Carbohydrate utilization by juvenile silver perch, Bidyanus bidyanus (Mitchell). IV. Can dietary enzymes increase digestible energy from wheat starch, wheat and dehulled lupin?

The effects of exogenous digestive enzyme supplements on the digestibility of wheat starch or diets containing either wheat or dehulled lupin (Lupinus angustifolius var. gungurra) by silver perch (Bidyanus bidyanus) were investigated. In the first experiment, Natustarch (α-amylase supplement specific to starch) was added at three nominal concentrations (0, 50, 100 or 150 mg kg⁻¹ diet) to diets containing either raw or 100% gelatinized wheat starch (30% dietary inclusion content) and fed to silver perch. The apparent digestibility coefficients (ADCs) for dry matter, starch and energy were calculated. The action of Natustarch on the diet and in the digestive tract was also investigated. The addition of Natustarch to diets containing raw and gelatinized wheat starch led to an average increase in reducing sugar content of diets of 67% and 340% respectively, indicating that the α-amylase was more efficient at hydrolysing wheat starch in the gelatinized form. Gelatinized wheat starch was digested more efficiently than raw wheat starch. However, although the addition of Natustarch at > or = 50 mg kg⁻¹ led to a significant increase in digestibility of raw wheat starch; the digestibility of gelatinized wheat starch, which was already high, was not further improved. Leaching due to immersion in water caused a minor loss of α-amylase activity from diet pellets treated with Natustarch (approximately 13% after 5 min). The α-amylase activity in the anterior section of the intestinal tract of silver perch fed diets containing Natustarch was not affected, indicating that the α-amylase had been denatured by the acidic conditions in the stomach of silver perch. In the second experiment, diets containing wheat or lupin (at the 30% inclusion content) were treated with Natugrain-blend [an enzyme supplement containing β-glucanase and β-xylanase, specific to non-starch polysaccharides (NSPs)] at three nominal concentrations (0, 75, 150 or 300 microliter kg⁻¹) and fed to silver perch. ADCs for energy and protein were calculated. The addition of Natugrain-blend had no effect on dry matter, energy or protein digestibility of the diets or ingredients.