Yeast Slurry pH as an indicator of yeast Autolysis
By Moritz Kallmeyer
Chief Brewer Drayman’s Craftbrewery, Silverton Pretoria, January 2004
At the end of fermentation, beer pH is usually between 4.1 and 4.40. An increase in stored yeast slurry pH above this end of ferment beer pH is a good indicator of yeast autolysis. Protease’s and other low molecular weight compounds of an acidic nature are also released which causes an initial sharp drop in pH during the first 24 hours of storage at 4°C. However, when a slurry is monitored right from the end of harvest through to re-pitching, there is a gradual increase in pH – especially from day three onwards.
Other compounds in the stored slurry that impacts negatively on subsequent fermentations are:
- An increase in FAN (free amino nitrogen) especially from day three onwards;
- A decrease in the stored glycogen *content (time and temperature dependant);
- An increase in the trehalose *content which is indicative of a response to increasing stress.
The authors concluded that the measurement of slurry pH is a simple, quick and inexpensive method to determine the quality of yeast slurry. The use of slurries showing high pH values impacts negatively on beer foam and beer flavour of the beer produced.
- Trehalose (a non-reducing disaccharide / carbohydrate) is primarily a stress protectant at the cell membrane for organisms like yeast because it pocesses the ability to act like water (particularly during dehydration) as a water replacement.
- Yeast will accumulate trehalose earlier during its growth when the major sugar in the wort is maltose
- It appears to play an energy reserve roll (as a storage carbohydrate) in cell sporulation.
- In brewing the stress of high gravity, osmotic stress, high temperatures, low levels of water activity and ethanol toxicity all induce synthesis of trehalose.
- Threhalose is normally accumulated later in the growth of yeast than glycogen, at the decelerating phase.
- Normal percentages in a cell is 1-2% of the dry weight, 5-10% is a little uncomfortable but higher than 10% is unacceptable except in dry yeast as this is essential in allowing the dehydration to occur successfully.
- Some reports say that a high trehalose concentration in yeast at pitching improves viability, enhance the carbohydrate utilization rate and increase the production of higher alcohols such as isoamyl and isobutanol.
- Glycogen is a storage carbohydrate essential in the lag phase for sterol synthesis, which is essential for good brewing yeast and thus a healthy fermentation.
- Glycogen is accumulated in yeast early, while the glucose / sugar content is still high – it levels off as the yeast enters the stationary phase.
- Glycogen degradation is accelerated when yeast is exposed to air during yeast handling or disturbed excessively between brews – with resulting poor yeast performance.