Milling Case

Objective:

This experiment aims to investigate the particle size distribution of ground rice sample produced using blade mill and hammer mill.

Introduction:

Dry food materials consist of different size of particles. Therefore, it is significant to categories these particulate materials by determining their particle size distribution (PSD).

Particle size distribution (PSD) is commonly used in food industry which refers to the fraction of particles within a size range in a population of particles. It categorizes particulate materials and defines a 'mean particle size'. PSD is important to control food quality and processing. For example, particle size affects the pellet durability (Abdollahi et al., 2012). The durability is higher when the grind is finer.

In food industry, solid raw materials and intermediate products are usually submitted to size reduction operations. It aims to have portion control, obtain pieces and particles of defined shapes, facilitate separation of different parts of a material and obtain a desirable product texture. Size reduction is generally used for fine grinding of chocolate mass, fine mashing of baby food, milling of cereal grains to produce flour and so on.

Hence, milling is the common unit operation used for size reduction, especially in agricultural, food and pharmaceutical industries to provide a series of particles within a size range. During milling, the initial shape, size and strength of samples influence the speed for milling and the type of milling machine used (Pienti et al., 2013). Different milling machine apply different type of force. In addition, different speed of milling produces different size of comminutes particles.

Generally, there are four applied forces for size reduction that are compression, impact, attrition and cutting to produce different range of particle size reduction. The crushing rolls that use compressive forces have at least two heavy steel cylinders which can resolve towards each other to nip and pull the feed for size reduction. The roller surface is designated to be either smooth or rough to increase friction and trap the particles between the rolls.