| The resistance of Listeria monocytogenes, Salmonella spp. and Escherichia coli 0157:H7 to high pressure processing (HPP) in an acidic whey protein beverage (pH 3.8) was evaluated. Five strains of each pathogen were grown to stationary phase at 35°C following two 24-hour transfers. Cells were harvested by centrifugation (2450 rpm, 25°C, 15 min), washed and re-suspended in 0.85% (w/v) sodium chloride. The whey beverage was inoculated with approximately 6--7 log10 CFU/mL with one of the cocktails. Samples were subjected to pressurization treatments for 2, 4, 6, or 8 min at pressures ranging from 200 to 500 MPa depending on organism sensitivity to HPP. Cell survivors were enumerated by serial dilution of the beverage in buffered peptone water (to reduce acidic beverage effect) and plated on tryptic soy agar with 0.6 % yeast extract (TSAYE) and selective media: Modified Oxford Medium (MOX) for L. monocytogenes, Sorbitol MacConkey agar (SMAC) for E. coli, and Xylose Lactose Tergitol 4 agar (XLT-4) for Salmonella spp. D-values were calculated from log transformed survival curves. The E. coli 0157:H7 cocktail exhibited the greatest resistance to high pressure over L. monocytogenes and Salmonella spp. A shelf life study of E. coli 0157:H7 in a whey beverage containing 50 ppm nisin, 50 ppm nisin and 0.04% (w/v) potassium sorbate, 0.04% (w/v) potassium sorbate and a control (without nisin and/or potassium sorbate) was performed. The beverage was stored at 25°C following HPP at 400 MPa for 4 minutes. Samples were assayed by spread plate on TSAYE and SMAC on days 0, 1, 3, and 5 following pressure treatment. Five mL of the remaining sample was mixed with sterile double strength tryptic soy broth with 0.6 % yeast extract (TSBYE) for enrichment and incubated at 35°C for 48 hours. Enrichment was streaked on TSAYE and SMAC to confirm E. coli O157:H7 growth. Antimicrobial containing samples treated with HPP were more effective at inactivation of E. coli O157:H7. At day 1 of storage all antimicrobial samples contained E. coli O157:H7 counts below the detection limit of 1 CFU/mL by plating and were undetectable by enrichment. Unpressurized controls contained E. coli O157:H7 counts above 6 log CFU/mL10 at day 5 on TSAYE, reconfirming this organism's ability to persist in an acidic environment. Overall, HPP was effective in reducing E. coli O157:H7 6.79 log10 CFU/mL in a low pH whey beverage. The acidity of the whey beverage alone was not enough to reduce E. coli O157:H7 counts 1 log after 5 days of storage. |
