Aeration of model gels: Rheological characteristics of gellan and agar gels

Journal of Food Engineering 107 (2011) 134–139

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Research Note

Aeration of model gels: Rheological characteristics of gellan and agar gels Shipra Tiwari, Suvendu Bhattacharya ⇑ Food Engineering Department, Central Food Technological Research Institute, Council of Scientific and Industrial Research, Mysore 570020, India

a r t i c l e

i n f o

Article history: Received 9 February 2011 Received in revised form 18 May 2011 Accepted 24 May 2011 Available online 1 June 2011 Keywords: Gellan Agar Aeration Compression Stress relaxation

a b s t r a c t Model food sols containing gellan and agar at different solid concentrations were aerated. The rheological characteristics of the setted gels were determined. These were the compression characteristics (fracture strain/stress/energy, firmness and the total energy for compression) and the stress relaxation characteristics (extent of relaxation and relaxation time). Fracture strain of gels increased due to aeration indicating that the aerated samples exhibited a delayed fracture due to the presence of incorporated air. The extent of increase in fracture strain for aerated agar gels over the corresponding non-aerated samples is 1.7–23.1% while it is 4.3–15.1% for gellan gels. The extent of relaxation increased due to aeration while relaxation time (k) decreased. Relaxation time for aerated agar and gellan gels is 9.6–40.7% and 19.6– 37.6% lower than the non-aerated samples. At a low concentration of hydrocolloid, the effect of aeration was usually non-significant (at p 6 0.05) while prominent changes in rheological behaviour occurred at a high concentration. Aeration of food gels offered the possibility of rheological modifications to suit the consumer requirements in developing specialty fabricated gels. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction A gel is an intermediate state between a solid and a liquid, and it is possible to include flexibility in texture, structure and attributes related to consumer acceptance in food gels. Examples of such gel modifications include hydro-gel (Kong, 2005), fractal-particle gels (Walstra, 2003), filled gel (Steeneken and Woortman, 2009), heat-set protein gels (Bhattacharya and Jena, 2007), multicomponent gels (Aguilera and Rademacher, 2004; Banerjee and Bhattacharya, 2011), etc. These special gels can offer some particular advantages such that they become different in their functional properties compared to the well known gel systems like jam, jelly, gelatin gels, etc. Among the common hydrocolloids used for developing food gels, agar and gellan finds many applications. Gelation of agar is a thermo-reversible process (Armisén and Galatas, 2000). Agar requires heat to bring them into dispersion; on cooling, the hot dispersion sets to a gel (Stanley, 2006). In aqueous dispersion at high temperatures, gellan polymers are in a disordered single-coiled state. Cooling of gellan sol promotes gel formation. Coil-helix conformational transition occurs in a temperature range from 30 to 50 °C (Horinaka et al., 2004). The concept of introduction of air or an aerated gel is an interesting phenomenon wherein certain properties of gels may be modified affecting their texture, structure/microstructure and functional properties (Zúñiga and Aguilera, 2008; Labbafi et al., ⇑ Corresponding author. Tel.: +91 0821 2513910; fax: +91 0821 2517233. E-mail address: [email protected] (S. Bhattacharya). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.05.036

2007). The consumer appreciation due to novelty value and obtaining a good volume with the same mass are the other advantages of aerated foods. The addition of air to a gel may be conducted in several ways such as whipping or by compressed air. Tailor made cellular solid foods originating from dry hydrocolloid gels, enzymatically produced agar-starch sponges and freeze dried agar-texturized fruit gels have been reported (Rassis et al., 1997; Nussinovitch et al., 1998, 2004). Zúñiga and Aguilera (2008) have also indicated that aerated food gels may increase the spectrum of possibilities for texture formation and perception, flavour encapsulation and/or release, delivery of bioactive molecules and the control of satiety. Aerated food gels improve gelatin based fruit jellies and marshmallows (Baziwane and He, 2003; Zúñiga and Aguilera, 2009). Less calorie-dense food products (Rolls et al., 2000), gas-filled spongy gels for unique textural properties for food and medical applications (Nussinovitch et al., 1992), and specialty future food product for slow release of water, flavour or nutrients to be bio-available in human system (Hermansson, 2007) have been reported. Aerated chocolate (Haedelt et al., 2007), encapsulated product and aerated structure to facilitate mastication, enzyme accessibility and enhancement of flavour delivery systems are also possible (Lau and Dickinson, 2005). It appears that the aerated gels possess a good scope for food industries in developing unique products with flexible rheological characteristics and sensory attributes. Therefore, the objective of the present study was to determine the rheological attributes of aerated agar and gellan gels at different concentrations by employing compression and stress relaxation tests.

S. Tiwari, S. Bhattacharya / Journal of Food Engineering 107 (2011) 134–139

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Fig. 1. Schematic diagram of aeration set-up. All dimensions are in mm.

2. Materials and methods 2.1. Materials Gellan gum (GelriteÒ, G-1910, Sigma–Aldrich, USA) and agar (Loba Chemie, Mumbai, India) were used as the gelling agent.

2.2. Preparation of gel and aeration Preliminary trials were conducted to understand the phenomenon of setting of gellan and agar gels to ascertain the minimum concentration that forms a well set gel, and the maximum level up to which pouring of sol is possible. The results obtained from the preliminary trials were used to plan the actual experiments. Dispersions of gellan gum at different concentrations (1%, 2%, 3%

and 4%, dry solid basis) were prepared by dispersing the appropriate amount of gum in water which included hydration of the gum for 1 h with continuous stirring. This followed heating of the sol in a water bath, maintained at a temperature of 90 °C for 1 h. Aeration of gels was conducted by passing air from a compressor for 10 min to the sol employing the aeration set-up (Fig. 1). It consisted of a 250 ml beaker in which 150 ml of sol was transferred. Air was allowed to flow through the pores of the set-up at a gauge pressure of about 50 kPa. Compressed air came out as small bubbles through an inverted container of outer diameter 55 mm. The upper part of this container had 25 distributed pores of about 1 mm each in diameter. While conducting aeration, the sol was maintained at a temperature of 65 °C by using a water bath. After aeration, the hot dispersions were poured into plastic petri plates (internal diameter and height were 48 and 11 mm, respectively) up to the brim and allowed to cool to room temperature (about 25 °C) to

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A

40

e: Aerated

30

a: Aerated Force (N)

Force (N)

60

A

f: Non-aerated 20

30 10

b: Non-aerated 0

0

2

4

6

Distance (mm)

0

8

0

4

6

8

Distance (mm)

4 30

2

B

B 3

Force (N)

Force (N)

20

g: Aerated

2

c: Aerated

10

h: Non-aerated 1

d: Non-aerated 0

0

100

200

300

400

Time (s)

0 0

100

200

300

400

Time (s)

Fig. 2. Sample (A) compression and (B) stress relaxation curves for non-aerated and aerated agar (a, b, c and d) and gellan (e, f, g and h) gels at 2% concentrations. The dotted lines show different zones.

form gels. They were kept undisturbed with covers for 16 h after which they were manually removed from the mould and were subjected to further studies. Agar gels with solid concentrations of 1%, 2% and 3% were also used for aeration; these gels were prepared in the same manner as it was done for gellan gels. The process of aeration was repeated twice. Non-aerated gellan and agar gels of similar concentrations were used as control samples.

2.3. Compression and stress relaxation testing of gels Gel samples (47 mm in diameter and 11 mm in height) were subjected to uniaxial compression by using a flat-bottomed circular base stainless steel plate of 100 mm in diameter at a compression speed of 1 mm s1 employing a texture-measuring instrument (Model # TAHD, Stable Microsystems, Surrey, UK) up to an engineering strain of 0.5. Five lubricated samples were examined each time, and the experiment was repeated twice. The software provided by the manufacturer was used to calculate the different rheological parameters from individual forcedeformation curves. The fracture force is associated with the occurrence of the first major fracture; it was noted as the force at which the sample showed a sudden drop in force by P1 N; corresponding stress and strain values were reported to be fracture stress and strain, respectively (Ravi et al., 2007). Fracture strain was calculated as the ratio of the height of the sample at the first major fracture and the initial height of sample. Corresponding stress values

were obtained by dividing the fracture force by the initial crosssectional area of sample. Fracture energy was obtained as the area under the force-deformation curve till the first fracture point.Axial compression of a cylindrical sample between parallel plates causes the specimen height to decrease and its diameter to increase (Hamann and MacDonald, 1992). Thus, the commonly used engineering or apparent compressive strain ðeE Þ was calculated by Eq. (1):

eE ¼

Dh h0

ð1Þ

where h0 was the initial height of the sample and Dh was the change in height due to compression. The engineering or apparent compressive stress was calculated from the force–time plot by knowing the force F(t) at time t and the cross-sectional area of the sample (A0) prior to compression (Eq. (2)):

rE ¼

FðtÞ A0

ð2Þ

In stress relaxation test, a gel sample was initially compressed at a crosshead speed of 1 mm s1 to a desired engineering strain of 0.2 (Saha and Bhattacharya, 2010). Later, sample was allowed to relax up to 300 s while keeping the strain constant. Stresses at the beginning and end of relaxation (initial stress, r0 and residual/equilibrium stress, re), respectively, were noted. The extent of relaxation was calculated by using Eq. (3). The relaxation time (k) was obtained as the time required for the stress to decay to a

137

Non-aerated

1100

Aerated

900 2

Firm ness (N m m -1 )

200

100

0 12

30 20 10

30 20 10 2

3

1

2

3

1

2

3

90 60 30 0

3

40

1

40

3

To ta l energ y fo r co m pressio n (m J )

Fra cture stress (kPa )

Fracture energy (m J)

1

F r a c tur e str a in (% )

D ensity (kg m -3 )

S. Tiwari, S. Bhattacharya / Journal of Food Engineering 107 (2011) 134–139

600 400 200 0 1

Agar (%)

2

3

Agar (%)

Fig. 3. Compression characteristics of non-aerated (filled bar) and aerated (hollow bar) agar gels at different concentrations.

value of e1 (0.368) of the initial stress. As gellan gels with concentrations P2% showed a slow decay, the relaxation time was extended up to 2000 s. Three samples were tested each time and the whole experiment was repeated twice.

Extent of relaxation ð%Þ ¼

r0  re  100 r0

ð3Þ

2.4. Experimental design and statistics One variable at a time approach was followed to prepare various concentrations of hydrocolloid gels; these were 1%, 2%, 3% and 4% for gellan, and 1%, 2% and 3% for agar gels. All gel preparation processes were repeated twice, and results were expressed as mean ± standard deviation (SD). Duncan’s Multiple Range Test (DMRT) was applied to determine the significant differences at p 6 0.05 (Little and Hills, 1979). 3. Results and discussion 3.1. Compression The sample compression curves for non-aerated and aerated gels made from agar and gellan (at 2% concentration) are shown in Fig. 2. These curves show five major zones: (a) a short linear zone depicting elastic deformation; (b) non-linear plastic deformation zone associated with a marked increase in the magnitudes of force indicating densification in addition to exclusion of water; (c) reaching the peak resistive force at which a major fracture occurs; (d) rapid decrease in force after exhibiting fracturing with simultaneous loss of water and (e) marginal increase in force when the fractured gel reorients, and offers increased resistance due to further densification. Each of these regions possesses characteristic

mechanical–rheological property, and the extent of these zones varies with the nature and concentration of hydrocolloid, pH, etc. The aerated samples are placed at higher positions than that of non-aerated ones meaning that the former samples can offer higher resistive forces during most of the phases of compression (Fig. 2). The densities of the aerated agar gels are lower than that of non-aerated samples (significant at p 6 0.01) (Fig. 3). The fracture strain of agar gel increases with an increase in concentration and due to aeration. The increase in fracture strain for aerated agar gels over the corresponding non-aerated samples is 1.7–23.1% in the range of concentration studied. It means that the aerated samples exhibit a delayed fracture due to the presence of air cells that possibly provide plasticity in the samples. The firmness (or slope of the initial phase of compression), an indication of the toughness of the sample, is similar to elasticity; the 3% sample exhibits nearly three times higher firmness values compared to 1% and 2% agar gels; similar trend is also true for fracture stress. A progressive increase in fracture energy and total energy for compression occurs with an increase in agar concentration while aerated samples always exhibit higher values compared to non-aerated samples. Generally, at a low concentration of hydrocolloid, the effect of aeration is non-significant while prominent significant changes occur at higher concentrations. The results for gellan gels (Fig. 4) indicate that the fracture strain of aerated gellan samples are significantly higher (p 6 0.01) than that of non-aerated samples while showing a decreasing trend with an increase in the concentration of gellan. The extent of increase in fracture strain for aerated gellan gels is 4.3–15.1% when compared to corresponding non-aerated samples. The fracture energy, firmness, fracture stress and the total energy for compression show a marked increase with an increase in gellan concentration and the aerated samples always display higher values.

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S. Tiwari, S. Bhattacharya / Journal of Food Engineering 107 (2011) 134–139 Non-aerated 1200

50

Fracture strain(%)

Density (kgm-3)

Aerated

1000

1

2

3

1

4

400

2

3

4

75

300

Firmness (N mm-1)

Fracture energy (mJ)

30

20

800

200

100

50

25

0

0 1

2

3

4

1

2

1

2

3

4

800

120

600 80

Total energy for compression(mJ)

Fracture stress (kPa)

40

40

0

400

200

0 1

2

3

4

Gellan (%)

3

4

Gellan (%)

Fig. 4. Compression characteristics for non-aerated (filled bar) and aerated (hollow bar) gellan gels at different concentrations.

3.2. Stress relaxation The sample stress relaxation curves for agar and gellan gels are shown in Fig. 2. These curves usually possess three zones; the first zone shows a sharp nearly linear decrease followed by a non-linear rather slow decrease in the second zone. In the last zone, the force decay curve is asymptotic meaning that it approaches a residual or equilibrium stress. In the case of gellan gel (Fig. 2), the decay curve for the aerated gels is always at a higher position than that of nonaerated samples. On the other hand, the aerated agar gel though showing higher values in the beginning phase of stress relaxation, exhibits lower values at the latter phase. The extent of relaxation decreases with an increase in agar/gellan concentration while the aerated samples show higher extent of relaxation but lower relaxation time (Fig. 5). Relaxation time for aerated agar gels is 9.6–40.7% lower than that of corresponding non-aerated samples; these values are 19.6–37.6% for gellan samples. It is expected that the air cells present in the gels are filled with liquid possibly arising out of syneresis. Hence, the presence of liquid in the air cells makes the aerated gels to relax more as an ideal liquid shows the highest extent of relaxation. Zúñiga and Aguilera (2009) have studied the effect of gas (air, nitrogen and helium) on the gas-filled gelatin gels. Increasing pro-

tein concentration increased density and fracture values and lower gas hold up. Gas-filled gels have been reported to be weaker and less ductile than control samples. The decrease in stress and strain is 70–80% and 40–65%, respectively. Nussinovitch et al. (1992) have observed that carbon dioxide incorporated gels from agar and alginate gels can maintain mechanical integrity while carrageenan gels fail to do so. Significant decrease in compressive behaviour of carbon dioxide incorporated agar gels has been attributed to local rupture produced during bubble formation and growth. In the present study, an increase in the fracture strain has been observed while there is a decrease in relaxation time due to aeration. Conventional methods to incorporate air bubbles into a liquid or viscoelastic medium include blowing air through nozzle. Most gels experience syneresis which is a slow, time-dependent de-swelling resulting in an exudation of liquid. In aerated gels, part of the syneresis may be trapped inside multiple gas bubbles and thus may not appear on the surface of the product during storage. The expulsion of liquid is generally related to large deformations and fracture during oral processing (van der Berg et al., 2007). The presence of bubbles is likely to change the fracture pattern of gels in the oral cavity and release of water.

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Non-aerated

120

120

Emtent of relaxation(%)

Extent of relaxation (%)

Aerated

100

80

80

40

0

60 1

2

1

3

2

2100

1400

1400

Relaxation time (s)

Relaxation time (s)

2100

700

3

4

Gellan (%)

Agar (%)

700

0

0 1

2

3

Agar (%)

1

2

3

4

Gellan (%)

Fig. 5. Relaxation characteristics for non-aerated (filled bar) and aerated (hollow bar) agar and gellan gels at different concentrations.

4. Conclusions Aeration of sol prior to gel formation in agar and gellan systems at different concentrations have indicated rheological changes in the formed gel that can be assessed by large-deformation compression as well as stress relaxation studies. The compression curves of these gels exhibit five major zones while it is three for stress relaxation characteristics. The fracture strain and firmness of gels increases with an increase in concentration and application of aeration process. The extent of relaxation decreases with an increase in agar or gellan concentration while the aerated samples show higher extent of relaxation due to the presence of air cells that is filled with liquid arising out of syneresis.

Acknowledgement The work has been funded under the Network Project (SIP 002) of Council of Scientific and Industrial Research (CSIR), New Delhi, India.

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