Cryopreservation of Yeast Cultures

Abstract

Yeast cultures are held in long-term storage in the National Collection of Yeast Cultures (NCYC; Norwich, UK) by two methods: freeze-dried in glass ampules and under liquid nitrogen using glycerol as a cryoprotectant. Freeze-drying is a generally accepted method for yeast storage, having the advantages of conferring longevity and genetic stability, as well as being suitable for easy worldwide postal distribution of the cultures in glass ampules. However, preservation by freeze-drying tends to be much more labor intensive than storage in liquid nitrogen and requires a higher level of skill to produce an acceptable product. Strain viabilities are generally low, typically being between 1 and 30%, as compared to >30% for those of yeast preserved frozen in liquid nitrogen. There are also several yeast genera, including Lipomyces, Leucosporidium, and Rhodosporidium which have particularly low survival levels and frequently cannot be successfully freeze-dried by the standard method. However, some improvements have been made recently using trehalose as a protectant (1, 2). Techniques for freeze-drying yeasts can be found in Chapters 4.