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Incubation Incubate the prepared dishes, inverted, at 35±10C for 72±3hours.

Counting the colonies Following incubation, count all colonies on dishes containing 30 -300 colonies and record the results per dilution counted.

# Calculation

• a)

When the dishes examined contain no colonies, the result is expressed as; less than 1x101 bacteria per g or ml.

• b)

When the dishes (dilution 1in 10) contain less than 30 colonies, the result

is expressed (30x10=3x102).

as:

less than

3X102

c)

When the colonies are more than 30, count the colonies in both plates of a dilution and calculate the average,

retaining only two significant digits multiply by the inverse of corresponding dilution to obtain

and the the

number of bacteria per g or ml. Example: dilution 1/100 dish 1: 175 colonies

208 colonies

# Calculation: 175+208=383/2=191Æ190=x100

Result: 1.9x104 bacteria per g of food.

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