Impact of variability in fishmeal physicochemical properties on the extrusion process, starch gelatinization and pellet durability and hardness

The purpose of this study was to identify physicochemical properties of fishmeal (FM) with impact on specific mechanical energy, starch gelatinization, pellet durability and pellet hardness. The effects of variation in 18 physicochemical properties were assessed based on standardized extrusion, drying and coating conditions. Multivariate models (R2 = 0.907–0.970) were established based on partial least squares regression. Increased level of specific mechanical energy was associated with decreased water-holding capacity (P=0.008), decreased cumulative mean particle size (P=0.020), increased value of flow-figure (P<0.001), increased degree of protein hydrolysis (P<0.001) and increased level of the water soluble protein fraction with molecular weight less than 0.2 kDa (P=0.022). Improved degree of starch gelatinization was associated with decreased water-holding capacity (P<0.001) and increased degree of protein hydrolysis (P<0.001). The variation in FM physicochemical properties resulted in a large span in pellet hardness (4.9–133.4 N) covering the range from poor to high physical quality. Improved hardness was associated with decreased water-holding capacity (P=0.069), decreased pH (P=0.001), decreased level of non-soluble protein (P<0.001), increased level of salt (P=0.002), and increased level of the water soluble protein fractions with molecular weight 5–6 kDa (P<0.001), 20–35 kDa (P<0.001) and above 60 kDa (P=0.006). The underlying physical and chemical mechanisms are discussed. The study documents the complexity of FM as a protein ingredient with significant impact on the extrusion process, starch gelatinization and physical pellet quality. The FM specifications normally used on the world commodity market inadequately describe the technical quality of a FM.