Binding and pelleting characteristics of alfalfa
Tabil, Lope Galindo
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Alfalfa pellets have been known to generate dust and fines because of repeated handling and transport. During transport, this product loses or gains moisture which may affect its quality. Therefore, there is a need to manufacture pellets which are not prone to breakage but still can be palatable to animals. Experiments were conducted to determine the effect of chop quality, process and machine variables on the quality of alfalfa pellets. Three different qualities of alfalfa chops namely, low, intermediate and high quality, were used in the experiments. The chemical components and physical characteristics of alfalfa chops and grinds were determined. Pellets were made from alfalfa grinds and from alfalfa grinds mixed with binders. Durability and hardness of pellets were evaluated after manufacture and also after a period of storage at 28°C and 90% relative humidity. To study their compressibility, alfalfa grinds were compressed in a single pelleter and the tensile strength of pellets was determined by diametral compression test. Of four models considered, the Cooper-Eaton model provided the best fit to the particle compression data (volume or density of pellets versus applied pressure). Parameters determined from the Heckel and Cooper-Eaton models indicated that grinds from low quality alfalfa chops densified readily by particle rearrangement. Grinds from intermediate and high quality alfalfa chops produced pellets which required lower yield stress for plastic deformation (PY) and were, therefore, more compressible. The tensile strength of pellets increased with compression pressure during pellet formation and peaked at 100 MPa. Conditioner temperatures of 92°C and above and conditioned grind moistures of 8.5 to 10% w.b. were ideal for pelleting. High durability pellets were produced in a high length-to-diameter ratio die. High temperature of pellets extruded out of the pelleter die resulted in high durability. The particle size of the grinds, die speed and pellet temperature were regressed to pellet durability, such relationships could be used for pellet mill control. Collagen protein, hydrated lime, lignosulfonate, bentonite and pea starch were the binders used with three chop qualities at inclusion rates depending on their cost. The durability of pellets made from low quality alfalfa chops was improved by addition of binders. However, the effect of binders on pellets made from intermediate and low quality chops was negligible. Hydrated lime and pea starch were identified as potential binders. Additional pelleting trials conducted showed that an inclusion rate of 0.5% of either hydrated lime or pea starch was adequate for improving the durability of low quality alfalfa pellets. Extending the conditioning time from the usual 17-22 s to 35-40 s did not affect binder performance nor did it improve the durability or hardness of pellets. Experiments were also conducted to determine the effect of high humidity storage on the physical characteristics of pellets. Moisture absorption data were modeled using the exponential model. Pellet durability decreased when the absorbed pellet moistures exceeded 10 or 12%. Pellet hardness was sensitive to moisture change. In summary, the results from these experiments revealed that the physical quality of dehydrated alfalfa pellets was a function of both process and machine variables. Alfalfa chop quality also affected pellet quality and results showed that binder usage could help rectify variations in pellet durability.