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Dehydrated Culture Media

MILK AGAR

Code: CM0021

a nutrient medium enriched with milk solids for the determination of the viable micro-flora of dairy and water samples

Typical Formula*

gm/litre

Yeast extract

3.0

Peptone

5.0

Milk solids (equivalent to 10ml fresh milk)

1.0

Agar

15.0

pH 7.2 ± 0.2 @ 25°C

 
* Adjusted as required to meet performance standards

Directions
Suspend 24g in 1 litre of distilled water. Bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes.

Description
Oxoid Milk Agar is made to a formula corresponding to the official medium described in Dept. of Health Memo. 139/ Foods1. It is recommended for performing the plate count test on milks, rinse waters, milk products and ice cream, etc.

It also complies with the recommendations of EUROGLACE (EEC Ice cream Industries) submitted to the EEC Commission for the examination of ice cream2.

Statutory tests for milk must be carried out exactly as described in the appropriate Statutory Instrument e.g. Statutory Instrument 1963, Food & Drugs, Milk and Dairies: The Milk (Special Designation) (Amendment) Regulations. Ditto. (1965).

Technique
The sample bottle is inverted 25 times and after flaming the mouth, some of the milk sample is discarded - while the remainder is re-shaken thoroughly and used for the preparation of decimal dilutions in 1/4 strength Ringer solution (BR0052).

For milk, dilutions of 1/10, 1/100 and 1/1000 are prepared and 1ml of each pipetted aseptically into separate Petri dishes. 10ml of molten Milk Agar, cooled to 45°C, is then added to each dish and the contents mixed by a combination of rapid to-and-fro shaking and circular movements lasting 5-10 seconds. The recommended procedure is five to-and-fro movements followed by five circular movements in a clockwise direction, succeeded by five to-and-fro movements at right angles to the first set followed by five anti-clockwise circular movements.

No more than fifteen minutes should elapse between preparation of the dilutions and pouring the plates. The plates are allowed to stand on the bench for about an hour and then transferred to the incubator, where they are incubated in an inverted position for 2 days at 35°C or 3 days at 30°C.

Appreciably higher counts may be obtained after incubation at 22°C and 30°C than at 35°C 3,4,5.

After incubation the colonies are counted, within four hours, and the result expressed as plate count per ml.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared medium at 2-8°C.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Straw coloured gel

Quality control

Positive control:

Expected results

Escherichia coli ATCC® 25922 *

Good growth; straw coloured colonies

Negative control:  

Uninoculated medium

No change

* This organism is available as a Culti-Loop®

Precautions
Make sure that the procedures and media used in milk product testing comply with the National Regulations required for each country.

References
1. Dept. of Health (1987) Memo. 139/Foods.
2. Klose J. (1968) Susswaren. 14. 778-782.
3. Davis J. G. (1959) `Milk Testing’ 2nd ed., Dairy Industries Ltd., London, pp. 175-187.
4. Thomas S. B. and Jenkins E. (1940) Proc. Soc. Appl. Agric. 38-40.
5. Wilson G. S. (1935) `Bacteriological Grading of Milk’ HMSO, London.

 
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