Formation of gizzard erosion-inducing substance in heated casein-histidine mixture

Formation of a toxic substance inducing gizzard erosion (GE) was examined by using mainly a casein-histidine mixture as a simple model of fish meal. When a casein-histidine mixture was heated at 135 degrees C for three or five hours or at 160 degrees C for one hour, it induced severe GE. Changes in pH of histidine solution seemed to have little effect on the production of the toxic substance. Addition of a heated mixture of 30 mg of histidine and 150 g of casein (per kg of diet) failed to induce GE. A mixture of 750 mg of histidine and 150 g of casein was enough to produce a maximal toxicity. A heated mixture of histidine and isolated soybean protein, ovalbumin or gluten induced severe GE. A heated mixture of gelatin and histidine had a somewhat milder toxicity. On the other hand, zein and yeast treated in the same way showed only a slight toxicity. Two kinds of pre-treatment of casein before the addition of histidine were proved to be ineffective to inhibit the formation of the toxic substance. The toxicity of heated casein-histamine was rather lower than that of heated casein-histidine. Studies were also made on releasing of the toxic substance from heated casein-histidine into the soluble fraction by acidic hydrolysis and precipitation of this substance at acidic pH.