INVITRO DIGESTIBILITY SIMULATING THE PROTEOLYSIS OF FEED PROTEIN IN THE MIDGUT GLAND OF GRASS SHRIMP (PENAEUS-MONODON)

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 digestive-tract 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-degrees-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.