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Food Hydrocolloids 23 (2009) 1853–1863

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Food Hydrocolloids

journal homepage: www.elsevier .com/locate/ foodhyd

Deformation and fracture of emulsion-filled gels: Effect of gelling agentconcentration and oil droplet size

Guido Sala a,b,*, Ton van Vliet a,c, Martien Cohen Stuart d, Fred van de Velde a,e, George A. van Aken a,e

a TI Food & Nutrition (formerly known as Wageningen Centre for Food Sciences), P.O. Box 557, 6700 AN Wageningen, The Netherlandsb Wageningen University & Research Centre, Centre for Innovative Consumer Studies, P.O. Box 17, 6700 A Wageningen, The Netherlandsc Wageningen University & Research Centre, Department of Agrotechnology and Food Science, Bomenweg 2, 6703 HD Wageningen, The Netherlandsd Wageningen University & Research Centre, Laboratory for Physical Chemistry and Colloid Science, P.O. Box 8038, 6700 EK Wageningen, The Netherlandse NIZO food research, Texture Department, Kernhemseweg 2, P.O. Box 20, 6710 BA Ede, The Netherlands

a r t i c l e i n f o

Article history:Received 25 November 2008Accepted 12 March 2009

Keywords:Carrageenan gelsGelatine gelsWPI gelsEmulsionsLarge deformationsOil droplet sizeStrain-dependency

* Corresponding author. Wageningen University &Innovative Consumer Studies, P.O. Box 17, 6700 AA WTel.: þ31 317 482 482; fax: þ31 317 483 777.

E-mail address: guido.sala@wur.nl (G. Sala).

0268-005X/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.foodhyd.2009.03.002

a b s t r a c t

The effect of the ratio between the modulus of the oil droplets and that of the gel matrix (varied bychanging gelling agent concentration and oil droplet size) on the large deformation properties of gela-tine, k-carrageenan and whey protein isolate (WPI) gels was studied at different compression speeds. Theeffect of gelling agent concentration and oil droplet size on strain-dependency of modulus and visco-elastic properties was also studied. An increase in the concentration of gelling agent resulted in densergels with more bonds between structural elements. This induced an increase of both Young’s modulusand fracture stress for all gels. With increasing gelling agent concentration, polymer gels (gelatine and k-carrageenan) became less strain-hardening, and the particle gels (WPI) even became strain-softening.The effect of a decrease in the oil droplet size on the Young’s modulus was generally according to the Vander Poel theory, unless when the oil droplets were aggregated. Moreover, a decrease in oil droplet sizeinduced a decrease of the fracture strain in gels with non-aggregated bound droplets. The extent of thesechanges was shown to depend on the gelling agent concentration. The effect of a decrease of the oildroplet size on other fracture parameters and in other gel systems was minor. With decreasing oildroplet size gelatine gels with unbound droplets and WPI gels became more viscous and less elastic.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

In a previous article we studied the effect of compression speedand oil concentration on the large deformation properties ofgelatine, k-carrageenan (polymer gels) and whey protein isolate(WPI, particle gels) emulsion-filled gels (Sala, van Vliet, Cohen Stuart,van Aken, & van de Velde, 2009). The experimental data reportedin that article were obtained for systems with constant gellingagent concentration and oil droplet size (i.e. with constant ratiobetween the modulus of the oil droplets and that of the gel matrix).

For emulsion-filled gels, changes in gelling agent concentrationand oil droplet size both result in variations in the ratio M(¼G0f/G0m)between the modulus of the filler (G0f ¼ 4g/d, where g ¼ surfacetension and d ¼ average diameter of the oil droplet) and that of thegel matrix (G0m). The effect of M on the small deformation

Research Centre, Centre forageningen, The Netherlands.

All rights reserved.

properties of emulsion-filled gels was discussed by van Vliet(van Vliet, 1988) on the basis of the Van der Poel theory (van derPoel, 1958; Smith, 1974, 1975). For bound droplets, the effect of oilvolume fraction (4) on G0 (modulus of the filled gel) is directlyrelated to M. For droplets stiffer than the matrix (M > 1), the largerthe ratio, the larger the increase of G0 with increasing 4. For dropletsless stiff than the matrix (M < 1), the lower the ratio, the larger thedecrease of G0 with increasing 4. In Fig. 1 the effect of M on

a ¼�

dðG0=G0mÞd4

�f/0

(1)

is shown as derived from the Van der Poel theory. For gels withM < 1, the minimum value of a is �1.67. This is also the value of G0/G0m for gels with unbound droplets as proposed by van Vliet (vanVliet, 1988). Alpha increases with increasing M and for M / N itreaches the asymptotic value of 2.5.

In heat-set WPI gels a decrease of the oil droplet size at constantwhey protein concentration (i.e. an increase of the ratio betweenthe modulus of the oil droplet and that of the matrix) resulted ina higher compressive stress for gels containing emulsions stabilised

-3

-2

-1

0

1

2

3

0.01 0.1 1 10 100 1000 10000

G'f/G'

m

2.5

-1.67

α

Fig. 1. Effect of the ratio between modulus of the filler and that of the matrix on alphaat 4/N according to the Van der Poel theory. G0m: storage modulus of the gel matrix;G0 f: storage modulus of the filler. The dotted line represent the maximum value fora (2.5), the dashed line the minimum (�1.67). �1.67 is also the value of a for unbounddroplets.

Table 1Volume-surface average diameter (Sauter diameter) and pH of the emulsions usedfor the preparation of the filled gels.

Emulsifying agent d3,2 (mm) G0 droplet (kPa) Ey droplet (kPa) pH

1 wt% WPI 4.25 19 56 7.701.10 73 218 7.440.45 178 533 7.80

3 wt% WPI aggregates 4.90 16 49 7.502.55 31 94 7.580.40 200 600 7.70

2 wt% Lactoferrin 3.75 21 64 5.361.35 59 178 5.32

2 wt% Tween 5.15 4 12 4.240.90 22 67 4.240.45 44 133 4.18

The G0 of the droplet was calculated by G0 ¼ 4g/d2 whereby the surface tension g wastaken to be 20 mN/m for protein stabilised droplets and 5 mN/m for Tween-stabi-lised droplets. The Young’s modulus (Ey) was Ey ¼ 3G0 .

G. Sala et al. / Food Hydrocolloids 23 (2009) 1853–18631854

by WPI (i.e. bound droplets) (McClements, Monahan, & Kinsella,1993). For gels containing emulsions stabilised by SDS, Tween 20and Triton X-100 (i.e. unbound droplets) no effect of the oil dropletsize on compressive stress was observed.

In a previous article on cold-set, acid-induced WPI gels con-taining oil droplets bound to the matrix we confirmed theconjectures of van Vliet regarding the effect of a variation of M onsmall deformation properties (Rosa, Sala, van Vliet, & van de Velde,2006). In that study the fracture stress and the fracture strain werefound to be independent of the droplet size. Compared to the studyby the group of McClements, the range of oil droplet size taken intoconsideration in this study was smaller.

In heat-set soybean protein isolate gels containing oil dropletsstabilised by the same protein a decrease in oil droplet size resultedin an increase of the Young’s modulus and the fracture stress (Kim,Renkema, & Van Vliet, 2001). The fracture strain was not affected byeither the oil droplet size or the oil content. The oil droplets presentin the gels were extensively aggregated.

The effect of oil droplet size and gelling agent concentrationon the large deformation properties of agar gels filled with oildroplets stabilised by polyglycerolesters of fatty acids wereinvestigated by compression measurements (Kim, Gohtani, Mat-suno, & Yamano, 1999). At all agar concentrations the presence ofthe oil droplets induced a decrease of the fracture stress, whichwas larger for larger droplets. The presence of the oil dropletsand their size did not affect the fracture strain. The oil dropletswere extensively aggregated and microscopy observationsrevealed empty spaces between the gel network and the oildroplets.

Based on the findings reported in literature, the ratio betweenthe modulus of the oil droplet and that of the matrix appears tohave a clear effect on the large deformation and fracture propertiesof emulsion-filled gels. In the present work the effect of gellingagent concentration, oil droplet size and compression speed on thelarge deformation properties and viscoelastic behaviour of emul-sion-filled gels, with varying droplet–matrix interactions, wasstudied. The chosen systems were gelatine, k-carrageenan andwhey protein isolate (WPI) gels containing emulsions made ofmedium-chain triglycerides oil and stabilised with differentemulsifying agents (WPI, Tween 20, lactoferrin and WPI aggre-gates) to control droplet–matrix interaction.

2. Materials and methods

2.1. Materials

Porcine skin gelatine PBG 07 (bloom 280, isoelectric point 8–9)was kindly provided by PB gelatines (Vilvoorde, Belgium).k-Carrageenan was kindly donated by CP Kelco (Lille Skensved,Denmark). The k-carrageenan sample consisted of 93% mol k-unitsand 7% mol i-units, as determined by NMR spectrometry (van deVelde, Pereira, & Rollema, 2004). Powdered whey protein isolate(WPI, Bipro�) was obtained from Davisco International Inc.(La Sueur, MN, USA). Tween 20 (Polyoxyethylene sorbitan mono-laurate, in the text referred to as Tween) was obtained from Sigma(Sigma–Aldrich Chemie BV, Zwijndrecht, The Netherlands). Lacto-ferrin was kindly donated by DMV International (Veghel, TheNetherlands). Medium-Chain Triglycerides (MCT) MIGLYOL 812N(neutral) oil was purchased from Internatio BN (Mechelen,Belgium). Potassium chloride (p.a.) was obtained from Merck(Darmstadt, Germany). Glucono-d-lactone (GDL) was kindlydonated by Purac (Gorinchem, The Netherlands). All materials wereused without further purification. All solutions were prepared withdemineralised water.

2.2. Sample preparation

2.2.1. EmulsionsEmulsions stabilised with different emulsifying agents were

used for gel preparation in order to vary the interactions betweenoil droplet and gel matrix. The procedures for the preparation of theemulsions have been described before (Sala, van Vliet, CohenStuart, van Aken, & van de Velde, 2009). In the present study the oildroplet size of the emulsions was varied. The emulsions preparedwith each emulsifying agent were homogenised at different pres-sures in order to obtain the desired droplet size. The droplet sizedistribution of the emulsions was measured using a MalvernMastersizer 2000 (Malvern Instruments Ltd., Malvern, UK). Thedroplet volume-surface average or Sauter diameter (d3,2) and othercharacteristics of the emulsions used for the preparation of thefilled gels are reported in Table 1.

2.2.2. GelsGelatine (4, 6, 8, 10 wt%) and WPI (3, 5, 7, 9 wt%) gels were

prepared in demineralised water. k-Carrageenan (0.6, 1.0, 1.4,1.8 wt%) gels were prepared in a 30 mM KCl solution. Samples offilled gels were prepared at two different gelling agent concentra-tions per gel type (4 and 10 wt% for gelatine, 3 and 6.75 wt% forWPI, 0.6 and 1.8 wt% for k-carrageenan) with 4 of 0.21,

Table 2Ratio Ef/Em for the different combinations between emulsions and matrices.

Gel matrix/d3,2 (mm) Emulsifying agent

1 wt% WPI 3 wt% WPI aggregates 2 wt% Lactoferrin 2 wt% Tween

4.25 1.10 0.45 4.90 2.55 0.40 3.75 1.35 5.15 0.90 0.45

Gelatine4 wt% (5 kPa) 11 44 107 – – – – – 2.3 13 2710 wt% (25 kPa) 2.3 8.7 21 – – – – – 0.5 2.7 5.3

k-carrageenan0.6 wt% (6.5 kPa) 8.7 34 82 – – – 10 27 – – –1.8 wt% (88 kPa) 0.6 2.5 6.1 – – – 0.7 2.0 – – –

WPI3 wt% (6 kPa) – – – 8.2 16 100 – – – – –6.7 wt% (40 kPa) – – – 1.2 2.4 15 – – – – –

–: combination not studied.

G. Sala et al. / Food Hydrocolloids 23 (2009) 1853–1863 1855

corresponding to an oil concentration of 20 wt%. The procedures forthe preparation of the gels have been described before extensively(Sala et al., 2009). The final pH of the WPI gels was about 4.8. InTable 2 the ratio’s between the Young’s modulus of the oil dropletsand that of the gel matrix are reported for all combinations studied.

The large deformation properties of the gels were determinedby uniaxial compression between two flat lubricated plates usingdifferent constant deformation speeds. True strain (3H) and truestress (st) were calculated as follows:

3H ¼ZH

H0

1H

dH ¼ ln�

HH0

�(2)

st ¼FA

(3)

where H0 is the initial height of the specimen, H the actual heightduring deformation, F the force measured during compression andA the corresponding cross-sectional area of the specimen.Compression was applied at different, constant deformation speeds(0.05, 0.1, 0.5, 1, 2 and 4 mm/s) up to a strain of 80%. For an extensivedescription of the method we refer to Sala et al. (Sala et al., 2009).The effect of gelling agent concentration and oil droplet size onstrain-hardening behaviour was studied by superimposing thestress vs. strain curves of gels for which these parameters werevaried and observing possible changes in the shape of the curves;stresses were normalised by the Young’s modulus. The method forthe determination of the recoverable energy (RE) can be foundelsewhere (Sala et al., 2009). RE was defined as:

RE ¼ Ws

Wc(4)

where Wc is the work necessary to compress the samples up to 25%strain (calculated from the area below the stress vs. strain curve)and Ws the work released by the gel specimen after removing thestrain.

The microstructure of the samples was studied by ConfocalLaser Scanning Microscopy (CLSM). For an extensive description ofthe procedure followed, see (Sala et al., 2009). Coalescencephenomena occurring after mixing the emulsions with the gellingagents solutions were studied by analysing the CLSM pictures of thegels (image size of 39.7 � 39.7 mm). The diameter of the dropletspresent in one image was measured with a ruler. The obtained sizedistribution of the oil droplets present in the analysed image wascompared to the droplet size distribution of the emulsion.Furthermore, CLSM pictures of different gels containing the sameemulsion were compared and increases of the diameter related tocoalescence were visually estimated.

3. Results

3.1. Effect of gelling agent concentration in gels without oil droplet

For all gels, an increase in the gelling agent concentrationinduced an increase in both the Young’s modulus and the fracturestress (Fig. 2). For WPI gels, also the fracture strain increased withincreasing gelling agent concentration, while for the polymer gelsthe effect of the gelling agent concentration on this parameter wasminor. The effect of compression speed on the Young’s modulusand fracture properties has been discussed before (Sala et al., 2009).The compression speed remarkably affected the fracture propertiesof both gelatine and k-carrageenan gels. This effect of speedincreased with increasing gelling agent concentration (Figs. 2.1Band 2.2B). For WPI gels the dependency of the fracture stress oncompression speed increased also with increasing gelling agentconcentration, while the effect of the compression speed on thefracture strain was not affected by the concentration (Fig. 2.3B). Forall concentrations, the difference between the lowest and thehighest fracture strain was about 0.15 and the strain did notincreased sequentially with compression speed.

With increasing gelling agent concentration, polymer gelsbecame less strain-hardening (Figs. 3.1 and 3.2). For k-carrageenangels, at the highest concentration tested (1.8 wt%) the stressincreased almost linearly with strain, i.e. at the mentionedconcentration the gels were not strain-hardening. With increasinggelling agent concentration WPI gels became strain-softening atintermediate strains (Fig. 3.3). For all gels, the effect of gelling agentconcentration on strain-hardening was not affected by thecompression speed (results not shown).

The recoverable energy slightly increased with increasinggelling agent concentration for WPI gels, and decreased for k-carrageenan gels (Fig. 4). For gelatine gels, the recoverable energywas not affected by the gelling agent concentration (Fig. 4). For allgels, the recoverable energy was not affected by the compressionspeed (results not shown). In Table 3 the effects of the gelling agentconcentration on large deformation and fracture properties aresummarised. Table 4 shows the effects of compression speed on thelarge deformation and fracture properties of gels with differentgelling agent concentrations.

3.2. Effect of oil droplet size in gels with different gelling agentconcentrations

Variations in the oil droplet size did not remarkably affect theaggregation of the oil droplets embedded in the gel matrix previ-ously described (Sala, van Aken, Cohen Stuart, & van de Velde,2007; Sala et al., 2009). In gelatine and WPI gels, the oil dropletswere not aggregated, irrespective of the emulsifying agent used for

0

10

20

30

0.0 1.0 2.0 3.0 4.0 5.0Compression speed (mm/ s)

Mo

du

lu

s (kP

a)

1A

Conc.

0

20

40

60

80

100

120

Fracture strain

Fractu

re stress (kP

a)

Conc.

Speed

1B

0

10

20

30

40

50

60

70

80

90

100

Compression speed (mm/ s)

Mo

du

lu

s (kP

a) Conc .

2A

0

20

40

60

80

100

120

Fracture strain

Fractu

re stress (kP

a)

Speed

Conc.

2B

0

10

20

30

40

50

60

70

80

90

100

Compression speed (mm/ s)

Mo

du

lu

s (kP

a)

Conc.

3A

0

20

40

60

80

100

120

0.0 0.5 1.0 1.5 2.0

0.0 1.0 2.0 3.0 4.0 5.0

0.0 1.0 2.0 3.0 4.0 5.0

0.0 0.5 1.0 1.5 2.0

0.0 0.5 1.0 1.5 2.0Fracture strain

Fractu

re stress (kP

a)

SpeedConc.

2B

Fig. 2. Effect of compression speed on Young’s modulus (A) and fracture points (B) of gelatine (1), k-carrageenan (2) and WPI (3) gels at different concentrations. The concentrationsof the different gels are reported in the ‘Material and methods’ session.

G. Sala et al. / Food Hydrocolloids 23 (2009) 1853–18631856

emulsion preparation. In contrast, in k-carrageenan gels both theemulsions stabilised by WPI and those stabilised by lactoferrinwere strongly aggregated. A comparison between the CLSMmicrographs of gelatine and k-carrageenan gels containing thesame emulsions stabilised by WPI (Figs. 5A and 5B) and witha Sauter diameter of 0.45 mm revealed extensive coalescence in thek-carrageenan gels in addition to aggregation. This phenomenonwas not as evident at larger droplet size and for emulsions stabi-lised by lactoferrin (results not shown). Furthermore, it was notobserved when studying the effect of 4 on the large deformation

properties of the same gels at constant oil droplet size (d3,2 about1 mm) (Sala et al., 2009). Nevertheless, some coalescence of the oildroplets embedded in k-carrageenan probably occurred also atlarger droplet size and for emulsions stabilised by lactoferrin.

Coalescence in k-carrageenan gels was a consequence of theaggregation occurring in the emulsions upon addition of KCl. Thissalt was used for gelling k-carrageenan and, therefore, used toadjust the salt concentration of the emulsions to be added to k-carrageenan. As a result of coalescence, the diameter of oil dropletsembedded in k-carrageenan gels was about a factor 2–4 larger than

0

1

2

3

4

5

6

True strain True strain

True strain

0

0.4

0.8

1.2

1.6

2

No

rm

alised

stress

No

rm

alised

stress

No

rm

alised

stre

ss

0

0.4

0.8

1.2

1.6

2

0 0.5 1 1.5 2 0 0.5 1 1.5 2

0 0.5 1 1.5 2

Conc.

Conc.Conc.

1

3

2

Fig. 3. Effect of gelling agent concentration on the slope of the stress vs. strain curve for different gels. (1: gelatine; 2: k-carrageenan; 3: WPI) at a compression speed of 1 mm/s.

Table 3Summary of the results obtained for gels without emulsion droplets with respect tothe gelling agent concentration.

Type of gel Gelling agent concentration

sf 3f Ey Ef(3f) RE

Gelatine þþþ þþ þþþ – – 0k-carrageenan þþþ þ þþþ – – – –

G. Sala et al. / Food Hydrocolloids 23 (2009) 1853–1863 1857

the Sauter diameter measured after preparation of the emulsions,just before preparing the gels. The ratios between the modulus ofthe droplet and that of the gel matrix (M ¼ G0f/G0m, effective M)were consequently smaller than those calculated with the originalSauter diameter of the emulsions (nominal M). Rough estimates ofthese ratios, taking into account the coalescence of the oil droplets,are reported in Table 5.

0

20

40

60

80

100

120

0 2 4 6 8 10 12Gelling agent concentration (wt%)

RE

(%

)

Fig. 4. Effect of gelling agent concentration on recoverable energy for different gels:A: gelatine; -: k-carrageenan; :: WPI. (compression speed: 1 mm/s).

WPI þþþ þþþ þþþ – – þ

sf: fracture stress; 3f: fracture strain; Ey: Young’s modulus. Ef(3f): modulus asa function of strain. þ denotes an increase of the parameter with gelling agentconcentration. –indicates a decrease of the parameter. More þ or – correspond toa stronger effect. 0: no effect.

Table 4Summary of the results obtained for gels without emulsion droplets with respect tothe compression speed.

Type of gel Compression speed

sf 3f Ey Ef(3f) RE

Gelatine þþþ þþþ 0 0 0k-carrageenan þþþ þþ 0 0 0WPI þþþ 0 þþþ 0 0

sf: fracture stress; 3f: fracture strain; Ey: Young’s modulus. Ef(3f): modulus asa function of strain. þ denotes an increase of the parameter with speed. More þcorrespond to a stronger effect. 0: no effect.

The effect of oil droplet size variations was found to depend ondroplet–matrix interactions, the microstructure of the gels and M.For non-aggregated bound droplets (e.g. in gelatine gels with WPI-stabilised emulsions (Fig. 6) and in WPI gels (Fig. 10)), a decrease of

Fig. 5. CLSM images of a gelatin gel (A) and a k-carrageenan gel (B) containing a WPI-stabilised emulsion with an oil droplets size of 0.45 mm (f: 0.21; image size 159 � 159 mm).

Table 5Ratio Ef/Em for the different combinations of emulsions and k-carrageenan gels aftercorrection to take into account oil droplet coalescence.

Emulsifying agent

1 wt% WPI 2 wt% Lactoferrin

d3,2 (mm)

8.5–17 2.2–4.4 0.9–1.8 7.5–15 2.7–5.4

0.6 wt% (6.5 kPa) 4.3–2.2 16.8–8.4 41–20.5 4.9–2.5 21.7–10.91.8 wt% (88 kPa) 0.3–0.2 1.2–0.6 3–1.5 0.4–0.2 1.6–0.8

0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0Compression speed (mm/ s)

Mo

du

lu

s (kP

a)

05

1015202530354045505560

0.0 1.0 2.0 3.0 4.0 5.0Compression speed (mm/ s)

Mo

du

lu

s (kP

a)

Lower diam.

Lower diam.

2A

1A

Fig. 6. Gelatine bound: effect of compression speed and oil droplet size on Young’s modulusoil; -: 4.22 mm; :: 1.09 mm; C: 0.47 mm).

the droplet size induced an increase of the Young’s modulus anda decrease of the fracture strain. In agreement with the Van der Poeltheory, the Young’s modulus was higher for larger M. For both typesof gels, the fracture stress was not affected by a decrease of the oildroplet size. For gelatine gels, the effect on fracture strain waslarger at low gelling agent concentration. For WPI gels, this effectwas larger at the higher gelling agent concentration.

For gels with bound and aggregated oil droplets (k-carrageenangels with lactoferrin-stabilised emulsions), an effect of a decrease inthe oil droplet size was observed at low gelling agent concentration

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0

Fracture strain

Fractu

re stress (kP

a)

0

20

40

60

80

100

120

0.0 0.5 1.0 1.5 2.0

Fracture strain

Fractu

re stress (kP

a)

Speed

Speed

Lower diam.

2B

1B

(A) and fracture points (B) of gelatine gels with 4 wt% (1) and 10 wt% (2) gelatine (A: no

0

2

4

6

8

10

Compression speed (mm/ s)

Mo

du

lu

s (kP

a)

0

5

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15

20

25

Fracture strain

Fractu

re stress (kP

a)

05

1015202530354045505560

Compression speed (mm/ s)

Mo

du

lu

s (kP

a)

0

20

40

60

80

100

120

0.0 1.0 2.0 3.0 4.0 5.0 0.0 0.5 1.0 1.5 2.0

0.0 1.0 2.0 3.0 4.0 5.0 0.0 0.5 1.0 1.5 2.0

Fracture strain

Fractu

re stress (k

Pa)

2A 2B

1B1A

Speed

Speed

Fig. 7. Gelatine unbound: effect of compression speed and oil droplet size on Young’s modulus (A) and fracture points (B) of gelatine gels with 4 wt% (1) and 10 wt% (2) gelatine (A:no oil; -: 5.12 mm; :: 0.90 mm; C: 0.45 mm).

G. Sala et al. / Food Hydrocolloids 23 (2009) 1853–1863 1859

(Fig. 8.1A). At low gelling agent concentration the Young’s modulusof the gels increased with decreasing droplet size, while fracturestress and fracture strain slightly increased. At high gelling agentconcentration M was in the range 0.2–3 (Table 5). Due to aggre-gation, the presence of oil droplets in k-carrageenan gels with highgelling agent concentration caused a decrease in the gel modulus,the effect being independent of the oil droplet size. Probably, thehighly deformable droplet aggregates rather than the individualdroplets act as the filler particles here, effectively lowering thevalue of M (Fig. 8.2A). For gels with unbound droplets (gelatine gelswith Tween-stabilised emulsions and k-carrageenan gels withWPI-stabilised emulsions) variations in the droplet size had noeffect on either the Young’s modulus or the fracture behaviour(Figs. 7 and 9). For these gels, M is always much smaller than 1,independently of the oil droplet size. For k-carrageenan gels, thelack of effect of oil droplet size on large deformation properties canalso be ascribed to the aggregation and coalescence of the oildroplets.

For gels with non-aggregated bound droplets, the effect of a sizereduction on fracture properties resembled that observed withincreasing 4 at constant size (Sala et al., 2009). In order to confirmwhether in these gels a reduction in oil droplet size could mimic anincrease in 4, these parameters were varied together (in gelatinegels with a gelling agent concentration of 4 wt%). Samples with4 ¼ 0.21 and containing an emulsion with d3,2 ¼ 3.1 mm showedcomparable Young’s modulus and fracture properties as sampleswith 4 ¼ 0.11 and containing an emulsion with d3,2 ¼ 0.41 mm(Fig. 11).

At the compression speeds tested, the oil droplet size did notsignificantly affect the strain-hardening behaviour of the gels

(results not shown). As expected on the basis of the results pre-sented for the effect of the gelling agent concentration (Fig. 3), theemulsion-filled gels with higher gelling agent concentration wereless strain-hardening.

The effect of the oil droplet size on the recoverable energyvaried in the different gel systems (Table 6). For gelatine gels withbound droplets (stabilised by 1 wt% WPI), the recoverable energyslightly increased with decreasing oil droplet size, whereas itstrongly decreased in gels with unbound droplets (stabilised by2 wt% Tween). For k-carrageenan gels, the recoverable energyincreased with a decrease of the oil droplet size for both bound(stabilised by 2 wt% lactoferrin) and unbound (stabilised by 1 wt%WPI) droplets. For WPI gels this parameter decreased withdecreasing oil droplet size. In Table 7 an overview is given of theeffects of the oil droplet size on the large deformation behaviour ofthe emulsion-filled gels.

4. Discussion

The effect of compression speed and presence of oil droplets inthe gel matrix on large deformation properties was extensivelydiscussed in Sala et al. (Sala et al., 2009). In this article we focus ourdiscussion on the effects of gelling agent concentration and oildroplet size, i.e. on the effect of the ratio between the modulus ofthe filler and that of the matrix (M).

An increase in gelling agent concentration results in denser gels,with more structural elements and more bonds between them. Ofcourse these gels are stiffer; this can be seen from the increase inYoung’s modulus and fracture stress observed for all gels (Table 3).We ascribed part of the effect of the compression speed on the

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Fig. 8. k-Carrageenan bound: effect of compression speed and oil droplet size on Young’s modulus (A) and fracture points (B) of k-carrageenan gels with 0.6 wt% (1) and 1.8 wt% (2)k-carrageenan (A: no oil; -: 3.17 mm; :: 1.32 mm).

G. Sala et al. / Food Hydrocolloids 23 (2009) 1853–18631860

fracture parameters of the gels to friction between structuralelements of the gel network (Sala et al., 2009). The effect ofcompression speed on fracture parameters observed in the presentwork with increasing gelling agent concentration (Table 4) can alsobe primarily related to increased friction phenomena. For gelatinegels, an increase in gelling agent concentration will correspond toa higher density of physical bonds. Upon compression it will thentake longer to unzip these bonds and to fracture the gel. This willresult in a higher fracture stress and strain for higher compressionspeed.

A higher density of bonds in gels with a higher gelling agentconcentration will also decrease the extensibility of the polymerchains in polymer gels and of the randomly oriented backbones inparticle gels. Furthermore, a higher density of bonds in the gelnetwork will suppress strain-induced crystallisation (Groot, Bot, &Agterof, 1996). This probably explains the decrease of the strain-hardening behaviour observed for the gelatine and the k-carra-geenan gels. For WPI gels the strain-softening behaviour observedwith increasing WPI concentration (i.e. with increasing gelmodulus) is in accordance with the findings reported by otherauthors (Pouzot, Nicolai, Benyahia, & Durand, 2006).

From the recoverable energy (Fig. 4), we see that k-carrageenangels become more viscous (at the applied strains of 0.25) withincreasing concentration. A more viscous character of k-carra-geenan gels with increasing gelling agent concentration was alsoobserved by other authors as an increase of the tan d (Bayarri,Duran, & Costell, 2004). Since the KCl concentration in k-carra-geenan gels was kept constant (30 mM) with increasing gellingagent concentration, a probable reason for the observed

phenomenon may be an imperfect formation of cross-links due toa decreasing KCl/k-carrageenan ratio.

For gels with non-aggregated, bound oil droplets, the effect ofthe oil droplet size on the Young’s modulus follows the Van der Poeltheory (Table 8): a decrease of the oil droplet size corresponds toa stiffening of the droplets and, therefore, results in larger increasesof the Young’s modulus as compared to gels with larger droplets atthe same 4. In contrast, the Young’s modulus of gels with unbounddroplets does not change, which supports the approximationproposed by van Vliet (van Vliet, 1988) to set the effective modulusof unbound oil droplets to zero, whatever the size.

For gels with aggregated, bound droplets, and at nominal (takinginto account droplet coalescence) M > 1, the effect of oil dropletaggregation can be twofold. Firstly, the aggregation of the oildroplets can cause an increase of the effective volume of the fillers,resulting in an increase of the Young’s modulus of the filled gels.Secondly, when the modulus of the aggregates is lower than theYoung’s modulus of the gel matrix (hence the effective M is <1) theeffect of oil droplet aggregation can be a decrease of the Young’smodulus (Sala, van Aken, Cohen Stuart, & van de Velde, 2007). Therelative importance of these two effects will clearly depend on theextent of aggregation, so that the combined effect is hard to predict.

In this study no significant difference was observed in thefracture strain between bound and unbound droplets. Elsewhere(Sala et al., 2009) we showed that bound droplets are more effec-tive than unbound droplets in decreasing the fracture strains offilled gels. Furthermore, the experimental results obtained forgelatine and WPI gels (i.e. gels in which the oil droplets were notaggregated) show that for gelatine gels with bound droplets the oil

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Fig. 9. k-Carrageenan unbound: effect of compression speed and oil droplet size on Young’s modulus (A) and fracture points (B) of k-carrageenan gels with 0.6 wt% (1) and 1.8 wt%(2) k-carrageenan (A: no oil; -: 4.22 mm; :: 1.09 mm; C: 0.47 mm).

G. Sala et al. / Food Hydrocolloids 23 (2009) 1853–1863 1861

droplets are more effective in decreasing the fracture strain in gelsat a higher M only (compare Fig. 6.1B with Fig. 6.2B and Fig. 7.1Bwith Fig. 7.2B). For WPI gels, the oil droplets were more effective indecreasing the fracture strain in gels with higher gelling agentconcentration, i.e. at lower M (Table 2; compare Fig. 10.1B withFig. 10.2B). According to the theory developed by Gao et al.(Gao,Lelievre, & Tang, 1995), unbound stiff fillers in a soft gel areexpected to induce a larger stress concentration than bound or lessstiff fillers. Our results do not fit the theory of Gao et al. (Gao et al.,1995), which predicts the reverse of what we found.

To study the effect of the stiffness of the oil droplets on stressconcentration phenomena, a comparison between gels containingemulsions with different droplet size would be an option. However,experimental design presents a substantial problem. A decrease ofthe oil droplet size at constant 4 corresponds to a significantincrease in the number of filler particles and, hence, to a decreasingdistance between droplets. As a consequence, more interferencewill occur between stress concentration regions and such an effectcannot be accounted for.

The decrease of the oil droplet size in gels at fixed gelling agentconcentration and 4 corresponds to stiffening of the droplets and toan increase of their number. Decreasing the oil droplet diameter bya factor 10 roughly corresponds to an increase of the dropletmodulus by the same factor (Table 1), but the number of dropletsincreases by a factor 1000. The Van der Poel theory, that holds inprinciple for small deformations, considers only the stiffness and 4

of the fillers for the prediction of their effect on the modulus of thefilled gel and not their number (van der Poel, 1958; Smith, 1974,1975). For large deformations, the filler particles should also be

considered as defects in the gel matrix which lead to local stressconcentration. Upon deformation, the stresses near the fillerinterface may become larger than the strength of the bonds in thegel matrix at smaller deformation. Therefore, the filler particlesmay act as crack initiators (if their size is larger than the intrinsicdefect length of the gel matrix). In this case, the number and thesize of filler particles are likely to be important parameters.

For gels with bound, non-aggregated droplets, a decrease of thedroplet size and the related increase in number of droplets has thesame effect on Young’s modulus and fracture strain as observed foran increase in 4 (Fig. 11) (Sala et al., 2009). For unbound and foraggregated droplets, the increase in the stiffness and the number ofdroplets as a result of a decrease in oil droplet size does not appearto affect the fracture behaviour of the gels. This supports the findingthat unbound droplets are less effective stress concentrating nucleithan bound droplets (Sala et al., 2009).

For gelatine gels with unbound droplets, a decrease in oil dropletsize and the related increase in the number of droplets results ina remarkable increase of the energy dissipated by friction, as shownby the observed decrease in recoverable energy (Table 6). In otherwords, gelatine gels with small unbound droplets show at largedeformation a relatively more viscous character. The same obser-vation holds for WPI gels. Similar effects regarding the recoverableenergy were observed for both gels as a result of an increase in 4.

In a previous article on the effect of oil content and deformationspeed on the deformation and fracture properties of emulsion-fil-led gels we proposed an interpretation of the obtained experi-mental results on the basis of the different mechanisms regardingcompression speed dependency, stress concentration and energy

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Fig. 10. WPI bound: effect of compression speed and oil droplet size on Young’s modulus (A) and fracture points (B) of WPI gels with 3.0 wt% (1) and 6.75 wt% WPI (A: no oil; -:4.88 mm; :: 2.52 mm; C: 0.39 mm).

G. Sala et al. / Food Hydrocolloids 23 (2009) 1853–18631862

dissipation upon deformation (Sala et al., 2009). The same mech-anisms can also explain the effects of a decrease in oil droplet sizeon Young’s modulus and fracture behaviour (Table 8). The overviewgiven in Table 8 is similar to that previously proposed for the effectof an increase in 4 on the large deformation properties (Sala et al.,2009), and suggests that, from a phenomenological point of view,a decrease in oil droplet size has the same effects as an increase in 4.For gels with bound, non-aggregated droplets, the similarity doesnot only concern the phenomena occurring, but also the observed

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Fig. 11. Gelatine bound: Young’s modulus vs. compression speed (A) and fracture points (B)with different droplet size (A: no oil; -: 4: 0.21; d3,2: 3.1 mm; C: 4: 0.11; d3,2: 0.41 mm;

effects on large deformation properties. As a consequence, for thesegels variations in the oil droplet size represent a tool to engineergels with different 4 but the same fracture behaviour.

5. Conclusions

As expected, an increase in the gelling agent concentrationresults in a denser gel structure with a higher number of bondsbetween the structural elements forming the gel network. This

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of gelatine gels with 4 wt% gelatine and different amounts of WPI-stabilised emulsions:: o: 0.21; d3,2: 0.41 mm).

Table 6Effect of droplet size on recoverable energy for the different combinations between emulsions and matrices (compression speed: 4 mm/s).

Gel matrix/d3,2 (mm) Emulsifying agent

1 wt% WPI 3 wt% WPI aggregates 2 wt% Lactoferrin 2 wt% Tween

RE (%)

4.25 1.10 0.45 4.90 2.55 0.40 3.75 1.35 5.15 0.90 0.45

Gelatine4 wt% (5 kPa) 89 90 92 – – – – – 82 66 5610 wt% (25 kPa) 92 93 93 – – – – – 96 90 89

k-carrageenan0.6 wt% (6.5 kPa) 55 54 59 – – – 56 58 – – –1.8 wt% (88 kPa) 49 49 55 – – – 48 51 – – –

WPI3 wt% (6 kPa) – – – 48 43 38 – – – – –6.7 wt% (40 kPa) – – – 56 52 45 – – – – –

–: combination not studied; in brackets the Young’s modulus of the gels is reported.

Table 7Summary of the results obtained for filled gels with respect to the oil droplet size.

Type of gel Decreasing droplet size

sf 3f Ey Ef(3f) RE

Gelatine bound4 wt% 0 –– þþþ 0 þþ10 wt% þ – þþþ 0 þþ

Gelatine unbound4 wt% 0 0 0 0 ––10 wt% 0 0 þ 0 ––

k-carrageenan bound0.6 wt% þþ 0 þþþ 0 þþ1.8 wt% þ 0 0 0 þþ

k-carrageenan unbound0.6 wt% þ 0 –– 0 þþ1.8 wt% 0 0 0 0 þþ

WPI bound3 wt% 0 0 þ 0 ––6.7 wt% þþ –– þþ 0 ––

sf: fracture stress; 3f: fracture strain; Ey: Young’s modulus. Ef(3f): modulus asa function of strain. þ denotes an increase of the parameter with decreasing oildroplet size. – indicates a decrease of the parameter. More þ or – correspond toa stronger effect. 0: no effect.

Table 8Summary of the mechanisms explaining the effect of the oil droplet size on fractureproperties and Young’s modulus.

Type of gel sf 3f Ey

Gelatine bound Cb, Db Cb Db

Gelatine unbound Cu, Du Cu Du

k-carrageenan bound Cb, Db Cb Db

k-carrageenan unbound Cu, B, Du Cu, B Du

WPI bound A2, Db, (Cb) A2, Cb Db

sf: fracture stress; 3f: fracture strain; Ey: Young’s modulus. A2: induced viscous flowof the matrix; B: friction between structural elements; C: stress concentration; D:van der Poel theory (for C and D b refers to the phenomena occurring in gels withbound dropelts; u refers to phenomena occurring in gels with unbound droplets). Anextensive description of the mechanisms indicated by A–D is given in Sala, G. et al.(2009).

G. Sala et al. / Food Hydrocolloids 23 (2009) 1853–1863 1863

causes an increase of both the Young’s modulus and the fracturestress. In particle gels (WPI gels) also the fracture strain increaseswith increasing gelling agent concentration. This can probably beascribed to a decrease of the size of the inherent defects present inthe gel structure.

More importantly, by varying the gelling agent concentrationand the oil droplet size, the ratio between the modulus of thedroplets and that of the matrix can be modulated. Variations of thisratio affect only the Young’s modulus and fracture properties of gelscontaining bound, non-aggregated properties. For these gelsa decrease in the droplet size allows to decrease the amount of oil,while maintaining the same Young’s modulus and fractureproperties.

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