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Washing and thawing
This section applies to washing the product only, not washing the process plant. The objective of washing is to remove and separate unwanted components (dirt or residual peel, brine used for preservation) from the food in order to ensure that the surface of the food is in a suitable condition for further processing. Unwanted components can include soil, micro-organisms, pesticide residues, salts, etc. Raw materials (e.g. fish and meat) may be received in frozen state. Thawing will then be needed before further processing is carried out.
Field of application
Washing is widely applied as a first processing step for root crops, potatoes, cereals, fruits and vegetables. Thawing is widely applied in fish processing and it is used in some other sectors such as the production of ready-to-eat meal.
Techniques, methods and equipment
There are many types of machines and systems that have been adapted for the material to be cleaned. Washing can be carried out by vigorous spraying with water, or by immersion with the aid of brushes, or by shaking and stirring. Sometimes cleaning substances are added. Warm water may be used but this might accelerate chemical and microbiological spoilage unless careful control of the washing time and process is carried out. The dirt, once loosened, usually differs so greatly from the product that the actual separation of dirt and product is normally straightforward (for example by sedimentation). Semi-processed vegetables may arrive at the plant kept in strong brines, in this case the excess salt needs to be eliminated by rinsing with running water. Thawing by contact with the outside environment is slower than the opposing freezing under otherwise equal driving force conditions. Accelerating the thawing by using water saturated hot air may cause a rapid growth of micro-organisms on the surface layers of the thawed product, as well as hindering reabsorption of thawed water, thus creating the unsightly and often nutritionally wasteful drip loss. The use of microwave energy, which is not transferred by conduction through the thawed food layers, is a faster and less damaging thawing process. The traditional thawing of frozen fish and meat takes place under running water. In this case, the unpacked meat or fish are put in iron crates and completely immersed in pools with water. Thawing by sprinkling is also applied. Desalting and defrosting are carried out simultaneously.