Introduction
It has been shown (1,2) that the survival levels of yeasts stored in liquid nitrogen at -196C are high and superior to those obtained with other preservation methods. Good strain stability following cryopreservation is also recorded.
Storage at temperatures -130 C below which molecular activity does not occur, can be achieved by storage in the liquid phase of nitrogen or in mechanical refrigerators operating at these temperatures (3) and are to be preferred to storage at high temperatures. The method described here is for storage in liquid nitrogen. Cultures may be stored in polypropylene cyotubes, glass vials or polypropylene straws. This method uses straws and has the following advantages:
-Economies of space (5-6 straws can be stored in each cryotube).
-Prevention of seepage of liquid nitrogen into the samples; liquid nitrogen may penetrate the washers of polypropylene cryotubes, but cannot penetrate the sealed straws.
-Polypropylene drinking straws are cheap and readily obtainable throughout the world.
-Polypropylene is very much safer to use than glass vials.
-The straws are available in different colours, allowing colour-coding of samples.
-The culture can be recovered for use by removal of a single straw, the rest of the samples remaining frozen.