The most important factor affecting the quality and health of yeast in storage is the maintenance of adequate glycogen reserves. Glycogen is a carbohydrate similar to malt starch and serves as an internal energy reserve for yeast. The cells glycogen content indirectly affects the cells ability to uptake nutrients and perform a healthy fermentation. Glycogen is especially important during the initial phase of fermentation acting as an energy source for the synthesis of sterols.
Sterols are critical cell wall components required for cell permeability which enables the cell to uptake metabolites. Cells are depleted of sterols throughout fermentation, and must synthesize them at the beginning of subsequent fermentations.
Yeasts also rely on glycogen for an energy source to survive periods of starvation. Lacking a source of carbohydrates, yeast will utilize glycogen reserves to fuel metabolism. If metabolism is not slowed dramatically during storage, glycogen reserves will quickly become depleted rendering the culture unsuitable for re-pitching. Warm temperatures, exposure to oxygen, agitation, and trub all stimulate yeast metabolism and should be kept to a minimum.
Environmental stresses including alcohol stress, osmotic stress, and carbon dioxide toxicity can also affect the health of stored yeast and should be minimized. Yeast should only be stored 10 to 14 days under optimal conditions. The longer a culture is stored, the more glycogen reserves are used. As glycogen reserves fall, so does vitality and ultimately viability.
Keys to Successful Yeast Storage:
- Keep the yeast cold, 34°F (1°C).
- Store in sterile, vented stainless steel container.
- Store under a 3 to 4 inch layer of low alcohol, low hop beer.
- Store yeast from only lower alcohol beers.
- Minimize trub.
- Minimize exposure to oxygen.
- Store under a blanket of CO2 with minimal positive pressure.
- Use as soon as possible, preferably within 3 to 4 days.
- Test for viability, cell count, and contamination prior to pitching.