Experiments were performed using commercially processed
creamy peanut butter purchased at a local grocery store (Seoul,
Republic of Korea) and stored at room temperature (22 ± 1 C). The
composition of the peanut butter (in the order listed on the product
label) consisted of roasted peanuts, sugar, hydrogenated vegetable
oil (cottonseed, soybean and rapeseed oil) and salt. The nutrition
facts label indicated 16 g of fat, 7 g of protein, 6 g of total carbohydrate
and 150 mg of sodium per each 32 g serving. Thirty g samples
of peanut butterwere aseptically placed in sterile 100ml Pyrex glass
beakers. For inoculation, 0.3 ml of culturewas added to each sample
and thoroughly mixed for 2 min with a sterile spoon to ensure even
distribution of the pathogen. Uniform distribution of inoculum was
confirmed by similar log CFU counts (log 5e6 CFU/g) on xylose
lysine desoxycholate agar (XLD; Difco) that were obtained from 1 g
sub-samples of inoculated peanut butter taken from three randomly
selected locations. After inoculation, approximately 5 g was
removed to obtain 25 g of inoculated sample. Water activity of
inoculated peanut butterwas 0.26 and pHwas 5.99. Aw and pHwere
measured by an Aqualab model 4 TE aw meter (Decagon Devices,
Pullman, WA) and a pH meter (Mettler-Toledo, Switzerland).