Many fish species do not tolerate starch – especially uncooked starch. But starch comes with many of the alternative plant-based protein ingredients. Also starch can be an important “binder” to help achieve pellet durability. Firstly, realise that starch does not truly “gelatinise” during extrusion – there is not enough water present for the swelling and unravelling of granules that characterise the gelatinisation reaction. In addition, the starch is easily damaged by excessive “shear” during extrusion. So we promote conditions that will “cook” the starch – achieving sufficient temperature with sufficient water, and with sufficient time, but without excessive shear – therefore use of a preconditioner can be a major advantage. Another option – if we require starch for pellet binding – is to choose a starch that cooks and binds more effectively at a lower temperature – such as use of tuber starches instead of grain starches. Therefore effective selection of both the source and the amount of starch can be used to optimise the processing and nutritional characteristics of the product formulation.
During extrusion, the protein dena-
Effect of Density on Sink / Float Pellet Behaviour
Sea Water (3% Salt)
> 600 g/L
580 to 600 g/L
540 to560 g/L
520 to 540 g/L
480 to 520 g/L
< 480 g/L
< 440 g/L
Figure 2 turation reaction is not unlike that of starch – that is, with respect to good “functional” protein (here referring to protein functionality from a physical, rather than nutritional, perspective). The globular proteins unravel and, under the right conditions (optimal moisture content and temperature), can cross-link. Therefore “functional” protein contributes to binding and pellet durability. But many of the traditional fish meals, while good nutritionally,
contain denatured protein, and contribute little to the “binding” function. So while the scarcity of traditional marine proteins (eg fish meals) is an issue for nutritional balance, the substitution of “functional” plant proteins can have the added benefit of assisting pellet durability – as long as the process promotes rather than destroys that functionality (temperature and shear not excessive).
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