How important is density of product during fish feed pelleting

Fish digestive system is very short (for example: it is 10 times shorter that swine digestive system) and that is why fish feed must be light. Both dimensions and shape of pellets must be adjusted to size of fish. Besides, in water pellets must behave according to habits of fish: sink or float on surface of water.
And this feature of feed depends on pellet density.

Quality requirements of fish feed

There are carnivorous fish ( salmon, trout), omnivorous fish (carp) or herbivorous fish (crucian). Selecting a proper formula for a given kind of fish, we should pay attention to their nutrition habits. Bed fish usually consume food at the bed of water basin and eat 6 times slower than carnivorous. The food for them should not move in the water.

Raptorial fish catch the food during slowly fall onto bed and that is why this kind of fish does not eat close to the bed.Shrimps and other crustaceans nourish only at the bed of basin. This is the reason why their food must not dissolve in water within few hours.

Kinds of food for fish

Depending on product density we can get food with different properties, which are appropriate for their nourish habits, which are explained below:

• food floating on surface of water – for warmwater fish
• slowly sinking food – for raptorial fish, which like cold water
• sinking food, water resistant, for warmwater fish
• sinking food, extremely water resistant, for shrimps and other crustaceans

In growth phase fish can need pellet up to 12 different sizes, from 0,05 mm to 12 mm. If the given food is too big or too small, fish will not eat it.

Production process

On density (specific weight) depends if pellet will float on surface of water or will sink. Floating pellets have the specific weight about 450-550 g/dm³, and sinking pellets - 550-650 g/dm³. Before pelleting (or expansion) components must be conditioned (mixed) with use of steam and water.

Conditioning temperature is 80 - 90^oC, content of moisture in mixture directly before pelleting is 15-16%, if expander will be used then 18-20%, and if extruder will be used then 25-30%

Pellet mill

During the pelleting process product is squeezed through perforated die, and its density always exceeds 650g/dm³. That is why this way of pelleting is best for sinking pellet producing.

Pellet mill preceded by expander.

In production line before pellet mill is installed an expander. In expander, through a short period of time, raw materials are processed in very high temperature (just like in extruder).Produced in this way pellet is better resistant to abrasion, than product that was only pelleted.

Extruder or expander with perforated die.

To avoid inconveniences connected with work of usually used pellet mill or pellet mill with expander, an expander equipped with heating and cooling system was created. Due to the expander we can affect the expansion and, controlling the temperature, also density of the product.If we compare pellets produced in 3 different production processes, it will appear, that the food made using extruder sinks 50% slower than food produced only with normal pellet mill.

The density can be modified by adding an oil to formula. Depending on amount of added oil the food will float or sink, and density of mixture produced basing on the same formula can be modified, so that it can have density from 430 – 550 g/l.

Final conclusions

The condition of a proper fish breeding is adjusting of the density of pellet to nourish habits of individual species. Choosing the proper density of pellets we will save on cost of feeding, and we will avoid exceeding pollution of water, what is result of not eaten food dissolving. Food producer can affect the density of fodder using extruder or expander, and due do proper conditions of raw materials treatment.