Most raw materials contain some components which are inedible or have variable physical characteristics. Processing techniques such as sorting, grading, screening, dehulling, trimming and destemming are necessary to obtain the required uniformity of the raw material for further processing. Metal check may also be needed to identify any metal particles in the raw material.

Field of application

These processing techniques are widely used as a first step in the processing of fruits and vegetables (legumes), as well as being used for 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.

Description of techniques, methods and equipment

a) Sorting, screening

Sorting and/or screening (dry and wet) are/is the separation of raw materials and/or food slurries into categories on the basis of shape, size, weight, image and colour. The size sorting and dry 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 is aimed at ensuring preserving only good quality fruit, is 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.

b) Grading

Grading is the assessment of a number of characteristics of a food to obtain an indication of its overall quality. Grading is normally carried out by trained operators. Meats, for example, are examined by inspectors for disease, fat distribution, carcasse size and shape. Other graded foods include 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.

c) Dehulling

Dehulling is mainly associated with the removal of hulls from legumes and the shells from cocoa beans. It is also applied in soybean crushing processes. The ease of dehulling of legumes depends on the thickness of the seed coat and can be achieved via wet or dry methods. The wet method involves 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”.

d) Destemming/Destalking

Destemming/destalking is mainly associated with the removal of grape stems, according to the type of wine being produced.

e) Trimming

Trimming is used for the removal of either inedible parts or parts with defects, or for cutting the raw material to a size that is suitable for further processing. Trimming can be carried out manually or by rotating knives.

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  • Sorting