Pitching rate is adjusted according to wort gravity and the viability and composition of the yeast. It is important that the yeast is evenly distributed in the wort.
The method of pitching depends on:
- Yeast storage and handling procedures (e.g. slurry or pressed yeast).
- Excise regulations and indulgences
- The time taken to fill each fermenter.
- The number of brew lengths required to fill a fermenter.
- Good distribution of the yeast in the wort ensures effective use of dissolved oxygen.
Range of Values
- 1·7 – 3 3 gm wet weight yeast (pressed or centrifuged) is pitched per litre (equivalent to 0·6 – 1·2 lb wet weight per barrel) for worts of initial gravity 1030° to 1060°.
- Pitching rate can be increased (up to a maximum of 10 – 25% over norm) to compensate for variations in yeast viability and composition. This should only be used as an emergency measure.
- Temperature of pitching should be 15·5 – 17°C (60° – 63° F) (see Topic 18. Fig. 2)
The volume/weight of slurry or pressed yeast added to the fermenter is adjusted according to the number of cells per ml of slurry or the yeast dry weight per gram of pressed yeast.
Some methods of pitching are:
- Yeast slurry is injected into the wort main to the fermentation vessel (using single or repeated ‘doses’) There are various strategies according to the number of brew lengths per fermentation vessel
- Pressed yeast is slurried with warm wort, ie. approximately 21° C (70° F). and the slurry allowed to rise in the skip prior to pitching. The yeast is then added to the fermentation vessel prior to filling or during early stages of filling. If worts are blended to final gravity in the fermenter, the yeast is pitched into the strongest wort
- Yeast can be pitched straight after collection (particularly in the case of direct vessel to vessel transfer with cylindroconical vessels).
- The practice of alternate cycling of yeast between low gravity and higher gravity fermentations is sometimes favoured.
- The number of cells per ml of yeast slurry is counted by microscopic examination of a dilute suspension (EBC Analytica Microbiologica method 220.127.116.11 – see Appendix 1)
- The wet weight of yeast per ml of slurry is determined by centrifuging the slurry and weighing the pellet obtained (EBC Analylica Microbiologica method 2.2 1. see Appendix 1).
- The weight of yeast per gm of pressed yeast or slurry is measured by drying and weighing (EBC Analytica Microbiologica method 2.2.1, see Appendix 1)