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.

Setup

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

Results

The following chart shows the yeast growth as billion new cells per initial extract (malt extract and added glucose)

Yeast growth over malt content in starter wort

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: CH_1.61O_0.56N_0.16 (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:

Starter Apparent Attenuation over Malt Content

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.

Conclusion

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.

This experiment accompanies Yeast Growth in Hopped Wort and it examines the effect that roasted malts have on yeast growth. I did this experiment about a month ago but haven’t found time to write about it until now. The result was that roasted malts do have an impact on yeast growth but it is unclear if that’s due to less fermentable sugars or the inhibitory effects of the melanoidens.

Setup

Just like the hopped wort experiment I prepared 500 ml purely DME based wort and 500 ml DME+carafa wort. For the latter 40 g DME and 40 g Carafa II special (ground to a powder) were boiled with water. The resulting wort was filtered and adjusted to 10 Plato extract content.  An extract potenial of 75 % was assumed for the carafa. That means that the resulting wort is equivalent to wort from a grist with ~43% carafa II and ~57 % pale malt.

To create worts with increasing amounts of Carafa both worts were mixes as follows:

• 100 DME wort: no carafa
• 67% DME wort + 33% Carafa+DME wort: equivalent to about 15% Carafa in the grist
• 33% DME wort + 67% Carafa+DME wort: equivalent to about 28% Carafa in the grist
• 100 Carafa+DME wort: equivalent to about 43% Carafa in the grist

The mixed worts were inoculated with about 2 ml WY2042 yeast slurry. The counted cell density was 0.15 B/g extract. All starters were 250 ml wort in 500 ml flasks and left open while on the stir plate.

For this experiment the resulting yeast was not filtered and dried.

Results

The results are shown below.

Yeast growth over roasted malt content

There is a clear relationship between roasted malt content and resulting yeast growth: the more roasted malt the less yeast was grown per extract.

Conclusion

At roast malt levels found in most dark beers (10 % or less) the impact on yeast growth is expected to be minimal and such worts should be suitable for yeast propagation. Extremely high levels of roasted malts should be avoided as the impact on yeast growth can be significant.

The experiment did not asses whether the reduced yeast growth was due to reduced fermentability (roasted grains provide less fermentable extract)  or the inhibitory effect that melanoidens are known to have on yeast metabolism.

In this experiment I looked at the influence that iso-alpha acids have on yeast growth. This was motivated by curiosity and the fact that there are a number of brewers, like me, who are using leftover wort from brewday for future starters.

Setup

For this experiment I prepared 500 ml of unhopped wort and 500 ml of heavily hopped wort. Both worts had an initial extract content of about 11 Plato. The hopped wort was boiled for 30 min with the addition of 4 g of German Magnum (13% alpha acid) pellets. The unhopped wort was also boiled for the same amount of time.

According to the Tinseth IBU model the hopped wort should have 290 IBU, but it is likely that the actual IBU number is less due  iso-alpha acid solubility and or isomerization limitations.

By mixing these two worts starters with increasing bitterness were created ranging from

• 100% unhopped wort
• 67% unhopped and 33% hopped wort
• 33% unhopped and 67% hopped wort
• 100% hopped wort

All starters were inoculated with 4 ml of yeast slurry from the same yeast culture. The initial cell density was estimated as 0.05 B/g, which is fairly low. All starters were left open and stirred at 20 C.

Yeast dry weight was determined by filtering a known amount of well suspended yeast in starter beer through 1 micron nominal filter paper. The filtered beer was clear which means that all yeast was held back by the paper. The paper and yeast was then dried using a microwave until all water was removed from the yeast. The paper and dried yeast was the weighed using a balance scale with a resolution of 0.01 g.

Results

The chart below shows the growth as B/g for the 4 different starters

Yeast growth over starter IBU levels

Because the yeast in the control flocculated and made counting difficult its yeast count was estimated using the dry weight of a filtered amount of suspended starter assuming 20 Billion per gram.  I’m fairly confident that this is correct since the the yeast from the 33% and 65% hopped wort starter both had a dried yeast cell density of ~ 20 B/g which also matches previous dried yeast measurements for WY 2042.

Yeast from the 100% hopped wort starter showed with 14 B/g slightly less cells per dried weight but the overall yield in yeast biomass was still less than the yield in less hoppy starters.

Biomass Yield Over IBU

When looking at the biomass yield there is a constant decline as the IBU level of the starter wort is increased.

Conclusion

While hopped wort has a negative effect on yeast growth it is only significant at very high hopping levels. Thus the use of Pale Ale, Pilsner and even most IPA worts for brewing yeast propagation should not pose any problems. Very highly hopped worts may show a noticeable decline in yeast growth and are thus less well suited for yeast propagation.