Yeasts

180. - Fermentation of molasses. Into a common test tube or any glass vial, place a solution made by mixing one spoonful of molasses with ten spoonfuls of water. Rub up a little compressed yeast in water and put a few drops into the tube of molasses water. Set aside in a warm place and let it stand for about 24 hours. At the end of this time a vigorous fermentation will be seen. The liquid will have become somewhat cloudy, numerous bubbles can be seen rising through it, a froth forms on top, and a mass of sediment soon collects at the bottom. The bubbles are the carbon dioxide which is escaping into the air; the sediment at the bottom is the growing mass of yeast, and the alcohol, which looks like water, is dissolved in the liquid and is, of course, invisible.

181. - Proof of the nature of the gas. Prepare two tubes. In tube A place molasses and water inoculated with several drops of yeast, as in the last experiment. Put the cork in place and insert the other end of the tube into a second tube underneath the surface of some clear limewater. Set aside in a warm place until vigorous fermentation occurs. Note the bubbles of gas that arise from the fermenting tube and bubble up through the limewater. The limewater soon becomes turbid, showing that the gas contains carbon dioxide.

182. - Rub a bit of yeast cake in a little water so as to make a slightly cloudy solution. Place a drop of the solution upon a microscope slide, cover with a cover glass, and examine first with a 2/3- inch objective. Note that the water seems to be filled with very minute dots. Study with a higher power (1/6 in. objective). Examine the yeast cells, noting the shape and comparative size.

183. - Growing yeast. With a glass rod, remove a drop of the sediment from growing yeast prepared as in Expt. 180. Place the drop on a slide, cover with a cover glass, and study as in the previous case. Remove some of the yeast found floating on the surface and study in the same way. Note that the yeast cells are in groups. Make a sketch of several groups, showing buds of various sizes. Note any other differences you can see between this growing yeast and the compressed yeast cake.

184. - Effect of boiling. Prepare two test tubes of molasses and water and inoculate each with a drop of yeast. Plug with cotton. Place one test tube in water and boil for ten minutes, and then leave both test tubes side by side in a warm place for two days, and determine whether the boiling has been sufficient to kill the yeast.

185. - Wild yeast. Prepare several test tubes of molasses and water as described and without plugging with cotton leave exposed in various places for two or three days. Determine by the appearance of bubbles whether fermentation occurs. If any change takes place in the liquids, examine with a microscope to determine whether yeasts have found entrance from the air or whether some other microorganisms are growing in the solutions. Commonly, bacteria will be found more abundant than yeasts.

186. - Effect of age on yeast. Obtain an old sample of dried yeast cake. Prepare two tubes of molasses and water and inoculate one with a small quantity of the old yeast cake and two others with a similar quantity of a fresh cake. Set aside in a warm place and determine in which fermentation starts sooner, and in which it is the more vigorous. Examine with a microscope after fermentation begins, to see if either contains other organisms besides yeast.

187. - Action of yeast on bread. Mix up a little flour and water to about the consistency of dough for bread making and divide into three lots, A, B, and C. Into A and B place a little compressed yeast. A and B are then to be placed in a warm place for five or six hours, while C, without the yeast, is to be baked at once. After A and B have risen under the influence of the yeast, bake B at once in the oven, while A is to be thoroughly kneaded and then baked. Compare the results of A, B, and C, noticing the difference in the textures of the bread.

188. - Over raising. Mix another lot of dough with yeast in the same way and allow it to rise in a warm place for twelve hours or more. Test with litmus paper to see if it is acid. Bake and taste to see if it has become sour.

189. - Repeat Expt. 187, comparing cold milk, scalded milk, and water as the liquid to be used in the dough. Note which dough sours first.

190. - Bread raised by wild yeast. Put a small amount of salt in a little milk and then allow it to stand in a warm place until a froth appears. Mix it with flour to make a dough and set aside to rise. Does the dough rise as rapidly and as satisfactorily as when yeast is used? Does the baked dough have the same taste?

191. - Kumiss. Into a quart of milk, put two tbsp. of granulated sugar and add about 1/16 of a compressed yeast cake. Put in a warm place and leave for 24 hours. Cool and taste. This will be kumiss or fermented milk. Is it sour?

192. - Prepare equal weights of the following sugars: a. sucrose b. maltose c. lactose d. glucose e. levulose

Dissolve each in small equal quantities of distilled water. (These solutions are for the next experiment.) Note any changes in appearance which may occur, and the rapidity with which each sugar dissolves. Using a small measured fraction of each solution can you detect any difference in sweetness?

193. - a. Prepare small, equal weights of compressed yeast cake and mix one portion with each sugar solution. Turn the mixture into fermentation tubes and allow them to stand in a uniformly warm place.

b. Note in which tube fermentation begins and the rapidity with which it proceeds.

c. Compare the volume of gas collected during a unit length of time.

d. What do you conclude as to the relative fermentative qualities of the sugars used?

e. Of what significance is this?