Nisin, an antimicrobial peptide used in food preservation was evaluated for thermal stability. Nisin retained antimicrobial activity after having been heated at 121��C for seven hours. Anhydrous nisin and concentrated nisin solutions (5-10 mg/ml) were still active after having been heated up to 190��C or 230��C. When exposed to high temperatures of 200 or 250��C for up to one hour, nisin lost all antimicrobial activity. After 3.5 hours of heating at 121��C an inactive nisin degradation product, designated nisin L, was isolated. In comparison to native nisin, nisin L had reduced activity against Pediococcus pentosaceus FBB-61 and no activity against Bacillus cereus T vegetative cells or spores. Structural changes to nisin L were studied by high performance liquid chromatography (HPLC), ion spray mass spectroscopy (MS), ��H nuclear magnetic resonance spectroscopy (NMR) and circular dichroism (CD) spectroscopy and compared to original nisin. There was no difference in molecular weight between nisin and nisin L. Both MS spectra contained nisin, with average molecular weight (MW) of 3354 daltons (D), and hydrated nisin with average molecular weight of 3372 D. Nisin L had higher proportion of hydrated molecules, and it had molecules with more then one water addition. Proton NMR analysis of nisin L indicated that dehydrobutyrine 2 and dehydroalanine 5 residues had been altered, and that several new hydrogen resonances appeared. Water additions in nisin L are likely to have occurred at dehydroresidues, making them inactive. Nisin L was found to be more polar, as would be expected for a more hydrated peptide. Analysis of CD spectra indicated that nisin L had smaller content of a helix and therefore lesser membrane spanning capability. Tandem mass spectroscopy of original nisin revealed that it was hydrated at lysine 34 residue. / Graduation date: 1996