Physical quality of pelleted animal feed. 2. Contribution of processes and its conditions
M. Thomas a, D. J. van Zuilichemb and A. F. B. van der Poela
a Department of Animal Nutrition, Wageningen Agricultural University, Wageningen Institute of Animal Sciences, WIAS, Marijkeweg 40, 6709 PG, Wageningen, Netherlands
b Department of Food Science, section Process Engineering. Wageningen Agricultural University, VLAG, Graduate School of Food Technology, Bomenweg 2, 6700 EV, Wageningen, Netherlands
The effects of changes in process parameters and their effect on pellet quality in terms of hardness and durability are discussed. The pelleting process in this respect is the combination of conditioning, pelleting and cooling. The parameters discussed with respect to the conditioning process are process variables such as steam and water and system parameters such as residence time and pressure.
Parameters during the pelleting process that can be adjusted or influence pelleting properties of a feed mash include layout (e.g. flat-bed vs. ring-die pellet press) and dimensions, roller and die assembly and die velocity of the pellet press. The effect of the changes in one or more parameters and its effect on pellet quality (durability and hardness) is however often a matter of the judgement and experience of the operator. For instance, a certain increase in the amount of steam added to a feed mash generally improves pellet hardness and durability. Increasing the amount of dissipated power in the feed mash, generally has a similar favourable effect on pellet quality. The extent to which pellet hardness and durability rise is however dependent on feed formulation and other parameters as temperature of the feed mash and cooling air characteristics. The latter parameters can be measured but their relation to the pelleting process is often not clear.
Pellet quality, as affected by cooling, is mainly determined by the bed-height of the pellets in the cooler, pellet-size, air-flow and air-characteristics during the cooling period.
The use of modern conditioning systems to operate at a wide range of processing and system variables means that pellet quality is dictated more by equipment than by diet formulation.
However, use of such conditioning systems must always be justifiable in terms of cost and pellet quality, rather than the dictates of fashion.
It is concluded that the amount of steam is a more decisive factor than steam pressure. In addition, it seems that the individual steam supply of a factory has a larger influence on the measured pellet hardness and durability than would be expected from theoretical relationships concerning steam in an ideal situation. Although water has binding properties as well, it is concluded that steam is far superior to water in producing good quality pellets. The additional heat included in the meal permits changes in physico-chemical properties which lead to more durable and hard pellets. Equipment which incorporates some form of hold-time enhances the possibility to incorporate more liquids, without detrimental effects on pellet quality. The use of pressure to alter physico-chemical properties of the feed in combination with water and heat, and the use of pressure to pre-densify the feed mash prior to pelleting seems to be important in obtaining a good quality pellet.
Pelleting is not the sum of conditioning, pelleting and cooling steps, but should be considered as an integral system which performance is dependent on interrelations between the three unit operations: conditioning, pelleting and cooling. However, these interrelations with respect to the different formulations used, are still poorly understood. The use of decision support systems and process optimization procedures may nowadays greatly enhance the opportunity to obtain the best possible quality of pelleted feeds with minimal use of labour and energy for a given feed formulation.
Keywords: Conditioning; Expander conditioning; Pelleting; Cooling; Hardness; Durability; Pellet quality
Animal Feed Science and Technology, Volume 64, Issues 2-4, February 1997, Pages 173-192