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Calculating and Reporting Plate Counts

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Department
Food Science
Course
FOOD 3230
Professor
Unknown
Semester
Fall

Description
Calculating and Reporting Plate Counts Step 1: Understand the formula Result (CFU/g or CFU/ml) = Initial dilution × Subsequent dilution × Ave count / Volume (ml) Where: Initial dilution is the first dilution which dilutes the original solid or liquid sample in buffer. Normally the initial dilution is 1:10. In Lab 1 shrimp tests, the initial dilution was prepared by putting 25g shrimp into 225ml buffer. Subsequent dilution is the following 10 fold dilutions. In Lab 1, they were prepared by transferring 1ml sample from initial dilution (in stomach bag) to test tube with 9ml buffer. T-2 fur-3er dilu-4ons were prepared in the same way. So the dilutions in there test tubes were 10 , 10 and 10 . Ave count is typical colony number on the plate. If 2 or more parallel tests are performed, the number is average number of each test. Volume (ml) is volume of diluted sample added on the plate. Normally, pour plate method requires 1ml and spreading method need 0.1ml. Step 2: Choose suitable dilution(s) for calculation Note: You DO NOT need to calculate all colony number from all dilutions. Only colony number between 30 and 300* (or 15 and 150 for Mould) should be counted and calculated. Ex.1 Pour plate (1ml per plate) Dilution Colony number Result (calculation) -1 10 -2 >300 3 10 68 6.8x10 CFU/ml (100x68/1) 10 -3 5 Ex.2 Spreading (0.1ml per plate) Dilution Colony number Result (calculation) 10 -1 >300 -2 4 10 -3 68 6.8x10 CFU/ml (100x68/0.1) 10 5 If plates from two adjacent dilutions are between 30 and 300, both colony amounts from two dilutions should be considered. [ ] [ ] Result = Ex.3 Spreading (0.1ml per plate) Dilution Colony number Result (calculation) 10 -1 282 2.9x10 CFU/ml ( /0.1) 10 -2 35 -3 10 4 Sometimes none of dilutions have colony number between 30 and 300. A) All numbers are less than 30. We choose the dilution with colony number closest to 30 and calculate. Ex. 4 Spreading (0.1ml per plate) Dilution Colony number Result (calculation) -1 3 10 -2 12
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