proteins, lipids, sugars, and enzymes) that contribute to the consistency and nutritional value of bread.
The part that forms the sticky mass however contains the insoluble proteins (the gliadines and the
glutenins) and is called gluten.
Flour in cooking
Flour is often added to mixtures for two reasons – to thicken a liquid (where the flour starch has an
important role) or to give strength to dough (where the insoluble flour proteins play a role).
Flour as a thickener
How it thickens: When flour as added to a hot liquid to thicken a sauce, the starch granules in the flour
will take up water and start to swell. The liquid molecules entering the granule break up some of the
bonding holding the starch molecules together, opening up the structure. The more water that enters
the granules, the more the granules will open up, so the more water can enter. The sauce will thicken
because firstly the liquid exterior to the granules is reduced, so less “free water” is available to let the
sauce freely flow. Additionally, these swollen starch granules impede the ease with which the liquid
can flow, further thickening the sauce. At around 50-60 °C, known as the gelatinisation temperatures,
the individual granules have swelled so much that lose their previously ordered structure and just
becomes an unorganised mixture of starch and water molecules.
As temperatures are heated further, from about 80°C, the swollen granules will leak amylose and
amylopectin molecules into the surrounding liquid. The long amylopectin molecule is especially
effective at thickening the sauce. At this point the sauce is at its thickest.
Left: raw starch granules. Middle: starch granules that have swollen on heating. Right: starch
molecules released from starch granules after further heating
III/V - 2 (of 3)
Thinning out: As temperatures reach above 93 °C, the sauce will start to thin. Excessive heat for too
long, heating at too high a temperature, or vigourous stirring cause the fragile starch granules to start
to break up into smaller fragments. Although this will cause more amylose to be released, the overall