PURE AND IMPURE FOODS. 



397 



of Agriculture, is this! The yeast, 

 which is really a mass of tiny plants, 

 has reproduced again and again, and in 

 this growth has fed on the sugar of the 

 liquid and given off alcohol and carbon 

 dioxid. This phenomenon is called alco- 

 holic fermentation, and is essentially the 

 same as that which "raises" a loaf of 

 bread. Such fermentation is by no means 

 the only kind which occurs in common 

 life. The souring of cider into vinegar, 

 for instance, is due to another kind. In 

 that case a variety of microscopic plant de- 

 velops in the cider, and in so doing pro- 

 duces acetic acid, which gives vinegar its 

 characteristic taste. This is called acetic 

 fermentation. Similarly, if another va- 

 riety of bacteria get a chance to develop 

 in sweet milk, they give rise to lactic 

 fermentation, during which is produced 

 the lactic acid which turns the milk 

 sour. Rancidity of butter is due to the so- 

 called butyric fermentation. Here the bac- 

 teria yield butyric acid, which gives the 

 butter its disagreeable taste and odor. 



These microscopic plants and many oth- 

 ers are widely distributed in the air, and 

 often find their way accidentally into dif- 

 ferent materials, where they grow and 

 multiply, causing fermentation; just as 

 thistle seeds, for instance, are blown about 

 in the air until they lodge in some favor- 

 able spot and grow. At other times spe- 

 cial forms of ferments in so-called "pure 

 cultures" are purposely added to some ma- 

 terial, just as seeds of larger plants are 

 purposely sown in the garden. Thus pure 

 cultures of certain microscopic organisms 

 are added to cream to improve the flavor 

 of butter and make it uniform in quality. 

 This insures a special fermentation in- 

 stead of the accidental fermentation which 

 would otherwise occur. 



It is a peculiar feature of fermentation 

 that the microscopic plants which cause it 

 affect a much larger quantity of the ma- 

 terial on which they feed than goes to their 

 own development. Thus yeast converts 

 much more sugar into alcohol and car- 

 bon dioxid than it consumes in its own 

 growth and reproduction. When the fer- 

 mentation ceases, the yeast plant remains ; 

 in other words, the fermentation has been 

 produced without changing the nature of 

 the agent producing it. In the same way 

 enzyms (chemical bodies which also cause 

 fermentation) bring about the change 

 without being themselves changed. Though 

 so much has been learned in recent years 

 concerning fermentation, there still remain 

 many things to be explained. We know 

 what changes take place and under what 

 conditions, but just why they take place 

 is not clear. It is a remarkable fact con- 

 cerning ferments that the substances they 

 produce, in time, put a stop to their activ- 



ity. Thus the alcohol produced by the 

 yeast, in time, is sufficient to hinder the 

 growth of the yeast plant and ultimately 

 to kill it. If the products of this activity 

 are removed, the ferments resume work. 



Keeping the above facts in mind, it is 

 easy to understand the leavening effect of 

 yeast in dough. The yeast, "working" in 

 the warm water and flour, feeds on sugar 

 originally present or else produced from 

 the starch by diastase, grows and spreads 

 throughout the dough, at the same time 

 giving off carbon dioxid gas, which forces 

 its way between the tenacious particles of 

 gluten and lightens the dough. 



Scientifically speaking, yeast is a minute 

 fungus of the genus saccha romyces. A 

 single plant is a round or oval one-celled, 

 microscopic body which reproduces in 2 

 ways : either by sending out buds which 

 break off as new plants, or by forming 

 spores which will grow into new plants. 

 It grows only in the presence of moisture, 

 heat, and nutritive material. 



Yeast is literally as old as the hills. 

 Like mold spores and other micro-organ- 

 isms, it must be present in the atmosphere, 

 for if a dish of malt extract, originally 

 free from yeast, be exposed to the air, al- 

 coholic fermentation, such as could be pro- 

 duced only by yeast, will soon set in. 

 Such yeast is known as "wild yeast," and 

 all our yeasts have been cultivated from it. 



A piece of dough containing yeast is 

 called "leaven." Raising bread with leaven 

 is still carried on in some regions of Eu- 

 rope. Wet, or potato, yeast, so common in 

 this country before the days of patent 

 yeast, was made by cultivating wild yeast 

 or yeast obtained from a baker or else- 

 where in a decoction of hops or potato and 

 water. Some of the material thus obtained 

 was mixed with the dough. The "barms" 

 so much used in Scotland are made by 

 letting yeast grow in malt extract and 

 flour. Brewers' and distillers' yeasts are 

 taken from the vats in which malt extract 

 has been fermenting. Compressed yeasts 

 are made by growing yeast plants in some 

 sweet liquid, then drying the material to 

 check their growth, and pressing it. Some- 

 times a little starch is added to make the 

 little cakes keep their shape. The strength 

 of any yeast depends on the care with 

 which it is made and preserved. Ordinary 

 brewers' yeasts are likely to be full of the 

 bacteria which set up lactic or other fer- 

 mentations in the bread and give it a disa- 

 greeable odor and taste. They are very 

 susceptible to changes in the weather, and 

 can not be always relied on. Compressed 

 yeasts, if carefully made, are more uniform 

 in strength and composition. Usually a 

 few bacteria other than yeast are allowed 

 to remain, as the acid taste they give the 

 bread is considered an advantage. 



