The rheological behaviour of fruit purees was measured at different temperatures (20ºC–40ºC) in a rotational viscometer. The rheograms were fitted with the Power Law or Ostwald de Waele model, the HerschelBulkley model, the Casson model, and the Cross model. The best adjustment was obtained with the Cross model, except for systems fruit purees 2 and 4, which were fitted satisfactorily with Power Law model. Flow curves exhibited at all temperatures a pseudoplastic character after applying a shear stress higher than a critical value.
International Journal of Food Properties, 11: 321–329, 2008 Copyright © Taylor & Francis Group, LLC ISSN: 1094-2912 print / 1532-2386 online DOI: 10.1080/10942910701359424 RHEOLOGICAL CHARACTERIZATION OF COMMERCIAL BABY FRUIT PUREES E Álvarez, M.A Cancela, N Delgado-Bastidas, and R Maceiras Chemical Engineering Department, ETSEI, University of Vigo, Vigo, Spain The rheological behaviour of fruit purees was measured at different temperatures (20ºC– 40ºC) in a rotational viscometer The rheograms were fitted with the Power Law or Ostwald de Waele model, the Herschel-Bulkley model, the Casson model, and the Cross model The best adjustment was obtained with the Cross model, except for systems fruit purees and 4, which were fitted satisfactorily with Power Law model Flow curves exhibited at all temperatures a pseudoplastic character after applying a shear stress higher than a critical value Keywords: Viscosity, Purees, Pseudoplastic, Cross model, Power law model INTRODUCTION Commercial baby food is a good nutritional complement for the suckling baby and they are very useful for parents Milk as unique food from the six months does not provide energy and nutrients that baby needs at this age, in addition, as their digestive functions have matured, new foods must be included in his diet, following regulated norms The habitual form to introduce a complementary feeding must be replaced each milk intaking that baby receives by different components from the complementary feeding (cereals baby food, fruit puree, vegetable puree) These replacements must be done of one by one, with a sufficient interval so that baby accepts new foods, and hereby to confirm his tolerance before introducing a new food, this process is for giving time to the adaptation of his organism It is very important in this period, to allow that the amount of food can vary from a day to another one and from one week to another one, according to the appetite of the baby In diverse circumstances, infants need to feed with homogenized infantile foods These foods can be made up of: vegetables, fruits, meats, fish, milky or mixes, whose exclusive purpose is to establish an infantile nutritional regime The nutritious composition and norms of quality, production and elaboration of these products are collected in the Director 96/5/CEE, which demands that these products must be elaborated following strict norms of control of quality and with a suitable nutritious value.[1] Received 31 October 2006; accepted 23 March 2007 Address correspondence to M A Cancela, Chemical Engineering Department, ETSEI, Rúa Maxwell, s/n, University of Vigo, Vigo 36310, Spain E-mail: chiqui@uvigo.es 321 322 ÁLVAREZ ET AL Fruit purees are elaborated with varied fruits (peach, apple and banana, or others like apricot, orange or pineapple), and enriched or not with vitamins Water or juices are added to fruits Moreover these can include milk, cheese, biscuits, cereals and sugar that they will indicate in the label Their ingredients are controlled strictly; this guarantees the quality and the nutritious contribution of these purees to obtain a correct grown of baby They avoid the existence of preservatites or pesticides to offer a first quality product The fruits purees produced commercially have increased in popularity during the last years, due to nutritional benefits for the infants, as also by the comfort in the handling and consumption of these products For reasons described previously and because it is very important for the market in Europe, it is necessary to assure the quality of baby foods On the other hand, for the food industry is essential to have knowledge of some parameters as apparent viscosity among others, because the flow behaviour plays an important role in the design and optimization of the processes, control process, quality control[2–5] within the food industry in particular for the derivatives of fruits A point of considerable interest is the effect of temperature upon the flow properties Various studies made have looked and analyzed the effect of temperature on apparent viscosity for different fruits juice and fruit purees.[6–11] In addition, some further investigations have characterized the flow behaviour of samples of commercial banana and peach baby foods [12] In these studies were observed the behaviour of flow of the different samples, in which it was found that the flow parameters were changing significantly according to the manufacturer and the type of sample All the made studies confirm non-Newtonian behaviour of many of fruit purees analyzed.[7,13–16] The present paper was one focused in the study of the effect of temperature upon the rheological behaviour of different kinds of commercial fruit purees In this study some rheological models were used to determine the relationship between shear stress as function of shear rate to different temperatures Rheograms were fitted according to the following models: Power Law or Ostwald de Waele model, Herschel-Bulkley model, Casson model, and Cross model MATERIAL AND METHODS Infant Foods (commercial fruit purees) The ingredients of the puree fruits are showed following and the physicochemical characteristics are refers in Table Puree Apple (76%), peach (18%), honey (4%), apple juice concentrated and vitamin C, without gluten Puree Banana (35%), water, orange (17%), sugar, biscuits without gluten (7%), lemon, rise starch, and vitamin C Puree Peach pulp (27%), water, milk, sugar, creamed fresh cheese (6%), apricot, rise starches and maize, lemon juice, apple concentrated, rice flour, cream, and vitamin C Puree Apple (67%), pineapple (19%), pear (4%), sugar, and cereals (3%) (rice, maize, tapioca) Puree milk (40%), apple, pineapple, banana, and sugar RHEOLOGICAL CHARACTERIZATION OF FRUIT PUREES 323 Table Nutritional information per 100 g and pH Proteins (g) Hydrates (g) Sugars (g) Fats (g) Saturated (g) Pectine (g) Na (mg) Ca (mg) Vitamin C (mg) Water content (g) pH Puree Puree Puree Puree Puree 0.3 — 15.0 0.4 — — 2.0 — 11 77.3 3.57 0.8 24.7 19.0 1.4 0.4 1.4 10 — 35 75.1 3.62 1.4 21.8 16.4 0.9 0.6 0.6 14 — 35 78.1 3.65 0.5 8.5 — 0.1 — 1.2 6.2 — 25 78.7 3.41 2.9 20.5 — 3.2 — — 37 130 — 79.8 3.68 Flow Properties Measurements The rheological properties of fruit purees were carried out using rotational viscometer (Haake VT550/MV3, Searle type system) This viscometer is equipped with two coaxial cylinders, thereby that it has an inner cylinder rotating in a fixed outer cylinder The gap width between two cylinders was 1.45 mm Radius and length of the rotating cylinder were 10.1 mm and 61.4 mm, respectively Thermostatic bath was used to control the working temperature with a precision of ± 0.1°C Shear stress was analyzed as function of shear rate from 18 to 445 s−1 with an uncertainly of ± 0.001, while the temperature was kept fixed On the other hand, also there was analyzed the effect of the temperature on the rheological behaviour to different shear rate Measurements were carried out within temperature range from 20–40°C in measures In order to investigate the reproducibility of the results, two replicates were made for most of the experiments and the reproducibility was ± 5% on average RESULTS AND DISCUSSION Rheograms Shear flow curves of commercial fruit puree samples in a temperatures range of 20–40°C are shown in Figure In this Figure, it can be observed that there is a decrease in shear stress when the temperature increases Therefore, the suspensions exhibited nonNewtonian and pseudoplastic behaviour; thereby that η must decrease with the shear rate and with the temperature as it is observed in the Figure Likewise, in the Figure can be seen rheological behaviour of all systems of commercial fruit purees at 20 and 40°C All samples exhibited shear-thinning behaviour and therefore the shear stress decreases with the temperature Rheological Parameters The Cross model described well the flow behaviour of systems fruit purees for each temperature, as compared with other models However, in the case of systems fruit Purees and the model that better fitted the experimental data was the Power Law 324 ÁLVAREZ ET AL 90 80 τ (Pa) 70 60 50 40 30 20 100 200 300 400 500 γ (s ) –1 Figure Shear Stress vs shear rate for system fruit puree at all temperatures: (ᮀ) 20°C; (᭹) 25°C; (Δ) 30°C; (᭜) 35°C; (∇) 40°C η (Pa.s) 0 100 200 300 400 500 γ (s–1) Figure Viscosity vs shear rate for system puree at all temperatures: (ᮀ) 20°C; (᭹) 25°C; (Δ) 30°C; (᭜) 35°C; (∇) 40°C Systems purees 1, 3, and Systems purees 1, 3, and were fitted with Cross model, and the adjusted determination coefficient in all cases was higher than 0.998 The Cross model was represented as: ηa =η∞ ⎛ ⎞ η0 −η∞ ⎟ ⎜ +⎜ m⎟ ⎜⎝ + α γ ⎟⎠ c (1) Where γ is the shear rate (s−1), ηa the apparent viscosity (Pa s), h0 the zero-shear rate viscosity (Pa s), h∞ the infinite-shear rate viscosity (Pa s), ac (sm) is time constant, and m is dimensionless constant The constant αc is expressed by the ratio k1/k0, where k0+k1 γn is the rate constant for the rupture of linkages; the parameter ac is related to the relaxation time of the structural species responsible for shear thinning and the onset of shear-thinning RHEOLOGICAL CHARACTERIZATION OF FRUIT PUREES 325 20°C 40°C η (Pa.s) η (Pa.s) 2 0 100 200 300 400 500 100 200 300 400 500 γ (s–1) γ (s–1) Figure Viscosity vs shear rate for all systems analyzed at: 20 and 40°C (᭢) Puree 1; (᭹) Puree 2; (᭜) Puree 3; (ᮀ) Puree 4; (᭛) Puree behaviour.[17,18] A high value of ac implies a relatively large shear dependent contribution to structural breakdown.[19] The exponent m is related to the power law exponent “n” (flow behaviour index).[20] Systems in which h0 >> h∞ the Cross equation reduces to the power law model or may be approximated to the Bingham model.[21,22] Newtonian fluids have m = 0, while fluids that exhibit shear thinning have small< 1, positive exponents Moreover, such as >> h∞ the Cross model was used with only three adjustable parameters (assuming h∞ ≈ 0).[17,23] As a zero-shear viscosity was not obtained on these samples the h0 was estimated to lower shear rate, whereby was used the relation Log η vs Log γ This rheological model adjusts the experimental values reasonably well for the different purees (Figure 4a) Furthermore, the time constants (αc) are larger for the system puree with values between 1.1333 and 0.7265 (sm), among 20 and 40°C respectively On the other hand, systems puree and presented a stable behaviour with respect to relaxation time and temperature, presented values between 0.6907–0.7280 and 0.7726–0.7597 (sm), respectively Moreover, the exponent “m” in all cases was a positive exponent lower than 1, therefore there is deduced that these systems exhibit a shear thinning behaviour 40°C 40°C η (Pa.s) η (Pa.s) 2 1 0 100 200 γ (s–1) (a) 300 400 500 100 200 300 400 500 γ (s–1) (b) Figure Viscosity vs shear rate for different systems fitted with rheological models: (᭢) Puree 1; (᭹) Puree 2; (᭜) Puree 3; (ᮀ) Puree 4; (᭛) Puree 326 ÁLVAREZ ET AL Table Parameters of the Cross model for systems puree 1, 3, and Systems T (ºC) αc (sm) m χ2 R2 Puree 20 25 30 35 40 20 25 30 35 40 20 25 30 35 40 0.691 0.702 0.710 0.727 0.728 1.133 0.831 0.772 0.751 0.727 0.773 0.746 0.743 0.747 0.760 0.641 0.655 0.660 0.672 0.686 0.690 0.696 0.690 0.679 0.659 0.756 0.751 0.748 0.747 0.739 2.0E-05 0.99929 3.27E-03 0.99872 6.0E-05 0.99938 Puree Puree The flow behaviour was largely affected by temperature In the Table can be observed parameters of the Cross model that were obtained from the adjustment of the experimental data of the systems mentioned previously Equation was used to fit systems by means of the Cross model Systems fruit purees and As can be seen in Figure 4b, the Power Law model described well the flow behaviour of systems puree and puree 4, for each temperature, the adjusted determination coefficient being in all cases higher than 0.999 The Power Law model was represented as: η = K •γ n −1 , (2) where K is the consistency coefficient, and n is the flow behaviour index The values of the flow behaviour index and consistency coefficient, n and K, are reported in Table It can be observed that n has a value less than 1, indicating pseudoplasticity For flow curves of system puree the flow behaviour index varies from 0.374 to 0.414 Whereas, flow curves Table Parameters of the Power Law model for systems puree and System T (ºC) n K Puree 20 25 30 35 40 20 25 30 35 40 0.374 0.380 0.390 0.405 0.414 0.286 0.282 0.268 0.254 0.247 16.706 14.882 12.556 10.299 9.007 26.109 24.675 24.487 24.543 24.125 Puree RHEOLOGICAL CHARACTERIZATION OF FRUIT PUREES 327 of system puree the flow behaviour index varies from 0.286 to 0.280 Figure shows the influence of the temperature on consistency coefficient and flow behaviour index From a practical point of view, we have decided to describe effects of temperature and total solids content on consistency and flow behaviour index by one combined model 0.3 0.3 0.29 0.29 n 0.28 0.28 0.27 0.27 0.26 n (a) 0.26 0.25 0.25 0.24 0.24 25 T( °C) 30 21 35 40 21.4 Solid % (b) 26.5 26.5 26 26 25.5 K K 25.5 25 25 24.5 24.5 24 24 25 30 T (° C) 21 35 40 21.4 Solid % Figure Response surfaces for the effect of temperature and total solids on rheological parameters of puree 328 ÁLVAREZ ET AL Table Regression coefficients of the models for rheological parameters of puree and Parameter N K a0 a1 a2 b1 b2 b3 0.3911 47.3254 −12 768 96 −10240 2.8545 −3.4707 −1.0371 10–3 0.1097 1.1333·10–5 −1.1467 10–3 In the literature, some authors[24,25] used the combined effect of temperature and total solids content on thses parameters to describe the flow behaviour of fruit purees In this work, a response surface methodology was selected to investigate this effect on rheological parameters of purees and 4, and a quadratic polynomial regression model was assumed for predicting the individual variables The model proposed for rheological parameters is: y = a0 + ∑ i S + ∑ bi T , i (3) where a0 is a constant; and, and b i are regression coefficients of the model The regression coefficients are shown in Table The predictive models developed for n and K were considered adequate because they had satisfactory levels of R2 (R > 0.9999) The model indicated that temperature has significant effect on behaviour index This observation is also verified from canonical analysis of response surface Figure shows the response surfaces for the effect of temperature and total solids on rheological parameters for puree CONCLUSIONS From this study, it can be determined that temperature significantly affected the flow characteristics of all cases analyzed Systems purees 1, 3, and could be accurately described with 3-parameters of the Cross model, which were sensitive to change in temperature On the other hand, systems and were fitted well by the Power Law In the same way, such as in the case of systems fitted by Cross model, Power Law parameters (behaviour 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