Which Carbs Do Yeast Like Best?

All simple sugars have the same formula C6H12O6. Complex sugars and carbohydrates are chains of these sugars connected together (linearly, like beads on a string or branched, like a tree). The simplest of these is sucrose, or table sugar, which consists of two simple sugar units—one glucose and one fructose. Do the specific sugar structures matter? Does it matter whether individual sugars are bonded together or separate?

To understand how different sugar substrates are utilized by S. Cerevisiae (yeast), we can measure the amount of CO₂ produced. If you recall the stoichiometry for fermentation, for every mole of glucose, yeast cells will produce two moles of CO₂, which makes a quantification of sugar metabolism fairly straightforward. While scientists have invented a number of devices to quantifiably measure the rate CO₂ production resulting from fermentation in yeast, these devices are not practical for classroom settings. Here we will use a basic 15ml conical tube (such as Falcon or Corning brands) with gradation markings as a device to measure CO₂ production in response to a variety of carbon sources.

• Prepare sugar solutions at 40% w/v (40g per 100ml of H₂O)
• Heat water baths and/or hot plates to 40 °C
• Warm all solutions to be used
• Puncture 2-4 holes in caps of 15ml conical tubes using a 21g needle
  • As CO₂ is produced, it will displace the yeast-sugar solution via the holes punctured in the tube cap, and provide a volumetric measurement of gas produced over time.
• Immediately before experiment, prepare the 7% yeast solution in H₂O

Setup

Set up your tubes and water bath in such a way that the gas produced in each fermentation will rise to the tops of the tubes and volumes of gas can be marked with a permanent marker or waxed pencil.

In the example at right, plastic tubes were used with a few holes poked into each plastic cap. The tubes were filled with yeast solution and food source (e.g. glucose), covered, and inverted quickly into the water bath. The samples should be held inside the tubes and displaced into the water bath as gas is produced.

A similar setup can be used with test tubes and stoppers with holes or using a glass slide to invert each tube into the bath before sliding off the slide and allowing the fermentation gas produced to displace the yeast solution. You may also be able to use a typical gas collection apparatus from a chemistry laboratory at your school.

Finally, volumes of gas can be determined by emptying a marked tube and filling it with water exactly to the mark. Then that water can be poured into a graduated cylinder of appropriate size to determine the volume of gas produced.