|technology > raw materials handling > sorting, screening, grading|
Sorting, screening, grading, dehulling, trimming, destemming
metal particles in the raw material.
|Field of application|
|Techniques, methods and equipment|
|a) Sorting, screening|
meat, eggs and fish. The screening of grain is, for example, applied in the malting industry to select even sized grains for the malting process. Besides dry screening/sorting, a wet screening of slurries is also widely used in the food industry to separate different components.
cleaning of agricultural raw materials separates solids into two or more fractions on the basis of different sizes, usually by sieving or screening. Size sorting is especially important for food products which have to be heated or cooled, as large differences in size might lead to an over-processing or under-processing of the product.
Sorting also allows the separation at first sight of some (undesirable) additional material (e.g.leaves, stones) or inappropriate raw material (immature or rotted berries), and aims at ensuring that only good quality fruit is preserved and passed through for further processing.
For size sorting, various types of screens and sieves, with fixed or variable apertures, can be used. The screens may be stationary, rotating or vibrating.
Shape sorting can be accomplished manually or mechanically with, for example, a belt- or roller-sorter.
Weight sorting is a very accurate method and is therefore used for more valuable foods (cut meats, eggs, tropical fruits, certain vegetables).
Image processing is used to sort foods on the basis of length, diameter, and appearance, i.e. surface defects and orientation of food on a conveyor.
Colour sorting can be applied at high rates using microprocessor controlled colour sorters.
cheese and tea. In some cases the grading of food is based on laboratory analyses results.
In the wine industry, grading also covers the necessary classification of the grapes harvested according to their degree of maturity (for example, sugar content). Many characteristics cannot be examined automatically and trained operators are employed to simultaneously assess several characteristics in order to produce a uniform high-quality product. Grading is more expensive than sorting (which looks at only one characteristic) due to the high costs of the skilled personnel required.
soaking the legumes in water for a few hours, draining, drying, milling and then blowing them with air to remove the seed coat. In the dry method, oil is mixed with the seeds by passing them through emery-coated rollers to abrade the surface. This is common practice for legumes with particularly tough seed coats.
The dehulling of soybeans is always preceded by a heating step, which enhances the removal of the hulls. To remove the shells from cocoa beans, the beans are first broken between adjustable toothed rollers. The broken pieces are subsequently separated in fractions by sieving. Each fraction is treated with a stream of air that carries the light shell pieces away. This breaking and fanning process is often referred to as “winnowing”.
cutting the raw material to a size that is suitable for further processing. Trimming can be carried out manually or by rotating knives.