When choux paste is being baked any air that has been beaten into the paste will expand and the water will be converted to steam. The expanded air and steam try to escape but to a large extent are prevented from doing so because both are trapped and retained by the gluten matrix and uncoagulated film of egg albumen. The egg albumen (protein) is extensible and will be inflated and distended by the internal pressures of the gases - air and steam.
Expansion of the paste only ceases when the egg albumen coagulates and both it and the gluten film lose their extensibility and gas-holding powers. Egg albumen coagulates at high temperatures and loses its extensibility and so the temperature of the paste at which the eggs are added is an important factor in getting maximum volume. The presence of strong films of uncoagulated egg protein in the paste when it goes into the oven is of the utmost importance for the achievement of good volume in the baked product. If the eggs are added to the paste before it has been allowed to cool adequately the fluidity of the eggs is quickly lost and the penalty is lower volume. Pastes can be left to cool to 2441 °C (75-106 °F) naturally, by stirring with the beaters of the machine or by being spread onto a cold clean slab or tabletop. An economic advantage of cold paste is that it requires the addition of less egg and will still give a good volume product.
The consistency of the paste for choux pastry is another of the critical factors in controlling the volume of choux products. It also plays a prominent role in the appearance of the product. Ideally the paste should be as soft as possible but without causing the resultant pastry case to be of poor shape.
If the paste is too stiff when making eclairs they become unattractive in appearance and exhibit harsh surface cracks and breaks. If the paste is too soft they need more baking to dry them out otherwise they will collapse on being taken from the oven. Even if they are adequately dried out to prevent them from collapsing they will have a lower volume and look squashed in appearance. When the consistency is correct they are beautifully rounded, and have no harsh breaks, bursts or cracks to detract from their appearance.
For cream bun shells, the more they crack and burst, the better the product appears. To obtain this cracked shape the paste can be made a little softer than for eclairs. Again, however, if the paste is made too soft the buns will be of poor shape and can collapse. So although it is a good plan to make choux paste slightly softer when making cream buns, the difference is only slight and must not be overdone. When baking choux buns it is essential to create steam under sufficient pressure to aerate or inflate them as fully as possible before the coagulation of the proteins and before surface drying or crust formation occurs. This is achieved by baking the paste in a comparatively hot oven at 227 °C (440 °F), so that steam pressure is built up rapidly to expand the paste and to hold it in that condition until the coagulation of the proteins and the crust formation.
Frozen and spray-dried egg as well as liquid eggs are regularly used in the production of choux pastry. Care must be taken when using spray-dried egg that the egg has not been kept too long since in storage egg proteins are degraded through the activity of bacteria. Frozen eggs should be defrosted before use. Some adjustment in recipes may be required when using frozen egg as there may have been some change in egg viscosity as a result of the freezing/thawing operation.
See also 12.2.
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