Iodine is commonly used to test your starch conversion during the mash phase of brewing. But did you know that you can use it as a quick test to check the health of your stored yeast slurry before pithing?
So why would we?
During the deceleration and stationary phase of fermentation, nutrients start running out and yeast cells start anticipating for a starvation period. They do that by utilizing some wort sugars to create storage carbohydrates that will help them survive when food runs out.
Glycogen is a storage carbohydrate that will remain within the cell (up to 40% of the cell’s dry weight) and will be broken down to glucose to help your yeast survive while stored. It will also be critical during the lag phase of the next fermentation where cells use it for energy to get their membrane ready for more nutrient uptake and multiplication.
The glycogen uptake may be relatively slow but will be accelerated in the presence of oxygen.
Since Glycogen is a polymer of glucose, similar to starch’s amylopectin, it will react with iodine. If it turns black, it means that your yeast is still packed with its storage food and is theoretically still healthy and happy.
On the other hand, if your yeast is short on glycogen, it may mean that your yeast has been stored for too long, or in bad conditions, and you may be facing unhealthy yeast and a potentially high proportion of dead cells. This is obviously no good news for the upcoming fermentation.
An Iodine test is a practical and quick way of checking up on your yeast and has the benefit of being feasible during busy brew days. It can indicate that a methyl blue cell count is necessary if not reactant.