Dry brewing yeast typically contains less than 7% water. Stringent quality standards are applied during manufacturing to avoid microbial contamination. The remarkable stability of dry yeast allows for significant QC testing in order to ensure less than 1 bacterium or wild yeast cell detected per million cells of brewing yeast. For most strains, 1g of dry yeast contains a minimum of 5 billion viable cells, but the number will vary slightly from batch to batch.
Dry yeast is packaged under vacuum in 500g packs or 11g sachets and must be stored dry, below 4˚C (39˚F). Exposure to humidity and oxygen will affect the viability and vitality of the yeast. Do not use soft packs or sachets that appear to have lost their vacuum. Once a pack or sachet is open, use immediately for best results. If kept sealed (or re-sealed) under vacuum and stored under appropriate conditions, dry yeast can be used until the indicated expiration date, which is typically 2-3 years after manufacture.