THE STORY OF THE YEAST PLANT. 

 Professor F. C. Harrison. 



We have all heard of yeast, but perhaps, not very many know that 

 yeast is a plant — a very dififerent plant, however, from what we usually 

 see. It has no stems, no leaves, and no roots ; it is not even g-reen; 

 it is so small that a single plant cannot be seen by the naked e3'e. In 

 order to see it, we must use a powerful magnifying- instrument, called a 

 microscope. If we examined a yeast plant by means of a microscope, we 

 should see that full grown plants were round, oval, or egg-shaped, and 

 so small that 5,000 of them placed end to end would be about an inch long. 

 Figures 31 and 32 will give some idea of the shape of this plant, and con- 

 vey a hint as to its size, as the illustrations are photographs of yeast 

 plants magnified 1000 times. 



Most of us eat bread every day ; but only few of us stop to think that 

 we are indebted to the yeast plant in a large measure for the flavour 

 and digestibility of the "Staff of Life. ' The baker kneads, or mixes, his 

 flour, water, and yeast and then leaves it in a warm place, which favours 

 the growth of the yeast. In a very short time, the yeast begins to grow 

 by feeding upon the sugar m the flour, and in so doing changes the sugar 

 into alcohol and a gas, commonly called carbonic acid gas, which is 

 familiar to us all in ginger ale and other aerated drinks. 



The gas formed from the decomposition of the sugar by the yeast 

 plant in the dough, is unable to get out, owing to the sticky nature of the 

 kneaded flour It is held in small bubbles, the form of which can be seen 

 on looking at a \ iece of bread, the small holes being the spaces which are 

 made by the gas bubbles in the dough. The heat in the oven acting 

 upon these bubbles causes them to expand, or grow large, and thus pushes 

 the particles of flour apart, so that the loaf when baked is much larger 

 than the piece of dough before baking. 



The alcohol, a liquid formed, as stated above, by the yeast plant 

 acting upon the sugar in the flour, may be smelt, if an opening is made in 

 the dough when it has risen ; but most of this substance is evaporated, 

 or driven away, by the heat in bakmg, and only a very little of it is re- 

 tained in the bread. 



Thus we see that by the action of the yeast the particles of flour are 

 divided and subdivided, giving a large surface for the digestive fluids to 

 act upon when the bread is eaten ; and for this reason, bread is more 

 digestible than cakes made with baking powder or sour milk and soda. 



The use of yeast for making bread is very old. We know that the 

 Jews were acquainted with the use of " leaven," or yeast ; for we read 

 that Lot " did make them a feast and did bake unleavened bread." 



And the use of yeast for making wine is even more ancient ; for we 

 learn that Noah, the second father of mankind, planted a vineyard and* 

 made wine. 



The Chinese also knew of the use of yeast for bread and wine 

 making; for about the vear 2000 B.C.Ching Noung, a Chinese philosopher, 



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