Re: Why do yeast produce differing amounts of CO2 with different sugars

Hi Judy -

There are a few possibilities for the result you saw. I'll give some explanation as to how you may arrive at an answer.

Each of the foodstuffs you mention contains different carbohydrates. When yeasts metabolize carbohydrates they release CO₂ gas, which you then collect in the balloon.

Some possibilities to consider in your system:

• Yeasts metabolize different carbohydrates with differing efficiencies.
• Yeasts may not have the machinery needed to metabolize some carbohydrates.
• Something in each of the foodstuffs could be toxic to the yeasts so they don't metabolize the sugars as well.

To find content of specific sugars, try querying the Nutrient Lab Database at the USDA to detemrine grams of each kind of sugar in the foodstuffs. Select the option for "100 grams" when you enter queries of (1) honey, (2) maple syrup or (3) granulated sugar (same as table sugar). For the granulated sugar, you can also determine the main carbohydrate by looking on the label of most packaged sugars.

Carbohydrates have different formulae - the chemical formula for glucose is C₆H₁₂O₆; that for maltose is C₁₂H₂₂O₁₁. Which do you think weighs more, a molecule of glucose or a molecule of maltose? Expand this to the number of grams of each carbohydrate you have in your foodstuffs - it would be nice to know how many "molecules" of glucose, fructose, maltose or other sugars are present. To do this, you'll need to convert from the gram amounts to moles, a mole being 6.022 X 10²³ molecules of a substance. So...

1. Look up the chemical formula for each sugar listed in your Nutrient Lab results.
2. The USDA database will tell you the number of grams in 100grams of your foodstuff. Knowing this information, with the molecular formula, you should be able to convert this amount to moles of each carbohydrate.
3. Determine the volume occupied by 100 grams of each foodstuff. You can make this determination in the following way
   1. Place a clean graduated cylinder on a metric scale & "tare" or re-zero the scale. If you can't re-zero the scale, note the weight of the graduated cylinder.
   2. Now add 100 grams of honey, maple syrup or sugar to the cylinder, and stop when the scale reads 100 grams, or the original weight + 100 grams.
   3. Note the volume on your graduated cylinder.
4. With the moles of each sugar, and volume occupied by 100 grams, you can calculate a molarity for each sugar.

You can now assess the production of gas relative to the molarity of sugars in your solutions. If you vary these concentrations, what impact does it have on the gas produced? A simple way to do approach this question is to dilute each of your starting foodstuffs and add the same amount of the diluted material that you did for the undiluted material. What happens if you dilute your honey or maple syrup 5-fold and 10-fold (i.e., 1.0 milliliters (mL) of honey + 4mL of warm water, and 1mL of honey + 9 mL of warm water).

Some further questions you might investigate:

• What is the difference between a monosaccharide and a disaccharide (try looking up these words in an introductory biology or chemistry book).
• Which of the sugars you found in your nutrition search are monosaccharides or disaccharides?
• What is known about how well bakers yeast (species Saccharomyces cerevisciae) can use monosaccharides and disaccharides?

Lynn Bry, Moderator MadSci Network