Manufacture of processed cheese

The manufacture of PCPs involves the following major steps:

1. Formulation, deciding on the different types and levels of ingredients to be included.

2. Cleaning and comminution of the cheese

3. Blending with ES, water and optional ingredients

4. Processing (heating and shearing) of the blend

5. Homogenization of the molten blend (optional)

6. Hot packing and cooling.

i. Selection of Natural Cheese

Selection is determined chiefly, by type, age, acidity, flavour, body, texture and composition of the lots of cheese available. Some successful operators make their selections entirely on the basis of flavor, body and texture. The proper choice of the cheese must establish uniformly desirable flavor, body, slicing properties and composition. These are attained by mixing many lots of cheese.

In practice any type of cheese without exception can be used. During the entire history of processed cheese there remains scarcely a single type of cheese which has not been investigated. In practice, for economic reasons, factory technique and consistency in quality, cheese which is available in plentiful quantities giving little difficulty in processing and little waste in cleaning is used. The most important types are the hard and semi- hard types, such as, Cheddar, Gouda, Edam, Emmentaler,and Provolone to mention but a few. Due to their relatively high dry matter content and their high intact protein content as structure building matter, they guarantee to the processed cheese the necessary stability. Amongst these types which are easy to process and which cause few complications one can name Cheddar. For this reason,Cheddar is the most widely used type of raw material for processing throughout the world.

It is generally accepted that old cheese alone is not suitable for processing as it results in a cheese having a loose and grainy texture, weak body, poor slicing properties and being liable to fat separation. On the other hand, very young cheese gives a viscous, glutenous mix while young cheese yields a stable emulsion having a smooth texture, firm body and good slicing properties. Old cheese may be blended in with the latter in small amounts to give added flavour to the product. Usually approximately 75% of the blend consists of short held cheese (1-3 months) and the rest consists of cheese 5-6 months old. It has been found that 15% current (tough) cheese may be incorporated in batch taking care to avoid lumps in the finished product, but acid current cheese should not be used as colour defects ranging from straw to deep pink are then likely to occur. Acid cheese should be at least 8 weeks old when used and it is best not to use over 12%. As regard flavour, cheese on the acid side can be an asset. High acid cheese tends to produce grainy texture.

In a successful blending of a batch of cheese for processing it has been suggested that at least three, and often up to a dozen, different kinds of bulk cheese may be required. It was further recommended that the cheese should be of three different stages of ripening:

1. Young cheese 25%, 1-3 month old to impart a cohesive quality to the product.It must not have acid character.

2. Short held cheese 50%, from 3-4 months in age, the cheese must not have rubbery texture, provides mellowness and flavor.

3. Aged cheese- aged 6 months or more- 25% provide flavor.

Thus, it can be seen that young cheese is used for its desirable body texture, and slicing properties and older cheese is used to impart satisfactory flavor. Also, by using a combination of young and old cheese, the average storage time necessary to produce a desired flavor is less than when only medium ripened cheese is used. This practice also caters for the early usage of young cheese of poor curing properties before definite flavour defects are evident.

Many cheese unfit for retailing, i.e. with broken or damaged rinds or cheese invaded by mould after sampling can be salvaged for processing. It has been reported that up to 5% of inferior cheese may be blended with good cheese with no noticeable ill effects, but that putrid cheese should never be used as even 1% in a blend is detectable in the finished product.

ii. Trimming and Grinding

The first step after the cheese has been selected is to trim off all non- edible portions.In processed cheese the trimming is not done as deeply as in case of natural table purpose cheese. This is due to the following:

a) The surface layer is inferior in flavour, and may be unclean, and

b) Beyond this, additional rind may be removed because it is so firm and dry as to be unpalatable.

In processing, only the first of these reasons applies, rind need not be discarded,solely because it is too firm, because grinding and heating will remedy this condition.The rind may be removed by knives. Another method is to soften the rind by tempering.

a) Tempering: Cheese is held in the blending room until it approaches a temperature of 15-21 °C. This may require 48-72 hrs. Warming the cheese in the blending room softens it and facilitates the cleaning, cutting and grinding operations. It has added advantage of reducing the amount of heat required to bring the cheese to its final cooking temperature

b) Trimming: The first step in cleaning is the removal of paraffin and bandage.The first rough cleaning removes most of the soiled areas on the surface of the cheese. The cheeses are then moved to the next area, where all inedible portions of the cheese are scraped of or cut away.

c) Trimming losses: The amount of rind removed from a normal, undamaged cheese is determined by the flavour and consistency of the rind itself. The surface areas of cheese in storage absorbs odours from boxes and storage rooms, the areas affected must be removed in cleaning. A critical operator assumes that any portion of the surface that is not suitable for consumption in its natural state is undesirable for processing. Damaged rinds, cracked surfaces containing mould growth. Mouldy areas surrounding trier holes, spots damaged by box nails and the like are entirely removed. Weight losses in cleaning may range from a few ounces in current cheese to several pounds in an old cheese which may have been damaged by excessive mould growth or other from rind injury. It is minimized by flexible wrapping. The rind can also be softened by exposing the cheese to steam for a short time and then removing the surface by scraping.

iii. Processing

Processing refers to the heat treatment of the blend, indirectly or by direct steam injection, while constantly agitating. Processing has two main functions: 1) to kill any potential pathogenic or spoilage microorganisms, and thereby extend the shelf life of the product; 2) to facilitate physico-chemical and microstructural changes which transform the blend to an end product with the desired characteristics and physico-chemical stability.

The cheese is grinded directly into the cooker. For grinding it is usually necessary to cut the cheese into strips 2-3 inches square. The order of addition of the ingredients varies with plant practices, cooker type, overall plant design, and duration of cooking.A typical order of addition is: ground cheese, a dry blend of emulsifying salts and optional dairy ingredients (e.g. skim milk powder), water and flavours. When the cooking time is relatively short, the ES may be dispersed in a portion of the water added at the beginning of the processing. This approach minimizes the time required for the ES to dissolve during cooking, increases the concentration of ES during early cooking and thereby enhances the effectiveness of the ES in promoting the desired physico-chemical changes in the blend. After the preset time, the remaining water may be added manually, delivered by metering pump, or drawn in by vacuum inside the vessel. Flavours may be added later in the process to minimize the loss of volatile flavour compounds.

Processing may be performed in batch cookers (e.g. Stephan, Damrow) or continuous cookers (e.g. Kombinator, Votator) connected to water, steam and vacuum. During processing, the ground cheese remains crumbly until a temperature of about 49 °C is reached. Beyond this point the cheese becomes sticky, stringy and plastic and may effectively retain (or reincorporate) the fat without the addition of an emulsifying agent. In the early stages of heating there is usually some “oiling off”, especially at temperatures where the fat is fluid and before the plastic condition has been reached, i.e. between 35 and 49°C. If the conditions are correct, this free fat will be reincorporated into the cheese by the stirring at temperatures beyond 49°C. Factors affecting fat separation are 1) age of cheese, 2) acidity, 3) water content, 4) velocity of agitation.

The temperature-time treatment in batch processing varies (e.g.70-95°C for 4-5 min), depending on the formulation, extent of agitation, the desired product texture and shelf-life characteristics. The bacteria in the vegetative state are killed by these temperature treatments. However, a temperature >130 °C may be required to kill some spores. A temperature of 140 °C can be achieved in continuous cookers by virtue of their design, e.g. scraped surface tubular heat exchangers which maximize the surface area of contact between the heating medium (e.g. steam, oil or hot water) and the blend, and ensure sufficient agitation to prevent burn-on of the blend on the heat transfer surface. In continuous cookers, the blend is, typically, heated to 140 °C for 5 s and then cooled to 70-95 °C by flash evaporation of moisture due to a pressure drop, or by passing through scraped –surface tubuler coolers. The product is then held at this temperature for 4-15 min to allow adequate time for interaction of the different blend ingredients, the desired physico-chemical changes to occur and the development of the desired textural characteristics. The blend thickens progressively with holding time at 70-95 °C.

iv. Cooking Technique

The oldest method is heating in a steam jacketed kettle equipped with double action agitators. Steam pressures up to 30lbs/ sq. inch may be used in the jacket. A definite weight of the selected, ground cheese and the calculated and weighed amounts of emulsifying salt and other ingredients are placed in the kettle. The required amount of water and possibly cheese colour are added. Heating and agitation are preferably started before the kettle is completely loaded. The water and emulsifying salts should be present before the heating has progressed very far. The heating and stirring are continued until a temperature of 75 °C is reached. At that stage there should be no free fat and the hot cheese should be semi-fluid with quite a pronounced tendency to draw string. As soon as this condition and the temperature of 75 °C have been reached, the cheese may be discharged from the kettle for packaging. No holding is necessary for the sake of pasteurization, since the packaged product will cool slowly and will be within an effective pasteurizing range for some time.

v. Packaging

The cheese is packed as soon as possible after heating to avoid aerial contamination.It has been reported that 18-21° C is the best temperature for the cooling room,the packages should not be placed under refrigeration until they have attained these temperatures.

The main requirements for packaging materials for process cheese are:

1) moisture retention,

2) impermeability to oxygen,

3) no off-flavour side effects, and

4) close adherence to the cheese

The following packaging materials were tried: tin, tin foil, cellulose films, glassine, coated aluminium foil and laminates of aluminium foil and parchment paper. It has been reported that tins and parchment paper impregnated with foil are the best.