional Mechanisms of Maltodextrins:A ReviewIoannis S. ChronakisPhysical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, Box124, S-221 00 Lund, SwedenReferee: Dr. Charles W. Baker, The Sugar Association, Inc., 1101 15th Street, N.W., Suite 600,

Washington, D.C. 20005

ABSTRACT: Compositional, physicochemical, and structural properties of maltodextrins andthe most important advances that have been made are critically reviewed. Individual topicsfocuses on the maltodextrin production, carbohydrate composition, and dextrose equivalentdetermination, factors that alter the polysaccharide properties, the molecular arrangement, themechanisms and complex physicochemical changes of maltodextrins such as water interaction(hygroscopicity, precipitation, turbidity, bound and free water) and the role of molecular inter-actions for a network formation. Of particular importance is the information concerning thenetwork structure of maltodextrins gels (degree of crystallinity, crystallite size, aggregation) andthe involvement of linear and branched chains for the network formation. Rheological propertieshave become a desirable tool to predict and understand their structural and functional properties,in single and in mixed systems with other macromolecules. These advances are assessed togetherwith the structural development of food products and processes. Their main food applications,particular advantages, recent commercial directions, and modifications together with potentialproblems are also discussed. As food ingredients, maltodextrins are a valuable production tool,but still with considerable promises. Nevertheless, a more detailed knowledge of the propertiesof maltodextrins is necessary in order for their use to be considered as sufficiently effective anddesirable in a number of known food applications and for novel development purposes.

KEY WORDS: maltodextrins, processing, water interactions, network microstructure, vis-coelasticity, gelation, functional properties, reduced-fat foods, and reduced-calorie foods.

I. Introduction ...................................................................................................................... 600II. Description of Maltodextrin System ................................................................................ 601

A. Production ................................................................................................................... 601B. Compositional Characteristics .................................................................................... 602C. Dextrose Equivalent (DE) Determination .................................................................. 602

III. Properties .......................................................................................................................... 604A. Physicochemical Characteristics ................................................................................. 604B. Polymer Water Interaction of Maltodextrins ............................................................. 605

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I. INTRODUCTION

Hydrolysis of starch by means of heatand acid,97 or specific enzymatic treatments,46or combined acid and enzyme hydrolysis,63yields a spectrum of depolymerized oligo-mers. The hydrolyzed products mainly con-sists of D-glucose, maltose, and a series ofoligosaccharides and polysaccharides (suchas maltose oligosaccharides, maltotriose, andmaltotetraose mixtures). The wide range ofhydrolyzates available are described in termsof their dextrose equivalent (DE) value,which is a measure of the total reducingpower of all sugars present relative to glu-cose as 100 and expressed on a dry weightbasis. Therefore, a degradation product witha high dextrose equivalent has been sub-jected to a greater degree of hydrolysis thanone of a lower DE.

Maltodextrins are hydrolysis products ofstarches with DE lower than 20 (for DE > 20the term syrup solids or dextrins is used).They represent a mixture of saccharides witha broad molecular weight distribution be-tween polysaccharides and oligosaccharides

and are available as white powders mostly orconcentrated solutions. In contrast to nativestarches, the maltodextrins are soluble inwater.

The addition of maltodextrins as foodadditives has been introduced the last 25years. Maltodextrins can be classified ascarbohydrate-based bulking macromolecu-lar replacements123 (like glucose polymers,modified sugars, and mixed hydrocolloids),and substituted on an equal-weight basisprovide 4 kcal or 16.8 kJ/g.6 Maltodextrinswith low DE values are claimed to display inpart the desirable organoleptic characteris-tics of fat, and from the mid to late 1980sthey have received considerable attention fordeveloping fat- and calorie-reduced prod-ucts. Some of their important functional prop-erties include bulking, gelling, crystalliza-tion prevention, promotion of dispersibility,freezing control, and binding.18

The intent of this article is to brieflyreview the current knowledge and recentresearch developments regarding maltodex-trins. The article focuses on factors that af-fect the physicochemical properties, the struc-

1. Hygroscopicity and Storage .................................................................................. 6052. Turbidity ................................................................................................................ 6063. Precipitation ........................................................................................................... 6064. Free and Bound Water .......................................................................................... 608

IV. Mechanism of Network Formation of Maltodextrin Gels............................................... 610V. Microstructure Organisation of Maltodextrins Network ................................................. 612

VI. Rheological Properties ..................................................................................................... 614A. Single Systems ............................................................................................................ 614

1. Low Amplitude Oscillatory Studies ...................................................................... 6142. Creep and Stress Relaxation Measurements ......................................................... 6183. Large Deformation Properties ............................................................................... 619

B. Steric Exclusion Phenomena in Mixed Maltodextrin-BiopolymersSystems ....................................................................................................................... 6211. Blending Laws ....................................................................................................... 6212. Gelation and Distribution of Water in Binary Systems ....................................... 6223. Large Deformation Properties of Low Fat Spreads using Maltodextrins ............ 626

VII. Food Applications ............................................................................................................ 627VIII. Future Directions .............................................................................................................. 629

References ........................................................................................................................ 631

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tural-functional behavior, and the quality ofmaltodextrins. The considerable effort ontheir processing conditions, product applica-tions, and food developments processes andtheir specific advantages are presented. Itcan contribute to bridge relations and gapsbetween maltodextrins properties and theirbehavior in real food systems. It could pro-vide ideas and introduce areas where it maybe necessary to design and perform detailedstudies.

II. DESCRIPTION OFMALTODEXTRIN SYSTEM

A. Production

According to Alexander,3,4 the presenttechnology involving starches and other car-bohydrates probably originates with the earlywork of Richter and co-workers, who wereissued patents in 1976. Several types of pro-prietary equipment and modern installationsare used today for a desirable conversion ofstarch. These aspects have been reviewedrecently.5,18,78

The acid conversion process consists oftreating a suspension of purified starch witha small amount of strong acid at a fairly hightemperature.113 Hydrochloric acid 0.02 to0.03 M is usually used and temperatures of135 to 150C for 5 to 8 min are applied.Measurement of pH (range 1.6 to 2.0) is nota very sensitive means of controlling acidaddition and is normally performed volu-metrically or by conductivity measurements,while adjustment of DE is carried out byvarying the reaction temperature in a nor-mally fixed time.18 When sufficient saccha-rification has taken place, the acid is neutral-ized, and the mixture is filtered, decolorizedand concentrated to the required solids con-tent. In modern methods, the conditions arearranged to keep the time of conversion asshort as possible in order to minimize sidereactions with partial degradation resulting

in bitter taste, off colors and dextrin haze(retrogradation) on storage.18 Today, the acidhydrolysis is particularly recommended forproduction of dextrins with DP < 5 (glucosesyrups).

In order to have a full continuous hy-drolysates production process, the use ofcontinuous conversions catalyzed by en-zymes or combinations of acid and enzymicprocesses are placed. The actual process usedfor the production of maltodextrin is oftenpatented, and typically involves mixing en-zyme and starch slurry, heating at the gela-tinization temperature of starch (75C) hold-ing there for a fixed time, and then heating toa higher temperature (105C) or acidifyingthe product (pH 3.5) to inactivate the en-zyme.5,16 The optimum conditions (i.e., tem-perature, pH) for a particular enzyme fre-quently depend on the organism from whichit is produced. Finally, the soluble materialis separated from the insoluble fibers bycentrifugation and neutralized for subsequentspray drying under vacuum.

Enzyme-catalyzed conversion withmostly a-amylase (1,4--D-glucan glucano-hydrolase, EC 3.2.1.1) from Bacillus subtilisand pullulanase (pullulan 6-glucanohydro-lase, EC 3.2.1.41) are now used for pro-duction of gelling maltodextrins.18,48,73,76,129-Amylase is an endo-acting enzyme hy-drolyses the (14)-linkages in -d-glucansbut cannot hydrolyze -(16)-linkages atthe branch points. As a result, maltodextrinsproduced by -amylase, an extensive hy-drolysis of amylose but only a partial hy-drolysis of amylopectin, takes place.18 How-ever, a low amount of high-molecular-weightamylose still remains.21 The maximum ac-tivities of the -amylases are usually in theacid region between pH 4.8 and 6.5, but theactivity-pH profile and location of the pHoptima differ depending on the enzymesource, with examples of pH optima rangingfrom pH 3.5 to pH 9.0. The pH optimum forplant and microbial -amylases is generallylower than for the animal -amylases.76

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Pullulanase is specific for 16 linkages in-D-glucans and therefore acts as a de-branching enzyme to provide a series of(14) linked --D-glucopyranose oligosac-charides. Most pullulanases have pH optimabetween pH 5.0 and 7.0 and usually tem-perature optima of 45 to 50C.76

Enzyme catabolized or a combination ofacid and enzymic hydrolysis of starch havedistinct advantages compared with the acidprocess. The hydrolysis obtained is morespecific, depending on the enzyme or thecombination of enzymes selected, and agreater flexibility in the final composition ofthe product is usually achieved. Enzymicprocesses provide a greater amount of fer-mented sugars and less formation of unde-sirable components from thermal process-ing, while there is no need to remove saltsformed during acid neutralization.131 It canbe conducted at wider pH values and lowertemperatures and pressures (an economicadvantage of requiring less energy), whilethe processes are easier to control. Never-theless, the use of enzymes for starch hydro-lyzate is not a completely continuous pro-cess, and several attempts have been madeto use enzymes insoluble by immobilizationtechniques.38,76 In addition to the practicalproblems using immobilized form of theenzymes on porous matrices, an economicquestion also arises. Improved characteris-tics of enzymes can be expected to expandwith the use of genetic engineering.

B. Compositional Characteristics

As a digestion product from starch,maltodextrins contain linear amylose andbranched amylopectin degradation products(Figure 1). Maltodextrins, therefore, are con-sidered as D-glucose polymers in which theindividual -D-glucopyranosly residues arejoined by (1 4)-linkages to give linearchains with a degree of (1 4, 16)-linkedor (1 6)-linked branch points. The Dex-trose Equivalent is an inverse measure of thenumber of anhydro -D-glucose units, thus,

for instance, a maltodextrin of DE 5 corre-sponds to a polymeric species of 20 glucosemolecules (degree of polymerization, DP).However, varying the DE among maltodex-trins polysaccharides does not necessarilymean that they differ only in dextrose con-tent. Moreover, maltodextrins with the sameDE value can have very different properties,that reflect the composition of the compo-nents rising from the hydrolysis reactions.The type of starch (maize, oats, rice, tapioca,potato, etc.) is also an important factor deter-mining the molecular segments of maltodex-trins. The ratio of linear amylose chain mol-ecules to branched amylopectin variesaccording to the source of starch. The major-ity of starches contain between 15 and 35%of amylose.76 Wheat and rice starch havecharacteristics generally similar to those ofmaize sources. On the other hand, waxystraches differ and are made up entirely ofbranched amylopectin molecules. As a re-sult, maltodextrins derived from waxy maizestarch consist exclusively of amylopectin,contain very little linear amylose, and do notexhibit the retrogradation phenomena typi-cal of common corn starch oligosaccharides.90These maltodex-trins have an average mo-lecular weight comparable to other polysac-charides and much less than that of conven-tional amylopectin. Stability to retrogradationdue to some characteristics (size and impu-rities) of the amylose that inhibits itsreassociation is present in starch from po-tato.18 Alkali-modified cassava starch105 isalso a good source of maltodextrins of DE.2023

In addition, native starches differ in watercontent, and in addition amylose and amy-lopectin several noncarbohydrate compo-nents are present, such as lipids, proteins,and minerals.

C. Dextrose Equivalent (DE)Determination

Any method for reducing sugar determi-nation can be used; however, traditionally

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FIG

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the Lane and Eynon titration83 has been usedto determine the content of reducing sugarsand still is the method of choice for someindustrial applications. It is a non-stoichio-metric reaction in which approximately 5.0equivalents of cupric ions are required tooxidize 1.0 mol of reducing sugar. Theirconcentration is monitored titrimetrically,compared with a reference to standard tables,and calculated as a percentage of the drysubstance.127 Careful control of the heatingis required, and for the most accurate analy-sis two titrations are necessary. Table alter-ations have also been reported improvingthe original method.42 The exact procedurehas been published by the Corn RefinerAssociation (Method E-26).18 However, be-cause the reaction is not entirely stoichio-meteric, the theoretical DE of saccharides isalways lower that the observed with DE undercertain conditions with other main disadvan-tages being the time consumed, the stan-dardized reagents, and the expertise required.

Rapid determination of dextrose equiva-lent of maltodextrins (and glucose syrups)could be made by cryoscopy, which is ameasurement of freezing point depression.44The cryoscopic approach is a response to thenumber of moles of material in solution. It isunaffected by the presence of high-molecu-lar-weight materials such as residual en-zymes, proteins, etc. It can be affected by thepresence of low-molecular-weight inorganicsalts (i.e., ash).

Nevertheless, the use of oligosaccharidefractionation by gel permeation chromatog-raphy is now recommended as the bestmethod for characterization of starch hy-drolysates, and the determination of dex-trose equivalent is based on the actual com-position of oligosaccharides.80 Fractionationof starch and its hydrolysis products usingBio-Gel P-2,126 microsperical cellulose,43 andporous glass beads (CPG-10)81 have beenalso reported. Additionally, a number ofspecific assay methods have been developedfor the quantitation of individual oligosac-

charides, among colorimetric methods, whichis used for the gross determination of totalcarbohydrate content or total reducing sugarcontent.77 Such assays use alkaline 3,5-dinitrosalicylic acid,17 alkaline ferricyanide,79or alkaline picric acid.96

1H-NMR studies could also be involved.The anomeric proton region of a maltodextrinspectrum usually comprises four doubletsassigned to H(1), the anomeric protons at 1,6branch points and 1,4 linkages (their ratiocangive the degree of branching), and reduc-ing end groups in the and configura-tions. The DE values of maltodextrins canbe determined from the combined intensityof the resonances from and reducingend-groups relative to the total anomeric sig-nal.53

III. PROPERTIES

A. Physicochemical Characteristics

As a digestion product of starch, themaltodextrins contain linear and branchedamylose and amylopectin degradation prod-ucts the size of which extends from oligo-mers to macromolecules. In the sol state thesemolecules are hydrated and expanded, andthe extended helical regions are interruptedby short, disordered regions. At high con-centrations helices aggregate, forming crys-talline domains. Therefore, maltodextrinshave a significant portion of average chainlength long enough to form thermally re-versible gels. The sol-gel transition is a slowprocess accompanied by dehydration andcombined with the growth of helices of suf-ficiently long molecular chains or chain seg-ments.107 The transition depends on the tem-perature, concentration, time, and structuralpeculiarities.

The gelation of maltodextrins is a weaklycooperative process, with the standard Gibbsfree energy of gelation not strongly influ-enced by the degree of cooperativity.120 Con-

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sidering the low entropic effect, smallchanges in enthalpy result in the formationof thermodynamically stable gel structures.Low elasticity, small mechanical stability,high rigidity and turbidity are characteristicsof these gels. Despite the high rigidity of themaltodextrin gels the heat changes associ-ated with the melting of gels are small.132 Aparticular characteristic of the gel matrix isthe unbound state of the greater part of thewater.

Acetylated maltodextrins do not givesolid NMR signals and melting peak indifferential scanning calorimetry, as observedin the formation of highly ordered domainsof the gel network.29,121 Hence, acetylationstabilize the maltodextrins in solution andno aggregation or gelation have been ob-served.

Variations in DE values results inmaltodextrins with varying physicochemi-cal properties. Hygroscopicity, solubility,osmolality, and their effectiveness to reducethe freezing point increase with increasingDE, while viscosity, cohesiveness and coarse-crystal prevention increase as DE decreases.92It is possible, however, by altering the tem-perature of hydrolysis to produce malto-dex-trins preparations that have similar DE val-ues but different proportions of high-andlow-molecular-weight saccharides.57 Differ-ences in these saccharide profiles are ex-pected to yield maltodextrins with differentphysicochemical properties. In particular,solubility and solution stability will be influ-enced by high-molecular-weight compo-nents, while viscosity, crystallization, andsweetness will depend on the amount of low-molecular-weight components.

Maltodextrins are suitable ingredients toreplace fat in foods59,60,123 and contribute and/or reproduce the fat like mouthfeel in a va-riety of products. This sensation presumablyoriginates from the three-dimensional net-work of submicron particles in structuredwater layers, that function as the structure offat. The network is loosely associated and

the particle gel structure has a large surfacearea and high degree of water immobiliza-tion. For instance, acid hydrolyzed starchunder specific conditions can form crystal-lites that are essentially intact and organizedlaterally.47 This system has low degree ofassociation and the aggregates are very smallparticles (about 0.02 m). In a continuousoil phase this loosen association linked tothe fat crystals and deform similarly.123 Aswell, the particle size of irregularly shapedmaltodextrin aggregates are 3 to 5 m indiameter, approximately the same size as thefat crystals8,99 which presumably contributeto the fat-like mouthfeel. Maltodextrins havethe capacity to participate in Maillard reac-tions and can be used as nonbrowing carriersfor drying sensitive products.82,92,136

B. PolymerWater Interaction ofMaltodextrins

1. Hygroscopicity and Storage

Hygroscopicity is one of the most im-portant properties determining the shelf-lifeand storage stability of maltodextrins. Theeffect of different relatives humidities (vary-ing from 40 to 95%) has been investigated.104Samples exposed to environments of 75%relative humidity and more behave pastry intexture even after 6 days, while at lowerrelative humidities (40 to 60%) samples at-tained equilibrium moisture level after 18days of storage without undergoing any vis-ible textural changes. According to Radostaand co-workers,101 if the maltodextrins arestored above a definite critical water activitythey stick together and change their statefrom powder to a sorption gel. Thus, themaltodextrins have to be stored under suchconditions to prevent the change from pow-der to sorption gel (Table 1).

Absorption of moisture is well acceptedin maltodextrins; however, the reason is notquite solved104 and more analysis is required.

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TABLE 1Critical Water Activity of the Transitionfrom Powder to Gel Under SorptionConditions

Water activity

Pn 0.45 0.66 0.79 0.93

240 P P P P G16 P P P G G9 P P G G S

Note: Pn : the molecular composition of themaltodextrin. P: powder, G: sorption gel, S:syrup, P G: transition from powder tosorption gel.

From Ref. 101 with permission.

Donuelly,40 after studying the hydroscopicityof different D-glucose polymers, suggestedthat the presence of compounds likemaltotriose and maltotetraose imparts highhygroscopicity to sugar mixtures. On theother hand, individual studies71 suggest thatmoisture absorption increases smoothly withdecreasing molecular weight, while sugarscontaining a high-molecular-weight fractionachieved equilibrium moisture sooner thanthe corresponding low-molecular-weightfraction.

2. Turbidity

Turbidity is considerably important be-cause it influences the film-forming prop-erty capable of good retention of volatilecompounds during the spray-drying process,which is preferable for flavor encapsulation.Samples with the same level of DE showedsubstantial differences in their dispersibiltyin water, while the low DE values display arelatively low turbidity.104 Mainly, the greatertendency of amylose to retrograde compar-ing with amylopectin led to the formation ofhaze or precipitation at higher concentra-tions. Factors that influence haze formationare the type of starch used and the method-

ology adopted for liquefaction for malto-dextrins preparations. The use of turbiditymeasurements is also a suitable method forthe determination of the rate of formation ofthe precipitate.75

3. Precipitation

The precipitation of maltodextrins solu-tions at various temperatures is importantfor the storage stability, shelf-life, productprocessing, and obviously the final qualityof food preparations. Kennedy and co-work-ers have extensively investigated the stabil-ity of aqueous solutions of maltodextrins(DE 14, 18, 1525) required for clinical feedpreparations.73 They found that the stabilityis frequently poor, with precipitation occur-ring at 25 to 4C at a rate that shows anegative temperature coefficient.73,74 Theparticle distribution is non-Gaussian, withapproximately 90% of the particles havingdiameters less than 3.5 m, the sphericalparticles ranging from 1.0 to 8.5 m and themean diameter was found to be 2.4 m. Inorder to promote the storage stability of suchaqueous maltodextrins, the presence of inor-ganic ions should be avoided by the use ofdistilled water rather than water purified byion exchange.74 The addition of D-glucoseand surfactants, despite the fact that theyaffect retrogradation of starch and amylose,show only a limited degree of improvementin storage stability. Furthermore, the adjust-ment of pH of the product in intervals of 3.0to 3.5 by citric acid provides a valuable sta-bilization effect.

Analysis of the precipitate formed fromsolutions of maltodextrins (DE 14,18) showthat oligosaccharides with degrees of poly-merization of 11 and above are the majorcomponents, with no evidence for the pres-ence of small oligosaccharides with degreesof polymerization up to 7, while the fractionwith DP values above 20 diminishes slightlywith storage time.75 This is in agreementwith the results by Gidley et al., who ob-

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served that amyloses with mean DP valueslower than 110 were particularly unstableand precipitated/retrograded faster than mol-ecules having larger molecular weights (i.e.,mean DP values 250, 300) that tend to giveturbid suspensions or gels, while the mini-mum DP required for precipitation is 8 to9.54

The above studies74 support that precipi-tation arises through a mechanism similar tothat which causes retrogradation in amylose,namely, alignment of linear molecules, viahydrogen bonding, to give aggregates thatultimately precipitate. Branched structuresinterfere with the formation of these aggre-gates, prevent retrogradation, and increasethe stability of maltodextrin solutions. Theprecipitation on storage of maltodextrinssolutions was increased after extended hy-drolysis time and temperature or increasedenzyme concentration using -amylase.73This is simply explained as -amylase willtend to attack the highly branched starchmaterial readily to liberate linear oligosac-charides, and a greater degree of hydrolysiswill result in more linear structures presentin solution.These investigations also foundthat a combined use of -amylase andpullulanase enzymatic treatments can pro-duce better products in terms of oligosac-charide composition and by selection of thereaction times, and storage stability.

The use of DE as a method for describ-ing maltodextrins is not appropriate as wellas in the prediction of storage stability.74 Analternative method of defining maltodextrinmaterials in terms of their oligosaccharidecomponent composition obtained by gel fil-tration chromatography was not a full proofmethod for predicting the shelf-life. Sampleswith similar oligosaccharide component spec-tra can have very different precipitation timesand storage behavior varying from 1 day tomore than 2 years.74

However, it has been suggested thatamylopectin may play an important role instarch retrogradation via interaction of its

branched chains that have average chainlengths in excess of the minimum DP re-quired for retrogradation.54 Therefore,maltodextrins precipitation is more complexand arises from a mechanism that facilitatesboth linear and branched fractions. Such ef-fects are discussed extensively at the pre-cipitation from potato maltodextrin-gelatinmixed solutions.67 The amount of malto-dex-trin precipitated (M) was proportional to thesquare of its initial concentration and to thefirst power of gelatin concentration (M = k[maltodextrin]2 [gelatin]), indicating thatgelatin drives substantially self-associationand aggregation of maltodextrins when bothpolymers are present in a single liquid phase.This dependence support reasonably for theinitial rate of a two-coil to double helix tran-sition for linear maltodextrin chains in thepresence of gelatin. These helices act asnuclei for ordering of shorter segments thatalso grow significantly by the addition ofbranched species. Furthermore, 1H NMRstudies show that the precipitated maltodex-trin is higher in molecular weight and in thedegree of branching than the material re-maining in solution while it redisolves atabout 80C, behavior that obviously has comefrom the amylopectin fraction of the originalstarch.

In summary, the following model for theextent precipitation of maltodextrin frommixed solutions was proposed:67 the pres-ence of another polymer will drive conver-sion of maltodextrin from the disordered stateto the more compact helical conformation.Within the maltodextrin rapid phase growthof large aggregates by the addition ofbranched material to the linear helices willcontinue (a kind of synergistic interactions)until the ordered core becomes totallyscreened by the disordered fringe. Finally,phase-separation and macroscopic precipi-tation will occur in mixed solutions pro-moted by large branched aggregated clus-ters. Phase separation and precipitation mayhave also occurred when both polymers con-

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centrations are high, before any significantordering.

4. Free and Bound Water

During gel formation a certain amountof water is absorbed in the system. The partof the water that underlies maltodextrin-waterinteractions can be characterized as boundwater, while the remaining, which does notinteract with the polymer, could be denotedas pure free water. These interactions areinfluenced by the chemical composition, thechemical modification of maltodextrins (crys-tallinity, ratio between linear and branchedmolecules), the concentration, the sol geltransition, the temperature, and the otherundissolved substances and show character-istic peculiarities. Information for the con-tent and distribution of water in maltodextrinshas been provided from NMR, differentialscanning calorimetry, water vapor sorptionisotherm techniques, infrared spectroscopy,and ESR spectroscopy studies.90,101,102 Thenon-freezability of a portion of the water inmaltodextrin solutions and gel was taken asa measure for polysaccharide-water interac-tions. A comprehensive review about poly-mer-water interactions of maltodextrins isgiven elsewhere,103 and only the main find-ings will be mentioned here (Figure 2).

1. The quantity of polysaccharide-waterinteractions is largely influenced fromthe concentration of the maltodextrin.As for starches, the portion of boundwater in relation to total water contentrises with increasing maltodextrin con-centration. Therefore, as the structur-ing effect of the maltodextrin on waterincreases, the number of structural ele-ments or aggregates increases, but with-out changing their micro-structure.

2. It is characteristic that the physicalmicro-structure or the state of aggrega-tion of the polysaccharide (powder,

xerogels, solutions, sol-gel transitions,gels) do not influence the water inter-actions and the hydration states. Theyall contain the same amount of boundwater at equal polysaccharide concen-trations. A very weak modification ofthe free water results due to the transi-tion from solution to gel. The aggrega-tion state (size and shape of the matrixcomponents) does not change theamount of bound water and the mo-lecular mobility of water. Linear orbranched structures of the moleculesinfluence insignificant the polysaccha-ride-water interactions.

3. The mean molecular mass compositionand distribution of molecular masses inmaltodextrins are also responsible forthe physical properties of water. At highconcentrations the interactions betweenhigh molecular mass polysaccharidesand water dominate, while at more di-luted and liquid systems the interactionbetween oligosaccharides and waterincrease. The higher the degree of po-lymerization the higher the bound wa-ter at high polysaccharide concen-trations. In low molecular massmaltodextrin fractions this relation isreversed with decreasing polysaccha-ride concentrations. Thus, the oligosac-charides stabilize the water interactionsin solutions and gels, whereas thepolysaccharides increase the polymer-polymer interactions. However, highmolecular maltodextrin fractions con-tain amounts of bound water that areindependent on concentration.

4. With increasing temperature (1 to 60C)a slight decrease of bound water wasfound. Structuring of maltodextrin so-lutions at low temperatures did not re-sult in detectable change in the amountof bound water.

Maltodextrins gels are able to include upto 9 g water/g dry mass inside the gel matrix;

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FIG

URE

2.In

fluen

tial v

aria

nts

on p

olym

er-w

ater

inte

ract

ions

and

thei

r effe

ct in

diff

eren

t sta

tes.

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Ref.

103 w

ith pe

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ion.)

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nevertheless, the water that shows real inter-action with maltodextrin amounts is muchless only 0.3 to 0.5 g/water/g dry mass (wa-ter binding capacity).101,102,103 Hence, the mainpart of the water in a maltodextrin gel mustbe located within the gel matrix. Conclu-sively, the binding of water molecules bymaltodextrins gel network is weak, permit-ting a very high molecular mobility of boundwater and fast exchange with free water.

It has been found that the degree of hy-dration of the starches depends on the sourcefrom which they were isolated and increasesin the series of hydrolyzed starch malto-dextrin < pea starch < wheat starch < maizestarch.50,51 As the degree of maltodextrinhydration is lower and the mobility of thewater molecules in the bound state is higher,the probability of forming stable clustersis higher. These clusters act as nuclei forthe formation of the maltodextrins gel net-work.

IV. MECHANISM OF NETWORKFORMATION OF MALTODEXTRINSGELS

The mechanism of network formation(or precipitation, as discussed previously)for maltodextrins can be inferred from thegelation of starch that is based on the cre-ation of co-axial double helices by 1,4-linked-D-glucan chains and the lateral aggrega-tion of these intermolecular associa-tions.29,34,67,89,119,120 Maltodextrin gels resultfrom coupling interactions between solubleamylose molecules and sufficiently branchedand linear chains of amylopectin mol-ecules.29,67,89,119,120 The hydrated linear amy-lose fractions that have a conformation char-acterized by extended helical regions andinterrupted by short disordered regions areresponsible for the initiation and accelera-tion of the gelation. In pure amylose gelsconformational ordering occurs from the

helicaljunctions of chains with average anhelix length longer than 50 to 70 residues,while shorter oligomers than DP 6 have beenfound to co-crystallize with longer chains.34Long outer linear segments of branchedamylopectin molecules also interact with theamylose chains. These helical species aggre-gate to form crystalline domains, which areembedded in a polymer solution with disor-dered chain segments. Because a portion ofthe molecule is sufficiently long and can beinvolved in the formation of several crystal-line domains, at proper concentrations anaggregated network in such domains repre-sents the junction zones of the polysaccha-ride.108 The junction zones extend over verysmall dimensions and despite the heterophasestate the mechanical properties of themaltodextrin gels corresponded to those of asingle phase system.

This synergistic interaction mechanismbetween different fractions is consistent withstudies where the importance of high mo-lecular weight stable helices capable of form-ing ordered domains as essential constitu-ents of the three-dimensional network havebeen established, as well as the formation ofshorter structures by cooperatively associat-ing with the oligomers.29,67 Analogous ef-fects have been observed from proton NMRand differential scanning calorimetry bySchierbaum and co-workers.120,121 Dea andco-workers also similarly confirmed thatmaltodextrin gels are apparently composedof a network of high-molecular-weightbranched molecules further stabilized byinteractions with short linear chains.21Debranching of maltodextrin gels and theanalysis of the different fragments by size-exclusion chromatography shows that thehigh-molecular-weight fraction was fully ex-cluded (minimum DP 60) and composedentirely of branched molecules derived frompartial depolymerisation of amylopectin.21The low-molecular-weight fraction (approxi-mate DP range 10 to 60) contain principally

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linear chains, presumably originating fromamylose and the outer chains of amylopec-tin. The high-molecular-weight fraction ofmaltodextrins is capable of forming gels,unlike the amylopectin from which it de-rived. The gelation of pure amylopectin oc-curs more slowly than the gelation of amy-lose and seems to involve formation andsubsequent crystallization of helices over alength of DP.15,34,54,112 Such studies clearlyindicate that the linear low-molecular-weightfraction must be sufficiently involved in thenetwork structure of the polysaccharide andcan facilitate structuring of branched spe-cies. Finally, the initial phase of the interac-tions between linear and branched molecules,the rapid aggregation, and the network for-mation are generally followed by a longperiod of slow structural rearrangements,strongly dependent on concentration andchain length (Figure 3).121

The above conclusions deviate from themodels presented on pure high-molecular-weight amylopectin-amylose systems,89where incompatibility has been shown. Otherauthors, therefore, attribute the gelation prop-erties of maltodextrins to the mutual incom-patibility between linear and branched chainsas are amylose and amylopectin chains.41The composition of maltodextrins, however,and the interacting components, deviate fromthe model results on the pure mixtures. It isalso known that amylopectin in the amylose-amylopectin water system can favor the for-mation of amylose aggregates, which are thestructural elements for maltodextrin gels.52,121Earlier suggestions also deduce that the struc-tural elements inmaltodextrin hydrogels areconnected to weak, unstable structures bysecondary forces.19

It should be noted that the associationbetween specific side chains, aggregation

FIGURE 3. Time dependence of sol-gel transition of maltodextrin-solution as characterized by low-resolutionNMR (d), wide angle X-ray scattering (*), shear modulus (s), (20% w/w), and DSC-measurements (8% w/w)(x). (From Ref. 121 with permission.)

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production, and gel formation was acceler-ated in the presence of partially disintegratedamylose, rather than in the presence of amy-lopectin disintegration products.119,121 Theeffect is more pronounced in the low con-centration systems, while in the nongellingsolutions amylose as well initiates the for-mation of a gel structure. These observa-tions confirm the suggestion that the linearfraction in the dissolved state is responsiblefor initiation and acceleration of gel forma-tion. It seems reasonable then to concludethat compatibility between soluble amy-lose and branched chain molecules resultingin the formation of a mixed maltodextrin gelstructure.

The peculiar gelling properties ofmaltodextrins were attributed to preferential-amylase action in the amorphous regionof the starch granule, leading to extensivehydrolysis of amylose but only to a partialhydrolysis of amylopectin.21 Hydrolysis by-amylase before and during gelatinizationof starch is likely to occur preferentially inthe amorphous regions of the granule be-cause of the protection provided by self-association of chains in the crystalline re-gions. In addition, low cooperativity ofinteractions as well as low molecular -glu-cosidic chains may be responsible for thistype of gel behavior.22,29,54,120

V. MICROSTRUCTUREORGANIZATION OFMALTODEXTRINS NETWORK

As provided by small and wide-angleX-ray scattering, the maltodextrin gels, in-dependent of DE and carbohydrate profile,contain crystalline structures. The regionshave the same crystalline structure asB-polymorph seen in naturally occurringstarches of tubers and roots, includingsamples of aggregated so-called retrogradedstarch.49,107,108,120 The crystalline regions instarches are generally supposed to be due to

the crystallinity of the amyloses, which alsoapplies for maltodextrin gels. These crystal-line domains consist of right-hand-ordereddouble helices aggregated into disc-like elec-tron density inhomogeneities with maximumdiameter of 280 nm, height (thickness) be-tween 28 and 36 nm, and radius of gyration89 nm (Figure 4).107 The anhydroglucoseunits in these double helical arrangementspacked in a hexagonal unit cell (a = b = 1.85nm, c (fiber repeat) = 1.04 nm, a = b = 90 = 120).115

Nevertheless, the crystalline domains inthe disk-like regions are not consist of idealcrystals, but lattice distortions exist in theinterior of the inhomogeneities that are com-posed of many microcrystallites. Approxi-mately 10 to 16% of the carbohydrate chainsmay be involved in these crystallites,121 whichare 16 to 17 m in size, independent ofconcentration.49 Branching of the polysac-charide chains should cause such lattice dis-tortions. The crystallites are embedded in aphase containing the noncrystallising partsof maltodextrins, which contain amorphouspolymer chains and water. The majority ofthe water between the crystalline domains isbound water. However, in spray-driedmaltodextrin powder and in the nongellingsystems crystalline structures cannot be de-tected.108 Other conclusions support thatamylopectin makes up the crystalline regionsof the maltodextrins, as they are in a less-degraded form than amylose molecules.21

With increasing concentration of malto-dextrin, a higher order is developed in thegel. Nevertheless, the crystallite sizes andthe crystallinity of maltodextrin gels as cal-culated from two different reflections ofX-ray diffraction measurements do not de-pend on the concentration of the polymer(Table 2).51 Therefore, increasing the con-tent of maltodextrin in the system should notmodify the concentration and the structureof aggregates but further increases only thenumber of structural elements.58 As previ-ously discussed, the mobility of water inside

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FIG

URE

4.St

ruct

ural

mod

el o

f the

mal

tode

xtrin

gel

(the

disc

s are

drawn

in pr

ofile)

. (Fr

om R

ef. 10

7 with

perm

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.)

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and outside these aggregates is different,while such aggregates may interact with eachother to give rise to structures of higher or-der. Information on the shape of gel-formingmolecules was also obtained by the mea-surements of electron microscopy (Table 3).The structural elements seem to have theform of rotary ellipsoids, whereas substruc-tures have a globular form.50,118

VI. RHEOLOGICAL PROPERTIES

A. Single Systems

1. Low-Amplitude OscillatoryStudies

The relatively low molecular weight ofmaltodextrins makes them very soluble, andsolutions can be prepared at 50% w/w sol-ids. Depending on the concentration and theDE value, preparations range from opaquesolutions and pastes to thermally reversiblegels.117 Recently, the gelation of malto-dex-trins as a function of DE, setting tempera-ture, and polymer concentration have stud-

ied.29,88,66-69 It was found that the tempera-ture dependence for the formation of a self-supporting gel was effectively independentof DE, thus suggesting a similar pattern ofintermolecular bonding for the network-form-ing chains. By contrast, a sharp increase inthe concentration dependence of gel forma-tion with decreasing degree of polymeriza-tion emphasized the necessity for long, lin-ear macromolecular chains, serving asnucleation sites, for the development of acontinuous structure.

Using a theoretical approach developedby Clark and Ross-Murphy,32 the networkformation of various maltodextrins has beenstudied. By this method the modulus can befitted with the concentration by the follow-ing mathematical expression of the cascadeformalism:33

= ( ) ( )[ ]G g cRTM f 1 1 22 (1)In this equation g = 1 for an ideal rub-

ber,45 and the factor takes higher values forbiopolymer systems where there is a sub-

TABLE 2Crystallite Size L(hkl) Calculated from Two Different Reflections andCrystallinities (b) of Maltodextrin Gels of Different Concentrationsas Observed from X-Ray Diffraction Measurements

%w/w concentration hkl L, nm b, arbitrary units

30.2 100 12.0 1.00121 8.4

30.7 100 12.0 1.05121 7.7

34.0 100 12.0 1.10121 8.4

36.9 100 14.0 1.25121 8.4

38.8 100 12.0 1.30121 7.7

39.4 100 12.0 1.45121 8.4

43.5 100 12.0 1.45121 7.7


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stantial enthalpic contribution to the elastic-ity of the network.32 The term RT refers tothe change of entropy per mole of networkchains, and the ratio of c to M is the numberof moles of polymer per unit volume. Thetheory also assumes that only a fraction ()of the available bonding sites (functionality,f) will react, and of those a proportion (ex-tinction probability, ) will be unable to sup-port the imposed stress, with the parameter being a function of , fm and . Further-more, the thermal reversibility of physicalcross-links was taken into account by intro-ducing a dimerization reaction between freeand associated sites, determined by an equi-librium constant K, and a minimum criticalgelling concentration (co) below which thebiopolymer is unable to form an infinitenetwork:61

c M f Kf fo

= ( ) ( ) 1 2 2 (2)The least squares fit to the experimental

points is shown as a solid line in Figure 5a.A good quality cascade fit for the SA-2 gelsproduces a high value of co (about 20.2%),

a result that is expected due to the shortpolymeric chains(DP 35). In accordancewith the small degree of polymerization ofmaltodextrin chains, the application of cas-cade model suggests that there are on aver-age 2.7 functional points per molecule, avalue that is well below the functionality(f 10) used to describe the network forma-tion of high-molecular-weight gelatin oragarose samples.31 On the other hand, theinterplay of entropic-enthalpic forces in themalto-dextrin associations produces a highlyenthalpic network (g = 3.8), when comparedwith the more entropic nature of otherbiopolymer gels.28,29,66 Overall, the highlyordered enthalpic aggregates, whose micro-structure bears no resemblance to that of anentropic rubber network, are in agreementwith the idea of crystalline domains in amaltodextrin gel.107

The molecular weight distribution is alsodetermining the concentration dependenceand the rheological properties of maltodex-trins. A commercial potato maltodextrinsample (Cerestar, 1906) where the chromato-gram shows that there is a core of high

TABLE 3Structure Forming of Maltodextrins-Solutions As Revealed byTransmission Electron Microscopy

Concentration

State 15% 20% 25%

80C-solution Without any structure, single microspheres 40 to 60 m

3 h/6C Microspheres Microspheres Microspheresweak gel Very small chains 50 m 20 nm

chains, incomplete comb-likecombs structure 0.6 to 2 nm

24 h/6C Microspheres Microspheres Microspheresgel 30 to 40 m 20 to 30 m 20 to 30 m

incomplete combs combs and thick-walled combsand clusters 2 nm clusters 3 to 5 nm and clusters 2 to 3 nm

Note: The differences of the crystallite sizes in accordance with the vlues from previousTable 2 was attributed to different sample preparations techniques.


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FIGURE 5. (a) Concentration dependence of G (0.1 rad/s) for SA-2 maltodextrin gels at 5C. The solid linetraces a cascade fit with the following parameters: g = 3.8, f = 2.7, and co = 20.2%. (From Ref. 88 withpermission.) (b) The blending of two cascade treatments in a concentration-storage modulus continuum forthe maltodextrin gels. The dashed line shows an earlier attempt to fit the full set of points with a single cascadetreatment. (From Ref. 29 with permission.)

A

B

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molecular weight chains (about 6% between2 105 and 5 106) was also investigatedunder dynamic oscillation measurements.29These chains are capable of forming orderednuclei for the development of a three-dimen-sional network. Shorter chains are not likelyto participate in the initialization of multi-functional junction zones, with an averagehelix length of about 70 residues being re-quired for commencing of the nucleationprocess.34 However, it was found that linearoligomers with a MW down to 1000 cancrystallize with segments of preformed heli-ces thus contributing to the mechanicalstrength of the network.54 Therefore, the ef-fect of addition of short species is significantfor this batch of potato maltodextrin becauseit contains about 32% more material than thetypical preparation within the MW rangefrom 1000 to 5000, that is, for molecules of7 to 35 glucose units. The two-step develop-

ment of shear modulus as a function of con-centration in Figure 5b is due to the transi-tion from high-molecular-weight assembliesto large aggregates comprising long helices,and short linear and branched chains. Thealgorithm of the cascade model (Eq. 1) alsofit the two curves. These experimental andcalculated evidences are entirely consistentwith the proposal of thermally stable longhelices acting as the structural units of thismaltodextrin network, which, however, en-courages at high levels of solids the closepacking of shorter, thermally metastable seg-ments around the central core of helices.

The length and therefore the stability ofthese associations is limited by the length ofthe shorter partner, which converted to thedisordered state at lower temperatures thantheir longer counterparts, hence producingthe two-tiered melting profile of Figure 6.Along these lines, the first wave of structural

FIGURE 6. Controlled heating of maltodextrin gels (% w/w) from 5 to 95C at a scan rate of 1 deg/min(frequency of 1.6 Hz; 1% strain). (From Ref. 29 with permission.)

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loss during a heating run abolishes the bulkof peripheral associations leaving intact theintermolecular associations of long strands.Therefore, networks at the beginning of thesecond melting process should comprisesparsely cross-linked structures of high-mo-lecular-weight chain segments in the man-ner envisaged for maltodextrin concentra-tions of 20% and below.

In another commercial sample, likePaselli SA-6, Lycadex, and Optagrademaltodextrins, the elastic moduli (G) hasbeen found to vary linearly with the concen-tration (do not follow the cascade algo-rithm).30,66 Probably, shorter SA-6 chains(compared with the higher average chainlength of SA-2) create a network in a differ-ent way by agglomeration of aggregatedhelices with dominant frictional forces be-tween adjacent particles of aggregated heli-ces (co 10% w/w). The data for Optagradeis also entirely different in form. Networkdevelopment was not obtained at concentra-tions below 13% w/w, and the concentra-tion dependence with the logarithm of theelastic modulus could be fitted by two dif-ferent straight lines.30 A possible interpreta-tion of this difference arises from the factthat Optagrade is a mixture of maltodextrinand corn starch. Extended chains create thelong range structure by agglomeration ofaggregated helices among the short helicesand starch granules, producing a more het-erogeneous arrangement. Similarly a non-dendric structure may be presumed for theLycadex maltodextrin but the critical gellingconcentration is much higher (27% w/w).30The botanical origin may be also responsiblefor differences found in gelation kinetics.For instance,41 Paselli SA-2 (DE 2.8) frompotato starch and N-Oil II maltodextrin (DE3-5) from tapioca starch are clearly true gelswith comparable properties, however, thekinetics of gelation was dramatically differ-ent. Thus, under the same conditions anddispersions concentrations, SA2 maltodextrinshow gelling behavior after 35 h, while for

N-Oil II was after 15 h. The cloudness ob-served was attributed to a liquid-liquid phaseseparation between linear and branchedchains due to mutual incompatibility thatappeared relatively quickly in both systems.Individual studies also found linearity be-tween the dependence of shear modulus witha concentration of maltodextrin for only alimited range (15 to 25%). Beyond this lim-iting concentration there is a maximum ag-gregate density, whereas the number of struc-tured crystallite components are increasedfurther.132

As a final point, it is evident that thebotanical origin and the production proce-dures are particularly responsible for thedifferences in the structural and functionalproperties of maltodextrins networks thatdrastically related to the final product prop-erties. Further studies are needed in order tounderstand the factors determining the dif-ferences in the gel structure and in the vis-coelastic properties of various originmaltodextrin systems.

2. Creep and Stress RelaxationMeasurements

Under creep studies maltodextrins showa typically relaxation behavior of thermallyreversible polysaccharide and proteins gels.Nevertheless, two different patterns ofbehaviour observed on passing from con-centrated to dilute gels.51,52 In very concen-trated gels the rate of relaxation process areidentical, independent of the concentration(the irreversible deformation amounts toabout 2%). In this case an increase in thecontent of polysaccharide in the system re-sults only in an increase in the number ofstructure elements, while the concentrationwithin each element stays unchanged.51 Indilute gels the rate of relaxation depends onthe concentration (the extent of irreversibledeformation is about 7%) and indicates thatthe systems become more uniform at lower

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concentrations. Such differences cannot bedetected from the NMR data, possibly dueto differences in the volume of the structureelements, which determine the rate of re-laxation in these methods. The same relax-ation properties were observed for amy-lopectin-amylose-water mixtures at ageingtimes of 2 and 48 h after the preparation ofgels.52 In particular, the mechanical-relax-ation rates of a constant 5.5% w/w amylosemixture containing increasing concentra-tions of amylopectin (0 to 40% w/w) werestudied. At concentrations of amylopectinin the range 0 to 5.8% and above 12%,there is little concentration-dependence,while at the same time that the concentra-tion of amylopectin ranges from 5.8 to 12%the relaxation rates increases. Such differ-ences in relaxation time could be explainedas the result of a variation in the interactionbetween aggregates, the structural elementsof the system. A similar complex characterwas also obtained in amylose, amylopectin,and their mixtures from the spin-spin relax-ation time of water molecules using pulseNMR.52 Nevertheless, the level of molecu-lar structure that is involved in the relax-ation of stress or creep effects cannot beestablished yet.

3. Large Deformation Properties

Variation of gel strength under compres-sion of maltodextrins show a rapid increasein yield stress with increasing concen-tration.21 Below a certain concentration(i.e., 25% w/w) the gels remain elastic aftercompression, while at higher concentrationsthe gels are increasingly more brittle andfragment after compression. This is indeedanother indication that at higher concentra-tions part of the polysaccharide is acting asa filler within the gel network, rather thanbeing involved in the tertiary structure, aspreviously addressed. Generally, higher con-centrations of maltodextrins tend to be more

brittle, while lower concentrations have aslimmy rather than a creamy character.Their short, pseudoplastic texture was attrib-uted to the low degree of association amongthe aggregates of submicron particles (about0.02 m), when compared with continuous,polymer gel networks that many gumsform.108

Nevertheless, investigation of the behav-ior of maltodextrins under compression test-ing in existent food systems is more compli-cated. The processing of various productapplications (dairy mixtures, low-fat spreads,etc.) is usually achieved within the tempera-ture range of 60 to 75C from which thepolymeric ingredients dissolve or denaturewithout undue depolymerization.93,94 How-ever, recent work on water continuous low-fat spreads, in which maltodextrins are usedas structuring ingredients, suggests that theaforementioned thermal treatment cannotproperly dissolve the starch hydrolysates.27Checking the effect of temperature on hy-dration at various maltodextrins dispersed inwater at 75C, it was found that the polymercontent was 75 2% of the originalmaltodextrin concentration. This may evenexplain why inconsistent results were ob-tained among HTST-treated maltodextrins(90C for 15 s), with samples left for 5 minat 85C. During cooling spreads from thesecond preparation are stronger (almost by5 kPa) than those cooled directly to ambienttemperature, because additional stress-bearing maltodextrin chains becomesoluble.

The mechanical properties of the dis-solved fraction of maltodextrin chains andtheir contribution to the overall networkstrength were evaluated by analyzing themafter compression testing. As shown in Fig-ure 7, samples showed very weak structures,reminiscent of thick viscous products. Itseems that the polymeric fraction dissolvedat 75C is comprised of low-molecular-weight maltodextrin chains that do not ad-equately develop the elastic component of a

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FIGURE 7. Force-deformation profiles of 250 g/kg maltodextrin samples dissolved at 95C for 15 min (1),dispersed at 75C and HTST treated (2), dispersed at 75C and centrifuged (3450 g, 30 min) to produce aprecipitate (3), and a supernatant (4). Before the compression analysis samples were left at 5C for 24 h. (FromRef. 27 with permission.)

network structure. The tightly packed swol-len particles of the precipitate, however, showsome structure that clearly distinguishes themfrom the liquid-like response of the superna-tant, with the matrix managing to withstandcompression up to 7 kPa in a rather discon-tinuous breaking pattern due to the absenceof a homogeneous network. By contrast com-pression of the samples, which have beencompletely dissolved at 95C, gives the sharpbreakdown profile typical of strong biopoly-mer gels, thus emphasizing the importanceof the remaining 25% of undissolvedmaltodextrin to the formation of an integralnetwork. Further attempts to see if pasteur-ization (HTST treatment) reclaims any ofthe undissolved polymer indicate thatmaltodextrins are not dissolved completelyby this process (only about 74% of the struc-ture is recovered in Figure 7).

Incomplete dissolution of maltodextrinduring commercial production, however,might have a beneficial effect on the rate ofstructure formation on cooling. As discussedpreviously, it has been shown that retrogradedamylose can accelerate gelation of hydro-lyzed potato starch, with its linear chainsfacilitating nucleation and then cooperativeassociation with the branched, shorter struc-tures of amylopectin.119 Because a fractionof the maltodextrin sample remains undis-solved during a conventional HTST treat-ment, its appropriateness as a seeding mate-rial in product development was alsoinvestigated.27 The presence of ordered struc-ture in a maltodextrin solution reduces thegelation time dramatically, by almost 50%.However, it remains to be seen if inclusionof a small amount of seeding amylose ormaltodextrin in the aqueous phases becomes

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an extra help in the product developmentand how this factor could be involved inproduct applications.

B. Steric Exclusion Phenomena inMixed Maltodextrin-BiopolymersSystems

The studies of maltodextrin with sys-tems such as gelatin, sodium caseinate, andmilk protein have been investigated exten-sively in order to gain a better understandingof the macromolecular organization and thephase structure of binary systems relevant tothe food industry.29,68,69,88 Mixed solutionsshow signs of bulk phase separation aftercentrifugation at temperatures where the in-dividual components remain stable as disor-dered coils. Thus, concentrated preparationsresolve into two liquid layers at equilibriumwhose composition defines a cloud pointcurve, or produces an insoluble maltodextrinprecipitate.

1. Blending Laws

Recently, some progress has been madein the understanding of how biphasic gelsbehave in terms of phase continuity, phaseinversion, and, above all, solvent distribu-tion between the two phases.25 It is based onthe assumption that either bulk phase sepa-ration to equilibrium takes place first withgelation then occurring subsequently andindependently in each phase or the fastestgelling component does so prior to the estab-lishment of a true thermodynamic equilib-rium with subsequent gelation of the second,slower gelling species. A number of theo-retical treatments from the realm of syn-thetic polymers were adapted for use inbiopolymer networks, namely, (1) the appli-cation of blending laws to the phase sepa-rated biopolymer gels was attempted, takinginto account the complication of solvent pres-

ence as a third component that can partitionitself between the two polymer constituents,(2) the modulus development as a functionof concentration (cascade formalism) wasderived from the relationship between equi-librium shear modulus and number of elas-tically effective network chains consideringthat gel formation due to noncovalent inter-actions between biological macromoleculesis described by a monomer-dimmer equilib-rium, and (3) the Flory deswelling theorywas applied to biopolymer gels assumingpermanent networks on the basis of stressrelaxation and dynamic oscillatory evidence.

The mathematical modeling of smalldeformation modulus of binary gels has beenattempted initially, as a function of changingpolymer composition in the blend, by Clarkand co-workers.31 It related the mechanicalproperties of the composite and bulk compo-nents via the equations of the Takayanagiblending laws:

G G Gc x y= + ( ) 1 (3)

1 1G G Gc x y= + ( ) (4)

where Gc, Gx, and Gy are the moduli of thecomposite, X-phase and Y-phase polymers,respectively, and f is the volume of phase X.For Gx > Gy and the polymer X forming thesupporting matrix the above approach givesan isostrain or upper bound behavior (Eq. 3),whereas phase inversion in the system, withthe polymer X being now the discontinuousfiller, results in the so-called isostress orlower bound model (Eq. 4). The analysisalso measured the relative affinity of twopolymers for water in a composite gel, as-suming a thermodynamic equilibrium be-tween the two gelled phases, and uses a pa-rameter p that divides the ratio of solventfraction in one phase (Sx) to the original(nominal) concentration of the appropriatepolymer (x) by the corresponding ratio ofthe other polymer (1 Sx,y):

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p S x S yx x

= ( ) [ ]( )1 (5)However, if a permanent gel network isformed at one concentration and then takento a different concentration by intro-duction or removal of solvent (swelling ordeswelling), the initial and final moduli (Giand Gf) are related to the initial and finalconcentrations (ci and cf) by:

G F c ci f i f= ( )2 3/ (6)The relevance of this behavior to analy-

sis of mixed gel moduli is that unless thesystem has already separated into discretephases in the sol state, the first component togel will do so at its original, nominal con-centration across the whole system. Subse-quent gelation of the second componentwithin the pores of the existing gel will thencreate a separate (discontinuous) phase,making a portion of the solvent unavailableto the polymer in the original (continuous)network. This removal of solvent can beregarded as deswelling of the continuousnetwork, raising its modulus, but to a valuesubstantially lower than would have beenattained if phase separation to the same phasevolumes had occurred prior to gelation.

2. Gelation and Distribution ofWater in Binary Systems

Recent investigations on systems of di-rect practical relevance, which one compo-nent of is maltodextrin, has shown evidenceof formation of biphasic gels by both mecha-nisms as discussed above (i.e., phase separa-tion followed by gelation and formation of asecond phase within an existing network).

From Figure 8 it is evident that low pro-tein concentrations in the blend allow themaltodextrin to form a weak network sur-rounding the stronger inclusions of proteinparticles, whereas at higher levels of sodium

caseinate (above 12.5%) a weak protein struc-ture is created that is capable of suspendingthe stronger maltodextrin particles.88 In Fig-ure 9 the calculated bounds and the experi-mental results from Figure 8 have been re-plotted as a function of the parameter p usingEq. 5. Modeling of the rearrangement ofwater between the two polymeric constitu-ents has demonstrated that the water parti-tion values are profoundly affected by thephase inversion in the co-gels. Clearly, theamount of water held in each phase changesdramatically as the system goes through aphase inversion. With maltodextrin as thecontinuous phase, the polysaccharide keepsone and a half times more solvent than theprotein (log p 0.17; p 1.5). When themacromolecular maltodextrin assemblies areincorporated in a discontinuous arrangement(filler); however, the caseinate moleculesmanage to invert the solvent-to-polymerdistribution in each phase (log p 0.19;p 0.6). In particular, the proportion of sol-vent associated with the maltodextrin phaseis reduced, as it ceases to be the supportingphase and becomes the discontinuous filler.Obviously, water diffuses in the anisotropicmedium seeking osmotic equilibrium, butthe decline in the amount of solvent kept inthe maltodextrin phase with the reduction inits surface-to-volume ratio (following phaseinversion) argues for mixed gels trappedkinetically from equilibrium solutions. Oth-erwise, the equilibrium value of relativeaffinity of the two polymers for water shouldnot be affected by the geometrical rearrange-ment of their phases in a binary mixture.

The theoretical postulate of initial phaseseparation and subsequent gelation of thetwo components separately in their ownphases has been shown to describe well thesteric exclusion phenomena between a com-mercial milk protein and maltodextrin.29 Theformation of milk protein or maltodextrincontinuous gels allowed the resolution oftwo different patterns of water distributionin the blend. Solvent fractions derived from

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FIGURE 8. Reproduces the computerized output of calculated bounds for the 15% w/w SA-2 maltodextrinseries with sodium caseinate (SC) plotted against the solvent fraction in the SA-2 phase (polymer X).Composite bounds of a maltodextrin continuous network run from the top left to the bottom right corner of thegraph, whereas composite curves for a caseinate continuous phase stretch from the bottom left to the top rightcorner of the same figure, with both traces crossing at a single point (Gx = Gy = Gc). Experimental points aremarked on the maltodextrin (j) and sodium caseinate (d) continuous bounds. (From Ref. 88 with permission.)

the calculated upper and lower bounds wereused for analysis of water partition betweenthe two phases, yielding p 1.7 (log p 0.23) for the intercepts in the milk-continu-ous systems, whereas the data beyond thephase inversion point (maltodextrin-continu-ous systems) are better fitted with a value ofp 1.1 (log p 0.04). In particular, theproportion of solvent associated with the milkprotein phase is reduced as it ceases to be thesupporting phase and becomes the discon-tinuous filler. A simple explanation of this

behavior is that the water-binding capacityis not only a reflection of the individualproperties of each polymer, but also dependson the geometrical organization of thecomposites microstructure. Because it hasbeen demonstrated that the maltodextrin in-clusions are spherical (Figure 10), it has beenproposed that the increase in the amount ofsolvent associated with the maltodextrinphase is due to phase inversion, and the en-suing increase in its surface-to-volume ratioas the water tries to diffuse in the anisotropic

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FIGURE 9. Calculated lower bounds for the composite mixtures of 15% w/w maltodextrin series as a functionof log p, with the concentrations of sodium caseinate (%) plotted to the right of the corresponding bounds.Experimental moduli are plotted on the maltodextrin (j) and sodium caseinate (d) continuous bounds andrelative values of solvent partition at both sides of the phase inversion point are indicated by the arrows. (FromRef. 88 with permission.)

medium.29 Obviously, by accepting diffu-sion to osmotic equilibrium as the mecha-nism behind water rearrangement, the ap-proximately round-shaped filler wouldexpose relatively less surface for a givenvolume, thus reducing its intrinsic relativepower of attraction for solvent. Moreover, itfollows that the difference in p values is theresult of phase-separated gels trapped awayfrom equilibrium conditions, because theequilibrium value of relative affinity of

the two polymers for water should not beaffected by the geometrical rearrangementsof their phases in a binary mixture.

Furthermore, the kinetic (deswelling)approach to explicit analysis of water parti-tion between two demixed polymers has beenutilized to describe the cold-setting aqueouspreparations of thermally processed gelatin/maltodextrin68,69 and that, in the hydrationstate, form similar species of comparablefunctionality.

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FIGURE 10. Reinforcement (Gcomposite to Gmatrix) of maltodextrin at concentrations between 0% and 12%embedded in a milk protein gel matrix. Comparison of the experimental results (j) with the predicted behaviorobtained from the Kerner equation for spherical, plate, and rod-like inclusions of maltodextrin filler in thecomposite gel. (From Ref. 29 with permission.)

Overall, the appearing picture of the ef-fect of polymer conformation on the state ofphase separation in binary mixtures relevantto the food industry has as follows:24,29,68,69,88conformationally similar species in solutionlike the disordered coils of gelatin andmaltodextrin tolerate each other at low con-centrations in a monophasic solution. Dur-ing cold-setting, the faster-gelling polymerin each mixture develops its continuous net-work prior to ordering of the second compo-nent. Maltodextrin after ordering claim extrasolvent and as a result deswell to a certainextent the protein network, but the systemsremain under kinetic control with slow dif-fusion of water from the faster to the slowersetting component with time. In both sys-tems, at higher concentrations (i.e., at com-binations above the phase inversion point)the kinetic effect is swamped by the enthalpicdisadvantage of polymer segments being

surrounded by others of a different type andphase separation occurs in solution. Thispersists in the gel state and produces a singlepattern of water partition throughout theconcentration range, but it is difficult to sayif the systems have now reached thermody-namic equilibrium.

Dealing with the question of kinetic in-fluences vs. thermodynamic equilibrium, ithas been demonstrated that thermodynamicincompatibility between the conformation-ally dissimilar species of thermally unfoldedglobular molecules of milk protein and dis-ordered chains of maltodextrin promotes anearly phase separation in solution and thenin the gel state, that is, at both sides of thephase inversion point. As a result, there is animmediate reinforcing effect of the malto-dextrin filler on the milk protein gel thatwas not observed in the case of gelatinand maltodextrin, where composite values

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below the phase inversion point remain closeto that of the continuous gelatin network atits nominal concentration. Hence, above all,both deswelled and phase-separated networksseem to be under kinetic control with thesolvent continuously seeking (but not achiev-ing within the experimental time constraints)osmotic equilibrium.

Moreover, phase separation occurs moreeasily with mixed maltodextrins systems withuncharged polymers (i.e., at low locust beangum concentration), compared with the caseof a charged polymer, that is, maltodextrin/carboxymethylcellulose.7 Thermodynamicincompatibility of maltodextrins (DE 12, 17,19) with the presence of proteins isresponsibile as well for reducing the heatstability of caseinate-stabilized emulsions.36Enhancement of thermodynamic incompat-ibility was more evident at higher tempera-tures or at higher molar mass of polysaccha-ride.

The mixed gel phase continuity, phaseinversion, and solvent distribution in non-equilibrium arrangements should be heavilygoverned by the thermal history that the blendis subjected to. Recent studies found thatdifferent rates of gelation reveal a trend inthe partition of solvent between the constitu-ent structures in the development of com-posite modulus, and in the polymer compo-sition at which phase inversion occurs.70Rheological measurements and light micros-copy work on the gelation and phase separa-tion of gelatin-maltodextrin solutions (quenchcooling and controlled cooling) confirms theshift in the phase inversion point when slowgel rates are employed. The gradual coolingdiminishes the competition between gela-tion and steric exclusion and makes the rateof phase separation faster than that of gela-tion. Again, to what extent this state of equi-librium had been disturbed by gelation dur-ing cooling of the phase-separated proteinsolution remained largely unresolved.

Obviously, there is a straightforwardpositive relationship between performance

characteristics of the biopolymer networksand maltodextrin polysaccharide that can beused to manipulate the rheological behaviorand water immobilization of one gelling agentat the expense of the other. A good under-standing of the above blending laws and ofthe technical approaches involved will alsoassist the food scientist in placing the devel-opment of novel products on a sound tech-nological basis.

3. Large Deformation Properties ofLow-Fat Spreads UsingMaltodextrins

The mechanical strength and the gela-tion rate of a maltodextrin structure on low-fat spreads depends on the molecular weightof the polymer and the heat treatment. Thus,the effect of gelling hydrolysate chainlengthon the mechanical properties of water con-tinuous spreads was recently briefly investi-gated by gradually replacing the control prod-uct formulation (DE 3), with lowermolecular weight homologues (DE 6 or8 or 12). The systems were further weak-ened in accordance with the additional re-duction in the overall chain length of themaltodextrin blend (30%, 60%, and almosttotal loss of cohesion, respectively).27 More-over, it was shown that products hydrolyzedat the same dextrose equivalent (e.g.,1906Cerestar and N-LiteD, DE 3) have a differ-ent degree of branching in the molecule andform dissimilar gel networks and dissimilarproducts at the same material concentra-tions.30 The partition, length, and stability ofless tightly packed swollen particles, thatinterfere in a such way that the intermolecu-lar structure, among the contributions of high-molecular-weight fraction determine theirperformances.

Some background understanding of thephase behavior and rheology of the milkprotein-maltodextrin low-fat spreads with andwithout thickening agents (such as xanthan,

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