morphological, thermal, pasting, and rheological properties of barley starch and their blends

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Morphological, Thermal, Pasting, andRheological Properties of Barley Starchand Their BlendsMahesh Gupta a , Amarinder Singh Bawa a & Anil Dutt Semwal aa Defence Food Research Laboratory , Siddhartha Nagar, Mysore,IndiaPublished online: 19 May 2009.

To cite this article: Mahesh Gupta , Amarinder Singh Bawa & Anil Dutt Semwal (2009) Morphological,Thermal, Pasting, and Rheological Properties of Barley Starch and Their Blends, International Journalof Food Properties, 12:3, 587-604, DOI: 10.1080/10942910801947763

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International Journal of Food Properties, 12: 587–604, 2009Copyright © Taylor & Francis Group, LLCISSN: 1094-2912 print / 1532-2386 onlineDOI: 10.1080/10942910801947763

587

MORPHOLOGICAL, THERMAL, PASTING, AND RHEOLOGICAL PROPERTIES OF BARLEY STARCH AND THEIR BLENDS

Mahesh Gupta, Amarinder Singh Bawa, and Anil Dutt SemwalDefence Food Research Laboratory, Siddhartha Nagar, Mysore, India

Native barley starch, as well as its blends with corn, wheat, and rice starch at differentratios of 75:25, 50:50, 25:75 were examined in terms of morphology, thermal, pasting,rheological, and retrogradation properties. Amylose content varied between 10.9–41.4% inrice, corn, wheat, and barley while it ranged from 18.02–38.40% in blends of barley starchwith rice, corn, and wheat. A rapid visco analyzer showed that barley starch and its blendshaving low amylose content exhibited higher peak viscosity, breakdown, and setback thanthe high-amylose-containing starches and their blends. Amylose content was found to benegatively correlated with swelling power while it exhibited nonlinear relationship with sol-ubility index. The transmittance of starch suspension stored at 4°C decreased during storageup to 6 days. Barley starch granules were largest (<110 mm) in size followed by wheat (<30mm), corn (<25mm) and rice (<20mm) starches. Gelatinization temperatures (To, Tp, Tc) andenthalpies of gelatinization (DHgel) of starches from different sources also differed signifi-cantly. Corn and rice starches showed higher transition temperatures in general than thosefrom wheat and barley; however, they showed higher DHgel values. Barley starch showed ahigher tendency towards retrogradation than the cereal starches. Barley starch showedhighest peak G¢, G² and lower tan ð than corn, rice and wheat starches during the heatingcycle. This study showed that the magnitude of changes in their properties during blendingdepends on the amylase content and morphological characteristics.

Keywords: Barley, Corn, Rice, Wheat, Starch, Morphology, Gelatinization, Thermalproperties, Rheology, Retrogradation.

INTRODUCTION

Starch is stored as discrete semicrystalline granules in higher plants. Starch consistsof two main components: mainly linear amylose and highly branched amylopectin. In thearea of carbohydrate polymers, starch is an important example that is currently enjoyingan increased attention due to its usefulness in different food products. Starch is the majorpolysaccharide in plants and exists in the form of granules comprising of amylose andamylopectin chains in a relatively ratio of average 20:80.[1] But the amylose content canvary from 17–65% depending on the botanical origin of the starch. The exact location ofamylose within the granule interior and the extent and nature of its interaction with amy-lopectin is still unclear. The physicochemical properties and functional characteristics that

Received 22 September 2007; accepted 29 January 2008.Address correspondence to Amarinder Singh Bawa, Director, Defence Food Research Laboratory,

DRDO, Siddhartha Nagar, Mysore 570 011, India. E-mail: [email protected]

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588 GUPTA, BAWA, AND SEMWAL

are imparted to the starch systems and their uniqueness in various food applications varywith the biological origin.[2] Starch granules vary in shape, size and composition depend-ing on their botanical origin.[3] Extracted starches from cereals are used extensively infood and non-food industries. These starches are physically and chemically modified tomeet the properties demanded by industry. Morphological and ultra-structural changesduring starch gelatinization have been observed by scanning electron microscopy (SEM)and transmission electron microscopy (TEM) in potato, waxy maize, normal maize, barley,oat, wheat, and cassava starches.[4,5] Morphological and structural changes at various tem-perature and moisture combinations have been shown to influence rheological behavior andfunctionality of starch gels.[6,7] Starches from various plant sources, such as wheat, maize,barley and rice, have received extensive attention in relation to structural and physicochemi-cal properties.[8,9] Starch contributes greatly to the texture properties of many foods and iswidely used in food and industrial applications as a thickener, colloidal stabilizer, gellingagent, bulking agent, water retention agent, and adhesive. Starches are good texture stabilizerand regulator in food systems,[10] but limitations like low shear stress resistance, thermalresistance, thermal decomposition, and high retrogradation have limited their use in someindustrial food applications. These shortcomings can be overcome by chemical and physicalmodification of starches.[3] This can also be overcome by explore the possibility of blend thedifferent starch sources for change starch properties comparable with modified one.

Native starch blends are increasingly applied in food industry to make starchy food witha desired rheological property, texture, storage stability, or to replace modified starch.[11]

In contrast to single starch systems, starch blends generally exhibit unique Visco-elastic orphysical properties that cannot be directly interpreted by the final apparent amylose con-tent or mixing ratio. [11,12] This may be because of the differences in swelling and solubilityproperties between dissimilar starches,[13,14] and the uncertainties involved in intermolecularassociation.[15,11] The interactions between starch materials that control the texture-relatedstructure of starch composites in food are dependent on the experimental or processingconditions. The modified starches generally exhibited better paste clarity, stability andincreased resistance to retrogradation.[16] In chemical starch modification, cross-linkingand substitution are used to give thickeners that have the desired rheological propertiesduring storage and shipment.[17] Interest in new value-added products to the industry hasresulted in many studies being carried out on the rheological and textural properties ofcorn and potato starches.[18,19,20,12] The objectives of the present study was to study themorphological, thermal, rheological, pasting and gel strength characteristics of barleystarch and blended with corn, rice and wheat starch at different levels that was comparableas that of modified starch properties.

MATERIAL AND METHODS

Barley grains and high-amylose cultivar were procured from Punjab AgricultureUniversity, Punjab-India. Barley grains were conditioned to 14% (db) moisture content.The barley grains were ground to flour in a laboratory centrifugal mill (Retsch GmbH,Germany) and passed through a 60-mesh (British standard-250 microns) sieve to obtain180–220 micron flour. Barley starch was isolated by the method described.[21] Wheatstarch was isolated using the method[22] of Lee et al. 2001. Corn and Rice starches wereprocured from S. D. Fine Chemicals Ltd. India. Analytical grade sodium hydroxide andhydrochloric acid were procured from Glaxo India Limited, Mumbai (India) and ethanolwas obtained from Hayman Ltd., Essex (U.K).

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PROPERTIES OF BARLEY STARCH AND THEIR BLENDS 589

Starch Blends

Starch from different cereal sources like Wheat, Rice, and Corn was blended withthat from Barley in the ratios 75:25, 50:50, 25:75. These blends were accessed for theeffect on physicochemical, morphological, thermal, rheological, retrogradation, and gelstrength properties.

Physicochemical Properties

Amylose content of the starches was determined using the method described.[23]

Swelling power and solubility of starches were determined using 2% aqueous suspensionof starch by the previous method.[24] Light transmittance (%) of pastes from barley starchblended with other starches was measured by the method earlier described.[25] A 2% aque-ous suspension of starch was heated in a boiling water bath for 30 min with constant stirring.The suspension was cooled for one hour at 30°C. The samples were stored for five days at4°C and % transmittance measured every 24 h at 640 nm against a water blank with aShimadzu UV-1601 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Fluidityvalue was determined by the method described.[26]

Morphological Properties

Scanning electron micrographs of Barley starch blended with other starches wereobtained with a scanning electron microscope (Jeol JSM-6100, Jeol Ltd., Tokyo, Japan).Starch samples were suspended in ethanol to obtain a 1% suspension and were sprinkledon double stick tape fixed on an aluminum stub, and the starch was coated with gold-palladium (60:40). An accelerating potential of 5kV was used during micrography.

Thermal Properties

Thermal properties of blended starches were analyzed using DSC-821e (MettlerToledo, Switzerland) equipped with a thermal analysis data station. Starch (3.5mg, dwb)was weighed into a 40-μl capacity aluminum pan (Mettler, ME-27331) and distilled waterwas added with the help of Hamilton micro-syringe to achieve a starch-water suspensioncontaining 70% water. Samples were hermetically sealed and allowed to stand for 1 hourat room temperature before heating in DSC. The DSC analyzer was calibrated usingindium and an empty aluminum pan was used as reference. Sample pans were heated at arate of 10°C per min from 20 to 100°C for corn, wheat, rice, and barley starch. Onset tem-perature (To); Peak temperature (Tp); Conclusion temperature (Tc) and Enthalpy of gelati-nization (ΔHgel) were calculated.

Pasting Properties

Pasting properties were analyzed using a Rapid Visco-Analyzer (RVA) (model 3D;Newport Scientific Ltd, Sydney, Australia). Viscograms of starch was monitored usingstarch–water suspensions (6 and 8%, w/w, dry starch basis, 28.0 g total weight). Each sus-pension was tested under the same temperature–time conditions: heating from 50 to 95°Cat 6°C/min (after an equilibration time of 1 min at 50°C), a holding period at 95°C for 5min, cooling from 95 to 50°C at 6°C/min and a holding phase at 50°C for 2 min. The

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constant rotating speed of the paddle was 160 rpm. Pasting parameters were automaticallycomputed and reported.

Rheological Properties

A small amplitude oscillatory rheological measurement was made for starches frombarley starch blended with other starches, with a dynamic rheometer (Paar Physica, MCR-100, Anton Paar, Germany) equipped with plate and cone system (4 cm diameter). Thegap size was set at 1000 μm. The strain and frequency were set at 2% and 5 Hz, respec-tively for all determinations. The dynamic rheological properties such as storage modulus(G′), loss modulus (G′′) and loss factor (Tan d) were determined for starch and its blends.Starch suspensions of 20% (w/w) concentration were loaded on the ram of rheometer andcovered with a thin layer of low-density silicone oil (to minimize evaporation losses). Thestarch samples were heated from 25°C to 85°C at the rate of 3°C/ min and cooled from85°C to 25°C at the rate of 5°C/ min.

Retrogradation Properties

Starch suspension (2%, w/w) was heated at 85°C for 30 minutes in a temperaturecontrolled water bath, followed by rapid cooling in an ice water bath to room temperature.The starch sample was stored for 24, 48, and 120 hours at 4°C. Syneresis was measured as% amount of water released after centrifugation at 5000 rpm for 15 minutes.

Gel Strength

An aqueous starch suspension (14%) barley starch blended with other starches washeated from 30–90°C and held for 20 min at 90°C, then cooled at 50°C. This cooked pastewas used to prepare starch gels for compression testing. Gels were prepared using starchesfrom different sources by slight modification of the earlier method.[27] The cooked paste asdescribed above was poured in a dish and cooled at 30°C for 1 h and stored in refrigeratorat 4°C. After every 24 h, a piece of gel (1cm3) was cut and loaded on the base plate of theTexture analyzer (Lloyd Instruments LR 30K, UK). The flat probe (3.5-cm diameter) traveleda distance of 6 mm at a test speed of 25mm/min, for each compression. The maximumforce (N) required to compress the sample was recorded.

Statistical Analysis

Data analysis for Duncan multiple comparison test and response optimization weredone using Statistica statsoft ver8.0 statistical package.

RESULTS AND DISCUSSION

Physicochemical Properties

Amylose content of native starches was varied between 10.9–41.4% rice, corn,wheat and barley starches respectively, as barley starch blended with rice, corn, andwheat at different ratios the amylose content was varied in between 18.02–38.40%.

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The amylose content during blended was comes between 10–40% range, was impor-tant in this study Table 1.

The swelling power and solubility of native barley starch was 10.43g/g and 18.40,for wheat was 14.05g/g and 12.72, for corn was 14.185g/g and 11.12 and for rice was17.74g/g and 9.92, respectively. As barley, starch blended with wheat, corn and rice swell-ing power increases as amylose content decreases and vice versa with solubility swellingpower and solubility index was varied between 11.20–15.12g/g and 10.45–17.73 respec-tively shown in Table. 1. Both swelling power and solubility of all starches tested increasedas the temperature increased. It has been reported that on the molecular level, the SwellingPower and solubility of the starch granule is influenced by many factors, including amy-lose to amylopectin ratio and contents, molecular mass of each fraction, degree of branching,conformation length of outer branch of amylopectin, and the presence of other compo-nents such as lipids and proteins.[28] Waxy starch exhibited the highest degree of swelling,supporting the idea that reduced amylose content relates to greater swelling. Because theswelling behavior of cereals has been related to amylopectin,[29] the high Swelling Powersuggested a less rigid granular structure of waxy starch compared with that of non-waxystarches. In fact, it has been reported[6] that waxy starches have a more open structure thatallows rapid water penetration, swelling and solubility. Swelling power and Amylose con-tent has negatively correlated with each other. The Fluidity values of native barley, wheat,rice and corn was 95.0, 92.0, 90.0, and 96.0, respectively; the fluidity values of all thestarches depends upon their morphological characteristics. As size of the starch granulesincreases fluidity value also increases means it produce resistance toward floe behaviourResults are shown in Table 1.

The Transmittance (%) at 640 nm was measured from the 2% heated starch slurrywas obtained up to 6 days stored at 4°C. the results showed that transmittance value wasdecreased during storage. In case of native barley starch transmittance value decreasedfrom 70.14 to 3.0, for wheat was decreased from 81.6 to 9.3, for corn was decreasedfrom 57.6 to 12.1 and in case of rice, was decreased from 63.11 to 7.3 as barley starch

Table 1 Summary of physicochemical properties of barley starch and their blends (n = 3).

Starch sourceAmylose

content (%)Swelling

power (g/g)Solubility

index

Fluidity value (ml)

100 ml = 40 sec

Transmittance at 640 nm (%)

1st day 3rd day 6th day

Barley starch 41.40e 10.43 a 18.40 d 95.0 c 70.14 c 10.11 a 3.01 a

Wheat starch 25.80 b 14.05 c 12.72 b 92.0 a 81.61 d 36.70 c 9.31 b

Corn starch 24.10 b 14.185 c 11.21 a 96.0 d 57.60 a 33.42 c 12.10 d

Rice starch 10.90 a 17.74 d 9.92 a 90.0 a 63.11 b 29.77 b 7.33 b

Barley: Wheat 75:25 38.40 d 11.20 a 17.73 c 94.0 b 74.32 c 39.71 c 10.4 c

50:50 33.92 c 12.31 a 15.11 c 93.0 b 76.77 c 41.20 d 9.31 b

25:75 30.11 c 13.09 b 13.70 b 92.0 a 78.12 d 46.36 d 11.31 c

Barley: Corn75:25 37.13 d 11.45 a 16.21 a 95.0 c 66.33 b 25.12 b 9.06 b

50:50 32.70 c 12.51 a 14.92 b 96.0 d 60.11 a 30.40 b 11.90 c

25:75 30.10 c 13.25 b 13.10 b 97.0 d 58.91 a 34.11 c 9.16 b

Barley: Rice75:25 32.92 c 12.92 a 15.17 c 93.0 b 68.02 b 36.02 c 7.20 b

50:50 25.72 b 14.78 c 12.03 b 91.0 a 62.14 b 30.19 b 5.10 a

25:75 18.02 a 15.12 c 10.45 a 91.0 a 61.37 b 27.87 b 5.40 a

Values with different alphabets in column are significantly different (p ≤ 0.05).

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blended with these wheat, corn and rice starches at different levels results showed thesame decreased trend as shown in Table. 1. It is well known that when starch suspensionis heated, leaching of amylose (linear fraction) occurs, and upon cooling, micro-crystalsare formed and turbidity appears.[26] Factors responsible for lower transmittance instarches during storage have been previously identified by many researchers[30,25] andinclude aggregates made of leached amylose, amylase, and amylopectin chain length,intra or inter molecular bonding, granule swelling, and granule remnants. Starch gelsundergo structural transformation in terms of chain aggregation, recrystallization duringstorage, and these changes are referred to as retrogradation. Short-term development ofcrystallinity is attributed to molecule organization and crystallization of the amylosefraction [31]

Morphological Properties

Morphological characteristics, such as shape and size of the starch granules,exhibit significant differences. Barley starch granules are smooth–surfaced, oval, andirregular or cuboidal-shaped while corn, rice, and wheat starch granules are angular,pentagonal and angular; and spherical and lenticular–shaped, respectively. Corn, riceand wheat starch granules are less smooth–surfaced than barley starch granules. Barleystarch granules are largest (<110 μm) in size followed by wheat (<30 μm), corn (<25 μm)and rice (<20 μm) starches. The variation in the size and shape of starch granules isattributed to the biological origin.[2] The morphology of starch granules depends onthe biochemistry of the chloroplast or amyloplast, as well as physiology of theplant[32]. The granular structures of barley, corn, rice and wheat starches show signifi-cant variations in size and shape when viewed by SEM. Scanning electron micro-graphs of the starch granules from various plant sources. The granule size is variableand ranges from 1 to 110 μm. The average granule size ranges from 1–20 μm forsmall and 20–110 μm for large barley starch granules. The extent of variation in thegranular structure of starches from cultivar to cultivar is significantly higher in barley.The average size of individual cornstarch granules ranges from 1–7 μm for small and15–20 μm for large granules. The rice starch granules range from 3–5 μm in size.Barley starch granules have been observed to be oval and irregular or cuboidal inshape. The starch granules are angular-shaped for corn, and pentagonal and angular-shaped for rice. At maturity, wheat endosperm contains two types of starch granules:large A- and small B-type. A-type granules are disk-like or lenticular in shape withdiameters ranging from 10 to 35 μm. On the other hand, B-type starch granules areroughly spherical or polygonal in shape, ranging from 1–10 μm in diameter. It hasbeen observed under a scanning electron microscope, the surfaces of the granulesfrom corn, rice and wheat appear to be less smooth than potato starch granules. Li et al.[33]

observed the presence of ‘‘pin holes’’ and equatorial grooves or furrows in large-sizedcorn starch granules. Fannon[34] has shown the presence of large protuberances (200–500μm) on the surface of potato starch granules, using atomic force microscopy. Theindividual granules, in the case of rice starch, develop into compact spherical bundlesor clusters, known as compound granules, which fill most of the central space withinthe endosperm cells. Physicochemical properties, such as percent light transmittance,amylose content, swelling power and water–binding capacity were significantly corre-lated with the average granule size of the starches separated from different plantsources.[35]

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Thermal Properties

Table 2 shows gelatinization properties determined using DSC. DSC parametersrecorded were onset To, Peak Tp Final Tc gelatinization temperatures, gelatinizationenthalpy ΔH. Gelatinization temperatures and enthalpies associated with gelatinizationendotherms varied between starches. To was 59.79–70.12°C for native barley, wheat, riceand cornstarches and was varied between 58.11–68.56°C for barley starch blended withwheat, rice and corn at different ratios. Tp was 64.06–73.85°C for native barley, wheat,rice, and corn starches and was varied between 61.55–70.92°C for barley starch blendedwith wheat, rice and corn at different ratios and Tc was 68.42–78.20°C for native barley,wheat, rice, and corn starches and was varied between 67.14 barley, wheat, rice and cornstarches and was varied between 67.14–74.33°C for barley starch blended with other,ΔHgel was 10.62–26.3J/g for native starches and 3.32–23.06J/g for blended starches (Figure 1).Mixed starches showed lower onset and peak temperatures than native starches withthe same amylose content. The gelatinized temperature range (Tc-To) of native starch was8.0−16.7°C, whereas that of blended starch was 5.7–14.28°C. Waxy barley starch showedhigher gelatinization temperature and enthalpy than normal barley starch.[36] The enthalpyof gelatinization reflects the loss of molecular order[37] and gelatinization temperature isconsidered a parameter of crystallite perfection.[29] Because amylopectin plays a majorrole in starch granule crystallinity, the presence of amylose lower the melting temperatureof crystalline regions and the energy for starting gelatinization.[38] More energy is neededto intimate melting in the absence of amylose-rich amorphous regions.[39] This correlationindicated that starch with higher amylose content has more amorphous region and lesscrystalline, lowering gelatinization temperature and endothermic enthalpy. Native andmixed starches with the same amylose contents as native starches showed clearly differentgelatinization onset and peak temperature. The difference in gelatinization propertiesbetween native and mixed starches are due to varied homogeneity. Fredriksson[40]

reported that a wide temperature range implies a large amount of crystals with varied

Table 2 Summary of thermal properties, syneresis (%), and gel strength (N) of barley starch and their blends (n = 3).

Starch source

Thermal properties Syneresis (%) Storage(hrs)

To(°C) Tp(°C) Tc(°C) ΔHgel J/g 24 hrs 72 hrs 120 hrs 24 hrs 72 hrs 120 hrs

Barley starch 59.79 a 65.35 b 76.46 d 26.3 d 1.18 a 2.97 a 6.73 a 1.12 a 1.52 a 3.90 b

Wheat starch 60.19 a 64.06 b 68.42 a 18.93 b 2.21 b 4.37 c 6.54 a 1.01 a 1.39 a 3.72 a

Corn starch 70.12 d 73.85 d 78.20 d 10.92 a 7.10 e 8.01 e 8.65 d 1.32 b 2.82 d 6.72 d

Rice starch 67.50 c 70.8 c 75.5 c 10.62 a 1.93 b 3.88 b 8.43 d 1.89 d 2.55 d 4.67 c

Barley: wheat 75:25 60.32 a 63.53 b 71.13 b 23.06 d 1.97 b 3.19 a 7.11 b 1.03 a 1.37 a 3.97 b

50:50 60.14 a 62.74 a 67.14 a 18.14 b 2.01 b 3.58 b 7.87 c 1.11 a 1.45 a 4.2 c

25:75 58.11 a 61.55 a 67.50 a 19.11 c 2.37 b 3.67 b 7.93 c 1.21 b 1.50 a 4.37 c

Barley: corn75:25 64.64 b 74.54 d 78.92 d 10.70 a 3.14 c 4.57 c 6.90 a 1.37 b 1.88 b 3.92 b

50:50 63.34 b 68.30 c 70.02 b 7.12 a 4.37 c 5.78 d 7.50 b 1.45 c 2.01 c 4.88 c

25:75 68.56 c 70.92 c 74.33 c 3.32 a 5.03 d 5.97 d 7.79 c 1.52 c 2.14 c 4.92 c

Barley: rice75:25 65.17 b 69.46 c 73.67 c 19.74 c 1.20 a 3.01 a 6.67 a 1.31 b 1.69 b 3.3 a

50:50 62.59 b 64.87 b 70.41 b 19.56 c 1.31 a 3.12 a 6.99 a 1.52 c 1.92 b 4.1 c

25:75 60.14 a 62.46 a 66.04 a 17.69 b 1.32 a 3.52 b 7.06 b 1.72 d 2.01 c 4.8 c

To = onset temperature, Tp = peak temperature, Tc = end set temperature ΔHgel = Enthalpy of gelatinization(dwb, based on starch weight). Values with different alphabets in column are significantly different (p ≤ 0.05).

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stability. The lack of homogeneity of ordered inside structures caused a broader gelatini-zation range.[41]

74.33 °C for barley starch blended with other, ΔHgel was 10.62–26.3J/g for nativestarches and 3.32–23.06J/g for blended starches (Fig. 1). Mixed starches showed loweronset and peak temperatures than native starches with the same amylose content. Thegelatinized temperature range (Tc-To) of native starch was 8.0–16.7°C, whereas that ofblended starch was 5.7–14.28°C. Waxy barley starch showed higher gelatinization tem-perature and enthalpy than normal barley starch.[36] The enthalpy of gelatinization reflectsthe loss of molecular order[37] and gelatinization temperature is considered a parameter ofcrystallite perfection.[29] Because amylopectin plays a major role in starch granule crystal-linity, the presence of amylose lower the melting temperature of crystalline regions andthe energy for starting gelatinization.[38] More energy is needed to intimate melting in theabsence of amylose-rich amorphous regions.[39] This correlation indicated that starch withhigher amylose content has more amorphous region and less crystalline, lowering gelatini-zation temperature and endothermic enthalpy. Native and mixed starches with the sameamylose contents as native starches showed clearly different gelatinization onset and peaktemperature. The difference in gelatinization properties between native and mixed starchesare due to varied homogeneity. Fredriksson[40] reported that a wide temperature rangeimplies a large amount of crystals with varied stability. The lack of homogeneity ofordered inside structures caused a broader gelatinization range.[41]

Figure 1 Representative thermal endotherms of native Barley, Wheat, Corn, and Rice starches and their blends.

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Pasting Properties

Pasting viscosity profiles of starches analyzed by using RVA. RVA profiles variedwith the type of starch and its blends in different ratios. The peak viscosity indicates thewater holding capacity of starch and refers to the maximum viscosity reached during theheating and holding cycle. It can be affected by the molecular structure of amylopectin.[42]

Starch water concentration, lipids, residual proteins,[43] granule size,[44] and instrumentoperating conditions.[45] The average peak viscosities were in the range of 345 to 1240RVU for native barley shown in Figure 2, wheat, rice and corn starches and were variedbetween 226.58–326.83 RVU for barley starch blended with wheat, rice and corn at differentratios. The pasting temperature of native barley starch was 77.75°C, for corn starch was70.4°C, for rice starch was 75.25°C and for wheat starch was 66.4°C. When barley starchblend with cornstarch the pasting temperature varied between 73.1–75.1 and whenblended with rice starch pasting temperature was varied between 70.5–73.8°C and withwheat starch pasting temperature was 70.50–71.85°C. The peak viscosity of native andblended starches was depend upon its amylose content, As amylose content increase from10.9–41.4% the peak viscosity was also increased from 192.2 to 324.33 RVU for nativeand blended starches as shown in Table 3.

Figure 2 Representative Pasting profiles of native Barley, Wheat, Corn, and Rice starches and their blends.

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Pasting properties of starch are affected by amylose and lipid contents and bybranch chain-length distribution of amylopectin. Amylopectin contributes to swelling ofstarch granules and pasting, whereas amylose and lipids inhibit the swelling.[29] Further-more, the amylopectin chain-length and amylose molecular size produce synergisticeffects on the viscosity of starch pastes.[46] Thus the effects of the structural features on thepasting properties of starches are rather complex. The RVA results showed that the past-ing temperature of all starches were higher than the onset gelatinization temperatures (To)determined by DSC. The differences ranged between ∼3°C of rice and more than 30°C ofwheat and barley starches. In general, waxy cereal starches had lower pasting tempera-tures, higher peak viscosity, and lower setback viscosity than the normal starch counter-parts. The increase in viscosity observed during heating of starch in water was mainlyattributing to the swollen granules and also to the amount of solubilized carbohydrateswith reference to amylase.[47] However, further continuous heating and shearing at a hightemperature (95°C) promotes the weakening and susceptibility of the starch granules toshear damage (disruption).[48] In fact, the difference in branch chain length distribution ofamylopectin, crystallinity, granular size distribution, and presence of other componentslikely play an important role in differences in pasting properties among starches. The asso-ciation between amylose and amylopectin molecules in mixed starches was different fromthat of individual starches, which induced specific chain interactions (between molecules,starch granules, swollen granules, and granule fragments) during heating, and each starchgelatinized independently of the other, thus providing double-peak viscosities. However,this phenomenon was not observed in mixtures made with high amylose starch and waxystarch nor between non-waxy starch mixtures. It is plausible that the very long branch-chains of amylopectin mimic amylose to form helical complexes with lipids and inter-twine with other branch chains to hold the integrity of starch granules during heating andshearing.[49] Sasaki et al.[50] investigate that lower amylase content is associated withhigher peak viscosity. Reduced amylose content starch relates to greater swelling. Greaterswelling reduces the quantity of free water and is associated with higher pasting viscosity.[51].Leloup et al.[52] reported that a minimum amylose to amylopectin ratio of 0.43 was neededto maintain the gel network during heating in water. At lower amylose content, the structure

Table 3 Summary of pasting properties of barley starch and their blends (n = 3).

Starch source Peak 1RVU Trough Set back Break down Final viscosity Pasting temp.

Barley starch 324.33 d 252.75 d 169.58 d 71.58 b 422.33 d 77.75 d

Wheat starch 256.00 b 152.10 c 127.00 c 104.00 c 279.01 b 66.4 a

Corn starch 240.00 b 95.00 b 145.10 d 167.00 d 190.20 a 70.4 b

Rice starch 192.20 a 138.00 c 78.11 b 97.00 c 166.22 a 75.2 d

Barley: wheat 75:25 287.58 c 76.00 b 58.33 a 57.58 a 328.33 c 71.8 b

50:50 279.50 c 86.33 b 60.33 a 53.17 a 351.92 c 68.8 a

25:75 261.00 b 85.25 b 68.25 b 55.75 a 353.50 c 70.5 b

Barley: corn75:25 284.92 c 73.00 a 58.92 a 51.92 a 231.92 b 73.1 c

50:50 276.58 c 69.58 a 58.42 a 57.00 a 228.00 b 75.0 d

25:75 258.67 b 72.83 a 60.33 a 53.83 a 233.17 b 75.1 d

Barley: rice75:25 279.75 c 73.33 a 60.92 a 46.42 a 234.25 b 73.3 c

50:50 243.08 b 70.83 a 58.08 a 52.25 a 228.92 b 71.4 b

25:75 226.83 b 71.75 a 60.42 a 55.08 a 232.17 b 70.5 b


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of starch gel is easily disrupted by heating. Amylose suppresses swelling and maintainsthe integrity of swollen starch granules.[6] Because starch swelling is mainly a property ofamylopectin[29] waxy starch swells rapidly and swollen granules degrade at lower temper-ature, indicating that waxy starch rapidly develops viscosity but cannot maintain the sta-bility of paste viscosity. The increase in viscosity during cooling is induced byleaching out amylose rearranging and forming a thin amylose gel layer.[38] This suggeststhat starch with lower amylose content decreased the amount of leached out amylose, sup-pressing viscosity during cooling. The breakdown (peak viscosity minus viscosityafter holding at 95°C (HPV) is caused by the disintegration of gelatinized starch granulestructure during continuous stirring and heating.[43] The differences among individual ricestarch samples and their blends in breakdown are related to differences in rigidity of swollengranules.[53,54,55] Moreover, amylose has a marked influence on the breakdown viscosity,which is a measure of the susceptibility of cooked starch granules to disintegration.[56]

Rheological Properties

At the earlier stages of heating, i.e., slightly before 60°C, the increase in G′ was rel-atively slow and the starch suspension was transformed into a ‘‘sol,’’ and the amylosemolecules were dissolved from the swollen starch particles.[57] At a certain temperatureabove 60°C (>To), G′ of all the heated starch suspensions increased rapidly (as the starchgranules kept swelling) to a maximum G′, tan ð (G″/ G′), G″ max shown in Figure 3. Theinitial increase in G′ was attributed to the interplay of the following factors: the progres-sive swelling of starch granules that finally become a close-packed network[52,12]; the sol-ubilized amylose that was released during the heating process, and the increase in gelvolume. In this study, the temperature TG′max for native and blended starches variedfrom 63.7 to 75.4° C and from 65.6 to 74.1°C for blended starches, respectively (Table 4).The rheological properties of starch mixtures varied to a large extent with respect to starchsources (Table 4). Svegmark and Hermansson[2] reported that, during the heating cycle,the difference in the G′, G″, and tan ð should be due to the difference in starch granularstructure. Moreover, at G′max and TG′max all starch concentration systems studied dur-ing our experiment exhibited G′ values >5000 Pa and tan ð values <0.2, and these areparameters of starch gels.[7] The gelatinized starch granules strengthened the gel networkformed by amylose[58,59] it illustrates the results from selected representative profiles andcompares the effects of different ratios in between individual native barley starch and theirblends with wheat, corn and rice starches on changes in storage modulus, G′, loss modulus,G″, during heating-cooling cycles. The parameters recorded of all starch suspensions aregiven in Table 4. Moreover, these effects showed similar changing patterns as the temper-ature increased. It was observed that G′ increased to maxima between 63.7–75.4° C andfrom 65.6–74.1°C for native and blended respectively and the sol-to-gel transition wasreinforced by strong interactions among the swollen starch particles[57] when the leachedamylose formed a gel network. Keetls and Van Vliet[60] indicated the decrease in elasticityafter reaching G′ max can be attributed to the melting of the remaining crystalline regionin the granules (or disintegration of close-packed swollen granules) that result in the gran-ules becoming softer. Furthermore, differences in amylose contents in individual nativerice starches and their respective mixtures are probably the main reason for the observeddifferences in G′ and G″. For starch mixtures, Depending on the proportion of the nativebarley starch in the mixture, the G′ peak maxima for barley starch blends were generallybetween those of their individual starch components at both ratios.

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During cooling, the G’ values of all native starch systems and their correspondingstarch blends decrease during cooling. When heated starch pastes were cooled from 85 to25°C, a quick development of gel formation took place due to intermolecular associationin polymer-rich regions[61,8] and amylose aggregation. In addition, a general decrease in

Figure 3 Representative Rheological properties of native Barley, Wheat, Corn, and Rice starches and theirblends (B = Barley, C = Corn, W = Wheat, R = Rice).

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tan ð was observed. Among individual native starches, barley starch exhibited relativelythe highest G′, G″ values and lowest tan ð, which proved the formation of the most rigidgel structure at both total starch concentrations. The blend made of barley: wheat, 75:25ratio showed the highest G′ and G″ values. These results show that T G′ max, G″ and tan ðare likely governed by amylose content. The rheogram that shows the influence of differentratios on change in loss modulus, G″ during cooling proved that the system was not stableand the phenomenon can be probably attributed to a lack of the rigidity and integrity ofcooked granule. However, appreciable increases in storage modulii for the related blendsof waxy starches occurred at both ratios (Table 4). Tan ð varied significantly among indi-vidual native starches and their corresponding starch mixtures. This variation was ascribedto the difference in G′ and G″. Mixtures made of low, intermediate and high amylose contentdeveloped generally G’ values between those of their individual components. A similar con-clusion was drawn earlier in studies on starch mixtures from different botanical sources.[62]

Retrogradation Properties

The retrogradation of gels prepared from native barley, wheat, corn, rice, and theirblended starches at different ratios was measured by determining % syneresis during storage at4°C. The % syneresis value of gelatinized starch gels from the native to blended starches dif-fered significantly. For barley it was increased from 1.18–6.73%, for wheat, it was 2.21–6.54%,for corn, it was 7.10–8.65% and for rice, it was 1.93–8.43%. The same trend was followedwhen barley starch blended with others at different levels shown in Table. 2.

Native barley, wheat and rice starches showed lower values for % synersis than thenative cornstarch that may be attributed to the rigid granular structure as shown in Figure 4,and presence of lipids in starch[63]. The differences in amylose content of barley, wheat,rice, and cornstarches might have also affected the retrogradation properties. Waxy starchhas been reported to retrograde slowly but pea and potato starches with high amylose con-tent.[64,65] Native corn and wheat starch gels showed higher % syneresis values than rice

Table 4 Summary of rheological properties of barley starch and their blends (n = 3).

Starch source

Heating Cooling

TG′(°C)Peak G′

(Pa)Peak G′′

(Pa)Peak tan d

Peak G′(Pa)

Peak G′′(Pa)

Peak tan d

Barley starch 75.4 d 37100 a 5270 a 0.142 c 26400 b 3340 c 0.126 c

Wheat starch 69.3 b 29400 a 5280 a 0.179 c 17210 a 2440 b 0.142 d

Corn starch 63.7 a 34100 b 6400 c 0.187 c 21200 b 3030 c 0.143 d

Rice starch 67.8 b 22300 a 8590 d 0.385 d 11200 a 1870 a 0.167 d

Barley: wheat 75:25 72.1 c 95300 d 6270 b 0.065 a 65400 d 1120 a 0.017 a

50:50 69.1 b 72200 c 5350 a 0.074 b 32200 c 2410 b 0.075 b

25:75 68.7 b 64800 b 4470 a 0.068 a 27200 b 1840 a 0.068 b

Barley: corn75:25 70.0 c 67600 b 5800 b 0.085 b 36600 c 4070 d 0.111 c

50:50 67.3 b 83400 d 5350 b 0.064 a 50200 c 3260 c 0.065 b

25:75 65.6 a 90400 d 5870 b 0.065 a 60700 d 2200 b 0.036 a

Barley: rice75:25 74.1 d 71700 b 5350 a 0.075 b 44600 c 3860 d 0.086 b

50:50 69.2 b 70300 b 6050 b 0.086 b 51103 c 3550 c 0.069 b

25:75 67.3 b 89600 d 6170 b 0.069 a 54200 d 3710 d 0.068 b


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and barley (Table 2). As barley, starches blended with corn, wheat and rice at differentlevels the syneresis values of gels from that increased progressively during storage. Nativestarches from the corn and wheat starch having large sized granules showed higher %syneresis values while those barley and rice having small sized granules showed lowersyneresis values (Table 2). The retrogradation properties of starches are indirectly influ-enced by the structural arrangement of starch chains with in the amorphous and crystallineregions of the ungelatinized granule which in turn, influence the extent of granule break-down during gelatinization and the interactions that occurs between starch chains duringgel storage.[66] The rapid initial rate of retrogradation was related to the loss of networkedamylose[65] to the development of amylose aggregates, and the bonding of granule rem-nants into assemblies by amylose and amylose aggregates.[30] It seems logical to mentionthat starch retrogradation is governed by a consecutive three-step mechanism that involvesnucleation, propagation and maturation. For native starch and mixed starches lower pasteclarity was explained by the presence of chain polymers that are resistant to retrograda-tion. In fact, the network of solubilized waxy starch was retained much of the amylose thatleached out of the non-waxy starch, contributing to the lower paste clarity.[67]

Gel Strength

In order to investigate the gel strength the starch gel was prepared with 14% starchsuspension of native barley, wheat, corn and rice starches and their blends. The hardnessvalues were measured at 24hrs, 72hrs and 120 hrs after storage at 4°C. The results areshown in Table 2. It can be observed that the hardness increased with the storage timeand thus with the extent of starch retrogradation. The results showed that native ricestarch exhibit the highest hardness (firmness) value followed by corn, barley, and wheat

Figure 4 Scanning electron micrographs (SEM): (a) rice, (b) wheat, (c) corn, (d) barley (bar = 10 mm).

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starch. As barley, starch blended with corn, rice, and wheat at different ratios the gelstrength values increased during storage up to 120 hrs at 4°C, Table 2. Starch with highapparent amylose content gave pastes with high elasticity and greater rigidity. This obser-vation was linked to a greater retrogradation tendency. Thus, the retrogradation rate and thedegree of retrogradation orders were Corn > Rice > Barley > Wheat. Barley starch gelationwas a much slower process, i.e., the amylopectin recrystallization was slower compared tothat of amylose, which re-associates rapidly during cooling similar results were earlierdescribed.[68] Corn and rice starch gel showed highest compression force of 6.72N and4.67N, respectively while barley and wheat starch gel showed the lower compression forceof 3.9N and 3.72N. While in case of barley and wheat blended starches at different levelcompression force varies between 3.97–4.37N, barley and corn blended starches was variedbetween 3.92–4.92N and barley and rice blended starches was varied between 3.3–4.8N.The differences in compression force values of gels from different starches may be due tovariation in their rheological and morphological properties. Hopkins and Gormley[27] alsoreported higher compression force values for potato starches having high peak viscositymeasured by Visco-amylograph and phosphorus content. The gel compression force valuesincreased with storage duration. During storage, the starch molecules rearrange thus stiff-ness and rigidity of the granules increases.[69]

CONCLUSION

It is possible to formulate starch blends from unmodified starches that possess at leastsome of the desired characteristics of modified starches. It appeared that starch moleculesfrom different starches interact to produce the attributes of the blends and that at least someof the interactions occur very early in the heating process, before gelatinization and pastingoccur. The amylose content of native barley, corn, wheat and rice starches varied widely.During thermal analysis using DSC, two characteristic endotherms peaks were observed insome cases at different temperature intervals. Measurement of G′ was used to study thedevelopment of the network structure of the gel. The rate of development of storage modu-lus, G′ and loss modulus G″, are concentration dependent. We conclude that TG′ max, G′and tan ð are certainly governed by amylose content. Amylose content suppresses starchswelling and appears to play a critical role in determining starch-pasting properties usingRVA. Transmittance values of native and blended starch suspensions stored at 4°C gener-ally decreased during storage up to day 6. The results indicated that the retrogradation ten-dency was the lowest in low amylose and highest in high amylose starch. Amylose contentplays the most important role in changing the elasticity of the studied starchy systems aftercooling up to 5°C, and during aging. Low amylose blends of 14% were most effective atreducing the rate of gel firmness as shown by the values of hardness when mixed with lowand non-waxy starches. Starch mixtures could have a place in the food industry where allnatural products are desirable and in the food and other industries, where reduction in theuse of chemicals for modification is desirable.

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morphological, thermal, pasting, and rheological properties of barley starch and their blends

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