Fish meal replacement by plant protein substitution and guar gum addition in trout feed. Part II: Effects on faeces stability and rheology.

In the explosively growing aquaculture industry, the challenge of sustainability includes a requirement to produce more fish than are consumed by stock. A promising avenue of research is the substitution of the fish meal in feeds with plant protein. However, there are inherent risks in the development of such feeds, and serious consideration should be given not only to nutritional content, but also to the mechanical quality of resulting faecal wastes. The present study uses a plate on plate rheometer running in different flow modes (creep, oscillation) to assess the rheological properties of wastes resulting from different diets.All faeces were shown to be thixotropic in nature, independent of diet composition. Details of dietary composition influence the consistency and the characteristic stresses at which faecal structure changes from a viscoelastic solid into a viscoelastic liquid. Furthermore, in linking active food components with mechanical properties of chyme faeces, rheological studies may be used to understand and counteract some problematic properties. Substituting 100% of fish meal with plant proteins leads to faeces that rapidly disintegrate into very fine solids which threaten the viability of aquacultural operations. Adding just 0.3% of the rapidly hydrating non-starch polysaccharide, guar gum (GG), significantly increased the viscosity and elastic modulus of the faeces. These mechanical improvements increase the size of the resulting particles and the effectiveness with which they can be removed, thereby leading to optimized water quality. GG addition was sufficiently effective to counter the stability and particle size effects of a 50% substitution of fish meal, but could not mitigate those of a 100% substitution, wherein dissolution effects of an unknown emulsifier are suspected.