I had my students conduct a fermentation experiment manipulating sugar amounts (number of sugar packets in solution) and measuring CO₂ production via capture in a balloon and measuring circumference (very common HS lab experiment). I also had them conduct a trial with two packets of Equal sweetener. To everyone's surprise two packets of the "zero calorie" sweetener produced as much CO₂ as four packets of sugar. Can anyone shed light on why this would have occurred? Equal contains maltodextrin and dextrose in addition to the aspartame. I'm assuming that yeast are able to metabolize one or more of these substances, but I haven't been able to find a more detailed explanation.
Your sugar substrate was sucrose. Yeast cells metabolise this by secreting an enzyme, invertase, which splits the disaccharide into glucose and fructose both of which can be fermented by yeast to produce CO2.
According to this site
Equal Original (blue packaging) is a zero calorie sweetener that contains aspartame and acesulfame potassium as its sweetening ingredients along with a bulking agent.
Equal Original PACKETS: dextrose with maltodextrin, aspartame, acesulfame potassium
This agrees with what you say - that the sweetener that you used contained dextrose - and leaves me very confused. Dextrose is another name for glucose so how can this contain zero calories?
The answer lies in the legal definition of zero calories = less than 5 calories per serving (presumably per package in this case.) So let's assume that by adding two packs you were adding 8 calories equivalent of glucose.
Calorie content of carbohydrate = 4 calories per gram
Two packs could contain 8 calories = 2 g glucose.
The molecular weight of glucose = 180
therefore 2 g = 2/180 moles
if completely fermented to CO2 each mole -> 6 moles CO2 12/180 moles CO2 = 0.75 litres of CO2.
I conclude that there is enough glucose in two packets of Equal to generate a reasonable amount of CO2.
Yeast cannot ferment maltodextrins. It is possible that aspartame (aspartyl-phenylalanine-methyl ester) could be metabolised, but I think the dextrose/glucose solution is the best explanation.
EDIT: I meant to say, just to illustrate that this is a significant amount of glucose, standard rich yeast medium is 2 g glucose /100 ml (2 % w/v).
EDIT #2 My back-of-an-envelope calculation has a fundamental error in it, but luckily I am the first to spot it. If glucose is being used fermentatively, then only one third of the carbon will end up in CO2, with the remainder going to ethanol. This means 0.25 litres instead of 0.75 litres. Yeast is so oriented towards fermentation that even in the presence of oxygen the fermentative pathway will dominate.
Equal is marketed as a "zero calorie" sweetener, with respect to human digestion. The sweetening agents are aspartame (Asp-Phe; a dipeptide) and acesulfame K. The maltodextrin and dextrose are probably bulking agents to give the product a free-flowing, poweder consistence.
The "fermentables" in question are:
- Dextrose. This is another name of glucose, and is the most easily metabolized sugar in the Equal packet.
- Maltodextrin is an oligomer of D-glucose linked by $\alpha$(1,4) linkages. If this were the disaccharide of glucose-glucose, or maltose, then yeast could readily ferment this. However, yeast cannot hydrolyze maltodextrin and therefore cannot metabolize it.
- Aspartame is a dipeptide which can easily be fermented.
Therefore, yeast will happily metabolize dextrose and aspartame, but not the other two agents.
I don't have a definitive answer to this, but a little over a decade ago I was in an undergraduate lab that had a similar thing happen - a small amount of metabolism of a "control" group of bacteria fed artificial, sugar/calorie free sweeteners instead of sugar.
Our running theories at the time were:
- Contamination. Always a problem in laboratory experiments, and especially undergraduate lab experiments.
- Trace contaminants. "Zero calorie" isn't actually 0, it's just small, and hardy microorganisms may be able to do just fine with small amounts of nutrients. If there's another factor limiting their growth, that may result in the identical CO₂ results.
- Some intermediate compound or the like that is useful for metabolism. As you said, possibly the ability to use maltodextrin or dextrose.
I can confirm the results of this experiment. My classes routinely show that yeast ferment 20% aspartame more efficiently than 20% glucose. I came here to find answers, but none so far are satisfactory. Even if aspartame is a 'mixture' of dipeptides and maltodextrin, a 20% solution of aspartame should contain much less glucose than a 20% solution of glucose.
The only thing I can think of is that at the concentrations we are using (20% glucose + 50mg/mL yeast) fermentation is actually inhibited. The small amount of glucose contaminants (?) in aspartame support more efficient fermentation.