Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Food Technology

First Advisor

Homer W. Walker


In one series of studies, a simple well-plate technique was utilized to determine the effect of various metals on the growth of microorganisms in media containing different polyphosphates. Aspergillus flavus and four Gram-positive bacteria were almost completely inhibited by media containing 1% of various alkaline polyphosphates, whereas four Gram-negative bacteria were not. Significant differences were observed between the type of polyphosphate added, the type of metal added, and the species of Gram-positive bacterium inhibited. A method for determining the amount of calcium and magnesium in water was modified to detect free Mg[superscript]2+ by replacing EDTA with phosphate. The addition of free Mg[superscript]2+, but not Mg[superscript]2+ chelated by tetrasodium pyrophosphate, permitted the growth of Bacillus cereus on a medium containing tetrasodium pyrophosphate. It is speculated that polyphosphates specifically inhibit Gram-positive bacteria because the polyphosphates have higher affinities than the cation-binding sites within Gram-positive cell walls for essential metal cations;In another series of studies, Listeria monocytogenes F5069 was suspended in either Trypticase soy-0.6% yeast extract broth (TSBYE) or sterile, whole milk and heated at 62.8°C in sealed thermal death time tubes. Severely heat-injured cells were recovered in TSBYE when incubated within sealed thermal death time tubes because of the formation of reduced conditions in the depths of the TSBYE after heating. Also, the use of anaerobic "Hungate" techniques significantly increased recovery in Trypticase soy yeast extract broth containing 1.5% agar (TSBYEA), compared with aerobically incubated controls. The exogenous addition of catalase, but not superoxide dismutase, slightly increased the recovery of heat-injured cells in TSBYEA incubated aerobically. Thermotolerance increased with increasing time and temperature of sublethal heat shock; however, cells grown at 43°C were more heat resistant than cells heat shocked at 43°C or cells grown at lower temperatures. Under the conditions of the present study, high levels of Listeria monocytogenes could survive the minimum low-temperature, long-time process required by the U.S. Food and Drug Administration for pasteurizing milk. The possibilities that injured cells can be recovered in selective media under strictly anaerobic conditions and heat shock proteins cause thermotolerance by preventing the denaturation of catalase and superoxide dismutase are discussed.



Digital Repository @ Iowa State University,

Copyright Owner

Stephen John Knabel



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242 pages