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The microstructure of cheese has a large influence of its functional properties (Joshi, Muthukumarappan, et al., 2004a). The functionality of Mozzarella cheese refers to the key attributes such as meltability, free oil formation and stretchability that make it suitable for use as a pizza topping (Rowney et al., 2004). Functionality has found to be strongly influenced by the pH, calcium to casein ratio as well as the extent of casein solvation (Kindstedt, 2004).

2.2.3.1 Unmelted Mozzarella

In order to facilitate even distribution on products as well as uniform melting, Mozzarella is generally shredded or diced (Yun, Kiely, Kindstedt, & Barbano, 1993). Therefore a key functional property of unmelted Mozzarella is in regards to its ability to be shredded. Shreddability is a term used to describe a number of characteristics including the ease with which the cheese is shredded, the integrity of the shreds, whether even shredding occurs or fines are formed, and whether the shreds remain free formed or matt together after being shredded (Kindstedt, 1995). Shreddability is of importance due to the major applications, such as its use on pizzas, requiring the cheese to be in a shredded state. If the cheese is too soft and wet, there are problems such as clogging and matting during shredding. However, problems also exist when the cheese is too dry and hard, with the shreds shattering due to brittleness of the cheese (Bertola et al., 1996b). The firmness of Mozzarella can be controlled by manipulating the density and the structure of the casein matrix (Rankin et al., 2005). This is generally done by altering the calcium content and the pH of the curd. Manipulating these two parameters, especially during the draining step, can have a significant influence on the firmness of the finished cheese (Lawrence et al., 1984).

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Unlike many cheeses, flavour is not a very important attribute for Mozzarella for pizzas; rather a lack of flavour is desired (Kindstedt, 1993a).

2.2.3.2 Melted Mozzarella

Due to Mozzarella generally being consumed in a molten state (Bertola et al., 1996a), many of the key functional properties relate to how it behaves when melted.

The use of Mozzarella as a pizza topping is the most important application that the cheese is put to due to the sheer volume used for this purpose. Many large pizza chain restaurants have very specific quality parameters that they give their Mozzarella suppliers. These generally relates to properties such as browning, melting, blistering and free oil formation.

2.2.3.2.1 Meltability

Meltability can be defined as the ease with which cheese can flow when heated (Muthukumarappan, Wang, & Gunasekaran, 1999). The meltability of Mozzarella is due to the combined effect of fat and the balance between protein to protein and protein to water interactions (McMahon & Oberg, 1998). It relates to the cheese’s ability to form a continuous melt with no individual particles present (McMahon et al., 1993). The melting process is a progression from the firm solid individual shreds of cheese to a semi-solid mass. When subjected to heating, at first the cheese does not change shape but does rapidly increase in temperature (Rankin et al., 2005). The cheese then reaches the softening point, which is the temperature the cheese begins to flow. The cheese matrix collapses and the individual shreds matt together to form a semi-solid mass. Once the cheese has completely melted the change in height is minimal and the temperature of the cheese approaches that of the heated environment. Meltability is generally expressed in

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terms of the decrease in the height of a sample or the increase in the sample area (Kindstedt, 1993a).

Initially after manufacture Mozzarella has unacceptable melting properties as it forms a tough, elastic, nonhomogeneous, semi-solid mass that has a granular appearance (Kindstedt, 1995). As the cheese ages it give a more desirable melt as the water holding capacity has increased and there is no separation of water, it has greater stretching and elastic properties. However, Mozzarella becomes excessively soft and fluid after extended aging of over 4 or so weeks, depending on the composition and process.

There is a trade-off between meltability and the firmness of Mozzarella. This can be controlled by the manipulation of the calcium content of the cheese (McMahon & Oberg, 1999). At higher calcium contents Mozzarella is firm but becomes less firm and increasingly more meltable as the calcium content decreases. Meltability is affected by casein solvation, temperature, age, level of proteolysis, fat and moisture levels (Everett & Auty, 2008). Any factor that reduces the casein-casein interactions will cause an increase in the meltability of Mozzarella (Everett & Auty, 2008).

2.2.3.2.2 Free Oil Formation

The release of oil during cooking is a key functional property related to the use of Mozzarella on pizzas. If an insignificant amount of free oil is released the pizza will lack a characteristic sheen and the cheese becomes dehydrated causing excessive browning and burning to occur (Yun, Kiely, Kindstedt, et al., 1993). This is due to the oil creating a hydrophobic film that prevents evaporating occurring from the cheese (Kindstedt et al., 2004). However, if there is an excessive release of free oil it gives the pizza an undesirable appearance and is considered a defect (Kindstedt & Fox, 1991). The release of free oil when Mozzarella is heated is a property that needs to be carefully controlled, with insufficient amounts and excessive amounts being undesirable (Kindstedt et al., 2004). The formation of free oil when Mozzarella is heated is governed by the coalescence of fat

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globules within the paracasein channels and the disassociation of the paracasein fibres, which causes them to collapse and flow (Kindstedt, 2007). The collapse of the paracasein fibres allows the pooled liquid fat in the channels to flow and merge together.

The tendency of the fat globules to coalesce and form free oil when the cheese is melted is related to the strength of the emulsion (Kindstedt, 1995). The casein present in the cheese is the main emulsifying agent, and is thus a key factor affecting the formation of free oil.

The fat in dry matter of the Mozzarella influences the amount of free oil that is formed (Kindstedt, 2007). The higher fat content means that there is a greater interruption of protein-protein interactions which causes the paracasein fibres to collapse and flow more readily. The storage and aging of Mozzarella has an effect on the amount of free oil that is released from the melted product. The amount of free oil present in Mozzarella increases from about 36% 3 days post manufacture to 50% by day 21 (Kindstedt & Fox, 1991). This increase indicates the extent of change occurring during the aging process.

The salt content of the cheese also has a significant effect on the quantity of free oil released (Kindstedt, 2007). Increasing the salt content of Mozzarella reduces the free oil formation. Another factor that influences the release of free oil in Mozzarella is the screw speed during the stretching process (Renda et al., 1997). This is due to its role in the dispersion of fat throughout the cheese.

2.2.3.2.3 Stretchability

One of the key functional properties of Mozzarella, that made it the cheese of choice for pizzas, is the ability to stretch when melted (McMahon & Oberg, 1998). Although having no distinct rheological definition, the stretch can be referred to as the ability to form

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fibrous strands which extend under tension. This ability of Mozzarella to form strings when stretched is its most distinguishing property (Ak & Gunasekaran, 2003).

Consumers of pizza products, whether they are from a restaurant or from a supermarket freezer, expect the Mozzarella on the topping to have a reasonable stretchability and elasticity whilst being in a semisolid form. This is due to the image advertising products containing Mozzarella generally depicting strands of melted cheese on the packaging (Ak & Gunasekaran, 2003).

The stretchability of Mozzarella is governed by the casein associated calcium present in the cheese. If the calcium to casein ratio is too high the curd will tear and fracture during stretching, while too little results in a complete loss of structure and stretch (Kindstedt, 2002). The amount of calcium present in the curd is dependent on how much is lost at whey drainage; however, the distribution of the calcium within the curd is dependent on the pH at the time of stretching.

2.2.4.2.4 Browning & Blistering

The appearance of Mozzarella changes dramatically from application to a pizza, through baking and cooling (Metzger, Barbano, Rudan, Kindstedt, & Guo, 2000).

Excessive browning is of concern due to many pizza manufacturers using temperatures of over 260°C (Matzdorf, Cuppett, Keeler, & Hutkins, 1994). The browning of Mozzarella when heated is a property that is of specific interest to pizza manufacturers. The browning occurs due to residual galactose and lactose in the cheese that undergo Maillard browning when subjected to heat (Johnson & Olson, 1985). Small peptides and amino acids serve as reactants for the reducing sugars to undergo the browning process (Kindstedt & Guo, 1997b). These small peptides and amino acids are produced by the starter culture in the milk. If Mozzarella is made using direct acidification without the presence of starters, the

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cheese does not brown to the same extent as when starter cultures are used (Oberg et al., 1991).

The blistering of Mozzarella on pizzas is another functional property related to the appearance of a baked product that needs to be controlled. Blisters occur when water vapour bubbles under the cheese surface (Kindstedt, 2007). The extent of blistering depends on the tensile strength and viscoelastic properties of the melted cheese.

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