Also posted here
Ever wondered what would happen if you let a yeast culture on agar grow? The result is called giant yeast colonies which in the past have been used to distinguish yeast strain. Differences in metabolism, flocculation characteristics and genetic stability lead to differences in the appearance of the giant colonies.
So I wanted to give it a try myself. Brewing Techniques featured a nice article on that topic which pointed out that the growth medium needs to be much thicker than the thin agar medium that is commonly used for Petri dishes. Not having deep mycological Petri dishes I used 4 oz canning jars. The growth medium was regular strength brewing wort solidified with agar. The BT article suggests using gelatin but that didn’t work for me, likely because I forgot about the advice not to autoclave the gelatin.
The agar surface was inoculated with very small amount WLP 002 (English Ale) yeast and allowed to grow for a few weeks at about 15-20 C (60-68 F). The result is shown below.
Apparent are “growth rings” which are considered typical for highly flocculent yeast. Another interesting feature is the wedge shaped change in yeast appearance. This is likely caused by a mutation that happened to a cell at the tip of the wedge which caused it and the cells originating from it to grow differently than the other cells in the colony.
While growing giant yeast cells has little application in practical brewing it is one of those fun things that can be done with supplies that I have in the brewery anyway and I also plan to gow and document the giant colonies of other strains in my yeast collection.