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Ask the Expert Your aquafeed processing questions answered
Q: We are getting more floating pellets in the days post production. What is the cause and how do we prevent it? This phenomenon is common, especially in feeds which require some degree of expansion for optimal fat inclusion, such as salmonid diets. Primary causes are described here, and all are exacerbated when there is considerable variation in piece density. • Residual pellet heat immediately post processing will always make pellets sink faster, as they tend to wick moisture in much more rapidly, so 100% sink post production does not provide a guarantee of 100% sink in the days to come. Where pellets have started to float in the days following processing, if you reheat pellets a little, you will find increased sink rate again – albeit temporary while they are warm! In this case you need to bump your density up to account for the change, either during the extrusion process or by adding more fat. Once fully cooled, the retrogradation (recrystalizing) of starch in the days immediately post extrusion likely also plays a part here, with its impact on water absorption potential and thus ability for water to wick into pellet core. • Moisture changes. More common in older dryer types or where
the spreader is poorly adjusted, moisture levels can vary by three percent or more across a bed. In this situation and particularly in combination with varied piece density, some of the higher moisture pellets may begin to float as moisture equalizes in the days post extrusion. • Oil leaking. When the extrusion process is too dry and SME too low, pellets tend to be “rod like” with poor internal cell structure. This leads to lack of bonding surface area for the oil to adhere to within the pellet, meaning that coated oil will leak over time, even if the pellets appear dry immediately post coating. High SME pellets have a finer cell structure, increasing the surface area for bonding and are less prone to splitting on hydration. If pellets leak oil during storage, they lose weight and will therefore be at an increased risk of floating. If this is the cause of your density changes, it is usually easy to recognize through the oil leaking from bags. • Density changes caused by vacuum coating. Why? When vacuum coating, the air is first sucked out of the pellet. During the coating process, if for example the pellets have potential to hold 25% fat, but are only coated with 20% fat, then after the release of the vacuum, the fat forms a partial seal preventing
Peter Hutchinson is a Technical Editor at Aquafeed.com, the owner – director of ENH Ltd., New Zealand, and an aquaculture feed consultant. Send your questions to: pete@aquafeed.com
complete ingress of air to refill the five percent void remaining. Over time air migrates past the oil barrier, re-expanding voids which collapsed during release of the vacuum and increases the potential to float. Imagine your pellet is a sponge within a plastic bag, compressed with all the air evacuated – now make a small hole in the bag and watch the sponge re-expand. The key to reducing latent float is obviously maximizing density to that which will still enable required oil absorption, while also maintaining suitable SME. Having said this, if you have wide variation in piece density, it can be nearly impossible
Aquafeed: Advances in Processing & Formulation Vol 12 Issue 1 2020