With deep research of feed industry, more and more researchers and manufacturers put the emphasis not only on ingredient quality and formula but also on feed processing technology to improve feed quality. Because feed processing technology can flexibly regulate the hardness of pellet feed. The hardness of pellet feed is the important indicator of pellet appearance quality and also has certain influence on animal production performance. In the whole feed processing technology, besides feed formula, factors influencing pellet hardness mainly include feed ingredients grinding process, expansion of raw material, feed ingredients mixing, water adding and fuel injection process, steam modulating process, dies in pelletizing, post curing & post spraying and drying & cooling.
I. Ingredients Grinding
The pulverization fineness of feed ingredients plays the decisive role in feed pellets hardness. Generally speaking, the finer the particle size the better the starch gelatinization, which can ensure stronger bonding of feed ingredients. Thus feed pellets have good hardness and are not easy to be broken. In actual production, the pulverization fineness should be adjusted depend on animal production performance and diameters of ring die holes. For poultry feed pellets, the particle size of raw materials should be bigger with average diameter of 800-900μm; for baby pig pellets, the particle size is around 400-500μm; for finishing pig pellets, the particle size is reduced to 600-700μm; for fish and aquatic feed pellets, the particle size is below 250μm.
For poultry feeding, the pellets should be with greater hardness and lower pulverization rate to reduce feed wastage. To improve the hardness of poultry feed pellets, you can regulate the proportion of coarse, medium and fine particle size in the mixing of grinded ingredients as follows: coarse particle size with the diameter of above 900μm are less than 15%; medium particle size of 700μm are round 35%; fine particle size of below 500μm are more than 50% including no less than 25% fine powder of below 250μm. The starch in the fine powder can be fully gelatinized in the conditioning process, which can enhance the bonding of particle sizes in pellets. So these coarse, medium and fine particle sizes are bonded together to form big pellets to improve the pellet hardness and reduce product pulverization rate.
For pig feeding, the hardness of pellets should be moderate. Too hard, the product palatability and production performance get lower; too soft, pulverization rate gets higher and production performance gets lower, which results in increased wastage. In the proportion of grinded materials, the particle sizes of 700-500μm should be over 70%, and particle size of below 250μm should be over 20%. Thus, this distribution of particle sizes contributes to forming pellets, improving pellet performance quality and keeping the right pellet hardness and lower differentiation rate.
For fish feeding, the proportion of crushed materials depends on fish physiological characteristics. Particle sizes of 250μm should be not less than 85%. Moreover small particle sizes are beneficial to pellet forming and stability in the water. Fish feed pellets are always in high hardness because of the demand for high stability of fish feed in water.
II. The Expansion of Raw Materials
The expansion of raw materials can bring the following advantages for pellet making like destruction of anti-nutritional factors, killing bacteria, eliminating hazardous material, denaturation of proteins and starch gelatinization. Gelatinized starches have significant influence on the hardness of pellets. Puffed materials now are mainly used in the production of high-grade baby pig feed and special aquatic feed. For special aquatic feed, the expansion of raw materials can increase starch gelatinization degree to enhance hardness of shaped pellets, which improves pellet stability in the water. For baby pig, feed pellets should be crisp but not too hard to benefit its feeding. However, due to high starch gelatinization degree, baby pig feed pellets are formed in strong hardness. So we should adopts other ways to lower pellet hardness.
III. Feed Ingredients Mixing, Water Adding & Fuel Injection
Raw materials mixing can strengthen uniform distribution of various feed particle sizes to ensure formed feed pellets in basically identical hardness. Water adding technology in the mixer is still being actively sought. In hard feed pellet production, adding 1%-2% water in the mixer is beneficial to improving the stability & hardness of pellet feeds. But the increased water brings negative effect on drying & cooling feed pellets and also is bad for pellet storage. In the wet pellet feed production, you can add 20%-30% water in powder and 10% water in the mixing, which is easier than adding water in conditioning. Feed pellets made of high-moisture materials are featured with low hardness, wet & soft, good palatability, which can improve livestock and poultry production performance. In the large-scale farming enterprises, the wet pellet feed can be adopted. Because it can not be stored which means wet pellets should be immediately used after production. Adding fats in the mixing is generally adopted to meet the nutrition needs of animals. Adding 1%-2% fats does not significantly reduce the pellet hardness but adding 3%-4% fats will significantly reduce the granule hardness. So the amount of adding fats should be controlled.
Steam conditioning is the key part in feed pellet production. Conditioning effect directly affects granule internal structure and appearance quality. Steam quality and conditioning period are two vital factors that affect conditioning effect. Highly dry and saturated steam can provide more heat to increase the temperature of materials, which makes starch gelatinized. The longer the conditioning time the higher gelatinization degree of starch. Therefore formed granules are with compact structure, high stability and strong hardness. For animals and poultry feeding, you can adjust steam addition to keep conditioning temperature from 70 to 80℃. Through adjusting length of conditioner, paddle angle and rotating speed, you can keep conditioning period for around 30 seconds. For fish feed, double layer or multilayer jacketed conditioner is always employed to improve temperature and prolong conditioning period, which contributes to better strengthening fish feed pellet stability in the water and hardness.
V. Ring die
The aperture and compression ratios of ring die can prominently affect granule hardness. Adopting ring dies with the same aperture but different compression ratios, increase of pellet hardness will accompany with the the increase of compression ratios. With the suitable compression ratios, you can produce pellets with right hardness.
The length of pellet also has distinct effect on pellet pressure bearing capacity. For pellets of the same diameter without any defects, the longer the pellet the greater the hardness. So adjusting cutting blades position can ensure suitable granule length to keep the same hardness. Diameter section shape of granule also has certain effect on granule hardness. 8 shape section has stronger pressure bearing capacity and can test higher hardness level than circular section. In addition, material of ring dies also influences granule appearance and hardness. Pellets made by ordinary steel ring die have significant difference compared with that made by stainless steel ring die.
VI. Post Curing and Post Spraying
Post curing and post spraying are rarely employed in livestock and poultry feed production process, but widely in fish and specific aquatic feed production. Post curing can fully gelatinize starch in pellets. Starch gelatinization can strengthen pellet internal structure to avoid water infiltration, which improves aquatic pellet hardness and stability in the water. Post spraying is used in hot pellet feed spraying technology. Fats or others that sprayed will penetrate interior of granules to reduce the pellet hardness but improve its stability in the water.
In order to prolong storage period and meanwhile ensure pellet quality, it is necessary to dry and cool feed pellets. In pellet hardness test, we desperately cool the same feed pellets for many times in different cooling periods: 5 minutes, 10 minutes and 15 minutes, then test the granule hardness. It has shown that for lower hardness pellets, cooling period has little affect on its hardness, but for greater hardness pellets, the longer the cooling period the lower the hardness. Probably due to internal moisture loss, pellet brittleness gets increased, which lower its hardness. Compared with fast cooling in 3 minutes with large air flow( with whole ventilation door) and slow cooling in 20 minutes with small air flow( with two-thirds of ventilation door), we find that in the former situation, the hardness of pellets become lower and surface cracks become more than that in the latter situation. In addition, particularly worth mentioning is that crushing big pellet into small particle can significantly reduce its hardness.
In feed pellet making process, factors that affect pellet hardness are more that the above. Believe that more and more methods of regulating granule hardness can be mastered by people. In the future we can produce kinds of pellet feed favored by animals!