Article pp.93-104 du Vol.25 n°2 (2005)

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ARTICLE ORIGINAL ORIGINAL PAPER

Some rheological properties of baby fruit desserts

L. Juszczak¹*, T. Fortuna

RÉSUMÉ

Propriétés rhéologiques de desserts aux fruits pour bébés

Sept desserts aux fruits pour bébé ont été testés pour déterminer certaines de leurs propriétés rhéologiques et physicochimiques. Les études rhéologi- ques comprenaient la détermination de courbes d’écoulement à taux et contrainte de cisaillement contrôlés, et celle de courbes décrivant la dépen- dance de la viscosité apparente envers le temps et la température. Les desserts différaient par leurs propriétés physicochimiques et ont montré un comportement rhéofluidifiant et thixotropique. Un nombre statistiquement significatif de corrélations linéaires a été trouvé entre les propriétés physicochimiques et les paramètres de modèles décrivant les propriétés rhéologi- ques des desserts.

Mots clés

propriétés rhéologiques, desserts aux fruits.

SUMMARY

Seven baby fruit desserts were tested to determine some of their rheological and physicochemical properties. Rheological studies included determination of flow curves at a controlled shear rate and shear stress, and curves describing the time and temperature dependence of apparent viscosity. The desserts differed in physicochemical properties and showed a shear-thinning behaviour and thixotropy. A number of statistically significant linear correlations were found between the physicochemical properties and the parameters of models describing the rheological properties of the desserts.

Keywords

rheological properties, fruit desserts.

1. Department of Analysis and Evaluation of Food Quality – Agricultural University – 122 Balicka Str. – 30-149 Kraków – Poland.

* Correspondence : fax : +48 12 662-47-46 ; e-mail : [email protected]

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1 – INTRODUCTION

Knowledge of the rheological properties, among them viscosity, of food raw materials and final products is of major importance to food technology. Such properties must be considered when designing manufacturing processes.

Rheological studies are used in quality control and help to understand the structure of complex food systems. They can also be employed to produce the right consistency of final products.

Consistency is an important, though often underestimated, characteristic of desserts intended for babies who develop reflexes connected with the chewing and swallowing of food. A baby dessert frequently constitutes a mixture of fruit pulps, with or without an addition of starch as a thickening agent, so it exhibits similar properties to the fruit pulp(s) from which it was obtained. Such products have a complex structure in which solid particles of various sizes and shapes are suspended in a continuous phase composed of a colloidal solution of pec- tins and a solution of dissolved substances: sugars, organic acids and salts (GENOVESE and LOZANO, 2000). The rheological properties of such dispersions depend on the characteristics of the continuous phase its viscosity, chemical composition, polarity, and concentration of electrolytes; on the properties of the solid phase size, shape, and volume of particles; and on electrorheological effects (GENOVESE and LOZANO, 2000; PELEGRINE et al., 2002).

In general, fruit pulps are classed as non-Newtonian fluids exhibiting a shear-thinning behaviour (RAO et al., 1974; LOZANO and IBARZ, 1994; GUERRERO

and ALZAMORA, 1997; BHATTACHARYA, 1999; PELEGRINE et al., 2002), a yield stress (GUERRERO and ALZAMORA, 1997; BHATTACHARYA, 1999; PELEGRINE et al., 2002), and thixotropy (LOZANO and IBARZ, 1994; BHATTACHARYA, 1999).

The present work aims to analyse chosen physicochemical and rheological properties of baby fruit desserts and to establish correlations between the physicochemical characteristics of the desserts and the parameters of the models used to describe their rheological properties.

2 – MATERIALS

The study material included seven samples of baby fruit desserts (identified by symbols P1 to P7). The desserts were:

– Leader Price Poland: apple-peach (P1), apple-banana (P2), apple-straw- berry (P3);

– Bobovita Poland: apple-carrot (P4), apricot-apple-grape-banana (P5);

– Gerber S.A. Poland: tropical fruit (P6), apple-peach (P7).

The products contained the following thickeners (according to the informa- tion provided on the labels): P1, P2 and P3 potato starch, P4 and P5 wheat starch and maize starch, P6 and P7 tapioca starch.

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3 – METHODS

3.1 Physicochemical properties

The samples were analysed to determine the following: dry matter content (PN-90/A-75101/03), density by pycnometric method (PN-90/A-75101/02), total extract content by refractometric method (PN-90/A-75101/02), total sugar and reducing sugar content by Lane-Eynon’s method (PN-90/A-75101/07) and total ash content (PN-90/A-75101/08).

3.2 Rheological properties

The measurements were made using a rotational rheometer Rheolab MC1 (Physica Messtechnik GmbH) with coaxial cylinders (bob diameter 25 mm, cup diameter 27.12 mm). The dessert samples were placed in a measuring element, thermostated and relaxed for 5 min. The following curves were generated:

1. Flow curves with a controlled shear rate (CSR) at a temperature of 25 ± 0.2 °C. The shear rate was increased from 1 to 500 s^-1 during 5 min, then maintained at 500 s^-1 for 2 min, and decreased from 500 to 1 s^-1 during 5 min.

The power law model was used to describe experimental data, and the area of the thixotropy hysteresis loop was determined;

2. Flow curves with a controlled shear stress (CSS) in the range of 0 to 50 Pa during 5 min at a temperature of 25 ± 0.2 °C. The experimental curves were fit- ted with the Herschel-Bulkley model;

3. Curves showing the time dependence of rheological behaviour (shear stress vs. shear time) at a temperature of 25 ± 0.2 °C and a constant shear rate of 100 s^-1 during 900 s. Experimental data were described by the Weltman model (RAO, 1999):

τ = A – B . ln t

where: τ –shear stress [Pa]; A–initial shear stress (at t = 1s) [Pa]; B – thixotropy breakdown coefficient; t - time [s];

4. Curves showing the temperature dependence of rheological behaviour (apparent viscosity vs. temperature) in the range of 10 to 40 °C at a constant shear rate of 1 s^-1. The experimental behaviour was described by the Arrhenius equation (RAO, 1999):

ηap = η_∞ · exp(E_a/R·T)

where: ηap – apparent viscosity [Pa·s]; η_∞ - material constant [Pa·s]; E_a - flow activation energy [J/mol]; R - gas constant [J/mol·K]; T - absolute temperature [K].

The significance of differences between the samples was determined using a one-way analysis of variance and the values of the least significant differences were calculated at p = 0.05 (LSD_0.05).

To find interrelations between the physicochemical properties of the dessert samples and the parameters of the models describing their rheological proper-

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ties, linear correlation coefficients were computed and their significance was tested using the t-Student test.

4 – RESULTS AND DISCUSSION

4.1 Physicochemical properties

Table 1 shows the results of determinations of the basic physicochemical parameters of dessert samples. The sample P5 had the highest dry matter content while the sample P7 contained the smallest amount of dry matter.

Table 1

Basic physicochemical parameters of baby fruit desserts.

Tableau 1

Paramètres phisicochimiques fondamentaux de desserts fruités pour bébés.

Density, an important characteristic of fruit pulps which is of interest to process engineers, exhibited smaller differences among the samples and was the greatest, again, for P5. The value of density increases with increasing content of soluble substances and decreases with increasing temperature (RAMOS and IBARZ, 1998; TSEN and KING, 2002).

The statistical differences in extract content were greater than the differences in density. The value of this parameter was the highest for P5 – the sample with the greatest dry matter content and density, and the lowest for P3 having a low dry matter content and an average density. The density of such products as fruit pulps is influenced not only by extract content but also by the amount of

Sample Dry matter [g/100g]

Density [g/cm³]

Soluble solids content

[^oBrix]

Total sugars content [g/dm³]

Reducing sugar content [g/dm³]

Ash content [g/100g]

P1 13.18 1.0548 13.2 195.9 164.3 0.1640

P2 15.55 1.0648 14.3 116.1 189.6 0.2183

P3 13.23 1.0601 11.5 197.0 176.0 0.1594

P4 15.40 1.0582 13.5 107.5 188.4 0.4245

P5 19.27 1.0727 17.5 124.6 115.7 0.3666

P6 14.35 1.0655 14.4 199.8 119.3 0.2480

P7 12.61 1.0509 12.1 197.3 186.6 0.1498

LSD_0.05 10.05 0.0056 10.2 110.8 110.6 0.0121

LSD_0.05 – least significant difference at p=0.05.

LSD_0.05 – plus petite différence significative à p=0,05.

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solid particles and the density of their packing which depends on the size and shape of the particles. As shown by LOZANO and IBARZ (1994), fruit pulps differ in the shape of solids.

The levels of total sugars and reducing sugars were the highest for the sample P5 having the greatest dry matter and extract contents. The lowest levels of reducing sugars were found in P1 with the lowest extract content. A very low level of reducing sugars in the sample P6 may be attributed to the specific composition of that dessert. The amount of sugars dissolved in the continuous phase considerably affects its viscosity.

The ash content of the dessert samples ranged from 0.1498 to 0.4245 g/

100 g and was the highest for P4. Mineral components in the form of soluble salts influence the rheological properties of the continuous phase, and in the form of ions participate in electrorheological interactions, thus affecting the rheological properties of fruit pulps.

4.2 Flow behaviour

Figure 1 shows the flow curves of selected desserts, obtained in the controlled shear rate (CSR) mode, and Table 2 lists the parameters of the power law model and shows the area of the thixotropy hysteresis loop. All the samples behaved as non-Newtonian fluids and exhibited shear-thinning characteristics, which confirms earlier observations concerning fruit pulps (RAO et al., 1974;

GUERRERO and ALZAMORA, 1997; PELEGRINE et al., 2002) and baby desserts (BARBOSA-CÁNOVAS and PELEG, 1983). Shear stresses had the highest values for the dessert P5 and the smallest for P1, which corresponds with the value of consistency coefficients (table 2) ranging from 6.22 Pa.sⁿ (sample P1) to 52.98 Pa.sⁿ (sample P5). This range is fairly wide and the values exceed those defined by BARBOSA-CNOVAS and PELEG (1983) for banana desserts thickened with tapioca starch (5.4 – 6.9 Pa.sⁿ).

0 50 100 150 200 250

0 100 200 300 400 500

Shear rate [s^-1]

Shear stress [Pa]

P5 P2 P1 P6

Figure 1

Flow curves (CSR mode) of baby fruit dessert.

Courbes d’écoulement (mode CSR) des desserts de fruits pour bébés.

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Table 2

Parameters of rheological models describing flow curves.

Tableau 2

Paramètres des modèles rhéologiques décrivant les écoulements.

The samples differed significantly in the values of consistency coefficient (K) although the desserts came from three producers only. One of the factors that affect the consistency coefficient is the temperature of storage. As observed by ALONSO and ZAPICO (1996), the values of consistency coefficient and yield stress of baby fruit desserts decrease with increasing storage temperature.

The values of the flow behaviour index (table 2) ranged from 0.23 to 0.40, which indicates a considerable pseudoplasticity of the desserts. The flow behaviour index was the lowest for the sample P5 having the greatest consistency coefficient and the highest for the sample P1 whose K was the smallest. The values mentioned are lower than those reported by BARBOSA-CÁNOVAS and PELEG (1983). The dessert samples (except P6) exhibited various degrees of thixotropy. The sample P5 had the largest area of the hysteresis loop (table 2) and differed greatly from the other samples. No statistically significant differences were found between the samples P2 and P3. In the case of P6 the curves for the increasing and decreasing shear rate intersect which results in a nega- tive value of the loop area, indicating a considerable rheological stability of that sample. Also the dessert P7, produced by the same manufacturer, showed a substantial stability, as demonstrated by the small area of the thixotropy hysteresis loop (table 2).

Figure 2 shows the flow curves obtained with a controlled shear stress (CSS) for chosen dessert samples. The course of the curves confirms the differences in the rheological properties of the desserts. The sample P5 containing the

Sample

Power law model (CSR mode) Area of thixotropy hysteresis loop (HA) [Pa/(s·cm³)]

Herschley-Bulkley model (CSS mode) Consistency

coefficient (K) [Pa·sⁿ]

Flow behaviour

index (n) R²

Yield stress (τ 0HB) [Pa]

Consistency coefficient

(K) [Pasⁿ]

Flow behaviour

index (n) R²

P1 16.22 0.40 0.9996 128 1.07 19.47 0.37 0.9998

P2 11.57 0.38 0.9997 209 1.11 10.11 0.42 0.9976

P3 13.56 0.35 0.9996 177 0.28 14.66 0.33 0.9990

P4 23.61 0.24 0.9863 291 1.72 15.45 0.35 0.9839

P5 52.98 0.23 0.9860 1078 0.53 43.01 0.44 0.9894

P6 15.23 0.38 0.9830 – 9 0.59 21.15 0.24 0.9862

P7 17.54 0.25 0.9937 48 0.66 16.57 0.29 0.9863

LSD_0.05 11.25 0.01 31 0.35 12.77 0.04

LSD_0.05 – plus petite différence significative p=0,05.

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highest levels of dry matter and extract (table 1) reached the assumed shear stress at the lowest shear rate, which corresponds with its behaviour in the CSR mode. This indicates that the dessert P5 is the most resistant to the shear stress applied. The sample which attained the assumed shear stress at the greatest shear rate was P1. The Herschel-Bulkley model, whose parameters are listed in Table 1, and the power law model used for the CSR mode gave a similar degree of fit to the experimental data. The dessert samples showed yield stress in the range 0.23 (P3) to 1.72 (P4) with small statistical variation. These values are lower than those reported by BHATTACHARYA (1999) for mango and pineapple pulps and considerably lower than the values obtained by GUERRERO

and ALZAMORA (1997) for banana pulp. They are also smaller than the yield stress values established by IBARZ et al. (1993, 1996) for non-clarified persimmon and sloe juices. The magnitude of yield stress depends on the method of its determination. Experimental methods (stress relaxation, stress-strain plot) are known to give more correct results than the extrapolation of flow curves and the use of rheological equations. In the case of the latter methods the controlled shear stress mode is more sensitive and yields more correct results. The consistency coefficient of the Herschel-Bulkley model for the CSS flow curves had the greatest value for the sample P5 and the smallest value for P1. Statistical differences, however, were much smaller than in the CSR mode. No significant differences were found between the samples P1 and P2, and between P3, P4 and P7. The values of the flow behaviour index in the Herschel-Bulkley model ranged from 0.24 to 0.44, which confirms a substantial pseudoplasticity of the desserts. However, comparison of the flow behaviour indexes of the two models mentioned did not reveal any pattern. The values of consistency coefficient and flow behaviour index were affected not only by the kind of the model but also by the mode in which the experimental curves were generated and the difference in the shear rate range: the maximum rate was 500 s^-1 in the CSR mode and under 100 s^-1 in CSS.

0 20 40 60 80 100

0 10 20 30 40 50

Shear stress [Pa]

Shear rate [s]-1

P1 P2 P5 P6 P7

Figure 2

Flow curves (CSS mode) of baby fruit dessert.

Courbes d’écoulement (mode CSS) des desserts de fruits pour bébés.

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4.3 Time-dependent rheological behaviour

Shear stress versus shear time at a constant shear rate (100 s^-1) is plotted in figure 3 for some dessert samples. The parameters of the Weltman model used to describe the experimental data are listed in Table 3. The degree of fit differed between the experimental curves. The samples P6 and P7, that were rheologi- cally the most stable, exhibited low values of the coefficient of determination (R²) and the lowest values of the parameter B (coefficient of thixotropic breakdown). The values of shear stress were the greatest for the sample P5 and the smallest for P1 nad P7. This corresponds with the values of the parameter A in the Weltman model (table 3) which reflect the values of shear stresses in the first second of the test. In the case of P5 the highest values of shear stress at a constant shear rate correlate well with the highest values of physicochemical parameters (table 1). In contrast, no significant differences in the value of the parameter A were observed between the samples P1 and P7, and between P2, P3 and P4. The sample P5 exhibited the highest value of the parameter B (coefficient of thixotropic breakdown), which correlates well with its largest area of the thixotropy hysteresis loop (table 2). The samples P6 and P7 proved to be the most stable. No statistically significant differences in B values were found between the samples P1, P2 and P3, and between P6 and P7. The magnitude of the loop area (table 2) showed a high linear correlation (0.9902) with the coefficient of thixotropic breakdown (B) in the Weltman model. Thixotropic properties of fruit pulps have been observed earlier by BHATTACHARYA (1999) and LOZANO and IBARZ (1994). According to BHATTACHARYA (1999) the Weltman model best describes the shear stress-shear time curves with the parameters of this equation being dependent on the shear rate assumed (they increase as the shear rate increases). LOZANO and IBARZ (1994) found that the differences in the parameters describing the thixotropic behaviour of fruit pulps cannot be attributed solely to the extract content. Investigating peach and plum pulps with the

30 60 90 120 150 180

0 300 600 900

Time [s]

Shear stress [Pa]

P1 P4 P5 P6

Figure 3

Time-dependent flow behavior of baby fruit dessert.

Comportement rhéologique des desserts en fonction du temps.

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same extract contents they observed that the pulps exhibit different values of such parameters, which they related to the different shape and size of solid particles. The Weltman equation was also considered by other authors to best fit the experimental data. Those included ALONSO et al. (1995) who studied the thixotropy of baby foods. In addition, ALONSO and ZAPICO (1996) found that the values of the parameter A of the Weltman model decrease with the time of storage.

Table 3

Parameters of Weltman and Arrhenius models.

Tableau 3

Paramtres des modles Weltman et Arrhenius.

4.4 Temperature-dependent rheological behaviour

Examples of apparent viscosity-temperature curves are shown in figure 4 and the parameters of the Arrhenius equation employed to describe the experimental data are given in table 3. The course of the curves differed among the dessert samples. The values of apparent viscosity were the greatest for the dessert P5 and the smallest for P1. As demonstrated by the values of flow activation energy, the desserts had a different sensitivity to temperature increase.

This agrees with STEFFE (1996) according to whom the higher the flow activation energy, the more temperature-sensitive a system is. Flow activation energy (E_a) was the greatest for the sample P5 having the highest apparent viscosity, and the lowest for P1. In the case of the former this greatest E_a value correlates well with the highest dry matter and extract contents (table 1). The E_a values determined in this study are comparable with those reported for mango pulps contai-

Sample

Weltman model Arrhenius model

Initial shear stress (A)

[Pa]

Thixotropy breakdown coefficient (B) R²

Material constant (η_∞)

[Pas]

Flow activation energy (E_a)

[kJ/mol]

R²

P1 153.35 11.33 0.9843 0.2138 19.00 0.9921

P2 176.79 11.70 0.9951 0.0657 13.17 0.9978

P3 173.88 11.36 0.9639 0.1612 11.21 0.9960

P4 174.00 12.65 0.9842 0.0331 15.45 0.9975

P5 217.50 14.84 0.9996 0.0590 16.43 0.9968

P6 191.49 10.47 0.8862 0.0439 15.19 0.9551

P7 153.25 10.54 0.9338 0.1632 11.47 0.9849

LSD_0.05 114.14 10.56 0.0270 10.77

LSD_0.05 – plus petite différence significative à p=0,05.

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ning similar amounts of extract, but much lower than the E_a values of concentrated fruit juices showing Newtonian properties (RAO, 1999). This is consistent with KROKIDA et al. (2001) who observed that the values of flow activation energy for non-Newtonian fluids are considerably lower than for Newto- nian fluids with the same concentration of solids.

4.5 Correlation between physicochemical and rheological properties Table 4 shows the coefficients of linear correlation between the physicochemical properties of fruit desserts and their rheological characteristics. Dry matter content and extract content correlated significantly with the consistency coefficient of the power law and Herschel-Bulkley models, the area of the thixotropy hysteresis loop, flow activation energy and the parameters of the Weltman model. Density correlated significantly with the same rheological parameters apart from K in the power law model. Total sugar content correlated with the same rheological parameters as dry matter (except K in the Herschel-Bulkley model) and with the flow behaviour index of the Herschel-Bulkley model. The other physicochemical parameters exhibited a weaker correlation with the rheological characteristics. No statistically significant correlation was observed between the physicochemical properties and the flow behaviour index of the power law model or the yield stress.

0 10 20 30 40 50 60 70

10 20 30 40

Temperature [^oC]

Apparent viscosity [Pa

. s]

P1 P2 P5 P6

Figure 4

Temperature-dependent flow behavior of baby fruit dessert.

Comportement rhéologique des desserts en fonction du température.

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Table 4

Coefficients of linear correlation between physicochemical parameters of desserts and parameters of models describing their rheological properties.

Tableau 4

Coefficients de corrélation linéaire entre les paramètres physicochimiques des desserts et les paramètres des modèles décrivant leurs propriétés rhéologiques.

5 – CONCLUSION

The baby fruit desserts exhibited different physicochemical properties and showed a shear-thinning and thixotrotropic behaviour. The desserts differed in the values of consistency coefficients, the degree of thixotropy and the sensitivity to temperature increase. The sample with the greatest dry matter and extract contents had the highest consistency coefficient and apparent viscosity.

Several significant correlations were established between physicochemical properties and the parameters of the models describing rheological properties.

Dry matter [g/100g]

Density [g/cm³]

Soluble solids content [^oBrix]

Total sugars content [g/dm³]

Reducing sugars content

[g/dm³]

Ash content [g/100g]

K [Pa·sⁿ] 0.8698* 0.6469 0.7789* 0.7572* 0.5930 0.6730**

n – 0.4447 – 0.0446 – 0.2736 – 0.4048 – 0.6537 – 0.5945

HA [Pa/

(scm³)]

0.9098* 0.6884** 0.8020* 0.8286* 0.7053** 0.5982

0HB [Pa] 0.0543 -0.2495 0.0055 0.1285 0.1355 0.5035

K_HB [Pasⁿ] 0.8010* 0.7051** 0.7929* 0.6438 0.3672 0.5238

n_HB 0.6639 0.4306 0.5325 0.7589* 0.7870* 0.3193

[Pas] – 0.6563 – 0.6584 -0.5916 – 0.6434 – 0.0349 – 0.7873*

E [kJ/mol] 0.7740* 0.7208** 0.7078** 0.6953** 0.1527 0.8391*

A [Pa] 0.9167* 0.8394* 0.8833* 0.7962* 0.4507 0.5589

B 0.9041* 0.7070** 0.8391* 0.8016* 0.6299 0.5690

* significant at p=0.05 / significatif à p=0,05.

** significant at p=0.1 / significatif à p=0,1.

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