In Gaul and Spain, where they make a drink by steeping [grain]... - they employ the foam which thickens upon the surface as a leaven : hence it is that the bread in those countries is lighter than that made elsewhere.
If it was lighter than, say, that of the Romans, it is because bread leavened with yeast typically is lighter than that made with sourdough, but also does not last as long.
A casual reading of Pliny's statement might suggest that all Gauls ate yeast-leavened bread – that is, a fine, lighter bread – and always had. But in fact almost the opposite is true.
For one thing, Pliny was writing of the Gaul of his time; that is, Gauls under Roman rule. One must be very cautious in extrapolating any of his comments to apply to the Gauls who, for centuries, lived as masters of what is now France. Also, even in his time, his comment almost certainly applied to a limited part of Gaul. For over a millennium, “yeast” would mean brewer's yeast, and Pliny specifically is referring to the foam which came from early beer. But beer was only known in certain, mainly northern, regions. Much of Gaul at this point was essentially Roman, and the Romans often were ignorant of beer.
Above all, many of the grains the Gauls had used in the past and some they still used in Pliny's time simply did not leaven very well. Finds at Acy-Romance echo others in pre-Roman Gaul:
...a wide range of grains including spelt, emmer, einkorn, barley, millet and oats. The only grain used to make bread (an unleavened naan-like flatbread) was spelt. The other grains were ground up in a mortar and eaten in preparations like porridge or soup.
(The History Blog)
Phylarcus, in a rare description of bread in Gaul from pre-Roman times, describes it as being “broken” (rather than torn or cut) which also suggests that it was flat, unleavened bread.
It is in fact very likely that the Gauls had eaten, and, under the Romans, often still ate, more unleavened than leavened bread and even ate their grains as gruel more often than as bread. The flatbread may have been cooked on a hot stone (as it had been in Swiss neolithic villages and later would be by early Scottish hunters) or under the embers of a fire (as it long would be in parts of France).
Gauls built their granaries on piles to protect them against rodents and humidity and used them for cereals, dried meats, dried fruits etc. In the first century B.C.E. these became less common, as grain was stocked in warehouses in oppida.
...The Gaul produced many iron tools and are credited with inventing the sickle, allowing them to harvest and store large quantities of hay. The adoption of the rotating millstone divided by 15 the time to produce a kilo of flour. The period is marked by an unquestionable agricultural revolution.
(Cité des sciences)
The Gauls also (like the Germans) used underground silos, which had the advantage of hiding grain from the enemy during conflicts, as well as effectively preserving it. This method was still being used in coastal areas of Africa, Spain and Italy during the nineteenth century (Benoit).
Two different authors also mention a complex threshing machine used by the Gauls. Both descriptions date to after the Roman conquest, but the fact that this apparatus remained particular to Gaul for several centuries suggests that it was, at the least, a native Gallic invention.
In the vast domains of the provinces of Gaul a large hollow frame, armed with teeth and supported on two wheels, is driven through the standing corn, the beasts being yoked behind it; the result being, that the ears are torn off and fall within the frame. In other countries the stalks are cut with the sickle in the middle, and the ears are separated by the aid of paddle-forks.In the fourth century, Palladius described a similar mechanism:
A vehicle is therefore made, which is borne on two small wheels : the square superficies of this is strengthened with boards, which extending to the outside may render it more roomy at the top. The height of the boards of the cart is less at the fore end: there small teeth, proportioned to the size of the ears, are set in a line, bending backwards towards the upper part. Behind the same vehicle there are formed two very short poles, like those of sedans : there the ox is yoked to the vehicle, with his head towards it, a gentle beast, which may not be apt to exceed the orders of the driver. When he begins to drive the vehicle, all the ears of corn being laid hold of by the small teeth, are taken into the cart, the straw being cut off and left; the ploughman who follows, generally directing the height or lowness of it: and thus, by going and returning a few times, the operation is performed in a few hours
The Gauls then were sophisticated farmers, but rarely, if ever. sophisticated bakers. Pliny's reference to yeast is tantalizing in showing that at least some Gauls had pushed the art of bread-baking further than the known grains would have suggested. But no additional data has so far been found to clarify it. Nor is it clear if the Gauls in question had always made such bread, or if this practice was an incidental side-effect of Roman influence on the bread in Gaul, Which would be profound.
FOR FURTHER READING
Pliny the Elder, The Natural History of Pliny, ed Bohn, Vol 4 1856
Strabo, The Geography of Strabo, Vol 1, 1892
Legrand d'Aussy, Pierre, Jean-Baptiste, Histoire De La Vie Privée Des Français, vol 1 1782
Palladius, Rutilius Taurus Aemilianus, The Fourteen Books of Palladius RutiliusTaurus Æmilianus, on Agriculture ed Thomas Owen 1807