Effects of red yeast inclusions in diets for salmonids and extrusion temperature on pellet tensile strength: Weibull analysis

Enhancement of fish feed extrusion-technology and improvements of technical pellet quality requires knowledge about causal connections amongst variables of processing condition and raw materials, and how these connections can be elucidated by measurements. Weibull analysis of feed pellet tensile strengths may represent one possible measurement tool for this purpose; this analysis is evaluated here and compared with the Holmen durability test. Diets added pre-treated red yeast Xanthophyllomyces dendrorhous (Basidiomycetes) that differ in the degree of enzymatic cell wall digestion (45, 70 and 97% CWD) were extruded at two temperatures, 100 and 140 °C. Pellet expansion ratio (32%) was independent of both temperature and yeast CWD. Bulk density and sinking velocity corroborated accordingly, with only marginal difference in density (640 kg m−3) and no effect of the independent variables on sinking velocity (10.7 ms−1). Average Holmen durability was significant higher (P<0.01) for pellets extruded at 100 °C (97.2%) than at 140 °C (96.4%), and the difference increased disproportional for testing times of 90 and 120 s. Tensile stresses at failure were Weibull distributed with values for the modulus m in the range 4.3–6.6, and the scale parameter σ0 in the range 360–430 kPa. Inclusions of yeast cells with digested cell walls (97% CWD) improved tensile strengths significantly, and the positive effect was evident at both extrusion temperatures. Pellets extruded at the highest temperature showed significant lower strength. Weibull analysis suggests that the effect of temperature is caused by differences in anisotropy in these feeds.