Static ultra-high pressure (200, 270, and 340 MPa) and sinusoidal and step-pressure (0-270 MPa) pulses were compared by determining their effects on bacteria and yeast survival counts. Samples were commercial pineapple chunks used as is and fresh cut pineapple and pineapple juice inoculated with Saccharomyces cerevisiae 2407-1a. Surviving bacteria and yeast counts were determined using plate count agar (PCA), acidified potato dextrose agar (PDA) and yeast extract dextrose agar (YEPD). Commercially acceptable reduction of microflora for pineapple (greater than 2.5 log cycles) were observed at static pressure of 340 MPa for 15 min at three temperatures (~4, 21, and 38°C). Longer process times (40 and 60 min) at this pressure did not result in further significant count reductions. Lower pressure treatments, 270 and 200 MPa for 15 min, at these temperatures reduced the microbial population of pineapple by 1-2 log and less than one log, respectively. Static pressures treatments of 270 and 340 MPa at 21°C were found to reduce bacterial and yeast counts by 5-log cycles on inoculated fresh cut pineapple. Total bacteria and yeast counts remained <25CFU/plate after a 1/10 sample dilution during 60 days of storage at 4°C. Sinusoidal and step-pressure pulses were compared with static pressure treatments for the inactivation of S. cerevisiae 2407-1 a in pineapple juice. No inactivation was observed after 40-4,000 fast sinusoidal pulses (10 cycles/s) at 4-400 s in the 235-270 MPa range. Static 270 MPa treatments at 40 and 400 s resulted in 0.7 and 5.1 decimal reductions, respectively. Slower 0-270 MPa step pulses at 0.1 (10 pulses), 1 (100 pulses) and 2 (200 pulses) cycles/s with 100 s total on-pressure time resulted in 3.3, 3.5, and 3.3 decimal reductions, respectively. A comparable static pressure treatment resulted in 2.5 decimal reductions. Changing the on-pressure/off-pressure time ratio showed that treatments with longer on-pressure time were more effective. Step pressure pulses (0-270 MPa) on inoculated pineapple chunks for 100s at 0.1 (10 pulses, T3) and 2 (200 pulses, T5) cycles/s were more effective than a 100 s static 270 MPa (T1). After 30 days of refrigerated storage T1 samples showed >10³CFU/g in bacteria and yeast counts, while T3 and T5 had < 10CFU/plate after a 1/10 sample dilution. Both 5 and 10 min step pulsed 0-270 MPa treatments were as effective as 15 min static 270 and 340 MPa. Treatments T3 and T5 for 5 min yielded 0-4 CFU/plate after a 1/10 sample dilution during 30 days of refrigerated storage and recovery of pressure-injured cells was not observed. Static 15 min pressure at 270 MPa and 340 MPa resulted in 0-10 and 0-8 CFU/plate after a 1/10 sample dilution during the same 30 days of refrigerated storage. Step pressure treatment seems to be more effective than static pressure for inactivating bacteria and yeast in pineapple; recovery of pressure-injured cells was not observed under refrigerated storage conditions. / Graduation date: 1997