Dynamic and steady shear rheology of fruit puree based baby foods

Transcription

1 Dynamic and steady shear rheology of fruit puree based baby foods Jasim Ahmed*, Hosahalli S Ramaswamy Department of Food Science and Agricultural Chemistry, McGill University, Macdonald Campus, Lakeshore Road, Ste Anne de Bellevue, Quebec H9X 3V9, Canada E*-mail:jahmed2k@yahoo.com The rheological behaviour of 3 strained pureed baby foods (apple, apricot and banana) was studied under small amplitude oscillatory and steady shear measurement in the temperature range of 5 to 80 C. Oscillatory shear data indicated weak gel-like behaviour of baby foods with the elastic modulus predominating over the viscous one (G2>G3 ) in the frequency range of 0.1 to 10 Hz. A power model was used to describe the changes of dynamic modulii (G) with frequency (ω) for whole temperature range. The values of b decreased with temperature for apple and apricot puree, whereas an increasing trend was noticed with banana puree. Under steady-shear deformation tests, the rheograms were adequately described by the Herschel-Bulkley model and purees exhibited shear-thinning characteristics. Comparison of dynamic and steady viscosities of fruit purees indicated that dynamic viscosity (η*) were much higher than steady-shear viscosity (η). The Cox-Merz rule between η* and η was followed by the apricot puree in the lower temperature range (5-35 o C) while a modified Cox-Merz rule described other puree samples including the apricot puree in the higher temperature range. Keywords: Apple puree, Apricot puree, Banana puree, Complex viscosity, Shear viscosity, Cox-Merz rule Baby foods are available in semisolid form in wide variety of commercial preparations to give diversity with respect to taste and nutritional requirements. The flow characterization of baby food is very important with respect to eating characteristics of the product: it must be thick enough to hold in spoon however, it should not be so thick as to have difficulty in swallowing (Steffe and Ford 1985). Rheological properties of pureed foods have been studied extensively and in most cases steady shear viscosimetry has been used to characterize the rheological properties (Duran and Costell 1982, Rao et al 1986, Ramaswamy et al 1994, Ahmed and Ramaswamy 2004, 2006a, b, 2007). Rheological characteristics of fruit products are influenced by structural polysaccharides. The available carbohydrates like starches and sugars, and an unavailable fraction made of pectin, cellulose, and hemi-cellulose are present in fruit (Lee and Mattick 1989). In addition, protopectin converts to colloidal polygalacturonic acids at suitable conditions, which ultimately form a gel with sugar and acid (Young and How 1986). Generally, fluids with suspended particles have a complex structure, which is sensitive to shear. Hence, steady-shear viscosimetry is not always ideally suited especially when one is interested to probe viscoelastic properties of an unperturbed dispersion. Small amplitude oscillatory shear (SAOS) tests afford the measurement of dynamic rheological functions, without altering the internal network structure of materials tested (Gunasekaran and Ak 2000) and are far more reliable than steady measurement (Bistani and Kokini 1983). Some linear viscoelastic material functions measured from oscillatory testing can be related to steady shear behaviour (Steffe 1996) by providing more reliable relationships especially at low frequencies and shear rate. Several empirical relations have been proposed that relate the viscometric functions to linear viscoelastic properties. Cox and Merz (1958) found that the complex viscosity (η*) and steady shear viscosity (η) of polymeric materials is nearly equal when the frequency (ω) and shear rate are equal. This simple empirical rule is valid for simple flexible polymers (Kulicke and Porter 1980) while it may fail for structured fluids (Alhadithi et al 1992). Therefore, the applicability of Cox-Merz rule to polymeric structured fluid or complex food systems is debateable. Various modifications of the rule have been suggested either introducing effective shear rate or shear stress equivalent inner shear rate and tested for various polymeric (Doraiswamy et al 1991, Gleissle and Hochstein 2003) and food materials (liquid and semisolid) with or without yield stress (Bistany and Kokini 1983, Dus and 579 Kokini 1990, Rao and Cooley 1992, Gunasekaran and Ak 2000). The rule can be applied to foods where oscillatory measurement is more convenient over steady shear measurement. Limited studies are available on commercial baby food characterization in terms of rheology/texture and possible changes in food components during handling and storage. The objectives of the present work were to: i) study the dynamic and steady shear rheology of fruit puree based baby foods as a function of temperature; and ii) test the applicability of Cox-Merz rule for the fruit purees at selected frequency ( Hz) and shear rate (0.1 to 10/sec) range. Material and methods Fruit puree samples: Three different commercial strained fruit puree baby foods (apple, apricot and banana) were purchased from a local supermarket. Fruit puree based baby food samples were randomly purchased to obtain an accurate representation of the commercial product. The puree jars were stored in a cool, dry and minimal light environment and jars were opened within 40 days of manufacturing as shown in the label. All samples were tested within 12 h after opening and kept refrigerated between uses. Total soluble solids (TSS) and ph of the samples were determined using a hand refractrometer (Atago, Japan) and phmeter (Accumet AB 15, Fisher Scientific,

2 580 Ottawa, ON), respectively. TSS of the commercial apple, apricot and banana based baby foods were 13.6±0.3, 13.8±0.4 and 23.0±0.3 o Brix, respectively while the ph values were found to be 3.8±0.05, 3.9±0.06 and 4.2±0.04, respectively. Product specifications provided by the manufacturer based on a 64 ml serving size are: apple (carbohydrate 9.8 g, sugar 7.8 g); apricot (carbohydrate 11 g, modified corn starch-amount not mentioned); banana (carbohydrate 15 g, sugar 12 g, starch 1.8 g and protein 0.7 g). The moisture contents of the samples (measured by standard oven drying method) were 89, 86 and 84% for apple, apricot and banana puree, respectively. It is evident from the formulation that carbohydrates and sugars (except apricot) account for the principal food components in the baby foods. Rheological measurements: Both small amplitude dynamic measurement and steady shear measurements were performed using a controlled-stress rheometer (AR 2000, TA Instruments, New Castle, DE, USA) with attached computer software (Rheology Advantage Data Analysis Program, TA). A 60 mm parallel plate attachment was used with a gap of 1 mm. The AR 2000 System is based on efficient peltier temperature control and temperature was monitored (±0.1 o C) during the experiments. A solvent trap was used to minimize moisture loss during the tests. For each test, a measured volume (approx. 2 ml) of well-mixed sample was transferred to the rheometer plate. The test temperature was selected at wider range between 5 and 80 o C. The linear viscoelastic range was determined by stress sweep tests at constant frequency of 1 Hz and based on that the applied stress for all experiments was selected. The instrument was programmed for the set temperature and equilibration for 10 min (relaxation step) followed by two-cycle oscillatory frequency changes from 0.1 and 10 Hz and back. Each time a new sample was used for rheological measurement. All rheological measurements were carried out in duplicate and the experimental dynamic rheological data were obtained directly from the software. For steady flow measurements, the rheometer was programmed for the set Fig 1. Mechanical spectra of fruit puree based baby foods at 20 o C temperature and equilibrated for 10 min following which a two-cycle programmed shear changing from 0.1 to 100 /sec in 5 min (upward) and back to 0/sec in next 5 min (downward). In addition, a few experiments were carried out at extremely low shear rates ( to 10/sec) at selected temperatures to verify the variation in yield stress data obtained at previously mentioned shear rate and low shear rate ranges. Rheological parameters (shear stress, shear rate and shear viscosity) were obtained from the software. Various rheological flow models based on shear stressshear rate were tested (Newtonian, Bingham, Casson, power law, Herschel Bulkley). The best fit model was selected on the basis of standard error, which is defined as:...(1) where, X m is the measured value; X c is the calculated value; n is the number of data points and range is the maximum value of X m the minimum value. Statistical analysis: Statistical analysis was carried out using Minitab statistical software package (Minitab Inc 2000). Trends were considered significant while means of compared sets differed at p < 0.05 (Student s t-test). Results and discussion Dynamic rheological behaviour: Fig. 1 illustrates the mechanical spectra of selected fruit-puree based baby foods. The elastic modulus, G (Pa) values were substantially higher than the viscous modulus, G (Pa) in the frequency range studied, and both modulii increased with frequency. Such behaviour is an indication of typical weak gel formation (Rao and Cooley 1992, Rao 1999, Youn and Rao 2003). Apple puree exhibited higher G throughout the frequency range (0.1 to 10 Hz) whereas apricot puree showed the lowest. Apple and banana purees had similar viscous behaviour, and the rheograms were almost superimposed (G s -ω curve) at lower frequencies. However, apricot puree showed lower values of G at studied frequency range. The tangent of phase angle (δ) (a ratio of viscous to elastic modulus) measures energy lost relative to the energy stored in a cyclic deformation. A phase angle of 90 o indicates that the material is fully viscous and a fully elastic material is characterized by phase angle value of 0 o. At 20 o C, banana puree behaved as viscous fluid (phase angle value, δ, o ) compared to apple (δ = o ) and apricot puree (δ = o ). Effect of temperature on the dynamic shear modulii of pureed baby foods is illustrated in Fig. 2. The G values of apple puree ranged between 227 and 738 Pa in the temperature range of 5-80 o C. G -ω curves of apple puree were almost similar in the temperature range between 5 and 50 o C with insignificant (p>0.05) change especially at higher frequencies (Fig. 2). The values of G ( Pa) decreased as temperature increased (p>0.05) in the frequency range of 0.1 to 10 Hz, except at 80 o C, where an increasing trend was noticed. A sharp increase in both dynamic modulii at ~80 o C could be attributed to starch gelatinization and interaction of other ingredients. The nonsystematic trend of G is also likely to contribute by the interaction of these ingredients during different degrees of thermal treatments. Magnitudes of G of apricot puree were found to vary between 52.9 and 274 Pa while G varied between 8.01 and 67.3 Pa in the temperature range 5-80 o C. Apricot puree showed a continuous decrease in both G and G with

3 581 Fig 2. Dynamic shear modulii of puree based baby food samples at selected temperatures temperature except at temperature range between 50 and 65 o C (Fig.2). Obviously, the starch gelatinisation in apricot puree during heating at 50 and 65 o C could have contributed to an increase of viscoelasticity which further decreased at 80 o C. This type of behaviour is common for cereal starches where the pasting temperature (above gelatinization temperature) attains a peak viscosity and falls somewhat as temperature rises further (Woolfe 1992). The G value of banana ranged from 237 to 1070 Pa, while the G ranged from 44.6 to 341 Pa. However, no systematic trend was observed for either G or G as function of temperature (Fig. 2). Both G and G decreased as temperature increased from 5 to 35 o C and, thereafter, values were not systematic. The starch gelatinization of banana puree initiated at 50 o C and the degree of gelatinization increased when temperature was increased from 65 to 80 o C. Increase in both G and G supported gelatinization and viscoelastic characteristic of banana puree. Change of phase angle (δ) data with temperature (Fig. 3) further confirmed the viscoelasticity and starch gelatinization of puree samples during heating. In apricot puree, phase angle was not affected (p>0.05) by temperature. The phase angle of apple puree decreased systematically as temperature increased except at 80 o C whereas δ of banana puree initially decreased from 5 to 35 o C followed by a sharp increase as temperature increased further and exhibited a peak value at 65 o C. During the temperature ramp, apple and banana puree exhibited more viscous characteristics (higher δ values) at 80 o C and at temperature range of o C, respectively. An increase in phase angle confirmed the presence of more liquid components (G ), which resulted either by modification or liquefaction of specific starches or other additives present in the baby food samples. In addition, heating of starch in presence of other ingredients leads to a composite gel microstructure, with the swollen starch acting as filler and significantly changing the rheological characteristics. The dynamic rheological modulii were fitted to a power model as suggested by Rao and Cooley (1992) for tomato paste:...(2)...(3) Linear regression of log ω vs log G and log G was carried out for all temperatures and the magnitudes of slopes and intercepts (power parameters) were computed. Regression parameters obtained are reported in Table 1. It is evident from the table that both elastic and viscous modulus-frequency data fitted the power model adequately (SE<0.10) except two cases where SE >0.20. Test samples ex- Fig 3. Effect of temperatures on phase angle of fruit puree based baby foods at frequency of 1 Hz

4 582 Table 1. Effect of temperature on dynamic rheological parameters of fruit puree baby foods Puree type Temp., o C G G a b c d Apple Apricot Banana G : Elastic modulus, G : Viscous modulus hibited a weak gel characteristic with small magnitudes ( ) of positive power slopes (Table 1). It is reported earlier that a true gel is characterized by a zero slope of the power law model while concentrated and viscous materials exhibit positive slopes (Ross-Murphy 1984). The power model intercepts obtained from log ω vs log G were much higher (a>c) than that of log ω vs log G. Regression parameters did not follow any particular order with temperature. Rao and Cooley (1992) reported similar weak gel characteristics of tomato paste (26-30 o Brix) with the slope ranging between 0.12 and Among the three test samples, the power model slope of elastic component (b) for banana puree significantly (p<0.05) differed from others while no significant variations (p>0.05) were noticed among power model exponents of viscous components (d). In general, all three purees behaved similarly in viscous components. Steady shear data: Fig. 4 illustrates the steady shear rheogram of fruit purees at selected temperature. There were significant differences (p<0.05) between upward (0.1 to 100/sec) to and downward Fig 4. Steady shear rheogram of baby foods at selected temperatures (100 to 0.1/sec) steady flow data during the shearing cycles. The upward data was is the yield stress, γ is the shear rate (/ more scattered due to the structural rigidity of the virgin sample and did not ad- (Pa.s n ), and n is the flow behaviour index sec), K is the consistency coefficient equately fit any particular rheological (dimensionless). model (data not shown). The associated Magnitudes of the rheological model standard errors were significantly high (e.g. τ o and K) parameters are presented in and the models were rejected. However, Table 2. Puree samples indicated the the downward flow data (following the existence of yield stress at studied shear upward shear history) were more reliable rate ranges (0.1-10/sec). Concentrated (Fig. 4) with low standard errors; hence suspensions or complex food materials only the down curve data were reported. usually show yield stress due to significant particle-particle interactions and Such a behaviour and use of down curve data has been reported in several previous crowding. The existence and values of the studies (Ahmed 2004, Ahmed and yield stress for various fruit puree have Ramaswamy 2004). Some researchers have been reported (Duran and Costell 1982, suggested performing a quick initial shearing of test samples before the actual material property representing the transi- Canet et al 2005). The yield stress is a measurement to obtain consistent steady tion between solid like and liquid like shear data. The Herschel-Bulkley model behaviour, and is related to the minimum (equation 4) was found the best-fit model shear stress corresponding to the first for the flow curves over the wider temperature range based on the estimated Yield stress varied significantly among evidence of flow (Doraiswamy et al 1991). errors (Table 2). The Herschel-Bulkley puree samples (Table 2). The magnitudes model is represented as: of yield were similar to earlier reports...(4) (Duran and Costell 1982, Rao et al 1986). The consistency index (K) decreased with where, τ is the shear stress (Pa), τ o

5 583 Table 2. Herschel-Bulkley flow model parameters for fruit based baby foods at selected temperatures Puree type Temp., o C t o (Pa) K (Pa.s n ) n (-) SE Apple Apricot Banana SE : Standard error selected frequency and shear rate range. The relationship between dynamic complex viscosity (η*) and the shear viscosity (η) data in the frequency range of 0.1 to 10 Hz and shear rate of 0.1 to 10/sec were studied for the fruit puree samples. In order to take a closer look at the variations of both dynamic complex viscosity (η*) and the shear viscosity (η) data with frequency/shear rate, their ratios (η -/η) were computed. Surprisingly, the Cox-Merz empirical rule was found to be applicable for apricot puree in the temperature range 5-35 o C. The two plots (Fig. 5, Apricot 5 o C and 35 o C) show the Cox- Merz superposition (Cox and Merz 1958) of η and η* plotted at equivalent values of shear rate and frequency. In the literature, there are examples of the Cox-Merz rule being valid for pure liquid and certain polymers (Baird 1980). As shown in Fig. 5, the Cox-Merz rule does not appear an increase in temperature for apricot and banana puree while no specific trend was found for apple puree. Moreover, K for apple puree was significantly (p<0.05) different from other samples. Flow behaviour index (n) varied significantly (p<0.05) over a wide range of values (0.17 to 0.43) among the puree samples and temperature confirming pseudoplastic nature of fruit puree. Values of both K and n were in close agreement with the literature values for fruit puree (Duran and Costell 1982, Rao 1999). Shear stressshear rate data for apple puree increased as temperature increased from 5 to 80 o C whereas, shear stress-shear rate data of both apricot and banana puree decreased systematically as function of temperature (Fig 4). These observations were not in close agreement with oscillatory shear measurement. The observed differences could be attributed to breakdown of food structure at higher shear rate compared to oscillatory measurement where textural breakdown is found to be insignificant. Lack of fit for Hershel-Bulkley model at and above 65 o C for apple puree (Table 2) could be due to significant structural modification of ingredients. Applicability of Cox-Merz rule: With the advent of rheometer interfaced with computers it became easy to obtain directly dynamic and steady shear data at Fig 5. Applicability of Cox-Merz rule for pureed baby foods

6 584 to hold for other puree samples or even apricot puree in higher temperature range (80 o C). The dynamic viscosity function predominates over shear viscosity function and the values of η* higher than η at almost all the temperature ranges and behaved differently in the similar range of frequency/shear rate for each puree. It is proposed that during oscillatory experiments the inner deformation increases due to the presence of rigid particles in suspensions (Gleissle and Hochstein 2003). However, most of the researchers (Grizzuti et al 1993, Hsiao et al 1990) advocated that the defect structure in a liquid crystal system varies over time in a steady shear experiment until a steady-state viscosity and most likely will return to the equilibrium defect structure of the unsheared state. The Cox- Merz rule can only be applied if both steady shear and the dynamic oscillation experiments are probing a similar defect structure. This could be achieved by shearing the sample before dynamic oscillation measurement (Fujiwara et al 1991). The difference in the studied samples is attributed by the shear characteristics and response of complex food structure during different types of shearing. The deviation has been explained by non-covalent interactions which determine the stability of the gel systems in literature (Ross-Murphy 1995). A direct power shift factor (α) type relationship was used by Rao and Cooley (1992) and Canet et al (2005) when power law parameters, especially the flow behaviour index was in a narrow range. This method seemed not to work (except apricot) for the present puree samples because of large differences in the power law parameters of the η*-ω and those of the η-γ. Similar observation for some commercial food samples was earlier reported by Bistany and Kokini (1983). However, close examinations of rheograms in Fig. 5 indicate good qualitative similarity of slopes between steady and complex viscosity curves. These slopes did not represent conventional flow behaviour index n, rather n-1. The tabulated (n-1) values for the different conditions differ only by about 25% at the most (Table 3). The divergence from the Cox-Merz rule was, however, rectified by applying a nonlinear modification (Equation 5). The Table 3. Power law parameters for η versus γ and η* versus ω at selected temperatures Puree type Temp., o C η =K(γ) n-1 η* =K*(ω) n*-1 K (Pa.s n ) (n-1) K* (Pa.s n ) (n*-1) Apple Apricot Banana Table 4. Power model coefficients of equation (5) Sample Temp., o C C α R 2 SE Apple Apricot Banana SE : Standard error modified equation was used to relate them under different conditions and found to be fitted adequately (R and SE 0.248) (5) Values for the multiplicative constant C and the power index α are reported in Table 4. The power indices of the baby foods ranged between 1.15 and 1.47 with an average value of 1.22±0.06. Samples with α value close to unity show linear relationships between steady shear and complex viscosities. The C values indicate the difference between the complex and steady shear viscosity. Apricot puree exhibited the least differences between two viscosities while substantial differences were noticed for apple and banana puree samples. Temperature has a significant effect on power model parameter, C, which is also evident from the Fig. 5. Power model parameter C showed non-systematic pattern when temperature was increased. For all purees, the changes in α were only marginal with an except for apricot at 65 and 80 o C. This means that the different curves could be shifted to nearly belong to one band when the scale factor C is applied to them. Conclusion Viscoelastic nature of fruit puree based baby foods was confirmed by dynamic and steady shear measurement over the temperature range of 5 to 80 o C. Based on dynamic shear data, fruit purees exhibited a weak-gel nature while the shear

7 585 thinning behaviour was confirmed by steady shear measurements. All 3 pureed samples behaved differently with temperature. The Cox-Merz rule was not directly applicable for most of samples except apricot puree at lower temperatures; however the divergence from the Cox-Merz rule was described by applying a nonlinear modification. Acknowledgement The research was partially funded by a grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). References Ahmed J Rheological behaviour and colour changes of ginger paste during storage. Int J Food Sci Technol 39: Ahmed J, Ramaswamy HS Response surface methodology in rheological characterization of papaya puree. Int J Food Properties 7:45-58 Ahmed J, Ramaswamy HS 2006a. Viscoelastic properties of sweet potato puree infant foods. J Food Eng 74: Ahmed J, Ramaswamy HS 2006b. Viscoelastic and thermal characteristics of vegetable puree based baby foods. J Food Process Eng 29: Ahmed J, Ramaswamy HS Dynamic rheology and thermal transitions in meatbased strained baby foods. J Food Eng 78: Alhadithi TSR, Barnes HA, Walters K The relationship between the linear (oscillatory) and nonlinear (steady-state) flow properties of a series of polymer and colloidal systems. Colloid Polym Sci 270:40-46 Baird DG Rheological properties of liquid crystalline solutions of polypphenyleneterephthalamide in sulfuric acid. J Rheol 24: Bistani KL, Kokini JL Comparisons of steady shear rheological properties and small amplitude dynamic viscoelastic properties of fluid food materials. J Text Stud 14: Canet W, Alvarez MD, Fernandez C, Luna P Comparisons of methods for measuring yield stresses in potato puree: effect of temperature and freezing. J Food Eng 68: Cox WP, Merz EH Correlation of dynamic and steady flow viscosities, J Polym Sci 28: Doraiswamy D, Mujumdar AN, Tsao I, Beris AN, Danforth SC, Metzner AB The Cox-Merz rule extended: a rheological model for concentrated suspensions and other materials with a yield stress. J Rheol 35: Duran L, Costell E Rheology of apricot puree: characterization of flow. J Text Stud 13:43-58 Dus SJ, Kokini JL Prediction of the nonlinear viscoelastic properties of a hard wheat flour dough using Bird-Carreau constitutive model. J Rheol 34: Fujiwara K, Masuda T, Takahashi M Rheological properties of a thermotropic liquid crystalline copolyester. Nihon Reoroji Gakkaishi 19:19-24 Gleissle W, Hochstein B Validity of the Cox-Merz rule for concentrated suspensions J Rheol 4: Grizzuti N, Moldenaers P, Mortier M, Mewis, I On the time-dependency of the flow-induced dynamic moduli of a liquid crystalline hydroxypropylcellulose solution. Rheol Acta 32: Gunasekaran S, Ak MM Dynamic oscillatory shear testing of foods selected applications. Trend Food Sci Technol 11: Hsiao BS, Stein RS, Deutscher K, Winter HH Optical anisotropy of a thermotropic liquid crystalline polymer in transient shear. J Polym Sci 28: Kulicke WM, Porter RS Relation between steady shear flow and dynamic rheology. Rheol Acta 19: Lee CY, Mattick LR Composition and nutritive value of apple products. In:Processed apple products. Downing DL (ed). AVI Publ, New York. p Ramaswamy HS, Basak S, Vande Voort FR Effect of strawberry concentrate on applesauce rheology. Can Agric Eng J 36: Rao MA Rheology of fluid and semisolid foods: Principles and applications. Aspen Publ, Maryland, p 433 Rao MA, Cooley HJ, Nogueira JN, McLellan, MR Rheology of apple sauce: effect of apple cultivar, firmness, and processing parameters. J Food Sci 51: Rao MA, Cooley HJ Rheological behaviour of tomato pastes in steady and dynamic shear. J Text Stud 23: Ross-Murphy SB Rheological methods. In:Biophysical methods in food research. Chan HWS (ed). Blackwell Scientific Publ, London Ross-Murphy SB Rheology of biopolymer solutions and gels. Blackie/Professional, London Steffe JF Rheological methods in food process engineering. Freeman Press, USA Steffe JF, Ford EW Rheological techniques to evaluate the shelf-stability of starch-thickened, strained apricots. J Text Stud 16: Woolfe JA Sweet potato: An untapped food resource. Cambridge University Press, Cambridge, UK p 642 Youn KS, Rao MA Rheology and relationship among rheological parameters of cross-linked waxy maize starch dispersions heated in fructose solutions. J Food Sci 68: Young CT, How JSL Composition and nutritive value of raw and processed fruits. In:Commercial fruit processing. Woodroof JG, Luh BS (eds). AVI Publ, New York. p Received 07 July 2006; revised 30 March 2007; accepted 13 June 2007