The Science of Macro Ingredients to Produce Floating Fish Feeds

The feeds for fish can be categorized as floating feeds, slowly sinking feeds, and sinking feeds. Each of these types of feeds are either made by exploring different types and amounts of ingredients, for instance, by controlling starch levels in the feed formulations or by varying the processing parameters, for instance, by modifying the screw profile, changing the screw speed, etc. Floating feeds are mainly made using an extrusion process whereas sinking feeds are or can be made using either extrusion or pelleting process. For attaining floatability, starch and pelleting process are the main drivers of making feeds, we will focus on the mechanism of how each of these macro ingredients affects the buoyancy or floatability of the feeds in the upcoming sections of this communication.

For any material/object to float in a liquid, the buoyant force (net upwards force exerted by the liquid) should be equal to the weight of the water displaced by that object. For example, if there are two objects of different masses but the same volume, then the amount of water displaced by both objects is the same. The object with a lighter mass would experience more upward buoyant force as compared to the object with a higher mass. If the buoyant force is more than the weight of the liquid displaced (which is the same in this case as both objects have the same volume), then the object would float. Therefore, for a heavier object to float, it has to displace more water, which is only possible by increasing its volume. And as the volume increases, the object has to be more stretched or expanded, and that in turn would lower the density of that material. So, there is a higher chance for a lower-density material to float as compared to higher density material (for the same volume of material).

Using the above concept for producing floating feeds, it is important for the feed to have a low density. This can be achieved by having an expanded product i.e. a product with a higher volume. Thus, it is important that we understand the functionality of ingredients, especially starch as an ingredient in feed formulations. Starch is a biopolymer which literally means that It is a polymer (large molecules made up of smaller repeating units of molecules), of biological origin or that which is naturally found in nature. Starch molecules consist of repeating glucose molecules. Polymers exhibit a viscoelastic nature and starch molecules exhibit the same.

Viscoelasticity is an important factor that affects expansion because both viscosity and elasticity are needed for the best expansion characteristics. In an expanded extruded product, the melt viscosity affects the longitudinal expansion, whereas melt elasticity affects the axial (radial) or diametral expansion (Alvarez-Martinez et al., 1988). Starch is majorly composed of two fractions – amylose and amylopectin. Generally, cereals have around 25% amylose and 75% amylopectin. Amylose is essentially a linear molecule, whereas amylopectin has a branched chain. Both amylose and amylopectin differ in their viscoelastic properties. Amylose is more rigid than amylopectin and amylose can form a gel in boiling water whereas amylopectin does not have that property.

The differences in the chain structure of amylose and amylopectin cause it to crystallize differently. Synthetic polymers show that linear polymers are crystallized more easily than branched polymers made of the same monomers. Using that analogy, it can be extrapolated that amylose crystallizes much faster than amylopectin. The faster crystallization of the amylose is what gives it a better gelling property. Further, the crystal melting temperature of amylose is much higher (115-120°C) than amylopectin (above 55°C) (Maurer, 2009), and that allows amylose to be in a gel state as compared to amylopectin. The lower molecular weight amylose is unable to trap the steam produced from water during extrusion due to venting and thus does not contribute to expansion.

During feed manufacturing, starch undergoes gelatinization and it affects both the main constituents of starch. Expansion of fish feed occurs due to the ability of starch to melt and trap the water vapor bubbles in that viscous matrix and to hold on to the expanded shape after exiting from the die and being cooled. Thus, it is evident that for an expanded feed, a viscous matrix capable of trapping water vapor bubbles and retaining the shape after being cooled is needed. The viscosity of the melted matrix is contributed by amylopectin and the rigidity is provided by amylose. Thus, higher the amylopectin content, higher the viscosity of the melted matrix and higher is the expansion of the product (Colonna et al., 1989). So, a balance of amylose and amylopectin is essential for good expansion characteristics of feeds.

Proteins added in the formula generally inhibit the expansion of the extrudates (fish feeds in this case). They dilute the starch and thus reduce the ability of the extensibility of starch. Generally, expansion occurs due to high molecular weight components (amylopectin). Proteins do not expand as starch because they have low molecular weight as compared to starch. So, during extrusion, the protein molecules align themselves in the direction of melt flow and that is difficult to pull apart (expand). Higher concentrations of protein (like > 40% in texturized vegetable proteins), however, expand because the protein molecules link together to form larger structures as they flow through a die channel. They aggregate and form higher viscosity complexes, which serve to form crude films and retain some of the expanding water vapor. To counteract the lower cross-sectional expansion due to protein addition in lower amounts, starches with high amylopectin or modified starches may be used.

Fats/oil is an important ingredient in fish feeds. Fats and oils are composed of molecules known as triglycerides, which are esters composed of three fatty acid units linked to glycerol. An increase in the percentage of shorter-chain fatty acids and/or unsaturated fatty acids lowers the melting point of a fat or oil. These act as lubricating agents and thus decreases the shear inside the extruder and significantly reduce the degree of starch gelatinization. This effect leads to decreased expansion of the extrudate (feed). Generally, exogenous fat/oil addition has more impact in reducing the expansion than inherent native fat/oil. The addition of oil > 2% can start affecting the expansion of fish feeds (Bortone, 2004). Fats with higher melting points may be chosen to minimize fat migration onto packaging during transit and storage.

Floating aquatic feeds require enough expansion of feeds to enable enough volume, which can displace the water equal to the weight of the feed. Starch, due to its ability to expand is the primary driver of expansion. Other ingredients in the feed formula also affect the expansion. Careful selection of ingredients and their ratios in the feed formula is therefore critical in producing floating feeds. However, the process conditions too have to be in synergy to achieve the right quality of floating feeds.