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Dehydrated Culture Media

POTATO DEXTROSE AGAR (EP/USP/JP/BP)

Code: CM0139

 for the detection and enumeration of yeasts and moulds in butter and other dairy and food products

Also for the preparation of Aspergillus niger for the Harmonised Microbial Limit Tests from EP/USP/JP (enumeration test) 

Formula

gm/litre

Potato extract

4.0*

Glucose

20.0

Agar

15.0

pH 5.6 ± 0.2 @ 25°C

 
*4.0g of potato extract is equvalent to 200g of infusion from potatos

Directions
Suspend 39g in 1 litre of water (purified as requested). Bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes. Mix well before pouring.

In order to suppress bacterial growth it is sometimes desirable to acidify the medium to pH 3.5. This can be done by adding 1ml of Lactic Acid 10% SR0021 to each 100ml of sterilised medium at 50°C. The medium must not be heated after the addition of the acid; this would result in hydrolysis of the agar and destroy its gelling properties.

Description

Food
A suitable medium for the isolation and count of yeasts and moulds in dairy products1 or those occurring on the surface of fresh meats, cured meats and sausage products and other foods 2. This medium is suitable for the detection and enumeration of heat resistant moulds in thermally processed fruits and fruit products 2.

Work carried out in cooperation with CSIRO Melbourne had shown that the minerals present in agar could influence the pigment formation of certain fungi. Where pigment production is a critical part of the identification of the fungus it is clearly important to stabilise this characteristic. The agar used in Potato Dextrose Agar is carefully screened to ensure correct pigment production by fungi such as Fusaria spp.

Pharma
A suitable medium for the preparation of Aspergillus niger. Potato Dextrose Agar conforms to the parameters of the harmonised Microbial Limit Tests from EP/USP/JP (enumeration test)3,4,5,.

Additional information about this and other products in our harmonised range can be obtained by downloading our Harmonised Culture Media Portfolio.

Technique

Food
After sterilising the reconstituted medium adjust the reaction to pH 3.5 by adding 1ml of Lactic Acid 10% (SR0021) to each 100ml of medium at 50°C. Do not reheat after acidification.Prepare dilute emulsions or suspensions of the product to be tested, make pour-plates in the usual manner, and incubate for 5 days at 21°C. Count the number of yeast and mould colonies.

If a non-selective medium is required, it is suggested that Potato Dextrose Agar may be used without added acid or, alternatively, one may use a general purpose mycological medium such as Malt Extract Agar (CM0059).

Pharma
Grow Aspergillus niger  on Potato Dextrose Agar (CM0139) at 20-25°C for 5-7 days or until good sporulation is achieved.

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: Off-white, free-flowing powder
Prepared medium: Light straw coloured gel

Quality Control

Food
Compare with previous lot/batch using pasteurised and raw milk samples, incubated at 32-35°C for 48 hours.

Positive control:

Expected result

Aspergillus fumigatus ATCC® 9197 *

White mycelium, blue green spores

Negative controls:

 

Uninoculated medium

No change

At pH 3.5 Bacillus subtilis ATCC® 6633 *No growth
* This organism is available as a Culti-Loop™

Pharma

Positive control:

Expected result

Aspergillus brasiliensis ATCC® 16404 *

growth

Negative control:

 

Uninoculated medium

No change

* This organism if available in a Quanti-Cult™ format

References
1. American Public Health Association (1992) Standard Methods for the Examination of Dairy Products. 16th Edn. APHA Inc. Washington DC.
2. American Public Health Association (1992) Compendium of Methods for the Microbiological Examination of Foods 3rd Edition. APHA Inc. Washington DC.
3. US Pharmacopoeia 30 (2008)
4. European Pharmacopoeia 6.1. 2.6.12 Microbiological Examination of Non-Sterile Products: Harmonised Method: Microbial Enumeration tests 2.6.13 Microbiological Examination of Non-Sterile Products: Test for Specified Micro-organisms. B. Harmonised Method. (2008).
5. Japanese Pharmacopoeia. 15 th Edition. (2006)

 
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