The drink was cervisia, the Gaul's primitive beer. Since beer was already known to the Egyptians and (at least by reputation) to the Greeks, it is already a mystery why a long list of writers in Latin - as late as Gregory de Tours - found it necessary to describe the drink, as though it were a novelty. But in terms of bread, a more relevant mystery is: why did the Romans never adopt this leavening method?
It is unlikely that even all Gauls used it since, both Pliny and Strabo write of many using millet, a grain which does not leaven well, if at all, and so could barely have produced bread, much less light bread. Still, Pliny mentions their spelt, which is a form of bread wheat and the Romans greatly increased the use of bread wheat (triticum aestivum) in the region. Either of these would have produced a lighter bread when leavened with yeast.
As it happens, Pliny lists the Roman methods for leavening bread, one of which involved millet, which, if kneaded with must, could be kept for a year; the same was true of wheat-bran. "When required for making bread, these cakes are first soaked in water, and then boiled with the finest spelt flour; and it is generally thought that this is the best method for making bread." He describes another made with barley and water, before going on:
At the present day, however, the leaven is prepared from the meal that is used for making the bread. For this purpose, some of the meal is kneaded before adding the salt, and is then boiled to the consistency of porridge, and left till it begins to turn sour. In most cases, however, they do not warm it at all, but only make use of a little of the dough that has been kept from the day before.
In other words, the Romans used sourdough to leaven their bread. Leaving one to wonder why, if they knew that bread from Gaul and Spain was lighter, did they not make at least some bread with the same method? This is all the more striking given the Roman love of luxury and variety.
Whatever the case, sourdough also became the main leavening agent in France and would remain so for over a millennium. This had one advantage, certainly: bread made with sourdough lasts far longer than bread made with yeast. In medieval times, rents were often paid in bread, something which would have been hard to do with yeast-leavened bread and its short shelf life.
Probably for most of the Middle Ages even scholars had long forgotten that the Gauls had leavened their bread with the foam from beer. This is ironic, given that beer itself quickly became a standard drink under the Franks and was even made by monks. But in 1560, Bruyerin, having cited Pliny's earlier statement, wrote that the custom endured among the Flemish "which is why theirs is lighter than other bread." And so, in the sixteenth century, at least some of the French knew that one obtained a lighter bread by using yeast.
And yet, not only did French bakers not adopt this method, some actually fought it.
Marie de Medici (Henri IV's Italian queen) is thought to have introduced rolls made with both milk and yeast, rolls which as a result were known as "the queen's bread"; as a class such bread was known as "softish bread" - pain mollet. Others imitated this, to the irritation of ordinary bakers, who put it out that using yeast in bread was actually harmful. In March 1668, the Faculty of Medecine decided to examine the question; the result was that they banned its use. The question then went before the French Parlement (more of a supreme court than a judicial body) which first sought the lieutenant of police's opinion. He too condemned its use, along with that of milk (which had already been used in bread in the fourteenth century). Nonetheless the Parlement decided on March 21, 1670, that yeast could be used in bread (though only that of Paris).
If this quarrel sounds comic, it also seemed so to La Condamine, who later immortalized it in a long poem on "Pain Mollet". Speaking of Guy Patin, a famous doctor, he wrote:
He concluded that Death flew
On the wings of pain mollet
As a practical matter, the use of yeast became standard in luxury breads, but would not, for a long time, be so in the more common breads.
In the latter half of the eighteenth century, Dr. Malouin, one of the great French experts on bread, wrote: "The discovery of the use of yeast in making dough is a remarkable epoch in baking, in facilitating the making of bread, because it makes the dough rise faster, and because one is less obliged to work it, if one has put in yeast." As discoveries go, this one might have been considered a long time in coming, especially since Malouin himself cites Pliny's text.
Nor was this the end of the story. All the way into the nineteenth century, yeast was referred to in French as "artificial leavening", despite being as natural in origin as sour dough. And today? It is not rare to find French artisan bakers who consider sourdough the true traditional leavening agent, and yeast a foreign innovation, still to be regarded with suspicion.
FOR FURTHER READING:
The Natural History of Pliny, Bohn, 1856 Volume 4
Jean-Baptiste Bruyerin, De re cibaria: libri XXII.
Charles-Marie de La Condamine"Le Pain Mollet", Choix des poésies de Pezai: Saint Péravi et La Condamine 1810
Malouin, Descriptions des arts et métiers: XIX, 673, [1 bl.] p., 10 f. de pl, 1771 ed, Jean-Elie Bertrand
Anne Claude Philippe Caylus (comte de, Correspondance inedite du comte de Caylus avec le P. Paciaudi, theatin (1757 ...