Rheology, Microstructure, and Functionality of Cheese

Chapter 10

Rheology, Microstructure, and Functionality of Cheese K. Muthukumarappan, G.J. Swamy Department of Agriculture and Biosystems Engineering, South Dakota State University, Brookings, SD, United States

10.1 INTRODUCTION Cheese is a fermented milk-based food product, produced in a wide range of flavors and forms throughout the world (Fox and McSweeney, 2004). Although the primary objective of cheese-making is to conserve the principal constituents of milk, cheese has evolved to become a food of haute cuisine with epicurean qualities, as well as being highly nutritious. Discussions on cheese, in terms of the relationship between its rheology, microstructure, and functionality increase the complexity. This chapter makes a substantial impact to an understanding of this complexity. It highlights limitations and contemplations in assessing and directing research on rheology, microstructure, and functionality. It also bridges the gaps in our understanding of the concepts and comprehensively scrutinizes the methods, theories, and applications of cheese.

10.2  RHEOLOGY OF CHEESE 10.2.1 Definition Rheology is defined as the science of deformation and flow of matter. The term originates from the Greek word “rheos” meaning “to flow.” The name was coined in 1920 by Eugene C. Bingham, a professor at Lafayette College (­Steffe,  1996). Rheology is applicable to all types of materials, from gases to solids. In food science, rheology is used to define the consistency of different products based on the viscosity (thickness, lack of slipperiness) and elasticity (stickiness, structure). Therefore, in practical applications, rheology denotes viscosity measurements, characterization of flow behavior, and determination of material structure. Advances in Food Rheology and Its Applications. http://dx.doi.org/10.1016/B978-0-08-100431-9.00010-3 Copyright © 2017 Elsevier Ltd. All rights reserved.

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There are numerous areas where rheological data are needed in the cheese industry which are summarized in Table 10.1

10.2.2  Basic Concepts Rheology deals with the relationship between strain, stress, and time (Blair et al., 1947). Strain and stress are closely related to deformation and force. Strain accounts for the size effect on material deformation due to difference in dimensions of sample whereas stress accounts for the size effect on applied force due to difference in cross-sectional area of samples. Using strain and stress, rheologists are able to obtain true material properties independent of the sample size and geometry and compare test results for samples of different sizes and geometries. The concepts are described in Table 10.2 (Gunasekaran and Ak, 2002).

10.2.3  Uniaxial Testing Uniaxial testing is a popular method for instrumental evaluation of cheese texture. Uniaxial compression is the most popular test for estimating the rheological properties of cheese. This test is popular probably because it is easy to execute and there is no need for sample gripping (Luyten et al., 1992). ­Nearly all compression tests on cheese are done using one of the versatile instruments commonly referred to as Universal Testing Machine (UTM). The UTM provides precise control of deformation while accurately measuring force. ­Mechanical properties usually calculated from uniaxial compression tests on cheese ­comprise modulus of deformability, fracture stress, fracture strain, and work to fracture. UTMs can be employed to carry out compression as well as tension, bending, and shear tests (Velmurugan et al., 2004). Uniaxial tension is simply the opposite of uniaxial compression. The main difference exists in the strain rate. When a specimen is deformed at a constant speed, the strain rate decreases in tension and increases in compression test. Uniaxial tension tests are considered not suitable for routine measurements since they are more difficult to execute due to lengthy sample preparation and difficulty of gripping (Wise, 1961).

10.2.4  Stress Relaxation Stress relaxation is a fundamental test to study viscoelasticity and it can be performed in uniaxial tension, compression, shear, bending, and torsion. At isothermal conditions, when a constant strain is applied to a viscoelastic material, the stress is essential to uphold that strain is not constant, however it decreases with time. This decrease in stress at constant strain is termed as stress relaxation. Relaxation experiments are of two types: stress relaxation preceded by a sudden step strain (often applied to solids) and stress relaxation after a cessation of steady flow (applied to liquids) (Gunasekaran and Ak, 2000).

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TABLE 10.1  Applications of Rheology in the Cheese Industry Important areas

Application in cheese industry

Reference

Process engineering calculations involving equipments such as pipelines, pumps, extruders, mixers, coaters, heat exchangers, homogenizers, and online viscometers

Quantifying real time rheological flow properties inline based on synchronized measurements of velocity profiles using an ultrasound velocity profiling (UVP) technique with pressure difference (PD) technology

(Wiklund et al., 2007)

Determining ingredient functionality in product development

The effects of calcium on the rheology of cheese at different stages of manufacture were assessed using stress controlled rheometer and it was reported that calcium had a significant effect on the rheology.

(Solorza and Bell, 1998)

Intermediate or final product quality control

The rheological properties of mozzarella cheese were determined by different deformation tests. The results showed that rheology can be used as a quality control tool and is closely correlated with the overall texture, sensory attributes, and ­microstructural changes during cheese processing

(Muliawan and Hatzikiriakos, 2007)

Shelf-life testing

Water is known to decrease the hardness or firmness of cheese and therefore the rheological property is an indication of spoilage

(Lee et al., 2004)

Evaluation of food texture by correlation to sensory data

Investigation of the sensory and rheological properties of young cheeses to identify perceived cheese texture. It was concluded that there was a strong correlation between the sensory and rheological properties which is useful to understand cheese texture

(Brown et al., 2003)

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TABLE 10.2  Terminologies Used in Rheology Terminology

Description

Strain

When a material is subjected to an external force, individual points of the body will move relative to one another causing a change in the size and shape of the material. Thus, strain (deformation) is the measure of such a change in size and shape.

Stress

Stress is defined as force per unit area over which the force is applied. Thus, the unit of stress in SI system is Pa (= N/m2). Two types of stress can act on a surface: normal stress and shear stress. Normal stress acts perpendicular to the surface whereas shear stress acts parallel to the surface. Normal stress is categorized as tensile and compressive based on the directions of force and unit normal vector of the surface. In tension these two vectors are in the same direction (angle 0°) while in compression they are in opposite directions (angle 180°). In simple shear the force is applied tangentially to the surface.

Strain rate

The strain rate is simply the time derivative of strain. The concept of strain rate is necessary to describe flow behavior of materials. In flow situations, since the strain will attain very large values with increasing time, it is preferred to discuss material behavior in terms of stress–strain rate rather than stress–strain.

Young’s modulus

It is a measure of material’s resistance to axial deformation. It represents the stiffness of the material to an applied load. The larger the stiffness, the higher the force or stress needed to cause a given deformation or strain

Proportional limit

It is the highest stress at which stress is directly proportional to strain. Hooke’s law applies up to the proportional limit. The proportional limit also marks the start of nonlinearity in the stress–strain curve

Elastic limit

It is the maximum stress the material can sustain without any measurable permanent strain remaining upon the full release of load. Thus, the material will come back to its original shape/size when the stress is removed

Yield point

A small increase in stress above the elastic limit results in a relatively large increase in strain. The sample is perpetually malformed even if the load is reduced to zero. This yielding is designated as the yield stress

Resilience

It is the amount of energy absorbed by a material in the elastic range

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In nonfood applications, stress-relaxation tests can be carried out for a long time. The test duration for foods is restricted due to deterioration of sample which may take place before the test is completed. The deterioration may be a result of physical changes like moisture exchange with environment, microbial activity, and chemical and biochemical changes such as enzymatic browning in fruits/oxidation in oil-containing foods. The apparent relaxation times (ie, the time required for the stress to relax to 1/e, or ∼37%, of its initial value) of cheeses vary from 0.5 to 375 s (Heldman et al., 2006).

10.2.5  Creep and Recovery of Cheese Application of a constant can create deformation. If the material, after some deformation, eventually resists further deformation, it is considered a solid. However, if the material flows indefinitely, it is considered a fluid. A creep test provides characteristic behavior of the viscoelastic material. A creep test may be executed in various configurations such as compression, tension, shear, and torsion (Huang et al., 2003). Generally, constant stress is administered to the material while the resultant strain is recorded as a function of time. Compression is the most common creep test in cheese studies. Table 10.3 summarizes the creep test done for different types of cheese. The objective of creep tests is to calculate material properties D(t) and J(t) from the experimental strain versus time data The time-dependent compliance for linear materials is given by D(t ) =

ε (t ) σ0

where ε(t) is the tensile or compressive strain. The shear creep compliance is given by J (t ) =

γ (t ) τ0

where γ (t) is the shear strain and τo is the applied shear stress. Most often, the creep response of cheese is described by the Kelvin–Voigt model, Peleg model, respectively, and Maxwell model (Gunasekaran and Ak, 2002; Rao et al., 2014). n   −t   ε (t ) t = D0 + + ∑Di 1 − exp     τi   σ0 η1 i = 2 

where D0 is the instantaneous compliance, t is the time, η1 is the Newtonian viscosity, 1/η1 is the slope of the linear portion of the creep curve, Di is the delayed compliance of respective Kelvin–Voigt element, and τi is the retardation time of respective Kelvin–Voigt element.

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TABLE 10.3  Modeling of Rheological Properties of Various Cheese Objective

Model

Result

Reference

Cheddar cheese

To observe the linear viscoelastic response of cheese at temperature of 40°C and stress of 1119.5 Pa

Six-element Kelvin model

It was obvious that the viscoelasticity index has the ability to differentiate the meltability of Cheddar cheeses at different ages and various fat levels

(Kuo et al., 2000)

Regular- and reduced-fat pasteurized process cheese

To analyze the linear viscoelastic properties of regular- and reduced-fat pasteurized process cheese during heating and cooling

Maxwell model

The viscosity distribution of Maxwell model elements was higher for reduced-fat cheese by a factor of 1.6–4.7 compared to the regular-fat cheese. This shows that the higher moisture content in the reduced-fat process cheese neither slackened the protein matrix nor softened the cheese even though higher moisture is suggested to cheese manufacturers to recompense for textural flaws in reduced-fat cheeses

(Subramanian et al., 2006)

Mozzarella cheese

To observe the effect of methocel as a water binder on the linear viscoelastic properties of mozzarella cheese during early stages of maturation

Six-element Voigt–Kelvin model

The skim milk Mozzarella with 0.2% Methocel was softer with higher creep and recovery compliance because of enhanced water holding capacity

(Subramanian et al., 2003)

Cheddar cheese

To investigate the stress-relaxation characteristics

Generalized Maxwell and Peleg models

Peleg model well described the stress-relaxation behavior of cheeses above 50°C and 8 element Maxwell model predicted better than 3 and 6 element Maxwell models. The stress-relaxation experiment differentiated the viscoelastic nature of different cheeses. This was due to fat content reduction, increase in moisture, melting temperature, and age of cheese

(Venugopal and Muthukumarappan, 2001)

PART | II  Product Specific Studies in Rheology

Cheese type

Objective

Model

Result

Reference

Process cheese

To investigate the influence of high and low levels of calcium and phosphorous content, residual lactose, and salt-to-moisture (S/M) ratio on the viscoelastic properties of eight types of process cheeses

Six-element Kelvin–Voigt model

Process cheeses with high Ca and P content and high S/M ratio were significantly harder (P < 0.05) (higher storage and loss modulus, and lower creep and recovery compliance) compared to low Ca and P content and low S/M ratio process cheeses.

(Biswas et al., 2008)

Edam cheese

To compare the influence of using two probes (a 100-mm plate and a 5-mm spherical probe) during a compression test on the trends of hardness changes during ripening of Edam cheese.

Peleg model

By means of Peleg’s model, the extent of material relaxation and the initial rate at which the stress relaxes were calculated. The values of cheese hardness within the ripening period determined by the probes strongly correlated among themselves and also with the sensorially evaluated rigidity.

(Bunˇka et al., 2013)

Siahmazgi cheese

To assess the rheological specifications during ripening

Peleg model

The elasticity nature was greater than the viscous nature of samples in terms of the domination of storage modulus over loss modulus during ripening

(Farahani et al., 2014)

Minas Frescal cheese

To study the rheological properties

Peleg model

Higher numerical values of ε indicate greater deformability as ε measures the deformation that occurs before the cheese ruptures. The higher numerical values obtained in this study indicate less deformable cheeses. The deformability may be related to the chemical structures of cheese components and also by small variations in the processing technology

(Magenis et al., 2014)

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Cheese type

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The term

n



 −t     is called the creep function and denoted by ψ(t) i 

∑ Di 1 − exp  τ

 i=2 To linearize and present the creep behavior of foods, the Peleg model can be applied. Using constants k1 and k2, the Peleg model describes the creep function as

ψ (t ) = D(t ) − D0 −

t t = η1 k1" + k2" t

A material, under a constant stress, has two strain components. The first one is an elastic component that happens instantaneously followed by immediate relaxation on release of the stress whereas the second one is a viscous component which increases with time, as long as the stress is applied. The Maxwell model for creep/constant-stress situations claims that strain increases with time in a linear fashion. The Maxwell model creep is given by the following equation: t  γ (t ) = γ 0  1 +   λ

10.2.6  Linear and Nonlinear Viscoelasticity The rheological behavior of cheese is viscoelastic. The simplest type of viscoelastic behavior is linear viscoelasticity, where the material is only slightly perturbed from its equilibrium state. Curves of frequency-dependent storage modulus, G(w) and loss modulus, G0(w) are commonly used to describe linear viscoelastic behavior. The small amplitude oscillatory shear (SAOS) measurements are commonly used to study the linear viscoelasticity of cheese (Gunasekaran and Ak, 2000). The main feature of SAOS tests is that, due to small stress/strain used, they can be categorized as objective/nondestructive tests appropriate for penetrating material configuration and structure development during different processes. There are four major experimental variables in any dynamic test: strain (or stress), frequency, temperature, and time. Thus, different types of dynamic tests can be set up changing one or more of these experimental variables. Table 10.4 shows a better understanding of these concepts. Processing operations such as extrusion and stretching & molding operation during Mozzarella cheese manufacture involve large and rapid deformations that cannot be modeled using the theory of linear viscoelasticity. The deformations involved in nonlinear viscoelasticity are neither small nor slow. Table 10.5 represents measurement of nonlinear viscoelasticity.

10.2.7  Cheese Stretchability Stretchability is unique to Mozzarella and other pasta filata style cheeses. It is the property that allows Mozzarella cheese to form fibrous strands when heated

TABLE 10.4  Dynamic Tests Used for Measurement of Nonlinear Viscoelasticity Description

Stress/Strain sweep

• In this test, the moduli are measured as a function of increasing strain while the frequency is fixed. • The objective of a strain sweep test is to determine the critical point beyond which the dynamic shear moduli (G*, G9, G0) become dependent on the input variable, strain. • The strain sweep test is the first step in dynamic mechanical analysis. It is carried out before a frequency sweep test to lay down the boundary levels of strain for frequency sweeps.

Example

Reference (Patarin et al., 2014)

The cheese samples were exposed to increments of strain from 0.001% to 5% at a frequency of 1 Hz. The selected frequency relates to the order of magnitude of the typical times of the stresses that the cheeses experience during chewing. The curves typically displayed a plateau at small strains, the LVR, and then quickly decreased. The plateaus of G9 and G0 describe the LVR of the cheeses. The LVR limit strain (γlin) marks the end of the LVR, where G9 and G0 begin to decrease. (Continued)

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Dynamic test

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TABLE 10.4  Dynamic Tests Used for Measurement of Nonlinear Viscoelasticity (cont.) Description

Frequency sweep

• It is the most versatile rheological test to characterize the viscoelastic behavior. • In this test, a sinusoidal strain (or stress) of fixed amplitude is imposed on the material and the dynamic moduli are determined over a wide range of frequencies. The resultant plot is also known as the “mechanical spectrum” of the material. • Contemporary rheometers are capable of measuring dynamic properties from 0.01 to 100 Hz but with advanced rheometers one can conduct oscillatory measurements at frequencies as low as 10–5 Hz.

Example

Reference (Farahani et al., 2014)

Elastic and loss moduli of Siahmazgi cheese as a function of shear strain (oscillating frequency 0.1 Hz, temperature 25°C) The viscoelastic parameters including storage modulus (G9), loss modulus (G0), and the ratio between the viscous and elastic properties of the material, phase angle tangent (tan δ) were determined under various frequency ranges (0.1–100 Hz) in a linear viscoelastic region. The lower the tan δ value (closer to 0), the less the cheese flows.

PART | II  Product Specific Studies in Rheology

Dynamic test

Description

Temperature sweep

• The temperature-sweep test involves measurement of dynamic moduli over a temperature range at constant frequency and constant strain (or stress) amplitude. • Temperature sweeps can be carried out in a ramp or stepwise fashion. • If the ramp mode is employed, then the rate of temperature change and frequency of oscillation must be selected carefully. • Temperature sweeps are essential to investigate phase transitions. • The temperature sweep test is also helpful to detect changes that would occur at rather high, and possibly inaccessible, frequencies if measurements were made at room temperature.

Example

Reference (Vogt et al., 2015)

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Dynamic test

Cheddar and Mozzarella cheese samples were heated from 20 to 70°C at a rate of 1°C per minute and were subjected to an oscillating frequency of 1 Hz. The complex viscosity (η*) was monitored to identify how each of the cheese samples responded to heating. Based on the results of the temperature sweeps, an Arrhenius plot of the complex viscosity against the temperature was constructed. The complex viscosity for all three cheese samples was found to decrease as the temperature increased. The complex viscosities of the Cheddar samples were observed to decrease at a greater rate than the Mozzarella sample.

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(Continued)

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TABLE 10.4  Dynamic Tests Used for Measurement of Nonlinear Viscoelasticity (cont.) Description

Time sweep

• Time-sweep measurements are made isothermally at constant strain/stress amplitude and frequency. • It is also known as a “gel cure” test, and may be carried out with a temperaturesweep experiment to examine changes in rheological behavior due to combined effects of time and temperature. • Generally, the oscillation frequency is set at 1 Hz. • Time-sweep measurements are very useful in monitoring the build-up or breakdown of structure.

Example

Reference (Ong et al., 2012)

A dynamic time sweep (7200 s) analysis at angular frequency of 5 rad/s (0.8 Hz) and 1% strain was used to analyze the changes in storage modulus (G9) as the milk coagulated. The time taken for the cheese–milk to first reach a G9 of 140 Pa was recorded and used as the cutting time in cheese making experiments.

PART | II  Product Specific Studies in Rheology

Dynamic test

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TABLE 10.5  Methods Used for Stretchability Measurement Nonlinear viscoelasticity measurement concept

Description

Pipkin diagram

Pipkin proposed that material behavior at various frequency– strain amplitude (w –γ0) regimes can be depicted and Tanner plotted a dimensionless quantity, λw (Deborah number) versus a characteristic strain amplitude which is also dimensionless, A (=λ, the Weissenberg number) called “Pipkin diagram” At low frequencies, because the shear rate varies slowly with time, the deformation approaches that of simple, steady shear. As the frequency is increased, the stress will begin to lag the strain, as the material will exhibit viscoelasticity. At very high frequencies the response becomes more and more elastic. It has been suggested that at some critical strain amplitude, for molten materials the melt will slip at the wall, causing the stress signal to become erratic in oscillatory shear. Therefore, when the slip happens, it may be difficult to distinguish the effects of slip and nonlinear viscoelasticity. Large amplitude oscillatory shear (LAOS)

The LAOS flow occurs usually when strain amplitude, γo is more than unity. The LAOS test is particularly useful for characterizing nonlinear viscoelasticity, because the Weissenberg number (proportional to the strain rate amplitude) and the Deborah number (proportional to the frequency) can be varied independently.

Spectral analysis

The spectral analysis is the most direct way to evaluate LAOS data. For nonlinear viscoelasticity, σ(t) is not sinusoidal and σ(t) cannot be described in terms of two functions of frequency (modulus and loss angle or G9(w) and G0(w)). A few cycles after starting the test, the shear stress normally becomes a standing wave that can be represented using the Fourier series.

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and stretched. Cheese stretchability is defined as “the ease and extent to which melted Mozzarella can be drawn to form strings.” Stretchability of Cheddar type hard and semihard cheeses are also occasionally reported as a way of comparing the effect of manufacturing variables among cheeses. Empirical and instrumental methods can be used to measure the stretchability; however the empirical method is widely used. Table 10.6 describes the methods used for stretchability measurement (Fife et al., 2002; Apostolopoulos, 1994).

10.3  CHEESE MICROSTRUCTURE Cheese microstructure is the three-dimensional arrangement of the casein micelles that link together into bands and chains. These micelles create a viscoelastic protein network in which moisture, fat globules, minerals, and bacteria are distributed (Impoco et al., 2007). One of the major influencers of texture and functional properties of cheese is the microstructure. It also has an impact on the physicochemical, transport, and nutritional properties of cheese (­Impoco et al., 2012). Fluorescence spectroscopy has been efficaciously employed to assess ­molecular-level interactions between fat and proteins in numerous food-based emulsions. It has also been applied to observe structural changes in cheese and nonenzymatic browning in milk and other dairy products (Herbert et al., 2000). Tryptophan in the cheese casein is a naturally occurring fluorescent substance. The fluorescent properties vary in a hydrophobic and hydrophilic environment (Karoui and Dufour, 2003). Fluorescence spectroscopy is also employed by researchers to observe the spectra of tryptophan. This enables to predict the microstructure of any type of cheese. Simultaneously, it can also point out ­Maillard browning and oxidative stability of processing cheese while in storage period (Dufour et al., 2001).

10.4  CHEESE FUNCTIONALITY 10.4.1  Factors Affecting Functionality The factors that affect the functionality of cheese ranges from the initial material, that is, milk to the postprocessing parameters. The factors are presented in Fig. 10.1

10.4.1.1  Milk Properties Milk, the primary raw material, has a direct effect on the functional properties of cheese. The standardization of milk (casein-to-fat ratio) is also responsible for cheese structure; else the cheese may be too soft or too hard. The melting point of milk fat changes seasonally and is directly related to the melting and stretching of cheese at high temperatures. The buffering capacity of milk is primarily due to colloidal calcium phosphate, soluble phosphate, citrate,

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TABLE 10.6  Disadvantages of Schrieber Test Method

Description

Empirical methods

One of the oldest methods to test stretchability is with the help of a fork. The “fork test,” as it is called, is performed by picking up a lump of melted cheese vertically with a fork until the bulk of the cheese strands break The length of the strands at failure is taken to indicate stretchability. Results of a fork test are only suitable for sample-to-sample comparison at the same location

Instrumental methods

The objective test accounts for the applied force or stress (the ease of stretch) and the failure deformation or strain (the extent of stretch). A prerequisite for a proper tensile test is a proper grip on the sample. For cheese, a good grip is not easily achieved as the material is soft. Therefore, they tend to deform at the grips and the stress concentration around the grip areas also leads to failure at the grips. This complicates data analysis. Additional problems are due to high test temperature and the difficulty in measuring the applied stress. Since the fibrous strands that form continuously yet randomly will thin out and break, typical stress profiles are very jagged.

Vertical elongation

Tensile tests that vertically strain a cheese sample until failure reflect how intuitively the stretch is perceived, in effect mimicking the fork test. It is the most popular tensile test configuration. Common problems associated with stretch tests are the effect of deformation rate and temperature change during the test. The cheese strands can cool rapidly because they are exposed to room temperature and, more importantly, are thinning out. Low-fat Mozzarella does not stretch well, as indicated by the poor stretch length and quality but low-fat Cheddar is better than low-fat Mozzarella

Horizontal extension

Cheese is stretched horizontally even though the test device operates as if it were a tensile test. For tensile tests, the sample is normally held vertical, but the problem of sample sagging under gravity occurs. It is overcome in this method by surrounding samples with a densitymatching medium. This also serves to maintain temperature uniformity and to prevent food samples from drying out

Compression tests

Compression is a rather unconventional testing mode to investigate stretchability. Apostolopoulos suggested using a “compressive elongation test.” (Apostolopoulos, 1994) Though it is true that elongational viscosity can be related to stretchability, a uniaxial test is more suitable because the compressive-elongation test measures a biaxial property. Furthermore, the viscosity will only provide information on “resistance to flow.” The more important “extent of stretch” cannot be obtained from this type of test.

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FIGURE 10.1  Factors affecting functionality of cheese.

bicarbonate, and casein. Depending on pH and temperature, approximately twothirds of the calcium is colloidal and the rest is in the form of solution. The proportion of colloidal calcium phosphate and pH influence the stretchability of cheese. Also studies have showed that calcium and phosphorus levels decrease with reduction in pH, thereby increasing meltability. Homogenization of milk reduces fat-globule size, alters the fat globule membrane, and creates a new fat–water interface predominantly containing caseins that can make fat globules more stable. Additionally homogenized milk can increase cheese yield. However, the adverse effects of homogenization (at high pressures ∼6.7 MPa) include poor body, texture, reduced stretchability, and meltability. Milk proteins play a vital role in the functionality of cheese. The hydrolysis of αs1-casein and β-casein affects functional properties during maturation.

10.4.1.2  Cheese Manufacturing Procedure 10.4.1.2.1  Starter Culture The idea behind adding the culture is for acid production, proteolytic activity, and utilization of sugars such as galactose, glucose, and lactose. The rate of acid production is critical as it can affect cheese composition and meltability. The proteolytic activity of the starter culture affects rheological and textural properties of cheese through slow but progressive breakdown of caseins during storage. Some species are not capable of fermenting galactose, contributing to

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Maillard browning of cheese during cooking while some cultures reduce the extent of browning. 10.4.1.2.2 Coagulants Coagulants can be enzymes or acids. The enzymes used for coagulation can be classified into four types based on their source as: animal coagulant (rennin), microbial coagulant (enzymes from R. miehei and C. parasitica), plant coagulant (extracts from Cynara cardunculus and Calotropis procera), and fermentation produced chymosin (pure chymosin). The major acids used as coagulants include malic acid, citric acid, acetic acid, hydrochloric acid, and phosphoric acid. According to the literature, about 6% of the coagulant added to milk is ­active in the cheese curd (Gunasekaran and Ak, 2002). During the primary proteolysis, caseins break into peptides followed by the secondary proteolysis where the peptides are fragmented into smaller peptides and free amino acids by the starter culture enzymes. Direct acidification of milk influences the functional properties of cheese depending on the type of acid used and pH. Cheese viscosity is believed to decrease with decreasing pH. 10.4.1.2.3  Curd Handling The coagulated mass is cooked to enhance syneresis and the temperature at which the curd is cooked in the whey affects the rheological properties. A higher cooking temperature reduces the moisture content and shrinks the curd. It also enhances the metabolic activity of bacteria in the curd, increasing lactic acid production and lowering pH, further contracting the curd. This action makes cheese acidic, hard, crumbly, and dry. The curd is salted and pressed to form the cheese block and this process is also responsible for their characteristic texture. Pressing promotes matting of the curd particles into a contiguous, firm mass (Cheddar cheese) or prevents curd particles from matting so that an open texture results (cheshire cheese). 10.4.1.2.4  Cooking, Stretching, and Cooling The cooking and stretching process is unique to cheeses such as Mozzarella cheese. The pH and temperature are critical factors that affect their characteristic properties. Mozzarella cheese curd is normally cooked at 40°C or higher to remove moisture and cause inactivation of starter culture. Such high temperature lowers cheese meltability, and stretchability. However, when the temperature is reduced to 35°C, it results in a softer cheese with a higher level of proteolysis after the cheese is made. For optimal stretching, there is an optimal combination of pH and temperature. The curd temperature is generally about 55–60°C. For instance, curd at pH 5.1–5.4 should be placed in hot water at 70–82°C for stretching. Higher stretching temperature increases inactivation of proteolytic organisms while reducing

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primary and secondary proteolysis during aging. The method of curd stretching affects the cheese properties. For example, extruder stretching resulted in a cheese with lower meltability and no detectable free oil. Stretched curd is cooled in chilled water-cooling towers to limit the growth of undesirable microorganisms which may lead to soft-body texture defect and gas holes. Soft-body defect renders cheese soft and pasty with poor shredding qualities and excessive meltability. Cooling continues to occur when Mozzarella cheese is placed in brine for salting and eventually leads to variations in cheese meltability, stretchability, and free-oil formation at different locations within the block.

10.4.1.3  Composition of Cheese 10.4.1.3.1  Moisture Content Moisture is a major constituent and comprises more than one-third of the cheese mass. The moisture content in cheese is affected by various factors such as method of cooking, temperature, and salt content. For instance, in mozzarella cheese, slow screw increases the manufacturing time resulting in lower moisture content cheese. Greater the moisture content, the softer the cheese and the better its meltability; however, it has poor shredability. 10.4.1.3.2  Fat Content Fat content varies from 20 to 33% in semihard and hard cheeses. Fat in cheese is present as globules contained within the protein matrix network and hence they are considered as “fillers” that influence the rheological, functional, textural and sensory properties of cheese. The size and distribution of fat globules have a significant effect on the properties of cheese. The average fat-globule size varies from about 1.5 to 4 µm and three-dimensional evaluations indicate that the higher the fat content, the higher the number of large fat globules and the higher the average fat-globule size. Higher fat content allows cheeses to melt better, but it may be more difficult to shred. A lot of studies have focused on the reduction in fat content based on consumer interest in a low-fat diet, but fat reduction is also associated with increase in moisture, protein, calcium, phosphorus, and ash content and decrease in milk nonfat portion, fat in dry matter, and salt-moisture ratio. As the fat ­content decreases, changes in physical properties and flavor lower the cheese quality. Low-fat cheese also tends to form a dry film on the surface during heating limiting cheese meltability. Several technological changes have been proposed, including use of fat replacements, to improve functional properties of low-fat cheeses. These changes have met with only limited success. Nonetheless, consumer acceptance of lower fat cheeses has only been tepid. Therefore, some hard-cheese plants are adding extra cream to make higher fat cheeses that offer improved functional properties.

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10.4.1.3.3  Salt Content Salt can have a major effect on properties of cheese in spite of being a minor constituent. In addition to enhancing taste, it controls moisture content, microbial growth, and acidity. Saly is added directly (Cheddar cheese) or by placing the cheese in brine after the mixing and molding step (mozzarella cheese). In general, cheese with a salt content greater than 2% melts poorly and is less stringy. Insufficient proteolysis due to high salt content can cause a curdy texture in certain cases. The effect of salt on the functionality of cheese is also related to the changes in water-binding capacity. A low salt level and high moisture content can make cheese pasty and off-flavored. 10.4.1.3.4 pH The slightest change in pH affects the functional properties dramatically. Cheese turns brittle when pH is less than 5.0. Softness and meltability are also affected by pH. The pH is not the singular dominant factor but a combination with temperature and other ingredients affect meltability.

10.4.1.4  Postproduction Processes 10.4.1.4.1 Aging Aging is essential for the cheeses to develop their functional properties and flavor. The ripening period stages from few weeks to 24 months. The proteolytic hydrolysis of intact caseins into peptides and free amino acids is one of the driving forces for changes in functional characteristics of cheeses during aging. Enzymes from several sources contribute to proteolysis. These sources are as follows: milk (plasmin), coagulant (rennet, chymosin, etc.), starter, secondary starter, and nonstarter microorganisms. Breakdown of caseins during proteolysis leads to reorganization and weakening of the protein matrix and enables the fat globules enmeshed within the matrix to be released such that they coalesce when cheese is heated, thus increasing meltability Mozzarella cheese is considered an unripened cheese. The high-temperature mixing–molding step during its manufacture partly inactivates the coagulant. Browning of mozzarella is also affected by aging. 10.4.1.4.2 Freezing Freezing of cheese increases shelf life and preserves color, flavor, and nutritive value. Commercially, cheeses are frozen to stop ripening and prolong shelf life; however, freezing may induce undesirable physical and organoleptic changes. Cheddar and Mozzarella cheeses when exposed to different freeze–thaw conditions became softer and melted better after freezing and thawing. 10.4.1.4.3  Heat Processing Temperature has a profound effect on meltability and stretchability. The heating temperature (70–200°C) has a linear relationship with intensity of browning but the browning is more related to heating duration. At low temperature,

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prolonged heating (70°C for 20 min) does not brown the cheese, but a brief exposure to high temperature (1–3 min at 200°C) results in significant browning. Increased viscosity during heating is thought to be the protein aggregation by hydrophobic interactions among the caseins. As cheese is heated, the protein matrix adsorbs energy that influences the interactions that maintain the ­protein structure. As a result of the opposing temperature dependences, proteins ­unfold in the 60–80°C. During heating, accumulated protein aggregations in the ­casein matrix change the moisture distribution within the protein matrix. Local ­hardness and uneven distribution of moisture in the protein matrix decrease cheese meltability.

10.4.2  Melt and Flow Properties Cheese is an important ingredient in pizza and it is exposed to high temperatures to melt and flow as consumer preference depends on the quality of melted cheese. Therefore, characterization of melt and flow properties of cheese is extremely critical for successful use of cheese as an ingredient. Meltability may be defined as “the property of cheese shreds to fuse together upon heating” or “the ease and extent to which cheese will melt and spread upon heating” depending on the use of the ingredient and measurement criteria. Melt and flow properties can be measured by empirical and instrumental techniques.

10.4.2.1  Empirical Test 10.4.2.1.1  Schreiber Test Remove two thick or three thin cheese slices from the sliced production run every 10 min and stack them to give a 0.5-cm (3/16-in.) thickness. Then insert a sharp-edged copper cylinder or round cookie cutter with 41-mm (1.6-in.) inside diameter into the slices and push out a sample onto the center of a clean glass Petri dish. Set this thin-walled 15- × 100-mm dish with a cover marked with an identification number in a kitchen oven, preferably electric, at 232°C (450°F) for exactly 5 min. Using thermal safety gloves, remove the plates and set them to cool on a flat surface for about 30 min. Then center them over a concentrically numbered target-type graph. Look through the uncovered glass Petri dish and record numerically the outer edge of the flow line. As the cheese melts uniformly and easily, its diameter and flow line number increase. Cheeses attaining a value of 4 or higher are acceptable. Cheeses with values below 4 are rejected and corrective action is immediately instituted. A dark brown discoloration indicates the presence of sugar or high pH. Table 10.7 shows the shortcomings of the Schreiber test method (Gunasekaran and Ak, 2002). 10.4.2.1.2  Modified Schreiber Test Muthukumarappan et al. conducted the Schreiber test at different oven temperatures (60–232°C) and used different heating surfaces (Petri dish, aluminum

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TABLE 10.7  Advantages of UW Meltmeter Test Problem

Description

Excessive heat treatment

During pizza baking, evaporative cooling effect due to moisture in the crust and other ingredients keeps the overall cheese temperature well below the oven temperature. Therefore, most cheeses get scorched and show brown or black discoloration especially at the edges when heated at 232°C

Uncontrolled heating

The cheeses undergo nonuniform temperature distribution when heated in an oven. As the outer edges begin to flow, this thin layer then gets heated further to even higher temperature, causing both moisture loss and scorching. Moisture loss during heating may adversely affect measurements if the heat and mass transfer properties of the cheeses being tested are different. This condition is further aggravated by the excessive heating discussed previously. Cheeses may also develop a thin surface film due to exposure to air.

Measurement of flow line

This is one of the simplifying elements of the Schreiber test. However, the measurement of flow line indicated by the leading edge of the melted cheese flow is appropriate only if the melted cheese spreads evenly into a circular pattern. This occurs with some regular-fat natural cheeses. Many other natural and process cheeses, especially lower fat types, spread very unevenly when heated. In such cases, the leading edge flow line measurement gives totally misleading data

plate, stainless-steel plate) (Muthukumarappan et al., 1999). They measured both the cheese flow line per and the cheese spread area. The spread area was determined by a computer vision method. Different heating surfaces were used to determine if thermal and surface tension properties would have an effect on the extent of flow. Based on this investigation, they proposed that the Schreiber test for Mozzarella should be performed at 90°C for 5 min on an aluminum plate and that the melted spread area should be measured as an indicator of cheese meltability. Gunasekaran et al. replaced the convective oven with direct conduction heating via the metal plate on which the cheese disk is heated and allowed to flow (Gunasekaran and Ak, 2002). Removing the oven reduced the overall cost and space requirements and also the sample was more easily accessible for spread length and area measurements. The conduction-heating test is faster and allows continuous cheese melt/flow measurement. Further, this system could be adapted to make multisample measurements, enabling more consistent cheese meltability measurements to be faster and more efficient.

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10.4.2.2  Objective Tests 10.4.2.2.1  Steady Shear Viscometry The test is carried out used a Brookfield viscometer along with a T-bar spindle. The viscometer reading in relative scale (in %) was measured as a function of cheese temperature. As the viscometer geometry and speed are arbitrary while the temperature distribution is not uniform, this test is of limited value. Steady shear viscometry is inherently unsuitable for measuring cheese viscosity due to fat separation. As cheese is heated, the fat melts and lubricates the stationary and rotating cylinders (or plates) to such an extent that they slip past each other. The entire molten cheese mass is either left in the middle between the concentric cylinders (or plates) or rotates en masse. For fresh and soft cheeses, steady shear viscometry has been applied successfully to characterize their flow properties. 10.4.2.2.2  Capillary Rheometry Capillary rheometry is a well-developed test and has structured data a­ nalysis procedures, making it a natural choice for cheese viscosity measurement. ­Slippage problems due to fat separation interfere with viscosity measurements of Cheddar and American process cheeses. The requirements for valid measurements using a capillary rheometer are as follows: (1) isothermal flow; (2) negligible radial flow; (3) negligible wall slip; (4) fluid is incompressible; (5) flow is laminar; and (6) minimal end effects. Major problems in testing cheeses using a capillary rheometer are the presence of viscoelastic effects and end effects. The fat separation further complicates the situation. Therefore, correction factors must be applied which is time-consuming and tedious, as it requires multiple tests using capillaries of different diameters. These problems along with expensive test instrumentation prevented exploration of cheese melt properties using the capillary rheometer. 10.4.2.2.3  Squeeze-Flow Rheometry This test is performed by eliminating friction between the sample-­compression platen interfaces. The test procedure is also simple and straightforward— compressing the sample axially between two lubricated plates in a uniaxial instrument. Under this configuration, assuming perfect slip, shear stress at sample–platen interfaces is zero. This method is suitable for cheese meltability evaluation for the following reasons. The first one is that melt is a biaxial phenomenon, hence, biaxial elongational viscosity determined from test adequately describes the melt characteristics. The other one is that the presence of slip at sample–platen interfaces caused by fat separation is not only a prerequisite for a proper test but also is incorporated in the calculation of results. Biaxial elongational viscosity decreases with biaxial strain rate and this validates the strain-rate thinning behavior of cheeses.

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10.4.2.2.4  UW (University of Wisconsin) Meltmeter The UW Meltmeter performs lubricated squeeze-flow tests (Wang et al., 1998). The device is made of aluminum and has a movable outer cylindrical annulus (75-mm outer diameter; 30-mm inner diameter). The annulus can be moved up and down by a lever around a 30-mm diameter stationary center cylinder. The stationary cylinder is equipped with an electric heater operated by a temperature controller. At the start of a test, the lever arm is raised such that the annulus is up, forming a 30-mm-diameter, 7-mm-deep sample well over the stationary cylinder. A sample of the same size as that of the sample well is placed in the well. The top of the sample should be flush with the top surface of the annulus, which serves as a platform for the melted cheese to flow and spread. The top of the cheese surface is covered with a 66-mm-diameter lubricated circular plate attached to a linear variable differential transformer (LVDT) rod. The role of the circular plate is to: (1) effectively seal the plate–cheese interface, preventing any loss of moisture from the sample during heating (2) maintain constant contact with the cheese, enabling continuous monitoring of sample flow, and (3) apply the force required during a test to cause the melted cheese to flow along with the LVDT rod. The LVDT is supported separately and connected to a computer data acquisition system. The sample is heated to the test temperature. A fine thermocouple inserted into the sample before the test monitors cheese temperature within 1°C and controls the heater. Once the sample attains the desired temperature (60°C), the lever arm is lowered to bring the annulus down. Simultaneously, the sample is subjected to lubricated axial compression due to the weight of the circular plate and LVDT core. This causes the cheese to flow. Additional weight may be added or a lighter plate can be used, as required, to change force causing flow. The sample height versus time of flow data is continuously recorded. The UW Meltmeter can also be operated under constant deformation rate by removing the LVDT and circular plate and bringing the test platen of a uniaxial testing machine in contact with the sample and flow platform at the beginning of a test. When the sample reaches the test temperature, the lever arm is lowered and crosshead of the uniaxial testing machine is activated simultaneously to deform the sample at a constant rate. Table 10.8 presents the advantages of the UW Meltmeter test over the Schreiber test. The problems associated with the meltmeter are that each measurement takes too long, multiple tests cannot be performed simultaneously, and moving parts get clogged. Modifications were made to the meltmeter; however these changes are substantial and have not led to a device suitable for routine industrial use. The UW Meltmeter still is a suitable test device for R&D. 10.4.2.2.5  Dynamic Shear Rheometry The dynamic shear rheometry is used for characterizing rheological properties of foods. Applying this technique for cheese meltability evaluation has the problem

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TABLE 10.8  Types of Material Failure Due to Fracture Advantages

Description

Controlled heating

The sample is heated to a point higher than the melt temperature (60 or 65°C), which eliminates sample scorching at the edges

Uniform temperature

The sample is not allowed to flow until the entire mass is at a uniform temperature. This prevents uneven temperature distribution during heating in the oven and the concomitant nonuniform flow pattern

Prevention of moisture loss

Since the sample is tightly covered during heating and a layer of oil is applied in the UW Meltmeter, there is no moisture loss, which improves the consistency and accuracy of meltability data

Continuous monitoring of flow

The LVDT and circular disk maintain continuous contact with the sample during flow, allowing calculation of flow rate. In the Schreiber test, only the end of the flow is measured

of excessive slippage due to fat melting at elevated temperature. The problem has been dealt by using serrated plates or plates with a fine grade of sandpaper glued on, or by bonding samples directly onto a plate using a commercial adhesive such as cyanoacrylate ester. Another problem encountered is moisture loss, especially at high temperatures and during tests that take a long time but it can be rectified by applying a mineral oil or similar coating to the samples. 10.4.2.2.6  Helical Viscometry In this method, a rotational viscometer is used. A T-bar spindle is attached to the rotating part instead of a cylindrical spindle as used in a typical rotational viscometry. The T-bar is lowered inside a ground mass of cheese held at 60°C in a glass beaker. The torque to rotate the spindle at a certain speed (1 rpm) as the T-bar spindle is raised through the melted cheese is recorded. The peak torque recorded and expressed in relative units of the full-scale response of the viscometer is used as an index of meltability. For lack of a better method and due to its apparent objectivity by using a commonly available viscometer, several researchers have since used this method for reporting meltability.

10.4.3  Fracture Properties Fractures originate from debonding of atoms, nucleation, or growth and coalescence of microcracks and microcavities. A fracture is a failure mechanism that encompasses stable or unstable spread of a defect within the material structure. Table 10.9 classifies the material failure due to fracture.

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TABLE 10.9  Classification of Failure During Cheese Fracture Failure

Description

Brittle or quasi-brittle failure

Fracture occurs without significant irreversible strain

Ductile failure

Failure at large plastic strain at low temperature (∼1/4 of the melting temperature)

Creep failure

Failure at large plastic strain at high temperature (>1/3 of the melting temperature)

Fatigue failure

Failure due to monotonous loading either above or below yield stress. Classified as low, high, and giga cycle fatigue damage

Fracture-mechanics analysis is used to prevent fracture or spread of a defect. It also applies to adverse fracture in cheeses which may lower overall quality and customer expectation. Compression, tension, shear, torsion, and bending may be used to evaluate the fracture properties. Tension or bending is more appropriate for evaluating fracture properties as it is easy to witness the crack initiation and propagation. However, tension tests are challenging to execute with soft materials such as cheese. Hence, bending test is the desired test mode. Compression tests are conducted until sample failure, even when the objective of the test is not to estimate the fracture properties.

10.4.3.1  Notch Test A notch is utilized to test materials in tension and bending with known crack size. It is made by forcing a razor blade into cheese to a measured distance (the crack length). The cheese sample is prepared such that width (w) is twice the thickness (B), and the ratio of span (S = distance between supports) to B is 4 for large samples (Gunasekaran and Ak, 2002). The fracture toughness is calculated as G=

A  a Bwϕ    w

10.4.3.2  Cutting With Wire and Blade Cutting cheese with wire and blade is very popular. Wire cutters range from small tabletop units to large-scale cutters at the factory level. Cutting with wire and blade comprise fracture, plastic deformation, and friction. The elastic–plastic fracture mechanics theory deems that the material flows only in a small area around the crack tip. Therefore the stored and flow energies are limited. During wire cutting, it can be assumed that only the material in the vicinity of the wire undergoes plastic deformation. The total energy during cutting may be considered to comprise three major components: friction, flow, and fracture.

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10.4.3.3  Eye/Slit Formation and Growth Small and round holes in a cheese mass are a characteristic and desirable feature of Swiss, Gouda, and Edam cheeses. A nucleus is required in order for a hole to form. The eyes are formed primarily from CO2 (fermentation of propionic and citric acids) and N2 (dissolved in the cheese milk). Small air bubbles (N2 in milk) attached to curd particles may form as nuclei along with some impurities and small mechanical openings. The nuclei grow into eyes due to diffusion of CO2. The size, number, and distribution of eyes can be related to the time, quantity, intensity, and rate of CO2 production in cheese. Holes formed in cheeses such as Tilsit and Havarti are not called eyes and holes present in Cheddar-type cheeses is caused by spoilage organisms producing CO2, H2, or H2S indicating a quality defect. Even in cheeses where eyes or holes are customary, cracks are formed under certain conditions, pointing a quality defect.

10.4.4  Texture Properties Texture of foods is the “eating quality” of foods encompassing many properties of foods that excite our senses of sight, touch, and sound. The International Organization for Standardization defines texture of a food as “all the rheological and structural (geometric and surface) attributes of the product perceptible by means of mechanical, tactile, and, where appropriate, visual and auditory receptors.” Texture is the primary quality attribute of cheeses. The overall appearance and mouthfeel of cheeses are appreciated before their flavor. Cheeses offer a variety of textures, for instance, Mozzarella cheese is “stretchy” or “stringy” and Parmesan cheese is “crumbly.” The cheese wheel (Fig. 10.2) comprises five major sectors: flavor, texture, aroma, appearance, and taste. Make note of the texture sector that is further divided into classes and subclasses to list corresponding sensory attributes. Cheese texture is a reflection of its structure at the molecular level. The major structure-forming constituent is the casein matrix in which fat globules are entrapped and water is bound to casein and fills interstices of the matrix. This network structure is significantly influenced by the protein, fat and water content, and the biochemical activities that occur during storage. During manufacture of cheeses, the factors that can contribute to the eventual cheese texture are moisture content, acidity, temperature, and pH. For instance, higher curd scalding temperature leaves the curd springy, and the resulting cheese. Lower pH of milk at the time of enzyme addition results in harder cheese Texture of cheese changes continuously even after it is manufactured due to the proteolytic action of the residual enzymes. The most notable change with age is decrease in fracture strain and springiness and increase in creaminess. The factors that have an effect on cheese texture during ripening are as follows: (1) pH at which whey is drained from the curd as this determines the proportions of chymosin and plasmin in the cheese (2) salt-in-moisture ratio that controls, along with temperature, the activity of residual rennet and plasmin in

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FIGURE 10.2  Cheese wheel.

cheese, and (3) pH of cheese after salting. The textural changes in cheese during storage occur in two phases. In phase 1, the first 2 weeks after manufacture, there is a rapid change during which the casein network results in the softening of the cheese and in phase 2, the proteolytic changes are gradual and the protein matrix becomes less cohesive.

10.4.4.1  Empirical Texture Measurement 10.4.4.1.1 Crumbliness An empirical method based on uniaxial compression test to quantify the ­crumbliness of cheese was developed (Hwang and Gunasekaran, 2001). The crumbliness is a unique textural property of cheeses like Queso Fresco (Latin American white cheese) that are crushed and sprinkled on foods and then consumed. These cheeses maintain their integrity under heat, so they are used in casseroles, enchiladas, quesadillas, tacos, and other dishes that are broiled or baked before serving. The compression test is performed to 90% deformation

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at a speed of 1250 mm/min to crumble the cheeses. The crumbled samples are analyzed for their particle size using sieves with opening sizes ranging from 12.70 to 1.41 mm. A geometric mean diameter and total number of particles are calculated as follows:  n M log d  ∑ i i Geometric mean diameter = log −1  i =1 n    ∑ i =1M i  Total number of particles =

Mt exp(4.6σ ln2 − 3ln dgm ) βv ρ

where Mi is the mass (g) retained by ith sieve and di is the geometric mean diameter (mm) on ith sieve; Mt is the total mass (g); βv is the shape factor for calculating volume of particles (= π/6, assuming spherical shape); ρ is the particle density (g/cm3); σln is the lognormal geometric standard deviation of parent population by mass in natural logarithm; and dgm is the geometric mean particle diameter (mm) by mass. 10.4.4.1.2  Cone Penetrometer Cone penetrometer is one of the rapid empirical methods used to evaluate the consistency of wide variety of solid, semisolid, and nonfood products. It also allows direct measurement of properties such as hardness “on the spot” which avoids textural damage due to transfer of sample from its original packaging to the measurement cup. Also, the rigidity and firmness of cheese can be determined. Three modes of operations are possible with the cone penetrometer: (1) a cone assembly of specific dimensions and weight is allowed to sink into the sample, and the depth of penetration after a fixed time is measured; (2) a cone assembly of specific dimensions and weight is released into the sample, and the depth of penetration is measured when it comes to rest due to yield stress of the test material, and (3) a cone assembly of specific dimensions and weight is driven at a constant speed into the sample, and the force required for the cone penetration is recorded. The surface of the specimen must be smooth and flat. In the constant weight test the penetration will be quick initially, but will gradually slow down and finally come to rest. The penetration depth at rest (h) is used to calculate an apparent yield stress (σapp):

σ app =

Mg α π h 2 tan 2    2

where M is the cone mass, g the acceleration due to gravity, and α the cone angle.

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10.4.4.1.3 Stringiness Stringiness is the ability of the cheese to be peeled of as “cheese strands” by tearing at room or elevated temperatures. Stringiness may also be measured empirically by allowing the material to flow from a spoon or a funnel and determining the length of the thread formed. In certain cases, the stretchability of cheese is measured by the fork method.

10.4.4.2  Instrumental Texture Measurement 10.4.4.2.1  Texture Profile Analysis (TPA) The TPA test is a type of uniaxial compression test. The primary differences between the TPA test and uniaxial compression test are as follows: (1) unlike in compression tests, the TPA test is performed by subjecting a cylindrical specimen to a two-step compression. The first compression step, known as the “first bite,” is followed by a second compression, the “second bite.” This is to simulate the first two bites taken during chewing of the food. The two compression steps may be separated by an optional wait time; and (2) deformation used in the TPA test is often 70% or more. To imitate the chewing action more closely, a 90% compression is suggested. The uniaxial compression tests are terminated at or before macroscopic sample failure. The many textural parameters determined from the TPA curve are as follows: hardness, cohesiveness, adhesiveness, gumminess, springiness, and fracturability. Since cheese is a viscoelastic material, the rate of compression and time between first and second bites will affect the test results. The fracture strain of cheeses is in the order of 25–60%. In typical TPA tests, the cheese is compressed 70% or more of the sample initial height. Thus, the sample is compressed beyond its macroscopic failure. 10.4.4.2.2  Compression Test Uniaxial tests other than the TPA are also widely used in measuring cheese properties. The uniaxial compression test procedure is discussed in the uniaxial testing section. Among the textural attributes, firmness and springiness correlate well with test data. Cohesion of hard cheese can be measured by uniaxial compression, tension, three-point bending, cutting tests, and stress-relaxation test. Out of these, uniaxial tension is the best to quantify cohesive properties of hard cheeses. 10.4.4.2.3  Wedge Fracture Test In this test, a wedge is driven into a specimen until it is fractured by propagation of a crack in a stable manner ahead of the tip of the wedge. It is called the f-Wedge test to emphasize the fact that the material is fractured in a controlled manner, and to distinguish this test from other wedge tests that simply push a wedge through the sample more like a penetration. The controlled crack

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propagation is essential for accurate calculation of fracture energy. For the fWedge test, the fracture energy is calculated as follows: Fracture energy =

0.75( Eu 2 H 3 ) H a 4 [1 + 0.64   ]4  a

where E is the modulus of elasticity, H the half sample width, u the distance between the split ears of the sample where the wedge is forcing them apart, and a the length of one split ear. 10.4.4.2.4  Texture Map The texture map is a plot of fracture stress versus fracture strain of a product manufactured or tested at varying compositions, pH, and age. The texture map can be divided into four quadrants to represent various material textures. The products that fall in Quadrant 1, lower left, are soft and “short” and materials in this quadrant are labeled “mushy.” In Quadrant 2, lower right, the materials are soft but are “long” and are known as “rubbery.” The materials that have high fracture stress and fracture strain are “tough.” These are located in Quadrant 3, top right. The materials that are firm but have a small fracture strain are “brittle.” These will fall in ­Quadrant 4, top left. A texture map for some selected cheeses is presented in Fig. 10.3.

FIGURE 10.3  Texture map of cheese.

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