Food Fermentation: Bread

Food Fermentation: Bread

Bread is known to man even from about 4,000 B.C. Today, bread is a major food of the world and it supplies over half of the caloric and Vitamins B and E intake of the world’s population.

The basic ingredients

The basic ingredients in bread-making are flour, water, salt, and yeasts. In modern bread making

Some other additives such as ‘yeast food’, sugar, milk, eggs, shortening (fat) emulsifiers, anti-fungal agents, anti-oxidants, enzymes, flavoring, and enriching ingredients are also used.

Flour, the chief ingredient of bread is produced by milling wheat.  Flour contains starch (70%), protein (7-15%), sugar (1%), and lipids (1%).

Flour proteins are of two types, the first type is soluble in water and dilute salt solutions and is non-dough forming. It forms about 15% of the total protein in flour and include albumins, globulins, peptides, amino acids, and enzymes. The second type is gluten which contributes the 85% of flour protein and they are insoluble in aqueous media and are responsible for dough formation. Gluten form an elastic structure when moistened with water and holds the starch, yeasts, gases and other components of dough.

One third portion of gluten is alcohol soluble fraction known as gladilins and two thirds of gluten is not alcohol-soluble and known as the glutenins. Gladilins are of lower molecules weight than glutenins.

Yeast used for baking is Saccharomyces cerevisiae. The ideal properties of yeasts baking are:

(a) Ability to grow rapidly at room temperature of about 20-25°C;

(b) Easy dispersability in water;

(c) Ability to produce large amounts of CO₂ rather than alcohol in flour dough;

(d) Ability to resist autolysis when stored at 20°C; Good keeping quality

(e) Ability to adapt rapidly to substrates during dough making.

(f) High invertase and other enzyme activity to hydrolyze sucrose to higher glucofructans rapidly;

(g) Ability to grow and synthesize enzymes and coenzymes under the anaerobic conditions of the dough;

(h) Ability to resist the osmotic effect of salts and sugars in the dough;

(i) High competitiveness i.e., high yielding in terms of dry weight per unit of substrate used.

The yeast amounts vary from 2.0 -  3.0% of flour weight. The amount of yeasts used during baking depends on the flour type, Very ‘strong’ flours i.e., with high protein levels, require more yeast than softer ones.  Also baking systems which involve short periods for dough formation, need more yeast.

The roles of yeasts in bread-making are leavening, flavor development and increased nutritiveness.

Yeast ‘food’ contain a calcium salt, an ammonium salt and an oxidizing agent such as iodates, bromates and peroxide. The bivalent calcium ion strengthens the colloidal structure of the wheat gluten, ammonium is a nitrogen source for the yeast and oxidizing agent strengthens gluten by reacting with the proteins and enhances the ability to hold gas releases during dough formation.

Yeast food has the following composition: calcium sulfate, 30%, ammonium chloride, 9.4%, sodium chloride, 35%, potassium bromate, 0.3%; starch (25.3%).

Sugar is added as sucrose or fructose corn syrups,

(a) to provide additional carbon nourishment for the yeasts

(b) to sweeten the bread;

(c) for more rapid browning of the crust through sugar caramelization.  This allows greater moisture retention within the bread.

Animal and vegetable fats such as Butter, lard (fat from pork) or soy bean oil, are added as shortenings in bread-making at about 3% (w/w) of flour in order to yield

(a) increased loaf size;

(b) a more tender crumb; and

c) enhanced slicing properties.

Emulsifiers are used in conjunction with shortening to ensure a better distribution of shortening in the dough. Emulsifiers contain a fatty acid such as palmitic or stearic acid, which is bound to glycerol, lactic acid, sorbic acid or tartaric acid. Emulsifiers are added at 0.5% flour weight. Commonly used surfactants are calcium stearyl- 2-lactylate, lactylic stearate, sodium stearyl fumarate.

Milk is added to make the bread more nutritious, to help improve the crust color by sugar cearamelization and for its buffering value. Milk is added at a ratio of 1-2 parts per 100 parts of flour.

About 2% sodium chloride is usually added to bread for the following purposes:

(a) It improves taste;

(b) It stabilizes yeast fermentation;

(c) As a toughening effect on gluten;

(d) Helps minimize proteolytic activity;

(e) It participates in the lipid binding of dough.

Since salt has a retarding effect on fermentation, it is added only towards the end of the mixing.

Water is needed to form gluten, to permit swelling of the starch, and to provide a medium for the various reactions that take place in dough formation.

Amylolytic enzymes are required to breakdown the starch from flour into fermentable sugars. Flour is supplemented with malted barley or wheat to provide Alpha amylase or Fungal or bacterial amylase preparations may be added. Bacterial amy1ase from Bacillus subtilis is heat-stable and can survive the baking process. Proteolytic enzymes from Aspergillus oryzae are also used.

Mold-inhibitors (antimycotics) are added and the chief antimycotic agent added to bread to prevent fungal growth is calcium propionate, sodium diacetate, vinegar, mono-calcium phosphate, and lactic acid.

Bread is enriched with various vitamins and minerals including thiamin, riboflavin, niacin and iron.

Process of Bread-making

The processes of yeast-leavened bread-making may be divided into:

1.      Pre-fermentation (or sponge mixing): A portion of the ingredients is mixed with yeast and with or without flour to produce an inoculum. During this the yeast becomes adapted to the growth conditions of the dough and rapidly multiplies.

2.      Dough mixing: The balance of the ingredients is mixed together with the inoculum to form the dough. Maximum gluten development occurs.

3.      Cutting and rounding: The dough formed above is cut into specific weights and rounded by machines.

4.      First (intermediate) proofing: The dough is allowed to rest for about 15 minutes at about   27°C. This is done in equipment known as an overhead proofer.

5.      Molding: The dough is flattened to a sheet and then moulded and placed in a baking pan which will confer shape to the loaf.

6.      Second proofing: This consists of holding the dough for about 1 hour at 35-43°C at high humidity (89-95°C)

7.      Baking: During baking the proofed dough in the final pan is transferred to the oven where it is subjected to an average temperature of 215-235°C for 15-60 minutes. Baking is the final stage and it determines the success of all the previous steps.

8.      Cooling, slicing, and wrapping: The bread is depanned, cooled to 4-5°C, sliced (optional) and wrapped.

Baking

Bread is baked at a temperature of about 235°C for 45–60 minutes. During baking, temperature of the outside of the bread is about 195°C but the internal temperature never exceeds 100°C. The higher outside temperature leads to browning of the crust, a result of reactions between the reducing sugars and the free amino acids in the dough.  As the baking progresses and temperature rises gas production rises and various events occur as below:

•         At about 45°C the undamaged starch granules begin to gelatinize and are attacked by alpha-amylase, yielding fermentable sugars;

•         Between 50 and 60°C the yeast is killed;

•         At about 65°C the beta-amylase is thermally inactivated;

•         At about 75°C the fungal amylase is inactivated;

•         At about 87°C the cereal alpha-amylase is inactivated;

•         Finally, the gluten is denatured and coagulates, stabilizing the shape and size of the loaf.

The Three Basic Systems of Bread-making

There are three basic systems of baking that differ in the presence or absence of pre-fermentation.

(i) Sponge doughs: This is the most widely used. In the sponge-dough, a portion (60-70%) of the flour is mixed with water, yeast and yeast food in a slurry tank (or ‘ingridator’) during the pre-fermentation.  A spongy material develops due to bubbles caused by alcohol and CO₂ (hence the name). The sponge is allowed to rest at about 27°C and a relative humidity of 75-80% for 3.5 to 5 hours. During this period the sponges rises five to six times and collapses spontaneously. During the next (or dough) stage the sponge is mixed with the other ingredients. Then it is processed and baked.  

(ii) The liquid ferment system. In this system water, yeast, food, malt, sugar, salt and milk are mixed during the pre-fermentation at about 30°C and left for about 6 hours. After that, flour and other ingredients are added in mixed to form a dough. The rest is as described above.

(iii) The straight dough system: In this system, all the components are mixed at the same time until a dough is formed. The dough is then allowed to ferment at about 28-30°C for 2-4 hours and then the same process already describedfollows. The straight dough is usually used for home bread making.

The Chorleywood Bread Process is a modification of the straight dough process, which is used in most bakeries in the United Kingdom and Australia. The process is also known as CBP (Chorleywood Bread Process) where All the components are mixed together in 3-5 minutes, with added Fast-acting oxidizing agents and higher level of yeast added and no pre-fermentation time.

Role of Yeast in Bread-making

Leavening is the increase in the size of the dough induced by gases during bread-making. Leavening may be brought about in a number of ways such as Air or carbon dioxide forced into the dough, Water vapor or steam which develops during baking, Hydrogen peroxide added to release oxygen, Carbon-dioxide released by the use of decarboxylase enzymes or by the use of baking powder. Baking powder consists of 30% sodium bicarbonate mixed with leavening acids such as sodium acid pyrophosphate, monocalcium phosphate, sodium aluminum phosphate, monocalcium phosphate generate CO₂ on contact with water and this is suitable for cakes and other high-sugar leavened foods, whose osmotic pressure is too high for yeasts.

But generally bread is Leavened by yeasts.  During bread making, yeasts ferment hexose sugars mainly into alcohol and carbon dioxide and various other alcohols, esters aldehydes, and organic acids. The CO2 dissolves in the dough and the excess CO2 in the gaseous state begins to form bubbles in the dough. This formation of bubbles causes the dough to rise or to leaven. The total time taken for the yeast to act upon the dough varies from 2-6 hours.

Factors which effect the leavening action of yeasts

(i)                 The nature of the sugar available: When glucose, fructose, or sucrose are added these are utilized and when no sugar is added to the dough, the yeast utilizes the maltose in the flour. Thr most rapid leavening is achievable by using glucose.

(ii)              Osmotic pressure: High osmotic pressures inhibit yeast action. Salt is therefore added as late as possible during the dough formation process.

(iii)            Effect of nitrogen and other nutrients: The addition of minerals and a nitrogen source increases gas production. Ammonium, amino acids and peptides and thiamine act as nitrogen source.

(iv)             Effect on fungal inhibitors: Anti-mycotics added to bread are inhibitory to yeast. So the minimum level inhibitory to yeasts is used.

(v)               Yeast concentration:

Flavor development in bread

The aroma of bread is distinct from all other fermented foods because of the baking process. During baking the lower boiling point molecules escape and new compounds result from the chemical reactions taking place at the high temperature. The flavor compound found in bread are organic acids, esters, alcohols, aldehydes, ketones and other carbonyl compounds.

  

Rye bread and San Francisco sourdough are two distinct artisan bread styles.

Rye Bread

Rye bread is a dense, flavorful type of bread made with various proportions of flour from rye grain. Because rye flour naturally contains less gluten than standard wheat flour, the resulting loaves are typically closer in texture, darker in color, and carry a distinctively earthy, robust flavor profile. It is highly appreciated for its health benefits, offering significantly more dietary fiber and a lower glycemic index than white bread.

Next to wheat, rye is the second most common cereal grain used to make bread. Rye has properties that pose particular challenges when used in bread making.   Unlike wheat (Triticum aestivum), rye (Secale cereale) lacks the protein structure required to form a cohesive, viscoelastic gluten network. The proteins gliadin and glutenin are present in rye, but water-soluble and water-insoluble non-starch polysaccharides called arabinoxylans (pentosans) prevent them from linking effectively.

Rye contains a high concentration of pentosans.  Pentosans are a heterogeneous mixture of pentose-containing polysaccharides consisting mostly of xylose and arabinose.  They constitute as much as 10% of rye flour, which is four to five times more than that found in wheat.  Pentosans have high water-binding capacity and pentosans may interfere with gluten formation, giving an inelastic dough that retains gas poorly.   As a result, breads made with rye as the main grain typically have a small loaf volume and a dense crumb texture. In addition, rye flour contains more amylase than is present in wheat, and this amylase is particularly active at the temperature at which starch gelatinizes. This results in excessive starch hydrolysis in the dough and bread, giving a poor texture and further reducing loaf volume.

The addition of sourdough cultures to rye doughs can compensate for these complications. First, as the pH decreases due to the lactic fermentation, the pentosans become more soluble.   They begin to swell and form a gluten-like network that enhances dough elasticity and gas retention. In other words, at low pH, the pentosans do the role normally performed by gluten. In addition, the sourdough starter culture is stimulated by the availability of free sugars liberated from starch via the amylase. Also, this enzyme begins to lose activity at the low pHs during the sourdough fermentation, so excessive hydrolysis is prevented. Some sourdough bacteria also can ferment pentoses released from pentosans, producing heterofermentative end products, including acetic acid.

San Francisco Sourdough

San Francisco (SF) sourdough fermentation relies on a symbiotic culture of wild yeast (Saccharomyces exiguus) and lactic acid bacteria (Lactobacillus sanfranciscensis). This specific pairing, favoured by cooler local temperatures, produces high levels of acetic and lactic acids, resulting in the bread's signature chewy crumb and tangy flavour.  

San Francisco Sourdough is a world-renowned style of bread defined by its uniquely tangy flavour profile, chewy interior crumb, and deeply caramelised, blistered crust.  This famous bread gets its distinctiveness from local Lactobacillus bacteria (Lactobacillus sanfranciscensis or Fructilactobacillus sanfranciscensis) and wild yeast strains.  A sharp, sour tang characterises its flavour profile, and it has a perfectly crispy, crackly crust with a soft, chewy, and airy texture.

This is traditionally made using unbleached wheat flour, water, salt, and active sourdough starter.  This culinary tradition dates back to the 1849 California Gold Rush, when French immigrant bakers blended European baking techniques with the wild starters.

The evolutionary success of the Kazachstania humilis + Fructilactobacillus sanfranciscensis partnership comes down to a perfect division of food resources, ensuring they never compete with one another. Flour lacks simple sugars but is loaded with maltose. Fructilactobacillus sanfranciscensis aggressively consumes maltose via an enzyme called maltose phosphorylase. Kazachstania humilis is maltose-negative; it completely lacks the metabolic machinery to break down maltose. In a normal ecosystem, this yeast would starve. The bacterium breaks maltose down into glucose and releases the excess glucose into the dough. K. humilis utilises this free glucose to produce carbon dioxide and ethanol. the bacterial and yeast population stabilizes at a perfect 100:1 ratio (bacteria to yeast), allowing the starter to remain viable for centuries.

Property	San Francisco Sourdough	Conventional Bread
Primary Inoculum	Symbiotic wild culture (Lactobacillus sanfranciscensis + Kazachstania humilis)	Saccharomyces cerevisiae
Fermentation time	Long duration (12 to 24+ hours)	Short duration (1 to 3 hours)
pH Range	Acidic environment (pH 3.8 to 4.5)	Near-neutral environment (pH 5.3 to 5.8)

 Conventional bread uses intense mechanical energy (high-speed mixing) or chemical oxidizers (e.g., ascorbic acid) to force glutenin and gliadin proteins into disulfide bonds.  The matrix builds strength quickly but retains a uniform, elastic tension that traps gas in small, identical cells, creating a tight crumb structure. Sourdough dough development relies on biochemical time. Long autolysis windows allow native proteases to gently relax the protein chains. As L. sanfranciscensis generates organic acids, the drop in pH reaches the isoelectric point of wheat gluten. This alters the surface charges on the proteins, reducing their water solubility, increasing dough extensibility, and allowing the crumb to stretch into a wild, open, uneven hole structure.  

Conventional Mixing──► Forced Mechanical Shear ──► Rigid, Uniform Gluten ──► Uniform Closed Crumb

Sourdough Ferment    ──► Acid-Induced Relaxation ──► Extensible Matrix    ──► Irregular Open Crumb

In Sourdough, Phytic Acid (an anti-nutrient that binds tightly to essential minerals like Fe²⁺, Zn²⁺, and Mg²⁺, preventing human absorption) degradation occurs in an acidic environment with a pH of 4.5 to 5.5.  Conventional dough remains at high pH (~5.5) and leaves the phytic acid intact.

The Baking Protocol for San Francisco Sourdough

1. Mix and Autolyse - Mix flour and water and cover and let rest for 45 minutes. This allows enzymes to break down starches into maltose, priming the environment for F. sanfranciscensis.

2. Incorporate Starter and Salt  - add  stiff starter and salt  into the dough, squeeze and knead the dough for 5–7 minutes until it becomes smooth and holds its shape.

3. Temperature-Controlled Bulk Fermentation - Place the dough in a container and maintain a dough temperature of 24°C–26°C which allows K. humilis and F. sanfranciscensis growth. The dough increases in volume by roughly 30% to 50%.

4. Preshape and Bench Rest - Gently work the dough into a loose round shape and let it rest uncovered for 20 minutes to allow the gluten to set.

5. Final Shape and Structural Tension - Dust the top of the dough with flour, flip it over, and shape it, place inside proofing basket

6. Extended Cold Retard (The Flavor Window) - Seal the basket inside a plastic bag to prevent the dough from drying out and immediately transfer to a refrigerator kept at 3°C–5°C and allow to proof for 24 to 36 hours. At this low temperature, the yeast completely stops producing gas, but the bacteria continue to slowly convert maltose into acetic acid, creating the sharp sourdough tang.

7. Score and Bake - Pre-heat a heavy cast-iron oven at 245°C (475°F), take the dough directly out of the refrigerator, drop the cold dough into the hot oven, cover with the lid, and lower the oven temperature to 230°C . Bake covered for 20 minutes to trap steam, and them remove the lid and bake uncovered for an additional 20–25 minutes until the crust develops a deep mahogany color and blistered exterior.

References

1. Industrial Microbiology: An Introduction, M J. Waites, N L. Morgan, J S. Rockey, G Higton
2. Modern Industrial Microbiology and Biotechnology, Nduka Okafor, Science Publishers