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.
- See more at: http://www.saps.org.uk/saps-associates/browse-q-and-a/169-q-a-a-how-does-sugar-affect-yeast-growth#sthash.FiPSHrah.dpuf