Bread owes its flavor to fermentation.
FRENCH BREAD—particularly its principal representative, the baguette—is reproached nowadays for having less flavor than it used to and for drying out too quickly. The second criticism is unjust, for it neglects the fact that the baguette was a product meant for city dwellers who could buy what they needed several times a day at their local bakery; the crust was more important than shelf life. Yet many bakers today admit to being more concerned with the mechanical behavior of the dough than the taste and odor of the bread.
The flavor of bread comes from the cooking of the dough, in which various reactions combine to produce a crispy, flavorful crust, and from the action of the constituent molecules of a species of yeast known as Saccharomyces cerevisiae (brewer’s yeast) and the products of their fermentation in helping to form the volatile compounds found both in dough and in bread itself. To better understand this process, two Institut National de la Recherche Agronomique (INRA) groups in Nantes compared breads made by several different methods, with or without yeast and with or without fermentation.
The most common method, direct yeast fermentation, begins with the kneading of a dough composed of flour, water, yeast, and salt for about twenty minutes. The dough is then allowed to ferment for forty-five minutes (floor-time), at which point it is divided into lumps and fermented again for an hour and forty minutes (proofing) before being cooked at 250°C (about 475°F) for thirty minutes.
The sponge method (sometimes called sur’poolish or French starter) is identical except that the dough undergoes a process of prefermentation in a semi-liquid environment. Water is first combined with a smaller quantity of flour to obtain a preparation with the consistency of crêpe or pancake batter. This preparation is left to ferment for several hours. An additional measure of flour is then incorporated to restore a consistency equal to that of directly fermented dough, and the same procedure followed as with the traditional method.
A third method consists of cultivating beforehand a natural microflora composed of yeast and lactic bacteria. The starter thereby obtained—sourdough—is then used to initiate the process of fermentation in bread dough.
“Vinegar” Needed
As late as the mid-1980s studies of the volatile compounds in bread revealed no systematic qualitative differences from one type of bread to another, although tasters were quite capable of distinguishing between them. Only the proportion of certain volatile organic acids seemed to vary markedly. In particular, the sponge and sourdough methods yield acetic acid concentrations that are, respectively, two and twenty times greater than those obtained by direct fermentation. In the case of the sourdough method, lactic acid is also produced by the bacteria that colonize the dough along with the yeast.
Studies of direct fermentation extended these first researches by clarifying the action of yeast in bread, a complex environment that is modified by cooking. An initial comparison of breads obtained by direct fermentation with breads baked after inhibition of yeast, without the addition of yeast, or with yeast added just before cooking revealed the various compounds produced by the action of yeast in French breads. The use of yeast in fermenting dough is directly responsible for the presence of compounds such as 3-hydroxy 2-butanone, 3-methyl i-butanol, and 2-phenylethanol (which has the odor of a wilted rose).
In bread cooked without yeast, other components are more abundant than in ordinary bread, particularly monounsaturated and polyunsaturated aldehydes and alcohols such as pentanol and benzyl alcohol, which may result from the oxidation of lipids in flour (the analogue of foods that turn rancid), a process known to contribute to the flavor of bread. The constituent compounds of yeast appear to play only a weak role.
Finally, the complexity of the phenomena that produce the transformation of bread dough led the INRA biopolymers laboratory in Nantes to study uncooked dough as well, with or without yeast, fermented or not. In this case chemical analysis was combined with investigation of the olfactory properties of dough extracts: Volatile compounds were extracted by various solvents, separated by chromatography, and smelled by the experimenters through a tube.
What they discovered was a general increase in the concentration for several alcohols, ketones, esters, and lactones and a reduction in the concentration of aldehydes. They also found that yeasts produce higher levels of alcohol. But chromatographic analysis, combined with sensory evaluations, showed that these levels have little to do with the aroma of the fermented dough, unlike the aldehydes and two compounds that so far have not been identified.
Such studies are being published at a time when bean and soy flours, customarily added to bread as whitening agents, are used less and less often. When the dough is subjected to longer and more rapid kneading by mechanical means, these flours produce hexanol, which brings a stale, oily smell. In reviving neglected techniques of fermentation such as the sponge method, a new generation of bakers seeks to avoid the disadvantages of standardized production and to reinvigorate the market for a food that is as old as humanity itself.