Questions about identifying yeast, by their morphology as seen through a microscope, come up frequently in brewing forums. I also receive frequent emails from brewers hoping I can help them identify the yeasts in pictures from their brewery microscope.
Unfortunately, you cannot identify a yeasts species (or even genus) purely from an image. But, an image can be an important first step in identifying your yeast. Where brewers often run into trouble is that there is a lack of visual guides to help them learn the language of yeast morphology. This post is intended as an simple introduction to the basics of yeast morphology, and the language used to describe it.
The term “morphology” may be a new one for some brewers. The term “morphology” means the form and structure of organisms and their specific structural features. For yeast, this generally refers to four key characteristics that we can see through a microscope:
- Cell shape – the overall shape of a single cell.
- Cell division pattern – the pattern of the positioning of where yeast cells bud, and the shape of the buds themselves.
- Cell clustering patterns – the patterns formed when multiple yeast cells stick together.
- Vacuolation – the presence, number and size of visible vacuoles in a yeast cell.
I’ll cover each of these below.
1. Cell Shape
Yeast cells come in a range of shapes: circular/ovoid, apiculate and elongated.
2. Cell Division Pattern
With only one exception (Schizosaccharomyces), yeast divide by budding. What this means is that a new yeast cell (called the daughter cell) will be formed as a miniature yeast cell extending off of the mother yeast cell. While all yeast bud, they show two predominant budding patterns: end-budding (technically called polarized budding) and budding (or unpolarized budding).
3. Cell Clustering Patterns
In liquids (e.g. beer) most yeasts grow as single cells, with daughter cells breaking free of their mother as soon as budding is complete. However, some yeasts will undergo incomplete cell division, resulting in the formation of pseudohyphae (chains) of yeast cells.
The final cellular feature we can (sometimes) see are yeast vacuoles – specialized organelles (technically lysosomes) which are acid-filled compartments inside of the yeast cell used to breakdown food and cell wastes. Yeast can have a one or more vacuoles, and they can vary greatly in size. Some yeast’s vacuoles are too small to be seen under a microscope, while others are so large that they occupy most of the interior space of the yeast. Identifying vacuoles can help identify a yeast – but they can also be misleading. The size and number of vacuoles can change in some species of yeast, often in response to the availability of food.