After all preparation treatments, the shredded, minced, and weighted raw material is transported to the cooker, where, by interaction with an emulsifying agent and water, processing is performed. Processing involves heat treatment of the blend with direct and/or indirect steam under partial vacuum. The product is constantly agitated through a continuous or batch method. If processing is carried out discontinuously, (ie, in a kettle), the temperature can reach 71 to 95°C for a period of 4 to 15 min (Table 1), depending on various parameters (3); this heating also provides pasteurization. In newly developed cookers it is also possible to reach the temperatures up to 140°C. A kettle,

Figure 3. Transmission electron microscopy (TEM) of milkfat emulsification during cheese processing: f—fat; m—protein matrix; c—crystalline sodium citrate; p—calcium phosphate crystals; b—bacterium. Source: Ref. 28.

or cooker (Fig. 2), consists of two double-jacketed, round, stainless steel pans of various sizes (2 to 100 L), fitted with corresponding lids, three-stage switchable stirring equipment, and fittings for direct steam injection and vacuum draw. Double jackets enable indirect steam heating as well. There are specially designed units, similar to cutters used in meat processing, where cutting is completed prior to processing by the aid of rapidly rotating knives with simultaneous heating and homogenization of the product. In addition to this most common round design, the newer, horizontal, tube-shaped processing unit is also popular, particularly in the United States and Canada. This installation is fitted with one or two mixing arms (up to 4 m long); it is fed at one end and the final product is discharged at the opposite end, within 4 to 6 min. This type of unit is constructed for the batch operation, but it can be continuous and can process large capacities. A new programmed jacketed processor has been successfully developed (5), which is used to grind, mix, and process natural cheeses with other blend components, water, and emulsifiers using steam injection and vacuum, at 75°C for 5 min. The processed blend is discharged by either tilting the processor or by aseptic pumping to a packaging machine. This programmed batch processor acts via a punch card for blend formulation and cleaning in place.

By continuous processing, the blend is sterilized at temperatures of 130 to 145°C for 2 to 3 s in a battery of stainless steel tubes (1). Although continuous cheese cookers were developed as early as 1920, they are not commonly used in the processed cheese industry. The main reason is the great versatility in processed cheese products; therefore, changing over to a different product in continuous process, which requires intermediate cleaning, cannot be considered economical. A German patent describes a con tinuous process for simultaneous melting, homogenization, and sterilization in processed cheese production without application of pressure (25). A Japanese patent (26) describes a new method for the postprocessing heat treatment (to 100°C) of packed processed cheese, produced in the usual way.

A group of French authors (27) recently investigated the effects of blend variants and process conditions on protein-lipid interactions made by batch or extrusion cooker methods. Added emulsifying agents or premelted cheese mix increased lipid binding. Final cooling with slow mixing increased lipid binding in the extrusion cooker but not in the batch method. Proteolysis was greater in extruded than in the batch samples.

Regardless of the kind of facilities and technique of processing used, the emulsification of milkfat takes place. Transmission electron microscopy was used to observe the emulsification process (Fig. 3) (28).

The main chemical, mechanical, and thermal parameters in the cheese processing procedure are listed in Table 3. As evident from Table 3, the most important working conditions, which affect the processing and thus the quality of the final product, are as follows:

1. Temperature (heat induced by direct or indirect steam).

2. Duration of processing (depending on size and construction of the cooker, quality of raw material and blend composition, mechanical treatment, emulsifying agent used, desired keeping quality, etc).

3. Agitation (slow, at lowest speed of 60-90 rpm when producing processed cheese block, or fast, at 120-150 rpm for processed cheese spreads).

4. Acidity (pH) (a rather limited pH range; the increase of pH value, decrease of H +, causes better peptization of casein but can spoil keeping quality and flavor, whereas a decrease in pH value introduces

Figure 2. Universal machine for processed cheese production with cross section. Source: A. Stephan & Sons. Hameln, Germany.

Figure 3. Transmission electron microscopy (TEM) of milkfat emulsification during cheese processing: f—fat; m—protein matrix; c—crystalline sodium citrate; p—calcium phosphate crystals; b—bacterium. Source: Ref. 28.

Table 3. Chemical, Mechanical, and Thermal Forces as Regulating Factors in the Cheese Processing Procedure

Process conditions

Firm slicing processed cheese, block cheese

Spreadable processed cheese, processed cheese spread

Raw material Average age

Relative casein content Structure Emulsifying salt


Addition of water Temperature Duration of processing pH


Precooked cheese

Milk powder or whey powder





Young to medium ripe, predominantly young 75-90%

Predominantly long

Structure building not creaming, eg, high molecular polyphosphate C, SE, S7, PZ also citrate 10-25% All at once 80-85°C


5-15 min

Slowly (10-20 h) at room temperature Very light and carefully

Combination of young, medium ripe, overripe 60-75% Short to long

Creaming, eg, lower and medium molecular polyphosphate, S9, S9 special, S10, S90 20-45% In portions 85-98°C ( - 150°C) 8-15 min 5.7-5.9 Rapid 5-20% 5-10%

Advantageous 10-30 min

Rapidly (15-30 min) in cool air Intensive treatment

Source: Ref. 3, courtesy of Food Trade Press.

thickening and solidifying of cheese structure). Higher pH values also favor more rapid product deterioration, in the event of postpasteurization contamination.

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