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3. Chart showing variations in rate of fermentation produced by different quantities of yeast....

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4. Chart showing rate of fermentation of pure and mixed yeasts--‒‒‒‒

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THE FERMENTING POWER OF PURE YEASTS

AND SOME ASSOCIATED FUNGI.

INTRODUCTION.

The use of selected pure yeast cultures has become a common practice in certain industries, notably in brewing and distilling, and their value in other lines, as in cider manufacture and in bread making, is now recognized. Selected, pure races of yeasts have come into much more general use in the cider industry in Germany than in other countries, and when employed with the proper technique they have been found very beneficial. Their use in this industry has been practiced to a considerable extent for some years past in France, and less extensively in this country, yet there is every reason to believe that the industry would be greatly benefited by their employment.

As special agent of the Bureau of Chemistry the writer collected from foreign laboratories a number of tested yeasts and has also isolated a considerable number of yeasts from native and foreign sources. These have been tested in the fermenting of apple juice for several years past, incidentally in connection with other investigations, and pure yeast cultures have also been distributed quite widely in the United States for the past six years, some of which seem to be valuable for cider making. A description of these yeasts is given on page 25.

There is, however, a very large amount of work necessary on the practical details, as well as more critical scientific investigation, before pure yeast cultures can be sent to local manufacturers with a fully developed plan for their use in ordinary factory work. In fact, the question greatly needs at this time the assistance of well-equipped cider manufacturers in working out many details, and to those who wish to experiment with special yeast cultures these will be furnished, together with such data and instructions as are obtained in the experimental work.

TESTING PURE YEASTS.

The fact that a yeast is a pure culture does not settle the question of its value for cider making, hence when an apparently desirable

22050-Bull. 111-08- -2

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pure yeast has been isolated it is necessary to subject it to scientific and practical tests. The measurement of the yeast's activity as a chemical reagent in breaking down sugars in solution has been the scientific test used. This test is so devised as to obtain a direct time measure of the fermenting power of the yeast and to determine the products resulting from its growth in the liquor fermented. Only those yeasts which have been isolated in accordance with correct technique and have shown good characteristics, such as vigor, brightness of resultant liquor, etc., and thus warrant further investigation, are subjected to the chemical test in culture flasks.

CULTURE FLASK.

This test is accomplished by sowing the yeast cultures into flasks so prepared that the growth activities of the yeast may be observed, the loss of carbon dioxid estimated, and, after fermentation is is completed, the resultant products determined by analysis without complication of the results by the growth of other organisms. A flask prepared for this work is shown in fig. 1. It consists ordinarily of of a stout, clear bottle, or of an Erlenmeyer flask of 800 cc capacity. In this flask are placed 400 cc of a filtered must or other prepared culture medium, the composition of which has been determined by analysis. The flask or bottle is then stoppered with a perforated rubber stopper, through which is fitted the ventilating tube, shown in fig. 2. The stem of the tube, inserted through the stopper, should not extend down near the liquor, as when fermentation is active the slightest agitation will tend to throw the foam through the ventilating tube and the test will be lost. The outer end of the tube is curved down onto the neck of the bottle or flask and carries a control apparatus consisting of a flask-like enlargement of the tube, with a small inner tube which dips down into the control liquid contained in the enlarged outer portion of the ventilating

FIG. 1.-Culture flask for fermentation tests.

apparatus. A small funnel-shaped opening permits the gas to escape during fermentation.

This ventilating apparatus not only permits the gas to escape, but makes it possible to observe the activities going on within the flask and also protects the medium from contamination by air. The growth of the yeast is noted by the rate of passage of gas through the control liquid, which should be 5 to 10 per cent sulphuric acid. This device, when the control liquid is in place, makes it impossible for germs to enter if, by change of temperature, air should be drawn into the bottle or flask. It also

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The stopper carrying the control apparatus is fixed in the bottle or flask after the measured, or preferably weighed, quantity of culture medium has been placed in it, and the neck of the bottle or flask, including the cork, is closely wrapped with parchment paper held in place by a rubber band. The outer funnel opening is plugged with cotton. The whole apparatus, with its contents, is then sterilized by steam heat and set away, ready to be sown with the organism to be tested. For acid fruit juice one sterilization, heating for forty minutes in steam at 100° C., is sufficient, but for malt or like substances this should be repeated on two successive days.

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FIG. 2.-Detail of ventilation tube for culture flask.

When making this test, it is of the greatest importance to sow under conditions which, as far as possible, prevent the entrance of extraneous organisms. It is also important to sow all cultures with a standard loop of platinum wire prepared to carry a drop of uniform size. Otherwise, tests made at different times in the laboratory may show a different rate of fermentation at inception, not by reason of greater or less activity on the part of the yeast used, but because of the greater

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