RESULTS of the Yeast Fermentation Lab

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RESULTS of Yeast Fermentation Lab

As obtained from two Bio 101 lab sections at Thomas Nelson Community College
Hampton, Virginia USA
(Spring 2010)

Variable [Sucrose]

Conditions: 23°c, 3% dry yeast in tap water, plus variable amounts of sucrose.

Results: the Pasteur Effect, which was initially thought to be substrate inhibition, but later determined to be ATP-feedback inhibition of glucokinase, which is the first step in glycolysis. You see, when yeast have a lot of sugar to "eat," excessive ATP is produced, and feeds back to slow down glucokinase.

Variable Temperature

Conditions: 3% dry yeast in tap water, plus 5% sucrose.

Results: the ecological optimum temperature for bakers yeast is in the mid-30° range. It would be interesting to see if brewer's yeast would do better at a lower temperature since most breweries operate at 15-20°.

Variable Substrate

Conditions: 3% dry yeast in tap water, plus 5% of other sugars, or 5% with any of Sweet'n'Low, Splenda, or aspartame.

Results: Looking first at the three sugars, we see a pattern - that the more the sugar must be enzymatically prepared to enter glycolysis the poorer the substrate it is.

Initially it is surprising that the artificial sweeteners are so good at feeding fermentation until one reads the ingredients on the "zero-calorie" packets. The primary ingredient is glucose (aka dextrose)!

Toxin Effects

Conditions: 3% dry yeast in tap water, plus 5% sucrose. Bleach was 2.5 mL/100 mL culture; lead nitrate was made to 0.001M.

Results: these were the most counter-intuitive in that students would think these would stop everything. In fact, they cause controls to uncouple in the cells, which then run wild and soon die. (Poisoned people usually become feverish.)

pH Effects

Conditions: 3% dry yeast in tap water, plus 5% sucrose. The culture was normally pH 5, and was thus adjusted higher or lower by two pH units.

Results: while rising above the initial pH was detrimental to yeast's ability to ferment, going more acidic gave uncertain results as one class found it detrimental, while the other saw little adverse effect. Perhaps their adjusting buffers were different.

Effect of [Product]

Conditions: 3% dry yeast and 5% sucrose in tap water. To this was added various amounts of USP ethanol.

Results show that up to about 5% ethanol, the yeast suffer no product inhibition, but thereafter it becomes very pronounced - for any of a number of reasons (dehydration, membrane damage, etc.).