Is pH Drop a Valid Measure of Extent of Protein Hydrolysis?

Utilizing nine foods as substrates, protein digestibility as determined by the two-step, pH-drop procedure of Satterlee et al. was compared to the extent of hydrolysis at the end of the incubation period. The extent to which the substrate protein was hydrolyzed was found to be much less than the calculated digestibility, as expected. However, the digestibility and extent of hydrolysis did not correlate with one another in any reasonable way. It is concluded that, although pH drop can be used to follow the couree of hydrolysis of a single substrate, and in spite of a demonstrated correlation between pH drop and in vivo protein digestibility, pH drop alone cannot serve as a measuee of extent of protein hydrolysis for the comparison of different substrates.