Dehydrated culture media
WL Nutrient Agar
For the determination of microbial flora in beer fermentation processes and manufacturing.
- Cat. No.
- LC-1086
- Category
- Dehydrated culture media
- Format
- 500 g
- Storage
- 2 to 25 °C
- Minimum guaranteed shelf life
- 15 months
WL Nutrient Agar, based on the Green and Gray formulation, is recommended for the control of industrial fermentations, particularly the manufacturing of beer. With a pH of 5.5, true counts of beer yeasts can be made. With a pH of 6.5, the medium is ideal for bakery and distilled spirit yeasts.
Key features
Applications
Selective enumeration of microorganisms of the brewing industry.
Industry
Alcoholic beverages, fermentation, food.
Preparation
75 g in one liter of distilled water; boil for one minute, sterilize in autoclave at 121 °C for 15 minutes.
Principles and uses
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 green is the pH indicator. Bacteriological agar is the solidifying agent.
Technical specifications
| Solubility | Without residues |
|---|---|
| Appearance | Fine powder |
| Color of the dehydrated medium | Clear toasted with blue tint |
| Color of the prepared medium | Grayish-blue |
| Final pH (25 °C) | 5.5 ± 0.2 |
| Amount per litre | 75 g |
| Storage | 2 to 25 °C |
Formula
| Ingredient | g/L |
|---|---|
| Bacteriological agar | 15 |
| Bromocresol green | 0.022 |
| Calcium chloride | 0.125 |
| Dextrose | 50 |
| 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 |
Preparation and use
Preparation
Suspend 75 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. Cool to 45-50 °C, mix well and dispense into plates.
Instructions for use
Inoculate and incubate at a temperature of 30 °C and observe after 24-48 hours.
Both WL Nutrient (WLN) and WL Differential (WLD) formulae are used in conjunction, as with WLN Agar the bacteria may not be detected unless the number of yeast cells is very small. 1 plate of WLN and 2 plates of WLD must be used.
- The WLN Agar plate is incubated aerobically for the total plate count of yeasts.
- One of the WLD Agar plates is incubated aerobically for acetic acid bacteria: Flavobacterium, Proteus, thermophilic bacteria and others.
- The second WLD plate is incubated anaerobically for the investigation of lactic acid bacteria and species of Pediococcus.
All plates are incubated, in general, at 25 °C in the case of beer, and at 30 °C for bakery and malt alcoholic yeasts. Plates are incubated for 2-10 days and up to 2 weeks, according to the flora present. Counts are made at regular intervals during this period.
Quality control
Microbiological test
Incubation conditions: 30 °C / 24-48 h.
| Microorganism | Specification |
|---|---|
| Escherichia coliATCC 25922 | Moderate growth |
| Proteus mirabilisATCC 25933 | Moderate growth |
| Saccharomyces cerevisiaeATCC 9080 | Good growth |
| Lactobacillus fermentumATCC 9338 | Moderate growth |
| Saccharomyces cerevisiaeATCC 9763 | Good growth |
Bibliography
- Green, S.R. and P.P. Gray 1950. Paper read at American Society of Brewing Chemists Meeting. Wallerstein Lab. Commun. 12:43.
- Green, S.R. and P.P. Gray 1950. A differential procedure applicable to bacteriological investigation in brewing. Wallerstein Lab. Commun. 13:357.
- MacFaddin J.D. 1985. Media for isolation-cultivation-identification-maintenance of medical bacteria, vol. 1. p. 854-856. Williams & Wilkins, Baltimore, MD.
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The product
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