RHEOLOGICAL CHARACTERIZATION OF KETCHUP ABSTRACT

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1 RHEOLOGICAL CHARACTERIZATION OF KETCHUP P. BOTTIGLIERI, F. DE SIO, G. FASANARO, G. MOJOLI, M. IMPEMBO and D. CASTALDO, Accepted for Publication May 8, ABSTRACT Ketchup flow properties were investigated with a concentric cylinder viscometer in the temperature range.from 30 C to 80 C. A partial characterization of the product viscoelastic behavior at 30 C was also made by Strain test and Frequency test. The Harper and El Sahrigi equation (7 = acj eedrt), as well as the equation that links the apparent viscosity of ketchups to the serum viscosity and to the pulp content (% = qserum +A(%PJB), do not describe adequately the ketchup flow behavior that we found to be highly affected by the production technology and by the raw materials used. INTRODUCTION The flow behavior of fluid foods (fruit juices, purees and tomato pastes), which are mostly non-newtonian, has been the subject of numerous studies (Duran and Costell 1982; Costell and Duran 1983; Rao and Coley 1983; Gherardi et al. 1985) due to its impact on production technology. Among food fluids, tomato pastes are particularly important for their use in sauces and similar products. However, a relatively small amount of data is present in literature concerning flow properties of products like ketchups (Upsana Rani and Bains 1987). The goals of this research have been the following: (1) Characterization of ketchup rheological behavior by rotational viscometry. (2) Characterization of ketchup viscoelastic behavior by Strain test and Frequency test. (3) Correlation between rheometric parameters and other product characteristics. Experimental Station of the Food Preserving Industry (Parmu) Department of fruit juices and tomato products-via NuZionale 12 1/123-ANGRI (SA) ITALY. ENCO s.r.1. Via Parroco, I I-CHIRIGNAGO (VE) ITALY. To whom correspondence should be sent. Journal of Food Quality 14 (1991) All Rights 1991 by Food & Nutrition Press, Inc., Trumbull, Connecticut. 491

2 498 P. BO'ITIGLIERI er al. (4) Testing the applicability of the Harper and El-Sahrigi equation (77, = acb eeat) and of the equation describing ketchup viscosity as a function of serum viscosity and of pulp content (q, = qserum + A(%P)B) in order to describe the ketchup flow behavior. MATERIALS AND METHODS Samples of nine brands of ketchups, purchased in the supermarket, were examined for chemical and physical properties. Potassium sorbate (1000 p.p.m.) was added to all samples to avoid microbial spoilage. Analytical Methods Total solids (TS) were determined by oven drying under vacuum at 70" until constant weight was obtained. Percent of wet weight pulp (%P) was determined after centrifugation at xg for 15 min. Total pectic substances (TP) (g/( loog of dry-matter)) determined as described by Rouse and Atkins (1955). Briefly, the pectic substances were precipitated by mixing ketchup with ethyl alcohol 95% (1 : 10 wh). The suspension was centrifuged and the residue washed with 95% ethyl alcohol. After centrifuging again, the residue was added with 5 ml NaOH (1 M), brought to 100 ml with distilled water and allowed to stand for at least 15 min with occasional shaking. The solution was filtered and the filtrate used for the colorimetric determination. Carbazole (0.1 g/100 ml EtOH) (0.5 ml) and concentrated sulfuric acid (6 ml) were added per ml of extract. The red color produced was measured photometrically and compared to a standard curve obtained using galacturonic acid monohydrate as a standard. Bostwick consistency (Bc), reported in cm/30s, was determined by a Bostwick consistometer as described by Bourne Serum density (DS) was determined by a pycnometer at 25 C f 0.1 "C. Viscosity Measurements The viscosity measurements were performed with a Haake mod. RV20 viscosimeter equipped with the two working head M5OSC and CV20. The M5OSC head was equipped with the Haake MVII and the Haake NV sensors. The MVII rotor was used for the ketchup samples in the rotational regimes. Measurements on each sample were taken three times at four different temperatures (30 C 45"C, 60 C and 80 C f 0.l"C) in the range s-', The NV rotor was used for serum (obtained from depulped ketchups centrifuged at 12,500 x g for 15 min).

3 RHEOLOGICAL CHARACTERIZATION OF KETCHUP 499 All measurements were made in a thermostatic bath. The viscoelastic behavior was studied with a CV20 head equipped with two sensors which had different geometries, i.e., coaxial cylinder with conical ending (Haake ME30) and plate-plate (Haake PQ30). The viscoelastic measurements were made only at 30 C. Time-Dependent Experiments Time dependence of ketchup viscosity was determined by measuring the velocity of the structure build-up and breakdown. Briefly, the stress value in a concentrated fluid food is dependent by large scale structure whose dimension depends on the shear history of the product (Firth and Hunter 1976) (Wagstaff and Chaffey 1977). When the fluid is subjected to a high deformation rate, the surviving structure is smaller in size. Therefore, as a consequence of a sudden decrease of shear rate (y), the apparent viscosity (y) and the shear stress (7) firstly undershoots the equilibrium value and then slowly increases. This happens because the structure becomes larger as it approaches the equilibrium dimension corresponding to the new applied shear rate. The time occurring to develop the gross structure is a measure of the product thixotropy. A typical two step procedure (5 or 500 s-i) was employed to study the time dependence of the product viscosity (Table 1). Dynamic Oscillatory Experiment (Frequency Test and Strain Test) The dynamic oscillatory experiments were performed at a temperature of 30 C. When a sinusoidal strain, having a fixed amplitude and frequency, is applied to a viscoelastic material, there is a phase lag between the shear stress response and the applied strain. The phase lag is measured by phase shift 6, defined as 6 = arctg G"/G' or 6 = TABLE 1. TIME DEPENDENCE TEST OF KETCHUP VISCOSITY SEGMENT TYPE SHEAR RATE TIME TEMPERATURE (sec-') (min) RANGE ('C) 0 Start-segment Jump Eta-Time Jump Eta-Time Jump Eta-Time 5 - >

4 500 P. BO'ITICLIERI et ul. arctg r]'/r]", where G' is the storage modulus (elastic component), and G" is the loss modulus (viscous component), r]' and r]" are the components of the complex viscosity r]* - r]' - i 7". For a viscous fluid (Newtonian), r]' = r]" = o G' = o 6 = 7712 For an elastic fluid (Hookian), 7' = o G' = G* G" = 0 6 = o For a viscoelastic fluid, o < 6 < r/2, and S is a direct index of the relative importance of both the viscous and the elastic component. A frequency test was made to evaluate the viscoelastic behavior of the product. The oscillation frequency ranged from 0.1 to 5Hz, and the amplitude was fixed at a constant value of 2". A strain test was made both to evaluate the product behavior at increasing strain and to find the range of the linear viscoelasticity. The strain angle ranged from 0.5 to 10" and the frequency was 5Hz throughout the test. RESULTS AND DISCUSSION Correlations Between Physico-Chemical Data Table 2 lists the analyzed composition of the investigated samples. As expected, a good correlation was found between DS and TS (Table 3) for which R2 is Instead, no linear correlations between other chemical and chemical-physical parameters was found. The low correlation values (.0206 I R2 I ) demonstrate that the behavior of samples having tomato as the principal ingredient is not interpretrable in terms of any linear dependence between the various parameters reported in the Table 3. Rheological Behavior of the Ketchups The classical approach to characterize structural breakdown in a fluid is the measurement of the hysteresis loop, as suggested by Green and Weltmann (1943). If a sample is shear sensitive, the two curves, obtained at increasing and decreasing shear rate, do not coincide, but they depict the hysteresis loop, typical of the thixotropic substances. All samples we tested showed very small hysteresis areas (Fig. 1) at higher gradients, probably due to liquid-phase separation by centrifugation rather than to thixotropic behavior. A typical flow curve obtained on the sample 1 of Table 1 at the temperature of 30" k O.l"C, using a MVII rotor, is shown in Fig. 1. Measurements of r], versus time (timedependent experiment), for the same sample, were taken to confirm the lack of thixotropy and the independence from the time (Fig. 2). In Fig. 2 the fast

5 TABLE 2. ANALYTICAL CHARACTERISTICS OF TOMATO KETCHUP SAMPLE Bostwick Sample consistency Serum-Density Pulp Total Solids Total Pectin - (BC)- - (US)- -(%PI- - (TS)- - (TE)- no (cm) (9/cm3) (w/v) % (g/ (1OOg of dry matter) 1

6 502 P. BOTTIGLIERI et al. Parameters BC TP %P TS DS BC TP %P TS DS , Each element of the table represent the R2 value of the least square analysis of the linear fit between the corresponding parameters. upstart of the product clearly appears. To evaluate the variations of G and G with shear and shear rate, we did measurements under oscillatory regimen (strain test and frequency test). In the Strain test, the samples were tested changing the strain angle in the range from 0.5 to 10 degrees at a frequency of 5 Hz with a PQ 30 sensor. From the plot reported in Fig. 3 the nonlinearity of G and G all over the measurement range is observed. In other words, there is the absence of a linear elasticity region in the deformation range we explored. However, as seen in Fig. 3, when the strain becomes greater by increasing the angle, the ratio of the storage modulus (G ) to the loss modulus (G ) decreases and the phase shift angle (6) approaches to 90. This relationship means that by increasing the strain, the product looses its elasticity and approaches Newtonian behavior. All the products we tested showed this effect. This behavior can be ascribed either to a less structured product or to a structural instability in the strain range we choose for the measurements. The plastic behavior of the products can be evidenced under continuous shear from the analysis of the mechanical spectra obtained by scanning an imposed oscillation frequency (frequency test) An an example, Fig. 4 reports the traces of G, G and 6 as a function of w for sample 1 of Table 2. As seen in Fig. 4, by changing the shear rate, the product has a constant behavior. The profiles of G and G are parallel, with a slight dependence on the frequency. Both G and G show a tendency toward a limiting value when w approaches zero, while the phase angle remains almost constant. The samples tested in this experimental range showed storage modulus always lower than loss modulus.

7 RHEOLOGICAL CHARACTERIZATION OF KETCHUP m 8 55 K c m 0 FIG. 1. TYPICAL FLOW CURVE OF KETCHUPS MEASURED BY HAAKE ROTATIONAL VISCOMETER, AT THE TEMPERATURE OF 30 f 0. I T, USING A MVII ROTOR 11.0 A A AAA AAAA A ooooc I g 55 m 8 z > 0.0. I,eeeeeepeee

8 504 P. BOTTIGLlERl ef a/ l FIG. 3. STRAIN TEST : CHANGE OF THE SAMPLE WITH STRAIN ANGLE IN THE RANC GRADES AT 5 HZ (0 o o 0 ) = phase angle shift 6 (I...). loss modulus G (0 0) storage modulus G - P 100 = t e - o o ~ ~ o o ooooo o o 0000 t D ; I I....o b Y - I a a.. 4 In v w E 1 I I I l I I I I I I I I I I I I I I IIIIL 0 FR EOUENCY --w - (Rad.8- ) FIG. 4. FREQUENCY TEST : CHANGE OF THE SAMPLE WITH FREQUENCY IN THE RANGE Hz AT A CONSTANT OF 2 GRADES (0 ) ) phase angle shift 6 (iimm) loss modulus G (0 0) storage modulus G

9 RHEOLOGICAL CHARACTERIZATION OF KETCHUP 505 Studies on the Applicability of the Power Law Model and of the Herschel-Bulkley Model The power law equation (T = ky") was found to be an adequate model to describe the flow behavior of the samples (Table 4). Value of RZ were greater than The flow behavior index (n) was found to be between 0.23 and 0.30, with an average value of These values are less than those reported by Upsana Rani and Bains (1987). The flow behavior index was found to be temperature and concentration independent. Consistency index lies in the range between 6.47 and NS"/m2. As the temperature increases, the consistency index substantially decreases. With regard to the Herschell-Bulkley model (T = TO+k,ynl), Table 5 lists the yield stress values obtained by the model application on the ketchup samples. The yield stress (7,) ranged from to N/mZ. The Herschel-Bulkey model was found to adequately describe the rheological parameters for ketchup flow. Value of R2 improved as the yield stress increased. The same considerations previously made for n and K in the power law model hold also for n' and K' in the Herschel-Bulkley model. The yield stress (TJ was found temperature dependent as already observed by Trifiro et al. (1985) and Saenz and Costell (I 986); i.e., it strongly decreases when the temperature rises. The unusually low correlation we observed for the various ketchups versus others physico-chemical product parameters, i.e., total solids, pectin content, percent of sedimentable pulp (see Fig. 5), can be attributed to many factors. These factors include the heterogeneity of the materials constituting the products (Leoni et al. 1984), the difference in the product formulation between cold and hot breaks (these last generally show a wide range of viscosity as reported by De Sio et al. 1986), the different homogenization degree (Kertesz and Locoonti 1944; Crandall and Nelson 1975; Crandall et al. 1988; Castaldo et al. 1990) and finally the difference among the products in their pectic content (Luh and Daoud 1971). However, we found a good correlation between 7, and Bostwick consistency by using the exponential equation = ~f~~ (see Fig. 5), where A and B are constants, (for T = 30 C, A = 46.59, B = , R2= ), determined by the curve-fitting, and Bc is the Bostwick consistency. This finding confirms that the yield stress (T,), as already reported by Rao et al. (1983) for tomato concentrates, can be conveniently used as a quality control instead of the Bostwick consistency to which it is correlated. The Bostwick consistency is not a convenient index of product quality for two reasons. This parameter cannot be converted in fundamental rheological parameters inasmuch as it is influenced by many factors, such as superficial tension, wetting power and so on. Moreover, when the product concentration is high, it approaches a limiting value. The correlation between the yield stress T, and the product structure was already observed studying the effect of the homogenization pressure on the

10 TABLE 4. VALUE OF RHEOLOGICAL PARAMETERS THAT IDENTIFY THE FLOW OF TOMATO KETCHUPS AT DIFFERENT TEMPERATURES P W 0 2 $ R E a a - Sample no P O W E R L A W M O D E L T = 3OoC T = 45OC T = 6OoC T = 80 C n K n K n k n k

11 TABLE 5. VALUE OF RHEOLOGICAL PARAMETERS THAT IDENTIFY THE ITOW OF TOMATO KETCHUPS AT DIFFERENT TEMPERATURES ' ' Sample no H E R s c n E L-B u L K L E Y n o D E L T = 30 C T = 45 C T = 6OoC T = 80 C "1 kl % "1 kl 50 "1 kl 5, "1 kl ZO ,

12 508 P. BOTTIGLIERI er al I 1 1,Pectin (g%ts), o Bostwick (cm) L I I I I % Pulp (w/w) I FIG. 5. INFLUENCE OF PECTIN (A-A), BOSTWICK CONSISTENCY ( ) - LINE (********) AND PULP PERCENTAGE (w/w) (0-0) ON 70 (YIELD STRESS) AT T = 30 C ketchup consistency (D. Castaldo et al. 1990). Although the homogenization pressure does not alter the chemical composition of the product, it strongly effects the sedimentable pulp content which increases with pressure, at constant temperature, up to a value of 35%. The good correlation we found between the yield stress, To, and the Bostwick consistency, together with the observation that the G' and G" values approach a limiting value when the frequency, w, goes to zero (typical of a network structured product) demonstrate that T, can be a measure of the product cross-link.

13 RHEOLOGICAL CHARACTERlZATlON OF KETCHUP 509 Serum Rheological Behavior The sera obtained after centrifugation at 12,000 x g for 15, resulted to be Newtonian fluids, at the explored temperatures, for all the ketchups we examined. The low plasticity values (7" I 1.1 Nm-') we found have a little attendability due to the instrumental sensitivity in this range. The absolute viscosity values ranged from 4 to 25 mpa's. Effect of Temperature and Concentration on the Apparent Viscosity For all the tested samples, the value of activation energy of flow Ed (KcaVmol) was evaluated using a linear regression between In q, and 1/T, where q. is the apparent viscosity (mpa*s) at the shear rate (y) of 100 s '. The value of E,% lies in the range t KcaVmole. The Arrehenius relationship can be used to represent the effect of temperature on the rheological behavior: where E, is the flow activation energy (Kcal/mole), R is the gas constant ( cal/gmole), T is the absolute temperature and A is a constant. In addition, the following relationship could be used to correlate apparent viscosity and concentration.?-a = A1 x c A2 A single equation combining the effects of temperature (Eq. 1) and concentration (Eq. 2) at a fixed shear rate, was proposed by Harper and El Sahrigi (1 965): In order to obtain the values of, a, I3 and Ed, a multiple regression analysis was used at 4 = 100 s I. The analysis was made using as concentration parameters both total solids content and sedimentable pulp percentage. In both cases, low correlation coefficient values (Table 6) were found, due to the different composition of the samples. To correlate the apparent viscosity values of ketchup with its chemical and physical characteristics, the model proposed by Tanglertpaibul and Rao, was

14 510 P. BOTTIGLIERI el al used. This model comprises two different contribution, i.e., serum viscosity and pulp weight percentage (qketchup = qserum+a(%p)b). The low correlation coefficient values we found, confirm that also this model is not adequate to describe the ketchup flow properties (Table 7). TABLE 6. MAGNITUDES OF COEFFICIENTS IN HARPER AND EL SAHRIGI MODEL BARPER and EL SAHRIGI Model CONCENTRATION a B E, R2 (Kcal/mole) Total solids Pulp percentage (w/w) TABLE 7. REGRESSION COEFFICIENTS FOR THE RELATION: Temperature ("c) A B R CONCLUSIONS Ketchups are time-independent non-newtonian fluids that show a small thixotropy. The differences among the samples we examined regarded essentially the viscosity and yield stress values. We were not able to completely characterize the products in an oscillatory regimen inasmuch as we did not find a region of linear viscoelasticity in the explored strain range. The results show that ketchup have a viscoelastic behavior similar to the gels (cross-linked substances). It is dif-

15 RHEOLOGICAL CHARACTERIZATION OF KETCHUP 511 ficult to correlate ketchup reometric parameters (n, k and T ~ with ) other chemicalphysical properties (Bostwick, total pectin, pulp percentage, total solids) probably due to the effect of different production technologies on the ketchups fluidodynamic properties. REFERENCES BOURNE. M.C Food Texture and Viscosity: Concept and Measurement. pp , Academic Press, New York. CASTALDO, D., LEONI, C., DE SIO, F., FASANARO, G., RILLO, L. and VILLARI, G Sull omogeneizzazione del ketchup: Aspetti chimico-fisici e technologici. Ind. Cons. 65, COSTELL, E. and DURAN, L Physicochemical rheology of fruit juices and purees. Review paper-fluss. Obst. 12, CRANDALL, P.G., DAVIS, K.C., CARTER, R.D. and SADLER, G.D Viscosity reduction by homogenization of orange juice concentrate in a pilot plant taste evaporator. J. Food Sci. 53, CRANDALL, P.G. and NELSON, P.E Effect of preparation and milling on consistency of tomato juice and puree. J. Food Sci. 40, DE SIO, F., BOTTIGLIERI, P., LEONI, C., ACASTALDO, D., TRIFIRO, A. and FORTINI, G Comportamento reologico di passati di pomodoro ottenuti mediante trattamento termico Hot Break. Ind. Cons. 61, DURAN, L. and COSTELL, E Rheology of apricot puree: Characterization of flow. J. Texture Stud. 13, FIRTH, B.A. and HUNTER, R.J Flow properties of coagulated colloidol suspensions 111. J. Colloid Interface Sci. 57, GHERARDI, S., CASTALDO, D. and TRIFIRO, A Effect of liquefying enzymes on pear purees: relationship between analytical composition and rheological properties. Flussiges Obst. 12, and GREEN, H. and WELTMANN, R.N Analysis of the thixotropy of pigment vehicle suspensions: Basic principles of the hystensis loop. Ind. Eng. Chem., Anal. Ed. 15, 201. HARPER, J.C. and EL SAHRIGI, A.F Viscometric behaviour of tomato concentrates. J. Food Sci. 30, KERTESZ, 2.1. and LOCOONTI, J.D Factors determining the consistency of commercial canned tomato juice. Bull. 272, New York State Agr. Expt. Sta., Geneva, New York. LEONI, C., ALDINI, R., FORTINI, G. and ZANOTTI, A Sulle modalith di produzione del passato di pomodoro. Ind. Cons. 59, LUH, B.S. and DAOUD, H.N Effect of break temperature and holding time on pectic and pectic enzymes in tomato pulp. J. Food Sci. 36, RAO, M.A. and COOLEY, H.J Applicability of flow model with yield for

16 512 P. BOTTIGLIERI et al tomato concentrates. J. Food Proc. Eng. 6, ROUSE, A.H. and ATKINS, C.D Bulletin 570, University of Florida, Agricultural Experiment Station, Gainesville, Fla. SAENZ, E. and COSTELL, E Rheological behaviour of lemon products. Effect of temperature and concentration. Rev. Agrochim. Tecnol. Aliment. 26, TANGLERTPAIBUL, T. and RAO, M.A Flow properties of tomato concentrates: Effect of Serum viscosity and pulp content. J. Food Sci. 52, TRIFIRO, A., CASTALDO, D., GHERARDI, S. and MILIOLI, M.C Impiego di enzimi fluidificanti per la produzione di puree concentrate di mela. Ind. Cons. 60, UPSANA RANI and BAINS, G.S Flow behaviour of tomato ketchups. J. Texture Stud. 18, WAGSTAFF, I. and CHAFFEY, C.E Shear thinning and thickening rheology I and 11, J. Colloid Interface Sci. 59,53-83.