FEATURE
Extrusion of aquafeeds
by Gordon Young, FoodStream Pty Ltd, Australia and Dennis Forte, Dennis Forte & Associates, Australia
Extrusion technology provides a number of major benefits over the more traditional pellet milling processes commonly used for aquaculture feeds. In particular, extrusion can provide a much higher degree of control over the “cook” achieved, as well as better control of the product density (therefore controlling the floating/sinking characteristics).
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ut extrusion is a very complex process – and we only have “indirect” control over that process. That is, we have full control over some variables – eg screw speed and amount of water added. But there are other inputs over which we have “limited” control. For example, we specify a formulation, and within limits we control the specifications of ingredients that go into it – but ingredients do vary, so our process needs to cope with “normal variation”. We set up the extruder and die to a known configuration – but the machine and the die-plate wears, so that over time the process changes.
determine the feed conversion ratio (FCR) for the product. So to use extrusion effectively – and take full advantage of the opportunities offered by the technology – the extrusion process needs to be properly understood. This article discusses just a couple of the issues relating to successful extrusion of aquafeeds.
Extrusion chemistry & aquafeed ingredients
In the area of ingredients, the aquafeeds industry faces a series of contradictions.
In addition, we never get just one parameter changing during extrusion – if ingredients vary, it doesn’t just change the final product composition – it changes the rheology of the mix and therefore changes how the melt moves through the extruder and the die, which in turn affects the residence time and temperature developed in the melt, which changes degree of cook and expansion – and therefore affects digestibility and floating/sinking characteristics. Within this complex relationship, we need to achieve consistency – of nutrition, of digestibility, of physical characteristics. It is the cumulative effect of these parameters which ultimately
Marine meals provide the best nutritional basis, but are increasingly difficult to obtain and are therefore expensive. Selected plant sources of protein can provide required proteins, but tend to come with fibre and starch which can interfere with the extrusion process and affect digestibility. We generally require very high fat (energy) contents in the feeds – but high fat levels are a problem in extrusion. Following are some basics of “extrusion chemistry” that relate to the way we design aquafeed extrusion processes:
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10 | INTERNATIONAL AQUAFEED | March-April 2015