Identification of wine yeasts by microscopy or even viable plating is considerably more difficult than identification of bacteria. This is because the colony and cell morphology of a wide range of yeasts that are found in wine can be very similar to one another. For example, a large number of yeasts isolated from wine have spherical to ovoid cell morphology, and similar-looking opaque dome-shaped colonies. The only way to obtain a definitive identification of a yeast is to test its genetic make-up, using molecular biological techniques. This is a specialist type of analysis, not possible for a wine laboratory.
Bearing this in mind, there are still some tests that can be carried out to gain some information about yeast isolated from wine, or also about yeast cultures made up in the winery.
- Differential plating – Techniques of using different media for plating out yeast identified from wine, can provide information about their possible identification
- Total and non-viable yeast count – This method will allow you to count either the total number of yeast cells per mL of a culture, or to determine the percentage of viable cells in a culture. A microscope with 400x magnification and a haemocytometer are required for this method.
However, microscopic examination does not usually give sufficient information to identify the microorganisms responsible for an instability.
For example, most yeast found in wine are ovoid to spherical in shape, so examination of them under the microscope does not enable species identification.