7.1 What steps could be taken to prevent round doughnuts shrinking or collapsing within a few minutes of leaving the
The collapse of doughnuts after they leave the fryer is associated with the transition of the dough from a 'foam' to a 'sponge' (in the generic not specific cakemaking sense). In foams the gas bubbles or cells are separated from one another by a stabilising film of one form or another, e.g. protein, fat, emulsifier, while in a sponge the cells are interconnected and gases and liquids can readily pass through the matrix (see Fig. 21).
In the oven most baked products undergo the transition from foam to sponge and the gas contained within the individual cells diffuses out of the product into the surrounding atmosphere. The transition occurs at different parts of the dough piece at different times, depending on the heat transfer rate. Before conversion the pressure inside the air bubbles is greater than that of the surrounding atmosphere and this contributes to inflation of the product. At the moment of conversion from foam to sponge there is an equalisation of the gas pressure with fryer?
Fig. 21 Transition from foam to sponge during baking.
that in the surrounding atmosphere and this is seen as a slight shrinking of the volume of the baked product just before the end of baking.
The addition of some ingredients can delay the conversion of the foam to a sponge to a later time during the baking process. Sugar, in the form of sucrose, delays the gelatinisation of the wheat starch and subsequently the temperature at which the foam to sponge conversion is made. In some formulations the transition may not take place within the centre of the product before baking is completed. As a consequence the foam remains intact and as the temperature within the cells falls so does the internal pressure. At some point the external pressure on the product becomes greater than the internal pressure of the cells and the product shrinks. This is the collapse and wrinkling of the product that you see.
To avoid the problem you should reduce the sugar level in the formulation or reduce the ability of the dough to retain gas. It is well known that a mechanical shock delivered to many such products can be used to eliminate the problem, so simply banging the trays as you remove them from the fryer can reduce or even eliminate this problem.
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