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COMBINED EFFECTS OF CO2, pH AND HIGH HYDROSTATIC PRESSURE ON INACTIVATION OF Saccharomyces cerevisiae and Escherichia coli IN APPLE JUICE
Chair: Barry G. Swanson High hydrostatic pressure (HHP) is a nonthermal processing technique for food preservation. Cell structural disruption of Saccharomyces cerevisiae (ATCC16664), inactivation of S. cerevisiae and E. coli (ATCC 11229) by HHP treatment of microbial suspensions saturated with carbon dioxide, recovery of HHP treated E. coli, and inactivation of S. cerevisiae and E. coli enzymes are investigated in this research. Yeast exhibited an increasing number of bud scars, elongation, wrinkling, and pores on the cell surfaces as observed with scanning electron microscopy (SEM) when the high pressure treatment increased from 20 Kpsi to 60 Kpsi. Three log10 cycles of S. cerevisiae in a citrate buffer at pH 3.5 were inactivated by a pressure treatment of 40 Kpsi for 30 sec at 23C. Six log10 cycles of E. coli in the citrate buffer at pH 3.5 were inactivated by a pressure treatment of 35 Kpsi for 30 sec at 23C. The CO2 saturation of citrate buffer and apple juice suspensions reduced the pH of these suspensions. The HHP treatments of citrate buffers or apple juice inactivated less S. cerevisiae and E. coli than HHP treatments of citrate buffers or apple juice saturated with CO2. The acid pH of citrate buffer (pH 3.5) combined with HHP treatments more effectively inactivated S. cerevisiae and E. coli than HHP treatments of citrate buffer (pH 5.5). The recovery of HHP treated E. coli was greater on nonselective tryptic soy agar (TSA) than on selective violet red bile agar (VRBA). Before HHP treatment, thirteen enzymatic activities were detected in S. cerevisiae and E. coli using the APIZYME system kit (BioMerieux Vitek, Inc., Hazelwood, MO). After S. cerevisiae and E. coli were treated with 65 Kpsi pressure for 30 sec at 23C, lipase, cystine arylamidase, and chymotrypsin were inactivated in S. cerevisiae and esterase, esterase lipase, lipase, valine arylamidase, cystine arylamidase, trypsin, a-glucosidase, and b-glucuronidase were inactivated in E. coli. However, alkaline phosphatase, acid phosphatase, and naphthol-AS-BI-phosphohydrolase activities in both microorganisms were retained following HHP treatment. HHP inactivation of microorganisms can be explained by observed structural damage of microbial cells. The combination of CO2 or acid pH with HHP treatments appears to be useful for enhancing inactivation of S. cerevisiae and E. coli, or decreasing pressure required during HHP treatment. HHP inactivation of enzymes can be used to extend shelf life and increase food quality during storage, and may also contribute to inactivation of S. cerevisiae and E. coli. |