Predough
Gluten proteins affect the water absorption and viscoelastic properties of the predough. The role of proteins can be divided into two stages of dough formation: hydration and deformation. In the hydration stage, gluten proteins absorb water up to two times their own weight. In the deformation or kneading stage, the action of mixing causes the gluten to undergo a series of polymerization and depolymerization reactions, forming a viscoelastic network. Hydrated glutenin proteins in particular help form a polymeric protein network that makes the dough more cohesive. On the other hand, hydrated gliadin proteins do not directly form the network, but do act as plasticizers of the glutenin network, thus imparting fluidity to the dough’s viscosity.
Starch also affects the viscosity of predough. At room temperature and in a sufficient amount of water, intact starch granules can absorb water up to 50% of their own dry weight, causing them to swell to a limited extent. The slightly swollen granules are found in the spaces between the gluten network, thus contributing to the consistency of the dough. The granules may not be intact, as the process of milling wheat into flour damages some of the starch granules. Given that damaged starch granules have the capacity to absorb around three times as much water as undamaged starch, the use of flour with higher levels of damaged starch requires the addition of more water to achieve optimal dough development and consistency.
Water content affects the mechanical behavior of predough. As previously discussed, water is absorbed by gluten and starch granules to increase the viscosity of the dough. The temperature of the water is also important as it determines the temperature of the predough. In order to facilitate processing, cold water should be used for two main reasons. First, chilled water provides a desirable environment for gluten development, as the temperature at which mixing occurs impacts the dough’s hydration time, consistency, and required amount of mixing energy. Secondly, cold water is comparable to the temperature of the roll-in fat to be added later, which better facilitates the latter’s incorporation.
In-dough fat affects the texture and lift of predough. Although higher levels of dough fat may lower dough lift during baking, it also correlates with a softer end product. As such, the main function of in-dough fat is to produce a desirable softness in the final croissant.
Lamination
In laminated croissant dough, the gluten network is not continuous. Instead, the gluten proteins are separated as thin gluten films between dough layers. The formation of thin, well-defined layers affects the height of dough lift. Generally, laminated croissant dough contains fewer layers than other puff pastry doughs that do not contain yeast, due to the presence of small bubbles in the gluten sheets. Upon proofing, these bubbles expand and destroy the integrity of the dough layers. The resulting interconnections between different dough layers would over-increase dough strength and allow water vapor to escape through micropores during baking, consequently decreasing dough lift. The role of fat also influences the separation of layers, as will be discussed next.
Roll-in fat affects the flakiness and flavor of the croissant. In laminated dough, fat layers alternate with dough layers. As such, the most important function of roll-in fat is to form and maintain a barrier between the different dough layers during sheeting and folding. As previously stated, the ability for fat to maintain separation between folded dough layers ensures proper dough lift.
The type of roll-in fat used is typically butter or margarine. Butter and margarine are both water-in-oil emulsions, composed of stabilized water droplets dispersed in oil. While butter is appealing due to its high consumer acceptance, its low melting point, 32 °C, actually makes it undesirable for production purposes. The use of butter as roll-in fat during the lamination step will cause problems of oiling out during sheeting and fermentation if the temperature is not tightly controlled, thus disrupting the integrity of the layers. On the other hand, kinds of margarine are commonly used as roll-in fat because they facilitate dough handling. Generally, roll-in margarine should have a melting point between 40 °C and 44 °C, at least 3 °C higher than the fermentation temperature to prevent oiling out prior to baking. It is also important to consider the plasticity and firmness of the roll-in fat, which is largely determined by its solid fat content. Generally, a greater proportion of solid fat coincides with larger croissant lift. At the same time, the roll-in fat should have plasticity comparable to that of the dough, such that the fat layers do not break during sheeting and folding. If the fat is firmer than the dough, then the dough can rupture. If the fat is softer than the dough, then it will succumb to the mechanical stress of sheeting and potentially migrate into the dough.
Fermentation
Croissants contain yeast, Saccharomyces cerevisiae, which is incorporated during predough formation. When oxygen is abundant, the yeast breaks down sugar into carbon dioxide and water through the process of respiration. This process releases energy that is used by the yeast for growth. After consuming all of the oxygen, the yeast switches to anaerobic fermentation. At this point, the yeast partially breaks down sugar into ethanol and carbon dioxide. Once CO2 saturates the dough’s aqueous phase, the gas begins to leaven the dough by diffusing to preexisting gas cells that were incorporated into the predough during mixing. Yeast action does not produce new gas cells, as the immense pressure required for a single CO2 molecule to create a new gas bubble is not physically attainable.
In order to ensure the flaky texture of the croissant, it is important to balance the yeast activity with steam production. If the yeast overproduces CO2, then the well-defined layers may collapse. During the baking process, this would cause steam to escape too early from the bread, reducing dough lift and flakiness of the final product. Thus, to offset the negative effects of yeast on layer integrity and dough lift, croissants usually contain fewer layers than other puff pastries.
Baking
During baking, the transient gluten network turns into a permanent network. At higher temperatures, intermolecular disulfide bonds form between glutenin molecules, as well as between gliadin and glutenin. With more bonds being made, the gluten network becomes more rigid, strengthening the croissant’s crumb texture. Additionally, the baking process significantly stretches the dough layers due to the large macroscopic deformation that occurred during fermentation’s dough lift.
Starch undergoes gelatinization as a result of baking. Prior to baking, starch granules absorb a small amount of water at room temperature as it is mixed with water to form predough. As long as the dough’s temperature stays under the gelatinization temperature, this granule swelling is limited and reversible. However, once the baking process begins and the dough is exposed to temperatures above the gelatinization temperature, amylopectin crystallites become more disordered inside the starch granules and cause an irreversible destruction of molecular order. At the same time, starch gelatinization actively draws water from the gluten network, further decreasing the flexibility of the gluten. Currently, the extent of amylose leaching and granular structure distortion during the baking of croissants is still unknown.
Roll-in fat gradually melts as the temperature in the oven increases. Some of the melting fat can migrate into the dough, which could then interfere with gluten protein crosslinking. The fat phase also contributes to dough lift through gas inflation, which will be described next.
Water is converted to steam during the baking process, which is the main factor behind the leavening of the dough. The water for steam production comes from both the dough layers and the roll-in fat. As the fat melts, the continuous oil phase is no longer able to stabilize the water droplets, which are then released and converted to steam. Although the exact mechanism of steam entrapment is still unclear, it is likely a result of both steam expanding inside each dough layer and steam migrating to oil layers, where it inflates gas bubbles. The steam migration to oil phase is likely due to the smaller pressure differential required to inflate a bubble of steam in liquid fat than in solid dough. As the concentration of steam increases between dough layers, the increased pressure causes the dough to lift. It is important to note that during the entire baking process, only half of the water vapor contributes to dough lift, as the other half is lost through micropores and capillaries of interconnected dough layers.
Storage
The effect of gluten proteins during cooling and storage is still unclear. It is possible that gluten proteins influence croissant firming through the loss of plasticizing water, which increases the stiffness of the gluten network.
Starch plays a major role in the degradation of croissants during storage. Amylopectin retrogradation occurs over several days to weeks, as amorphous amylopectin chains are realigned into a more crystalline structure. The transformation of the starch causes undesirable firmness in the croissant. Additionally, the formation of the crystal structure of amylopectin requires the incorporation of water. Starch retrogradation actively draws water from the amorphous gluten network and some of the amorphous starch fraction, which reduces the plasticity of both.
Water migration influences the quality of stored croissants through two mechanisms. First, as previously stated, water redistributes from gluten to starch as a result of starch retrogradation. Secondly, during the baking process, a moisture gradient was introduced as a result of heat transfer from the oven to the croissant. In fresh croissants, there is high moisture content on the inside and low moisture content on the outside. During storage, this moisture gradient induces water migration from the inside to the outer crust. On a molecular level, water is lost from the amorphous starch fraction and gluten network. At the same time, water diffuses from the outer crust to the environment, which has less moisture. The result of this redistribution of water is a firming up of the croissant, caused by a decrease in starch plasticity and an increase in gluten network rigidity. Due to the presence of large pores in croissants, moisture is lost to the environment at a faster rate than bread products. As such, croissants generally become harder in texture at a faster rate than breads.
Fat also affects the quality of croissants in storage. On one hand, an increased amount of in-dough fat has been found to correspond to a reduction in crumb hardness immediately after baking. This is likely attributed to the high-fat content of croissants, as increased fat levels decrease moisture diffusion. On the other hand, although roll-in fat softens the croissant’s initial crumb, its effect on croissant hardness during storage is still unclear.
