Microorganisms and performance criteria for testing selective media 'DG18' for Enumeration of Yeasts and moulds used in Food and animal feeding stuffs microbiology according to ISO 11133.
ISO 21527-2 (2008) medium: DG18
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Quantitative control of Productivity
- Use a quantitative reference material or test suspension with a known number of Saccharomyces cerevisiae and Wallemia sebi.
- Dilute this material so 50 to 120 cfu is inoculated on the DG18 by spreading for example 0.1 mL.
- This can be a multi or single strain material.
- Do this also for the reference media (SDA).
- Incubate at (25 ± 1) °C for 5 days.
- Count the colonies on the medium under test (DG18) and the reference media (SDA).
- Calculate the productivity ratio PR. The criteria is: PR ≥ 0,5.
- If the PR exceeds 1,4 identify the reason.
- For this the total count of colonies obtained on the culture medium under test (DG18) is divided by the total count of colonies obtained on the defined reference medium (SDA).
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Qualitative control of Selectivity
- Use a reference material or test suspension with a known number of Escherichia coli and Bacillus subtilis subsp. spizizenii.
- Streak each test strain (Escherichia coli and Bacillus subtilis subsp. spizizenii) as a single line on the media under test (DG18).
- Multiple strains can be streaked on the plate as long no crossing occurs.
- Use a 1 µl loop in case of a concentration of at least 10000 cfu or a 10 µl loop in case of a concentration between 1000 and 10000 cfu.
- Incubate at (25 ± 1) °C for 5 days.
- Asses the amount of growth as, 0 for no growth, 1 for weak growth or 2 for good growth.
- For this medium (DG18) and standard (ISO 21527-2) combination the criteria is: "No Growth.
Best Practice DG18
Quantitative control of Productivity solid medium
Calculate the productivity ratio PR as follows: | PR | NS |
N0 | ||
Ns = total count test medium | ||
N0 = total count reference medium |
Control strain | WDCM number | Ref. media | Incubation | Criteria | Characteristic reaction |
---|---|---|---|---|---|
Saccharomyces cerevisiae | SDA | (25 ± 1) °C for 5 days | PR ≥ 0,5 | Characteristic colony/propagules according to each species | |
+ Wallemia sebi | |||||
Aspergillus restrictus | |||||
Eurotium rubrum | |||||
The best practice for Quantitative control of Productivity for media control of 'DG18' is based on the data displayed in this table according ISO 11133. |
Best Practice DG18
Qualitative control of Selectivity solid medium
Control strain | WDCM number | Ref. media | Incubation | Criteria | Characteristic reaction |
---|---|---|---|---|---|
Escherichia coli | (25 ± 1) °C for 5 days | No Growth | No Growth | ||
+ Bacillus subtilis subsp. spizizenii | |||||
The best practice for Qualitative control of Selectivity for media control of 'DG18' is based on the data displayed in this table according ISO 11133. |