Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Engineering Science (Interdepartmental Program)

First Advisor

John Henry Wells


The mechanism of fish protein solubilization by acid hydrolysis at elevated temperatures was investigated. Rate expressions for the solubilization were determined using pilot scale data with two different rate determination methods. A postulated mechanism was verified and estimated rates were evaluated on five independent verification experiments. A functional relationship between nonprotein byproduct concentration and whiteness of fish protein hydrolysate (FPH) was developed. Initial and set-point operating variables were calculated to produce FPH with acceptable color characteristics for food use. A batch reactor was used for duplicate hydrolysis trials conducted at three acid and three temperature levels for four hours. Samples were analyzed for soluble protein nitrogen, insoluble protein nitrogen, nonprotein nitrogen, and hydrogen ion concentrations as well as color. A decrease in the insoluble protein and hydrogen ion concentrations and an increase in the soluble protein and free ammonia concentrations are observed with increasing temperature. Observations also included the formation of free ammonia byproduct parallel to protein hydrolysis. A non-unimolecular "side reaction" describing the solubilization mechanism was postulated. Concentrations were analyzed with mid-point slope method and nonlinear fit of the raw data to determine reaction orders and reaction rate constants. Five sets of independent experiments were conducted at low acid levels and temperature step changes to verify the postulated mechanism and evaluate the rate determination methods. The mid-point slope method was found more accurate for determination of the reaction rates than the simultaneous nonlinear fit of the raw data. Observed and predicted values obtained using postulated mechanism compared well. However, very low initial acid concentrations were beyond the limitations of the postulated acid solubilization mechanism. The functional relationships between acid concentration and protein hydrolysate color were studied for determination of critical concentration and temperature levels to obtain hydrolysate products with acceptable color properties. A strong negative correlation, was observed between the lightness and nonprotein byproduct. Usefulness of the solubilization mechanism was shown in an example calculating the steady state temperatures and initial hydrogen ion concentrations for a fixed processing time, to produce FPH with desired whitness.