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200. Causes of Fermentation. — Complex organic compounds are often broken up into simpler compounds by the agency of growing plants. This process is called fermentation, and the organisms which effect it are called ferments. Different ferments give rise to different products, as alcohol, vinegar, etc. Most of these ferments are plants, but one at least is an animal; and this, strange to say, cannot live in contact with free oxygen, but flourishes in an atmosphere of hydrogen. In order that the ferment should grow, it must be supplied with proper food, especially with ammoniacal salts and alkaline phosphates. These are contained in the albuminous matter generally present in the liquid about to be fermented. In order that the fermentation should go on well, the temperature should be from 70° to 100°; at much higher, as at much lower temperatures, the vitality of the ferment is destroyed.
In many cases, spontaneous fermentation sets in without the apparent addition of any ferment: thus wine, beer, milk, etc., when allowed simply to stand exposed to the air, become sour or otherwise decompose. These changes are, however, not effected without the presence of vegetable or animal life, and are true fermentations. The sporules, or seeds of these living bodies, are always floating in the air, and on dropping into the liquid begin to propagate themselves, and in the act of growing evolve the products of the fermentation. If the liquid
be left only in contact with air which has been passed through a red-hot platinum tube, so that the living sporules are destroyed; or if the air be simply filtered by passing through cotton-wool, so that the sporules are prevented from coming into the liquid, it is found that the liquid may be preserved for any length of time without undergoing the slightest change.
The principal forms of fermentation are
(1) The alcoholic fermentation, producing chiefly alcohol and carbonic acid.
(2) The acetous fermentation, producing acetic acid.
201. Alcoholic Fermentation. Sugar, if dissolved in presence of yeast (which is a plant and a ferment), undergoes fermentation, evolving mainly alcohol and carbonic acid :
CH2012=4C,H,0 (alcohol) + 4CO2. About 6 per cent of the sugar undergoes a different change, part being used as nourishment for the yeast, and part forming glycerine and other compounds. The alcoholic fermentation is best effected at a temperature between 77o and 86o.
When the cereal grains are used for making alcohol, the starch of the grain is first converted into sugar. This change may be brought about by the action of sulphuric acid, as already explained (161); but, practically, it is usually effected by means of diastase.
202. The Formation of Diastase in Growing Plants, - We have learned that the seed contains a supply of starch which is the food of the embryo plant until it is able to derive its sustenance from the earth and the air. We have learned, too, that this starch must be converted into sugar, in order that it may be dissolved in the sap of the plant and carried where it is needed for the purposes of growth. Let us now see how the starch is changed to
sugar. The seed contains more or less of the nitrogenized compound, gluten (142). This, under the influence of heat and moisture, begins to putrefy, and a portion of it is converted into the ferment diastase. This has so powerful an action upon starch, that one part of it, at a temperature of 150°, is sufficient to convert 2000 parts of starch into dextrine and then into sugar.
We can now understand how a seed, like the acorn (Figure 31), germinates. If we cut the acorn open, we see the radicle (at the right in the figure), and the two thick cotyledons, or seed-leaves, containing the starch and gluten. In the moist warm soil the acorn absorbs a little water, and the gluten is thus changed into diastase, which converts some of the starch into sugar. The radicle absorbs
and uses it to make woody fibre.
It is thus enabled to thrust out a little root and begin to draw food from the earth, as well as from the starch in the seed. It next sends out the plumule, a little stem with its first leaves unfolding to the air and light. It
has now the means of deriving its nour
ishment both from the earth and the air, and is no longer dependent upon the seed, which soon withers and perishes. Both root and stem are now rapidly developed (145–154), and thus at length the acorn becomes the mighty oak.
203. Brewing. — The brewer avails himself of this natural change in the germinating seed, and calls into action on a larger scale the chemical influence of diastase. The grain commonly used for the purpose is barley. This is first moistened in heaps, and spread out in a dark room to heat and sprout. When the gluten has begun to be transformed into diastase and the starch into sugar, so that the germ is about to burst from the envelop of the seed, the growth is arrested by heating the grain, and thus killing the embryo plant.
The malt, as the barley is now called, is next bruised, and soaked in warm water in the mash-tub. The water dissolves first the sugar which has already been formed in the seed, and then the diastase. The latter acts upon the rest of the starch of the seed, and of any raw grain which may be added to the malt, converting it into sugar, so that little, except the husk of the grain, is left undissolved; and the liquor, or wort, has a decidedly sweet taste.
The wort is now heated to boiling, to stop the action of the diastase, and to coagulate the albumen which has been dissolved out of the grain. At the same time, hops are added to the wort; and these, besides giving a bitter aroma to the liquid, help to clarify it. The boiled liquor is then filtered, and drawn off into shallow vessels, where it is cooled down to about 60°. Yeast is now added, and the mixture is allowed to ferment for six or eight days.
This fermentation is never allowed to continue until all the sugar is converted into alcohol. From one-half to three-fourths of it is decomposed; and the rest is left to give a sweet taste to the beer, and also to keep it from souring in the cask.
The liquor is next put into casks, where it goes through a second and much slower fermentation, which is essential to its preservation. When this fermentation is com
pleted, the cask must be closed tight, to exclude the air, the oxygen
of which would cause acetous fermentation. During this second fermentation, the carbonic acid is generated, to which the liquor owes its sparkling effervescent character.
204. The Distillation of Spirits. — When fermented liquors are boiled, the alcohol they contain rises in the form of vapor, mingled with steam. If the boiling be performed in a close vessel, and the vapors be led by a pipe into a cold receiver, they condense into a liquid, containing a large percentage of alcohol. This process is called distillation, and the apparatus for carrying it on is called a still.
If the alcoholic liquor thus obtained be re-distilled, or rectified, a spirit may be obtained, containing 90 per cent of alcohol and 10 of water. This is the strongest commercial alcohol. If this be mixed with calcic chloride, which has a strong affinity for water (113), and then be re-distilled, pure or absolute alcohol may be obtained.
We have seen that, in making beer, the fermentation of the wort is checked before all the sugar is converted into alcohol. But, in the manufacture of spirits from grain, the fermentation is prolonged until all the sugar is transformed into alcohol and carbonic acid. To leave any of it unchanged, would not only involve a loss of spirit, but, during the subsequent distillation, might injure the flavor of the spirit obtained.
The alcohol obtained from the fermentation of sugar is sometimes called wine alcohol, or vinic alcohol, to distinguish it from a large number of similar compounds, known to chemists as alcohols. Its symbol is C,H,O.
205. Ether.- When equal weights of strong alcohol and oil of vitriol are boiled in a retort, a colorless, fragrant, and highly volatile liquid passes over, called ether, or sulphuric ether, CH,O. It is called sulphuric