Change of biscuit embryo during baking
There are three main changes you can see in a cookie embryo when it's baked:
(1) The product density decreases greatly, accompanied by the formation of sparse porous structure.
(2) the content was reduced by 1%~4%
(3) Surface color changes.
Although the nature of these changes is considered to be inconsistent,when the product passes through the oven, it mainly occurs in the above order, but it can also be seen that these physical and chemical changes have considerable crossover and overlap
Formation of structure
This occurs mainly in the first quarter or third of the baking process.These variations are temperature dependent and involve several aspects of the recipe and the shape of the cookie billet.Bubbles of gas and water vapor form, which expand, causing the dough to become much less dense.The sparse porous structure gives cookies a tasty texture.The formation of structure is often referred to as oven swelling.Continue until the end of baking the outer surface of the cookie crust brown dextrin formation.
The conditions for a large bulge are not fully understood, but variations involving the preform include:
· Heat starch and protein to expand, gelatinize, denaturate and solidify;
· Release of gas from chemical dispersing agent;
· Rising temperature expands the bubbles and increases the pressure of water vapor in the bubbles;
· Partial bubble bursting and merging
· Product surface water is lost by evaporation, accompanied by water migration to the surface and continued release in oven in the atmosphere
· The concentration of sugar solution increases with the increase of temperature;
· The consistency of sugar solution and fat decreases with increasing temperature.
Water reduced
Ideally, water loss should occur after the structure is fixed, but obviously this cannot be done in an entire biscuit.Water is lost only on the surface of the cookie, so water must migrate to the surface by capillary action and diffusion.These two phenomena can be accelerated by temperature gradients, so it is necessary to rapidly heat the entire product to 100 ° C during the baking phase.If the surface is heated too much and dries too quickly, the color changes prematurely and it becomes different to dry the biscuit sufficiently later without excessive coloring.
As starch and protein gels lose moisture, some shrinkage occurs, so some loss in product flaring or oven swelling is inevitable.In most cases, this is small compared to the collapse of the internal structure caused by bubble bursting or merging, but it is worth noting that this shrinkage continues until the cookie is completely burnt.
The moisture gradient of the biscuit billet increases during drying, and the starch/protein structure shrinks as the biscuit structure dries.Cookies are flexible enough to withstand these crumpling stresses when hot, but if there is a significant moisture gradient after the cookie leaves the oven, a phenomenon called "cracking" can occur.As the cookie cools and the water tends to equilibrate (moving from the wetter to the drier), the resulting crumpling stress may cause cracking, which is known as cracking.The best way to prevent cracking is to ensure that the total moisture content of the cookie is low and the moisture gradient is small.
Products with high fat or sugar to form a more plastic structure, cookies cooling stress is not obvious, so it is more important to control the content of thin crackers, sugar cake than its class.The ideal moisture content of a cookie is determined by two main factors.Too little moisture and the biscuits will smell burnt and get too dark in colour.
Color change
Although cookies may turn brown and yellow during baking, the color change here refers only to a darkening of the surface and a reduction in reflectance.The change in color is caused by several reasons.The Maillard reaction, a non-enzymatic Browning that involves an interaction between reducing sugars and proteins, produces an attractive brown-red color.This reaction occurs at 150~160°C，and only in the presence of moisture.It is not possible to reheat a baked biscuit so that the surface of the Maillard reaction can effectively add color.It can be seen that radiation heating is more important than convection heating for the biscuit surface to reach high temperature when it is relatively humid.Color formation is also often accompanied by dextrinization of starch and caramelization of sugar.When the temperature is higher, the cookies will burn.
It can also be seen that if the cookie structure is very sparse, the water migration to the surface is slow, so the surface temperature is locally increased, so it is easier to color.Therefore, a puff dough with a high degree of expansion is easier to color than a puff dough with a dense structure.Excess base, usually caused by too much sodium bicarbonate in the recipe, can turn the entire biscuit structure slightly yellow, a product that is extremely undesirable when no other colour is present.
