**3.2 Isolated digestive tract compartments**

An intermediate between *in vivo* and *in vitro* protein digestibility assays is the utilization of isolated digestive tract compartments (Polovic et al. 2010; Wickham et al. 2009). The experimental procedure requires an animal to be sacrificed immediately before the measurement. After that, the required compartment is surgically removed and either placed into a vial with a simulated fluid of this compartment, or sealed both sides with the salt mixture with substrate within. A variation of this technique implies the inversion of a digestive tract compartment inside out before sealing, so the resultant bag is put into a beaker with the substrate-containing solution, and the digestion products diffuse to the internal volume of such bag. Some researchers add glucose to the incubation medium to provide the tissue with energy.

The strong point of this approach is that the digestion and absorption systems of the intestine remain intact, and so the biological significance of the obtained results is very high, and the question on whether the digestion products really pass through the intestinal wall is immediately answered. The isolation of reaction products is also rather simple, as they are automatically separated from the source protein and mucosal proteins (if a sealed compartment bag is employed). The downsides of this method are, of course, high animal requirements and low suitability for high-throughput screening.
