252 M.J. Lewis 9. 2. 4 Forces of adhesion There will be interactions between particles, known as forces of adhesion and also be- tween particles and the walls of containing vessels. These forces of attraction will influ ence how the material packs and how it will flow. Some of the mechanisms for adhesive forces have been described as liquid bridging by surface moisture or melted fat: electrostatic charges molecular forces, such as van der Waals and electrostatic forces crystalline surface energy. Schubert(1987a) describes some of the models that have been used to quantify these forces, and the limitations of such models There is some indication that interparticle adhesion increases with time, as the material onsolidates. Flowability may be time-dependent and decrease with time 9. 2.5 Bulk properties Although the discussion so far has focused on individual particles, the behaviour of the collective mass of particles or bulk is very important in most operations. The bulk properties of fine powders are dependent upon geometry, size, surface characteristics, chemical composition, moisture content and processing history. Therefore it is difficult to put precise values on them and any cited values should be regarded as applying only to that specific circumstance, Peleg(1983) The term cohesive is used to describe the behaviour of powders, as they are influenced by forces of attraction(or repulsion) between particles. For powders that are cohesive, the ratio of the interparticle forces to the particles'own weight is large. This ratio is alse inversely proportional to the square of the particle size, which explains why small articles adhere to each other more strongly than large particles. Schubert(1987a)states that the majority of food particles are non-cohesive(and thus free flowing)only when the particle size exceeds 100 um. Increase in moisture content makes powders more cohesive and increases the size at which the transition from cohesive to non-cohesive takes place Some of the bulk properties will be considered in more detail 9.2.6 Bulk density and porosity The bulk density is an important property, especially for storage and transportation, rather than separation processes. It is defined as the mass divided by the total volume occupi by the material. This total volume includes air trapped between the particles. The volume fraction trapped between the particles is known as the porosity (e), where where ps and pb are measured solid and bulk densities. Methods for determining bulk density are described by the Society of Dairy Technology(1980)and Niro(1978). Terms used depend upon the method of determination and include loose bulk density and com- Some bulk densities of powders are given in Table 9.7. Further values are given by Peleg(1983), Hayes(1987)and Schubert(1987a). Peleg(1983)argues that the relatively252 M. J. Lewis 9.2.4 Forces of adhesion There will be interactions between particles, known as forces of adhesion and also be￾tween particles and the walls of containing vessels. These forces of attraction will influ￾ence how the material packs and how it will flow. Some of the mechanisms for adhesive forces have been described as liquid bridging by surface moisture or melted fat; electrostatic charges; molecular forces, such as Van der Waals and electrostatic forces; crystalline surface energy. Schubert (1987a) describes some of the models that have been used to quantify these forces, and the limitations of such models. There is some indication that interparticle adhesion increases with time, as the material consolidates. Flowability may be time-dependent and decrease with time. 9.2.5 Bulk properties Although the discussion so far has focused on individual particles, the behaviour of the collective mass of particles or bulk is very important in most operations. The bulk properties of fine powders are dependent upon geometry, size, surface characteristics, chemical composition, moisture content and processing history. Therefore it is difficult to put precise values on them and any cited values should be regarded as applying only to that specific circumstance, Peleg (1983). The term cohesive is used to describe the behaviour of powders, as they are influenced by forces of attraction (or repulsion) between particles. For powders that are cohesive, the ratio of the interparticle forces to the particles’ own weight is large. This ratio is also inversely proportional to the square of the particle size, which explains why small particles adhere to each other more strongly than large particles. Schubert (1987a) states that the majority of food particles are non-cohesive (and thus free flowing) only when the particle size exceeds 100 pm. Increase in moisture content makes powders more cohesive and increases the size at which the transition from cohesive to non-cohesive takes place. Some of the bulk properties will be considered in more detail. 9.2.6 Bulk density and porosity The bulk density is an important property, especially for storage and transportation, rather than separation processes. It is defined as the mass divided by the total volume occupied by the material. This total volume includes air trapped between the particles. The volume fraction trapped between the particles is known as the porosity (E), where E = PS - pb/ps (9.5) where ps and pb are measured solid and bulk densities. Methods for determining bulk density are described by the Society of Dairy Technology (1980) and Niro (1978). Terms used depend upon the method of determination and include loose bulk density and com￾pacted and compressed bulk densities. Some bulk densities of powders are given in Table 9.7. Further values are given by Peleg (19831, Hayes (1987) and Schubert (1987a). Peleg (1983) argues that the relatively