Flour – Protein and Starch
To this point, denaturation has been outlined for thermal and chemical techniques; however, mechanical denaturation of proteins via the application of shear is also important in the extrusion of breakfast cereals and snacks like Cheetos and kneading flour and water, essential in dough formation. Flour is mostly starch (> 80%) with a lesser extent of proteins (5-16% depending on cultivar) and obtained from numerous sources, including corn, rice, wheat, pea and many others.
However, their function differs greatly by source, and most are unsuitable for dough manufacturing, which is restricted to wheat, rye and barley. Focusing on wheat flour as it is the most widely cultivated cereal; it is refined to varying degrees creating different purities of starch and protein. Wheat kernels are comprised of an outer bran layer rich in fiber and micronutrients, the germ, which is 10% lipid, protein and micronutrients, and the endosperm contains >80 starch, <10% protein and is devoid of lipids and some of the micronutrients. Only in wholegrain and wholemeal flours are the bran and germ remaining and are less popular as the lipid in germ oxidizes, and the bran is an insoluble and unfunctional component
Flour is available in various types depending on the amount of protein and starch, which have different end uses and range from hard wheat, very high in protein (13-16%), to soft wheat, used for low-in-protein pastry (8-10%) and cake (5-8%) flours. Hard wheat, such as Durham wheat or semolina (coarsely milled durum wheat), is made into pasta, especially dried pasta, couscous, and puddings. Pasta requires a strong elastic network that, upon boiling, retains its shape and uses 1 part flour to 2 eggs for additional protein. Bread flour (12-14%) and 00 flour (11-13%), specific for pizza dough, require a strong viscoelastic dough.
Gliadin & Glutenin are Kneaded into Gluten Dough
The two primary proteins in wheat are gliadin, a globular protein containing intramolecular disulfide bonds, and the intermolecular disulfide cross-linked polypeptide, glutenin, which is high in molecular weight due to multiple polypeptide chains being cross-linked by covalent disulfide bonds. Glutenin is not a monomer; polypeptide chains are cross-linked by disulfide bonds adopting fibrillar tertiary (3o) and quaternary (4o) protein structures that provide unique viscoelastic properties. When wheat flour is mixed with water and kneaded (folded by hand), the globular proteins unfold, and new disulfide bonds form upon protein denaturation, which exposes Cys and Met, allowing them to partake in new covalent disulfide cross-linkages forming a continuous viscoelastic protein network between gliadin and glutenin which forms gluten the elastic network which embeds starch granules and holds air bubbles during rising through baking.
Long and Short Doughs
This stretchy dough is referred to as long dough. The long elastic dough made from hard wheat cannot substitute cake flour as the dough will lack the elasticity required to support the structure during baking and collapses with low air volume (fewer larger bubbles). Disulfide bond formation in wheat proteins determines the physical properties and protein functionality. The long stretchy dough is transformed into a short crumbly dough by the addition of shortening, which is any solid fat at room temperature and typically includes lard, vegetable shortening or butter.
The solid fat is interspersed in the gluten network preventing the formation of disulfide bonds, and the elastic nature is reduced because the fat crystals, in effect, ‘shorten’ the gluten network. Hence, incorporating butter into the water and hard flour results in a flakey and crumbly biscuit compared to chewy bread. Cake and pastry soft flours (5-8% protein), used in bakery products such as pies and puff pastries, require a less coherent protein network, allowing flour protein content as low as 5%. Additionally, the flour water butter mixture is not processed in the same fashion, as these doughs are done forming when the ingredients are mixed uniformly and not kneaded in the same way as bread or pizza dough. The characteristic crumble of pie crust and puff pasty is achieved by the low protein content coupled with more shortening fat than water and less kneading resulting in a short dough.
Lamination of Croissants
Croissants, the final flour product to be discussed, requires lamination, which rolls the dough into a thin sheet and covers it in a layer of shortening before folding it. Once folded, the new dough surface is again covered in a layer of shortening and, once again, is folded, exposing a new dough surface and the process is repeated numerous times with intermittent rolling into a thin sheet as it thickens during folding. Lamination uniquely applies fat to the dough, creating a gluten network that is shortened more in one direction (vertically), creating highly layered croissants. It is remarkable to think that the same ingredients in different ratios can be formulated into so many diverse products, and – this is just one cereal.