Physical Properties

2.4.1.1 Rheological analysis

Rheology is the study of the deformation and flow of matter (Laurati et al., 2009; Rao, 2007b; Tunick, 2000). Rheological responses occur at a macroscopic level but are influenced by the properties and changes occurring at a microscopic and molecular level (Genovese, Lozano, & Rao, 2007). The difficulty in rheology lies in linking the macroscopic rheological properties to the changes and properties at the microscopic and molecular level (Rao, 2007a). The microstructure of a material is related to its physical properties such as viscosity, texture, firmness and elasticity (Velez-Ruiz & Canovas, 1997).

The rheological properties of cheese refer to how it behaves relative to applied stress and strain such as during compression (O'Callaghan & Guinee, 2004). Cheese is classified as a viscoelastic material, in rheological terms, because it behaves with liquid and solid

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characteristics when subjected to stress or strain (Guinee, 2002b). The rheological properties of cheese are determined by its microstructure, composition, macrostructure and the physiochemical state of its components (O'Callaghan & Guinee, 2004). Being related to the structure and composition, the rheological properties also undergo significant changes during ripening (Rosenberg, Wang, Chuang, & Shoemaker, 1995). This is because the rheological properties are related to the structure, composition and strength of the bonds within the cheese network (O'Callaghan & Guinee, 2004) with a number of these parameters changing during storage.

The rheological properties of cheese are important due to their influence on the handling, texture and eating quality, the use of cheese as an ingredient, its ability to retain its shape as well is the ability of the cheese to retain gas if required (O'Callaghan & Guinee, 2004). The textural and functional properties of Mozzarella strongly influence consumer acceptability (Ak & Gunasekaran, 1997). These functional attributes are related to the rheological properties of the cheese.

2.4.1.1.1 Large strain

Large strain deformation tests are useful for food gels that hold their shape (Rao, 2007a). These tests include shear/torsion, cutting, compression tests (O'Callaghan & Guinee, 2004). These types of tests break down the gel network allowing comparisons to be drawn with sensory properties (Bowland & Foegeding, 1999). This type of testing has been used to assess the viscoelastic properties of cheeses (Holsinger, Smith, & Tunick, 1995).

Uniaxial compression

The most common type of rheological assessment of cheese involves the application of a linear uniaxial displacement (O’Callaghan, O’Donnell, & Payne, 2002). This typically involves subjecting a cube or cylindrical sample of cheese to large strains between two

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parallel plates and dynamically measuring both the displacement and force (Guinee, 2002c). Compression tests of cheese utilise equipment such as an Instron (Creamer & Olson, 1982) or TA XT texture analysers (Truong, Daubert, Drake, & Baxter, 2002). These devises allow a sample to be compressed at a fixed rate to a predetermined level while recording the force as a function of the displacement (Guinee, 2002c).

Uniaxial compression measurements yield information about the mechanical and fracture properties of a material (Wium, Qvist, & Gross, 2007a), with the maximum force recorded being related to the hardness at the level of compression applied and the fracture determined by the initial peak force. A sample that undergoes uniaxial compression is distorted in various directions simultaneously, with any fracture that occurs in the sample more likely to be the result of the shear force caused by the distortion of the sample (O’Callaghan et al., 2002).

The advantages of using uniaxial compression as a measurement technique for assessing the texture of cheese include: being simple, rapid, and can be applied to almost any cheese. One disadvantage of uniaxial compression is that to achieve reproducible results samples need to be cut to precisely the same size and dimensions (Guinee, 2002c), which can be a difficult task in cheese. The samples must also all be at the same temperature for comparisons to be made.

Uniaxial compression has been used to assess the hardness and fracture of a number of different cheeses including: Feta (Wium, Qvist, & Gross, 2007b), Cheddar (Ak & Gunasekaran, 1992), Gouda (Bertola, Califano, Bevilacqua, & Zaritzky, 2001), Swiss (Rohm & Lederer, 1992) and Mozzarella (Casiraghi, Bagley, & Christianson, 1985).

Uniaxial compression has been used to evaluate how different levers affect the textural properties of Mozzarella including: the effect of draw pH and storage (Yun, Hsieh, Barbano, & Kindstedt, 1994), the effect of the inclusion of whey deplete retentate (Brandsma & Rizvi, 2001).

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Texture profile analysis (TPA) involves measurements using a double bite uniaxial compression (Guinee, 2002c). This method was developed to imitate the compressive action of molar teeth during mastication (Bourne, 1978).

Figure 2.2: An example of a TPA curve obtained for Mozzarella taken from Tunick (2000). Figure 2.2 is an example of the texture profile analysis on Mozzarella, With a number of features identified including the fracture point (F) and the hardness (H) of the sample (Tunick, 2000). Beyond these two parameters, texture profile analysis can be used to calculate a number of other parameters including: the cohesiveness (A2/A1), springiness (S), adhesiveness (A) and gumminess (hardness x cohesiveness) (Bourne, 1978). One of the

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advantages of TPA is the possibility of correlating textural properties to sensory properties of a food (Agulheiro-Santos & Roseiro, 2012).

Texture profile analysis has been applied as a measurement technique to assess a number of different dairy gel systems including: whey protein gels (Tang, McCarthy, & Munro, 1995), yogurt (Sandoval-Castilla, Lobato-Calleros, Aguirre-Mandujano, & Vernon-Carter, 2004), processed cheese (Joshi, Jhala, Muthukumarappan, Acharya, & Mistry, 2004; Piska & Štětina, 2004), and Cheddar cheese (Mistry & Kasperson, 1998).

TPA has been used as a tool for evaluating how a number of factors affect the texture of Mozzarella including: assessing the impact of coagulant type (Yun, Barbano, & Kindstedt, 1993), the effect of coagulant concentration (Kindstedt, Yun, Barbano, & Larose, 1995), the effect of reducing fat particle size by homogenisation in reduced fat Mozzarella (Rudan, Barbano, Gu, & Kindstedt, 1998), the effect of milk pre-acidification (Metzger, Barbano, Kindstedt, & Guo, 2001) and the assessment of fat reduction (Rudan et al., 1999).

2.4.1.1.2 Small strain rheology

Small amplitude oscillatory rheology (SAOR) is a rheological technique also referred to as a dynamic rheological experiment (Rao, 2007a). This technique involves a sinusoidal oscillating stress or strain applied to a material with a frequency, ω (Aguilera, 1995). The measurements taken are the stress, strain and amplitude ratio during the oscillations. This type of dynamic rheological testing is useful for investigating characteristics of gels as well as gelation and melting properties (Rao, 2007b). This allows for the determination of the viscous and elastic component of a food product (Lucey, 2008). There are three types of dynamic tests that give useful information about gel systems: 1) frequency sweep at a fixed temperature;2) temperature sweep at a fixed temperature; and 3) shear rate sweep

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at fixed frequency and temperature (sometimes referred to as a time sweep). Based on these tests information can be gained to describe the organisation of the gel network (Bowland & Foegeding, 1999). The parameters that can be determined from these tests include the storage or elastic modulus, G’, the viscous or loss modulus, G”, as well as the loss tangent, tan δ (Lucey, 2008). The elastic modulus relates to the energy stored per oscillation cycle, the viscous modulus relates to the energy lost per cycle and the loss tangent relates to bond relaxation as a gel is deformed (Lucey, 2002). The use of rheological data along with information about the structure and properties of foods can lead to a greater understanding of the relationship between them (Genovese et al., 2007). The application of SAO to cheese is done so that the strain is within the linear viscoelastic region so that the structural breakdown that occurs is largely reversible (Everett & Auty, 2008). SAO has been applied to a wide range of cheese systems as a means of assessing their physical properties. These applications have included: assessing how the fat content of Cheddar cheese affects physical properties (Guinee, Auty, & Fenelon, 2000); the effect of the change in the calcium equilibrium in Cheddar (Lucey, Mishra, Hassan, & Johnson, 2005); the characterisation of the melt properties of an imitation cheese (Mounsey & O'Riordan, 1999); examination the role of moisture content of cheese analogues (Pereira et al., 2001); and the effect of different curd washing methods in Colby cheese (Lee, Johnson, Govindasamy-Lucey, Jaeggi, & Lucey, 2011).

The technique has been applied to Mozzarella to gain information about how the cheese changes with storage (Joshi, Muthukumarappan, & Dave, 2004d), temperature (Muliawan & Hatzikiriakos, 2007; Tunick, 2010) and composition (Joshi, Muthukumarappan, & Dave, 2004c; Sheehan & Guinee, 2004; Van Hekken, Tunick, Malin, & Holsinger, 2007).

Behaviour of Mozzarella during heating is of particular interest due to the majority of the cheese being used as a pizza topping. The rheological properties of Mozzarella are highly temperature dependent (Ak & Gunasekaran, 1996). Below temperatures of 60°C,

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Mozzarella behaves as a viscoelastic-plastic as it maintains its structure up until this point (Muliawan & Hatzikiriakos, 2007). After 60°C has been reached, the structure is completely broken and Mozzarella behaves as a viscoelastic fluid.

Rheology has been proven to be a useful tool for assessing the physical properties of cheese systems. Therefore the use of rheological methods to assess Mozzarella in this investigation is advantageous.

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2.4.1.2 Melt

As LMPS Mozzarella is predominantly consumed in a molten state (Bertola et al., 1996a); assessing the meltability of the cheese is of great importance. The melting properties are generally assessed by measuring the change in the dimensions of a fixed size sample after exposure to heating. The most common tests for evaluating the meltability of cheese are the Schreiber and the Arnott melt tests (Kuo, Wang, & Gunasekaran, 2000; Park, Rosenau, & Peleg, 1984). The Schreiber test involves measuring the maximum diameter that a sample of cheese spreads during heating (Muthukumarappan et al., 1999). It involves placing a plug of cheese on a Petri dish and heating at 232°C for a 5 minute period (Park et al., 1984). Once the melted cheese has been allowed to cool for 30 minute, the maximum diameter of the melted sample can be measured.

The Arnott melt test assesses the reduction in the height of a cylinder of cheese following heating (Ustunol, Kawachi, & Steffe, 1994). It involves exposing a cylinder of cheese to a temperature of 100°C for a period of 15 minutes and measuring the height before and after the heat treatment (Arnott, Morris, & Combs, 1957).

A modification proposed to the Schreiber test involves carrying out the test at a lower temperature of 90°C to prevent charring and to conduct the test on an aluminium plate, as it was found to have less variation than a Petri dish (Muthukumarappan et al., 1999). A number of other methods to evaluate the meltability of cheese have been used including UW Meltmeter (Kuo et al., 2000) and capillary rheometers (Smith, Rosenau, & Peleg, 1980).

Due to the end use of the majority of LMPS Mozzarella produced being as a pizza topping, an evaluation of the melt of samples in this project would be advantageous.

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