Yeast and Sourdough, a Lesaffre Expertise

Historical background

Yeast and sourdough have historically lived side by side in the world since ancient times, undergoing periods of preferential use, especially in western countries. In France, sourdough and yeast were used alternatively until the late seventeenth century, and then for different purposes until 1840: yeast in poolish (liquid sponge) for high-quality bread and sourdough for more common products. It was not until the late 1940s that the use of yeast was generalized, while sourdough regained popularity amongst the bakers from 2000.

Yeast and sourdough serving fermentation

Yeast and sourdough are both fermentative agents destined to make dough rise as a result of carbon dioxide production. Both also contribute to the development of the bread’s aromas.

Sourdough is associated with long fermentation practices and the resulting products demonstrate strongly typed flavor profiles. These characteristic flavors are generated by the production of organic acids, mainly acetic and lactic acids. The impact on rheology is special: it results in a lower specific volume and a thicker crust. The lower pH increases bread shelf life via an anti-fungal effect, and bread staling is slowed down by a reduction of the speed of starch retrogradation.

It was Louis Pasteur who demonstrated the role of yeast (Saccharomyces cerevisiae) in what was until 1857 the “mystery” of fermentation. Since then yeasts, just like sourdough bacteria, have been isolated, propagated and concentrated, even assembled. Bakers now have selective ferments, stabilized and concentrated.

The microorganisms they contain provide fermentative activities and aromatic profiles that are no longer haphazard but controlled. Their activity is standardized, so it is easy to use in baking whatever the environment. Far from being opposed to each other, sourdough and yeast are therefore complementary and now reunited for a better regulation of fermentation activity and a stronger aromatic signature.

The management of these microorganisms (yeast and bacteria) is essential for those who want to offer unique fermentative and aromatic solutions.

Scientific understanding of strains, a key element in the fermentation process

The genome sequencing of Saccharomyces cerevisiae is a relatively recent approach (1994-1997) which enabled a better understanding by differentiating and categorizing the available strains according to their profiles and properties. Global coverage and the mastering of strain isolation thus enables us to select the most promising yeast and bacteria for pure or combined use.

In recent years, the development of screening techniques enabled the filtering and selection of candidates showing the strongest potential regarding specific properties. These include the fermentation rate (also called kinetics), aromatic properties, resistance to osmotic pressure, salts and acids.

These candidates are propagated at the pilot scale in order to study their ability to reproduce for future industrialization and confirm the expected performances in baking applications. Finally, the combination of these microorganisms enables us to offer both ready-to-use and customized solutions.

Instead of storing cocktails of bacteria issued from multiple sources of stabilized sourdoughs, it is now possible and advisable to isolate and multiply the selected microorganisms. They can then be reassembled as needed to take advantage of the wealth of the planet in terms of biodiversity, be propagated freely and thus shared. One more way to become independent from a skill or an exclusive and variable environment. Stored at -80°C the ferments (yeast and bacteria) will be preserved in special conditions, then bred to be available to production sites.

Being able to control the fermentation chain in its entirety, from the selection of the strain to the sensory analysis of the baked products, is an asset that only a few actors on the market can claim to. This gives them a consolidated view and exceptional responsiveness at the service of their clients.

Integrating local specificities and technologies

Understanding microorganisms, yeasts as well as bacteria, knowing how to multiply them in secure and environmentally friendly conditions is necessary but not enough. It is important today to understand and integrate local specificities.

The type of flour substrates (wheat, rye, buckwheat, etc.) and ingredients used (sugar, growth inhibitors, etc.) play a critical role during fermentation. They led yeast producers to develop the appropriate strains: yeast for acid dough using rye flour, osmotolerant yeast for high sugar dough.

The type of process and fermentation (short, long, blocked,frozen and sponge & dough) pushed manufacturers to provide increasingly customized fermentation solutions. Recent developments include fast kinetics yeasts (short diagrams, Anglo-Saxon profiles, also called “high activity”) and yeasts sensitive to cold. The latter enable a temporary blocking of fermentation to ensure flexibility in work organization or movement of semi-finished product to baking terminals. They also allow selling ready-to-use chilled dough to households. Originally used for other types of fermentations, some yeast such as maltose-negative are now applied to bread-making. This kind of yeast which cannot assimilate the maltose substrate of the flour has a limited action in time: an ideal solution to stabilize dough at room temperature.

Applications (crusty bread, sweet bread, sweet goods, pizzas, steamed bread, Arabic bread, etc.) are also decisive in the choice of fermenting agents.

The same applies to desired effects (customized aromatic signature, mild or intense, shelf life, etc.) that guide research on sourdough, alone or in combination with yeasts: a search for symbiosis between a yeast strain and bacteria for “dual effect” sourdough.

The conditions of preparation have largely orientated developments in the field of sourdough: refreshments and development of the pre-ferment before inoculation into the final dough, a desire to separate flavoring and fermentation properties (use of deactivated sourdough in combination with yeast), direct incorporation (ready-to-use living sourdough), etc.

Logistical constraints (accessibility, mean consumption, changes in production rates, etc.) influence the final stabilization of the sourdough, and are critical variables taken into account in the development of different yeast forms: liquid ready-to-use, ideally suited for automatic dosing (solutions for industrial but also craft bakers), compressed (in block or crumbled), dried (ready-to-use or to rehydrate before incorporation in the dough), frozen (Frozen Yeast of Intermediate Humidity for frozen dough).

It is through their ability to appreciate global issues and adapt them to a local or individual scale, that some actors are able to develop the most effective solutions.