In-vitro digestibility simulating the proteolysis of feed protein in the midgut gland of grass shrimp ( Penaeus monodon)

Abstract

Due to feed ingestion, maximal organ weight was reached in 1 h after feeding for the foregut, and 5 h after feeding for the midgut where the highest protease activity was observed in the whole digestivetract of shrimp. The proteases in the midgut gland showed two pH optima at 7.5 and 4.0, that at pH 7.5 being higher. Feed proteins were digested in-vitro with crude enzyme extract from the midgut gland of grass shrimp at 30°C, pH 7.5 for up to 4 h and in two steps at pH 7.5 for 2 h followed by 2 h at pH 4.0. The latter increased feed protein hydrolysis to 1.12–1.56 times that at pH 7.5 for 4 h. The two-step hydrolysis by the shrimp midgut gland extract can serve as an in-vitro method to approximate feed protein digestibility. The tests showed the following order of digestibility: Artemia>predissolved casein>gluten, pelletized shrimp feed>unpelletized shrimp feed>brown fish meal>white fish meal>soybean meal>yeast, undissolved casein. They were significantly different ( P<0.05). The protein digestibility correlated with the amount of lysine and arginine in the total amino acid composition of the protein sources ( r=0.99) indicating that trypsin plays an important role in digestion in shrimp. Aromatic amino acid content had a positive correlation with the digestibility of water-soluble or native protein sources, but a negative correlation with that of the heat-denatured protein sources, indicating that hydrophobic interaction in heat-processed proteins decreased their digestibility.