Fish Nutrition & Feed Technology

Unit 10 - Methods of feed formulation and manufacturing

10.7.Advantages Of Extrusion Cooking

Extrusion cooking has several advantages over pelleting.

Versatility:

Any type of feed can be prepared.

Gelatinized Starch:

• Starch is the primary binder used in shrimp feeds.
• Inorder to activate binding functionality, starch must be cooked.
• In the presence of water and heat, starch granules swell, lose their crystalline structure, and become hydrated in a process known as “gelatinization”.
• In pelletiser complete gelatinization does not occur within the temperature range of 55-85 C.. In extruder, because of higher temperature, pressure and shear, complete gelatinization occur.

Water stability:

• Water stability of feed is very important to the shrimp farmer, as shrimps, are slow scavenger type bottom feeders.
• The longer the feed holds together the greater the chance it will be consumed.
• The average accepted stability is approximately 2 to 4 hr, however, extruded feeds have been produced with water stabilities of 12 hr without binders and 24 hr and longer with binders.

Sterilization:

• An extrusion cooker eliminates bacteria in the raw materials.
• A total bacteria plate count on an unprocessed shrimp feed formula had 1,200,000 microbes/g. A subsequent plate count of the extruded diet yielded 1,000 microbes/g. A general guideline is that feed mill products with a plate count of 10,000 microbes/g are considered pasteurized.
• The animal and fish meals used in the formulations are generally the ingredients containing microbes. Formulations that are high in fish meal and meat & bone meal benefit greatly from extrusion cooking due to the reduction in the total bacterial count.
• This helps maintain stable feed quality and may reduce disease occurrence in the cultured animal.

Density control:

• Control of density of the finished extruded product allows the feed miller to produce products that float, sink or sink slowly so that the optimum feed can be produced for the intended species.
• This is controlled by changing the expansion rate and pressure of the extrudate.

Mechanical durability:

• Mechanical durability is important as any breakage or degradation of the product before use results in fines. This fine affects water quality and reduces feed conversion.
• Extruded products have an internal matrix system, which tends to increase resistance to mechanical handling in screw conveyors, automatic feeders or simply bag handling.
• Pelleted feeds generate 2% - 5% fines in the handling of the feeds in bulk or in bags. Extrusion cooked feeds produce approximately 1% - 2% fines reducing the amount of waste material that enters the water.
• Fines are wastes because these particles are rarely eaten in a production pond. Decreasing the fines will increase the productivity and efficiency of the feed.

Storage Life:

• Extrusion cooked feeds have a prolonged storage time. The reason is that extrusion cooking tends to stabilize the raw materials by reducing bacteria, reducing oxidation, oils do not go rancid and vitamins in extruded feeds are more stable than in pelleted feeds.
• It is noted that overdosing of vitamins in an extruded diet is required due to heat liability of vitamins.
• However, the advantage of the vitamin stability partially overcomes this disadvantage, as pelleted feeds also required overdosing to get acceptable shelf lives.