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146 Enumeration of Yeasts and Moulds (Spatula Method)

Methods Type: Generic Methods
Key data
Number: 146
Analyte: Yeasts and Moulds (Spatula Method)
Matrix: Cereals, Cereal Products, Bread and Baked Goods, Pasta
Year of Approval: 1992
Scope: Cereals, Cereal Products, Bread and Baked Goods, Pasta
Principle: 
  • Since direct counting of the yeasts and moulds contained in or on the product under investigation is impossible, an indirect method has to be used.
  • The product under investigation is first mixed with a sterile physiological solution (6.1) in order to separate and suspend the microorganisms (8.1), where necessary, after grinding the product by means of blending equipment according to 5.2.1, 5.2.2 or 5.2.3.
  • A tenfold dilution series is made from this initial suspension (8.2).
  • Aliquots of the dilution stages are transferred onto the surface of a culture substrate that has previously been poured into sterile Petri dishes and has solidified (6.2.2).
  • The transferred suspension containing the yeasts and moulds is uniformly spread over the surface of the culture substrate.
  • In the course of incubation the yeasts and moulds can multiply and form colonies on the surface of the culture substrate.
  • After the end of incubation the "number of yeasts and moulds per gram of sample" is calculated from the number of colonies present on the surface of selected plates of the culture substrate (9.1).

Definition

The yeast and mould count is the number of those colonies, that become visible (9.1) after transfer of an aliquot of a suspension of the product under investigation to the surface of a suitable agar medium (6.2), spreading and subsequent incubation at 25 °C for 5-7 days.

NOTE: The distinction between yeast and mould colonies is made by macroscopic examination. Under the conditions described in this International Standard moulds usually develop flat or fluffy spreading colonies often with coloured fruiting or sporing structures. Yeasts develop matt or shiny round colonies usually having a regular outline and a more or less convex surface.

Very small colonies (after 5 to 7 days) can be due to bacteria. Check by microscopic examination.

 

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