WORT AGAR

Code: CM0247

A medium for the cultivation and enumeration of yeasts.

* Adjusted as required to meet performance standards

Directions
Suspend 50g in 1 litre of distilled water and bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes.
PROLONGED OR EXCESSIVE HEATING WILL DIMINISH THE GEL STRENGTH OF THE AGAR.

Description
Wort Agar is a general purpose mycological medium, equivalent to the medium described by Parfitt¹ and especially suitable for the cultivation and enumeration of yeasts. The medium which duplicates the composition of natural wort, is of an acidity which is optimal for many yeasts but inhibitory to most bacteria. Parfitt investigated the relative merits of wort agar and other media for the count of yeasts and moulds in butter, and recommended the use of dehydrated whey, malt or wort agar for the purpose. Scarr² employed a modified wort agar (’osmophilic agar’) for the examination of sugar products for osmophilic yeasts. Scarr’s technique is also used for the determination of osmophilic yeasts occurring in materials used in the manufacture of soft drinks.

Technique
For the microbiological examination of butter, make suitable dilutions in quarter-strength Ringer solution (prepared with Ringer Solution Tablets BR0052). Transfer 1ml of each dilution to a separate Petri dish; add 15ml of melted Wort Agar, cooled to 45-48°C; mix by rotary movements in a horizontal plane; allow the poured-plates to set (protected from the light) at room temperature for 30-50 minutes. Incubate in an inverted position, e.g. for 5 days at 25°C, and count the number of yeasts and mould colonies which develop.

For the examination of sugar products for osmophilic yeasts, dissolve dehydrated Wort Agar in a syrup containing 35 parts w/w of sucrose and 10 parts w/w of glucose. A suggested method is as follows:-
1. Weigh 350g of sucrose and 100g of glucose into a measuring container and add enough water to make up to 1 litre.
2. Warm to dissolve.
3. Add 50g of Wort Agar to syrup ensuring that the powder dissolves.
4. Sterilise by autoclaving at 110°C for 20 minutes.

Inoculate and mix as above. Scarr² recommends incubation at 27°C for 3-4 days for Zygosaccharomyces species and for 5 days for less common osmophilic yeasts. Colonies on the medium are well defined and normally opaque.

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

Quality control

This organism is available as a Culti-Loop®

Precautions
Do not remelt the solid agar, it will destroy the gel. The surface of the agar is soft but suitable for poured inocula.

References
1. Parfitt E. H. (1933) J. Dairy Sci. 16. 141-147.
2. Scarr M. Pamela (1959) J. Sci. Food Agric. 10(12). 678-681.