Date of Graduation


Document Type


Degree Type



Davis College of Agriculture, Natural Resources and Design


Human Nutrion and Foods

Committee Chair

Kristen E Matak


With mechanical fish processing, a substantial amount of protein is discarded as byproducts. Isoelectric solubilization and precipitation (ISP) is a process that recovers previously discarded protein using extreme pH shifts to solubilize and precipitate protein from byproducts. Typically, strong acids are used for pH reduction but have limited impact on bacterial load; therefore, organic acids were used during ISP processing to test the impact on Listeria ssp. concentrations. The goals of these studies were to determine the effectiveness of organic acids, specifically acetic and citric acids, to reduce L. innocua during the protein recovery process of ISP and to determine if L. innocua is an appropriate surrogate for L. monocytogenes in future ISP processing studies.;Headed, gutted rainbow trout were inoculated with L. innocua or L. monocytogenes, homogenized, and brought to the target pH with granular citric acid (pH 2.0 and 2.5) or glacial acetic acid (pH 3.0 and 3.5). Proteins were solubilized for 10 min at 4°C and insoluble components (skin, insoluble protein, etc.) were removed by centrifugation. The remaining solution was pH shifted to the protein isoelectric point (pH 5.5) with sodium hydroxide and precipitated protein was separated from the water. Microbial content for each component (proteins, insolubles, and water) was enumerated on both growth and selective media. The sums of the surviving cells from each component were compared to the initial inoculum numbers. No significant differences were observed between the selective and growth media (P > 0.05). Significant reductions were detected at all pHs (P < 0.05) using L. innocua. The greatest reduction in L. innocua cells was at pH 3.0 with glacial acetic acid, resulting in a net pasteurization effect (6-log reduction in Listeria populations) with a mean log reduction of 7.64 in the combined components, and a log reduction of 7.89 in the protein portion. With L. monocytogenes, significant reductions were detected at all pHs (P 0.05) detected in any of the components between treatments. The greatest reductions were at pH 3.0 with acetic acid, with a mean log reduction of 3.03 in the combined components, and a 3.53 log reduction in the protein portion. Data were compared between the two studies. Significant differences (P < 0.05) in recovery were found between the two species at pH 2.0 and 3.0, regardless of processing pH or acid type, with a greater recovery of L. monocytogenes. These results demonstrate that while organic acids possess anti-microbial potential in ISP processing, the variability in resistance between species indicates that L. innocua is not an appropriate surrogate for L. monocytogenes during ISP processing.