Question

1. After carefully diluting pasteurized milk samples to 1:10 and 1:100 a lab tech went to sleep and placed 1.1 ml in each plate rather than the normal 1.0 ml. The highest dilution produced 44 and 48 colonies on duplicate plates. Calculate the actual CFU/ ml of milk. 2. A sample of tomato was examined for the presence of an organism which produces a defect in canned tomatoes called flat-sour. Twenty five grams of tomato were diluted in 25 ml of water and heated to 150 Degrees F for 5 minutes, then cooled to 50 Degrees F. The sample was then diluted 1: 10 and 1 ml of that dilution was plated. The plates were incubated at 120 degrees F. The plates had colony counts of 27 and 29 respectively. What is the flat-sour count per g?

Answer 1

1) The milk sample placed = 1.1 mL in each plate

the colonies are 44 colonies by 1.1mL in 100 fold dilution

So in 1mL = 40 colones

so CFU /mL = 40 X 100 CFU /mL = 4000 CFU / mL

For 48 colones

CFU mL = 48 X 100 / 1.1 = 4363.3 CFU / mL

2) five grams of tomatoes / 25mL

Then diluted to 1:10 times , so in mLs = 250 mL total solution ( 5 grams / 250mL )

Out of this 1 mL is taken it will consist of 0.02 grams

so the flat sour count / g will be

a) colony count / gram = 27 / 0.02 = 1350 flat-sour count per g

b) colony count / gram = 29 /0.02 = 1450 flat-sour count per g