the effect of ph and temperature on the rheological behavior of dulce de leche, a typical dairy...

8/13/2019 The Effect of pH and Temperature on the Rheological Behavior of Dulce de Leche, A Typical Dairy Argentine Product

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The Effect of pH and Temperature on the Rheological Behaviorof Dulce De Leche, A Typical Dairy Argentine Product

CLARA 0 ROVEDO, PASCUAL E. VIOLLAZ, and CONSTANTINO SUAREZDepartamento de lndustriasFacultad de Ciendas Exactas y Naturales

Ciudad Universitaria1428, Buenos Aires, ArgentinaABSTRACT

The rheological behavior of commer-cial samples of Dulce de Leche (sweetmilk for household and confectioneryuse was studied Both products have thix-otropic and pseudoplastic characteristics.Apparent viscosity, determined at 25, 40,and W C , demonstrated that t h is magni-tu& decreased with increased tempera-ture. Activation energies were 9.9 and10.4 kcaYmo1 for household and confec-tionery t y p e s , respectively.Apparent viscosities of householdsamples were detennined at pH = 3.0,6.1, and 8.3. At acid pH, the apparentviscosity was lower than that of the origi-nal sample (pH = 6.1 ; at alkaline pHviscosity was higher than that of theoriginal. The effect of pH becomes morepronounced with time.From shear stress and shear rate flowdiagrams, the Herschel-Bulkley modelwas adequate for both types of Dulce deLeche investigated.(Key words: rheology, Dulce de Leche,Argentina)

INTRODUCTIONDulce de Leche is similar in composition tosweetened condensed milk and is typically pre-pared with milk concentrated at atmosphericpressure in the presence of added sucrose. Asmall amount of sodium bicarbonate is alsoadded to prevent protein coagulation. There aretwo ypes of this product: Dulce de Leche forhousehold use and of confectionery type. Thefirst is generally used as a dessert, and the

Received April 19, 1990.Accepted December 3, 1990.1991 J Dairy Sci 741497-1502

second is preferred for confectionery purposesbecause of its greater viscosity. This increasedviscosity results from the starch added duringits preparation.An important characteristic of Dulce deLeche is the texture, because textural defectsoften render the product unacceptable to theconsumer. Rheological characterization of theproduct is also important for operating evapora-tion equipment as well as for handling (12).Little information exists in the literature con-cerning the rheological behavior of commercialDulce de Leche. Most studies were conductedwith milk concentrates, with and without sugaraddition, and total solid contents less than 30(5). However, Hough et al. (9) has observedthat the flow properties of Dulce de Leche aremarkedly different from that determined byHiggs and Norrington (8) for sweetened con-densed milk. As pointed out by Hough et al.(9), such differences are due to the total con-centrationof solids, which in the case of Dulcede Leche, may vary from 54 to 70% dependingon the method of preparation.The objective of this study was to measureflow properties of commercial samples ofhousehold and confectionery Dulce de Leche,to evaluate the effect of temperature (25 to55'C) on rheological behavior, and to predictflow properties near the processing tempera-tures and for the cooling process.As pH has an important effect on proteinstability, its effect on the flow curves was alsoinvestigated using commercial samples ofhousehold DuIce deLeche. A wide range of pHwas investigated, even though alkaline pH is oflittle practical interest.

MATERIALS AND METHODSThe Dulce de Leche used for the experi-ments was manufactured by Gandara S. A.(Provincia de Buenos Aims, Argentina) and

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ROVEDO ET AL.498purchased in a supermarket. The compositionof household Dulce de Leche was 80 milk,16% sucrose, and 4 glucose syrup; the con-fectionery type has a similar composition plus2% starch as thickener.Total solid contents forboth samples was 71 . Seventeen samples,including six for replications, were tested.The pH of homogenized samples of Dulcede Leche was measured with a glass electrodeon a Metrohm pH meter E632 previously cali-brated with appropriate buffer solutions. Sam-ples of Dulce de Leche for household use wereused to study the effect of pH on the rheologi-cal curves. Four samples of different pH (3.0;4.7; 8.3; and 9.4) were prepared by adjustmentwith 10% HC1 and 10%NaOH solutions. ThepH of the untreated or original sample was 6.1.

The effect of time on the rheological behav-ior of Dulce de Leche at each pH condition wasalso studied. For each pH, the rheological curvewas measured after 24,48,72,96, and 120 h ofstorage at room temperature. No appreciablechange was observed.Vlscorneter

The flow properties of Dulce de Leche weremeasured with a concentric cylinder viscometer(Haake RV 12 Rotovisco) with a MOO mea-suring head. The h4V II type sensor system wasused for household type Dulce de Leche andthe MV III sensor system was used for theconfectionery type. It was not possible to use

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the same sensor for both products because con-fectionery Dulce de Leche was too viscous toobtain results within the shear rates range in-vestigatedThe viscometer was operately automatically.The shear rate was varied from 0 to 32 rpm.Shear s t r e s s versus revolutions per minute dia-grams were obtained in the following manner.Speed was continuously increased from 0 to amaximum speed in 1.5 min while simultaneously recording torque, then immediatelydecreased to the starting point (3 Wcycle).For each tested sample, this procedure wascontinuously performed until the ascendingspeed curve coincided with the descending oneto verify that thixotropic breakdown had beeneffective. nce such condition was reached, therevolutions per minute (W) and torque (S) ead-ings of the viscometer were converted to shearrate @) and shear stress 2) by means of thefollowing equations:

2 (Pa) = A x SD d) M x Wwhere the values of A and M are provided bythe manufacturer for each sensor system. Ap-parent viscosity P a s ) was obtained fromq = T/D.

RESULTS AND DISCUSSIONCharacteristic flow curves, in terms oftorque versus rotation speed corresponding toDulce de Leche for household use, are shownin Figure 1. It was observed that the major

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2 l i ~ 6 ; 16imelsl l 150 203-~---___5Figure 1. Experimental r h g ~ a m w e d by increas-ing and decreasing shear rate (3 min per cycle) of thesample of Mce de Lcche for household usc at 25C andpH 6.1. shear rateFigure 2 . Variation of shear sl ress with lime at constant

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RHEOLOGY OF DULCE DE LECHE 1499

OO 0 20 30Shear r o t e , D I


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1500 ROVED0 ET AL.TABLE 1. Rheolonical model, fitting parameters, and mean relative deviation modulus for Dolce de -he.'

Household o n f e c t i O n t r yTempera-Rheological model tare Q Po n P%2 b Po n p46( c)25 45.46E = v J + l b . D n 40 1 8 2(Herschel Bulldey) 55 9.6525 2657

e = + k D p 40 10.52(Heinz-Casson) 55 7.0625 56.81

22.19(Casson) 55 9.7725 .84~ = % + p , y D 40 39.1(Bingham plastic) 55 17.08

T~ = E (Ib-D) 40

25 0(Dower law) 55 0E = p(yLP 40 0

28.3815.695.7.86.7 1.5

2.451.84.775.13.81.62

57.5829.14

.6 84.6 1 .65.65 1.39

.3 1.38

.3 .89.4 1.512.89

.5 2.18.5 1.841o 7.831o 6.311o 5.75.45 6.06.46 4.38

4574.850. . .. . .. . .

47.8422.918.1279.4639.4114.7300

18.3 .43 .7836.7 .47 1.2415.76 .47 1.37. . . . . . . . .. . . . . . . . .. . . . . . . . .6.09 4.943.5 4.561.6 522

13.58 1.0 8.617.42 1.0 8.313.2 1.0 9.3582.62 .42 .7341.24 .44 1.47

~ 12.7 .46 5.67 0 15.68 .48 1.31lRheolo@cal parameters: EO = yield stress; p~ = consistency index; D = shear rate; and n = flow behavior index.~ P S predicted percentage.

reach the thixotropic breakdown. This resultagrees with that reported by Hough et al. (9).who found that during Dulce de Leche prepara-tion (temperature near ZWC), the productlosesits thixotropic characteristic.Flow c w e s of three original samples atdifferent temperatures (household Dulce deLeche) are shown in Figure 5 . The apparentviscosity was calculated for a shear rate rangefrom 1.35 to 28.35 s-l and correlated accordingto Arrhenius relationship:r( = A expWT [31

where E, is the activation energy (kcaVmol), Ris the gas constant, T the temperaturerK), ndA a constant. The value of E,was determinedat different shear rates from regression analysisof In q versus lfl. The mean value of was9.9 f 4 kcal/mol. The effect of temperature onthe flow curves of confectionery Dulce deLeche is shown in Figure 6; the meancorresponding to t h i s product was 10.4 f 3kcal/mol.Modeling of the Flow Curves

The flow curves of the original samples ofDulce de Leche (household, pH = 6.1 and

confectionery, pH = 6.0) at 25, 40, nd 55'Cwere fitted by means of the following mathe-matical models: Casson (3), Bingham (2),power law or Ostwaldde Waele (ll), Heinz-Casson (He&, 6 . and Herschel-Bulkley (7).The five equations are given in Table 1. The

10 20 30Shear rate,O C dFigure 5 . Effect of temperature on flow curves of Dulcede Leche for household use (untreated sample, pH 6.1).

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RHEOLOGY OF DULCE DE LECHE 1501

6-bUU

0LcI

-5 10 15Shear rate, D(S1

Figure 6. Effect of temperalure on flow aw es of Dulcede Leche for confectionery use @H 6.0 .

values of the parameters corresponding to eachequation were obtained by a nonlinear regres-sion method and are also given in Table 1. Thefit of the models to the experimental data wascalculatedby means of the mean relative devia-tion modulus defined as follows (10):

where ~i = measured shear stress, Zpi = predicted shear stress, N = number of observa-tions. The P values equal to or less than 5can be considered to represent a good fit to thedata (10). The comparison between experi-mental data and predicted curves is shown inFigure 7 or householdDulce de Leche in orderto give a clear picture of how well each modelfits. From probability values given in Table 1,for household Dulce de Leche, the Herschel-

10 20 30Shear ro t e D I s

Pigum 7. Cumparkon between experimental data andmathematical models for household Dulcc de Leche @H6.1, UT ; ower law (-h Haschel-Bulkley (-. . );Casson (-.-); Singham (---); and experimental (A).Bulkley model gave the best fit, although accu-rate fitting was also obtained with Heinz-Cas-son and Casson models. For confectioneryDulce de Leche, good fits were obtained withHerschel-Bulkley and power law models.Flow behavior index (n) resulting from thevarious tested models was less than unity, indi-cating that Dulce de Leche is a pseudoplasticfluid Data at 25, 40, nd 55C showed that nwas not appreciably affected by temperature.This weak influence of temperature on flowbehavior index was also observed by otherinvestigators for Dulce de Leche and other foodproducts (9, 13).

TABLE 2. Yield stress values.Tanperahm Household confecti~ilerycc) Pa)25 54.5 27.240 22.6 16.355 11.3 5.4

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1502 ROVED0 ET AL.The plastic behavior of Dulce de Leche wasdemonstrated; to initiate flow a shear stresshigher than a critical value, the yield stress, TO,had to be applied Yield stress values obtainedfrom the rheograms are shown in Table 2.Values in Table 2 differ from those calcu-lated from the models in Table 1. This lack ofagreement was also observed by other investi-gators (12) and may be attributed to the thix-otropic effect, which modifies the value of therheological parameters that define the flow be-havior. Each of the five models predictedvalues for both products which indicates that

this parameter depends on both the material andthe model selected. As was reported in theliterature (l l) , he values of TO calculated frommathematical models are not totally reliablewhen used for subsequent calculation of un-known rheological parameters.CONCLUSIONS

The apparent viscosity of Dulce de Leche(household use) is affected by temperature andpH of the sample. Acidification decreased theapparent viscosity, and alkaline pH increased it.For alkaline and acid samples the apparentviscosity increased with storage time.The Herschel-Bulkley model gave the bestfit for the flow data for household and confec-tionery Dulce de Leche. The comparison of theconsistency index,h, nd flow behavior index,n, for household and confectionery Dulce deLeche indicates that the latter has more consis-tency (larger h and is more pseudoplastic(lower n) because of the small amount of starchadded during its preparation. For the range ofshear rates investigated, the product showedplastic flow with measurable yield stress.

REFERENCESlBarbosa Canovas, G. V. and M. Peleg. 1983. Flowparameters of selected commercial semi-liquid foodproducts. J. Texture Stud. 14213.W New York NY.3Casson, N. 1959. A flow equ tion for pigmented-oilsuspension of the printing ink type.. Page 82 in Rhec-Press. New York. NY.4 Evans. E. W. 985. Interactions ofmilk components infood systans. Page 241 in Interactions of food c o m peents. G. G. Birch and M.G. Liodley, ed. ElsevierAppl. Sci. hbl., London, Engl. New York, NY.5 P d e z - M a r t i n , F. 1972. Influence of temperatureand composition on some physical properties of milkandmilk comxntrates. II. Viscosity. J. Dairy Res. 39:75.6Fincke, A. 1961. Beiirage ZUI Losung rheologischerProblane in der Schokoladentechnologie. Fh.D. Diss.,TH Karlsruhe. Germany (Quoting Heinz, 1959.)

7Hasche1, W. H., and R. Bulkley. 1926. Konziztem-mcssungen von gommibensollosugen. Kolloid-Zeitschr.39291; Proc. ASTM 26:621.8Higgs. S. J., and R J. Norrington. 1971. Rheologicalproperties of selected foods&. Rocess Biochem. 6:52.9Hough. G., 0. Mom, . Segura, and N. Calvo. 1988.Plow properties of Dulce de b h e , a typical Argentined iry product. J. Dairy Sci. 71:1783.10Loma1~0 , . I., A. S. Bakshi, and T.P. Labum. 1985.Evaluation of food moisture sorption isotherm equa-tions. Part II: milk, coffee, tea, nuts, oilseeds, spicesand starchy foods. LebensmWiss. Techuol. 18:118.11Ofoli, R Y., . G Morgan, and J. P. Steffe. 1987. Ag rheologicalmodel for inelastic fluid foodsJ. Texture Stud. 18:213.12Pauletti,M. .,A. Vernier, D. tecbina, N. Sabbag, E.Castelao. 1988. Caracterizaci6n reol6gica del Dulce de&he. Rev. Agroqah. Techool. Aliment. 28(2):303.13Ra0, M.A. M. C. Bourne, and H. J. Cwlcy. 1981.Flow properties of tomato concentrates. J. TextureStud. 12521.14Rciner,M. 1949. Deformation and flow. ewis, Lon-

15Sherman, P. 1970. Industrial rheology. Academic

2 B i ~ g b m , . C. 1922.Fhidity and plasticity. McGraw-

logy of dispersc systems. c. c. Mill, 4. ergamon

don, -1Press, New York, Ny; London, E n g L

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the effect of ph and temperature on the rheological behavior of dulce de leche, a typical dairy...

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