Extrusion technology to process aquafeeds
There are wide varieties of methods that can be used to process aquatic feeds. For examples, in early days the most common method to feed the fish was hand feeding of mixed, home-blended diets and trash fish. Then fish farmers started using cold forming of moist diets. Pelleting presses then became popular and a majority of the fish feed were processed using pellet mill technology.
Today extrusion technology is the method of choice to process aqua feed because of the benefits it offers to fish farmers.
Each method had its advantages and disadvantages.
The initial days of aquaculture were very simple which involved hand feeding of ‘trash fish’ or home-blended diets. These diets were simulated natural diets and were very palatable but the availability was not consistent and providing fresh feed every day was difficult.
Storage of the fresh fish was a problem.
Farmers need to store these fish in cold storage or use only fresh. At the same time disease and virus transmission was very common in fresh trash fish. Feeding the fresh trash fish in large ponds was very labor intensive and water pollution was a huge issue in the ponds.
Today extrusion technology is the method of choice to process aqua feed because of the benefits it offers to fish farmers.
Each method had its advantages and disadvantages.
The initial days of aquaculture were very simple which involved hand feeding of ‘trash fish’ or home-blended diets. These diets were simulated natural diets and were very palatable but the availability was not consistent and providing fresh feed every day was difficult.
Storage of the fresh fish was a problem.
Farmers need to store these fish in cold storage or use only fresh. At the same time disease and virus transmission was very common in fresh trash fish. Feeding the fresh trash fish in large ponds was very labor intensive and water pollution was a huge issue in the ponds.
Pelleted aqua feed
The next phase in aquatic feed processing was use of pellet mills. Pelleted aquatic feed offered several advantages over the hand fed natural diets.
For example, pelleted feed was denser and lower in moisture than the natural feed, so the long-term storage was better. Pelleting also reduced the segregation of the ingredients which was a very common problem in homemade diets.
Since feed was pelleted there was less waste of feed in the water and that resulted in some improvements in water pollution and quality. As a result, pelleting offered low processing cost and a higher processing rate to the aquatic feed industry.
Operation of a pellet mill was not a complex process. On the other hand it was very difficult to make floating fish feed with the pellet mill. This means there was no control over buoyancy. At the same time micro aquatic feed (less than 2.00mm) was not possible with the pellet mill. The cooking temperature in the pelleting process was too low to be adequate for destruction of anti-nutritional factors, pathogenic organisms and viruses in the feed.
Floating and sinking
Current aquatic feed manufacturing practices seem to fit into two simple categories; floating and sinking.
Today 100 percent floating feed is extruded and nearly 60 percent sinking feed is extruded. Whereas, the rest of the 40 percent sinking feed is still pelleted. This is just simply because extrusion cooking offers several benefits to the aquatic feed manufactures. Following is the brief description of the advantages of the extruded feed.
Hygienic quality of extruded feed
Ingredients are cooked at high temperatures and pressures.
Therefore, extrusion cooking provides hygienic processing of feed destroying the pathogens and most viruses and reducing the toxin levels in the feed ingredients.
In extrusion cooking avian influenza virus is destroyed during the processing (greater than 70ºC).
Also, Newcastle disease virus is inactivated by the extrusion and drying process where temperatures exceed 75ºC for one minute. Growth inhibitor, allergens and other anti-nutritional factors largely inactivated during extrusion cooking.
Table 1 shows the effect of extrusion on microorganism and their survival.
Option of producing floating or sinking feed
There are many aquatic species that are cultured today.
Some of them prefer to eat the pellet on the bottom of the pound where as some of species like to come on the surface to take pellet and some are column feeders (taking the feed as it slowly sinks through the water).
For examples catfish, carp and tilapia like floating feed where as shrimp and river crab require sinking feed.
On the other hand salmon, trout, sea bass/sea bream, and cod like to eat the pellet when it is slowly sinking. Picture 1 shows different species requirements for floating and sinking.
Extrusion allows buoyancy control to make floating, sinking or slow sinking feed by controlling the processing parameters on one extruder.
To make the pellet float in fresh or sea water requires <480g/l bulk density. In order to make pellet sink fast requires > 640g/l bulk density for sea water at 20C (three percent salinity) and > 600g/l for fresh water at 20C.
To make slow sinking pellet it requires 580-600g/l for sea water and 540-560g/l for fresh water. At neutral buoyancy pellets should have 520-540g/l for sea water and 480-520g/l for fresh water.
Extrusion cooking provides ultimate control of the density of the products and thus buoyancy properties are managed. Picture 2 shows a quick test for floating and sinking while making feed with extruder.
Ingredients are cooked at high temperatures and pressures.
Therefore, extrusion cooking provides hygienic processing of feed destroying the pathogens and most viruses and reducing the toxin levels in the feed ingredients.
In extrusion cooking avian influenza virus is destroyed during the processing (greater than 70ºC).
Also, Newcastle disease virus is inactivated by the extrusion and drying process where temperatures exceed 75ºC for one minute. Growth inhibitor, allergens and other anti-nutritional factors largely inactivated during extrusion cooking.
Table 1 shows the effect of extrusion on microorganism and their survival.
Option of producing floating or sinking feed
There are many aquatic species that are cultured today.
Some of them prefer to eat the pellet on the bottom of the pound where as some of species like to come on the surface to take pellet and some are column feeders (taking the feed as it slowly sinks through the water).
For examples catfish, carp and tilapia like floating feed where as shrimp and river crab require sinking feed.
On the other hand salmon, trout, sea bass/sea bream, and cod like to eat the pellet when it is slowly sinking. Picture 1 shows different species requirements for floating and sinking.
Extrusion allows buoyancy control to make floating, sinking or slow sinking feed by controlling the processing parameters on one extruder.
To make the pellet float in fresh or sea water requires <480g/l bulk density. In order to make pellet sink fast requires > 640g/l bulk density for sea water at 20C (three percent salinity) and > 600g/l for fresh water at 20C.
To make slow sinking pellet it requires 580-600g/l for sea water and 540-560g/l for fresh water. At neutral buoyancy pellets should have 520-540g/l for sea water and 480-520g/l for fresh water.
Extrusion cooking provides ultimate control of the density of the products and thus buoyancy properties are managed. Picture 2 shows a quick test for floating and sinking while making feed with extruder.
The use of wet material in the pellet
Extrusion allows raw aquaculture wastes and undercooked waste to be used in final feed without any problem.
A reclamation system to recover wet, under-processed product that cannot be recycled through the dryer as dry re-work are available to be used in extrusion processing.
These are closed loop systems designed for recycling wet product with extruders.
These systems allow us to use wet slurries at levels ranging from 22 to 42 percent (as a percentage of the dry recipe) depending upon the moisture contents of the slurries in single screw extruders.
In a twin-screw extruder, addition of wet slurries can be as high as 60 percent depending upon the final moisture contents of the wet slurry. Picture 3 shows a closed looped system that can be used for wet material addition during extrusion in preconditioner. Picture 4 shows addition of wet slurry (more than 40 percent) in the extruder barrel.
High fat pellets
The main purpose of lipids (fats and oils) in feeds are as an energy source; to increase palatability; provide essential fatty acids; carrier for fat soluble vitamins; modified texture; density control and dust reduction.
Fat level in fish diets can vary depending upon the species of the fish.
Extrusion allows raw aquaculture wastes and undercooked waste to be used in final feed without any problem.
A reclamation system to recover wet, under-processed product that cannot be recycled through the dryer as dry re-work are available to be used in extrusion processing.
These are closed loop systems designed for recycling wet product with extruders.
These systems allow us to use wet slurries at levels ranging from 22 to 42 percent (as a percentage of the dry recipe) depending upon the moisture contents of the slurries in single screw extruders.
In a twin-screw extruder, addition of wet slurries can be as high as 60 percent depending upon the final moisture contents of the wet slurry. Picture 3 shows a closed looped system that can be used for wet material addition during extrusion in preconditioner. Picture 4 shows addition of wet slurry (more than 40 percent) in the extruder barrel.
High fat pellets
The main purpose of lipids (fats and oils) in feeds are as an energy source; to increase palatability; provide essential fatty acids; carrier for fat soluble vitamins; modified texture; density control and dust reduction.
Fat level in fish diets can vary depending upon the species of the fish.
Some fish, like salmon, need feed with up to 40 percent fat in the pellet. Extrusion processes allow feed manufacturers to produce high fat feed.
With twin screw extruder a 20 percent fat can be added during extrusion of feed. Where as in single screw it is limited to around 12 percent fat addition during extrusion.
Additional fat is added to the pellet using different coating systems (vacuum or atmospheric). Extruded feed can absorb more fat externally in the coating steps as compared to pelleted feed. Nowadays aquatic feed is processed to contain almost 50 percent fat which is only possible by using extrusion technology.
Different Sizes of the Pellets
Different shapes and sizes of aquatic feed can be produced using extrusion technology.
Depending upon the size of the fish the pellet size can range from 0.6mm to 60mm. The size of the pellet is important for the fish from juvenile to adult. A very small fish is not going to eat 10mm pellet since the size of the pellet is bigger than the mouth. Picture 5 (behind the headline) shows different sizes of the feed made with extruders at Texas A&M University extrusion pilot plant. Picture 6 shows micro aquatic feed made using twin-screw extruder.
Manufacturing soft aquatic pellets
Some species like blue fin tuna prefer very soft pellet.
Extrusion processing allows us to make a very soft pellet (moisture content up to 30 percent in finished feed), which is not possible by pelleting or other methods.
This is accomplished using a preservation system in aquatic feeds (final product moisture of 16-28 percent) during extrusion processing.
Lower Aw, (water activity) below 0.70 can be controlled with humectants at 10-12 percent levels and by reducing pH to 4.0-4.5 with acids at 1-2 percent levels or with fish silage/solubles.
Mould inhibitors are also added at 0.2-0.5 percent level in the formulation.
Recent developments indicate that soft, gel-like aquatic pellets can be processed by coupling alkaline extrusion with a post-extrusion acid bath to adjust ph and set or ‘firm up’ the texture and pellet integrity.
Water stable pellet
Pellets that break down quickly in water will lose nutrients.
Some farmers hydrate feed in water and nutrient solution prior to feeding and require rapid and excellent water stability for handling and feeding purposes.
Pellets that breakdown quickly in the stomach of fish loses nutrients (during regurgitation) and may contribute to GDAS (Gastric dilation and air sacculitis) in certain species.
Extrusion processing allows us to make pellets which are very stable in water for a long duration. Picture 7 shows pellet soaked in the water and then pressed with spatula to see if it will break or not. If it breaks that means it will not be water stable.
Water absorption of pellets
Extrusion moisture content during processing can be controlled over a wide range, which helps to increase the water stability of the final feed.
These pellets absorb more water, retains shapes for longer time and results in reduced losses of nutrients. Water Absorption and integrity after hydration is very important. It benefits pellet technical qualities and fish health. Picture 8 shows pellets disintegrate in the water after some time.
Mechanical resistance of pellets
Feed made using extrusion technology is more resistant to mechanical durability and produces fewer fines in the finished feed during transportation.
Extruded feed has an internal matrix system which tends to increase resistance to mechanical handling of feed. Extruded feed produces approx one-to-two percent fines in the finished products during bulk handling, where as pelleted feed normally generates five-to-eight percent fines during handling in bulk or bag form.
Extrusion reduces 75 percent of the amount of fines which normally enters the water and ends up decaying on the bottom of the pond.
In other words, fewer fines means: increased water stability; clearer ponds; lower fatality rates; less undesirable bacteria growth; increased conversion rates (10 - 20 percent); higher production yields; improved digestibility and reduced feed cost. Picture 9 shows pellet made with extruder with very little or no fines.
With twin screw extruder a 20 percent fat can be added during extrusion of feed. Where as in single screw it is limited to around 12 percent fat addition during extrusion.
Additional fat is added to the pellet using different coating systems (vacuum or atmospheric). Extruded feed can absorb more fat externally in the coating steps as compared to pelleted feed. Nowadays aquatic feed is processed to contain almost 50 percent fat which is only possible by using extrusion technology.
Different Sizes of the Pellets
Different shapes and sizes of aquatic feed can be produced using extrusion technology.
Depending upon the size of the fish the pellet size can range from 0.6mm to 60mm. The size of the pellet is important for the fish from juvenile to adult. A very small fish is not going to eat 10mm pellet since the size of the pellet is bigger than the mouth. Picture 5 (behind the headline) shows different sizes of the feed made with extruders at Texas A&M University extrusion pilot plant. Picture 6 shows micro aquatic feed made using twin-screw extruder.
Manufacturing soft aquatic pellets
Some species like blue fin tuna prefer very soft pellet.
Extrusion processing allows us to make a very soft pellet (moisture content up to 30 percent in finished feed), which is not possible by pelleting or other methods.
This is accomplished using a preservation system in aquatic feeds (final product moisture of 16-28 percent) during extrusion processing.
Lower Aw, (water activity) below 0.70 can be controlled with humectants at 10-12 percent levels and by reducing pH to 4.0-4.5 with acids at 1-2 percent levels or with fish silage/solubles.
Mould inhibitors are also added at 0.2-0.5 percent level in the formulation.
Recent developments indicate that soft, gel-like aquatic pellets can be processed by coupling alkaline extrusion with a post-extrusion acid bath to adjust ph and set or ‘firm up’ the texture and pellet integrity.
Water stable pellet
Pellets that break down quickly in water will lose nutrients.
Some farmers hydrate feed in water and nutrient solution prior to feeding and require rapid and excellent water stability for handling and feeding purposes.
Pellets that breakdown quickly in the stomach of fish loses nutrients (during regurgitation) and may contribute to GDAS (Gastric dilation and air sacculitis) in certain species.
Extrusion processing allows us to make pellets which are very stable in water for a long duration. Picture 7 shows pellet soaked in the water and then pressed with spatula to see if it will break or not. If it breaks that means it will not be water stable.
Water absorption of pellets
Extrusion moisture content during processing can be controlled over a wide range, which helps to increase the water stability of the final feed.
These pellets absorb more water, retains shapes for longer time and results in reduced losses of nutrients. Water Absorption and integrity after hydration is very important. It benefits pellet technical qualities and fish health. Picture 8 shows pellets disintegrate in the water after some time.
Mechanical resistance of pellets
Feed made using extrusion technology is more resistant to mechanical durability and produces fewer fines in the finished feed during transportation.
Extruded feed has an internal matrix system which tends to increase resistance to mechanical handling of feed. Extruded feed produces approx one-to-two percent fines in the finished products during bulk handling, where as pelleted feed normally generates five-to-eight percent fines during handling in bulk or bag form.
Extrusion reduces 75 percent of the amount of fines which normally enters the water and ends up decaying on the bottom of the pond.
In other words, fewer fines means: increased water stability; clearer ponds; lower fatality rates; less undesirable bacteria growth; increased conversion rates (10 - 20 percent); higher production yields; improved digestibility and reduced feed cost. Picture 9 shows pellet made with extruder with very little or no fines.