Transformations during baking
Transformations during baking
Microbiological, colloidal, and biochemical processes develop during the baking phase. Microbiological processes. Fermentation processes have a major evolution during the dough proving phase; during baking, the microorganisms are partially inactivated by the oven temperature.
Only in the core of the product, where the temperature is not so high, a certain microorganisms activity is noted.
In the center, till the temperature reaches 35° C, the yeasts are also active, producing CO2. At 35° C, the alcoholic fermentation is at its maximum.
The yeast activity remains still high up to 40° C, then a significant decrease occurs, and at 50-53°C when the yeast metabolism is stopped.
Mesophilic lactic bacteria are inactivated at 35°C and thermophilic at 54°C, so in the first part of baking, lactic fermentation is still taking place.
Biochemical processes that occur during baking are:
– CO2 and ethyl alcohol production under yeast action ;
– lactic, acetic, and butyric acid production under lactic bacteria action;
– gelatinizing of starch, first in bread crust and then crumb, at 56 – 60°C; the starch is easily attacked by amylase, and as long as they are active, it is converted to dextrins, maltose, and glucose;
– the pentosanes reduction into soluble forms
– at 60-70°C the gluten proteins denaturation begins, and through their coagulation, the bread structure is stabilized
– proteolytic enzymes are resistant to temperature, so active up to 80- 85°C, and convert proteins into peptides, and these in amino acids
– in the initial stage of baking some of the starch-formed sugars are consumed by the yeast; the remainder (2-3% on the dry substance of the product) is caramelized on the crust of the product, and a part is
bound to the previously formed amino acids, forming the melanoidins. These products resulting from the transformation of sugars give the product the crust brown color, taste, and smell so specific to bread.
Influence of temperature on acrylamide content ( Ahrné et. All, 2007)
– in the crumb and crust of bread, volatile flavoring substances are formed, resulting from organic acids and ethyl alcohol, and evaporate giving an intense aroma of the finished product
Colloidal processes.
– gluten transformation – at 30°C gluten shows the highest water absorption capacity; this capacity decreases with increasing temperature, therefore at 60-70 °C the proteins are denatured, releasing the previously absorbed water.
– starch modification; the starch, unlike gluten, increases the water’s ability to absorb water at increasing temperature, reaching a maximum of 50-60°C, when the starch gels and its grains are broken.
Absorbing a lot of water, the granules inflate progressively, creating an internal pressure that destroys the protective coatings.
Therefore, in the temperature range of 50-70°C, there are simultaneous processes of protein denaturation and starch gelatinization, which explains how the dough is transformed into the crumb of bread.
During baking, air cells are inflated even more by the increase in the pressure of gases, vapor pressure of water, and additional CO2 leaving the water phase due to decreased solubility.
This is the oven spring. At some point during baking air cells rupture as the elastic properties of cell walls cannot cope with the inflation.
This gives the fine interconnected cell structure of bread. Many variations in baking performance and volume that have been noticed in aged flours have been assigned to alteration like native lipids due to oxidation and
the action of lipases.
Two well-known problems in bread making have been linked to variations in native lipid nature: lack of oven spring while proofer volume is fine and appearance of blisters on the surface of baked bread.
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