Dehydrated culture media
WL Differential Agar
Selective medium used in the control of industrial fermentation processes, especially in brewery.
- Cat. No.
- LC-1026
- Category
- Dehydrated culture media
- Format
- 500 g
- Storage
- 2 to 25 °C
- Minimum guaranteed shelf life
- 15 months
WL Differential Agar is a selective medium for the isolation and enumeration of microbial flora used together with WL Nutrient Agar (LC-1086) for the control of the manufacture of beer and other fermentation processes by yeasts. Both media are widely used in the industries of vinegar, bread yeasts, grape and wine-growing and distilled spirits. In the production of yeasts for the bakery and distillery industries, the pH of the media is adjusted to 6.5.
Key features
Applications
Selective enumeration and selective isolation of microorganisms of the brewing industry.
Industry
Alcoholic beverages.
Preparation
80 g in one liter of distilled water; boil for one minute, sterilize in autoclave at 121 °C for 15 minutes.
Principles and uses
The medium allows the selective multiplication of yeasts in fermentation liquids, which contain a microflora mix consisting of fungi and bacteria. When the number of yeasts present is relatively small, certain bacteria can also be detected.
Tryptone provides nitrogen, vitamins, minerals and amino acids essential for growth. Dextrose is the fermentable carbohydrate providing carbon and energy. Yeast extract is a source of vitamins, particularly of the B-group. Monopotassium phosphate is the buffer. Potassium, calcium and ferric chlorides all provide essential ions for the osmotic balance. Magnesium and manganese sulfates are sources of divalent cations. Bromocresol purple is the pH indicator. Bacteriological agar is the solidifying agent. The addition of 0.004 grams of cycloheximide converts the WL Nutrient formula into a differential medium, which inhibits the development of yeasts and molds while permitting the notable proliferation of the bacteria present in the fermentation liquids and subsequent identification and enumeration.
Technical specifications
| Solubility | Without residues |
|---|---|
| Appearance | Fine powder |
| Color of the dehydrated medium | Beige with blue tint |
| Color of the prepared medium | Blue |
| Final pH (25 °C) | 5.5 ± 0.2 |
| Amount per litre | 80 g |
| Storage | 2 to 25 °C |
Formula
| Ingredient | g/L |
|---|---|
| Dextrose | 50 |
| Bacteriological agar | 20 |
| Bromocresol green | 0.022 |
| Calcium chloride | 0.125 |
| Cycloheximide | 0.004 |
| Ferric chloride | 0.0025 |
| Magnesium sulfate | 0.125 |
| Manganese sulfate | 0.0025 |
| Monopotassium phosphate | 0.55 |
| Potassium chloride | 0.425 |
| Tryptone | 5 |
| Yeast extract | 4 |
Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.
Preparation and use
Preparation
Suspend 80 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 °C for 15 minutes.
Instructions for use
- Inoculate the initial suspension and/or diluted sample.
- Spread the inoculum with a sterile loop on the agar surface.
- Incubate the plates in an inverted position at a temperature of 35 ± 2 °C for 24-48 hours.
Time and temperature of incubation are important factors according to the type of yeast. In general, temperatures of 25 °C with the beer yeasts and 30 °C with the bread and other alcoholic fermentation yeasts are appropriate. The time of incubation varies from 2 to 7 days and up to 14 days, depending on the flora found.
Likewise, the atmosphere chosen for incubating the culture must be appropriate. The bread yeasts are incubated aerobically while the alcoholic fermentation yeasts are incubated anaerobically and in the presence of CO₂.
Quality control
Microbiological test
Incubation conditions: 30 °C / 24-48 h.
| Microorganism | Specification |
|---|---|
| Escherichia coliATCC 25922 | Good growth |
| Proteus mirabilisATCC 25933 | Good growth |
| Saccharomyces cerevisiaeATCC 9080 | Inhibited growth |
| Lactobacillus fermentumATCC 9338 | Good growth |
| Saccharomyces cerevisiaeATCC 9763 | Inhibited growth |
Bibliography
- Green and Grey. Wallerstein Lab. Comm. 13:357. 1950.
- Green and Grey. Wallerstein Lab. Comm. 14:169, 1951.
- Applicable to bacteriological investigation in brewing. Wallerstein Lab. Comm. 13:357.
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