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Effect of inulin on texture and clarity of gellan gels
Journal of Food Engineering 101 (2010) 381–385
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of inulin on texture and clarity of gellan gels Vasiliki Evageliou *, Georgia Tseliou, Ioanna Mandala, Michael Komaitis Department of Food Science and Technology, Agricultural University of Athens, 75 Iera Odos, 11855 Athens, Greece
a r t i c l e
i n f o
Article history: Received 12 April 2010 Received in revised form 13 July 2010 Accepted 14 July 2010 Available online 11 August 2010 Keywords: Gellan gel Inulin Stress Strain Young’s modulus Clarity
a b s t r a c t The effect of inulin on the gelation of 0.5 wt% low acyl gellan in the presence of increasing concentrations of potassium chloride was investigated by large deformation compression experiments and visible light absorbance. The sugar and salt concentrations varied from 0 to 15 wt% and 40–100 mM, respectively. Stress and strain at failure along with Young’s modulus were calculated from each compression curve. Samples prior to compression were refrigerated at 5 °C for 24 h. Reduced gel strength and firmness were observed for all inulin and salt concentrations. Increasing amounts of inulin resulted in an increase in gel strength which was greater for higher potassium concentrations. Elasticity values did not exhibit great diversion. Inulin at concentrations of 5 and 10 wt% led to less turbid gels than those with only gellan. Samples containing 15 wt% inulin gave absorbance readings greater than 1. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Fructans are one of the most abundant naturally found, in a wide variety of plants and in bacteria, storage polysaccharides. Inulin is the basic representative of this category and can be found in several vegetables, fruits and cereals (e.g. artichoke, onion, garlic, leek, bananas, wheat and barley). Industrially it is obtained by chicory roots (Franck and De Leenheer, 2002). Inulin is a polydispersed oligosaccharide, composed of fructose units bonded together by b(2-1) linkages with a degree of polymerization (DP) varying from 2 to 70. A starting glucose moiety may also be present. Although, it is regarded as a linear molecule, limited branching (1–2%) has been reported (De Leenheer and Hoebregs, 1994). Both the DP and the branching affect the functionality of inulin (Franck and De Leenheer, 2002). Over the last years, food industries use inulin in order to formulate innovative healthy foods. Due to its reduced caloric value, it is used as a substitute of fat and sucrose allowing an improvement of both taste and texture. Dairy (e.g. Guven et al., 2005; Torres et al., 2010) and bakery (e.g. Brien et al., 2003; Mandala et al., 2009; Zoulias et al., 2002) products are some of the major applications. In addition, it offers nutritional advantages due to its prebiotic properties as it is a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, and
* Corresponding author. Tel./fax: +30 210 5294691. E-mail address: [email protected] (V. Evageliou). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.07.023
thus improving the health of the host (Gibson and Roberfroid, 1995). Several studies on both the nutritional (e.g. Roberfroid, 1999, 2007) and rheological (e.g. Angioloni and Collar, 2009; Bishay, 1998; Bot et al., 2004; Kim et al., 2001; Zimeri and Kokini, 2003) properties of inulin have been reported. Regarding gelation, Kim et al. (2001) reported that inulin can gel by shearing or heating/ cooling an inulin suspension. In addition, on storage and for concentrations of inulin of 15 wt% or higher, a creamy, gel-like paste gradually develops due to formation and agglomeration of inulin crystals (Franck, 1998). The properties of mixed systems of inulin with other biopolymers like whey protein (e.g. Herceg et al., 2007; Glibowski, 2009), pectin (e.g. Giannouli et al., 2004), gelatin (e.g. Harrington and Morris, 2009) waxy maize starch (e.g. Zimeri and Kokini, 2003) and maltodextrins (e.g. van Duynhoven et al., 1999) have also been the subject of various investigations. This study focuses on inulin’s interactions with low acyl gellan, as they have never been studied previously. Low acyl gellan is a water soluble polysaccharide which forms transparent gels on cooling. Formation of threefold double helices, further aggregated to form junction zones, is stabilised by the presence of cations and a three dimensional network is developed. Gellan finds many applications in confectionary products where sucrose is usually present. Inulin, due to its properties, can replace sucrose and lead to new innovative products satisfying the consumers’ needs for healthier food. Evageliou et al. (2010a,b) studied, recently, the properties of gellan gel in the presence of various salts and sugars. In a continuation of these studies, the effect of inulin on the texture and clarity of low acyl gellan gels in the presence of potassium
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cation was investigated. Texture and clarity are very important organoleptic properties affecting the acceptance of a product by the consumers. The characterisation of texture was based on parameters (true stress, true strain and Young’s modulus) obtained from compression tests, whereas that of clarity on absorbance readings at the visible region (490 nm).
Table 1 Results from ANOVA analysis.
3. Results Fig. 1 shows the values of stress at failure with increasing concentrations of KCl for samples of 0.5 wt% gellan gels containing inulin at concentrations 0–15 wt%. Without inulin, addition of potassium chloride initially enhanced the network strength exhibiting a maximum at a concentration of 80 mM. For greater concentrations the strength decreased. When inulin was incorporated in the sample a major drop in strength was observed. In addition, statistic evaluation showed that for increasing concentrations of inu-
F
p
Stress at failure
Salt Inulin Salt inulin
3 3 9
77.827 522.126 25.944
0.0000 0.0000 0.0000
Strain at failure
Salt Inulin Salt inulin
3 3 9
7.302 3.219 2.003
0.0027 0.0509 0.1080
Young’s modulus
Salt Inulin Salt inulin
3 3 9
70.432 200.496 21.966
0.0000 0.0000 0.0000
Absorbance
Salt Inulin Salt inulin
3 2 6
250.458 21.434 4.318
0.0000 0.0000 0.0043
2. Materials and methods
60 50
Stress (kPa)
The deacylated gellan gum was provided by Sigma (Phytogel, P8169). KCl was from Merck. The inulin sample used was ‘‘Beneo™ GR inulin” (ORAFTI Group, Belgium) with a minimum average DP of 10. Distilled water was used throughout. Gellan was dispersed in distilled water at 90 °C under gentle agitation. Polysaccharide’s concentration in all samples was 0.5 wt%. After solvation, the appropriate amounts of KCl and inulin were added. The concentration of inulin varied from 0 to 15 wt% and that of the salt from 40 to 100 mM. For comparison reasons, samples of 5, 10 and 15 wt% inulin were also prepared by dissolving the appropriate amount of inulin in distilled water.Regarding the texture experiments, the resulted solutions were poured in cylindrical moulds of 16 cm height and 20 mm diameter and kept overnight in a refrigerator at 5 °C. Samples of 20 mm height were uniaxially compressed in an Instron Universal machine (Instron 1011, Massachusetts, USA) using a 40 mm diameter plunger at a fixed rate of 48 mm/min up to 70% deformation. Filter paper was placed between sample surfaces and plates in order to prevent the slippage of the gel. All measurements were taken at room temperature. The results shown are the mean value of four compression curves. Young’s modulus, true stress and true strain were the parameters that were calculated. ‘‘True strain” shows the extent of deformation and it is given by ln(L0/L) where L0 and L are the height of the sample before and during the compression. ‘‘True stress” is the stress corrected for the enlargement of the mean cross-sectional h area i of the sample during the compression and it is given by AF LL0 . F is the rupture force and A the initial cross-section area of the sample. The values for Young’s modulus were generated from the instrument. Stress and strain at failure are indicative of the network’s strength and elasticity, respectively and Young’s modulus is related to the network firmness (Sanderson et al., 1988). For the clarity experiments, polysterene cuvettes (4.5 ml, 1 1 cm) were filled with the gellan solutions. The cuvettes were kept at room temperature overnight and absorbance was measured at 490 nm with a double-beam UV–Vis spectrophotometer (Jasco V-530, Tokyo, Japan) using water as reference, according to the procedure of Tang et al. (2001). Each experiment was performed in triplicate. One and two way analysis of variance (ANOVA) and least significant difference tests (LSD) were carried out on the data in order to determine significant differences between the samples. The significant level was p < 0.05 throughout the study. Analysis of data was carried out with Statistica (Stat-Soft, Inc., Tulsa, OK, USA) and its results are presented in Table 1.
Degree of freedom
40 30 20 10 0 20
40
60 80 [ KCl] (mM)
100
120
Fig. 1. Stress values at failure of 0.5 wt% gellan gum gels with increasing concentrations of KCl after holding at 5 °C for 24 h in the presence of 0 wt% (N), 5 wt% (4), 10 wt% (j) and 15 wt% inulin (h). The error bars show standard deviation from compression of 4 replicates.
lin (same salt concentration), the gel strength showed a tendency to increase which became pronounced and significant at the highest salt concentration used. Moreover, the presence of inulin at concentrations of 10 and 15 wt% resulted in an increase in gel strength, which was more significant for the two highest concentrations of potassium chloride. The samples containing only inulin did not gel under the conditions of the present experiments. The elasticity of the resulting networks, expressed as the strain at the failure point, is seen in Fig. 2. The presence of inulin did not seem to statistically affect the elasticity of the gels, especially in the presence of 40 and 60 mM KCl, as no great differences were observed at the values of strain at failure. Fig. 3 shows the values of Young’s modulus for 0.5 wt% gellan gels as a function of KCl, in the presence of inulin at concentrations of 0–15 wt%. In a similar pattern as for stress at failure (Fig. 1), statistics showed that the presence of inulin resulted in significantly reduced modulus values. Moreover, 10 and 15 wt% inulin had no major effect on the firmness of gels in the presence of 80 and 100 mM KCl. Furthermore, for the same inulin concentration, a significant enhancement of firmness was seen when the two highest concentrations of salt were used. According to the performed statistical analysis (Table 1), the interactions of both salt and inulin
383
1.0
0.09
0.8
0.07
Absorbance (490 nm)
Strain
V. Evageliou et al. / Journal of Food Engineering 101 (2010) 381–385
0.6
0.4
0.05
0.03
0.2 0.01
0.0 20
40
60 80 [KCl] (mM)
100
120
Fig. 2. Strain values at failure of 0.5 wt% gellan gum gels with increasing concentrations of KCl after holding at 5 °C for 24 h in the presence of 0 wt% (N), 5 wt% (4), 10 wt% (j) and 15 wt% inulin (h). The error bars show standard deviation from compression of 4 replicates.
20
40
60 80 KCl (mM)
100
120
Fig. 4. Clarity of 0.5 wt% gellan gum gels with increasing concentrations of KCl after holding at 5 °C for 24 h in the presence of 0 wt% (N), 5 wt% (4) and 10 wt% inulin (j). The absorbance readings for samples incorporating only inulin at concentrations 5 wt% (x) and 10 wt% (d) are also included. The error bars show standard deviation of 3 replicates.
inulin in gels incorporating 40 and 60 mM salt. However, for the remaining salt concentrations, gels incorporating 5 wt% inulin were more clear than the ones with 10 wt% inulin. Moreover, increasing salt concentrations (same inulin concentration) resulted in significant clarity decrease. Once again, the statistical evaluation (Table 1) has shown that the observed clarity values depended on the interactions of salt and inulin concentration.
0.3
0.2 E (MPa)
0
4. Discussion
0.1
0.0
-0.1 20
40
60 80 [KCl] (mM)
100
120
Fig. 3. Young’s modulus values of 0.5 wt% gellan gum gels with increasing concentrations of KCl after holding at 5 °C for 24 h in the presence of 0 wt% (N), 5 wt% (4), 10 wt% (j) and 15 wt% inulin (h). The error bars show standard deviation from compression of 4 replicates.
concentration are significantly important for the determination of gel strength and firmness but not for the elasticity. The effect of inulin on gel clarity for the 0.5 wt% gellan gels, in the presence of increasing concentrations of potassium salt, is presented in Fig. 4. When 15 wt% inulin was present, the samples became opaque and the absorbance readings gave values higher than 1. Due to their deviation from Beer’s law these values are not reported. The absorbance readings for the samples containing only inulin (5 and 10 wt%) are also shown in this figure. Once again, the 15 wt% inulin sample exhibited absorbance readings greater than 1 (results not shown). According to the statistic evaluation (p < 0.05), clarity was not altered significantly by the presence of
When solutions of two biopolymers are mixed, interactions between their chains depend on the balance between the enthalpy and the entropy changes on mixing, being, therefore, either favourable (association) or unfavourable (segregation) (Piculell et al., 1994). Almost all biopolymer mixtures exhibit segregate interactions, unless there is an electrostatic drive to association. These usually result in phase separated networks where the components tend to exclude each other from their domains. Turbidity on mixing is indicative of phase separation in the solution state. Due to differences in density the resulting emulsion is resolved into two discrete layers when left standing or centrifuged. Studies on mixtures of a gelling and a non-gelling biopolymers, at concentrations where the pre-gel solution remains in a single phase, resulted in a variety of results. Tolstoguzov et al. (1974) studied the effect of dextran in the rate of conformational ordering of gelatine and the mechanical properties of the resulting gels. According to their study, small amounts of dextran led to a large increase in the rate of conformational ordering of gelatine. Harrington and Morris (2009) explored the possible generality of this behaviour in mixtures of gelatine with various soluble polysaccharides, among them inulin, and found no significant change in gel strength (Young’s modulus) for concentrations of inulin up to 15 wt%. In the case of low methoxyl pectin, incorporation of oxidised starch resulted in stronger, weaker or unaffected gel network depending on the degree of esterification of the pectin and the concentration of calcium ions (Picout et al., 2000). In a continuation of this work, inulin, among other polysaccharides, gave pectin gels with reduced values of G0 at 5 °C (Giannouli et al., 2004). Tara gum, guar gum and xanthan also affected the thermogelation of whey protein isolate
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as they led to stronger gels (Fitzsimons et al., 2008; Sittikijyothin et al., 2007). As seen from Figs. 1 and 3, the presence of inulin affected both gellan gel strength and firmness. As already mentioned, gellan gels are formed in the presence of cations. Their concentration strongly affects the mechanical properties of the resulting gel network (Grasdalen and Smidsrod, 1987; Sanderson, 1990) as initially increasing concentrations of cations lead to increased gel strength whereas at high concentrations of cations the strength decreases. Results from Fig. 1 are in good agreement with this behaviour as they showed that for concentrations of potassium ions up to 80 mM, progressively stronger gels were obtained whereas for greater salt concentrations, the gel strength decreased. Gels incorporating inulin exhibited significantly lower values of strength at failure. However, for each inulin concentration, the strength kept increasing with increasing salt concentration. Moreover, salt concentration was an important factor in the obtained behaviour as it affected the differences in gel strength among samples with increasing inulin concentration. Thus, the gel strength increased significantly at the highest salt concentrations used. The decrease in strength for gellan gels at high concentrations of salts can be attributed (Morris et al., 1999) to excessive helix– helix association into very large aggregates, which leads to a reduction in the total number of junction zones in the gel network. The results obtained in the present study suggest that the presence of inulin inhibited the aggregation of the gellan helices resulting in decreased gel strength. In addition, increasing salt concentration promoted phase separation. As a result, the increase in the effective concentration of the two biopolymers was greater. Thus, gelation of inulin became important and the increase in the gel strength was greater as salt concentration increased. This suggestion is supported by the fact that samples incorporating only inulin up to 15 wt% failed to give gels in the timescale of this experiment. As the ion requirement for gellan gelation depends on the concentration and the nature of the cosolute (Sworn and Kasapis, 1998), experiments with higher concentrations of both inulin and salt, than the ones used in the present study, seem worth planning. Based on this, we would also investigate if stress at failure exhibits or not a maximum at higher, than in gels without inulin, salt concentrations. This suggestion is also supported statistically as, according to Table 1, the interactions of salt and inulin are significantly important for the determination of gel strength. Regarding firmness (Fig. 3), when inulin was involved a drop in the values of Young’s modulus was observed. However, concentrations of inulin of 10 and 15 wt% did not seem to be as important, as in the case of strength, for the two higher salt concentrations. All previous studies regarding mixtures of biopolymers with inulin (e.g. Harrington and Morris, 2009; Giannouli et al., 2004) also showed a drop in the value of modulus, but in a smaller extent. However, they also showed no significant changes in the firmness of gels in the presence of inulin and for concentrations up to 15 wt%. Values of elasticity (Fig. 2) did not exhibit great differences in all samples, with or without inulin. The same observation was made when salts or sugars (glucose, fructose and sucrose) and their mixtures were present (Evageliou et al., 2010a,b). An additional observation was that, in the presence of inulin, the values for all studied parameters seem to be a linear function of salt concentration. In order to investigate this further, linear plots were applied to all data. Regression analysis, for increasing inulin concentration, showed r2 ranging from 0.731 to 0.996 (for true stress at failure), 0.338 to 0.588 (for true strain at failure) and 0.913 to 0.989 (for Young’s modulus). Regarding clarity (Fig. 4), increasing salt concentration, led to decreased clarity. In addition, it seems that for salt concentrations greater than 60 mM, clarity depended on the presence of gellan as
the obtained absorbance values are greater than those from the inulin samples and closer to the gellan ones. Inulin became important only at the concentration of 15 wt% since all the gellan–inulin samples showed absorbance values greater than 1. Tang et al. (2001) investigated the effect of sucrose and fructose on the clarity of gellan gels. They reported that both sugars led to clearer gels throughout the salt concentration range. This was attributed to the small contribution of the small sugar molecules to the turbidity of the system in comparison with the contribution resulting from the gelation of the gel components. In addition, they suggested that, the increased viscosity of the system in the presence of sugars hindered the gellan double helices to join junction zones. Therefore migration of the double helices was restricted and shorter junction zones were formed. In this study it was shown that inulin present in concentrations up to 10 wt% increased the gel clarity of gellan gels. This is in agreement with the previous suggestion. However, 15 wt% concentration of inulin led to an increase in the turbidity. This may also be explained by the reduction in gellan excessive aggregation due to its segregative interactions with inulin. According to our previous suggestion, those interactions were more pronounced at the highest concentration of inulin used, where inulin became the dominant factor in turbidity as samples with 15 wt% inulin, for all salt concentrations, had absorbance readings greater that 1, the same as the sample containing only 15 wt% inulin.
5. Conclusions The present study was focused on the effect of inulin on the clarity and texture of low acyl gellan gels. The elasticity of gellan gels was not altered significantly but both strength and firmness exhibited decreased values. Increased concentrations of inulin resulted in an increase in gel strength. The same pattern of behaviour was observed for clarity. This behaviour was attributed to segregative interactions between gellan and inulin which were more pronounced in greater salt concentrations. According to statistic evaluation, the interactions between salt and inulin concentration are significant for the determination of gel strength, firmness and clarity but not for elasticity.
Acknowledgements The authors would like to thank Mr. E.F. Anagnostaras for technical assistance.
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V. Evageliou et al. / Journal of Food Engineering 101 (2010) 381–385 Fitzsimons, S.M., Mulvihill, D.M., Morris, E.R., 2008. Large enhancements in thermogelation of whey protein isolate by incorporation of very low concentrations of guar gum. Food Hydrocolloids 22, 576–586. Giannouli, P., Richardson, R.K., Morris, E.R., 2004. Effect of polymeric cosolutes on calcium pectinate gelation. Part 2: dextrans and inulin. Carbohydr. Polym. 55, 357–365. Gibson, G.R., Roberfroid, M.B., 1995. Dietary modulation of the human colonic microbiota: introducing the concept of prebiotics. J. Nutr. 125, 1401–1412. Glibowski, P., 2009. Rheological properties and structure of inulin–whey protein gels. Int. Dairy J. 19, 443–449. Grasdalen, H., Smidsrod, O., 1987. Gelation of gellan gum. Carbohydr. Polym. 7, 371–393. Guven, M., Yasar, K., Karaca, O.B., Hayaloglou, A.A., 2005. The effect of inulin as a fat replacer on the quality of set-type low-fat yoghurt manufacture. Int. J. Dairy Technol. 58, 180–184. Harrington, J.C., Morris, E.R., 2009. Conformational ordering and gelation of gelatine in mixtures with soluble polysaccharides. Food Hydrocolloids 23, 327–336. Herceg, Z., Rezek, A., Lelas, V., Kresic, G., Franetovic, M., 2007. Effect of carbohydrates on the emulsifying, foaming and freezing properties of whey protein suspensions. J. Food Eng. 79, 279–286. Kim, Y., Fagih, M.N., Wang, S.S., 2001. Factors affecting gel formation of inulin. Carbohydr. Polym. 46, 135–145. Mandala, I., Polaki, A., Yanniotis, S., 2009. Influence of frozen storage on bread enriched with various ingredients. J. Food Eng. 92, 137–145. Morris, E.R., Richardson, R.K., Whittaker, L.E., 1999. Rheology and gelation of deacylated gellan polysaccharide with Na+ as the sole counterion. Prog. Colloid Polym. Sci. 114, 109–115. Picout, D.R., Richardon, R.K., Rolin, C., Abeysekera, R.M., Morris, E.R., 2000. Ca2+induced gelation of low methoxy pectin in the presence of oxidised starch. Part I. Collapse of network structure. Carbohydr. Polym. 43, 113–122. Piculell, L., Iliopoulos, I., Linse, P., Nilsson, S., Turquois, T., Viebke, C., Zhang, W., 1994. Association and segregation in ternary polymer solutions and gels. In: Phillips, G.O., Williams, P.A., Wedlock, D.J. (Eds.), Gums and Stabilisers for the Food Industry, vol. 7. IRL Press, Oxford, pp. 309–322.
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Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of inulin on texture and clarity of gellan gels Vasiliki Evageliou *, Georgia Tseliou, Ioanna Mandala, Michael Komaitis Department of Food Science and Technology, Agricultural University of Athens, 75 Iera Odos, 11855 Athens, Greece
a r t i c l e
i n f o
Article history: Received 12 April 2010 Received in revised form 13 July 2010 Accepted 14 July 2010 Available online 11 August 2010 Keywords: Gellan gel Inulin Stress Strain Young’s modulus Clarity
a b s t r a c t The effect of inulin on the gelation of 0.5 wt% low acyl gellan in the presence of increasing concentrations of potassium chloride was investigated by large deformation compression experiments and visible light absorbance. The sugar and salt concentrations varied from 0 to 15 wt% and 40–100 mM, respectively. Stress and strain at failure along with Young’s modulus were calculated from each compression curve. Samples prior to compression were refrigerated at 5 °C for 24 h. Reduced gel strength and firmness were observed for all inulin and salt concentrations. Increasing amounts of inulin resulted in an increase in gel strength which was greater for higher potassium concentrations. Elasticity values did not exhibit great diversion. Inulin at concentrations of 5 and 10 wt% led to less turbid gels than those with only gellan. Samples containing 15 wt% inulin gave absorbance readings greater than 1. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Fructans are one of the most abundant naturally found, in a wide variety of plants and in bacteria, storage polysaccharides. Inulin is the basic representative of this category and can be found in several vegetables, fruits and cereals (e.g. artichoke, onion, garlic, leek, bananas, wheat and barley). Industrially it is obtained by chicory roots (Franck and De Leenheer, 2002). Inulin is a polydispersed oligosaccharide, composed of fructose units bonded together by b(2-1) linkages with a degree of polymerization (DP) varying from 2 to 70. A starting glucose moiety may also be present. Although, it is regarded as a linear molecule, limited branching (1–2%) has been reported (De Leenheer and Hoebregs, 1994). Both the DP and the branching affect the functionality of inulin (Franck and De Leenheer, 2002). Over the last years, food industries use inulin in order to formulate innovative healthy foods. Due to its reduced caloric value, it is used as a substitute of fat and sucrose allowing an improvement of both taste and texture. Dairy (e.g. Guven et al., 2005; Torres et al., 2010) and bakery (e.g. Brien et al., 2003; Mandala et al., 2009; Zoulias et al., 2002) products are some of the major applications. In addition, it offers nutritional advantages due to its prebiotic properties as it is a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, and
* Corresponding author. Tel./fax: +30 210 5294691. E-mail address: [email protected] (V. Evageliou). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.07.023
thus improving the health of the host (Gibson and Roberfroid, 1995). Several studies on both the nutritional (e.g. Roberfroid, 1999, 2007) and rheological (e.g. Angioloni and Collar, 2009; Bishay, 1998; Bot et al., 2004; Kim et al., 2001; Zimeri and Kokini, 2003) properties of inulin have been reported. Regarding gelation, Kim et al. (2001) reported that inulin can gel by shearing or heating/ cooling an inulin suspension. In addition, on storage and for concentrations of inulin of 15 wt% or higher, a creamy, gel-like paste gradually develops due to formation and agglomeration of inulin crystals (Franck, 1998). The properties of mixed systems of inulin with other biopolymers like whey protein (e.g. Herceg et al., 2007; Glibowski, 2009), pectin (e.g. Giannouli et al., 2004), gelatin (e.g. Harrington and Morris, 2009) waxy maize starch (e.g. Zimeri and Kokini, 2003) and maltodextrins (e.g. van Duynhoven et al., 1999) have also been the subject of various investigations. This study focuses on inulin’s interactions with low acyl gellan, as they have never been studied previously. Low acyl gellan is a water soluble polysaccharide which forms transparent gels on cooling. Formation of threefold double helices, further aggregated to form junction zones, is stabilised by the presence of cations and a three dimensional network is developed. Gellan finds many applications in confectionary products where sucrose is usually present. Inulin, due to its properties, can replace sucrose and lead to new innovative products satisfying the consumers’ needs for healthier food. Evageliou et al. (2010a,b) studied, recently, the properties of gellan gel in the presence of various salts and sugars. In a continuation of these studies, the effect of inulin on the texture and clarity of low acyl gellan gels in the presence of potassium
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cation was investigated. Texture and clarity are very important organoleptic properties affecting the acceptance of a product by the consumers. The characterisation of texture was based on parameters (true stress, true strain and Young’s modulus) obtained from compression tests, whereas that of clarity on absorbance readings at the visible region (490 nm).
Table 1 Results from ANOVA analysis.
3. Results Fig. 1 shows the values of stress at failure with increasing concentrations of KCl for samples of 0.5 wt% gellan gels containing inulin at concentrations 0–15 wt%. Without inulin, addition of potassium chloride initially enhanced the network strength exhibiting a maximum at a concentration of 80 mM. For greater concentrations the strength decreased. When inulin was incorporated in the sample a major drop in strength was observed. In addition, statistic evaluation showed that for increasing concentrations of inu-
F
p
Stress at failure
Salt Inulin Salt inulin
3 3 9
77.827 522.126 25.944
0.0000 0.0000 0.0000
Strain at failure
Salt Inulin Salt inulin
3 3 9
7.302 3.219 2.003
0.0027 0.0509 0.1080
Young’s modulus
Salt Inulin Salt inulin
3 3 9
70.432 200.496 21.966
0.0000 0.0000 0.0000
Absorbance
Salt Inulin Salt inulin
3 2 6
250.458 21.434 4.318
0.0000 0.0000 0.0043
2. Materials and methods
60 50
Stress (kPa)
The deacylated gellan gum was provided by Sigma (Phytogel, P8169). KCl was from Merck. The inulin sample used was ‘‘Beneo™ GR inulin” (ORAFTI Group, Belgium) with a minimum average DP of 10. Distilled water was used throughout. Gellan was dispersed in distilled water at 90 °C under gentle agitation. Polysaccharide’s concentration in all samples was 0.5 wt%. After solvation, the appropriate amounts of KCl and inulin were added. The concentration of inulin varied from 0 to 15 wt% and that of the salt from 40 to 100 mM. For comparison reasons, samples of 5, 10 and 15 wt% inulin were also prepared by dissolving the appropriate amount of inulin in distilled water.Regarding the texture experiments, the resulted solutions were poured in cylindrical moulds of 16 cm height and 20 mm diameter and kept overnight in a refrigerator at 5 °C. Samples of 20 mm height were uniaxially compressed in an Instron Universal machine (Instron 1011, Massachusetts, USA) using a 40 mm diameter plunger at a fixed rate of 48 mm/min up to 70% deformation. Filter paper was placed between sample surfaces and plates in order to prevent the slippage of the gel. All measurements were taken at room temperature. The results shown are the mean value of four compression curves. Young’s modulus, true stress and true strain were the parameters that were calculated. ‘‘True strain” shows the extent of deformation and it is given by ln(L0/L) where L0 and L are the height of the sample before and during the compression. ‘‘True stress” is the stress corrected for the enlargement of the mean cross-sectional h area i of the sample during the compression and it is given by AF LL0 . F is the rupture force and A the initial cross-section area of the sample. The values for Young’s modulus were generated from the instrument. Stress and strain at failure are indicative of the network’s strength and elasticity, respectively and Young’s modulus is related to the network firmness (Sanderson et al., 1988). For the clarity experiments, polysterene cuvettes (4.5 ml, 1 1 cm) were filled with the gellan solutions. The cuvettes were kept at room temperature overnight and absorbance was measured at 490 nm with a double-beam UV–Vis spectrophotometer (Jasco V-530, Tokyo, Japan) using water as reference, according to the procedure of Tang et al. (2001). Each experiment was performed in triplicate. One and two way analysis of variance (ANOVA) and least significant difference tests (LSD) were carried out on the data in order to determine significant differences between the samples. The significant level was p < 0.05 throughout the study. Analysis of data was carried out with Statistica (Stat-Soft, Inc., Tulsa, OK, USA) and its results are presented in Table 1.
Degree of freedom
40 30 20 10 0 20
40
60 80 [ KCl] (mM)
100
120
Fig. 1. Stress values at failure of 0.5 wt% gellan gum gels with increasing concentrations of KCl after holding at 5 °C for 24 h in the presence of 0 wt% (N), 5 wt% (4), 10 wt% (j) and 15 wt% inulin (h). The error bars show standard deviation from compression of 4 replicates.
lin (same salt concentration), the gel strength showed a tendency to increase which became pronounced and significant at the highest salt concentration used. Moreover, the presence of inulin at concentrations of 10 and 15 wt% resulted in an increase in gel strength, which was more significant for the two highest concentrations of potassium chloride. The samples containing only inulin did not gel under the conditions of the present experiments. The elasticity of the resulting networks, expressed as the strain at the failure point, is seen in Fig. 2. The presence of inulin did not seem to statistically affect the elasticity of the gels, especially in the presence of 40 and 60 mM KCl, as no great differences were observed at the values of strain at failure. Fig. 3 shows the values of Young’s modulus for 0.5 wt% gellan gels as a function of KCl, in the presence of inulin at concentrations of 0–15 wt%. In a similar pattern as for stress at failure (Fig. 1), statistics showed that the presence of inulin resulted in significantly reduced modulus values. Moreover, 10 and 15 wt% inulin had no major effect on the firmness of gels in the presence of 80 and 100 mM KCl. Furthermore, for the same inulin concentration, a significant enhancement of firmness was seen when the two highest concentrations of salt were used. According to the performed statistical analysis (Table 1), the interactions of both salt and inulin
383
1.0
0.09
0.8
0.07
Absorbance (490 nm)
Strain
V. Evageliou et al. / Journal of Food Engineering 101 (2010) 381–385
0.6
0.4
0.05
0.03
0.2 0.01
0.0 20
40
60 80 [KCl] (mM)
100
120
Fig. 2. Strain values at failure of 0.5 wt% gellan gum gels with increasing concentrations of KCl after holding at 5 °C for 24 h in the presence of 0 wt% (N), 5 wt% (4), 10 wt% (j) and 15 wt% inulin (h). The error bars show standard deviation from compression of 4 replicates.
20
40
60 80 KCl (mM)
100
120
Fig. 4. Clarity of 0.5 wt% gellan gum gels with increasing concentrations of KCl after holding at 5 °C for 24 h in the presence of 0 wt% (N), 5 wt% (4) and 10 wt% inulin (j). The absorbance readings for samples incorporating only inulin at concentrations 5 wt% (x) and 10 wt% (d) are also included. The error bars show standard deviation of 3 replicates.
inulin in gels incorporating 40 and 60 mM salt. However, for the remaining salt concentrations, gels incorporating 5 wt% inulin were more clear than the ones with 10 wt% inulin. Moreover, increasing salt concentrations (same inulin concentration) resulted in significant clarity decrease. Once again, the statistical evaluation (Table 1) has shown that the observed clarity values depended on the interactions of salt and inulin concentration.
0.3
0.2 E (MPa)
0
4. Discussion
0.1
0.0
-0.1 20
40
60 80 [KCl] (mM)
100
120
Fig. 3. Young’s modulus values of 0.5 wt% gellan gum gels with increasing concentrations of KCl after holding at 5 °C for 24 h in the presence of 0 wt% (N), 5 wt% (4), 10 wt% (j) and 15 wt% inulin (h). The error bars show standard deviation from compression of 4 replicates.
concentration are significantly important for the determination of gel strength and firmness but not for the elasticity. The effect of inulin on gel clarity for the 0.5 wt% gellan gels, in the presence of increasing concentrations of potassium salt, is presented in Fig. 4. When 15 wt% inulin was present, the samples became opaque and the absorbance readings gave values higher than 1. Due to their deviation from Beer’s law these values are not reported. The absorbance readings for the samples containing only inulin (5 and 10 wt%) are also shown in this figure. Once again, the 15 wt% inulin sample exhibited absorbance readings greater than 1 (results not shown). According to the statistic evaluation (p < 0.05), clarity was not altered significantly by the presence of
When solutions of two biopolymers are mixed, interactions between their chains depend on the balance between the enthalpy and the entropy changes on mixing, being, therefore, either favourable (association) or unfavourable (segregation) (Piculell et al., 1994). Almost all biopolymer mixtures exhibit segregate interactions, unless there is an electrostatic drive to association. These usually result in phase separated networks where the components tend to exclude each other from their domains. Turbidity on mixing is indicative of phase separation in the solution state. Due to differences in density the resulting emulsion is resolved into two discrete layers when left standing or centrifuged. Studies on mixtures of a gelling and a non-gelling biopolymers, at concentrations where the pre-gel solution remains in a single phase, resulted in a variety of results. Tolstoguzov et al. (1974) studied the effect of dextran in the rate of conformational ordering of gelatine and the mechanical properties of the resulting gels. According to their study, small amounts of dextran led to a large increase in the rate of conformational ordering of gelatine. Harrington and Morris (2009) explored the possible generality of this behaviour in mixtures of gelatine with various soluble polysaccharides, among them inulin, and found no significant change in gel strength (Young’s modulus) for concentrations of inulin up to 15 wt%. In the case of low methoxyl pectin, incorporation of oxidised starch resulted in stronger, weaker or unaffected gel network depending on the degree of esterification of the pectin and the concentration of calcium ions (Picout et al., 2000). In a continuation of this work, inulin, among other polysaccharides, gave pectin gels with reduced values of G0 at 5 °C (Giannouli et al., 2004). Tara gum, guar gum and xanthan also affected the thermogelation of whey protein isolate
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as they led to stronger gels (Fitzsimons et al., 2008; Sittikijyothin et al., 2007). As seen from Figs. 1 and 3, the presence of inulin affected both gellan gel strength and firmness. As already mentioned, gellan gels are formed in the presence of cations. Their concentration strongly affects the mechanical properties of the resulting gel network (Grasdalen and Smidsrod, 1987; Sanderson, 1990) as initially increasing concentrations of cations lead to increased gel strength whereas at high concentrations of cations the strength decreases. Results from Fig. 1 are in good agreement with this behaviour as they showed that for concentrations of potassium ions up to 80 mM, progressively stronger gels were obtained whereas for greater salt concentrations, the gel strength decreased. Gels incorporating inulin exhibited significantly lower values of strength at failure. However, for each inulin concentration, the strength kept increasing with increasing salt concentration. Moreover, salt concentration was an important factor in the obtained behaviour as it affected the differences in gel strength among samples with increasing inulin concentration. Thus, the gel strength increased significantly at the highest salt concentrations used. The decrease in strength for gellan gels at high concentrations of salts can be attributed (Morris et al., 1999) to excessive helix– helix association into very large aggregates, which leads to a reduction in the total number of junction zones in the gel network. The results obtained in the present study suggest that the presence of inulin inhibited the aggregation of the gellan helices resulting in decreased gel strength. In addition, increasing salt concentration promoted phase separation. As a result, the increase in the effective concentration of the two biopolymers was greater. Thus, gelation of inulin became important and the increase in the gel strength was greater as salt concentration increased. This suggestion is supported by the fact that samples incorporating only inulin up to 15 wt% failed to give gels in the timescale of this experiment. As the ion requirement for gellan gelation depends on the concentration and the nature of the cosolute (Sworn and Kasapis, 1998), experiments with higher concentrations of both inulin and salt, than the ones used in the present study, seem worth planning. Based on this, we would also investigate if stress at failure exhibits or not a maximum at higher, than in gels without inulin, salt concentrations. This suggestion is also supported statistically as, according to Table 1, the interactions of salt and inulin are significantly important for the determination of gel strength. Regarding firmness (Fig. 3), when inulin was involved a drop in the values of Young’s modulus was observed. However, concentrations of inulin of 10 and 15 wt% did not seem to be as important, as in the case of strength, for the two higher salt concentrations. All previous studies regarding mixtures of biopolymers with inulin (e.g. Harrington and Morris, 2009; Giannouli et al., 2004) also showed a drop in the value of modulus, but in a smaller extent. However, they also showed no significant changes in the firmness of gels in the presence of inulin and for concentrations up to 15 wt%. Values of elasticity (Fig. 2) did not exhibit great differences in all samples, with or without inulin. The same observation was made when salts or sugars (glucose, fructose and sucrose) and their mixtures were present (Evageliou et al., 2010a,b). An additional observation was that, in the presence of inulin, the values for all studied parameters seem to be a linear function of salt concentration. In order to investigate this further, linear plots were applied to all data. Regression analysis, for increasing inulin concentration, showed r2 ranging from 0.731 to 0.996 (for true stress at failure), 0.338 to 0.588 (for true strain at failure) and 0.913 to 0.989 (for Young’s modulus). Regarding clarity (Fig. 4), increasing salt concentration, led to decreased clarity. In addition, it seems that for salt concentrations greater than 60 mM, clarity depended on the presence of gellan as
the obtained absorbance values are greater than those from the inulin samples and closer to the gellan ones. Inulin became important only at the concentration of 15 wt% since all the gellan–inulin samples showed absorbance values greater than 1. Tang et al. (2001) investigated the effect of sucrose and fructose on the clarity of gellan gels. They reported that both sugars led to clearer gels throughout the salt concentration range. This was attributed to the small contribution of the small sugar molecules to the turbidity of the system in comparison with the contribution resulting from the gelation of the gel components. In addition, they suggested that, the increased viscosity of the system in the presence of sugars hindered the gellan double helices to join junction zones. Therefore migration of the double helices was restricted and shorter junction zones were formed. In this study it was shown that inulin present in concentrations up to 10 wt% increased the gel clarity of gellan gels. This is in agreement with the previous suggestion. However, 15 wt% concentration of inulin led to an increase in the turbidity. This may also be explained by the reduction in gellan excessive aggregation due to its segregative interactions with inulin. According to our previous suggestion, those interactions were more pronounced at the highest concentration of inulin used, where inulin became the dominant factor in turbidity as samples with 15 wt% inulin, for all salt concentrations, had absorbance readings greater that 1, the same as the sample containing only 15 wt% inulin.
5. Conclusions The present study was focused on the effect of inulin on the clarity and texture of low acyl gellan gels. The elasticity of gellan gels was not altered significantly but both strength and firmness exhibited decreased values. Increased concentrations of inulin resulted in an increase in gel strength. The same pattern of behaviour was observed for clarity. This behaviour was attributed to segregative interactions between gellan and inulin which were more pronounced in greater salt concentrations. According to statistic evaluation, the interactions between salt and inulin concentration are significant for the determination of gel strength, firmness and clarity but not for elasticity.
Acknowledgements The authors would like to thank Mr. E.F. Anagnostaras for technical assistance.
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