Retrogradation (starch)
Encyclopedia
Retrogradation is a reaction that takes place in gelatinized starch
when the amylose
and amylopectin
chains realign themselves, causing the liquid to gel.
When native starch
is heated and dissolves in water, the crystalline structure of amylose and amylopectin molecules are lost and they hydrate to form a viscous
solution. If the viscous solution is cooled or left at lower temperature for long enough period, the linear molecules, amylose, and linear parts of amylopectin molecules retrograde and rearrange themselves again to a more crystalline structure. The linear chains place themselves parallel and form hydrogen bridges
. In viscous solutions the viscosity increases to form a gel. At temperatures between –8 and +8 °C the aging process is enhanced drastically.
Retrogradation can expel water from the polymer network. This is a process known as syneresis
. A small amount of water can be seen on top of the gel. Retrogradation is directly related to the staling
or aging of bread
Retrograded starch is linked to a reduction in colon cancer.. Retrogradated starch is less digestible (see resistant starch
).
Chemical modification of starches
can reduce or enhance the retrogradation. Waxy, high amylopectin, starches have also much lesser tendency to retrogradate. Also additive as fat
, glucose
, sodium nitrate
and emulsifier can reduce retrogradation of starch.
Starch gelatinization
Starch gelatinization is a process that breaks down the intermolecular bonds of starch molecules in the presence of water and heat, allowing the hydrogen bonding sites to engage more water. This irreversibly dissolves the starch granule...
when the amylose
Amylose
Amylose is a linear polymer made up of D-glucose units.This polysaccharide is one of the two components of starch, making up approximately 2-30% of the structure...
and amylopectin
Amylopectin
Amylopectin is a soluble polysaccharide and highly branched polymer of glucose found in plants. It is one of the two components of starch, the other being amylose.Glucose units are linked in a linear way with α glycosidic bonds...
chains realign themselves, causing the liquid to gel.
When native starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
is heated and dissolves in water, the crystalline structure of amylose and amylopectin molecules are lost and they hydrate to form a viscous
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
solution. If the viscous solution is cooled or left at lower temperature for long enough period, the linear molecules, amylose, and linear parts of amylopectin molecules retrograde and rearrange themselves again to a more crystalline structure. The linear chains place themselves parallel and form hydrogen bridges
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
. In viscous solutions the viscosity increases to form a gel. At temperatures between –8 and +8 °C the aging process is enhanced drastically.
Retrogradation can expel water from the polymer network. This is a process known as syneresis
Syneresis (chemistry)
Syneresis , in chemistry, is the extraction or expulsion of a liquid from a gel, as when lymph drains from a contracting clot of blood. Another example of syneresis is the collection of whey on the surface of yogurt...
. A small amount of water can be seen on top of the gel. Retrogradation is directly related to the staling
Staling
Staling, or "going stale", is a chemical and physical process in bread and other foods that reduces their palatability. Stale bread is dry and leathery.-Mechanism and effects:...
or aging of bread
Retrograded starch is linked to a reduction in colon cancer.. Retrogradated starch is less digestible (see resistant starch
Resistant starch
Resistant starch is starch and starch degradation products that escape digestion in the small intestine of healthy individuals. Resistant starch is considered the third type of dietary fiber, as it can deliver some of the benefits of insoluble fiber and some of the benefits of soluble fiber.Some...
).
Chemical modification of starches
Modified starch
Modified starch, also called starch derivatives, are prepared by physically, enzymatically, or chemically treating native starch, thereby changing the properties of the starch. Modified starches are used in practically all starch applications, such as in food products as a thickening agent,...
can reduce or enhance the retrogradation. Waxy, high amylopectin, starches have also much lesser tendency to retrogradate. Also additive as fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...
, glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, sodium nitrate
Sodium nitrate
Sodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...
and emulsifier can reduce retrogradation of starch.