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Updated: June 19, 2025
Bacteria and yeasts are both microscopic plants, and perhaps somewhat closely related to each other. In their general power of producing chemical changes in their food products, yeasts agree closely with bacteria, though the kinds of chemical changes are different.
In some of the examples to be mentioned the influence of bacteria is profound and fundamental, while in others it is only incidental. The fermentative industries of civilization are gigantic in extent, and have come to be an important factor in modern civilized life. The large part of the fermentation is based upon the growth of a class of microscopic plants which we call yeasts.
At this point we must observe for it is a matter of great importance that in the operations of the brewer there is always a time when the yeasts are in this state of vigorous youth of which we have been speaking, acquired under the influence of free oxygen, since all the worts and the yeasts of commerce are necessarily manipulated in contact with air, and so impregnated more or less with oxygen.
The whole of the great fermentative industries, in which are invested hundreds of millions of dollars, is based upon chemical decompositions produced by microscopic plants. In the great part of commercial fermentations alcohol is the product desired, and alcohol, though it is sometimes produced by bacteria, is in commercial quantities produced only by yeasts.
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They are dangers which must be guarded against. In former years the troubles from these sources were much greater than they are at present. Since it has been demonstrated that the different imperfections in the fermentative process are due to bacterial impurities, commonly in the yeasts which are used to produce the fermentation, methods of avoiding them are readily devised.
These "discoveries" have ranged the whole realm of unicellular life, bacilli, bacteria, spirilla, yeasts, moulds, protozoa, yet the overwhelming judgment of broad-minded and reputable experts the world over is the Scotch verdict of "not proven"; and we are more and more coming to turn our attention to the other aspect of the problem, the factors which cause or condition this isolation and assumption of autonomy on the part of the cells.
Each of these yeasts has its advantages, so that the one to select depends on the method preferred for the making of bread as well as the time that may be devoted to the preparation of this food.
Bacteria were not even yet recognised as a group of organisms distinct enough to be grouped by themselves, but were even by Pasteur at first confounded with yeasts. As a distinct group of organisms they were first distinguished by Hoffman in 1869, since which date the term bacteria, as applying to this special group of organisms, has been coming more and more into use.
This method of multiplication by simple division is the distinguishing mark which separates the bacteria from the yeasts, the latter plants multiplying by a process known as budding. While all bacteria thus multiply by division, certain differences in the details produce rather striking differences in the results. Other species divide only in one direction.
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