Fig. 107.
Fermentation tube
with culture of yeast.
Fig. 108.
Fermentation tube
filled with CO₂ from
action of yeast in a
sugar solution.
235. Respiration a breaking-down process.—We have seen that in respiration the plant absorbs oxygen and gives off carbon dioxide. We should endeavor to note some of the effects of respiration on the plant. Let us take, say, two dozen dry peas, weigh them, soak for 12-24 hours in water, and, in the folds of a cloth kept moist by covering with wet paper or sphagnum, germinate them. When well germinated and before the green color appears dry well in the sun, or with artificial heat, being careful not to burn or scorch them. The aim should be to get them about as dry as the seeds were before germination. Now weigh. The germinated seeds weigh less than the dry peas. There has then been a loss of plant substance during respiration.
Fig. 108a.
Yeast. Saccharomyces ceriviseæ. a, small colony; b, single cell budding; c, single cell forming an ascus with four spores; d, spores free from the ascus. (After Rees.)
236. Fermentation of yeast.—Take two fermentation tubes. Fill the closed tubular parts of each with a weak solution of grape sugar, or with potato decoction, leaving the open bulb nearly empty. Into the liquid of one of the tubes place a piece of compressed yeast as large as a pea. If the tubes are kept in a warm place for 24 hours bubbles of gas may be noticed rising in the one in which the yeast was placed, while in the second tube no such bubbles appear, especially if the filled tubes are first sterilized. The tubes may be kept until the first is entirely filled with the gas. Now dissolve in the liquid a small piece of caustic potash. Soon the gas will begin to be absorbed, and the liquid will rise until it again fills the tube. The gas was carbon dioxide, which was chiefly produced during the anaerobic respiration of the rapidly growing yeast cells. In bread making this gas is produced in considerable quantities, and rising through the dough fills it with numerous cavities containing gas, so that the bread “rises.” When it is baked the heat causes the gas in the cavities to expand greatly. This causes the bread to “rise” more, and baked in this condition it is “light.” There are two special processes accompanying the fermentation by yeast: 1st, the evolution of carbon dioxide as shown above; and, 2d, the formation of alcohol. The best illustration of this second process is the brewing of beer, where a form of the same organism which is employed in “bread rising” is used to “brew beer.”
[237. The yeast plant.]—Before the caustic potash is placed in the tube some of the fermented liquid should be taken for study of the yeast plant, unless separate cultures are made for this purpose. Place a drop of the fermented liquid on a glass slip, place on this a cover glass, and examine with the microscope. Note the minute oval cells with granular protoplasm. These are the yeast plant. Note in some a small “bud” at one side of the end. These buds increase in size and separate from the parent plant. The yeast plant is one-celled, and multiplies by “budding” or “sprouting.” It is a fungus, and some species of yeast like the present one do not form any mycelium. Under certain conditions, which are not very favorable for growth (example, when the yeast is grown in a weak nutrient substance on a thin layer of a plaster Paris slab), several spores are formed in many of the yeast cells. After a period of rest these spores will sprout and produce the yeast plant again. Because of this peculiar spore formation some place the yeast among the sac fungi. ([See classification of the fungi].)
238. Organized ferments and unorganized ferments.—An organism like the yeast plant which produces a fermentation of a liquid with evolution of gas and alcohol is sometimes called a ferment, or ferment organism, or an organized ferment. On the other hand the diastatic ferments or enzymes like diastase, taka diastase, animal diastase (ptyalin in the saliva), cytase, etc., are unorganized ferments. In the case of these it is better to say enzyme and leave the word ferment for the ferment organisms.