Yeast is a fungus and needs a supply of energy for its living and growth. Sugar supplies this energy. (Your body also gets much of its energy from sugar and other carbohydrates.)
Yeast can use oxygen to release the energy from sugar (like you can) in the process called "respiration". So, the more sugar there is, the more active the yeast will be and the faster its growth (up to a certain point - even yeast cannot grow in very strong sugar - such as honey).
However, if oxygen is short (like in the middle of a ball of dough), then yeast can still release energy from sugar, but in these conditions, its byproducts are alcohol and carbon dioxide. It is this carbon dioxide gas which makes the bubbles in dough (and therefore in bread), causing the dough to rise.
Alcohol is a poison (for yeast as well as for people) and so the yeast is not able to grow when the alcohol content gets too high. This is why wine is never more than about 12% alcohol.
WHY does an excess of sugar inhibit the yeast?
My guess would be that the osmotic concentration of the sugar gets so great that the yeast is unable to get enough water for growth.
As fresh yeast is more than 90% water, the single substance most needed for growth is water. As osmotic concentration increases, the water potential of the sugar solution gets more and more negative until it reaches a point where is lower than the water potential of the yeast cell contents and water tends to move OUT of the cell rather than IN. I do not know whether yeast cells are able to take up water actively, by expenditure of metabolic energy to pump the water against the water potential gradient.
I imagine that up to a certain concentration, the limiting factor is the amount of sugar available for respiration and synthesis of cell materials with the yeast able to take in more water than needed for growth. As the concentration of the sugar increases, although respiration and synthesis can take place faster, the uptake of water gets slower and slower until we reach a point where the rate of uptake of water becomes the limiting factor.
Which sugar is best for yeast growth?
"I tested four sugars (fructose, glucose, sucrose, and lactose). I concluded that sucrose made the yeast cells have the most foam. My question is why? I am especially curious about why glucose didn't make the yeast have the most foaming."
I wonder what concentration of sugar you used in each case? Was each sugar solution made up to the concentration eg the same molarity?
Basically, each sugar needs to be converted to glucose to enable it to feed into respiration and it is this process which produces the gas which causes the foaming.
Yeast is able to synthesise a range of enzymes to do this:-
Sucrose is a disaccharide:- GLUCOSE-FRUCTOSE = SUCROSE
Sucrase will split sucrose.
Isomerase will convert Fructose to Glucose.
Thus, 0.1M sucrose will yield 0.2M glucose (when ALL is converted to glucose).
Lactose is a disaccharide:- GLUCOSE-GALACTOSE = LACTOSE
Lactase will split lactose and Transacetylase will convert Galactose to Glucose.
However, I believe yeast does not have the gene for lactase and this is why the lactose sugar remains intact in 'Milk stout'.
So, I predict that lactose was bottom of your list, with the least foaming.
If a sugar is too concentrated, it will slow down the reaction. (This is why honey does not normally ferment.) So, you should be careful to only use dilute solutions in your experiment.
So, I suspect sucrose came out best in your test because it yielded twice as much glucose as the "same concentration" of glucose.
John Hewitson and Charles Hill