2.2. Sample preparation and inoculation
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 butter were aseptically placed in sterile 100 ml Pyrex glass beakers. For inoculation, 0.3 ml of culture was 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 5 – 6 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 butter was 0.26 and pH was 5.99. Aw and pH were measured by an Aqualab model 4TE aw meter (Decagon Devices, Pullman, WA) and a pH meter (Mettler-Toledo, Switzerland).