Fish are the most efficient farmed animals for converting feed nutrients into edible meat. The quality of such fish feed is crucial to ensure this goal. Fish feed is presented in the form of extruded pellets obtained through twin-screw extrusion. It is an agro-material whose composition, nutritional value, density, and size vary in order to suit the development stages, behaviour and therefore the nutritional requirements of various fish species. We focused this study on a carnivorous species called trout. Their diets must have high lipid content to provide a source of easily available energy (up to 30% w/w). That is allowed by injecting a mixture of vegetable and animal oils (i.e., rapeseed and fish oils) using a vacuum coating operation. However, pellets present a major defect, i.e., the oil leakage over time, contributing to various issues like water pollution and loss in the lipid intake expected for fish consumption.
This work describes a process approach to study the oil leakage phenomenon in adult trout feeds. A Stolz (France) MRSV 100 pilot vacuum coater was used for the study, and the latter was conducted in the form of an experimental design. Four coating parameters were studied: the stirring speed, the pressure inside the coater, its filling rate, and the time needed for going back to the atmospheric pressure. The properties measured on the obtained coated pellets were the oil leakage rate (OLR, in proportion to the pellet weight), their durability and their floatability. Coating conditions showed a real ability for influencing the OLR value, the latter varying at 40 °C from 2.7% to 1.2% depending on the coating conditions used. In fact, the vacuum level was the most effective factor for reducing this phenomenon. Indeed, a progressive reduction in OLR was observed as the pressure inside the coater was reduced at coating (Fig. 1a and Fig. 1b).