Grinding (milling) is used for the size reduction of solid dry material. It may also improve the eating quality and/or suitability of the material for further processing.
Milling is also used to crush cane sugar, to facilitate the extraction of sugar in sugar and rum factories.
Crushing covers, for instance, breaking the skin of berries and then crushing the berries to liberate the must. This process is necessary to facilitate the yeasts’ multiplication and also to conduct traditional macerations before pressing.
Field of application
Grinding/milling is applied in sectors in the food industry where dry solid materials are processed, for example, the animal feed industry, food products in flour milling industry, breweries, sugar industry, dairy industry (milk powder, lactose), etc.
Description of processing techniques, methods and equipment
A whole range of grinding/milling techniques and equipment are available for application with different types of food. Grinding/milling can be carried out dry or wet. In wet grinding/milling smaller particle sizes can be attained. Often dry grinding (milling) is combined with sieving or air classification, this results in particle size fractions. Generally cyclones are used as an integral part of the process to recover the particulate matter (dust) in extracted air. The recovered material is then reprocessed. Common types of mills used in the food industry are:
a) Hammer mills
A hammer mill consists of a horizontal or vertical cylindrical chamber lined with a steel breaker plate and contains a high-speed rotor fitted with hammers along its length. The material is broken apart by impact forces as the hammers drive it against the breaker plate.
b) Ball mills
The mill consists of a slowly rotating, horizontal steel cylinder, half filled with steel balls (2.5 – 15 cm in diameter). The final particle size depends on the speed of rotation and on the size of the balls.
c) Roller mills
The mill consists of two or more steel rollers which revolve towards each other and pull particles of the food material through the space between the rollers, the space being known as “the nip”. The size of the nip can be adjusted for different food materials
d) Disc mills
Disc mills consist of either a single rotating disc in a stationary casing or two discs rotating in opposite directions. The food material passes through the adjustable gap between the disc and the casing or between the discs. Pin and disc mills have intermeshing pins fixed on the discs and casing. This improves the effectiveness of the milling.