Monday, February 25, 2013
Side by Side Starters 3 of 4
Observations of the daily collected data.
When looking at the daily collected data, the first thing that really jumped out was the correlation between sugar consumed and cell count. Sugar is known to be a limiting factor in cell propagation, but seeing how well the two correlated was surprising. This opens up a new way to look at yeast growth. Trying to determine cell growth solely from the input conditions is not using all of the data available. If in addition the final sugar by weight is used a much better approximation of cell count can be achieved.
Both of the test vials followed very close to producing 12 billion cells for every gram of extract consumed. Another way to look at that number is relative to volume. For each degree Plato the cell density increases by 12 million per ml.
Comparing the two
Both the refrigerated cells and the new cells started consuming sugar almost immediately. Both had reduced the sugar in the wort by half in the first day.
The daily data shows that the refrigerated slurry out preformed the cells removed from a starter. This was a second big surprise. Common brewing knowledge would indicate that cells that have been in the refrigerator for a month will be starved, and will not preform well, however quite the contrary was the case here.
This unusual performance may be linked to glycogen reserves. At the start of fermentation yeast will build glycogen reserves. During the growth phase these are significantly depleted during cell division. At the end of fermentation the yeast will rebuild these reserves as they prepare for dormancy. (1) The cells taken from the refrigerator were allowed to ferment to completion, and even after a month in the refrigerator still had significant glycogen reserves to support cell division.
It seems that it is better to allow a starter to run to completion than to use the cells at high krausen.
(1) Fix, Principles of Brewing Science, p97 in the 2nd addition