2.2.3 Pelleting and extrusion

Pelleting, through compression, produces a dense pellet that sinks rapidly in water. Extrusion is a process through which the feed material is moistened, precooked, expanded, extruded and dried, producing a low-density feed particle which floats in water. Pelleting is less expensive and generally costs 10 to 12 percent less than extruded fish feeds. However, extruded or floating feeds are very popular with catfish farmers.

Pelleting involves the use of moisture, heat, and pressure to agglomerate ingredients into larger homogenous particles. Steam or hot water added to the ground feed mixture (mash) during pelleting gelatinizes starch, which aids in binding ingredients. Generally, an amount of steam is added to the mash to increase its moisture content to approximately 16 percent and temperature to about 85 C before passing through the pellet die; however, ingredient composition will influence these conditions. The moisture must be removed by proper cooling and ventilation immediately after the pellets leave the pelleting apparatus.

Pellet quality refers to resistance to crumbling and water stability. The amounts of fat, fibre, or starch in the formula can influence quality of the pelleted feed. Some ingredients, because of chemical or physical properties, do not have desirable pelleting quality and can be used only in limited quantity in pelleted feeds.

Additives that serve primarily as pelleting aids are frequently used in fish feed formulas to reduce fines and increase water stability, although research in fish feed technology has demonstrated that high-quality fish feeds can be made without binding materials by following good pelleting procedures. However, use of compounds such as hemicellulose and cellulose derivatives, lignosulphonates, bentonites, and others does allow the processor greater variation in ingredient selection and processing conditions to produce pellets of satisfactory quality.

Extrusion requires higher levels of moisture, heat, and pressure than pelleting. Usually, the mixture of finely ground ingredients is conditioned with steam or water and may be precooked before entering the extruder. The mash, which contains around 25 percent moisture, is compacted and heated to 135 to 175 C under high pressure. As the material is squeezed through die holes at the end of the extruder barrel, part of the water in the superheated dough immediately vaporizes and causes expansion. The low-density extruded particles contain more water than pellets and require more drying. Heat-sensitive vitamins are usually added topically after extrusion and drying. Extruded feeds are more firmly bound due to the almost complete gelatinization of the starch and result in less fines than pellets.

Extruded or expanded fish feeds have two definite advantages over pelleted feeds: the particles float and are more resistant to disintegration in water, and a floating feed allows the fish culturist to observe the condition of the fish and the amount of food consumed. A large percentage of the catfish farmers in the United States use expanded feeds.

Source: FAO

Pretty sure ya got air in floating feed's. If it is being expanded, then the water is vaporized out, something has to take it's place.

Density



Last edited by JKB; 11/04/12 09:13 AM.