Wort Agar, Granulated

Principle And Interpretation

Wort Agar is used for the cultivation, isolation and enumeration of yeast and moulds. According to Rapp (1), addition of certain dyes to Wort Agar allows differentiation between yeast and bacterial colonies. It is particularly well adapted for counting osmophillic yeast in butter, sugar and syrups, in lemonades and more generally in sweet or soft drinks. Wort Agar is a medium equivalent to the medium described by Parfitt (2) and equally suitable for the cultivation and enumeration of yeasts. 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 for the purpose. Scarr (3) employed a modified Wort Agar (Osmophilic Agar) for the examination of sugar products for presence of osmophilic yeasts. For more selective utilization, it is possible to adjust the pH to 4.5 or 3.5 by adding 10 ml/l of a 10% solution of lactic acid or tartaric acid before sterilization.

Yeasts grow well in culture media containing dextrose or maltose in an acidic environment. In this medium, peptic digest of animal tissue and malt extract provide nitrogenous and other nutrients for the growth of yeasts. Dextrin and maltose are the fermentable carbohydrates. The agar medium should not be re-liquified as it causes alteration with hydrolysis of agar at low pH and results in failure of agar to gel when cooled (4).

For the microbiological examination of butter, make suitable dilutions in quarter strength Ringer solution. Transfer 1 ml of each dilution to a separate Petri dish; add 15 ml of melted Wort Agar, cooled to 45-48°C, mix by rotary movements in a horizontal plane. Incubate the plates and subsequently count the colonies.