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employed for so long a time, a good product being obtained without sterilisation or centrifugalising.
According to Wortmann's researches the end products of fermentation, as well as the quantity of these, are the same, whether little or much yeast be added ; but the more yeast added to the must, the quicker is the fermentation. It is best when fermentation proceeds quickly, as, in consequence, the foreign germs present in the must are more completely suppressed; the fermentation may, however, be too violent if an excessive quantity of yeast is added ; the result of this is, that the must not only froths over easily and a part of it is thus lost, but, by the vigorous production of carbonic acid, bouquet substances are also carried off at the same time and the wine loses in quality.
With regard to the quantity of yeast to be employed, the following numbers given by Wortmann may serve as a guide : With light musts, i.e., musts containing about 18 to 20 per cent. of sugar, one can rely with certainty on the fermentation being controlled by the pure culture yeast and consequently on a good result if from À to į per cent. of yeast is added. By this is understood that to every 100 litres of fresh must is added to į litre of a must brought into vigorous fermentation by means of a pure yeast. Good results are also obtained with the larger addition of } to 1 per cent. But Wortmann found the effect of still larger additions of yeast to be too great with light musts. With heavy musts the addition of yeasts may be much larger, i.e., up to 1 per cent. or even more, without fear of bad results. If it is desired to re-ferment wines not thoroughly fermented, or sugared wines, the addition of pure yeast should be still greater, in fact 2 per cent. or over. The same holds good for the employment of pure yeast in the preparation of sparkling wine. If wines which have stopped fermenting are to be forced with pure yeast,
Wortmann recommends a yet greater addition of yeast, up to 10 per cent., according to circumstances.
The temperature of the fermenting room requires careful watching; it should be kept lower than it is otherwise usual to keep it, so that in fermentations to be carried out on the large scale with pure yeast, the fermenting may not be too violent.
It is, in addition, important to employ the yeast at the proper stage of development. The yeast must be in such a condition that, when placed in the must, it can immediately continue its development, so as to obtain the mastery in as short a time as possible. Therefore the practitioner obtains from the laboratory small quantities of yeast, which he then increases in definite quantities of must.
Since the number of wine yeast races brought into use is large, and the fermentation of grape must is limited to a few weeks in each year, a pure culture apparatus such as that which has been described for use in brewing for the continual production of yeast in mass is not applicable in this branch of work. Laboratories would require to have a large number of such pure culture apparatus, and even if they possessed them it would be impossible in the short time at disposal to produce the quantity of yeast necessary for the work. Consequently stations and laboratories dispose of the pure culture yeast in small quantities which the practitioner then increases for himself.
In cultivating the yeast, laboratories employ sometimes the concentrated must mentioned on page 80 and sometimes a must pressed from home-grown grapes ; the former is diluted with water. In cases where a specially vigorous yeast is required, e.g., in re-fermentations and fermentations of unfinished wines, the employment of well-nourished, non-aërated yeast is to be recommended.
Pure yeast is supplied to wine producers in a thin
liquid condition by the stations, e.g., by that in Geisenheim and by the fermentation laboratory at Klosterneuburg; the flasks contain as much yeast as is produced in 0:5 to 2 litres of culture liquid.
The Geisenheim station has given the following directions for the use of this pure culture yeast: The flask with the pure culture ought not to be more than two weeks old at the very most. Some days before the beginning of the actual vintage, about 12 litres of freshly prepared, not fermenting must from good, ripe, sound grapes are allowed to boil for about five minutes, being skimmed carefully meanwhile, and then allowed to cool down to the temperature of the room, the pot being covered. After cooling, the contents of the yeast flask should be poured into the must, the flask washed out several times with must, and the pot securely covered again, and placed away in a dust-free situation at the temperature of the room, until the must, after two to three days, exhibits violent fermentation. The must thus brought to fermentation is then put into the fresh must to be fermented, the quantity of which depends on the conditions at the time.
The selection of the pure yeast is of the highest importance in the preparation of sparkling wines. It affords the certainty that the after-fermentation under the extremely difficult conditions obtaining proceeds unaided, which was formerly more or less accidental and often caused the loss of large sums of money; the use of pure yeast further renders it possible to choose such yeast races as will produce little turbidity of the wine in bottle in spite of vigorous fermentation, as they separate easily and remain clinging to the cork.
The use of selected yeast races has also proved of value in the preparation of sweet wines. Its value lies chiefly in the fact that, by the aid of a yeast which is equally vigorous to resist and to ferment in a high concentratare of sugar and alcohol, wines may be produced with certainty which contain always the same amount of alcohol A further advantage is that these wines are ready sooner, and are finer toned and preserve better than those spontaneously fermented. W. Seifert has the merit of having first made researches in practice on this subject, and of having introduced the pure culture method into this branch of wine production in the large businesses in Austro-Hungary.
The Pure Culture System in the Manufacture of Cider. The employment of selected pure yeast in the manufacture of cider is no less important than in wine manufacture. Besides the above named, Jörgensen, Kayser and Nathan have worked specially at this subject. The procedure is the same as in the preparation of wine. The results are also very satisfactory here, and ciders prepared by means of pure wine yeasts assume a more or less vinous taste and smell.
The Pure Culture System in the Manufacture of Spirit and Pressed Yeast. The credit of bringing the pure culture system into general recognition in spirit manufacture is due to the station in Berlin. The yeasts prepared by P. Lindner (Races I. and II., especially the latter) are employed in numerous distilleries and have given good results.
During the last few years the principles of race selection and pure culture have been applied in the preparation of a lactic acid bacterium for use in the above-mentioned industry; a mass culture from this bacterium is used for souring in order to prevent the injurious butyric acid fermentation. The first to introduce this pure culture into practice was Fr. Lafar. The use of lactic acid bacteria for souring the yeast mash has been considered a necessary evil by all the leading technologists. Recently, Wehmer
has attempted to use lactic acid directly instead of the bacteria. A commercial lactic acid, not absolutely pure, can now be prepared somewhat inexpensively and he has obtained good results with it in practice as a souring material.
Finally, the pure culture system has been applied in recent times in the manufacture of pressed yeast. The Berlin station has been especially active on this subject and the use of Race V., there isolated, has spread to a great extent.
Other yeast species and races applicable to the manufactures mentioned have been isolated in the technical fermentation laboratories to be found now in every country.