Fish Feed Production

Aquatic food pellets conventionally sold for the raising of fish or other aquatic species have been generally of two types. The pellets have either been of sufficient density to allow them to sink in the water as would be necessary for the feeding of crustaceans or bottom feeding fish, or they have been of a reduced density to allow them to float on the surface of the water.

The floating type of pellet has typically been produced by an extrusion-expansion process rather than a conventional pelleting process because of the ability of an extruder to provide a significant decrease in bulk density of certain farinaceous-proteinaceous mixtures caused by gelatinization of the starch from the moisture, temperature and pressure conditions existing in the extruder. Conventional pellet mills, however, will normally not reduce the bulk density of these mixtures. Therefore, all floating aquatic food pellets of this type have heretofore been made on an extruder.

While those extruded products are highly satisfactory, it requires a substantial capital investment by a feed mill that is typically equipped only for the pelleting of feeds but not for extrusion. It would, therefore, be advantageous if a floating aquatic food pellet could be produced on a conventional pellet mill.

The expansion of certain grains such as by "popping" or "puffing" of the intact grain particle under heat and/or pressure is well known. Expansion of the particle by popping or puffing results from the increased vapor pressure in the intact grain particle because of vaporization of the existing moisture in the interior of the grain particle which is restricted by the pericarp of the grain particle. Eversion or popping in effect causes a "turning inside out" of the endosperm by rapid expansion of the starch material in the grain. This in effect causes a reduction in bulk density.

The use of expanded grain particles such as puffed or popped grains in producing floating aquatic food pellets provides a means of reducing the density of these materials to permit floating in aqueous mediums. It has, however, been determined that the expanded grains are unsuitable for a conventional pelleting process. Why this is the case is not completely understood, however, it is believed that an increase in pressure results in the pellet mill, when materials containing the expanded grains are pelleted, which causes a compression of the materials with an increase rather than a decrease in density. Also even if the density did not increase, the pellets still had poor structural integrity in water.

This difficulty in the pelleting of expanded grains in order to produce a floating aquatic food pellet, is overcome by incorporating a hardenable or vitrifiable matrix in the mixture containing the expanded grain particles prior to pelleting. During the pelleting process, with the application of heat and pressure, the matrix hardens or vitrifies and not only produces a pellet of satisfactory hardness and quality, but also has good structural integrity in water. Furthermore, the pellet with the hardenable or vitrifiable matrix has excellent floatability because it is believed that the hardenable matrix assists in lubrication of the die to prevent compaction of the pellet and thereby produce a pellet containing the expanded grain particles that will readily float.

A preferred matrix for use in the present process is a hardenable matrix comprising a liquid carbohydrate medium, water, a water soluble phosphorous source, a colloidal material and an alkaline earth oxide. This matrix will solidify or hardenupon the application of heat during pelleting to improve the structural integrity of the pellet in water. Furthermore, the matrix is believed to prevent compaction of the pellet with the expanded grain particles by sufficiently lubricating the die to prevent this problem. The pellet with the expanded grain particles therefore retains its floatability which is directly due to the presence of the expanded grain particles It is, therefore, an object of the present invention to produce a floating aquatic food pellet.

Thank you.
Enrique Diaz, P.E.
Sales/Service Process Engineer
R&D Equipment Company
Fort Worth, TX