In the Paris bookstalls we bought everything we could find as to the French practices in this respect, and furthermore we spent hours in the Bibliothèque Nationale studying the documents relating to it.
D. Allard, Professeur Départmental d'Agriculture, says in his book "Le Beurre": "It is generally remarked that in the regions which produce the finest and best-liked butter, la Normandie and la Bretagne, great care is taken to let the cream turn sour before it is churned. There is here certainly a result of fermentation, for one can, as we have said, impart these qualities to sweet cream by adding select ferments.
"Besides this, fermentation gives another advantage: it makes the cream easier to churn and increases the yield of butter.
"One must not go too far, however. The farmers know very well that the cream of a whole week gives a butter of unpleasant flavor.
"It is therefore the uniformity of fermentation that ensures uniformity in the production of butter; which explains the importance of this question."
Another writer, V. Houdet, Ingénieur-Agronome, Directeur de l'Ecole Nationale des Industries Laitières de Mamirolle, says in his book "Laiterie, Beuerrerie, Fromagerie" (fourth edition, 1912):
"No matter whether the cream has been obtained by letting the milk stand in a low temperature or by means of a separator, it does not, if churned at once, yield anything but a sweet butter, of pure taste but without bouquet and without _finesse_.
"In order that the butter may have the aroma, and particularly the nutty flavor which the consumers desire and which considerably increases its market value, it is necessary that the cream should ferment, should become soured, before it is churned, for it is particularly on this treatment, this _maturation_ (ripening), that all the qualities of the product depend.
"While the cream is fermenting, the sugar of milk it contains is changed into lactic acid which reacts in the measure of its production on the glycerides, saponifies them while liberating the volatile acids which impart to the butter its perfume and make it keep better.
"At the same time, as with all fermentation, it is necessary to stop in time; an excessive development of acid would yield a strong butter, rapidly undergoing a change and becoming rancid."