Microorganisms - Yeasts

Yeasts are fungi that usually do not build thread-like hyphea. However, some yeast can alternate between a yeast phase and a hyphal phase. They are called dimorphic. Yeasts are single cell units with a sexual (spore forming yeasts) and asexual propagation (budding and fission yeast). They are very common everywhere.


 

Yeasts have been used for over 4000 years in bakery and brewing. In most cases they are fungi that do not build thread-like hyphea. Some yeasts are able to build pseudohyphea or false hyphea. These are connections between budding yeasts. The yeast builds daughter cells (buddies). The daughter cell builds buddies, too, without losing the connection. Another possibility for reproduction is to build fissions (fission yeasts). Both, budding and fissions, are kinds of asexual reproduction. The sexual kind of propagation is building spores.

Figure 1: Schematic drawing of the building of a pseudohyphea [source: D. Graubaum, Berlin]

 

 

 

 

 

 

 

Yeasts are used in producing alcoholic and non-alcoholic beverage, in baking, in biotechnology (e. g. industrial ethanol production), for nutrition (nutritional supplements, probiotics, etc.) and in science (genetics and cell biology).

The best known yeast is the common bakery or brewing yeast, Saccharomyces cerevisiae, a budding and spore forming yeast. In brewing the yeast produces alcohol from grain and malt or other cereals. In baking the yeast produces gas in small bubbles, carbon dioxide, from sugar. This is the reason the dough rises to the double size. The baking yeast does not like high temperatures. During the baking process the heat kills the yeast and small air pockets remain in the dough. This gives the dough a spongy and fluffy texture.

 

Figure 2: Saccharomyces cerevisiae under a microscope (magnification: 400x); [Source: D. Graubaum, Berlin]

 

 

 

 

 

 

 

 

Yeast extracts are used as food additives. Most people know “marmite” or “vegemite”. For producing these additives, salt is given to a yeast suspension. This hypertonic solution breaks the cells down and the yeast begins to die by self-destruction. Another heating step completes the “marmite” production.

Yeasts are not welcome on every food as they often induce food spoilage. Some yeast species can grow under the presence of high sugar concentration, ethanol and low pH (< 5,0). They are often found on the surface of cheese, dairy product, juices and jam. The food gets an off flavour, a bitter taste or starts fermenting by producing gas or alcohol. An orange juice can turn into hard liquor. Nobody likes this for breakfast.

Some yeasts are even pathogenic. Candida albicans causes infections of the skin and of other parts of the body (urogenital tract and bloodstream).

 


This text was prepared by Prof. Diana Graubaum, Department of Food Microbiology, Beuth University of Applied Sciences Berlin, Berlin, Germany.

For further questions please refer to: graubaum@beuth-hochschule.de