Extrusion has been used to prepare animal feeds for many years and has proved to be an effective means to cook and shape raw ingredients into very specialized products, especially in the highly competitive pet food market where size, shape, mouth feel and degree of cook are important factors in the success of the product. During the past 25 years the commercial raising of fish for human food consumption has grown rapidly and this has opened up opportunities for the fish feed industry to produce special feeds for this application. Much of what has been practiced with the extrusion of feeds for other animals can be applied to fish feed.
The main conditions influencing the operation of the extruder are temperature and moisture. These conditions affect the starch and protein cooks and, since the starch cooks into an inflatable gel that binds the rest of the ingredients together, the degree to which the starch is cooked has a large bearing on the properties of the product.
In the raw state cereal starches are in the form of tiny particles, insoluble in water, and which act very much like miniature grains of sand when mixed with water. As the starch is cooked the granules start to swell and then burst, spilling out the molecules of starch from the tightly packed interior of the granule into a more loosely packed condition.
This occurs fairly early during the cooking conditions attainable within an extruder. Once this occurs the starch forms a gel which has binding properties that can glue together all the other purely solid particles in the formation and, in addition, the gel can be inflated. Some proteins form a gel similar to the way starch does, but if the protein is cooked too strongly it leaves the gel state and reverts back to the ordinary solid state.
There are a number of properties desired in animal feeds that are directly related to starch cook.
1. The animal feed should be digestible, which usually requires that the starch at least be cooked beyond the granule rupture stage.
2. The extruded product should be in discreet particles which hold together and do not fall apart into fines.
3. The particles should have the required bulk density which usually requires some degree of porous internal structure.
4. The particles should be able to absorb water yet continue to hold a definite shape and, if desired, still remain floating, or sinking.
The feed formulation can be fed raw into the extruder, and water or live steam, or both, are injected directly into the extruder to elevate the moisture. The live steam also contains heat. It, and the heat generated by friction as the shaft rotates through the material, elevates the temperature. The shaft forces the material against the discharge end of the barrel thereby placing it under pressure. There are small die openings in the plate covering the end of the extruder through which the moist hot material, now cooked, (actually it is pressure cooked) can exit. The elevated pressure within the extruder can keep all the water in a liquid state. Even the steam is compressed into liquid water. When the product exits into the atmosphere, the water immediately vaporizes. If the starch is sufficiently cooked to form a gel it will inflate with thousands of tiny pores which cause the product to be of low bulk density.
An extruder can be operated to provide a cook that causes the starch to form a gel that has a high affinity for absorbing water. This degree of cook is usually attained by extruding under moderate extrusion temperatures of 240 to 280°F and fairly high moisture levels of 27 to 33% moisture.
An extruder can also operate under conditions of a stronger cook such that some of the starch molecules begin to break down into molecules of dextrin. Partially dextrinized starch still forms a gel but it doesn’t absorb as much water as undextrinized starch and some of it is soluble in cold water. Therefore fish feeds that are bound together by mixtures of dextrin and starch, have different properties than fish feeds bound together by starch alone.
A fish feed manufacturer can take advantage of this by operating his extruder under conditions where he has minimal dextrin formation, if desired, or under conditions where varying levels of the starch is dextrinized. He can increase the dextrin level by operating at higher temperatures and lower moisture. At a temperature of 350°F and a moisture level of 20-23%, for example, a significant amount of the starch is converted to dextrin. The preparation of the feed also affects the cook within the extruder, particularly the particle size of the feed. Coarsely ground materials are harder to cook than finely ground materials. And the cook is not uniform with coarsely ground materials. Larger particles tend to have poorly cooked centers; the larger the particle, the greater the possibility the interior might be undercooked. The presence of fat in the formulation tends to reduce expansion. How fat is added can determine whether the product will float or sink. Fat added ahead of the extruder, or injected directly into the extruder, causes high density and will help the product to sink. Fat applied to the surface of finished product will not affect the expansion; it will make the product heavier but the interiors are still porous. If a formulation requires the addition of fat and it must float, it would help to add the fat after extrusion rather than before extrusion.
Even though extruder conditions have an influence on whether a product floats or sinks, the primary influence is controlled by the formulation itself. High levels of fat will insure that the product will sink. High levels of starch, or the addition of gelatinizable protein sources in the raw feed formulation, help to insure that the product will float. When extruding a shaped product it is important that the amount of expansion be controlled so that the product does not puff more than desired or less than desired or become distorted due to erratic puffing. In addition the product should have a structure compatible with the mouth feel desired in the final product. This is influenced not only by the cook in the extruder, but also by the exit moisture of the particles and by the cutter operation.
If the particles are extruded at high moistures (30%) they will lose some water when the internal moisture flashes into steam (approximately 3 percentage points) but the resulting particle at 27% moisture is still soft enough to collapse. This forms a tough skin around the surface and reduces the size of the pores within the particle.
After drying, this type of particle can have a tough and hard mouth feel and could be dense enough to sink. On the other hand, extruding at moistures of 20% to 25% can provide for a particle that becomes stiff after flashing, so that minimal collapse occurs. The particle now has a porous surface, larger internal pores, and a thinner wall of solid matter surrounding the pores. This makes for a softer mouth feel, and keeps the bulk density low so that the particle is more apt to float. The particles are cut by a rotating fly knife attached to the surface of the die plate. The fly knife should be set up to make a clean cut of the particles without distorting them. The degree of particle puffing is influenced by the land length of the die opening. The land is the thickness of the final opening. A die permitting maximum expansion will have a short land, usually 1/8”. A die used to minimize expansion will have a longer land. This can also help to influence whether a product floats or sinks.
The total die area regulates how much product can be physically discharged. In addition, the size of the individual openings also affect the horsepower consumed during extrusion. A set of small dies of a given total die area have a greater resistance to flow causing more back pressure and higher horsepower draw than a set of larger dies with the same total die area. This is due to the fact that less of the material comes into contact with the walls of the dies if it is all going through a smaller number of large dies. One of the ways a feed manufacturer can control the conditions under which he is going to extrude is to select dies with specific diameters and lands to provide for the horsepower draw and flow pattern he wants for that product.
If a feed manufacturer understands what is happening during extrusion he can take advantage of the parameters and make them work for him. He can impart the qualities he wants into his products by a judicious selection of feed preparation, extruder operating conditions, and mechanical set up of the knives and dies.