Efficiency of utilization of some potato starch modifications by rainbow trout ( Oncorhynchus mykiss)

Abstract

Six modifications were made on raw potato starch (RS) — starch hydrolysed with acid (AS), starch gelatinized at 70, 100, 120 or 134 °C (GS 70, GS 100, GS 120 and GS 134) and dextrin (Dx). Seven diets containing 20% RS or one of the modifications were formulated. Rainbow trout (initial weights 63 and 115 g for growth and digestibility trials, respectively) were fed to satiety once daily at a water temperature of 13.5 °C. A growth trial was conducted for 50 days. All methods of starch treatment used resulted in some degradation of polysaccharides and/or destruction of starch granules. Both destruction of granules (diet with GS 70 vs diet with RS) and degradation of the polysaccharides (diet with AS vs diet with RS) caused a significant increase in starch digestibility by trout. Joint action of these factors yielded the best results (diets with GS 100, GS 120, GS 134 or Dx). Digestibility of starch increased with increasing low-molecular weight polysaccharide content due to a rise in the temperature of the hydrothermic treatment up to 120 °C. Additional degradation of polysaccharides at a higher temperature of 134 °C or thermal dextrinization did not increase digestibility of starch. Feed gain ratios decreased progressively with increasing digestibility of starch. Hepatosomatic index and dress out percentage correlated to the degree of starch degradation but not to starch digestibility.