Free Access
Issue
Dairy Sci. Technol.
Volume 90, Number 5, September–October 2010
Page(s) 589 - 599
DOI https://doi.org/10.1051/dst/2010012
Published online 18 March 2010

© INRA, EDP Sciences, 2010

1. INTRODUCTION

Rheological properties obtained in the linear viscoelastic region are useful tools for the food industry. Elastic and viscous contributions to the internal structure of the sample can be obtained performing oscillatory measurements. Such studies provide an insight into the fundamental nature of the physical basis of food texture [2]. Cheese is a typical viscoelastic food whose structure can be considered as a casein network embedded with a dispersed phase of fat globules, minerals and water. The rheological behavior of cheeses is influenced by the components in the cheese matrix structure [19]. Also, the viscoelasticity of cheeses can be influenced by ripening because the structure of the cheese matrix is modified due to breakdown of the protein network during casein hydrolysis. During ripening, the changes in pH, moisture content and salt concentration can affect textural characteristics of cheeses [17]. For this reason, correlations between rheological parameters and physicochemical properties of cheeses over ripening of commercial cheeses have been reported [2, 5, 24].

Over the last decade, the production of low-fat cheeses has significantly increased. However, the removal of fat from the cheese has been shown to cause textural, functional and sensory defects. The use of fat mimetics in the cheesemaking process is an interesting strategy to improve the quality of low-fat cheeses [13]. Protein-based fat mimetics, such as Simplesse® or Dairy-Lo, consisting of microparticles of whey proteins have the property to give a sense of creaminess similar to fat [16].

Changes in some textural parameters (hardness, fracturability and chewiness) over ripening of several types of low-fat cheeses manufactured with Simplesse® and Dairy-Lo were published [7, 14, 15], but few studies about viscoelastic properties of those kinds of cheese have been reported [11, 25]. Moreover, viscoelastic behavior during a specified ripening period of a low-fat soft cheese containing Simplesse® has not been extensively studied.

The objective of this work was to study the change in the viscoelastic behavior during ripening of a commercial low-fat soft cheese containing whey proteins (Simplesse®) and to find the relationships between some rheological parameters and physicochemical properties.

2. MATERIALS AND METHODS

2.1. Cheese samples

Twelve low-fat soft cheeses (LFSC) that contained whey proteins as fat mimetics (Simplesse® D100, NutraSweet Co., Deerfield, IL, USA) were used for this study. Cheeses were manufactured at a local factory according to regional legislation [3], obtained by rennet coagulation of pasteurized skim milk, salted in brine to reach a final salt concentration of 0.41 ± 0.01% and packed in heat-shrinkable plastic bags. Cheese samples had a rectangular shape (28.7 ± 0.3 cm × 11.6 ± 0.3 cm × 7.4 ± 0.2 cm) and a weight of 3.0 ± 0.1 kg. Cheeses were transported in ice containers from the factory to the laboratory and were stored in a Tabai Comstar PR 4GM chamber (Tabai Espec Corp., Osaka, Japan) at 6 °C. Cheeses were sampled at different ripening times (1, 21, 48 and 76 days) in triplicate. Four slices (3 cm thickness) parallel to the smallest surface (11.6 ± 0.3 cm × 7.4 ± 0.2 cm) were obtained from the central zone of the cheeses. A thin layer was removed from slices, leaving an area of 9 cm × 6 cm and 3 cm thickness for sampling. Subsequently, six cubes of 3 cm side were obtained for each slice but only four cubes near each corner were used as samples. Finally, slices of 3 mm were cut from the cubes. Both cubes and slices were stored in plastic containers, to prevent dehydration, and held refrigerated until physicochemical and rheological testing.

2.2. Physicochemical analysis

Moisture content was measured with a microwave oven CEM AVC 80 (CEM, Attheus, NC, USA) according to the Association of Official Analytical Chemists procedure [1]. The initial fat content (5.75 ± 0.04%) was determined in duplicate using the standard method of International Dairy Federation [6]. Salt concentration was determined as proposed by Fox [4] with an automatic titrator model DL40RC (Mettler Instrumente AG, Greifensee, Switzerland). The pH was measured with an electrode for solid foods (pH Spear, OAKTON Instruments, Vernon Hills, IL, USA). Water-soluble fraction was extracted with a procedure developed by Kuchroo and Fox [8], and modified by Verdini and Rubiolo [24]. Total nitrogen content (TN) and water-soluble nitrogen content (WSN) were determined using the micro-Kjeldahl method with an automatic digestor model 430, a distillation unit model 322 (Büchi, Flawil, Switzerland) and the automatic titrator. Maturation index (MI) was expressed as a percentage of the WSN to the TN (MI = WSN × 100/TN). Two replicates were used in all analysis.

2.3. Rheological analysis

2.3.1. Stress and frequency sweeps

Disks (20 mm of diameter) were cut with a borer from the cheese slices and were used for the rheological analysis. Frequency sweeps were performed in the range of 0.01–10 Hz at 20.0 ± 0.5 °C using a stress controlled rheometer RheoStress 80 (Haake Inc. Instruments, Karlsruhe, Germany) with a plate-plate geometry test fixture. Diameter and gap were 20 and 2.5 mm, respectively. The region of linear viscoelasticity was determined prior to each frequency sweep performing stress sweeps at 10 Hz. A thin film of silicone oil (100 cp) was applied to the exposed sample edges to prevent water evaporation during measurements. Elastic (G′), viscous (G″) and complex (|G*|) moduli, complex viscosity (|η*|) and tangent of phase angle (tan δ) were measured at a fixed stress amplitude (318 Pa). Frequency sweeps were conducted in duplicate for each sample.

2.3.2. Modeling of the mechanical spectra

Frequency dependence of G′ and G″ was modeled with power-law equations (1) and (2) and Maxwell equations (3) and (4) [18]:(1) (2) (3) (4)where ω is the frequency, a, b, x and y are the power-law parameters, λ i is the relaxation time and G i is the relaxation modulus of the i th Maxwell element, respectively.

2.3.3. Kinetic analysis of the rheological parameters

A first-order kinetic model was assumed to represent the behavior of the rheological parameters derived from power-law and Maxwell equations during the ripening time [24]:(5)where P 0 is the initial value of the rheological parameter, P(θ) is the rheological parameter over ripening, θ is the ripening time expressed in days (d) and K is the kinetic rate constant (d−1).

2.4. Statistical analysis

Analysis of variance was performed and when the effect of the factors was significant (P < 0.05), the multiple ranks HSD Tukey test was applied (95% of confidence level). Linear regression was used to determine the corresponding parameters of power-law equation. Nonlinear regression analysis was conducted to obtain the parameters of five Maxwell elements, using the procedure proposed by Subramanian et al. [20]. Multiple regression analysis was applied in order to establish the relationships between rheological parameters and physicochemical properties. The complete statistical analysis was performed using Minitab 13.20 (Minitab Inc., State College, PA, USA).

3. RESULTS AND DISCUSSION

3.1. Physicochemical analysis

The initial moisture content of the LFSC was 52.80 ± 0.46%, and the moisture content did not change significantly during the studied ripening period (Tab. I). The high moisture content observed in LFSC is in agreement with the reports of other authors in several types of low-fat cheeses containing whey protein-based fat mimetics [7, 12, 14, 15]. Fat mimetics are known for their water-binding capacity, which may in turn explain the higher moisture content found in cheeses containing these materials [14]. Also, it has been suggested that these materials interfere with the shrinkage of the casein matrix, retarding the curd syneresis during the cheesemaking process [12]. However, moisture content of LFSC did not change significantly during the studied ripening period (Tab. I).

Table I.

Physicochemical properties over the ripening of a commercial low-fat soft cheese containing whey proteins.

The initial salt concentration of the LFSC was 0.41 ± 0.01%, and it increased significantly over ripening from 1 to 21 days and did not change significantly between 21 and 76 days (Tab. I). This result is expected because when cheeses are salted in brine, salt gradient develops from the surface to the center of the cheese, leading to an almost uniform salt distribution [10].

The initial pH of the LFSC was 5.27 ± 0.03, and the pH slightly decreased during ripening from 1 to 48 days and did not change significantly between 48 and 76 days (Tab. I). Residual lactose in cheese is fermented by lactic acid bacteria to several water-soluble organic acids. Upon dissociation, organic acids liberate H+, causing a decrease in pH. Buffering in cheese is related to the presence of proteins and inorganic constituents such as weak acids, bases and metal ion complexes [9]. The production of acid should lead to immediate solubilization of calcium and phosphate entrapped by the para-casein network, which would act as a buffer resisting the decrease in the pH [21].

MI of the LFSC increased significantly over ripening, from 3.60 ± 0.67% at the beginning of the maturation to 8.66 ± 1.05% at 48 days of ripening, but there were no significant differences between 48 and 76 days (Tab. I). The pH 4.6-soluble nitrogen content is an index of cheese primary proteolysis, which is more prominent during the first weeks of ripening. Subsequent hydrolysis of the large and intermediate-sized peptides results in the formation of smaller peptides and free amino acids that does not significantly influence pH 4.6-soluble nitrogen content [17]. Hence, larger changes in the pH 4.6-soluble nitrogen content are expected during the initial stages of ripening compared with the later stages [22].

3.2. Rheological analysis

3.2.1. Region of linear viscoelasticity

Values of |G*| and strain (γ) of the LFSC were plotted as a function of applied stress at the frequency of 10 Hz (Fig. 1). At any ripening time, values of |G*| were not affected by the magnitude of the applied stress and a linear relationship between stress and strain was observed. The linear relationship between stress and strain in the experimental conditions indicates that regardless of structural changes during ripening, LFSC behaved as a linear viscoelastic material during dynamic testing at stresses below 630 Pa.

thumbnail Figure 1.

Stress sweeps over the ripening of a commercial low-fat soft cheese containing whey proteins. Full symbols complex modulus (|G*|) and empty symbols strain (γ): (♦◊) day 1, (▲∆) day 21, (■□) day 48 and (●○) day 76 of ripening. Points represent individual values of one of two replications.

3.2.2. Mechanical spectra

Changes in G′ and G″ of the LFSC as a function of frequency are shown in Figure 2. Elastic modulus was greater than viscous modulus throughout the frequency range, indicating that LFSC showed a solid-like behavior at any ripening time. This mechanical spectrum describes the characteristic behavior of a viscoelastic solid [18].

thumbnail Figure 2.

Frequency sweeps over the ripening of a commercial low-fat soft cheese containing whey proteins. Full symbols elastic modulus (G′) and empty symbols viscous modulus (G″): (♦◊) day 1, (▲∆) day 21, (■□) day 48 and (●○) day 76 of ripening. Points represent individual values of one of two replications.

Changes in |G*| and |η*| of the LFSC as a function of frequency are shown in Figure 3. Values of |G*| and |η*| describe the total resistance to deformation of a material that is considered as an elastic solid or a viscous liquid. Complex modulus increased and complex viscosity decreased with frequency, showing the same pattern of viscoelastic response at any ripening time.

thumbnail Figure 3.

Complex modulus (|G*|) and complex viscosity (|η*|) over the ripening of a commercial low-fat soft cheese containing whey proteins. Full symbols |G*| and empty symbols |η*|: (♦◊) day 1, (▲∆) day 21, (■□) day 48 and (●○) day 76 of ripening. Points represent individual values of one of two replications.

Values of tan δ, that compares the amount of energy lost to the amount of energy stored during a test cycle, are shown in Figure 4. According to Gravier et al. [5], tan δ can be used to indicate the strong relationship between the viscous behavior and the degree of casein hydrolysis. Values of tan δ of LFSC were between 0.3 and 0.55, indicating that elastic properties predominate at any ripening time. However, values of tan δ increased with the ripening time, showing that viscous properties increased over ripening.

thumbnail Figure 4.

Tangent of phase angle (tan δ) over the ripening of a commercial low-fat soft cheese containing whey proteins: (♦) day 1, (▲) day 21, (■) day 48 and (●) day 76 of ripening. Points represent individual values of one of two replications.

3.2.3. Modeling of the mechanical spectra

Power-law and Maxwell parameters of LFSC are shown in Tables II and III, respectively. Coefficients a and b represent the magnitude of G′ and G″ at a frequency of 1 rad·s−1, and exponents x and y represent the slopes of the linear relationships between modulus and frequency. Coefficient a was higher than b, showing the predominance of a solid-like behavior at any ripening time. Also, exponent x was higher than exponent y, indicating that elastic properties were more sensitive to frequency changes than viscous properties. During cheese ripening, coefficients a and b decreased, while exponents x and y increased with ripening time. This result indicates that both elastic and viscous components were sensitive to frequency changes during maturation. These observations are in agreement with the decrease in the cheese matrix rigidity during ripening.

Table II.

Power-law parameters for elastic (G′ = aω x ) and viscous (G″ = bωy) moduli over the ripening of a commercial low-fat soft cheese containing whey proteins.

Table III.

Parameters derived from Maxwell model (λ i and G i for i = 1, …, 5) over the ripening of a commercial low-fat soft cheese containing whey proteins.

In general, the elastic contribution of each Maxwell element (Gi ) decreased as the ripening time increased. These results are expected because cheese becomes softer during ripening [19]. The proteolysis during ripening contributes to the softening of cheese because most casein breakdown products are water soluble and they cannot contribute to the framework provided by the protein matrix [23].

3.2.4. Kinetic analysis of rheological parameters

The behavior of the rheological parameters of the LFSC during the ripening time is shown in Figure 5. In order to establish the rate of decay of parameters a and b derived from power-law equation and parameters G i derived from Maxwell equation, kinetic rate constants were calculated (Fig. 5). These obtained results indicate that all parameters decreased with the same rate (P > 0.05) during the studied ripening period. In addition, both models appropriately characterized the mechanical spectra of LFSC over ripening (Tabs. II and III). For this reason, parameter a of power-law equation was arbitrarily selected to correlate physicochemical properties and rheological parameters.

thumbnail Figure 5.

Decay of rheological parameters obtained from power-law and Maxwell equations as a function of ripening time of a commercial low-fat soft cheese containing whey proteins. Lines represent fitted curves.

3.2.5. Correlation between physicochemical properties and rheological parameter a

Correlations between physicochemical properties (salt concentration, pH and MI) and the rheological parameter a of power-law equation are shown in Table IV. The best simple correlation was obtained between MI and a. The negative correlation indicates that when MI increased the rheological parameter a decreased. This observation reaffirms the importance of proteins in cheese texture that was previously published by other authors [2, 5, 24]. The proteolysis of LFSC produced an increase in the pH 4.6-soluble nitrogen compounds that cannot contribute to the protein network. This behavior is related to a decrease in the cheese matrix rigidity that becomes softer during ripening.

Table IV.

Correlations between physicochemical properties and rheological parameter a of power-law equation over the ripening of a commercial low-fat soft cheese with whey proteins.

The combination of MI with other physicochemical properties such as moisture content, salt concentration and pH using multiple correlations did not improve the results obtained when MI was used alone (Tab. IV).

4. CONCLUSIONS

This study allowed the characterization of the change in the viscoelastic behavior over ripening of a commercial low-fat soft cheese that contained whey proteins as fat mimetics (Simplesse®). This cheese behaved as a linear viscoelastic material during dynamic testing at stresses below 630 Pa and also, elastic properties predominated in the mechanical spectra. In general, viscoelastic parameters derived from power-law and Maxwell equations decreased with the ripening time and decayed with the same kinetic rate, showing that ripening contributed to changes in the structure of cheese matrix. The best correlation between physicochemical properties and rheological parameters was obtained between parameter a derived from the power-law equation and MI. The resulted regression equation can be used to predict the MI from rheological tests.

Acknowledgments

This work was done with the financial support of Universidad Nacional del Litoral, Consejo Nacional de Investigaciones Científicas y Técnicas and the Agencia Nacional de Promoción Científica y Tecnológica of Argentina. The authors acknowledge Daniel De Piante Vicin for technical assistance in the rheological tests and physicochemical determinations.

References

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All Tables

Table I.

Physicochemical properties over the ripening of a commercial low-fat soft cheese containing whey proteins.

Table II.

Power-law parameters for elastic (G′ = aω x ) and viscous (G″ = bωy) moduli over the ripening of a commercial low-fat soft cheese containing whey proteins.

Table III.

Parameters derived from Maxwell model (λ i and G i for i = 1, …, 5) over the ripening of a commercial low-fat soft cheese containing whey proteins.

Table IV.

Correlations between physicochemical properties and rheological parameter a of power-law equation over the ripening of a commercial low-fat soft cheese with whey proteins.

All Figures

thumbnail Figure 1.

Stress sweeps over the ripening of a commercial low-fat soft cheese containing whey proteins. Full symbols complex modulus (|G*|) and empty symbols strain (γ): (♦◊) day 1, (▲∆) day 21, (■□) day 48 and (●○) day 76 of ripening. Points represent individual values of one of two replications.

In the text
thumbnail Figure 2.

Frequency sweeps over the ripening of a commercial low-fat soft cheese containing whey proteins. Full symbols elastic modulus (G′) and empty symbols viscous modulus (G″): (♦◊) day 1, (▲∆) day 21, (■□) day 48 and (●○) day 76 of ripening. Points represent individual values of one of two replications.

In the text
thumbnail Figure 3.

Complex modulus (|G*|) and complex viscosity (|η*|) over the ripening of a commercial low-fat soft cheese containing whey proteins. Full symbols |G*| and empty symbols |η*|: (♦◊) day 1, (▲∆) day 21, (■□) day 48 and (●○) day 76 of ripening. Points represent individual values of one of two replications.

In the text
thumbnail Figure 4.

Tangent of phase angle (tan δ) over the ripening of a commercial low-fat soft cheese containing whey proteins: (♦) day 1, (▲) day 21, (■) day 48 and (●) day 76 of ripening. Points represent individual values of one of two replications.

In the text
thumbnail Figure 5.

Decay of rheological parameters obtained from power-law and Maxwell equations as a function of ripening time of a commercial low-fat soft cheese containing whey proteins. Lines represent fitted curves.

In the text