Protein Source for Fish Feed-IV. Change in Nutritive Value of Defatted Soybean Meal with Duration of Heating Time for Young Yellowtail.

In order to elucidate changes in the nutritive value of defatted soybean meal with the duration of heating time, the general and amino acid composition and trypsin inhibitor (TI) activity of several types of meal heated at 108°C for 0-40 min were determined, and the growth, feed conversion, protein digestibility, and body composition of young yellowtail Seriola quinqueradiata were also compared by feeding them with diets containing about 20% of each meal. The nutritive value of raw defatted soybean meal was extremely low: it had a high TI activity and high nitrogen solubility index, resulting in a low protein digestibility; its feeding caused weight loss in fish with inferior hematological characteristics and body composition. The nutritive value of the meal was markedly improved by heating it for 20 min, and was maximized by heating it for 30 min, but it decreased slightly after being heated for 40 min. The heating treatment hardly improved the general and amino acid composition of the soybean meal, while it caused a rapid inactivation of TI together with a marked improvement in protein digestibility, indicating that the improvement of its nutritive value by heating could be mainly attributed to an elimination of heat-labile growth inhibition factors such as TI. Also, the comparison in nutritive value between these heated soybean meals and soy protein concentrates suggests the continuing existence of heat-stable growth inhibition factors in the heated meal.