YEAST. 



fecuring it from the air ; and will keep frefti, found, and fit 

 for ufe for a long time. 



This is a fecret that might be of great ufe to the brewers 

 and diftillers here, who, though they employ very large 

 quantities of yeaft, feem to know no method of preferving 

 it, or raifing nurferies of it ; for want of which they fullain 

 a very confiderable lofs ; whereas the brewers in Flanders 

 make a very great advantage of fupplying the malt-diflillers 

 of Holland with yeaft, which is rendered laftiiig, and fit for 

 carriage, by this eafy expedient. Shaw's Leftures. 



Mr. Henry has repeatedly prepared an artificial yeaft, by 

 impregnating flour and water with fixed air, with which he 

 has made very good bread, without the afliftance of any 

 other ferment : and he propofes this method of procuring 

 frefh fermented bread at fea. 



The procefs is as follows ; Boil flour and water together 

 to the confiftence of treacle ; when the mixture is become 

 cold, fill a fmall caflc with it. This cafk is to be filled up 

 in the manner reprefented in Plate XV. Jig. 7. Pneumatics, 

 and defcribed under Pyrmoxt Water, for the impregnation 

 of water with fixed air ; and the procefs is to be conduced 

 in a fimilar way, except that the caflt is to be agitated as 

 often as the mixture rifes to about two-thirds of the capacity 

 of the funnel k ; and after each agitation, which fliould con- 

 tinue during feveral minutes, the unabforbed air is to be let 

 out, by withdrawing the plug from the orifice m, till that 

 part of the mixture which remained in tlie funnel has re- 

 turned into the cafk. The orifice at i fhould alfo be larger 

 than is neceflary in the other operations, on account of the 

 fuperior vifcidity of the mixture. When, after repeated 

 agitation, the mixture which has afcended into the funnel 

 does not fubfide into the call<, it may be fuppofed incapable 

 of abforbing more air. 



Pour the mixture, thus faturated, into one or more large 

 bottles, or narrow -mouthed jars ; cover it over loofely with 

 paper, and upon that lay a flate or board with a weight to 

 keep it fteady. Place the veffel in a fituation where the 

 thermometer will ftand from 70° to 80°, and ftir up the mix- 

 ture two or three times in twenty-four hours. In about 

 two days, fuch a degree of fermentation will have taken 

 place, as to give the mixture the appearance of yeaft. 



With the yeaft in this ftate, and before it has acquired a 

 thoroughly vinous fmell, mix the quantity of flour intended 

 for bread, in the proportion of fix pounds of flour to a 

 quart of the yeaft, and a fufficient portion of warm water. 

 Knead them well together in a proper veflel, and covering 

 it with a cloth, let the dough ftand for twelve hours, or till it 

 appears to be fufliciently fermented, in the above-mentioned 

 degree of warmth. It is then to be formed into loaves and 

 baked. 



Mr. Henry adds, that perhaps the yeaft would be more 

 perfeft, if a decoftion of malt were ufed inftead of fimple 

 water. 



When the operation is finiflied, the cafli, in order to pre- 

 vent its contrafting a difagreeable taint, fliould be well 

 waflied. Henry's Account of a Method of preferving Water 

 at Sea, &c. p. 26, 1781. 



Yeast, Chemical Properties of. The nature of yeaft has 

 been briefly difcuifed under the head of Fermentation. 

 Since that article was written, however, fome additional 

 experiments have been publifhed on the fubjeft, which deferve 

 to be noticed here. 



The aftive and effential principle of yeaft, as obfervcd under 

 the article Fermentation above alluded to, appears to bea fpecies 

 of gluten. When yeaft is kept for fome time in cylindrical 

 glafs vefl^els, a white fubftance, not unlike curd, feparates 

 and fwims upon the furface. If thia fubftance be removed, 



Voi. XXXIX. 



the yeaft lofes the property of exciting fermentation. Thi» 

 fubftance poflefles many of the properties of gluten, though 

 it diff'ers from it in others. Its colour is much whiter. 

 It has not the fame elafticity, and its particles do not adhere 

 with the fame force. It diflblves more readily in acids. 

 Dr. Thomfon confiders this as the real fermentative prin- 

 ciple, and thinks it may be confidered as gluten fomewhat 

 altered, and rendered much more capable of decompofition. 

 He thinks alfo that it exifted in the raw grain originally, 

 but underwent confiderable modifications by the malting 

 procefs, and perhaps others during the fermentation of the 

 beer from which it feparated. 



The fame diftinguiflied chemift ftates, in fupport of this 

 opinion, the experiments of Fabroni and Thenard. Fabroni, 

 by heating the juice of grapes, and pafling it through a filtre, 

 feparated an adhefive matter, which poflefied the properties 

 of gluten, and deprived of which the juice refufed to fer- 

 rnent, though it fermented as well as ufual when this prin- 

 ciple was again added. Thenard Ukewife found in the juices 

 of all the fruits he examined a fubftance fimilar to that de- 

 fcribed by Fabroni, and which, according to him, is abfo- 

 lutely the fame with pure yeaft. This fubilance is infipid, 

 does not change vegetable blues, is infoluble in water, lofes 

 three-fourths of its weight when dried, and is decompofed 

 like animal fubftances. When eight parts of it were dif- 

 tilled, they left 2.83 of charcoal, and yielded i 61 of water, 

 1. 8 1 of oil, and a quantity of ammonia, which, when fatu- 

 rated with muriatic acid, formed 1.46 of muriate of ammo- 

 nia. The gas obtained weighed 0.33, and confifted of one- 

 fifth of carbonic acid, and four-fifths of carburetted hydro- 

 gen, requiring li times its bulk of oxygen to confume it. 

 Nitric acid, even when much diluted, converts it into a fpecies 

 of tallow. With potafli it forms a foap, while ammonia is 

 difengaged. When mixed with fugar and a fufficient quan- 

 tity of water, fermentation takes place, carbonic acid is dif- 

 engaged, and a vinous liquor formed. By this aftion, the 

 ferment lofes the whole of its azote, and becomes incapable 

 of exciting fermentation when mixed with a new portion of 

 fugar. In farther corroboration ftill of this opinion. Dr. 

 Thomfon adduces an experiment of Kirchhoff, which he 

 thinks throws confiderable light on the nature of yeaft. 

 Barley-meal contains both gluten and ftarch. Pure ftarch 

 infufed in hot water is not converted into fiigar ; nor does 

 gluten become faccharine matter when heated in the fame 

 manner. But if a mixture of pure dried pulverized wheat- 

 gluten and potatoe-ftarch be infufed in hot water, the ftarch 

 is converted into fugar. During the procefs an acid is faid 

 to be evolved. The gluten is httle changed in appearance 

 or quantity, and may moft of it be feparated by filtration. 

 What is Angular, however, it is incapable of inducing the 

 fame change upon ftarch a fecond time. 



The following are the conftituents of yeaft, according to 

 Weftrumb, as quoted by Dr. Thomfon. From 15-360 

 parts he obtained, 



Befides 



