Thixotropy of Orange Concentrate and Quince Puree

zy zyxw THIXOTROPY OF ORANGE CONCENTRATE AND QUINCE PUREE A.M. RAMOS Departamento de Tecnologia de Alimentos Vniversidade Federal de Vicosa 6570.000 Vicosa, Brazil AND A . IBARZ' Departament de Tecnologia d 'Aliments Vniversitat de Lleida Av. Rovira Roure, 177 251 98 Lleida, Spain zyxw zyxw zyxwv zyxwv zyxwv (Manuscript received 1997; in final form March 16. 1998) ABSTRACT Orange juice with pulp and pectins is thixotropic at soluble solids concentrations of 55 and 60 "Brix in the range of temperatures from 0 to 20C and shear ratefrom 7.2 to 57.6 s-'. Quince pure'e is thixotropic at soluble solids concentrations of 12.3 to 28 "Brix in the range of temperatures between 0 to 20C and shear rate from 7.2 to 57.6 s-'. The thixotropic behaviour of orange juice and quince pure'e increases with increasing concentration and decreasing temperature and they can be described by the kinetic model proposed by Figoni and Shoemaker (1983): u = ae+( uoi- ue)exp(-kt) The thixotropic structure of orangejuice was destroyed by applying a shear rate of 57.6 s-'for 5 min, and for quince pure'e for 10 min. Quince puree shows a greater thixotropic character than orange juice, because it has a higher content of fiber, pulp and pectins and also because it shows a microscopic structure consisting of long particles and heterogeneous fibers. zyxwvutsrqp INTRODUCTION Various authors agree that fruit concentrate and puree are thixotropic and that this is related to the percentage of total pectins and fibers (Mizrahi and Berk ' To whom correspondence should be addressed Journal of Texture Studies 29 (1998) 313-324. All Righrs Reserved. "Copyright 1998 by Food & Nutrition Press, Inc., Trumbull, Connecticut 313 zyxwvutsrq zyxw zyxwvu 3 14 A . M . RAMOS and A. IBARZ 1970; Crandall et al. 1990; Ibarz and Lozano 1992) and also with pulp content (Mizrahi and Firstenberg 1975). The time dependence is related to the structural change due to shear. If the shear ceases, the viscosity returns to its initial value due to the structural recovery of the material. This process is reversible; it could pass from a state of gel to sol and from sol to gel. The structural factors that contribute to dependence of the time to the flow of a material are similar to those that contribute to pseudoplastic characteristic (Rha 1978). Therefore, thixotropy is the result of structural reorganisation, with a decrease in the resistance to flow (Barbosa-Cinovas et al. 1993). An example is the degree of torque required to stir the viscous liquids in a mixer with constant speed (Mohsenin 1996). The usual method to characterise thixotropy is to apply a constant shear rate and study the variation of shear stress with time, and fit the experimental data to models that describe that variation (Tung et al. 1970; Ibarz 1993). This work has the goals of determining the thixotropic behaviour of quince purke and orange juice at several concentrations, and study the effects of temperature and shear rate on this thixotropic behaviour and the influence of structure of these derived fruits on the thixotropy. z MATERIAL AND METHODS Raw materials were supplied by industries in the region of Lleida in Spain. The samples used are orange juice concentrate with soluble solids of 60 "Brix and quince purke at concentration of 12.3 "Brix. The samples of quince puree and orange juice were characterised by soluble solids concentration, pH, water activity and fiber (AOAC 1990); acidity and formol index (MAPA 1993); pectin (IFFJP 1984); pulp content ( D u r h and Jimknez 1980) and sugars (Garza er al. 1996). All of which were carried out five times. Samples of quince puree were obtained with several soluble solid concentrations through the process of vacuum evaporation (Rotavapor Resona Technics Lab0 Rota) at 50C. Orange juice concentrate was diluted with distilled water to obtain several concentrations of soluble solids. For the thixotropic characterization a Rotovisco RV-12 Haake viscometer was used. The sensors used were the NV (radii ratio Re/Ri= 1.02) and SV (radii ratio Re/R, = 1.14), that consist of cups with two rotors that allow to obtain different measuring ranges. A M500 torque measuring unit that could measure a maximum torque of 4.90 N.cm was used. A thermostatic bath was used to control the working temperature within the range 0 to 80C (f 0.2C). The instrument can be operated at 16 different speeds from 1 to 512 rpm which are changed stepwise with a selector switch (Haake 1983). zyx zy ORANGE CONCENTRATE AND QUINCE PUREE zyx 315 The experimental data were controlled automatically through the DECIPHER PLUS version 1.1 program (Decipher 1992) installed in a computer PC, connected to a DATA-TAKER 500 signals microprocessor (Datataker 1992) which is directly connected to the Haake viscometer. The thixotropy was characterised in samples of industrial quince puree at concentrations of 12.3; 16; 20; 24; 28 "Brix; and in samples of orange juice at concentrations of 45, 50, 55, 60 "Brix. For orange concentrate the sensor system NV was used in all determinations. In preliminary tests, the working temperatures of 0, 5, 10, 20C and shear rates of 7.2; 14.4; 28.8; 57.6 s-' were chosen. The sample was placed in the viscometer and excess sample is removed with a syringe or pipette. It is left for one hour to allow relaxation and the equilibration of temperature (BarbosaCanovas and Peleg 1983). The concentration, the working temperature and the shear rate of the sample are fixed, and the variation of the shear stress during a period of 10 min studied. Results were reported as average of three thixograms obtained for each condition, with an error less than 2%. A covering of low viscosity oil was applied at the edge of all samples, after loading, between the two cylinders. The variation of the shear stress with the time at constant shear rate was controlled automatically by means of a computer programmer that samples the shear stress every second. Experimental data obtained was fitted by Figoni and Shoemaker (1983) model through analysis of regression carried out with the statistical package STATGRAPHICS v. 7.0. zyxwvut zyxw RESULTS AND DISCUSSION Thixotropic Behaviour of Concentrated Orange Orange juice at 55 and 60 "Brix showed time dependence while juice at 45 and 50 "Brix did not. The thixograms for orange juices (60 and 55 "Brix) divided into two stages. The first, from 0 to 20 s, a rapid decrease in the shear stress, while in the second stage the decrease is slow. This could be explained by means of two mechanisms of structural break-down. The first mechanism could be due to the disintegration of irregular pulps into smaller and more homogeneous particles. The second mechanism is explained by the orientation of particles caused by the shearing action. The behaviour obtained for the other conditions were similar. Experimental values were fitted to Weltman (1943) and Figoni and Shoemaker models (1983). A better fit was obtained with the Figoni and Shoemaker model in all cases, so only the values obtained by this fit are given. Figoni and Shoemaker's model (1983) states that: 316 zyxwv zyxw A.M. RAMOS and A. IBARZ zyxwv zyx zy zy zy where a is the shear stress, a, is the shear stress equilibrium value which is reached after a long enough shear time (Pa), a,, is the initial shear stress, (aoi a,) represents the quantity of break-down structure for shearing, k, is a kinetic constant of structural destruction and 0 is the time of shearing. Thixotropic parameters of Figoni and Shoemaker model, and the determination coefficients, are listed in Table 1. The fits and the estimates of the parameters, a,,, a, (aoi - a,) and k,are significant with a probability level of 95%. Effect of Shear Rate on the Thixotropy The effect of shear rate on the thixotropic character of the juice is shown in Table 1. For a shear rate of 57.6 s-' the quantity structure break-down (aoiae)during shearing was greater than for shear rate of 7.2 s-I. A shear rate of 7.2 s-I showed a smaller value for the equilibrium shear stress, a,, which implies that it will show a smaller apparent final viscosity than the sample shearing at the higher shear rate. Those behaviours have also been detected in juice concentration at 60 "Brix. This could be explained by the fact that when applying a high shear rate, the shear stress obtained is greater, which implies higher differences between the initial stress and equilibrium stress. Chiralt ef al. (199 1) observed similar behaviours, working with Jijona turrbn. Effect of Temperature on the Thixotropy As temperature rises, thixotropy is reduced (Pascual 1974; Alvarez et al. 1989). In orange juice at concentration of 60 "Brix, structural break-down of samples decreases when the temperature increases, being less with decreasing shear stress. The effect the temperature is seen in Table 1. With the increase of temperature, equilibrium shear stress decreases, this indicates that it will show less apparent viscosity with the increase of temperature. Effect of Concentration on the Thixotropy The evolution of shear stress with time at shear rate of 28.8 s-' for orange juice at temperature of 1OC and concentrations of 55 "Brix and 60 "Brix was also studied. The difference from the variation of shear stress-time exists, especially in the initial times, where the structural destruction of sample to concentration of 55 "Brix was slightly more rapid. ORANGE CONCENTRATE AND QUINCE PUREE zyx 317 zyxwvut zyxwv zyx zyxwvuts zyxwv zyx zyxwv TABLE 1 . FIGONI AND SHOEMAKER MODEL PARAMETERS FOR ORANGE C ("Bnx) Y T (s.') ('C) % - 0, 011, ~ 55 (Pa) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML 72 10 3 7 8 ~ 0 7 26 1 t 0. I 1 I 7 f 0.6 0 I70 f 0,013 0.864 144 10 642k09 46.5 f 0. I 17 7f0.9 0 223 f 0,014 0.898 0 1065t07 92.2 f 0.1 14.2 f 0.6 0.101 f 0 . 0 0 7 0.891 5 951k11 74.7 f 0.1 20 7 f 1.0 0.182 f 0.0 12 0.889 10 994rt 1 9 60.6 t 0.1 38.9 f 1.8 0.600 t 0 030 0.943 20 665k19 43.7 f 0.1 21.8 k 1.8 0.580 f 0 050 0.832 576 10 1 0 7 6 k 0 9 83.9 2 0.1 23.7 k 0.8 0.225 f 0 010 0.944 7.2 0 102 2 k 0 . 6 80.9 f 0.1 21.3 f 0.5 0.083 k 0.003 0.962 5 6 8 9 2 1.1 49.1 t 0. I 19.8k 1.1 0.250t0017 0.883 _- _- -- _- 20 66.0 k 1.3 39.4 f 0. I 26.6 f I 2 0.205 f 0.013 0.903 0 117.32 1.5 94.9 f 0.1 23.4 f 1.4 0.195 f0.016 0.839 288 50 60 RL (Pa) 10 14.4 28.8 57.6 -- 5 85.3 t 1.3 65.6 f 0.1 1 9 . 7 t 1.2 0.161 f0.013 0.838 10 71.5 k 0 . 6 60.6 f 0.2 10.9 f 0.3 0.030 f 0.002 0.904 20 77.0 k 0 . 7 61.2 f 0. I 15.8f0.6 0.124f0.007 0.915 0 155.22 1.3 132.0 f 0.2 23.3 ? 1.2 0.134 f 0.009 0.874 5 130.82 1.4 103 4 f 0.2 27.4 k 1.3 0.144 zk 0.009 0.893 10 110.2 k 0 . 5 882fO I 0.1 10 f 0.003 0.972 20 1 0 7 . 0 t 1.6 * 22.1 f 0.5 75 7 0.1 31.3f 1.5 0.311 f 0 . 0 1 8 0.913 0 181.9 22.3 136.3 f 0.2 45.6 t 2.1 0.251 f 0.015 0.907 5 145.2 k 1.6 1 16.8 f 0.1 28.4 f 1.5 0.202 f 0.014 0.875 10 115.7f 1 . 1 98.3 f 0.2 17.3 t 1.0 0.073 f0.006 0.828 20 107.3 f 1.3 84.5 f 0.1 22.8 f 1.2 0.201 f 0.005 0.879 318 zyxw zyxwvutsrqp zyxwv A.M. RAMOS and A. IBARZ zy z The difference, a, - uer represents the quantity of juice structure that is degraded during the shearing. The values obtained for 55 "Brix juice at i. = 28.8s.' (14.2 to 38.9 Pa, Table 1) and for 60 "Brix juice at i. = 28.8s.' (22.1 to 31.3 Pa, Table 1) indicate an appreciable magnitude of thixotropy. At 5C and shear rate at 28.8 s-' (Table l), the equilibrium stress, u,, decreases with concentration, and the sample with more elevated concentration will show a greater apparent final viscosity. For the break-down structure parameter, a, - a,, this quantity is greater in the sample of orange juice at 60 "Brix, than the one which indicates that a higher quantity of structure has been destroyed during the shearing, k, values of sample at 55 "Brix are higher; these values imply that the speed of structural destruction of the sample is higher than the sample at 60 "Brix. Thixotropic Behaviour of Quince PurCe The shear stress obtained on quince puree at 28 "Brix at a shear rate of 57.6 s-' was extremely sensitive to the time of shearing, especially in the first 100 s. All the samples showed thixotropic characteristic. The shear stress, as well as the apparent viscosity reaches a constant value after the 300 s of shearing. For the puree at 12.3; 16; 24 and 28 "Brix similar thixograms were also observed; however, values of initial shear stress were directly influenced by shear rate. Similar behaviour has been obtained by Costell and D u r h (1978) and Costell et al. (1982) in apricot puree; Chiralt et al. (1991) in "Jijona turron"; Ibarz and Lozano (1992) in peach puree and Alonso et al. (1995) in fruit-based baby foods. zyxwvut zyxw zy Effect of Shear Rate on the Thixotropy Table 2 shows the influence of shear rate applied on the degree of thixotropy for quince puree at different concentrations at 1OC. The behaviour is similar to orange juice (Table 1) and agrees with results obtained by Chiralt el al. (1991). Effect of Temperature on the Thixotropy An increase of temperature reduces the thixotropy of quince puree samples. This is similar to orange juice at 55 and 60 "Brix (this work) and condensed milk (Alvarez ef al. 1989). In quince puree, structural degradation of the samples decreases when the temperature increases. The thixogram for the quince puree (28 "Brix, 57.6 s-I) also has two different stages, as that obtained for orange juice (this work) and "Jijona turron" (Chiralt et af. 1991). The first stage, which is characterised as a rapid decrease in the shear stress, is more extended and it is in an interval from 0 to 100 s. zyx zyxw ORANGE CONCENTRATE AND QUINCE PUREE 319 This difference shows the greater thixotropy of puree. Effect of Concentration on the Thixotropy A greater thixotropic behaviour can be seen at higher concentrations of the puree, while for the concentrations of 16 and 12.3 "Brix the time dependence is small and the thixotropy structure in these concentrations is weak. The effect of concentration has also been studied for the shear rates 7.2, 12.4 and 57.6 s.' at 1OC. Similar thixograms were obtained. However, values of initial shear stress are directly influenced by the shear rate applied and an initial shear stress increase with the shear rate, which implies a higher stress equilibrium. zyxwv zyxwvu TABLE 2. FIGONI AND SHOEMAKER MODEL PARAMETERS FOR QUINCE PUREE AT TEMPERATURES OF 1OC AT DIFFERENT SHEAR RATES AND CONCENTRATIONS SOLUBLE SOLIDS 7 c (S-') ("Brix) 7.2 12.3 16 20 24 28 444f02 I08 f 1 0 307 t 3 0 608f6O 837 f 9 0 14.4 12 3 16 20 24 28 28.8 57.6 (JOl (Je (Pa) zyxwvutsr (Pa) (JO, - (Je (Pa) kl R: (S.') 408tO 1 36?02 939206 140f04 244 0 f0 3 6 2 7 - 2 4 444 3 ? 0 7 l 6 3 9 f 1 2 241 + 8 0 596 0 2 1 2 0 104f0003 0118fOOO5 O125fO007 0117tOOO5 0 115t0006 0 887 0 952 0 923 0 944 0 936 486fO2 1293f09 363 f 4 0 654 f 8 0 919f I I 0 450tO 1 1134?0 1 274 6 If. 0 3 472 3 f0 8 663 I f 1 2 3.7 f 0.9 16.5 t 0 . 9 88 t 4 0 183 7.0 286 f 9.0 0. I09 k 0.009 0.183 f 0 . 0 1 3 0. I84 t 0.010 0.153 t 0.008 0.128 t 0.006 0 854 0 875 0 921 0 929 0 941 12.3 16 20 24 28 684fO4 1803tl 7 404 t 5 0 715 f 10 0 I082 f I3 0 61 4 f 0 1 1504fO I 318 2 2 0 5 520 9 f O 9 7536t12 7.0 f 0.4 29.9f 1.6 86 ? 4.0 195t9.0 328 ? 11.0 0.257 f 0.020 0.215 t 0 . 0 1 5 0.146 k 0.01 1 0 184fO.011 0.171 ?00.008 0 852 0 878 0 865 0 900 0 938 12.3 16 '0 24 28 I09 9 f 2 4 258f4O 439 f 5 0 789 ? I 1 0 l304+130 6 4 5 t O 1 45.3 f 2.3 70 f 3.0 I87 3 f 0 2 76 f 4.0 3627f06 5 8 0 9 f . 0 8 208 f 10.0 8 8 6 0 t l O 417f120 0.773 k 0.004 0.405 f 0.023 0. I I4 f 0.009 0.235 t 0.015 0239t0009 0 946 0 919 0 849 0 898 0960 zyxwv zyxwvu + zyxwv zyxw zyxwvu zyx zyxwvu 320 A.M. RAMOS and A. IBARZ The results were tested against two models (Weltman, and Figoni and Shoemaker). A better fit was obtained with the Figoni and Shoemaker model in all cases, so only the values obtained by this fit are given. Thixotropic parameters of the Figoni and Shoemaker model, as also the determination coefficients, are listed in Tables 2 and 3. The fits and the estimates of the parameters, sol, a, (aol - a,) and k,are significant at the 95% probability level. zyx TABLE 3. FIGONI AND SHOEMAKER MODEL PARAMETERS FOR QUINCE PUREE AT CONCENTRATION SOLUBLE SOLIDS OF 28 "BRIX AT SHEAR RATES AND DIFFERENT TEMPERATURES Y T 601 (s-') ("C) (Pa) 7.2 0 5 10 20 14.4 0 5 10 20 28.8 0 5 10 20 57.6 0 5 10 20 1042f 923f 837f 770f (Je 001 (Pa) 9 7 9 8 731f1 657 f 1 596 f 1 536 f I 1153f 11 1024 k 1 I 920 f 1 1 843f 9 808 f 1 699 f 1 663 f I 620 f 1 I309 f 12 1224 k 14 I082 f 13 993 f 10 888f 1 826 f 1 754 1 694 f 1 1463 f 19 I439 k 19 I304 f 13 1123f12 1042 f 2 992 -+ 1 886 f 1 811fl * - (Je kl (Pa) 312f 267f 241 f 233f 7 6 8 7 * 0093 0.003 0.088 k 0.003 0.1 15 f 0.006 0.100 f 0.004 1 R? 0.966 0.970 0.936 0.952 zyxw zyxw 344f 9 326 f 10 286f 9 223f 8 0.122 f 0.005 0.124 f 0.005 0.128 f 0.006 0.1 12 f 0.006 0.958 0.95 1 0.941 0.933 422 2 1 1 399 f I3 3285 11 299 f 10 0.161 f 0.006 0.170 f 0.008 0.17 1 f 0.008 0.195 f 0.008 0.965 0.995 0.938 0.949 421 f 17 447f 18 417f 12 0.188f0.010 0.255 k0.013 0.239 f 0.009 0.199f0.010 0.925 0.931 0.960 0.935 312k11 For purCe at lOC, the equilibrium shear stress, a,, increases with the increase at soluble solid concentration, such as with that of shear rate. For samples shearing at shear rate of 7.2 s", the equilibrium shear stress increases rapidly from 40.82 Pa, at 12.3 "Brix, to 596.0 Pa at 28 "Brix, while the equilibrium stress increases to 886 Pa at shear rate of 57.6 s-'. This means that the prolonged shear has different effects in accordance with the concentration of quince puree. Similar behaviour were obtained for purees and derived fruits (Chiralt et af. 1991; Lozano and Ibarz 1994). zyxwvu zyx z zyxw zyxw zyxwvu ORANGE CONCENTRATE AND QUINCE PUREE 32 1 For all concentrations of quince puree the parameter that measures the structural break-down the shearing (a,,- a,) increases of progressively with soluble solids content and with increasing shear rate. At a shear rate of 7.2 s-I, the parameter u,, - u, increases from 3.55 Pa at 12.3 "Brix to 241 Pa at 28 "Brix, and to 417 Pa for the shear rate of 57.6 s-I. The values of parameter ao,u, indicate that thixotropy of quince puree has an important magnitude. Table 2 shows that the constant k,, structural degradation rate, was almost unchanged with concentration when the sample was shearing at 7.2 and 14.4 s-I; while with higher shear rate, 28.8 and 57.6 s-', values k, decreases with the increase of the concentration. Shear rate being 57.6 s-', ki value varies from 0.773 s.I for 12.3 "Brix to 0.239 s-' to 28 "Brix, which indicates that the speed at which the structure for of the sample of 12.3 "Brix is destroyed is higher, than for 28 "Brix. Similar behaviour is observed for orange juice at concentrations of 55 and 60 "Brix in this work and in peach puree by Lozano and Ibarz ( 1994). Table 3 shows that a,, a,, - a,, have a similar behaviour to that obtained for orange juice. Those parameters decrease with the increase of temperature, while the constant k, increases with the increase of temperature. This behaviour was also observed by Pascual (1974) and Alvarez et af. (1989). zyxw zy zyx Comparison Between Quince Purke and Orange Concentrate Structural Microscopic analysis. Quince puree shows a greater thixotropic behaviour than orange concentrate. This could be caused by the higher level of solid soluble, fibers, pulps and pectins in quince puree. Costell ef al. (1982), Ibarz and Lozano (1992) obtained similar behaviour when studying texture and thixotropy, respectively, in fruit purees. For a more complete study of the thixotropic behaviour of the two samples, microscopic observations of their structures were carried out (Fig. 1). While orange pulp micrograph (Fig. la) showed a uniform and regular distribution of particles, quince pulp (Fig. lb) had longer particles and a heterogeneous fiber microscopic structure. As both orange and quince pulps were manufactured in the same processing line, differences in the microscopic structure could be attributed to inherent nature of cellular tissue for each fruit. These studies confirm the greater degree of thixotropic behaviour of quince puree. Lozano and Ibarz (1994) obtained similar behaviour studying thixotropy in plum and peach purees. ACKNOWLEDGMENT The author A . M . Ramos is very grateful to CoordenaGiio de AperfeiGoamento de Pessoal de Nivel Superiorde Brasil -CAPES- and Federal University 322 A.M. RAMOS and A . IBARZ zyxw of ViCosa-UFV, Brazil for the financial support received while at University of Lleida, Spain. zyxwv zyxw FIG. I . MICROGRAPH OF CONCENTRATED ORANGE (a) A N D QUINCE PUREE (b) PULPS Marked bar is equivalent to 50 pm. zyxw zyxwvu ORANGE CONCENTRATE A N D QUINCE PUREE 323 REFERENCES ALONSO, A.L., LARRODE, 0. and ZAPICO, J. 1995. Rheological behaviour of infant foods. J. Texture Studies. 26, 193-202. ALVAREZ, A . I . , MELCON, B., COSTELL, E. and ZAPICO, J. 1989. Evaluacion de la tixotropia en leche condensada. Influencia de la temperatura en el comportamiento reologico. Rev. Agroquim. Tecnol. Aliment. 29(4), 519-529. 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