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theses

High Pressure Processing (HPP) has gained acceptance as a technology that improves the safety of food products with minimal changes on its organoleptic properties. It is generally assumed that the internal pressure distribution in foods during HPP is uniform. This may not be true for solids with hard inclusions, like meats with bones or for particulate foods, which could compromise their safety and shelf-life. Numerical simulations of stress distribution in a solid with a hard inclusion showed existence of pressure and shear stress gradients. Our work aimed at determining if these gradients would affect microbial inactivation. Additionally we attempted to develop a pressure sensor and a method to monitor microbial inactivation in real-time. Model systems consisting of a gel with a wood rod inclusion, embedded glass wool, or plaster particles were inoculated with Listeria innocua or Saccharomyces cerevisiae and subjected to HPP. A FRET system composed of DPH and Nile red was investigated as a possible pressure sensor. Propidium iodide was used to monitor the inactivation of Enterobacter aerogenes. No differences were found in the inactivation of bacteria at different positions in the gel with wood inclusion. However, 2% glass wool decreased the inactivation of bacteria by 1 log cfu/g compared to pure gel. Higher inclusion volumes caused further drops in the inactivation levels. The yeast inactivation decreased by 1 log cfu/g with 2% plaster, but at 27% it reverted to the levels of the pure gel. It was determined that stress gradients formed very close to the inclusions, and that pressure and shear would affect each organism differently. The FRET system was not an adequate pressure sensor. Membrane rupturing was detected during pressurization and holding but not during depressurization. A larger-than-expected drop in fluorescence was observed after cycles at higher pressure and longer times. This drop couldn’t be explained by volume expansion alone, and the drop was not instantaneous. We suspect this drop in fluorescence came from reversible disassociation of ribosomes, occurring after cell wall and membranes rupture. Our research has begun to elucidate the mechanisms of microbial inactivation by HPP, which could help in designing more effective processes.

ETD_6188

Ph.D.

Includes bibliographical references

by Jose Antonio Maldonado Ugaz

doi:10.7282/T38054GD

ETD doctoral

The author owns the copyright to this work.