Melting

Objective

The objective of melting is to obtain a phase change from solid to liquid, in order to prepare the material for further processing (e.g. for fats, processed cheese) or to recover the melted fraction (i.e. in fat recovery).

Field of application

The main applications of melting in the food industry are in chocolate moulding, the production of processed cheese, the processing of oils and fats and the recovery of animal fat from meat residues.

Description of techniques, methods and equipment

For melting, processing kettles are used. These can be operated either batch-wise or continuously. Heating may be carried out by direct steam injection or indirectly by steam jackets. Processing kettles are available in various sizes and shapes. In continuous processing, scraped surface heat exchangers are applied. Some typical examples of melting processes are described below:

a) Melting in the production of processed cheese

Milled cheese and other ingredients are put into a processing kettle and heated to a temperature normally not less than 75°C to ensure a complete pasteurisation of the processed cheese. Agitation during processing is important for a complete emulsification of the processed cheese. The temperature and duration of the process depends on the type of processed cheese aimed for and the nature of the raw cheese.

b) Melting to recover fat from meat residues

To recover fat from meat residues two process methods are used: wet melting or dry melting.
In the wet melting process, the raw material is heated in the process kettle by direct steam injection to a temperature of about 90°C. This results in a watery phase and a fat containing phase. The phases are separated by decantation and centrifugation.
In the dry melting process, the raw material is indirectly heated in the processing kettle (a kettle with a steam jacket). All the water which evaporates is removed from the kettle (under vacuum). The liquid phase (molten fat) and dry phase (fat free meat residue) are separated by decantation.

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