Yet another experiment I did a while back. I wanted to see how yeast growth is affected by the ratio of malt extract and corn sugar. The motivation was not to find a way to save on the amount of malt extract needed for starters but to get an idea of how much yeast growth depends on the nutrients that are in wort in addition to the wort sugars.
Wort and a glucose (corn sugar) solution both with with an extract content of 10.6 Plato were prepared. Both solutions were mixed to yield growth media containing 100%, 75%, 50% and 0% malt extract. They were inoculated with WY2042 at a rate of about 0.3 B/g (billion cells per gram of extract or sugar).
This experiment was repeated with the addition of 3% DAP (diammonium phosphate as percent of extract weight) as an additional nitrogen source.
All starters were done under these conditions:
- ambient temperature: ~20 C
- volume: ~250 ml in a 500 ml flask
- stirred and covered with foil
The following chart shows the yeast growth as billion new cells per initial extract (malt extract and added glucose)
With and without the added nitrogen there is a steady increase in yeast growth as the percentage of malt in the stater wort is increased. Pure sugar solution, even with the added nitrogen, did not yield any significant growth which is likely due to other nutrients (vitamins, trace elements) that are found in wort. The addition of nitrogen boosted the yeast growth, however, yeast growth was still limited by the malt content and an increase of the latter also increased yeast growth in the presence of additional nitrogen.
The added nitrogen (3% of extract+sugar weight) was about the amount needed to grow 1.5 Billion new cells per gram of extract/sugar if these assumptions are made:
- yeast biomass formula: CH1.61O0.56N0.16 (source)
- dry cell density: 20 B/g
- DAP molar weight: 132 g/mol
Another interesting observation was the attenuation levels that each of the no DAP starters achieved:
The low attenuation of the all sugar starter is likely due to the virtual absence of growth in that starter. That resulted in a small yeast population which was not able to consume much of the sugar it had available. These starters were fermented for about 2 days. But even the 50% and 75% malt starters had lower attenuation than the 100% malt stater. That’s surprising since the apparent attenuation limit of the 50% malt starter is about 100% and the yeast population should have been large enough to completely consume all available sugars.
All malt wort provides vital nutrients for yeast growth. When these nutrients are diluted with sugar overall yeast growth suffers. The addition of additional nitrogen in the form of DAP (diammonium phosphate) can compensate for the reduced level of nutrients. But even with plenty of added DAP the best yeast growth was seen in all malt worts.