Means of Improving Pellet Durability
Pellet durability may be improved by manipulation of diet formulation; use of raw materials with good binding ability such as wheat, barley, rape and use of pellet binders will have an influence. Feed manufacturing practices will also have an effect on pellet durability and potentially involve less expense than raw material addition. Grinding of raw materials and conditioning of feed are regarded as the most influential factors affecting pellet quality.
There are several reasons to grind raw materials, it improves uniformity of mixing, increases absorption of steam and increases digestibility of feed. In terms of pellet quality grinding reduces the amount of large particles which can reduce pellet strength; it also increases surface area for feed particles to adhere. In other words a finer even grind can result in better pellet quality. Points to consider when grinding are;
- Screen hole size – appropriate for the grist (particle) size and pellet size required.
- Screen correct side to hammers – provides more efficient grinding.
- Hammer tip speed – faster will produce finer material.
In conclusion grinding needs to provide a fine, even grist for best pellet quality.
Conditioning has probably the most influence on pellet quality. It creates thermal, chemical and mechanical energy; the steam used during conditioning opens up starches and causes gelatinization and also plasticizes proteins and softens fibers. Optimal ‘cooking’ of feed will result in more durable pellets.
To ensure optimal conditioning the following points should be considered:
- Steam pressure into the conditioner needs to be kept low (<2 Bar) and constant as high pressure ‘blasts through’ the meal. Low pressure steam transfers heat to the meal more efficiently than high pressure steam.
- Steam temperature needs to be about 135 degrees centigrade to adequately condition the feed, ideally the temperature in the conditioner should be greater than 80 degrees centigrade.
- The dwell time of the meal in the conditioner will influence pellet durability, depending on the temperatures involved, meal retained longer will generally result in better conditioning.
- The level of meal in the conditioner will affect conditioning efficiency, too low and dwell time is reduced, too high and the mechanical effect of the conditioner is reduced.
- The point of addition of steam into the conditioner should be below the meal fill level if it’s above this steam may not penetrate the meal efficiently.
Along with grinding and conditioning, pelleting also has an influence on pellet quality. Meal should enter the pelleter correctly conditioned as this will aid ‘shaping’ the meal into pellets rather than cooking the meal via friction heat at the die. Excess friction heat in the die can produce a hard brittle pellet rather than a durable one, also conditioning the meal in the conditioning vessel is more cost efficient than in the pelleter.
Consideration should be given to:
- Cheaper dies are usually a false economy as they are likely to be lower quality resulting in uneven wear and therefore poor pellet quality and throughput.
- The number of die holes will affect the throughput and rate of wear of die.
- Die hole dimension will affect pellet quality, consider the effect of die length on compression and throughput, the longest die hole may not always give the best quality.
- Faster die speed will increase throughput but reduce pellet quality.
- Meal must be fed evenly across the full face of the die otherwise there may be uneven die and roller wear which results in poorly formed pellets reducing pellet quality.
Pellets should be tested for durability on an ongoing basis, the aim being to test the ability of the product to remain as a whole pellet from the mill to when it is presented to the bird. There are generally two mechanisms available which mimic field conditions. 1. The tumbling can – involves placing a weighed sample of material into a rotating chamber for a set period of time, usually 10 minutes at 50 r.p.m. 2. The Holmen tester – the weighed sample of pellets are pneumatically conveyed around a closed pipe, usually 30 seconds. The pellet durability index figure (P.D.I.) is calculated by measuring the amount of fines derived from the test as a percentage of the sample added.
The guideline durability for both measurements for 2 to 3mm pellets is as follows:
Test – Durability Index – Time
Tumbling Can – 98% – 10 minutes
Holmen Test – 95% – 30 seconds
- It is vital birds receive optimal feed intake to achieve optimal growth.
- Feed form has a significant impact on broiler performance.
- Improving feed form involves significant profit opportunities.
- Feed form may be improved at little cost by manipulating feed formulations or feed manufacturing practices.
- Grinding, conditioning and pelleting practices make a significant contribution to pellet quality.
- There are many means to improve pellet quality cost effectively.
- Assessing pellet quality at the mill is essential to ensure improvements made to pellet quality are maintained.