• blending of cheese with emulsifying salts, water and optional ingredients;

• processing of the blend;

• hot packing and cooling.

Formulation involves selection of the correct type and quantity of natural cheeses, ES, water and optional ingredients to give a PCP with the desired composition, textural and functional properties [192,193,194]. Processing refers to the heat treatment of the blend, by direct or indirect steam, with constant agitation until it is molten and uniform in consistency. In batch processing, the temperature-time combination varies (70-95 0C for 4-15 min) depending on the formulation, extent of agitation, desired product texture and shelf-life characteristics. In continuous cooking, the blend is heated to 130-145 0C for a few seconds, flash cooled to 90 0C and held for a further 4-15 min. Processing has two main functions:

1. It kills any potential pathogenic and spoilage microorganisms in the blend, and thereby extends the shelf-life of the product.

2. It facilitates the interaction of the different blend ingredients and the physico-chemical and microstructural changes necessary to transform the blend into a physicochemically stable end product.

The processed blend is conveyed from the cooker to the filling machine where it is packed hot prior to cooling and storage at 4 0C. Numerous packaging formats are possible through the use of specialised filling/moulding machines including individually wrapped portions (e.g. foil-wrapped triangles), blocks, sausage-shapes, cans, tubes and slices.

Added ES plays a crucial role in the formation of PCPs. The ES usually contain a monovalent cation and a polyvalent anion, with the sodium salts of citric acid and/or phosphoric acid being the most common types. In their absence, processing would generally lead to the formation of a heterogeneous, gummy, pudding-like mass that undergoes extensive oiling-off and moisture exudation during manufacture and on cooling. These defects are associated with heat- and shear-induced physical damage of the fat globule membranes in the natural cheese, liquefaction and coalescence of non-globular fat, and aggregation of the protein (paracasein) phase of the natural cheese in the blend. The addition of ES at levels of 1 to 3% (w/w) prevents such defects. While they are not emulsifying agents per se, the ES convert the insoluble cheese protein (paracasein) to sodium paracaseinate, which binds water and emulsifies the dispersed free oil droplets during processing. This conversion is mediated by two important functions of the ES:

1. Upward adjustment and stabilisation (buffering) of the blend pH (from -5.05.3 to 5.8-6.1).

2. Sequestration of calcium (Ca2+) from the cheese protein by ion-exchange with the Na+ ion of the ES (Fig. 1).

The combined effect of ES, heating and shearing then leads to structural transformation from a 'loose' oil-in-water (O/W) emulsion physically encased

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Fig. 1 Schematic showing the ion exchange function of added emulsifying salt. The emulsifying salt exchanges sodium for calcium on the insoluble cheese protein (paracasein) and thereby results in the destruction of calcium phosphate crosslinks between the strands of the casein matrix in the cheese.

within a particulate cheese paracasein matrix in the natural cheese to a 'finer' oil-in-water emulsion in a concentrated paracasein(ate) dispersion in PCPs.

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