11 November. 4pp in one bi-folded leaf. The top edge chipped with some losses to words, strengthened with archival tissue tape, else very good. A wonderful and long scientific letter written at a time when Pasteur was making many of his most important discoveries relating to bacteria, pasteurization and the fermentation process of wine. The letter (in French), reads, in part: : "[.] The claims of these gentlemen are based only on errors and I think I have demonstrated it very well this year as in 1866[.] You asked me for an opinion on two points, the action of oxygen and the temperature of the must. As for oxygen, note, as is exhibited in my studies on wine [Paris, Imprimerie impériale, 1866] that it is necessary to distinguish carefully between a brush oxidation and a very slow oxidation, the difference is enormous between these two modes of action: the first is almost always harmful[.] As for the heating of very sweet wines, here is the complete theory that you must have in mind to guide you in practice: 1° the presence of alcohol and acids in the wine has a great influence on the minimum degree necessary for conservation after heating, that is to say for the germs to be killed. The more acidic and alcoholic a wine is, the less you need to heat to kill the germs, the closer you get to the state of a [normal must], the more you must raise the temperature. If you have had difficulties in preserving a certain wort by heating it to a certain temperature, I urge you to try it out either by raising the temperature further or by adding a small amount of alcohol to the wort beforehand. To make myself clear, I invite you to consult my work on spontaneous generations. You will see that everything depends on the state of acidity, neutrality, or very low alkalinity of the environment for the determination of the degree of temperature proper to kill the germs. For example, I have shown that milk, which is weakly alkaline, requires a temperature between 100 and 110°. On the other hand, urine, which is a little acidic and which requires only a temperature lower than 100°, requires immediately a temperature at least equal to and higher than that if it is made neutral. In 1865 I experienced on very sugary white wines from Bergerac to stop their fermentation, I needed a temperature of 75°. Without a doubt, I would have had to go even further if the alcoholic fermentation that had already occurred had been less pronounced, that is, had introduced less alcohol into the wine[.]"
