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Effect of thermal and mechanical factors on rheological properties of high performance inulin gels and spreads
Journal of Food Engineering 99 (2010) 106–113
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Effect of thermal and mechanical factors on rheological properties of high performance inulin gels and spreads Pawel Glibowski * Department of Milk Technology and Hydrocolloids, University of Life Science in Lublin, Skromna 8, 20-704 Lublin, Poland
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 4 January 2010 Accepted 7 February 2010 Available online 11 February 2010 Keywords: Inulin gels Inulin spreads Rheology Texture ESEM
a b s t r a c t The aim of this study was analysis of a number of factors affecting the rheological properties of high performance inulin gels and spreads in comparison with commercial products. Inulin gels (20%, 25%, 30%), commercial and inulin model spread (20% canola oil, 20% inulin, 3% emulsifier) were analysed. Inulin particles in water environment absorbed water which caused an increase in viscosity of the inulin suspensions. Different temperatures of preparation, cooling rates and viscosity increase which appeared during the stirring of inulin suspensions did not significantly change the rheological parameters of the final gels in contrast to heating rates (p 6 0.05). In spite of rigid laboratory conditions high standard deviation for hardness and apparent viscosity showed how difficult the process of an inulin crystallization is to control. Rheological properties of inulin model spread exhibited thixotropic and shear thinning behaviour which made it similar to commercial spread. The applied structure destruction step in the manufacturing process should be applied to make inulin model spread comparable to commercial spreads. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Inulin is a carbohydrate consisting of GFn and Fm compounds, where n or m represents the number of fructose units (F) linked by b(2–1) bonds, with one terminal glucose (G) (Carabin and Flamm, 1999). Nowadays inulin may be purchased in different forms. Inulin extracted from chicory roots has an average degree of polymerisation (DP) of 10–12 and consists of wide spectrum of molecules with chain lengths from 2 to 60 units. The purification process of inulin extracts allow the obtainment of high performance (HP) inulin with an average DP of 23–25 and a molecular distribution ranging from 11 to 60 (Niness, 1999). Depending on the form, inulin powders have different taste (from slightly sweet for standard inulin which include all fractions to neutral taste for HP inulin) and different rheological properties (Chiavaro et al., 2007; Franck, 2002). Inulin has a great impact on human physiology. It reduces cholesterol and lipid levels in blood plasma, gives prebiotic and fiber effects and helps in calcium and magnesium adsorption (Coudray et al., 2003; Gibson and Roberfroid, 1995; Liong and Shah, 2005; Roberfroid et al., 1998; Schneeman, 1999). Application of inulin gelling properties in the food industry needs to pay special attention to the thermochemical conditions during the dissolution of inulin powders. In an acidic environment inulin undergoes hydrolysis however in pH 5 and higher it is quite stable. Inulin gels obtained in neutral or near neutral pH are firm * Tel.: +48 081 462 3349; fax: +48 081 462 3354. E-mail addresses: [email protected], [email protected] 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.02.007
(Glibowski and Wasko, 2008). Some authors indicated shearing conditions as an important factor in gelling properties of inulin HP however other studies did not confirm this (Glibowski and Gała˛zka, 2009; Kim et al., 2001). A crucial factor affecting rheological properties of inulin gels is heating time and temperature of preparation stage. Suspending inulin powder at 20–40 °C allows obtaining firm gels. Farther temperature increase causes deterioration of gel firmness as a result of decreasing the number of seeding crystals (Glibowski and Gała˛zka, 2009; Glibowski and Wasko, 2008). Inulin forms particulate gels by a network of small crystallites (Bot et al., 2004). When the crystallites are diluted at a high temperature i.e. above 80 °C the inulin gelation is inhibited (Bot et al., 2004; Glibowski and Wasko, 2008). Food technologists supported by food scientists are still searching for an alternative for butter, margarine and spreads. Butter has pour spreadability after taking it out of the refrigerator, a relatively high price and additionally influences on cholesterol and lipid levels in blood (Parodi, 2009). Margarines and spreads produced on the hydrogenated oils basis have high levels of trans fatty acids which increase concentrations of LDL cholesterol and reduce HDL cholesterol (Karabulut and Turan, 2006). The texture of inulin gels resemble solid fats. Because of this similarity inulin gel may be applied as fat mimetic (Bot et al., 2004). Bearing in mind healthy advantages, inulin gel seems to be an attractive alternative for butter and traditional spreads and margarines. Since inulin is considered as a health promoting ingredient it is often applied in many foods (Brennan et al., 2004; Brien et al., 2003; Hennelly et al., 2006; Mendoza et al., 2001; Paseephol et al., 2008).
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Rheological properties of inulin gels have been studied with usage of small and large deformation techniques (Bot et al., 2004; Glibowski and Wasko, 2008; Zimeri and Kokini, 2003). However information about the comparison of rheological properties of inulin and commercial spreads is rather scarce. Besides, to the best of my knowledge there is a lack of information available on the effect of heating and cooling conditions of inulin solution on the rheological properties of inulin gels. Therefore, the aim of this study was analysis of a number of factors affecting application in food industry i.e. temperature and time of suspension preparation, heating and cooling rates of inulin suspensions/solutions and wide study of rheological properties of HP inulin gels and spreads in comparison with commercial products. 2. Materials and methods 2.1. Materials Inulin FrutafitÒ TEX! was donated by Sensus Operations C.V. (Roosendaal, The Netherlands). Inulin was extracted from chicory root and its average degree of polymerisation is P23 (producer’s data). WPI (whey protein isolate) Extensor (95% protein in dry mass, producers data) was a gift from Lacma Sp. z o.o. (Nadarzyn, Poland). Canola oil and spread with 25% fat content was purchased from a local supermarket. 2.2. Effect of stirring time on apparent viscosity of inulin suspension Inulin (20%) was suspended in distilled water (20 °C) using a magnetic stirrer. This step took approximately 2 min. Subsequently inulin suspensions were homogenized by laboratory homogenizer H 500 (Pol-EkoAparatura, Wodzisław S´la˛ski, Polska). Homogenization was undertaken to dispose of any possible clumps. Homogenization lasted for 1 min at rotational speed of 10,000 min1. Immediately after homogenisation the solution was poured into a rheometer cup.
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within 21 min and 20 s and cooled at 20 °C reached 20 °C within 8 h and 20 min which gave a cooling rate 2.35 and 0.1 °C min1, respectively. All rheological and textural measurements for these samples were carried out at 20 °C. 2.4. Inulin model spread preparation Inulin model spread was prepared by inulin suspension followed by WPI (3%) dispersion. WPI was applied as an emulsifier. The procedure of inulin suspension was very similar to that described in Section 2.2. The suspensions were poured into a conical flasks and heated up to 70 °C in a water bath at a heating rate of 16 °C min1. Subsequently inulin solutions were homogenized with hot (70 °C) canola oil using laboratory homogenizer. To obtain stable emulsion homogenization lasted for 3 min at a rotational speed of 12,000 min1. Afterwards the emulsions were poured into final containers. All samples were stored at 5 °C in a thermostatic cabinet. The final composition of inulin spread was 20%, 20% and 3% inulin, oil and WPI, respectively. 2.5. Effect of structure destruction on rheological properties of the samples To analyse the effect of structure destruction on the rheological properties the structure of the samples of inulin model spread, commercial spread and inulin gels (20%, 25%, 30%) were destroyed. For this purpose inulin gels were obtained according to Section 2.4, but without emulsifier addition. Hot water (70 °C) was used instead of hot canola oil. Samples of the commercial spread were put into containers with special attention to avoid bubbles formation. All samples were analysed after 24 h storage at 5 °C (fresh samples). After analysis each sample was stirred with a glass rod (for 10 rotations to right and left side) and packed carefully in the container (first structure destruction (SD)). Special attention was paid to avoid bubbles formation. The same procedure took place for another 3 days running (second, third, fourth SD). All rheological and textural measurements for these samples were carried out at 5 °C.
2.3. Effect of temperature and time of suspension’s preparation, heating and cooling rates of inulin suspensions/solutions
2.6. Rheometry
Inulin (20%, 25%, 30%) was suspended in distilled water as it was described in Section 2.2 at two temperatures (20 or 40 °C). The temperature was maintained by MS 11 HS magnetic stirrer (Wigo, Piastów, Poland) with temperature control of the stirring liquid. To avoid temperature decrease (in case of 40 °C) during homogenisation, the suspension was poured into a warm homogeniser bottle and the bottle with the sample was immersed in a water bath at 40 °C. After the homogenisation step the suspensions were stirred using a magnetic stirrer for different periods (5 or 30 min). Subsequently inulin suspensions were heated in conical flasks in a water bath up to 70 °C. Temperature was controlled by a laboratory thermometer. Inulin suspension at temperature 20 °C reached 70 °C in a water bath at 100 °C within 2 min and 7 s which gave a heating rate 23.6 °C min1. Heating suspension at temperature 40 °C in a water bath at 100 °C gave 70 °C within 1 min and 42 s which gave a heating rate 17.6 °C min1. Heating suspensions in a water bath at 80 °C gave heating rates 11.0 and 8.3 °C min1 for the suspensions at 20 and 40 °C, respectively. The flasks were subjected to intense agitation during heating. Afterwards the solutions were poured into plastic cylindrical containers 35 mm in inner diameter and the lids were twisted on to prevent evaporation. A half of the containers were put into a thermostatic cabinet at 3 and the other half at 20 °C. Since the samples cooling at 3 °C reached 20 °C immediately they were relocated to 20 °C and stored overnight. Samples cooled at 3 °C reached 20 °C
Rheological measurements were conducted using a Haake RS 300 rheometer (Haake, Karlsruhe, Germany). Temperature control was maintained by a Haake DC30 circulator water bath (Haake, Karlsruhe, Germany). All rheological data were collected and calculated by Haake Rheowin software version 3.61.0004 (Haake, Karlsruhe, Germany). To study the effect of stirring time on apparent viscosity of inulin suspension rheological measurements were carried out using a concentric-cylinder-fixed cup (43 mm diameter) and rotating vane (22 mm diameter, 112 mm height) at 25 s1 shear rate for 15,000 s at 20, 40, 50, 60 and 70 °C. The experimentally chosen shear rate guaranteed stirring of the whole content of the cup. Measurement began when the sample was poured into the cup, 2 mL of oil was put on the surface of the sample to prevent evaporation, the lift moved and the vane took the measuring position (8 mm clearance to bottom). The time to achieve 40, 50, 60 and 70 °C for the stirred suspension/solution was 215, 353, 491 and 629 s, respectively. All other rotational and dynamic oscillatory rheological measurements were conducted using parallel plate geometry (both 35 mm diameter and serrated). These measurements were carried out at 1 mm gap. The apparent viscosity was measured at 20 s1 shear rate for 120 s. For analytical purposes the average value was calculated from the 90th, 105th and 120th second of measurement (Glibowski et al., 2008). In viscosity vs. shear rate measurements, shear rate was changed from 1 to 300 (s1) linearly in
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1 min then stayed for 30 s at 300 (s1) shear rate and subsequently changed again from 300 to 1 s1 also linearly. Strain sweeps were conducted at a frequency of 1.0 Hz. Frequency sweeps were conducted at the strain corresponded to the maximum found within the linear viscoelastic region of the studied material (Zimeri and Kokini, 2003). 2.7. Texture analysis Hardness analyses were performed according to the method previously described by Glibowski (2009). Briefly, samples were punched by a cylindrical probe (1 cm diameter) with the crosshead speed 1 mm s1 at 15 mm depth, using a TA-XT2i texture analyser (Stable Microsystems, Goaldming, England). The maximal peak value after punching the sample 15 mm down was considered as gel hardness. The analysis was performed without removing the samples from the containers. 2.8. Spreadability Spreadability was measured with a TTC Spreadability Rig (HDP/ SR) attachment using method previously described by Glibowski et al. (2008). Briefly, samples were filled into female cones. During the analysis samples were displaced to within 0.5 mm of the base of the female cone using a corresponding male cone attachment for the texture analyzer. Force expressed in Newtons was measured for the duration of the test, and spreadability was equated to the area under the curve. 2.9. Environmental Scanning Electron Microscopy (ESEM) Changes in inulin structure at high relatively humidity (RH) were examined with an environmental scanning electron microscope Qanta 200 (FEI, Eindhoven, The Netherlands) at 30 kV, using wet mode at 400–706 Pa. 2.10. Statistical analysis
Viscosity (Pa•s)
Hardness and apparent viscosity measurements were completed in three independent trials. Each analysis was performed in duplicate. Six samples of each spread were used for spreadability measurements. All data were analysed by the Statistical Analy-
3
20°C 40°C
2.5
50°C 60°C
sis System (SAS Enterprise Guide 3.0.3.414) using the ANOVA procedure for analysis of variance and Student–Newman-Keuls ttest for ranking the means. 3. Results and discussion High performance inulin is practically insoluble at low temperatures. Even at 50 °C solubility of HP inulin is about 1.2%. Above 50 °C solubility significantly increase up to 34% at 90 °C (Kim et al., 2001). When HP inulin is dissolved and/or heated at 60– 70 °C and its concentration is 15–20% it is sometimes difficult to unambiguously affirm by visual assessment whether one is dealing with a solution or a suspension. For this reason the most accurate term which will be used in this work for the samples with inulin heated at 60–70 °C is suspension/solution. 3.1. Effect of stirring time on apparent viscosity of inulin suspension Fig. 1 shows the effect of time and temperature of stirring on apparent viscosity for 20% inulin suspension/solutions. The higher suspension/solution temperature the lower apparent viscosity values which is connected with increasing inulin solubility (Kim et al., 2001). An increase in apparent viscosity with stirring time may be explained by water absorbance and swelling by inulin particles like in some gum solutions (Isikili and Karababa, 2005). Very similar behaviour for inulin was noticed by other authors (Bot et al., 2004; Toneli et al., 2008). Environmental Scanning Electron Microscopy allowed to register changes in inulin structure at high relatively humidity (Fig. 2). Inulin powder was put in the environmental at 95–100% RH. The micrographs show a transformation in inulin structure according to binding water. Simultaneously the particle volume increase can be noticed. Water absorbance by inulin particles and what follows suspension viscosity increase at temperatures making inulin dissolution impossible may result from the tendency to form crystal structure by inulin. Ronkart et al. (2009) using wide-angle X-ray scattering confirmed transformation of amorphic inulin into crystal forms at 75% RH and above. Commercial inulin is a spray-dried powder, and inulin in this powder is in an amorphous state (Ronkart et al., 2009). In a water environment inulin forms crystals and inulin crystals contain water in the structure (Andre et al., 1996).
70°C
2 1.5 1 0.5 0 0
3000
6000
9000
12000
15000
Time (s) Fig. 1. Apparent viscosity changes for 20% inulin suspensions/solutions at different temperatures.
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Fig. 2. ESEM micrographs of inulin showing the changes in time in particles shape in an environmental with high relative humidity (RH). (A) Time 00 0000 , 65% RH; (B) 50 3100 , 95% RH; (C) 60 5600 , 95% RH; (D) 120 3700 , 100% RH; (E) 130 0400 , 100% RH; (F) 140 3600 , 100% RH. Scale bar 100 lm.
3.2. Effect of temperature and time of suspension’s preparation, heating and cooling rates of inulin suspensions/solutions Food producers aspirate to obtain product with invariable sensorical properties. Rheological properties e.g. hardness and
spreadability are the most essential features perceived by the consumers (Prentice, 1972). After the first part of this study a question arises – do the viscosity increase and the preparation temperature have an influence on the rheological properties of the inulin gels? As crystallization processes take part in inulin
Table 1 Hardness and apparent viscosity of the samples contained 20% inulin as affected by stirring time, heating and cooling rates and preparation temperature.A Preparation temperature (°C)
Heating rate (°C min1)
Cooling rate (°C min1)
Hardness (N) Stirring time (min) 5
30
5
30
20
11.0
2.35 0.1 2.35 0.1 2.35 0.1 2.35 0.1
1.46ab ± 0.55 1.38ab ± 0.42 0.81cd ± 0.25 0.77cd ± 0.12 1.59ab ± 0.07 1.22bc ± 0.12 0.82cd ± 0.16 1.23bc ± 0.16
1.82a ± 0.30 1.44ab ± 0.37 0.70d ± 0.22 0.90cd ± 0.24 1.43ab ± 0.33 1.54ab ± 0.11 1.68ab ± 0.30 1.21bc ± 0.29
7.53abcd ± 2.63 4.63cde ± 1.68 3.83de ± 2.74 3.22de ± 2.02 8.46abc ± 0.54 4.02de ± 0.68 4.00de ± 2.43 6.76bcde ± 0.86
9.62ab ± 2.37 5.48cde ± 3.02 3.40de ± 2.55 3.89de ± 2.52 2.46e ± 0.75 2.60e ± 0.23 10.91a ± 1.26 6.67bcde ± 1.32
23.6 40
8.3 17.6
A
Apparent viscosity (Pa s)
Data are presented as means ± standard deviation. Means for the hardness or apparent viscosity with different superscript letters are significantly different, p 6 0.05.
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A
100000
Viscosity (Pa•s)
10000 1000
100 10 1 0.1
B
0
50
100
0
50
100
150
Shear rate (s-1)
200
250
300
200
250
300
100000
Viscosity (Pa•s)
10000 1000 100 10 1 0.1 150 -1
Shear rate (s ) Fig. 3. Flow curves for 20% inulin gel (hj), inulin (sd) and commercial (e) spreads. (A) Fresh sample and (B) 1 day after first structure destruction. Open symbols represent down curves, closed symbols represent up curves. Three replicate measurements of each flow curve were performed with little variation, only one example is displayed.
gel formation an influence of heating and cooling rates was also studied. The results of hardness and apparent viscosity of the inulin gels prepared at different conditions are shown in Table 1. Inulin suspension/solution after heating was cool down at different cooling
rates but it did not change hardness or apparent viscosity values significantly (p 6 0.05). Generally preparation temperature also did not considerably (p 6 0.05) affected on the rheological parameters. Stirring time before heating inulin suspension was statistically unimportant (p 6 0.05). As viscosity of the suspension was
Table 2 Hardness (N) of the samples as affected by structure destruction (SD).A
20% inulin gel 25% inulin gel 30% inulin gel Inulin spread Commercial spread A
Fresh sample
1 day after first SD
1 day after third SD
10 days after fourth SD
1.76a ± 0.09 5.55a ± 0.48 9.45a ± 0.30 6.78a ± 0.02 0.44a ± 0.09
0.17b ± 0.01 0.29b ± 0.02 0.76b ± 0.04 0.70b ± 0.07 0.42a ± 0.03
0.09b ± 0.00 0.15b ± 0.01 0.39c ± 0.04 0.41c ± 0.08 0.42a ± 0.03
0.10b ± 0.00 0.18b ± 0.01 0.49c ± 0.03 0.44c ± 0.07 0.43a ± 0.03
Values are means ± standard deviation. Means in the same row with different superscript letters are significantly different, p 6 0.05.
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P. Glibowski / Journal of Food Engineering 99 (2010) 106–113 Table 3 Apparent viscosity (Pa s) of the samples as affected by structure destruction (SD).A Fresh sample a
7.87 ± 0.42 5.35c ± 0.36 13.30c ± 0.45 15.44b ± 0.88 29.09a ± 0.65
20% inulin gel 25% inulin gel 30% inulin gel Inulin spread Commercial spread A
1 day after first SD
1 day after third SD
b
10 days after fourth SD
a
6.57 ± 0.35 6.63b ± 0.41 17.28b ± 0.81 18.51a ± 1.12 29.67a ± 0.67
8.56a ± 0.48 9.09a ± 0.37 21.03a ± 0.73 18.49a ± 1.33 29.96a ± 0.67
8.18 ± 0.38 8.51a ± 0.43 17.09b ± 0.54 18.59a ± 0.57 28.88a ± 0.53
Values are means ± standard deviation. Means in the same row with different superscript letters are significantly different, p 6 0.05.
considerably higher after 30 min mixing in comparison to 5 min mixing at 20 °C (Fig. 1) it also means that viscosity of the suspensions had insignificant influence on rheological data of inulin gels. Significant differences in analysed rheological parameters were affirm for different heating rates, however it was hard to find unambiguous tendencies. The highest heating rate caused signifi-
A
cant (p 6 0.05) hardness and apparent viscosity decrease in comparison to the samples heated at 11.0 °C min1. Although target temperature was the same (70 °C) and the flasks with inulin suspensions were under intense agitation during heating punctual overheating might happened and caused different inulin solubility extant. This supposition was confirmed by visual assessment done
1000000
G', G" (Pa)
100000
10000
1000
100
10 0.0001
0.001
0.01
0.1
0.01
0.1
Strain
B
1000000
G', G" (Pa)
100000
10000
1000
100
10 0.0001
0.001
Strain Fig. 4. Strain sweeps for 20% inulin gel (hj), inulin (sd) and commercial (e) spreads. (A) Fresh sample and (B) 1 day after first structure destruction. Open symbols represent loss moduli (G00 ), closed symbols represent storage moduli (G0 ). Three replicate measurements of each strain sweep were performed with little variation, only one example is displayed.
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right before pouring the solutions/suspensions into the containers. Inulin suspensions heated at 8.3 and 11.0 °C min1 (at 80 °C water bath) were milky ones and did not turn into clear, transparent solutions like those heated at 17.6 and 23.6 °C min1 (100 °C waterbath). High standard deviations deserve a special comment. Despite three independent trials, triplicate analysis for each trial and rigid laboratory regime in samples preparations average coefficient of variation for hardness was 21% and for apparent viscosity 24%. High standard deviation for rheological properties analysis of inulin gels were noted in other studies (Glibowski and Wasko, 2008; Toneli et al., 2008). This phenomena may be explained by the nature of the inulin gel formation which is a crystallization process and the formation of a three dimensional network by inulin crystals. The control of the nucleation and crystallization is very difficult (Stasiak and Dolatowski, 2008) and depending on the mutual
A
arrangement of the crystals, gels with different rheological properties are formed. 3.3. Rheological properties of inulin gel, inulin model spread and commercial spread – effect of structure destruction The third step of this study was the comparison of inulin gel, inulin model spread and commercial spread rheological properties. The above mentioned samples have thixotropic properties which were seen after the analysis of apparent viscosity curves (data not shown). Flow curve of inulin model spread show shear thinning behaviour of the analysed materials which makes it alike to commercial spread (Fig. 3A). When samples with inulin were subjected to shear forces during rheological analysis or by mixing with a glass rod, the structure was irreversibly broken (Table 2). Structure destruction (SD) in a inulin model spread made possible
1000000
G', G" (Pa)
100000
10000
1000
100 0,1
1
10
1 00
Frequency (Hz)
B
1000000
G', G" (Pa)
100000
10000
1000
100 0,1
1
10
100
Frequency (Hz) Fig. 5. Frequency sweeps for 20% inulin gel (h), inulin (s) and commercial (e) spreads. (A) Fresh sample and (B) 1 day after first structure destruction. Open symbols represent loss moduli (G00 ), closed symbols represent storage moduli (G0 ). Three replicate measurements of each frequency sweep were performed with little variation, only one example is displayed.
P. Glibowski / Journal of Food Engineering 99 (2010) 106–113
obtaining constant parameters similar to low fat spread (Fig. 3B, Tables 2 and 3). Increase in apparent viscosity values after serial SD of inulin gels and inulin model spread may be connected with partial inulin aggregates reconstruction. Existence of such aggregates was proposed by Bot et al. (2004). Fig. 4 shows strain sweeps for inulin gel, inulin model spread and commercial spread. Storage and loss moduli decreased drastically for the samples with inulin after exceeding 0.001 strain. This tendency probably results from a very weak bounds stabilizing structure of samples with inulin. Simultaneously G0 and G00 moduli are exceptionally high. Decrease of the dynamic moduli for the strains higher than 0.001 is characteristic for butter (Rohm and Weidinger, 1993). Similarity between fat and inulin gel behaviour was suggested by Bot et al. (2004) however if in fats like butter structure is stabilized by van der Waal’s attraction (Walstra and Jennes, 1984) inulin molecules are bounded by intermolecular hydrogen bonds (Andre et al., 1996). Strain sweep for commercial spread was linear throughout analysed strain range. Structure destruction in samples with inulin caused a significant decrease in analysed moduli values with reference to fresh samples although putting the sample on measurement elements and taking measurement position by the rheometer caused some sample damage (Figs. 4 and 5). G0 and G00 moduli values for the samples containing inulin were similar to 25% fat commercial spread ones, although 40% fat commercial spread had higher loss and storage moduli values (data not shown). Frequency sweeps confirmed solid-like behaviour (G0 > G00 ) of the analysed samples (Fig. 5). Significant difference between loss and storage moduli throughout the frequency range is typical for the gel (Steffe, 1996). Rheological analysis of inulin model spread proved the usefulness of the structure destruction step in industrial practice. In products with structure based on inulin the lack of the SD step would cause obtaining a product with unacceptable rheological properties change. The structure obtained after SD showed behaviour similar to commercial spreads (p 6 0.05) which was confirmed by spreadability analysis. The analysed spreadability (Ns) was 32.5 ± 3.7 and 31.1 ± 2.0 (means ± standard deviation) for inulin and commercial spread, respectively. 4. Conclusions Inulin suspension/solutions viscosity increase affected by stirring in a water environment, especially at low temperatures (20– 40 °C), results from water absorbance and swelling by inulin particles and this is a consequence of changes in inulin structure from amorphous to crystal one. Stirring inulin suspension/solutions at higher temperatures causes slower viscosity increase as a result of a partial inulin dissolution. Different preparation temperatures, cooling rates and stirring times did not change the rheological parameters significantly in contrast to heating rates, however, it was hard to find unambiguous correlations between heating rates and rheological tendencies. High standard deviation for hardness and apparent viscosity in spite of rigid laboratory conditions showed difficulty of the inulin crystallization control and rheological nature of crystal gels. Rheological properties of inulin model spread showed thixotropic and shear thinning behaviour which makes it to be like commercial spread. The applied structure destruction step caused inulin model spread to be similar to commercial spreads. Acknowledgement The scientific work was funded from resources for the science in years 2007–2010 as research project.
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References Andre, I., Mazeau, K., Tvaroska, I., Putaux, J.L., Winter, W.T., Taravel, F.R., Chanzy, H., 1996. Molecular and crystal structures of inulin from electron diffraction data. Macromolecules 29, 4626–4635. Bot, A., Erle, U., Vreeker, R., Agterof, W., 2004. Influence of crystallisation conditions on the large deformation rheology of inulin gels. Food Hydrocolloids 18, 547– 556. Brennan, C.S., Kuri, V., Tudorica, C.M., 2004. Inulin-enriched pasta: effects on textural properties and starch degradation. Food Chemistry 86, 189–193. Brien, C.M.O., Mueller, A., Scannell, A.G.M., Arendt, E.K., 2003. Evaluation of the effects of fat replacers on the quality of wheat bread. Journal of Food Engineering 56, 265–267. Carabin, I.G., Flamm, W.G., 1999. Evaluation of safety of inulin and oligofructose as dietary fiber regulatory. Toxicology and Pharmacology 30, 268–282. Chiavaro, E., Vittadini, E., Corradini, C., 2007. Physicochemical characterization and stability of inulin gels. European Food Research and Technology 225, 85–94. Coudray, C., Demigne, C., Rayssiguier, Y., 2003. Effects of dietary fibers on magnesium absorption in animals and humans. Journal of Nutrition 133, 1–4. Franck, A., 2002. Technological functionality of inulin and oligofructose. British Journal of Nutrition 87 (Suppl. 2), S287–S291. Gibson, G.R., Roberfroid, M.B., 1995. Dietary modulation of the human colonic microbiota – introducing the concept of prebiotics. Journal of Nutrition 125, 1401–1412. Glibowski, P., 2009. Rheological properties and structure of inulin – whey protein gels. International Dairy Journal 19, 443–449. Glibowski, P., Gała˛zka, A., 2009. Effect of shear forces on textural properties of inulin gels. Acta Agrophisica 13 (1), 67–76. Glibowski, P., Wasko, A., 2008. Effect of thermochemical treatment on the structure of inulin and its gelling properties. International Journal of Food Science and Technology 43, 2075–2082. Glibowski, P., Zarzycki, P., Krzepkowska, M., 2008. The rheological and instrumental textural properties of selected table fats. International Journal of Food Properties 11 (3), 678–686. Hennelly, P.J., Dunne, P.G., Sullivan, M.O., Riordan, E.D.O., 2006. Textural, rheological and microstructural properties of imitation cheese containing inulin. Journal of Food Engineering 75, 388–395. Isikili, N.D., Karababa, E., 2005. Rheological characterization of fenugreek paste (çemen). Journal of Food Engineering 69, 185–190. Karabulut, I., Turan, S., 2006. Some properties of margarines and shortenings marketed in Turkey. Journal of Food Composition and Analysis 19, 55–58. Kim, Y., Faqih, M.N., Wang, S.S., 2001. Factors affecting gel formation of inulin. Carbohydrate Polymers 46, 135–145. Liong, M.T., Shah, N.P., 2005. Production of organic acids from fermentation of mannitol, fructooligosaccharide and inulin by a cholesterol removing Lactobacillus acidophilus strain. Journal of Applied Microbiology 99, 783–793. Mendoza, E., Garcia, M.L., Casas, C., Selgas, M.D., 2001. Inulin as fat substitute in low fat, dry fermented sausages. Meat Science 57, 387–393. Niness, K.R., 1999. Inulin and oligofructose: what are they? Journal of Nutrition 129 (7S), 1402S–1406S. Parodi, P.W., 2009. Has the association between saturated fatty acids, serum cholesterol and coronary heart disease been over emphasized? International Dairy Journal 19, 345–361. Paseephol, T., Small, D.M., Sherkat, F., 2008. Rheology and texture of set yogurt as affected by inulin addition. Journal of Texture Studies 39 (6), 617–634. Prentice, J.H., 1972. Rheology and texture of dairy products. Journal of Texture Studies 3, 415–458. Roberfroid, M.B., van Loo, J.A.E., Gibson, G.R., 1998. The bifidogenic nature of chicory inulin and its hydrolysis products. Journal of Nutrition 128, 11–19. Rohm, H., Weidinger, K.H., 1993. Rheological behaviour of butter at small deformations. Journal of Texture Studies 24, 157–172. Ronkart, S.N., Paquot, M., Blecker, C.S., Fougnies, C., Doran, L., Lambrechts, J.C., Norberg, B., Deroanne, C., 2009. Impact of the crystallinity on the physical properties of inulin during water sorption. Food Biophysics 4, 49–58. Schneeman, B.O., 1999. Fiber, inulin and oligofructose: similarities and differences. Journal of Nutrition 129, 1424S–1427S. Stasiak, D.M., Dolatowski, Z.J., 2008. Efficiency of sucrose crystallization from sugar beet magma after sonication. Polish Journal of Natural Sciences 23 (2), 521–530. Steffe, J.F., 1996. Rheological Methods in Food Process Engineering, second ed. Freeman Press, USA. Toneli, J.T.C.L., Park, K.J., Murr, F.E.X., Martinelli, P., 2008. Rheological behavior of concentrated inulin solution: influence of soluble solids concentration and temperature. Journal of Texture Studies 39, 369–392. Walstra, P., Jennes, R., 1984. Dairy Chemistry and Physics. John Wiley & Sons, USA. Zimeri, J.E., Kokini, J.L., 2003. Rheological properties of inulin–waxy maize starch systems. Carbohydrate Polymers 52, 67–85.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of thermal and mechanical factors on rheological properties of high performance inulin gels and spreads Pawel Glibowski * Department of Milk Technology and Hydrocolloids, University of Life Science in Lublin, Skromna 8, 20-704 Lublin, Poland
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 4 January 2010 Accepted 7 February 2010 Available online 11 February 2010 Keywords: Inulin gels Inulin spreads Rheology Texture ESEM
a b s t r a c t The aim of this study was analysis of a number of factors affecting the rheological properties of high performance inulin gels and spreads in comparison with commercial products. Inulin gels (20%, 25%, 30%), commercial and inulin model spread (20% canola oil, 20% inulin, 3% emulsifier) were analysed. Inulin particles in water environment absorbed water which caused an increase in viscosity of the inulin suspensions. Different temperatures of preparation, cooling rates and viscosity increase which appeared during the stirring of inulin suspensions did not significantly change the rheological parameters of the final gels in contrast to heating rates (p 6 0.05). In spite of rigid laboratory conditions high standard deviation for hardness and apparent viscosity showed how difficult the process of an inulin crystallization is to control. Rheological properties of inulin model spread exhibited thixotropic and shear thinning behaviour which made it similar to commercial spread. The applied structure destruction step in the manufacturing process should be applied to make inulin model spread comparable to commercial spreads. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Inulin is a carbohydrate consisting of GFn and Fm compounds, where n or m represents the number of fructose units (F) linked by b(2–1) bonds, with one terminal glucose (G) (Carabin and Flamm, 1999). Nowadays inulin may be purchased in different forms. Inulin extracted from chicory roots has an average degree of polymerisation (DP) of 10–12 and consists of wide spectrum of molecules with chain lengths from 2 to 60 units. The purification process of inulin extracts allow the obtainment of high performance (HP) inulin with an average DP of 23–25 and a molecular distribution ranging from 11 to 60 (Niness, 1999). Depending on the form, inulin powders have different taste (from slightly sweet for standard inulin which include all fractions to neutral taste for HP inulin) and different rheological properties (Chiavaro et al., 2007; Franck, 2002). Inulin has a great impact on human physiology. It reduces cholesterol and lipid levels in blood plasma, gives prebiotic and fiber effects and helps in calcium and magnesium adsorption (Coudray et al., 2003; Gibson and Roberfroid, 1995; Liong and Shah, 2005; Roberfroid et al., 1998; Schneeman, 1999). Application of inulin gelling properties in the food industry needs to pay special attention to the thermochemical conditions during the dissolution of inulin powders. In an acidic environment inulin undergoes hydrolysis however in pH 5 and higher it is quite stable. Inulin gels obtained in neutral or near neutral pH are firm * Tel.: +48 081 462 3349; fax: +48 081 462 3354. E-mail addresses: [email protected], [email protected] 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.02.007
(Glibowski and Wasko, 2008). Some authors indicated shearing conditions as an important factor in gelling properties of inulin HP however other studies did not confirm this (Glibowski and Gała˛zka, 2009; Kim et al., 2001). A crucial factor affecting rheological properties of inulin gels is heating time and temperature of preparation stage. Suspending inulin powder at 20–40 °C allows obtaining firm gels. Farther temperature increase causes deterioration of gel firmness as a result of decreasing the number of seeding crystals (Glibowski and Gała˛zka, 2009; Glibowski and Wasko, 2008). Inulin forms particulate gels by a network of small crystallites (Bot et al., 2004). When the crystallites are diluted at a high temperature i.e. above 80 °C the inulin gelation is inhibited (Bot et al., 2004; Glibowski and Wasko, 2008). Food technologists supported by food scientists are still searching for an alternative for butter, margarine and spreads. Butter has pour spreadability after taking it out of the refrigerator, a relatively high price and additionally influences on cholesterol and lipid levels in blood (Parodi, 2009). Margarines and spreads produced on the hydrogenated oils basis have high levels of trans fatty acids which increase concentrations of LDL cholesterol and reduce HDL cholesterol (Karabulut and Turan, 2006). The texture of inulin gels resemble solid fats. Because of this similarity inulin gel may be applied as fat mimetic (Bot et al., 2004). Bearing in mind healthy advantages, inulin gel seems to be an attractive alternative for butter and traditional spreads and margarines. Since inulin is considered as a health promoting ingredient it is often applied in many foods (Brennan et al., 2004; Brien et al., 2003; Hennelly et al., 2006; Mendoza et al., 2001; Paseephol et al., 2008).
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Rheological properties of inulin gels have been studied with usage of small and large deformation techniques (Bot et al., 2004; Glibowski and Wasko, 2008; Zimeri and Kokini, 2003). However information about the comparison of rheological properties of inulin and commercial spreads is rather scarce. Besides, to the best of my knowledge there is a lack of information available on the effect of heating and cooling conditions of inulin solution on the rheological properties of inulin gels. Therefore, the aim of this study was analysis of a number of factors affecting application in food industry i.e. temperature and time of suspension preparation, heating and cooling rates of inulin suspensions/solutions and wide study of rheological properties of HP inulin gels and spreads in comparison with commercial products. 2. Materials and methods 2.1. Materials Inulin FrutafitÒ TEX! was donated by Sensus Operations C.V. (Roosendaal, The Netherlands). Inulin was extracted from chicory root and its average degree of polymerisation is P23 (producer’s data). WPI (whey protein isolate) Extensor (95% protein in dry mass, producers data) was a gift from Lacma Sp. z o.o. (Nadarzyn, Poland). Canola oil and spread with 25% fat content was purchased from a local supermarket. 2.2. Effect of stirring time on apparent viscosity of inulin suspension Inulin (20%) was suspended in distilled water (20 °C) using a magnetic stirrer. This step took approximately 2 min. Subsequently inulin suspensions were homogenized by laboratory homogenizer H 500 (Pol-EkoAparatura, Wodzisław S´la˛ski, Polska). Homogenization was undertaken to dispose of any possible clumps. Homogenization lasted for 1 min at rotational speed of 10,000 min1. Immediately after homogenisation the solution was poured into a rheometer cup.
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within 21 min and 20 s and cooled at 20 °C reached 20 °C within 8 h and 20 min which gave a cooling rate 2.35 and 0.1 °C min1, respectively. All rheological and textural measurements for these samples were carried out at 20 °C. 2.4. Inulin model spread preparation Inulin model spread was prepared by inulin suspension followed by WPI (3%) dispersion. WPI was applied as an emulsifier. The procedure of inulin suspension was very similar to that described in Section 2.2. The suspensions were poured into a conical flasks and heated up to 70 °C in a water bath at a heating rate of 16 °C min1. Subsequently inulin solutions were homogenized with hot (70 °C) canola oil using laboratory homogenizer. To obtain stable emulsion homogenization lasted for 3 min at a rotational speed of 12,000 min1. Afterwards the emulsions were poured into final containers. All samples were stored at 5 °C in a thermostatic cabinet. The final composition of inulin spread was 20%, 20% and 3% inulin, oil and WPI, respectively. 2.5. Effect of structure destruction on rheological properties of the samples To analyse the effect of structure destruction on the rheological properties the structure of the samples of inulin model spread, commercial spread and inulin gels (20%, 25%, 30%) were destroyed. For this purpose inulin gels were obtained according to Section 2.4, but without emulsifier addition. Hot water (70 °C) was used instead of hot canola oil. Samples of the commercial spread were put into containers with special attention to avoid bubbles formation. All samples were analysed after 24 h storage at 5 °C (fresh samples). After analysis each sample was stirred with a glass rod (for 10 rotations to right and left side) and packed carefully in the container (first structure destruction (SD)). Special attention was paid to avoid bubbles formation. The same procedure took place for another 3 days running (second, third, fourth SD). All rheological and textural measurements for these samples were carried out at 5 °C.
2.3. Effect of temperature and time of suspension’s preparation, heating and cooling rates of inulin suspensions/solutions
2.6. Rheometry
Inulin (20%, 25%, 30%) was suspended in distilled water as it was described in Section 2.2 at two temperatures (20 or 40 °C). The temperature was maintained by MS 11 HS magnetic stirrer (Wigo, Piastów, Poland) with temperature control of the stirring liquid. To avoid temperature decrease (in case of 40 °C) during homogenisation, the suspension was poured into a warm homogeniser bottle and the bottle with the sample was immersed in a water bath at 40 °C. After the homogenisation step the suspensions were stirred using a magnetic stirrer for different periods (5 or 30 min). Subsequently inulin suspensions were heated in conical flasks in a water bath up to 70 °C. Temperature was controlled by a laboratory thermometer. Inulin suspension at temperature 20 °C reached 70 °C in a water bath at 100 °C within 2 min and 7 s which gave a heating rate 23.6 °C min1. Heating suspension at temperature 40 °C in a water bath at 100 °C gave 70 °C within 1 min and 42 s which gave a heating rate 17.6 °C min1. Heating suspensions in a water bath at 80 °C gave heating rates 11.0 and 8.3 °C min1 for the suspensions at 20 and 40 °C, respectively. The flasks were subjected to intense agitation during heating. Afterwards the solutions were poured into plastic cylindrical containers 35 mm in inner diameter and the lids were twisted on to prevent evaporation. A half of the containers were put into a thermostatic cabinet at 3 and the other half at 20 °C. Since the samples cooling at 3 °C reached 20 °C immediately they were relocated to 20 °C and stored overnight. Samples cooled at 3 °C reached 20 °C
Rheological measurements were conducted using a Haake RS 300 rheometer (Haake, Karlsruhe, Germany). Temperature control was maintained by a Haake DC30 circulator water bath (Haake, Karlsruhe, Germany). All rheological data were collected and calculated by Haake Rheowin software version 3.61.0004 (Haake, Karlsruhe, Germany). To study the effect of stirring time on apparent viscosity of inulin suspension rheological measurements were carried out using a concentric-cylinder-fixed cup (43 mm diameter) and rotating vane (22 mm diameter, 112 mm height) at 25 s1 shear rate for 15,000 s at 20, 40, 50, 60 and 70 °C. The experimentally chosen shear rate guaranteed stirring of the whole content of the cup. Measurement began when the sample was poured into the cup, 2 mL of oil was put on the surface of the sample to prevent evaporation, the lift moved and the vane took the measuring position (8 mm clearance to bottom). The time to achieve 40, 50, 60 and 70 °C for the stirred suspension/solution was 215, 353, 491 and 629 s, respectively. All other rotational and dynamic oscillatory rheological measurements were conducted using parallel plate geometry (both 35 mm diameter and serrated). These measurements were carried out at 1 mm gap. The apparent viscosity was measured at 20 s1 shear rate for 120 s. For analytical purposes the average value was calculated from the 90th, 105th and 120th second of measurement (Glibowski et al., 2008). In viscosity vs. shear rate measurements, shear rate was changed from 1 to 300 (s1) linearly in
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1 min then stayed for 30 s at 300 (s1) shear rate and subsequently changed again from 300 to 1 s1 also linearly. Strain sweeps were conducted at a frequency of 1.0 Hz. Frequency sweeps were conducted at the strain corresponded to the maximum found within the linear viscoelastic region of the studied material (Zimeri and Kokini, 2003). 2.7. Texture analysis Hardness analyses were performed according to the method previously described by Glibowski (2009). Briefly, samples were punched by a cylindrical probe (1 cm diameter) with the crosshead speed 1 mm s1 at 15 mm depth, using a TA-XT2i texture analyser (Stable Microsystems, Goaldming, England). The maximal peak value after punching the sample 15 mm down was considered as gel hardness. The analysis was performed without removing the samples from the containers. 2.8. Spreadability Spreadability was measured with a TTC Spreadability Rig (HDP/ SR) attachment using method previously described by Glibowski et al. (2008). Briefly, samples were filled into female cones. During the analysis samples were displaced to within 0.5 mm of the base of the female cone using a corresponding male cone attachment for the texture analyzer. Force expressed in Newtons was measured for the duration of the test, and spreadability was equated to the area under the curve. 2.9. Environmental Scanning Electron Microscopy (ESEM) Changes in inulin structure at high relatively humidity (RH) were examined with an environmental scanning electron microscope Qanta 200 (FEI, Eindhoven, The Netherlands) at 30 kV, using wet mode at 400–706 Pa. 2.10. Statistical analysis
Viscosity (Pa•s)
Hardness and apparent viscosity measurements were completed in three independent trials. Each analysis was performed in duplicate. Six samples of each spread were used for spreadability measurements. All data were analysed by the Statistical Analy-
3
20°C 40°C
2.5
50°C 60°C
sis System (SAS Enterprise Guide 3.0.3.414) using the ANOVA procedure for analysis of variance and Student–Newman-Keuls ttest for ranking the means. 3. Results and discussion High performance inulin is practically insoluble at low temperatures. Even at 50 °C solubility of HP inulin is about 1.2%. Above 50 °C solubility significantly increase up to 34% at 90 °C (Kim et al., 2001). When HP inulin is dissolved and/or heated at 60– 70 °C and its concentration is 15–20% it is sometimes difficult to unambiguously affirm by visual assessment whether one is dealing with a solution or a suspension. For this reason the most accurate term which will be used in this work for the samples with inulin heated at 60–70 °C is suspension/solution. 3.1. Effect of stirring time on apparent viscosity of inulin suspension Fig. 1 shows the effect of time and temperature of stirring on apparent viscosity for 20% inulin suspension/solutions. The higher suspension/solution temperature the lower apparent viscosity values which is connected with increasing inulin solubility (Kim et al., 2001). An increase in apparent viscosity with stirring time may be explained by water absorbance and swelling by inulin particles like in some gum solutions (Isikili and Karababa, 2005). Very similar behaviour for inulin was noticed by other authors (Bot et al., 2004; Toneli et al., 2008). Environmental Scanning Electron Microscopy allowed to register changes in inulin structure at high relatively humidity (Fig. 2). Inulin powder was put in the environmental at 95–100% RH. The micrographs show a transformation in inulin structure according to binding water. Simultaneously the particle volume increase can be noticed. Water absorbance by inulin particles and what follows suspension viscosity increase at temperatures making inulin dissolution impossible may result from the tendency to form crystal structure by inulin. Ronkart et al. (2009) using wide-angle X-ray scattering confirmed transformation of amorphic inulin into crystal forms at 75% RH and above. Commercial inulin is a spray-dried powder, and inulin in this powder is in an amorphous state (Ronkart et al., 2009). In a water environment inulin forms crystals and inulin crystals contain water in the structure (Andre et al., 1996).
70°C
2 1.5 1 0.5 0 0
3000
6000
9000
12000
15000
Time (s) Fig. 1. Apparent viscosity changes for 20% inulin suspensions/solutions at different temperatures.
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Fig. 2. ESEM micrographs of inulin showing the changes in time in particles shape in an environmental with high relative humidity (RH). (A) Time 00 0000 , 65% RH; (B) 50 3100 , 95% RH; (C) 60 5600 , 95% RH; (D) 120 3700 , 100% RH; (E) 130 0400 , 100% RH; (F) 140 3600 , 100% RH. Scale bar 100 lm.
3.2. Effect of temperature and time of suspension’s preparation, heating and cooling rates of inulin suspensions/solutions Food producers aspirate to obtain product with invariable sensorical properties. Rheological properties e.g. hardness and
spreadability are the most essential features perceived by the consumers (Prentice, 1972). After the first part of this study a question arises – do the viscosity increase and the preparation temperature have an influence on the rheological properties of the inulin gels? As crystallization processes take part in inulin
Table 1 Hardness and apparent viscosity of the samples contained 20% inulin as affected by stirring time, heating and cooling rates and preparation temperature.A Preparation temperature (°C)
Heating rate (°C min1)
Cooling rate (°C min1)
Hardness (N) Stirring time (min) 5
30
5
30
20
11.0
2.35 0.1 2.35 0.1 2.35 0.1 2.35 0.1
1.46ab ± 0.55 1.38ab ± 0.42 0.81cd ± 0.25 0.77cd ± 0.12 1.59ab ± 0.07 1.22bc ± 0.12 0.82cd ± 0.16 1.23bc ± 0.16
1.82a ± 0.30 1.44ab ± 0.37 0.70d ± 0.22 0.90cd ± 0.24 1.43ab ± 0.33 1.54ab ± 0.11 1.68ab ± 0.30 1.21bc ± 0.29
7.53abcd ± 2.63 4.63cde ± 1.68 3.83de ± 2.74 3.22de ± 2.02 8.46abc ± 0.54 4.02de ± 0.68 4.00de ± 2.43 6.76bcde ± 0.86
9.62ab ± 2.37 5.48cde ± 3.02 3.40de ± 2.55 3.89de ± 2.52 2.46e ± 0.75 2.60e ± 0.23 10.91a ± 1.26 6.67bcde ± 1.32
23.6 40
8.3 17.6
A
Apparent viscosity (Pa s)
Data are presented as means ± standard deviation. Means for the hardness or apparent viscosity with different superscript letters are significantly different, p 6 0.05.
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A
100000
Viscosity (Pa•s)
10000 1000
100 10 1 0.1
B
0
50
100
0
50
100
150
Shear rate (s-1)
200
250
300
200
250
300
100000
Viscosity (Pa•s)
10000 1000 100 10 1 0.1 150 -1
Shear rate (s ) Fig. 3. Flow curves for 20% inulin gel (hj), inulin (sd) and commercial (e) spreads. (A) Fresh sample and (B) 1 day after first structure destruction. Open symbols represent down curves, closed symbols represent up curves. Three replicate measurements of each flow curve were performed with little variation, only one example is displayed.
gel formation an influence of heating and cooling rates was also studied. The results of hardness and apparent viscosity of the inulin gels prepared at different conditions are shown in Table 1. Inulin suspension/solution after heating was cool down at different cooling
rates but it did not change hardness or apparent viscosity values significantly (p 6 0.05). Generally preparation temperature also did not considerably (p 6 0.05) affected on the rheological parameters. Stirring time before heating inulin suspension was statistically unimportant (p 6 0.05). As viscosity of the suspension was
Table 2 Hardness (N) of the samples as affected by structure destruction (SD).A
20% inulin gel 25% inulin gel 30% inulin gel Inulin spread Commercial spread A
Fresh sample
1 day after first SD
1 day after third SD
10 days after fourth SD
1.76a ± 0.09 5.55a ± 0.48 9.45a ± 0.30 6.78a ± 0.02 0.44a ± 0.09
0.17b ± 0.01 0.29b ± 0.02 0.76b ± 0.04 0.70b ± 0.07 0.42a ± 0.03
0.09b ± 0.00 0.15b ± 0.01 0.39c ± 0.04 0.41c ± 0.08 0.42a ± 0.03
0.10b ± 0.00 0.18b ± 0.01 0.49c ± 0.03 0.44c ± 0.07 0.43a ± 0.03
Values are means ± standard deviation. Means in the same row with different superscript letters are significantly different, p 6 0.05.
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P. Glibowski / Journal of Food Engineering 99 (2010) 106–113 Table 3 Apparent viscosity (Pa s) of the samples as affected by structure destruction (SD).A Fresh sample a
7.87 ± 0.42 5.35c ± 0.36 13.30c ± 0.45 15.44b ± 0.88 29.09a ± 0.65
20% inulin gel 25% inulin gel 30% inulin gel Inulin spread Commercial spread A
1 day after first SD
1 day after third SD
b
10 days after fourth SD
a
6.57 ± 0.35 6.63b ± 0.41 17.28b ± 0.81 18.51a ± 1.12 29.67a ± 0.67
8.56a ± 0.48 9.09a ± 0.37 21.03a ± 0.73 18.49a ± 1.33 29.96a ± 0.67
8.18 ± 0.38 8.51a ± 0.43 17.09b ± 0.54 18.59a ± 0.57 28.88a ± 0.53
Values are means ± standard deviation. Means in the same row with different superscript letters are significantly different, p 6 0.05.
considerably higher after 30 min mixing in comparison to 5 min mixing at 20 °C (Fig. 1) it also means that viscosity of the suspensions had insignificant influence on rheological data of inulin gels. Significant differences in analysed rheological parameters were affirm for different heating rates, however it was hard to find unambiguous tendencies. The highest heating rate caused signifi-
A
cant (p 6 0.05) hardness and apparent viscosity decrease in comparison to the samples heated at 11.0 °C min1. Although target temperature was the same (70 °C) and the flasks with inulin suspensions were under intense agitation during heating punctual overheating might happened and caused different inulin solubility extant. This supposition was confirmed by visual assessment done
1000000
G', G" (Pa)
100000
10000
1000
100
10 0.0001
0.001
0.01
0.1
0.01
0.1
Strain
B
1000000
G', G" (Pa)
100000
10000
1000
100
10 0.0001
0.001
Strain Fig. 4. Strain sweeps for 20% inulin gel (hj), inulin (sd) and commercial (e) spreads. (A) Fresh sample and (B) 1 day after first structure destruction. Open symbols represent loss moduli (G00 ), closed symbols represent storage moduli (G0 ). Three replicate measurements of each strain sweep were performed with little variation, only one example is displayed.
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right before pouring the solutions/suspensions into the containers. Inulin suspensions heated at 8.3 and 11.0 °C min1 (at 80 °C water bath) were milky ones and did not turn into clear, transparent solutions like those heated at 17.6 and 23.6 °C min1 (100 °C waterbath). High standard deviations deserve a special comment. Despite three independent trials, triplicate analysis for each trial and rigid laboratory regime in samples preparations average coefficient of variation for hardness was 21% and for apparent viscosity 24%. High standard deviation for rheological properties analysis of inulin gels were noted in other studies (Glibowski and Wasko, 2008; Toneli et al., 2008). This phenomena may be explained by the nature of the inulin gel formation which is a crystallization process and the formation of a three dimensional network by inulin crystals. The control of the nucleation and crystallization is very difficult (Stasiak and Dolatowski, 2008) and depending on the mutual
A
arrangement of the crystals, gels with different rheological properties are formed. 3.3. Rheological properties of inulin gel, inulin model spread and commercial spread – effect of structure destruction The third step of this study was the comparison of inulin gel, inulin model spread and commercial spread rheological properties. The above mentioned samples have thixotropic properties which were seen after the analysis of apparent viscosity curves (data not shown). Flow curve of inulin model spread show shear thinning behaviour of the analysed materials which makes it alike to commercial spread (Fig. 3A). When samples with inulin were subjected to shear forces during rheological analysis or by mixing with a glass rod, the structure was irreversibly broken (Table 2). Structure destruction (SD) in a inulin model spread made possible
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Frequency (Hz)
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Frequency (Hz) Fig. 5. Frequency sweeps for 20% inulin gel (h), inulin (s) and commercial (e) spreads. (A) Fresh sample and (B) 1 day after first structure destruction. Open symbols represent loss moduli (G00 ), closed symbols represent storage moduli (G0 ). Three replicate measurements of each frequency sweep were performed with little variation, only one example is displayed.
P. Glibowski / Journal of Food Engineering 99 (2010) 106–113
obtaining constant parameters similar to low fat spread (Fig. 3B, Tables 2 and 3). Increase in apparent viscosity values after serial SD of inulin gels and inulin model spread may be connected with partial inulin aggregates reconstruction. Existence of such aggregates was proposed by Bot et al. (2004). Fig. 4 shows strain sweeps for inulin gel, inulin model spread and commercial spread. Storage and loss moduli decreased drastically for the samples with inulin after exceeding 0.001 strain. This tendency probably results from a very weak bounds stabilizing structure of samples with inulin. Simultaneously G0 and G00 moduli are exceptionally high. Decrease of the dynamic moduli for the strains higher than 0.001 is characteristic for butter (Rohm and Weidinger, 1993). Similarity between fat and inulin gel behaviour was suggested by Bot et al. (2004) however if in fats like butter structure is stabilized by van der Waal’s attraction (Walstra and Jennes, 1984) inulin molecules are bounded by intermolecular hydrogen bonds (Andre et al., 1996). Strain sweep for commercial spread was linear throughout analysed strain range. Structure destruction in samples with inulin caused a significant decrease in analysed moduli values with reference to fresh samples although putting the sample on measurement elements and taking measurement position by the rheometer caused some sample damage (Figs. 4 and 5). G0 and G00 moduli values for the samples containing inulin were similar to 25% fat commercial spread ones, although 40% fat commercial spread had higher loss and storage moduli values (data not shown). Frequency sweeps confirmed solid-like behaviour (G0 > G00 ) of the analysed samples (Fig. 5). Significant difference between loss and storage moduli throughout the frequency range is typical for the gel (Steffe, 1996). Rheological analysis of inulin model spread proved the usefulness of the structure destruction step in industrial practice. In products with structure based on inulin the lack of the SD step would cause obtaining a product with unacceptable rheological properties change. The structure obtained after SD showed behaviour similar to commercial spreads (p 6 0.05) which was confirmed by spreadability analysis. The analysed spreadability (Ns) was 32.5 ± 3.7 and 31.1 ± 2.0 (means ± standard deviation) for inulin and commercial spread, respectively. 4. Conclusions Inulin suspension/solutions viscosity increase affected by stirring in a water environment, especially at low temperatures (20– 40 °C), results from water absorbance and swelling by inulin particles and this is a consequence of changes in inulin structure from amorphous to crystal one. Stirring inulin suspension/solutions at higher temperatures causes slower viscosity increase as a result of a partial inulin dissolution. Different preparation temperatures, cooling rates and stirring times did not change the rheological parameters significantly in contrast to heating rates, however, it was hard to find unambiguous correlations between heating rates and rheological tendencies. High standard deviation for hardness and apparent viscosity in spite of rigid laboratory conditions showed difficulty of the inulin crystallization control and rheological nature of crystal gels. Rheological properties of inulin model spread showed thixotropic and shear thinning behaviour which makes it to be like commercial spread. The applied structure destruction step caused inulin model spread to be similar to commercial spreads. Acknowledgement The scientific work was funded from resources for the science in years 2007–2010 as research project.
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