total plate count TPC aerobic plate count APC

total plate count (TPC) aerobic plate count (APC) total mesophilic plate count • Total bacterial number is an important indication for food quality • Basic principle for TPC – Sampling size, 25 or 50 g • Solid sample – Add 9 times volume of buffer (25 g + 225 m. L wt: vol; 25 m. L + 225 m. L vol: vol) • Homogenize – Homogenizer or stomacher

Materials and methods • Food samples – store at room temperature • Depend on the food samples to decide the dilution range • Dilution range is from -1 to -4

Detection limit • No dilution – 1 CFU/ml; 10 CFU/g • -1 dilution – 10 CFU/ml; 100 CFU/g • However, we need 25 to 250 CFU in one plate • Thus, reliable detection limit is: • No dilution – 25 CFU/ml; 250 CFU/g • -1 dilution – 250 CFU/ml; 2500 CFU/g

• 培養基 – TSA • • Dilutions (every group has their own) 25 g + 225 m. L phosphate buffer Homogenize for 2 min Decimal serial dilution (1: 9)

procedures 1. Homogenize sample For milk, no homogenization 2. Decimally serial dilution 3. Place one m. L of the undiluted homogenate into a Petri-dish (0 dilution) 1. Add agar to mix the sample: 混稀 pour plate 4. Place 0. 1 m. L of the undiluted homogenate onto a Petri-dish (-1 dilution) 塗抹: spread plate 5. Place 0. 1 m. L of -1 dilution the homogenate onto a Petri-dish (-2 dilution)

• Incubate the plates at 37 C for 24 (spread) to 48 (pour) hours • Count the colony (25 -250 colony/plate) • Calculate bacterial number in food sample – Solid food: average CFU number x dilution x 10 – E. g. 56 x 103 x 10 = 560000 = 5. 6 x 105 CFU/g = 5. 75 log 10 CFU/g – Milk: average CFU number x dilution – E. g. 56 x 103 = 56000 = 5. 6 x 104 CFU/g = 4. 75 log 10 CFU/m. L