Disclaimer: Yeast hydration instructions can vary among suppliers. In the first instance, carefully follow the manufacturer’s instructions detailed on individual yeast packets. The guidelines given below represent a simplified generic yeast hydration procedure, which will optimise the hydration and fermentation capability of most Saccharomyces cerevisiae cultures. This generic procedure might not give optimal results with non-Saccharomyces cerevisiae or other non-Saccharomyces cultures. Furthermore, proprietary yeast reactivation preparations can improve yeast performance when inoculating suboptimal (highly clarified/high °Brix/°Baume) juices/musts. Consult your yeast supplier/manufacturer for specific information.
Equipment and materials required
- Mineral water/rain water/clean tap water (free of inhibitory substances such as chlorine). Chlorine can be removed by sparging/boiling
- Yeast (500 g; newly opened sachet or stored according to manufacturer’s recommendations)
- Diluted preservative-free (SO2) grape juice (pasteurised/sterilised)
- Proprietary nutrients (inactive yeast) for difficult musts
- Carboy or other vessel with large surface area, for hydration
As per the manufacturer’s recommendations. The yeast manufacturer has determined the optimal procedure for rehydrating and reactivating a particular type of dried yeast.
Generally, an inoculum of 0.25 g yeast/L (25 g/hL) juice to achieve a minimum 5×106 viable cells/mL. This is based on the ADWY containing 2×1010 cells/g. High solids musts are typically inoculated with 0.20 g yeast/kg (approx. 20 g/hL) due to the smaller liquid volume of musts.
For highly clarified juices, high °Brix/°Baume juices/musts or musts with inhibitory residual SO2, (>10-15 mg/L free SO2 / >30-50 total SO2) a higher inoculum may be used at up to twice the normal rate. Oxidative techniques, including addition of hydrogen peroxide, can be used to lower excessive residual SO2 (see Removal from and addition of sulfur dioxide to must, juice and wine and How to determine a hydrogen peroxide addition calculator)
Step 1. Rehydration
- Pre-heat water to 38-40°C; confirm temperature by measurement.
- Add water to a container such as a carboy or a vessel with a large surface area.
- Rehydrate ADWY by sprinkling it slowly and evenly over the surface of 5-10 times its weight in water (e.g. for 500 g sachet of yeast by suspending in 2.5 to 5 L).
- Avoid formation of yeast clumps. Clumping produces non-wetted/non-rehydrated yeast, and hence, inactive yeast. Gentle stirring (do not use a powered mechanical device, which can injure the cells) can help disperse some strains that show hydrophobic properties.
- Leave to stand for 10-15 minutes.
Step 2. Amelioration step
- Mix the partially settled rehydrated yeast by stirring.
- The yeast culture should preferably be slowly cooled to within 5-10°C of the juice/must to be inoculated; this amelioration process seems to be most important for difficult to ferment juices/musts.
- Cooling can be achieved by sequentially adding appropriate volumes of clean water or juice (with low residual SO2) from the tank to be inoculated over a period of 10-20 minutes. Steps of 5-10°C are usual.
- When water alone is used, the yeast should be inoculated immediately once the cooling steps have been completed to avoid inactivation of the cells. If this is not possible, add an equal volume of juice to the yeast culture and inoculate within several hours and before the sugars become depleted. Considerable foaming can, however, result.
Step 3. Inoculation
- The juice/must should equal or exceed 15°C when inoculated to advantage the yeast culture over indigenous strains.
- Active fermentation typically initiates within 24 hours. If not, check the proportion of budding and viable yeast by microscopic examination of a juice/must sample (Total and non-viable counts of yeast cells in a culture). If these indices are low, reinoculation might be required with a newly prepared yeast culture.
- Once active fermentation begins, the fermentation temperature can be controlled within a small range. Temperature changes exceeding 3-5°C per day have been reported to partially inactivate budding yeast.
Yeast should be stored strictly according to manufacturer’s recommendations, generally refrigerated; non-optimal storage conditions will result in loss of activity, which can lead to (unexpected) fermentation problems. Do not keep open sachets of yeast between vintages. For difficult to ferment juices/musts it is preferable to use new batches of yeast prepared with proprietary reactivation (inactivated yeast) preparations.
Suggested sites for further information
- Henschke, P.A. Preparing a yeast starter culture: fresh or dried yeast? Allen, M. (eds). Advances in juice clarification and yeast inoculation: proceedings of a seminar; 15 August 1996; Melbourne, Vic. Australian Society of Viticulture and Oenology: Adelaide, S.A : 17-21 ; 1997.
- Monk, P.R. Rehydration and propagation of active dry wine yeast. Aust. Wine Ind. J. 1: 35; 1986.