science of sourdough bread fermentation chemical reactions processes scientific names and latin

Explain From a Scientific Perspective: What Chemical Reactions Happen When Making Sourdough Bread?

The fabrication of sourdough bread, a gastronomic staple cherished across cultures, commences with a multifaceted dance of biochemistry and microbiology processes. It transpires in a series of biochemical transformations, influenced by a symbiotic consortium composed of various naturally-occurring yeasts, a genus of bacteria from the Lactobacillus family (lactic acid bacteria), and Acetobacter species (acetic acid bacteria).

Creating the ‘Sourdough Starter’ (Consociatio Fermenti): The initial operation involves the establishment of a sourdough starter, colloquially known as leaven. This is an environment conducive to microbial activity, consisting of a medley of flour (Triticum aestivum) and water (H2O). Exposing this mixture to the surrounding environment encourages the spontaneous accumulation and growth of naturally-occurring yeasts and bacteria, forming a symbiotic community. This instigates a biochemical process known as spontaneous fermentation.

Autolysis (Autolysis): Once the leaven demonstrates active signs of biological activity (usually after approximately a week), it’s amalgamated with additional flour and water. This blend is then allowed to rest during a period of autolysis. During this stage, the water imbues the flour particles, activating native enzymes – amylases (which catalyze the hydrolysis of starch into sugars) and proteases (which catalyze the hydrolysis of proteins into peptides and amino acids) – within the flour. The enzymatic reactions can be represented as follows:

Starch (Amylum) + H2O → (amylases) → Sugars (Monosaccharides)
Proteins + H2O → (proteases) → Peptides + Amino acids

Kneading and Fermentation (Mixtura et Fermentatio): Following autolysis, salt (NaCl) is added to the dough, which is then manually manipulated in a process known as kneading. At this point, the dough commences its initial fermentation, often referred to as ‘bulk fermentation’. The yeasts in the leaven metabolize the simple sugars resulting from starch breakdown into carbon dioxide (CO2) and thermal energy through a process referred to as alcoholic fermentation, though the ethanol production is typically minimal in sourdough compared to other fermentation processes. The chemical reaction for alcoholic fermentation can be described as:

C6H12O6 → 2CO2 + 2C2H5OH + Energy

In parallel, the Lactobacillus bacteria execute lactic acid fermentation, transforming simple sugars into lactic acid and additional carbon dioxide. Acetobacter species contribute to the acidic profile by producing acetic acid alongside lactic acid. The chemical equations for these fermentations are:

C6H12O6 → 2C3H6O3 + Energy (Lactic acid fermentation)
C6H12O6 + O2 → 2CH3COOH + 2CO2 + Energy (Acetic acid fermentation)

This complex fermentation process generates the unique sour note characteristic of sourdough bread and acidifies the dough, thereby deterring the proliferation of potentially harmful bacterial strains.

Proofing (Testatio): Post primary fermentation, the dough is fashioned into the desired form and allowed to undergo a second fermentation, colloquially referred to as proofing. Here, the microbial activity continues, producing even more carbon dioxide.

Baking (Coctura): After ample proofing, the dough is ready for the final transformative stage – baking. The application of heat brings about the expansion of trapped carbon dioxide and evaporation of any residual ethanol, contributing to the ‘oven spring’ or final inflation of the bread. Moreover, the thermal energy causes denaturation and subsequent reformation of the proteins within the dough, leading to a rigid, stable structure. Concurrently, the Maillard reaction, a non-enzymatic browning process, takes place between the amino acids and simple sugars on the dough’s surface, contributing to the enticing, golden-brown crust and enhancing the flavor profile of the bread.

The Maillard reaction can be simplified as:

Amino acids + Reducing sugars → (Heat) → Melanoidins (Brown pigments) + Flavor compounds

Cooling and Staling (Refrigeratio et Staling): Post-baking, the bread loaf is left to cool, permitting moisture to evenly disperse throughout the bread. Over subsequent days, a process referred to as ‘staling’ commences, where the starch molecules re-crystallize and the loaf progressively loses moisture, resulting in an altered texture.

The creation of sourdough bread is a fine orchestration of biological, chemical, and physical principles, acting upon a simple combination of ingredients to metamorphose them into a complex and varied gastronomic delight. Each phase in this process, with its unique transformations, contributes to the final taste, texture, and visual aesthetics of the sourdough bread. Mastery of these stages enables the production of a wide variety of bread characteristics.

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