Special ingredients or binders are usually used in steam pelleted feeds for crustaceans, which must remain intact in the water for several hours. Precooked (pregelatinized) starches from corn, sorghum, potato, palm nut, and tapioca have been used at levels of 10% to 20% of the formula to increase bond strength of the pellets. Wheat gluten and ground wheat have good binding properties. Special organic hydrocolloids, such as those presented in Table 5.4, (page 118) are used in quantities of 0.5% to 3% of the ingredient formula.
Extrusion requires more elaborate equipment and more energy for processing than pelleting (see Figure 5.3). Higher levels of moisture, ingredients is conditioned with swteam or water into a “mash” which may or may not be precooked before entering the extruder. The mash, which contains around 25% moisture, is compacted and heated to 104°C to 148°C under pressure in the barrel of the extruder. As the material ;is squeezed throuht die holes at the end of the barrel, and external pressure decreases, part of the water in the superheated dough immediately vaporizes and causes expansion of the feed particles. The extruded particles contain mor water than steam pelleted particles and require external heat for drying. Thus, after extrusion, the particles must pass through a drying tunnel to reduce moisture to a safe storage level. Heat-sensitive vitamins, especially ascorbic acid, are added in excess prior to processing or applied to the surface after processing. Extruded feeds are more firmly bound due to the almost complete gelatinization of the starch, and this results in fewer fines and longer water stability than pelleted feeds.