Factors Affecting Palatability of Pellet Feed—Hardness (PDI)

Factors Affecting Palatability of Pellet Feed—Hardness (PDI)

Lyu Jirong, Liu Feng

 

Technology Service Department of DadHank (Chengdu) Biotech Corp.

 

1 Influence of Crushing Process on Pellet Hardness

The crushing particle size of ingredients is the decisive factor for pellet hardness in crushing process. Generally, the finer the particle size of ingredients is crushed, the easier it is for starch to be gelatinized during conditioning, the stronger the bonding effect in pellet feed, the less likely the particle is broken and the harder the pellet feed is. In actual production, according to the production performance of different animals and the hole size of ring mold, the crushing particle size should be adjusted appropriately. The pellet size of chicken and duck feed should be coarse, the average crushing particle size should be 800-900 μm, while suckling pig feed should be fine, the average crushing particle size should be 400-500 μm, and fattening pig feed should be 600-700 μm. Fish feed and special aquatic products should be finer and no larger than 250 μm, generally. In poultry feed, it is generally required that the pellet feed should be harder and the pulverization rate should be low to reduce the waste of feed. In order to improve the particle hardness of poultry pellet feed, the particle hardness can be improved by adjusting the coarse, medium and fine particle proportions of ingredients. The proportion should not exceed 15% for coarse particle with a particle size of above 900 μm, is about 35% for medium particle with a particle size of about 700 μm, and requires more than 50% for fine particle with a particle size of less than 500 μm, which fine powder with a particle size of less than 250 μm is required to be no less than 25%. The starch in fine powder (with a particle size of less than 500 μm) can be fully gelatinized during conditioning and plays an important （adhesion ）role in the pelleting process, bonding coarse, medium and fine particles together into large particles, improving the hardness of particles and reducing the pulverization rate of products. In the production of pig feed, it is generally required that the particles hardness should be moderate. Too hard will reduce the palatability and production performance of products, and too brittle will increase the pulverization rate of products, reduce production performance and increase waste. In production of pig feed, it is generally required proportion of particle with a size between 500 and 700 μm to be more than 70% and proportion of particle with a size no larger than 250 to be more than 20%, which is beneficial to pelleting and improve the appearance quality of particles, and can also ensure the appropriate hardness and low pulverization rate of products. In the production of fish feed, on the one hand, the physiological characteristics of fish require that the particle proportion with a size below 250 μm should be not less than 85%; On the other hand, small particle size is beneficial to the formation of particles and their stability in water, and the particle of fish feed is relatively hard, which is because the stability of fish feed in water should be better and the particles should be dense. At present, all the fish feed are hard pellet feed, which should be developed in the direction of soft pellet feed.

 

2 Effect of Ingredients Extrusion and Expansion Process on Particle Hardness

Extrusion and expansion treatment of ingredients can destroy anti-nutritional factors, remove toxins, kill bacteria, eliminate harmful substances, denature protein, and fully gelatinize starch in the ingredients. The impact of gelatinized starch on particle hardness is significant. At present, expanded ingredients are mainly used in the production of high-end piglet feed and special aquatic feed. For special aquatic feed, after expanded, the degree of starch gelatinization increases, and the hardness of the formed pellet also increases, which is beneficial to improve the stability of pellet in water. For piglet feed, it is required that the pellets are relatively crispy and not too hard, which is conducive for pig to eat. However, expanded pig feed particles have a higher degree of starch gelatinization, so the pellet is harder. Therefore, other methods should be used to reduce the hardness of the pellets.

Aarseth et al. found that although extrusion of feed ingredients can increase their gelatinization degree, subsequent pelleting will make the overall structure of particles soft. Amornthewaphat and Attamangkune's study showed that the extrusion process can improve the water absorption of corn, the water absorption index and water solubility index of non- expanded corn-based diets were lower than those of expanded corn-based diets, and it was still unclear whether this is due to starch or fiber denaturation.

 

3 Effect of Mixing, Watering and Oil Spraying Processes on Particle Hardness

Mixing of ingredients can improve uniformity of various particle size components, which is advantageous for maintaining basic consistency in particle hardness and improving feed product quality. The adding water during the mixing process is still a problem being actively explored, and adding 1-2% water during mixing in the production of hard grain feed is beneficial to enhancing stability and hardness of the feed particles. However, the increase in moisture brings negative effects to drying and cooling of particles, as well as storage of products. In the production of wet pellet feed, up to 20-30% water（moisture） can be added to the powder feed, and about 10% water（moisture） can be added during mixing, which is easier than adding water during conditioning. Pellets formed with high-moisture materials have low hardness, are moist and soft, and have good palatability, which can improve the productivity of livestock and poultry. Large-scale feeding enterprises can use this type of wet pellet feed. Wet pellets generally cannot be stored and require immediate feeding after production.

In Moritz's （et al.) study, the moisture content of feed for broiler chick starter increased from 7% to 14% during the mixing phase, resulting in an increase in pellet durability index from 51% to 77%. In Lundblada's （et al.) test on the effect of adding water to pig feed during the mixing process, the powder feed in the mixer was added with 1.5%, 3.0%, 6.0% and 12.0% of water respectively, the pellet durability index increased from 91% to 93% as the moisture content of the powder feed Hemmingsen et al. found that high-starch ingredients such as barley, wheat, and hulless(hulled) oats were more prone to bind with water in 80℃ water than in wet steam at the same temperature, whereas protein ingredients such as soybean meal or rapeseed meal tended to bind with water in wet steam. Gilpin （et al.) discovered that low-quality steam with free water can improve the quality of pellets, but high-quality steam can not provide enough free water to form durable pellets when the initial moisture content of the powder feed was low. Therefore, the optimal moisture content for pelleting is related to factors such as steam quality, conditioning time, conditioning method, and initial moisture content of the powder feed.

Adding oils and fats during the mixing process is a common method of fats and oils addition in feed production workshops. The addition of 1%-2% fat does not significantly reduce the pellet hardness, while adding 3%-4% fat can significantly reduce the pellet hardness.

4 Effect of Ingredient Moisture Content and Pelleting Temperature on PDI of Pellet Feed

The moisture content of ingredients has a significant impact on the yield and processing quality of pellet feed. Adding a certain amount of moisture or steam during the pelleting process can form a thin layer of moisture on outside of each particle, making it easier to pass through the mold hole. It can also dilute the natural binder in ingredients, thereby improving the quality of pellet feed. The moisture content of ingredients before adding water or steam generally does not exceed 13%. If the moisture content is too high, it will impose certain restrictions on the addition of water or steam before pelleting. Usually, the ingredients need to be watered and heated by steaming before pelleting. If steam heating is not used, pelleting can still be achieved by adding only a few percent of water. However, the temperature difference before and after the materials passes through the mold is about 20-30℃. According to conventional methods, the temperature should be heated to around 80℃ in the conditioner to pellet, and the temperature difference before and after mold is 5-15℃. Additionally, comparing the two methods, the hardness of feed products made by cooked ingredients is about twice as high as that of adding water. From these situations, it can be seen that:

(1) After cooking (steaming), the ingredient fluidity improves and the resistance to pass through mold decreases, which enhances processing capability.

(2) By utilizing the gelatinization of starch and the heat-induced denaturation of protein, an increase in cohesive force can lead to the production of hard, high-quality pellets.

 

6 Effect of Pellet Mold on Pellet Hardness

Technical parameters such as aperture and compression ratio of the pellet mill ring mold can significantly affect the hardness of pellet. Pellet formed by using ring mold with the same aperture but different compression ratios show an obvious increase in hardness as the compression ratio increases. Choosing a suitable compression ratio ring mold can produce pellets with suitable hardness. Miladinovic et al. pointed out that the mold hole diameter and thickness of ring mold had a great influence on the quality of feed pellets. Test results show that when the hole diameter is 3.5mm, the thickness of the ring mold is 60mm and the feeding speed reaches 500kg/h, the durability of the feed pellets is highest (PDI=90.1%), while when the thickness of the ring mold is 50mm, the durability of the feed pellets decreases by 3.3%.

 

8 Effect of Drying and Cooling Process on Pellet Hardness

To extend the storage time of feed products and ensure product quality within a certain period, necessary drying and cooling treatment is required for the feed pellet. In the test of determining pellet hardness, it is found that the hardness of pellets with low hardness is not significantly affected by cooling time, while the hardness of pellets with larger hardness decreases as the cooling time increases after cooling the same product for 5, 10 and 15 minutes respectively. This may be because as the moisture inside the pellet dissipates, the brittleness of the pellet increases, thus affecting their hardness. In addition, when comparing the results of pellets subjected to rapid cooling (cooling time of three minutes with full wind gate open) and slow cooling (cooling time of 20 minutes with the wind gate closed to one-third), it is found that the former had a slightly lower hardness and an increased surface crack compared to the latter.

 

Tang Xing et al. (2015) found that as the cooling time of the cooling tower increased and the tempering temperature rose, the hardness of the feed increased significantly. When the cooling time of the cooling tower was between 8-12 minutes and the tempering temperature was between 75-95℃, the feed hardness increased significantly.

Wu Ying et al. (2010) found that cooling time and cooling air volume had a significant impact on the hardness of pellet feed, and the hardness of pellet feed increased significantly with the prolongation of cooling time and the increase of cooling air volume. The interaction between cooling air volume and cooling time had a significant effect on the hardness of pellet feed. The results indicated that in the production of pellet feed, reasonable adjustment of cooling time and cooling air volume based on factors such as particle diameter and season were necessary to ensure the hardness of pellet feed. Considering the physiological characteristics of the digestive tract of piglets and feed palatability, the suitable cooling time is 7-8 minutes and the cooling air volume is 21-23 m³/(t·min) for pellet feed hardness.