PLASTERING.
It is a Common Practice in Spain and in the southern part of France to plaster the wines, by adding more or less gypsum, or plaster of Paris. It is either thrown upon the grapes before or after crushing, or is added to the must. Gypsum is known to chemists, when pure, as calcium sulphate (sulphate of lime), but contains a certain amount of water of crystallization, and is generally found associated with other substances, such as rock salt, and calcium carbonate, or limestone. It is the commonest impurity found in spring water, and gives water its permanent hardness. Much has been written for and against the practice of plastering, and both sides of the question have strong advocates.
Objects.—There are many different reasons given for the practice, some of which are fanciful. It is claimed that it retards fermentation, and that red wines under its effects develop more color, because the marc can be left longer in the fermenting vat; that the froth of plastered wine is livelier and sooner disappears, which pleases the merchants; and that it has a preservative effect upon the wine. It is claimed by some that it renders the wine dryer and harsher, as it does, if used to excess, and by others, that it unites with a portion of the water of the juice, and renders the remaining juice richer in sugar. Again, it is added to neutralize a portion of the acid contained in the must.
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.
2. That plaster diminished the intensity of the color. (This, of course, refers to the direct effect.)
3. That the potassium bitartrate, one of the most useful principles contained in wine, is decomposed by plaster, and that potassium sulphate is formed, which remains in solution, and calcium tartrate, which is precipitated.
4. That potassium phosphate, also one of the salts naturally contained in wine, is equally decomposed by plaster.
5. That plastering profoundly modifies the nature of wines, by substituting for the potassium bitartrate a purgative salt in the proportion of from 3 to more than 7 grammes per litre.
And they demand the exclusion of all wine containing more than 4 grammes of the sulphate per litre.
And Mr. Carles, above quoted, concludes that, owing to the purgative effect of this salt, potassium sulphate, the quantity present should not exceed 2 grammes per litre, or half as much as the army commission allow.
Still later, we have the instructions of the Minister of Justice of France to the procureurs Généraux, issued in 1880, as follows:
After several judicial decisions relative to the sale of plastered wines, one of my predecessors expressed to the Minister of Agriculture and Commerce the desire that new experiments should be made in order to establish, if in the present state of science the immunity accorded to plastered wines by the circular of July 21, 1858, should be maintained.
Having examined the question, the consultation committee of public hygiene issued the following notice:
1. That the absolute immunity which plastered wines enjoy on account of the circular of the Minister of Justice dated July 21, 1858, ought no longer to be officially allowed.
2. That the presence of potassium sulphate in the wines of commerce, which results from, plastering the must, from the mixture of plaster or sulphuric acid with the wine, or from cutting with plastered wines, should only be tolerated to the maximum limit of 2 grammes per litre (about 117 grains per gallon).
In calling my attention to this notice, my colleague of agriculture and commerce informs me that he completely concurs.
He, therefore, instructs the officers to prosecute, under the laws against adulterations, the dealers who shall sell wine containing more than the quantity indicated of potassium sulphate, as dangerous to the health of the consumers.
Plastering Sherry—Quantity Used.—Mr. Vizitelli says that during his stay at Jerez, he paid particular attention to the plastering question, saw the gypsum applied in almost a hundred instances, and questioned the overseers in scores of vineyards. He states that within his own knowledge gypsum is by no means invariably used in the vinification of sherry, as already stated under the head of Sherry. And although applied in the majority of cases, but a few pounds per butt are used, say 6 lbs. at most in a dry season, and a little more than double that quantity in years when great dampness prevails. And he argues from the Montpellier experiment, already mentioned, where the committee added 40 grammes of gypsum to a litre of wine, and found only 1.240 grammes of sulphate of potash per litre where pure calcium sulphate was used, and 1.828 grammes where ordinary plaster was employed, that when the Spaniards add the amount which they do to the must in sherry making, no injury to the wine can occur. It may be proper to suggest, however, for the benefit of future inquirers, that wine, after insensible fermentation, contains but a small proportion of the potassium bitartrate which was contained in the grape, the greater part of it having been deposited with the lees and the marc. Wines do not contain tartar enough to furnish 2 grammes per litre of potassium sulphate, nor enough to act upon 1 gramme of pure gypsum. But it is far otherwise with grape juice. Now 6 lbs. of gypsum to one butt of wine of 108 Imperial gallons would be the same as about 5.5 grammes per litre, and if pure, ought to produce, on being fully satisfied with the acid potassium tartrate, as much as 8 grammes per litre of potassium sulphate, and Mr. Carles, above quoted, says that it does amount to from 4 to 7.5 grammes per litre in plastered wines.
Supposing the following to be the correct reaction, 1 gramme of pure gypsum ought to produce, with 2.6 grammes of cream of tartar, 1.477 grammes of sulphate of potash; and to produce the 2 grammes per litre of the latter would only require 1.353 of the former; and but a little more than 1 lb. of pure gypsum could safely be added to 100 gallons of must:
CaSO₄ + 2(C₄H₅KO₆) = C₄H₄CaO₆ + C₄H₆O₆ + K₂SO₄
As the gypsum is usually added to the pomace itself, or to the grapes before crushing, it is unsafe to argue from the effects produced by adding it to wine.
By Adding Water to must, the effects of plastering may be produced, if the water is hard by reason of the gypsum contained in it.