Sink / Float Phenomenon in aquafeeds By Dennis Forte, Dennis Forte & Associates Pty. Ltd.and Gordon Young,Food Industry Engineering
Aquatic feeds manufactured by extrusion are typically designed to either exhibit “100% Sink” or “100% Float” behavior (Figure 1). But sometimes a small proportion of the product may not exhibit the desired behavior i.e. a few of the pellets might float, instead of sinking, or vice versa. A related quality control issue is when the feed
Fig.1 The extrusion process used for the manufacture of aquatic feeds can generate product that either sinks or floats, or a mixture of both
sinks, as required when produced, but some time (days) later, a proportion of the feed becomes floating. This is referred to as the
Sink / Float Phenomenon. This article discussed the causes of these issues.
Archimedes' Principle states that the buoyant force on a submerged object is equal to the weight of the fluid displaced. A practical result of this is that an object will float if its density is less than the density of the surrounding fluid, or will sink if its density is greater than that of the surrounding fluid. Note it is the density of each individual pellet that is important – the Pellet Density. However, in the aquafeed industry, a more convenient measure is Bulk Density.
The Bulk Density is directly affected by Pellet Density – but also relates to the way the pellets pack within the sample container (Figure 2). Bulk Density is an indirect – and inexact – measure of pellet density, but it is a product parameter that can be easily measured and relates to the sink/float behavior – see Table 1 – so it is commonly used as a quality control parameter.
Aquafeed magazine is focused on advances in feed formulation and processing for aquculture.