Chemical Effects.—Maumené says that it transforms the potassium salts of the wine into insoluble lime salts and potassium sulphate, and this may have an important effect upon fermentation, for some chemists attribute to the acid potassium tartrate the property of holding ferments in solution, and that potassium sulphate, even with the freed tartaric acid, does not possess this power; that the carbonate of lime contained in the plaster, in neutralizing the acidity of the tartar, without doubt contributes to the precipitation of the ferment which this salt held in solution; and that during the neutralization, carbonic acid is disengaged, and the evaporation of the moisture carried up by the gas somewhat lowers the temperature. He supposes that all these causes combined retard the fermentation.
P. Carles (J. Pharm. Chim. {5}, 6, 118-123), says that the calcium sulphate acts on the potassium bitartrate in the juice of the grape, forming calcium tartrate, tartaric acid, and potassium sulphate, a large proportion of the last two bodies remaining in the wine. That without plastering, wine contains about two grammes per litre of pure tartaric acid, whilst after plastering, it contains double or treble that amount, and even more, according to the quantity of potassium bitartrate decomposed.
In order to make clear what this chemist says, in ordinary language, we will say that the gypsum acts upon the cream of tartar in the grape juice, sets free a portion of the tartaric acid existing in combination in it, and also forms tartrate of lime and sulphate of potash.
At first sight, therefore, it would seem that the addition of gypsum, or plaster of Paris, actually increases the acid, and this would be true if the gypsum consisted of pure calcium sulphate, but it always contains more or less calcium carbonate, and this substance, which is but another name for chalk, limestone, or marble, precipitates the free tartaric acid, and the carbonate of lime does what is generally claimed for the gypsum—diminishes the acidity of the wine. But if the calcium carbonate does not exist in sufficient quantity in the gypsum to precipitate all the tartaric acid set free, the opposite effect would be produced. Why not add marble dust at once?
The experiments given in Thudichum and Dupré’s work show that the amount of water withdrawn from must by the addition of even anhydrated plaster is so small as to be unworthy of notice, being only one-fourth the weight of the plaster used.
Effects on the Health.—This question was examined at Montpellier, in France, by a committee of chemists appointed by the court, and the results of their inquiries are frequently cited by those who are in favor of plastering:
1. That the plastered wine contains no new mineral substance.
2. That the quantity of plaster introduced into the wine may be considered null, because it is entirely changed into potassium sulphate, a slightly purgative salt, analogous in this respect to tartar.
Later, however, a commission was appointed by the Conseil des Armées, who reported as follows:
1. That by the taste, plastered wines could not be distinguished from the natural ones.