Sucrose increases FG?

Discussion in 'Recipe Editor' started by Brewer #49964, Jun 21, 2016.

  1. Brewer #49964

    Brewer #49964 New Member

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    I'm making a recipe with some sugar.

    Before adding 1kg of sucrose, the recipe editor calculates OG=1.049 and FG=1.010.
    After adding the sucrose to the recipe, OG is is 1.067 and FG is 1.014.

    The FG seems correct, but I think sucrose should decrease FG, not increase it. Since sucrose is completely fermentable, it should all end up as alochol (and CO2), thus lowering the FG.

    Is this a bug or am I missing something?

    The recipe: http://www.brewersfriend.com/homebrew/r ... first-try-
     
  2. Ozarks Mountain Brew

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    it raises the original gravity by adding weight but the attenuation of the yeast is set to 76.5%, if the attenuation was 100% you would be correct but its not, the fg is based on the brewhouse % and also the yeast attenuation %
     
  3. Brewer #49964

    Brewer #49964 New Member

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    Interesting point, I didn't think of that.

    The manufacturers typical attenuation will be correct for a malt based wort, because it contains complex sugars which more or less unfermentable to different strains of yeast. Applying the attenuation % to sucrose doesn't seems like a realistic way to calculate FG, as any yeast should attenuate simple sugars like sucrose nearly 100%(?)
     
  4. jmcnamara

    jmcnamara Well-Known Member

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    Right, but you have to take the yeast's alcohol tolerance into account.

    The sucrose added 18 points to the OG, but only 4 to the FG.

    From what I understand, sucrose is easy for yeast to eat through, so they go through that first. So you're right that they'd be eating that 100%. But, some of the more complex sugars they might leave because the poor guys are full and just tuckered out
     
  5. jeffpn

    jeffpn Well-Known Member

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    I suppose an experiment is in order. Dump a bunch of table sugar into some water, check the OG, shooting for 1.06 or near. Pitch some yeast, and check the FG when it's done. I'd be surprised if it's 1.000.
     
  6. Brewer #49964

    Brewer #49964 New Member

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    That would be an interesting experiment, I might do that. It's not unusual for wine to end below 1.000.

    I don't think only sucrose and water will work well, because there won't be any nutrients for the yeast. I think a wort with half the fermentables from sucrose and the rest from DME would work. Maybe with an all DME wort as a control.
     
  7. jmcnamara

    jmcnamara Well-Known Member

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    im no scientist, but i'd definitely go with a control so you can compare things.

    and, to keep the variables to a minimum, i'd also add the nutrients to the all DME batch. that way the only difference (or at least the main, intentional difference) would be the sucrose
     
  8. Brewer #49964

    Brewer #49964 New Member

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    Removing the sugar and adding pilsner malt to get OG=1.067 also results in FG=1.014.
     
  9. jmcnamara

    jmcnamara Well-Known Member

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    whatever the ratio of simple / complex sugars in the wort, the yeast can only eat through x% of them before they stop.

    with both sets of OGs and FGs, you're looking at about 80% attenuation. change the grist however you want, but i still contend that this particular yeast will only get through about 80% of the sugars.

    to use a football metaphor, 8 yards is 80% of the way to a first down, no matter where you start on the field. but, the further you start from the end zone (the higher the OG), the further you will end up from the end zone (FG), assuming that 8 yard gain is a constant
     
  10. Brewer #49964

    Brewer #49964 New Member

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    Last night I prepared the samples for the experiment described below.

    Actual OG was 1.042 for Sample A and 1.072 for sample B. The yeast used was a pack of S-23 which expired in 2014 (it's been in my fridge since 2013, so it's probably ok). The ~7.5 grams not pitched to the samples was boiled with the wort to provide nutrients.

    Hypothesis
    Sucrose is nearly 100% fermentable by brewers yeast. The typical attenuation percentage of brewers yeast applies to a typical all-malt wort, but not to a wort with increased amounts of simple sugars. Increasing the original gravity of a wort by adding sucrose will therefore not increase the final gravity of the beer.

    Test methodology
    Two samples A and B, will be prepared.

    1 liter of wort will be prepared from water and dried malt extract. DME will be added to boiling water to reach a specific gravity of 1.040. Dry brewers yeast will be added to the boil to serve as yeast nutrient. After boiling for 5 minutes the wort will be cooled and aerated. An equal amount of this wort will be transferred to two fermenters, labeled A and B. Care will be taken to transfer an equal amount of trub to both fermenters.

    Sample A: No modifications.
    Sample B: A pasteurized solution of sucrose in water will be added to the sample to reach a specific gravity of 1.070.

    2g of dry yeast will be sprinkled on top of each sample.

    Sample A and B will be fermented at the same temperature until fermentation is complete. Fermentation will be considered complete when SG has remained unchanged in both samples for >=48 hours.
    The final gravity of the samples will then be measured using the same hydrometer.

    Evaluation of results
    If the hypothesis is correct the following observation is expected:
    The final gravity of Sample A will be higher than or equal to the final gravity of Sample B.

    Potential problems with the experiment:
    1. If the OG of the sample is too high, the alcohol tolerance of the yeast may influence, the result. I expect 1.070 be low enough that this will not influence the results.
    2. The yeast may become “lazy” feeding on simple sugars and not ferment the more complex sugars from the DME. Another experiment including a sample where the sucrose is added after the yeast has fermented the DME should be able to show if this is the case.
    3. If the FG of the two samples are similar, measurment errors may influence the result. With 30 points seperating the OG of the samples, I don't expect this to be a problem.
    4. If either batch gets infected with a wild yeast, bacteria or even a different strain of brewers yeast the results will be invalid. I therefore cleaned and sanetized as I would normally do for a starter.
    5. Underpitching may cause low attenuation and invalidate the results. Despite the age of the yeast, I don't think it will influence the result as I pitched a large quantity relative to the amount of wort and the yeast has been stored cold. I do not expect overpitching to influence the experiment.
     
  11. Brewer #49964

    Brewer #49964 New Member

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    An update on my experiment:
    The SG of sample A has been unchanged at 1.009 (4.8P) for 48 hours, so it has finished fermenting.
    The SG of sample B has droped from 1.011 (7.8P) to 1.010 (7.3P) over the last 48 hours, so it's still fermenting (slowly).

    (These SG measurments where made using a refractometer. I don't trust the Plato to SG conversion formula, but the refractometer is consistant, so it's a quick way to check if fermentation has finished.)

    When both samples have finished fermenting I will take hydrometer readings.
     
  12. Brewer #49964

    Brewer #49964 New Member

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    Experiment results:
    Today the SG of sample B remained the same as measured 48 hours ago, so I considerer the fermentation to be complete.

    I measured the temperature of the samples to be 22.5 deg C. I therefore calibrated the hydrometer in water at the same temperature. It read 0.998. A correction of +0.002 will therefore be made to the FG measurements.

    Measured FG (uncorrected):
    Sample A: 1.005, 1.004, 1.004
    Sample B: 1.000, 1.001, 1.000, 1.000

    Average FG, corrected for hydrometer error:
    Sample A: 1.006
    Sample B: 1.002

    Conclusion:
    As sample A finished 4 points higher than sample B, the hypothesis is confirmed, adding sucrose to worth will not increase FG.
     
  13. jmcnamara

    jmcnamara Well-Known Member

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    those are some very interesting results. the finishing gravities of both samples seem really low to me, compared to an actual beer. especially sample B

    that yeast also survived quite well in the fridge all that time. 85 and 97% attenuation is great for fresh yeast

    thanks for going to the trouble of doing this and posting it
     

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