Description
GRAY (1951) has given a detailed description of the use of WL Nutrient Agar and WL Differential Agar in the microbiologial quality control employed in the beer-brewing industry.
Mode of Action
WL Nutrient Agar has a pH of 5.5, which is optimal for the enumeration of brewers’ yeast. If bakers’ or distillers’ yeast is to be examined, the pH should be adjusted to 6.5 (better yields).
When cultivating microorganisms from an alcoholic mash, tomato juice should be added to the medium. WL differential agar contains cycloheximide to suppress yeasts and any moulds which may be present; this medium allows reliable counting of all bacteria which may be encountered in the tests performed in brewery laboratories.
Typical Composition (g/litre)
Yeast extract 4.0;
casein hydrolysate 5.0;
D(+)glucose 50.0;
potassium dihydrogen phosphate 0.55;
potassium chloride 0.425;
calcium chloride 0.125;
magnesium sulfate 0.125;
iron(III) chloride 0.0025;
manganese sulfate 0.0025;
bromocresol green 0.022;
agar-agar 17.0
Preparation
Suspend 77 g/litre, if required dissolve the medium in a mixture of 400 ml clarified tomato juice and 600 ml demineralized water, adjust the pH to 6.5 if necessary, autoclave (15 min at 121 °C), pour plates.
pH: 5.5 ± 0.2 at 25 °C.
The plates are clear and blue-green.
Experimental Procedure and Evaluation
Dilute the sample material, spread 0.1 ml on WL Nutrient Agar and, if necessary, on WL differential agar.
Incubation: up to 2 weeks at 25 ° and, if applicable, at 30 °C aerobically. WL Differential Agar should be incubated both aerobically and anaerobically.
Count the number of colonies per plate and calculate the microbial count. Acetic acid bacteria, flavobacteria, thermobacteria, Proteus bacteria and other species grow on WL Differential Agar under aerobic conditions whereas lactobacilli
and pediococci proliferate under anaerobic conditions
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