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2006 12: 253Food Science and Technology International A. Tárrega, E. Costell and M. A. Rao

-carrageenan Addition.λConcentration andVane Yield Stress of Native and Cross-linked Starch Dispersions in Skimmed Milk: Effect of Sta

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Vane Yield Stress of Native and Cross-linked Starch

Dispersions in Skimmed Milk: Effect of Starch Concentration

and -carrageenan Addition.

A. Tárrega,1,* E. Costell1 and M.A. Rao2

1 Instituto de Agroquímica y Tecnología de Alimentos (CSIC), PO Box 73, 46100 Burjassot, Spain 2Department of Food Science and Technology, Cornell University, Geneva, NY 14456–0462, USA

The effects of starch concentration and -carrageenan addition on the yield stress values of native andcross-linked waxy maize starch-milk systems were studied. Static yield stress (0-S) and dynamic yieldstress (0-D) of each dispersion were measured using the vane method before and after breaking down itsstructure by shearing, respectively. Increases in values of 0-S with concentration of starch and -car-rageenan were higher for cross-linked starch than those of native starch. 0-D values of cross-linkedstarch also increased with the concentration of starch and -carrageenan. In contrast, for native starch

samples, 0-D values were very low and did not vary much with concentration of either starch or -car-rageenan indicating that the structures responsible for the yield stresses were highly sensitive to shearing.

Key Words: yield stress, vane method, starch, milk, -carrageenan

INTRODUCTION

The rheological behaviour of commercial dairydesserts (Batista et al., 2002; Tárrega et al., 2004 and2005a) or of their corresponding model systems (Wis-chmann et al., 2002; Depypere et al., 2003; Vélez-Ruiz

et al., 2005) in general, show time dependent, shear-thinning flow behaviour, with a measurable initialresistance to flow, as well as viscoelastic propertiestypical of structured gels. This behaviour is stronglyinfluenced by the particular characteristics of theiringredients, including: milk fat content, type and con-centration of starch and hydrocolloid, and their crossedinteractions. An important aspect of this behaviour isthe initial resistance to flow of these products. Oralthickness of the commercial dairy desserts showed agood correlation with the yield stress values (Tárregaand Costell, 2006) in agreement with the comments of van Vliet (2002) about the assessment of thickness of

products with a high viscosity or with a yield stress thatcan be related with the pressure (stress) required toproduce a significant flow.

*To whom correspondence should be sent (e-mail: [email protected]).Received 30 June 2005; revised 21 November 2005.

Food Sci Tech Int 2006; 12(3):253–260© 2006 SAGE PublicationsISSN: 1082-0132DOI: 10.1177/1082013206065551

The concept of yield stress as a true material property has been questioned (Barnes and Walters, 1985however its practical usefulness in process design andmodelling, and in quality control of foods is generallyaccepted (Nguyen and Boger, 1992). The existence of ayield stress in a material’s flow indicates that there is across-linked or other interactive structure which musbe broken down before flow can occur at an appreciable rate. Thus, the yield stress of a food can be considered to be the minimum stress required to initiateflow, and that it is a measure of the strength of thematerial network. There are several ways to evaluatethe yield stress of foods. Indirect methods involve theextrapolation of the shear stress–shear rate data tozero shear rate, with or without the help of a flowmodel such as Bingham, Casson or Herschel-Bulkley(Canet et al., 2005). The yield stress values thuobtained are dependent on the model used to fit thedata and on the range of shear rates applied, but they

are relatively easy to determine. Yield stress values ostarch dispersions heated in water or milk wereobtained by applying the Herschel-Bulkley and Cassonmodels (Acquarone and Rao, 2003; Nayouf et al., 2003Vélez-Ruiz et al., 2005; Tárrega et al., 2005b).

One direct and simple method to measure yieldstress of structured products is by vane rheometry. Theparticular advantages of the vane geometry are its simplicity of fabrication, ease of cleaning and more thananything else, minimal destruction of the sample structure during loading and its elimination of wall-slipeffects (Barnes and Nguyen, 2001). Vane rheometry



has been used to measure directly the yield stress of different foods (Qiu and Rao, 1988; Briggs et al., 1996;Rao and Steffe, 1997; Truong et al., 2002) as well asthat of aqueous starch dispersions. Genovese and Rao(2003) studied the structural differences in cross-linked, tapioca and amioca starch dispersions, by mea-suring yield stress before (0-S) and after breaking

down its structure (

0-D) with the vane method. Basedon earlier studies on dispersions (Michaels and Bolger,1962; Metz et al., 1979), those authors suggested thatthe 0-S was due to contributions of bonding, networkand viscous components. More recently, Genovese etal. (2004) studied the effect of sucrose or fructose onthe yield stress of gelatinised amioca starch dispersionsusing both the vane method and the indirect methodby extrapolation of shear stress–shear rate data fittedto the Herschel-Bulkley model.

Starch is frequently used in combination with car-rageenan in the formulation of dairy products. Whenstarch is heated in water, starch granules swell and after

cooling a viscous paste is formed, a biphasic structurecomposed of swollen starch granules in a continuousphase. In native starch, the continuous phase is essen-tially an aqueous solution of the amylose leached duringgranule swelling (Nguyen et al., 1998; Thebaudin et al.,1998). Native and modified starches show differentbehaviours during heating and cooling processes. Modi-fied starch shows higher thermomechanical resistanceand if the granules remain whole after pasting, the resul-tant system is an aqueous dispersion whose rheologicalbehaviour mainly depends on the granules volumetricfraction and on their rigidity or deformability (Tat-tiyakul and Rao, 2000; Nayouf et al., 2003).

Quantitative changes in the rheological response of these systems may be originated by substituting milkfor water (Matser and Steeneken, 1997; Tárrega et al.,2005b) or by adding different types of hydrocolloids(Umadevi and Raghavendra, 1987; Alloncle and Dou-blier, 1991; Liu and Eskin, 1998). For commercial dairyproducts, carrageenans, mainly - and -carrageenan,are considered to be the most suitable hydrocolloidsdue to their capability of combining into double helicesand to their interactions with casein, with which theyform network structures. -carrageenan moleculeshave a strongly anionic charge due to the presence of

three sulphate groups, and they do not form gels inaqueous solutions, causing only an increase in viscosity.However it has been shown that -carrageenan is ableto form gels in the presence of milk (Langendorff et al.,2000; Shchipunov and Chesnokov, 2003). According tothe former authors, -carrageenan is not capable of combining into double helices, however it shows attrac-tive interactions with casein micelles, that can inducebinding of the micelles and the formation of a car-rageenan/casein network on cooling.

The objectives of this work were to study the effectsof starch and of -carrageenan concentrations on the

vane yield stress values of both native and cross-linkedwaxy maize starch-skimmed milk dispersions.

MATERIALS AND METHODS

Materials

Two types of waxy maize starch: cross-linked starch(Purity®-W), native starch (amioca) from NationalStarch and Chemical Co., NJ, USA and -carrageenan(Satiagum™ ADC 25) from Degussa TexturantSystems, NE, USA were used in this study. Skimmedmilk was prepared by dissolving 12% (w/w) commer-cial skimmed milk powder in distilled water 24h inadvance.

Methods

Sample Preparation

The first lot of samples, containing a fixed amount of skimmed milk (80%) and varying in starch concentra-tion (4.0, 4.5, 5.0 and 5.5%) and in the type of starch(native and cross-linked starch) were prepared to studythe effect of starch concentration. A second lot of samples, containing fixed amounts of each type of starch (4.0%) and skimmed milk (80%) was preparedfor the evaluation of the effect of -carrageenan con-centration (0, 0.02, 0.03, 0.04, 0.05 and 0.06%). Samples(500g) were prepared as follows: a concentrated sus-pension of starch in water was heated to 50°C, whichwas below the gelatinisation temperature. The starchsuspension was added to the hot (87°C) milk-car-rageenan mixture, under magnetic stirring, until thefinal mixture temperature reached 80°C. The samplewas held at this temperature for 10min by heating in arotating round-bottom flask submerged in a thermosta-tic bath (Rotavapor, Büchi, Switzerland; Genovese andRao, 2003). Finally the sample was cooled by submerg-ing the rotating flask in a water-ice bath until thesample reached a temperature of 20°C (approximately10min).

Rheological Measurements

Samples were placed into a jacketed stainless-steelvessel (diameterϭ7.2cm, heightϭ11.7cm) connectedto a constant temperature circulator (Haake DC30-k20, Paramus, NJ, USA) at 5±1ºC for 2h to get thestructure formation. Rheological measurements werecarried out using a Haake Rotovisco RV30 viscometerequipped with a 6-blade vane impeller(diameterϭ4cm, heightϭ6cm) and monitored by aHaake Rotation v. 3.0 software. The 0-S of the undis-rupted sample was measured by recording the magni-tude of torque with the vane rotating at a constant

254 A. TÁRREGA ET AL.



speed of 0.05rpm. Recorded torque values were con-verted to vane stress values using the following equa-tions:

ϭᎏ

K

Tᎏ (1)

Kϭᎏ

2

D3

ᎏᎏH

Dᎏϩ ᎏ

1

3ᎏ (2)

where (Pa) is the vane stress, T is the torque and K isthe vane parameter that depends on the D (diameter)and H (height) of the vane impeller. After disruptingthe sample structure by shearing it from 1 to 258.6rpmin 10min up and 10min down, the yield stress wasmeasured again. This value was considered to be the0-D of the disrupted sample. Each measurement wasdone in duplicate.

Statistical Analysis

ANOVA, including two factors with interactions,was used to study the combined effect of starch typeand starch concentration on 0-S and 0-D values of starch-milk dispersions and to study the combinedeffect of starch type and carrageenan concentration on0-S and 0-D values of starch-milk--carrageenan dis-

persions. The Fisher test (␣ϭ0.05) was used to calculate the minimum significant difference. All calculations were carried out with the Statgraphics Plus v. 4.1software.

RESULTS

Shear Stress-time Curves

The shear stress-time curves for undisrupted anddisrupted samples were registered and some of themare represented as an example in Figure 1. The curveshowed an initial increase in stress that represents theelastic response of the material, followed by a decreaseof the stress values, associated with the gradual structure breakdown (Truong et al., 2002). Stress decay inthe disrupted samples was very small, due to therupture of the structural bonds during shearing (Genovese and Rao, 2003). 0-S and 0-D values wereobtained from the peak (or the maximum values) othe curves corresponding to the undisrupted and disrupted samples, respectively, and the angular deformation of the sample at this point (F) was calculatedusing the following equation:

Vane Yield Stress of Native and Cross-linked Starch Dispersions in Skimmed Milk 255

Time (min)

0 2.5 5 7.5 10

S h e a r s t r e s s ( P a )

45

40

35

30

25

20

15

10

5

0

Time (min)

0 2.5 5 7.5 10

S h e a r s t r e s s ( P a )

75

40

35

30

25

20

15

10

5

0

45

50

55

60

65

70

Figure 1. Vane stress versus time curves obtained at 0.05 rpm for native (a) and cross-linked (b) starch samples

with different compositions: 4% starch (), 5% starch (᭡) and 4% starch ϩ0.03% -carrageenan (᭹). Undisrupted sample (empty symbols) and disrupted samples (filled symbols).

(a) (b)



Fϭᎏ2

6

0ᎏN ·t (3)

where t (s) is the time required to reach the maximumstress and N corresponds to the rotational speed (rpm).The values of angular deformation at failure mainlydepended on the type of starch, being higher in thecase of native starch-milk dispersions than in cross-

linked starch-milk dispersions, indicating that the latterreached the failure point at lower angular deforma-tions and showed a more ‘brittle’ behaviour (Table 1).This observation is in agreement with the dataobtained by Genovese and Rao (2003) in aqueous dis-persions of cross-linked waxy maize, tapioca andamioca starches. They observed that cross-linked waxymaize starch dispersion yielded at lower angular defor-mation than tapioca and amioca starch dispersions.The values of deformation at failure were not signifi-cantly affected by the starch concentration (Table 1) orby the addition of -carrageenan (Table 2).


Table 1. Values of angular deformation at failure (F)

for undisrupted and disrupted starch samples fordifferent starch types and concentrations.

Starch

F (rad)

Concentration Undisrupted Disrupted

Starch Type (%) Sample Sample

Cross-linked 4.0 0.41 0.12

4.5 0.41 0.11

5.0 0.41 0.15

5.5 0.40 0.18

Native 4.0 1.63 0.99

4.5 1.74 0.90

5.0 2.07 0.87

5.5 1.80 0.99

Table 2. Values of angular deformation at failure (F)

for undisrupted and disrupted 4% starch samples fordifferent starch types and -carrageenan

concentrations.

-carrageenan

F (rad)

Concentration Undisrupted Disrupted

Starch Type (%) Sample Sample

Cross-linked 0.00 0.41 0.12

0.02 0.58 0.11

0.03 0.57 0.15

0.04 0.69 0.18

0.05 0.66 0.18

0.06 0.65 0.18

Native 0.00 1.63 0.99

0.02 1.66 0.92

0.03 1.55 0.78

0.04 1.64 1.09

0.05 1.63 1.02

0.06 1.44 0.92

Effect of Starch Concentration on 0-S and 0-D Values

The effects of starch concentration and type of starch, on 0-S and 0-D values were studied in thestarch-milk dispersions. For both parameters, the effectof the interaction, starch type–starch concentration wassignificant (Fintϭ47.41, pϽ0.001, for 0-S andFintϭ34.94, pϽ0.001, for 0-D) which means that theeffect of starch concentration on both parameters wasdifferent depending on the starch type. The 0-S valuesincreased with starch concentration for both native andcross-linked starch dispersions (Figure 2). The incre-ments were higher for cross-linked starch dispersions,which at 4 and 4.5% starch concentration showedlower 0-S values than native starch dispersions, whileat 5 and 5.5% starch concentration they were higher.The variation of 0-S stress values with starch concen-tration for native and cross-linked starch followed an

exponential function (Equations (4) and (5), respec-tively).

0-Sϭ10.3·e0.24·[starch concentration] R2ϭ0.951 (4)

0-Sϭ2.7·e0.75·[starch concentration] R2ϭ0.991 (5)

As expected, the values of yield stress after shearing(0-D, Figure 3) were lower than 0-S values. The 0-D

values for native starch samples were very low andthey did not vary with starch concentration, indicatingthat in these dispersions the structure responsible for

S t a t i c y i e l d s t r e s

s ( P a )

60

50

40

30

20

10

04 4.5 5 5.5

Starch (%)

Figure 2. Variation of 0-S values with starch concen-tration for cross-linked (ᮀ ) and native (᭝) starchsamples and the corresponding fittings to exponential

equations (—).



the yield stress was highly sensitive to shearing andthat most of the structural bonds were broken. Forcross-linked starch, the variation of 0-D values withstarch concentration followed the same trend as that of 0-S values, i.e., they increased following an exponentialfunction (Equation (6)).

0-Dϭ2.32·e0.73·[starch concentration] R2ϭ0.9961 (6)

According to Genovese and Rao (2003), when yieldstress is calculated from vane mixer data, the 0-S canbe considered to be the total failure stress of the undis-turbed starch dispersion and the magnitude of the dif-ference between 0-S and 0-D could be associated withthe stress required to break the bonds, 0-B, that con-tributed to the initial resistance to flow of the material.The ratio of this difference to the total yield stress, (0-

B/0-S), in the studied starch dispersions represents therelative contribution of the stress required to break the

bonding to the total stress necessary to initiate flow(Figure 4). The contribution of bonding was higher inthe native starch dispersions (70–80%) than in thecross-linked ones (10–20%), probably due to the roleof leached amylose in the former in the formation of bonds.

Effect of -carrageenan Concentration on 0-S and 0-D

Values

For both 0-S and 0-D values, ANOVA resultsshowed a significant interaction between -car-


D y n a m i c y i e l d s t r e

s s ( P a )

60

50

40

30

20

10

04 4.5 5 5.5

Starch (%)

Figure 3. Variation of 0-D values with starch concen-tration for cross-linked () and native (᭡) starchsamples. (—) Fit to exponential equation for cross-

linked starch.

( 0 - S

– 0 - D

) /

0 - S

Starch concentration (%)

1

0.8

0.6

0.4

0.2

04 4.5 5 5.5

Figure 4. Effect of starch concentration on the contribution of stress required to break the bonding (0

SϪ0-D) relative to the total stress necessary to initiate

flow (0-S) for cross-linked ( ) and native (ᮀ ) starchdispersions.

rageenan concentration and starch type (Fintϭ26.43 pϽ0.001 for 0-S and Fintϭ17.01, pϽ0.001 for 0-D). The0-S values increased with -carrageenan concentrationbut the magnitude of the increase depended on thetype of starch. Cross-linked starch samples, which contained intact granules, showed a higher increase of 0-

with -carrageenan concentration than native starchdispersions (Figure 5). The variation of 0-S with -carrageenan concentration followed a sigmoid function(Equation (7) for native starch and Equation (8) focross-linked starch) indicating that the 0-S valueincreased with the addition of -carrageenan up to alimit.

0-Sϭ R2ϭ0.990 (7

0-Sϭ R2ϭ0.992 (8

This behaviour can be explained by the ability of carrageenan bridging casein micelles. According to the

observations of Shchipunov and Chesnokov (2003), themodel’s network structure is composed of polysaccharide molecules and casein micelles acting as its nodesexplains well the effect of carrageenan in milk systemsFor - and -carrageenan, the stabilisation of the genetwork is achieved by cross-linking of chain fragmentbetween the micelles by double helices. For -carrageenan, gelling is ensured only by binding withcasein micelles. As the amount of -carrageenanincrease, the casein molecules get saturated and nofurther association with -carrageenan molecules canoccur, that would increase the gel network strength

136ᎏᎏᎏ

1ϩ100.03Ϫ[% carrageenan]/0.017

55.71ᎏᎏᎏ

1ϩ100.0168Ϫ[% carrageenan]/0.036



and consequently the 0-S values. On the other hand,the fact that the effect of -carrageenan concentrationon 0-S values showed a similar trend for both nativeand cross-linked starch may indicate that the increasein 0-S with -carrageenan concentration could beattributed in part to the increase in the effective starchconcentration, due to the ability of -carrageenan forbinding water.

The effect of -carrageenan addition on 0-D valueswas smaller than that on 0-S values and only in thecase of cross-linked starch samples 0-D valuesincreased significantly with -carrageenan concentra-tion (Figure 6), following an exponential function(Equation (9)):

0-Dϭ5.22·e25.1·[%carrageenan] R2ϭ0.974 (9)

The presence of -carrageenan may have increasedthe effective concentration of the intact cross-linkedstarch granules and this could explain the increase in0-D values by increasing -carrageenan concentration.

The ratio of 0-B (0-Bϭ0-SϪ0-D) to the total yieldstress (0-S) for dispersions with different -car-rageenan concentration (Figure 7) indicated that thecontribution of bonding increased due to the presenceof -carrageenan-casein network for both native andcross-linked starch dispersions. For the cross-linkedstarch dispersions, compared to the control sample(0% carrageenan), the relative increase in bonding washigh at 0.02% -carrageenan concentration and it didnot change much over the concentration range 0.03 to0.05%. Further, comparison with Figure 4 shows that-carrageenan-casein network played a major role in


S t a t i c y i e l d

s t r e s s ( P a )

Carrageenan (%)

125

150

75

50

25

0Ϫ0 0.02 0.03 0.07

100

0 0.01 0.04 0.05 0.06

Figure 5. Variation of 0-S values with -carrageenanconcentration for 4% cross-linked (ᮀ ) and 4% native

(᭝) starch dispersions, and the corresponding fittingsto sigmoid equations (—).

D y n a m i c

y i e l d s t r e s s ( P a )

Carrageenan (%)

45

50

35

30

25

0Ϫ0 0.02 0.03 0.07

40

0 0.01 0.04 0.05 0.06

2015

10

5

Figure 6. Variation of 0-D values with -carrageenanconcentration for 4% cross-linked () and 4% native(ᮀ )starch dispersions in milk. (—) Fit to exponentialequation for cross-linked starch.

the contribution of bonding of the cross-linked starchdispersions.

Texture Map

Traditionally the textural characterisation of vis-coelastic food has been carried out using compressionor torsion tests. Truong et al. (2002) observed that thegrouping of samples in the texture map generated froma compression test was similar to that generated from avane test. A plot of vane yield stress (failure stress)against angular deformation (or strain) can provide uswith a ‘texture map’ of the samples and it has beenused to describe some textural differences between dis-persions of different starch types (Genovese and Rao,2003). The plot obtained in this study (Figure 8) indi-

( 0 - S

– 0 - D

) /

0 - S

Carrageenan (%)

1

0.8

0.6

0.4

0.2

00 0.03 0.05 0.060.02 0.04

Figure 7. Effect of -carrageenan concentration onthe contribution of stress required to break thebonding (0-SϪ0-D) relative to the total stress neces-

sary to initiate flow (0-S) for 4% cross-linked ( ) and4% native (ᮀ ) starch dispersion.



cated that samples containing amioca starch showedmore ‘rubbery’ characteristics than samples containingcross-linked starch. For both starch types, increasingthe starch content or the carrageenan content of samples resulted in a decrease of the spreadability andthese effects were higher for cross-linked samples. Inthe case of cross-linked starch samples, those contain-ing carrageenan showed more ‘tough’ characteristicsthan samples without carrageenan.

CONCLUSIONS

Differences in plasticity found between native andcross-linked starch revealed important differences inthe structure of the starch–milk systems. For cross-linked systems, the increase of the structure strengthwith the starch concentration was higher than fornative starch systems. The presence of small amounts

of -carrageenan resulted in a large increase in thestructure’s strength. The differences in the magnitudeof this effect found among samples containing differentstarch types, indicated that starch still plays an import-ant role in the structure of this type of system. Theeffect of -carrageenan addition on the yield stressvalues can be explained by the sum of two effects: theability of -carrageenan to immobilise water, thusincreasing the effective starch concentration, and theformation of a -carrageenan-casein network, due tothe simultaneous presence of -carrageenan and caseinin the continuous phase.


Brittle Tough

Mushy Rubbery

S p r e a d a b i l i t y

0.06% C

5.5% S

4% S

0.02% C

0.06% C

0.02% C5.5% S

4% S

10

F a i l u r e

s t r e s s ( P a )

140

10.1

120

100

80

60

40

20

0

Angular deformation (rad)

Figure 8. Texture map for samples containing differ-ent starch concentration (triangles) and different car-rageenan concentration (squares). Cross-linked

starch (black symbols) and Amioca starch (whitesymbols).

ACKNOWLEDGEMENTS

We are grateful to MEC of Spain for financiasupport (Project AGL 2003–0052), for the fellowshipand for the aid to stay in Cornell University awardedto author Tárrega. To National Starch and ChemicaCo., NJ, USA and Degussa Texturant Systems, NE

USA for providing free samples of the materials, andto Dr Luis Durán for his invaluable advice.

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