tion is the best; though frequently the slimy dregswhich remain in the bottom of casks when the clearporter is racked off is sold for yeast. A great dealof the success of the fermentation depends upon thegoodness of the yeast, and the quantity used mustbe regulated by its freshness.
In the course of fermentation the liquid gains about20 degrees Fahr. (4° C.) above its ordinary tempera-ture. Donovan gives the following as a fair state-ment of the specific gravity and temperature of thewort of a five days’ fermentation, the original specificgravity being 1/050:—
First morning, temp., 70° Fahr., sp. gr. of wort, 1*050“ evening, “ 70° “ “ “ 1*050
Second morning, “ 72° “ “ “ 1*046
“ evening, “ 76° “ “ “ 1*032
Third morning, “ 80° “ “ “ 1*022
“ evening, “ 84° “ “ “ 1*012
Fourth morning, “ 88° “ “ “ 1*007
“ evening, “ 88° “ “ “ 1*005
Fifth morning, “ 88° “ “ “ 1*003
“ evening, “ 88° “ “ “ 1*001
The time occupied by the fermentation variesbetween three and nine days. Difference of seasonhas a great influence on the celerity or slowness ofthe fermentation. During the first few days thetuns are exposed as much as possible to the atmo-sphere ; after this time they are covered over tightlyto exclude the air. Two reasons have been allegedfor this arrangement: first, to prevent the escape ofthe carbonic acid, which, it is supposed, forwards theaction; and secondly, if suffered to diffuse itselfinto the atmosphere, it would carry off portions ofthe alcohol. For the first supposition some explana-tion may be offered. It seems that the carbonic acid,when confined, causes the greater attenuation of theliquid. It is known that the carbonic acid from fer-menting tuns, when conducted through fresh worts,causes fermentation ; though this is not so completeas when yeast is added. Pasteur has proved thatthis happens in consequence of a portion of the activeprinciple of the ferment being carried over mechani-cally, and that by this the fermentation is caused;carbonic acid generated from any other source havingno such effect. With regard to the second reason,it is scarcely admissible, as the fermentation is nearlyterminated before shutting down the tuns.
Distillers never take yeast from the fermentingtuns, but beat it into the liquid, being of opinionthat any such abstraction would render the fermen-tation less complete, and consequently diminish theproportion of spirit.
The best temperature for the wort, when let intothe fermenting tuns, is about 70° to 75° Fahr. (21° to24° C.); during the fermentation the temperaturebeing at the maximum elevation, between 82° and90° Fahr. (28° to 82° C.). Great care must be exer-cised to prevent the temperature exceeding 95° Fahr.(85° C.), in order to avoid acetification.
Difference in the time of the fermentation of wortsis almost always owing to variation in the quality ofthe yeast. As the fermentation proceeds the specificgravity of the wort diminishes, in consequence ofthe decomposition of the saccharine matter intocarbonic acid and alcohol, which is expressed by the
distillers as attenuation , and is the standard by whichthey judge of the success of the operation. Theamount of spirit the wash will yield may be knownfrom the determination of the specific gravity, ordegree of attenuation which has taken place.
Two reasons may be offered for the diminishedspecific gravity after fermentation.
1. The disappearance of the saccharine matter inthe fluid. The whole of the sugar, however, is notdecomposed, and consequently the specific gravityof the fermented wort would be proportionallygreater than the water in its ordinary state, as thequantity of undecomposed matter is more or less.
2. The results of the fermentation are carbonicacid, which escapes, and alcohol, which remains;the alcohol, having a less specific gravity than water,diminishes the density of the latter.
The aim of the distiller is to have the spec. gr.of the wort to correspond with water. When thisobject is gained, it does not however followthat the whole of the saccharine matter has beendecomposed. The weight of a volume of alcoholbeing much less than that of a corresponding volumeof water, it is evident, if both be commingled, thatthe resulting liquid will have a specific gravity lessthan the latter; and to make up the strength, so asto equal the density of water, a greater or lessamount of saccharine matter is requisite. In otherwords, a quantity of saccharine matter remains un-decoinposed, which raises the density considerablyhigher than that of water at ordinary temperatures.Generally, this quantity of undecomposed sugarincreases with the original strength of the worts, asthe alcohol generated in the proportionate bulk ofthose extracts is larger; and with it the decomposi-tion of the saccharine matter is partially arrested, inconsequence of the property which alcohol has ofpreventing eremacausis, or fermentation. This factis easily proved by adding some alcohol to a portionof the wort in active fermentation, so that the liquidwill indicate a specific gravity equal to, or a littlebelow water; the action of the ferment is arrested,and the sugar, necessarily, remains in the liquid intact.
If the solution be introduced into a retort, or smallcommon copper still, and about one-third of theliquid drawn off at the temperature of the water-bath, the spent wash in the still run once more intoa vessel, and a little yeast added, active decomposi-tion of the sugar sets in, and a corresponding quantityof alcohol and carbonic acid is produced. The caseis similar in the fermentation on a large scale: theannexed table shows this very clearly. The densityof the nine worts examined ranged about 1*045,and the degrees of attenuation to which they werereduced were as follows:—
1. Specific gravity after attenuation,— 1*0012
2. “ “ . 1*0045
3. “ “ . 1*0018
4. “ “ . 1*0000
5. “ “ . 1*0012
6. “ “ . 1*0045
7. “ “ . 1-0047
8. “ “ * . 1*0007
9. “ “ .. .. 1*0007