MEASURING ELONGATIONAL PROPERTIES OF MOZZARELLA CHEESE

MEASURING ELONGATIONAL PROPERTIES OF MOZZARELLA CHEESE

M.M. AK Istanbul Technical University

Chemical-Metallurgical Engineering Faculty Food Engineering Department

Maslak, Istanbul, 80626 Turkey

and

S. GUNASEKARAN

University of Wisconsin Agricultural Engineering Department

460 Henry Mall Madison, WI 53706

(Manuscript received April 12, 1994; accepted October 10, 1994)

ABSTRACT

A vertical uniaxial extension apparatus was developed for determining elongational properties of Mozzarella cheese over the course of one month of refrigerated storage. Cheese specimens suspended in hot oil (4OC) first 'underwent melting and then stretching under the influence of a constant force. Samples did not break even afier 400% extension. Strain rate and sample temperature both increased during a test. Transient elongational (TE) viscosity of Mozarella cheese decreased with increasing strain rate and sample temperature. Proteolysis of caseins in Moz- zarella, determined by gel electrophoresis, did not alter its TE viscosity to an extent measurable by our method.

INTRODUCTION

The investigation of elongational behavior of cheese, particularly Mozzarella, is of technological and scientific importance. Elongational properties will ultimate-

Journal of Texture Studies 26 (1995) 147-160. A11 Rights Reserved. 0 Copyright 1995 by Food & Nutrition Press, Inc., Trurnbull, Connecticut. 147

148 M.M. AK and S. GUNASEKAFUN

ly determine the behavior (quality) of cheese in many ingredient applications. However, measuring elongational properties is often a difficult task even in polymer rheology where the subject has received much attention due to its in- dustrial importance (Barnes et al. 1989).

In food rheology, elongational behavior of cheese has rarely been a subject of a fundamental rheological investigation (Olson and Nelson 1980). However, with the increasing use of cheese as a food ingredient, interest in elongational properties has grown in recent years. Some empirical methods were developed to determine stretching and melting of Mozzarella cheese (Kindstedt et al. 1989a; Pagliarini and Beatrice 1994). Recently a preliminary report was published on the objective measurement of stretchability of Mozzarella using a test developed to evaluate the spinnability of polymeric materials (Cavella et al. 1992). Apostolopoulos (19%) described a biaxial elongation (squeeze flow) test procedure to calculate the elongational viscosity of Mozzarella cheese.

Results of an attempt to develop an apparatus for measuring uniaxial elongational properties of Mozzarella cheese are reported in this study. The apparatus presented here may have potential for use in routine measurements in the dairy industry and also in scientific studies of cheese rheology.

EXPERIMENTAL

Sample Preparation and Test Procedure

Fresh (Q 3 days) rectangular blocks of brine-salted, low-moisture, part-skim Mozzarella cheese were obtained from a commercial cheese plant. Cheese blocks were stored in their original packaging in a refrigerator (642) until sample preparation (i.e., one day before testing).

A cheese block was cut into thin slices ( - 6 mm) parallel to the long axis of the block using a hand-operated slicer, and dumbbell-shaped samples were cut from the slices with a sharp-edged stainless steel template. Another template was used to make the central part of the specimen thinner. This part was considered as the test-section. Typical dimensions of the test-section were 6 mm in thickness, 7 mm in width and 60 mm in length.

A schematic drawing of the apparatus is shown in Fig. 1. One end of the sample was inserted into a special clamp which was attached to a load cell (Model LCL-l13G, 113 g maximum, Omega Engineering, Inc., Stamford, CT). The load cell was calibrated by hanging constant loads and used only to check the accuracy of force values obtained from the force balance (see below). The sample was lowered into an oil bath (60 f 2C) heated by circulating hot water through the hollow wall of the vessel made of transparent Lucite. The clamped end of the sample always stayed out of the oil to prevent melting and slipping of the specimen

MOZZARELLA CHEESE 149

Hole \ Load cell

Wheel -n Bearing

Light +-- sensor

FIG. 1. SCHEMATIC DIAGRAM OF THE APPARATUS USED IN VERTICAL ELONGATION TESTS

from the clamp. The vessel was insulated with Styrofoam. Tschoegl et al. (1970) have used a similar technique for determining the large deformation and rupture properties of wheat flour doughs in simple tension.

150 M.M. AK and S. GUNASEKARAN

The deformation-time data were obtained from the rotation of a wheel made of black-painted Lucite (12.7 cm in diameter) on which 72 holes (2.5 mm in diameter) 2.5 mm apart were punched. The wheel rotating on a ball bearing was connected to the end of the test-section of a specimen with a thread and a push- pin. As the cheese stretched under constant weight, it rotated the wheel, and a photocell system (infrared emitter and detector, Radio Shack, Fort Worth, TX) was activated with the passage of light through the holes. The voltage-time data acquisition was accomplished using DAS 16G High Speed analog 110 Board (Metrabyte Corp., Taunton, MA) and Easyest LX Software (Asyst Software Technologies, Inc., Rochester, NY) with a Compaq 386SX computer. Figure 2 shows an example of the-voltage-time output. A check on the accuracy of extension measurements was performed as follows: the wheel was rotated by a known amount (in terms of number of holes), and this was compared with the number of holes computed from the voltage-time output. The agreement was good to within 3%.

The maximum extension of the sample was limited to about 30 cm, which cor- responded to a Hencky strain of 1.6. Higher Hencky strain can be achieved by reducing the initial length of the test-section. The weight of the bottom part of a sample and the added weight were considered as the pulling-weight which caused stretching of the test-section. The range of added weights that could be used was limited due to possible detachment of the push-pin from the sample.

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FIG. 2. A PORTION OF A TYPICAL VOLTAGE-TIME OUTPUT FROM THE PHOTOCELL UNIT OF THE VERTICAL ELONGATION APPARATUS


The time that a sample spent in oil was about 3 min. This was not expected to alter the properties of the cheese. To check this assumption, tests were carried out using samples kept in oil for about 1 h at room temperature. Results indicated that oil contact had no significant effect on stretching properties of the cheese (Ak 1993).

It was found from pictures of stretched cheese specimens in uniaxial horizontal extension tests (Ak 1993; Ak et al. 1993) that the thickness of the elongated sample varied about 10% with position along the test section, indicating fairly homogeneous deformation. A similar variation in thickness may be expected in vertical elongation tests as well.

Experiments were performed on days 7, 14, 21 and 28 after the production date using two pulling-weights (0.14 and 0.18 N) with several specimens (n 2 10). Urea-polyacrylamide gel electrophoresis and image analysis of resulting bands in the gel were done as described in Ak el al. (1993).

Temperature Profile of a Specimen During a Test

The temperature of a specimen increased during a test (i.e., during softening and deformation). In separate tests, the temperature of cheese was followed by inserting a thermocouple into specimens without the pulling-weight. By remov- ing the pulling-weight we could record cheese temperature up to 150 s with no detectable extension taking place.

The two-dimensional transient heat transfer equation (Geankoplis 1983) was solved for the test-section (width-thickness plane) of the sample using finite element analysis (Algor, Inc., Pittsburgh, PA) with the properties available in the literature: heat transfer coefficient (h) = 60 W/mZK (Brodkey and Hershey 1988); thermal conductivity (k) = 0.38 W/mK (Sweat and Parmelee 1978); density (p) = 1140 kg/m3 (Sweat and Parmelee 1978); and heat capacity (c) = 2.68 M/kgK (Geankoplis 1983).

The finite element analysis was carried out for various initial thickness and width values for a total of four sets of dimensions. The first set of dimensions was that of specimens used in the experiments. Dimensions in other sets were deliberately selected smaller to approximate the decrease in cross-sectional area of a specimen during actual testing.

Data Analysis

The Hencky strain (E) is defined by (Munstedt 1975)

E = In (- L(t) LO


and the corresponding strain rate (i) by

(2 ) 1 dL L(t) dt

which was assumed constant throughout the test-section. In order to avoid an amplification of errors in the required differentiation of

the length data in Eq. 2, the experimental length-time values were smoothed by applying curve fitting using the following equation.

E = --

L(t) = Lo (1 - f > k l + kzt (3)

This equation was used for normalization and linearization of stress relaxation and creep curves of solid foods (Peleg 1980). In above equations, Lo and L(t) are, respectively, the initial and momentary length of the test-section of a specimen, and k, and kz are constants.

As shown in Fig. 3, Eq. 3 faithfully represented the experimental data. In this figure, t = 0 refers to the beginning of detectable stretching, not to the time of sample immersion into hot oil. The values of kl and kz in Eq. 3 were obtained

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FIG. 3. TYPICAL LENGTH-TIME DATA (CIRCLES) OBTAINED DURING VER- TICAL ELONGATION TESTS AND ITS REPRESENTATION BY EQ. 3 (LINE)


from linear regression analysis (Peleg 1980). This equation was used to obtain the acceleration term (dZL/dtZ) in the stress calculation given below.

The experimental stress UE was calculated from a force balance about the pulling-weight:

&Lo

where M is the mass of pulling-weight, g gravitational acceleration, FB the buoyant force on pulling-weight, and A, initial cross-sectional area of test-section. The weight of the test-section was not included in the pulling-weight since deformation was monitored at the bottom of the test-section. The calculated force dif- fered about 4% from the load cell output (Ak 1993).

The relationship between the stress and strain rate is defined as the transient elongational (TE) viscosity, qg (t,UE) and was computed using the following expression.

The word transient was used to signify that neither the strain rate nor the stress was constant during a test.

RESULTS AND DISCUSSION

The apparatus described in this work is inexpensive, easy to construct, and simple to operate. It provides a uniform temperature environment and, since the sample stays in oil during the entire test, there is no moisture loss from the sample. The test is usually completed within 3 min under the conditions described above.

From a rheological standpoint, the experiment is a creep test and the apparatus is of constant force type. In a proper creep test, one applies a constant stress, which is achieved by applying a force varying in proportion to cross-sectional area of a deforming specimen, and measures the elongation (Munstedt 1975). However, such tests (in tension) generally require both ends of the sample to be clamped. Gripping food samples in general and cheese in particular is difficult to accomplish (Bourne 1982; Luyten et al. 1992) and becomes far more difficult at high temperatures where the material softens. In some cases it might be possible to design grips with cooling facility (Dealy et al. 1976), but this adds


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complications to the apparatus. The design of the apparatus used here advan- tageously avoided gripping problems even at melting temperatures.

The difficulty of applying heavier weights needs to be overcome in order to expand the range of strain rates attainable. Tests with 1 or 5 g of added mass did not produce significantly different strain rates. To attain higher strain rates, the pulling-weight can be increased by cutting samples with larger bottoms using a special template.

The time for elongation to begin varied with the pulling-weight; i.e., elongation began earlier and consequently at a lower sample temperature when the heavier weight was applied. Hence, the response of cheese was influenced by the com- bined effects of sample temperature and applied weight. Using a standard weight can reduce such complications.

Good agreement between the measured and finite element method (FEM)- predicted center temperature of a specimen immersed in hot oil is shown in Fig. 4. This graph shows that sample temperature increases to about 40C in the first minute, which was, on average, the time required for detectable elongation to begin during a test. For times greater than 1 min, sample temperature in actual tests is expected to be higher than those shown in the graph because of thinning in the test-section as a result of deformation. Results show that the temperature of test-section increased probably up to 55C towards the end of a test. Thus, the

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FIG, 4. MEASURE (SQUARES) AND FEM-PREDICTED (CIRCLES) CENTER TEMPERATURE OF A CHEESE SPECIMEN IMMERSED IN OIL AT 6OC


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rheological results presented in this paper pertain to cheese temperature varying between 40 and 55C. Obviously, a modification in the test procedure or design of the apparatus to enable experiments at constant sample temperature would be a major improvement. Additional results of the FEM analysis are available in Ak (1993).

Since both the strain rate and the stress changed during a test, results are reported in the form of transient elongational (TE) viscosity (Eq. 5). The TE viscosity of Mozzarella as a function of strain rate is shown in Fig. 5 for 7-day-old cheese. The graph contains data points from several replicate runs. In general, the scat- ter of data was larger at higher strain rates, i.e., towards the end of stretching where the fit of length-time data to Eq. 3 also deteriorated. Similar plots were obtained at other testing days. The line in the graph represents the fit of experimental data to an equation of the form:

This equation was used only to reproduce experimental TE viscosity data from different test days (7, 14, 21 and 28) for common strains (Fig. 6 and 7).

The resistance of melted cheese to uniaxial extension was lower at higher strain rates (Fig. 5). The TE viscosity of Mozzarella cheese also decreased with increasing strain rate and temperature. A similar response was obtained with heavier

1000 ' 0.001 0.01 0.1 1

Strain rate (s -') FIG. 5. TRANSIENT ELONGATIONAL VISCOSTITY OF MOZZARELLA CHEESE

0.14 N; SEE TEXT FOR THE LINE IN THE GRAPH) AS A FUNCTION OF STRAIN RATE AT DIFFERENT AGES (PULLING-WEIGHT:

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FIG. 6. TRANSIENT ELONGATIONAL VISCOSTIY OF MOZZARELLA CHEESE

0.14 N) AS A FUNCTION OF STRAIN RATE AT DIFFERENT AGES (PULLING-WEIGHT:

Qg 7-d 0 14-d A 21-d (I 2 8 4 .A

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Strain rate (s-')

FIG. 7. TRANSIENT ELONGATIONAL VISCOSITY OF MOZZARELLA CHEESE

0.18 N) AS A FUNCTION OF STRAIN RATE AT DIFFERENT AGES (PULLING-WEIGHT:

weights except that magnitudes of the TE viscosity were larger. Smith et al. (1980) reported that the apparent viscosity of Mozzarella cheese, evaluated by capillary


rheometry, decreased with increasing shear rate and temperature. However, the elongational viscosity calculated in a biaxial compressive-elongation tests at a constant temperature (Apostolopoulos 1994) increased with the biaxial strain rate. Therefore, the increase in TE viscosity might be more due to the temperature effect than to the increasing strain rate. With our measurement procedure, however, if was not possible to separate the effects of the strain rate and temperature.

The TE viscosity of Mozzarella was not affected significantly by the age of cheese during one month or refrigerated storage (Fig. 6 and 7). The gel electrophoresis result shown in Fig. 8 indicates that cysl-casein was hydrolyzed to a greater extent than 0-casein. Image analysis of the bands showed that the relative intensity for cYsl-casein at day 28 was about 40% of its value at day 7 and that for 0-casein was about 70%. Apparently, the breakdown of proteins in commercial Mozzarella into large peptides did not affect its elongational properties to an extent measurable by our method. This result may point to a low sensitivity of the apparatus, as Kindstedt et al. (1989b) reported a significant decrease in apparent viscosity of Mozzarella, measured by an empirical method, over a same duration of storage. On the other hand, preliminary tests performed on young (7- to 17-day) and aged (8 months) Cheddar cheese with our apparatus showed that Cheddar cheese and aged Cheddar required more time to soften and stretch a given distance than Mozzarella of similar age and young Cheddar, respectively (Ak 1993). Thus, the method seems sensitive to differences in elongational properties of Cheddar and Mozzarella and also of Cheddar samples with marked age difference (i.e., > 1 mo).

FIG. 8. PROTEOLYSIS IN MOZZARELLA CHEESE USED IN VERTICAL ELONGA- TION TESTS

Left to right: Lanes 1 and 2 for day 7 sample; Lane 3 for day 14 sample; Lane 4 for day 21 sample; Lane 5 for day 28 sample; Lane 6 for casein standards; Lanes 7 to 1 1 are from

a replicate gel and labels are the same as Lanes 1 to 5.

158 M.M. AK and S . GUNASEKARAN

An important rheological property of ingredient Mozzarella cheese is its stretchability. One objective measure of stretchability of the cheese might be its strain at fracture. Horizontal extension tests on Mozzarella cheese showed that at 40C the sample did not break even after 650% extension (Ak et al. 1993). Vertical extension results presented here were in accord with this finding since samples did not break after 400% extension. Cavella et al. (1992) reported increasing elongation at break for Mozzarella cheese with increasing temperature, reaching a maximum of about 220% at 72C using a fiber spinning method. During part of this test, melted cheese is exposed to ambient conditions which may cause hardening of cheese due to cooling and moisture loss and affect the response. More importantly, temperature range, 57-83C, and strain rates, 2 4 s - ', reported by Cavella et al. (1992) were different from those attained in this work and in Ak et al. (1993). Apostolopoulos (1994) reported that the entanglement and cross- link formation of the protein molecules determines the integrity of the strings of cheese when pulled apart. Since the resistance to flow is directly related to the degree of entanglement, he suggested that elongational viscosity can be used to relate the stretchability of Mozzarella cheese. More research is needed to establish an objective definition for stretchability of cheese.

CONCLUSIONS

The importance of determining elongational properties of Mozzarella cheese as a measure of quality (e.g., stretchability) in dairy and food processing industries is well recognized. Inexpensive and simple apparatus presented in this study could be useful in determining stretching characteristics of the cheese. As a research tool, it can provide valuable information on elongational properties of different cheeses and on effects of various factors.

The transient elongational (TE) viscosity of Mozzarella obtained from a commercial cheese plant decreased as the strain rate and sample temperature simultaneously increased during a test. This might be a useful information in processing operations involving extensional deformation of melted cheese or flow of plastic curd during cheese manufacturing. Proteolysis taking place in Moz- zarella cheese during one month of refrigerated storage did not affect its TE viscosity to an extent measurable by the procedure described in this study.

ACKNOWLEDGMENTS

The National Dairy Promotion and Research Board, through the Wisconsin Center for Dairy Research, and the Scientific and Technical Research Council


of Turkey provided financial support for this work. We thank Dr. K. Muthukumarappan of the Agricultural Engineering Department, University of Wisconsin-Madison, for his help with the finite element analysis and David Bogenrief of the Wisconsin Center for Dairy Research for his help with the elec- trophoretic analysis.

REFERENCES

AK, M.M. 1993. Rheological Measurements on Mozzarella Cheese. Ph. D. thesis, University of Wisconsin-Madison, Madison, WI.

AK, M.M., BOGENRIEF, D., GUNASEKARAN, S . and OLSON, N.F. 1993. Rheological evaluation of Mozzarella cheese by uniaxial horizontal extension. J. Texture Studies 24, 437C453.

APOSTOLOPOULOS, C. 1994. Simple empirical and fundamental methods to determine objectively the stretchability of Mozzarella cheese. J. Dairy Res. 61, 405-413.

BARNES, H.A., HUTTON, J.F. and WALTERS, K. 1989. An Introduction to Rheology, Elsevier Science Publishers, New York.

BOURNE, M.C. 1982. Food Texture and Viscosity: Concept and Measurement, Academic Press, New York.

BRODKEY, R.S. and HERSHEY, H.C. 1988. Transport Phenomena: A Un@ed Approach, McGraw-Hill Book Co., New York.

CAVELLA, S . , CHEMIN, S . and MASI, P. 1992. Objective measurement of the stretchability of Mozzarella cheese. J. Texture Studies 23, 185-194.

DEALY, J . , FARBER, R., RHI-SAUSI, J. and UTRACKI, L. 1976. Experiences with a constant stress melt extensiometer. Trans. SOC. Rheol. 20(3), 455464.

GEANKOPLIS , C . J . 1983. Transport Processes and Unit Operations, Allyn and Bacon, Boston.

KINDSTEDT, P.S. RIPPE, J.K. and DUTHIE, C.M. 1989a. Measurement of Mozzarella cheese melting properties by helical viscometry. J. Dairy Sci.

KINDSTEDT, P.S., RIPPE, J.D. and DUTHIE, C.M. 1989b. Application of helical viscometry to study commercial Mozzarella cheese melting properties. J. Dairy Sci. 72(12), 3123-3128.

LUYTEN, H., van VLIET, T. and WALSTRA, P. 1992. Comparison of various methods to evaluate fracture phenomena in food materials. J. Texture Studies

MUNSTEDT, H. 1975. Viscoelasticity of polystyrene melts in tensile creep ex-

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periments. Rheol. Acta 14, 1077-1088.

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OLSON, N.F. and NELSON, D.L. 1980. A new method to test the stretchability of Mozzarella cheese on pizza. Proc. Marschall Italian cheese Seminar, Madison, WI.

PAGLIARINI, E. and BEATRICE, N. 1994. Sensory and rheological properties of low-fat filled ‘pasta filata’ cheese. J. Dairy Res. 61, 299-304.

PELEG, M. 1980. Linearization of relaxation and creep curves of solid biological materials. J. Rheol. 24(4), 451463.

SMITH, C.E., ROSENAU, J.R. and PELEG, M. 1980. Evaluation of the flowability of melted Mozzarella cheese by capillary rheometry . J. Food Sci.

SWEAT, V.C. and PARMELLE, C.E. 1978. Measurement of thermal conductivity of dairy products and margarines. J. Food Proc. Eng. 2 , 187-197.

TSCHOEGL, N.W., RINDE, J.A. and SMITH, T.L. 1970. Rheological properties of wheat flour doughs. J. Sci. Food Agric. 21, 65-70.

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MEASURING ELONGATIONAL PROPERTIES OF MOZZARELLA CHEESE