Physical and Mechanical leavening

Physical

By physical leavening we talk about the availability of liquids. What happens is upon heating of bakery products during proofing or baking water and other bound or unbound moisture will expand: the density is changing. When the dough becomes warm enough, water will start to escape from starch and protein cells inside the dough and turns into water vapour or steam. Upon continuous heating this steam will expand, until it escapes the dough matrix or is assimilated by one of the starch or protein cell upon gelatinising or denaturing. During this stage of expansion and fixation of the structure (gelatinising and denaturing) the density continuous to change, gas expands, cells expand and volume increases. In certain cases it can even lead to an increase of close to 15% volume increase, this however only happens in product stabilisation phase of baking: so a certain amount of that volume you will loose upon product cooling.

Mechanical

With mechanical leavening we talk about the incorporation of air. By mechanical force air can be incorporated during the preparation stage: mixing, stirring and whisking. By this actions the density of the dough will change and become lighter: a phenomenon that is very clear when working with foam cakes.

By having a certain amount of air inside bakery doughs, this air will expand upon heating/ warming and happen at an earlier stage than moisture turning into steam. By this means an initial expansion of gas (air) is set and cells can increase: this will cause flowing of dough (until it is halted by a baking shape or fixation by temperature of the dough) and increase of volume. As air is more volatile than water, this expansion will happen earlier and therefore in an earlier stage of the baking process.

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