Pyler says: Starch gels at baking temperatures
May 3, 2016
by E.J. Pyler and L.A. Gorton
Interactions with water define the chief aspects of starch functionality in all types of baked foods. When starch gelatinizes and pastes, it binds much more water than it does in its native state. During baking there is a net movement of water from the hydrated gluten to the gelatinized starch.
As gas bubbles within the dough expand and eventually rupture, the starch gel surrounding these bubbles increases in viscosity to aid formation of the final structure of the bread or other baked food. This gelatinization affects the crumb structure. During cake baking, structural setting also depends on starch gelatinization.
Other ingredients in the formula will interact with starch. Surfactants such as sodium stearoyl lactylate and monoglycerides will raise the pasting temperature of starch, increase viscosity and retard or prevent gel formation. Another emulsifier, diacetyl tartaric acid esters of monoglycerides, commonly called DATEM, raises starch’s gelation temperature but does not change the viscosity.
Starches compete with sugar for water in formulations. If the formula contains 50% sugar, the starch will be unable to change the mixture’s viscosity, and there will not be enough water available for gelatinization. Starch requires a minimum of 25 to 30% water to gelatinize and swell under normal processing conditions and temperatures.
Acid ingredients decrease paste viscosity and gel strength by cutting the length of starch chains. Fats and oils lower the temperature of maximum viscosity but not the viscosity itself. Salts have little effect, but amylase enzymes will break down starch molecules drastically.
Gelatinization is essential to unlocking starch’s functionality in foods, especially baked foods. During gelatinization, the hydrogen bonds that hold the micelles and granules together begin to weaken. As such bonds break, the tightly bound granules lose their structure and allow water to penetrate. The granules swell, and some of the starch leaches out.
The endothermic process known as gelatinization uses water to plasticize the crystalline starch. As the starch changes phase from crystalline to amorphous, it goes through a “glass” transition, glass being the scientific term for a uniform but amorphous mixture lacking a regular crystal lattice structure. With starch, the glass transition describes its change from one phase (opaque) to another (clear). If the quenching or rapid cooling temperature (T) drops below its glass transition temperature (Tg), the material becomes “glassy” and if above that temperature, “rubbery.” Starches from different sources have different ranges for T to Tg, and this difference will guide their use in food products (Patil 1991).
Gelatinized starch will retrograde over time, losing moisture and shrinking, thus causing baked foods to stale. The term “setback” is sometimes used to describe retrogradation.
During gelatinization, some amylose molecules leak out of the granules, either by rupturing completely or by seeping out of the granule at its equatorial groove (Tester and Morrison 1990). Upon cooling, amylose reassociates, either as a precipitate in environments with low starch concentration (<3%) or as a gel network if the starch concentration is high. The amylopectin remains behind, forming a skeleton of sorts to support the swollen granule. The same researchers reported that swelling is a property of amylopectin, although amylose when complexed with lipids can inhibit swelling. The recrystallation of the amylopectin, and the corresponding collapse of the granule, are recognized as an important cause of staling. Amylopectin crystallization is reversible upon heating to 100°C (212°F), restoring the paste state of starch.
But retrogradation of starch has other effects. It increases the viscosity, opacity and turbidity of the starch paste. It causes a skin to form on the surface and prompts syneresis, or weeping, of the paste. A variety of factors affect the retrogradation process including the starch concentration, its ratio of amylose to amylopectin, its vegetable source, cooking procedure, temperature, storage time, pH, cooling procedure and the presence of other ingredients, notably enzymes.
Bread firming and starch recrystallization are not synonymous although both occur during bread storage. Although the role of starch in firming has been extensively studied, protein plays a role in this reaction as well. Martin et al. (1991) proposed that bread firming results from hydrogen crosslinks between the continuous protein matrix and the discontinuous remnants of starch granules. During staling, as the crumb loses kinetic energy, the crosslinking interactions increase in number and strength.
Martin, M.L., Zeleznak, K.J., and Hoseney, R.C. 1991. A mechanism of bread firming. I. Role of starch swelling. Cereal Chem. 68 (5): 498.
Patil, S.K. 1991. Starches in bakery foods. AIB Tech. Bull. 13 (6).
Tester, R.F., and Morrison, W.R. 1990. Swelling and gelatinization of cereal starches. I. Effects of amylopectin, amylose and lipids. Cereal Chem. 67 (6): 551.
More on this topic can be found in “Baking Science & Technology, 4th ed., Vol. I,” Page 355, by E.J. Pyler and L.A. Gorton. Details are at in our store.