Dilutions

10X Dilutions

Generally, an initial 10X dilution is preparred from the original sample. Further dilutions are then made from this initial dilution. In order to make this 10X dilution, follow the guidelines below for sample size and amount of diluent to use.

GENERAL PRODUCTS		DAIRY PRODUCTS
Amt. of Sample +	Amt. of Diluent	Amt. of Sample +	Amt. of Diluent
10 mL or grams +	90 mL diluent	11 mL or grams +	99 mL diluent
50 mL or grams +	450 mL diluent	55 mL or grams +	495 mL diluent

Solid samples: Blend the sample with the diluent in a sterile blender for 2 minutes, or massage the sample with the diluent in a stomacher bag for 2 minutes.
Liquid samples: Add the sample to the diluent in a sterile container (such as a diluent bottle or stomacher bag) and shake approximately 25 times.

Remove 1 mL from this 10X dilution and place in the Easygel bottle. Swirl and pour into an Easygel petri dish. Incubate, and count colonies. Multiply the count by the dilution factor, which is 10.

For example, a count of 34 colonies would be multiplied by 10, resulting in a reading of 340 colonies per gram or per mL of sample.

100X Dilutions

GENERAL PRODUCTS		DAIRY PRODUCTS
Remove 10 mL from the 10X dilution and deposit in a new 90 mL diluent bottle. Shake 25 times.		Remove 11 mL from the 10X dilution and deposit in a new 99 mL diluent bottle. Shake 25 times.

Remove 1 mL from this 100X dilution and place in the Easygel bottle. Swirl and pour into an Easygel petri dish. Incubate, and count colonies. Multiply the count by the dilution factor, which is 100.

For example, a count of 45 colonies would be multiplied by 100, resulting in a reading of 4500 colonies per gram or per mL of sample.

1000X Dilutions

GENERAL PRODUCTS		DAIRY PRODUCTS
Remove 10 mL from the 100X dilution and deposit in a new 90 mL diluent bottle. Shake 25 times.		Remove 11 mL from the 100X dilution and deposit in a new 99 mL diluent bottle. Shake 25 times.

Remove 1 mL from this 1000X dilution and place in the Easygel bottle. Swirl and pour into an Easygel petri dish. Incubate, and count colonies. Multiply the count by the dilution factor, which is 1000.

For example, a count of 75 colonies would be multiplied by 1000, resulting in a reading of 75000 colonies per gram or per mL of sample.

Expected dilutions for various products

The following table recommends dilutions for total count, coliform, and yeast and mold testing. However, some of these products may require testing for other organisms also.

Dilutions should yield between 30 and 300 colonies per dish. This is for both accuracy and ease of counting.

For example, if the expected count is somewhere between 2500 and 250,000 colonies per gram or per mL, a dilution of 100X and 1000X should be prepared.

If you do not know what count to expect from your product, try running a 10X and 1000X dilution. This will give you a good idea of what to expect for future dilutions.

Dilutions Overview