Retrogradation is a reaction that takes place when the
amylose
Amylose is a polysaccharide made of α-D-glucose units, bonded to each other through α(1→4) glycosidic bonds. It is one of the two components of starch, making up approximately 20–25% of it. Because of its tightly packed Helix, helical struct ...
and
amylopectin Amylopectin is a water-insoluble polysaccharide and highly branched polymer of α-glucose units found in plants. It is one of the two components of starch, the other being amylose.
Plants store starch within specialized organelles called amyloplas ...
chains in cooked,
gelatinized starch realign themselves as the cooked
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diet ...
cools.
When native starch is heated and dissolved in water, the crystalline structure of amylose and amylopectin molecules is lost and they hydrate to form a
viscous
Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...
solution. If the viscous solution is cooled or left at lower temperature for a long enough period, the linear molecules, amylose, and linear parts of amylopectin molecules retrograde and rearrange themselves again to a more
crystalline structure
In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...
. The linear chains place themselves parallel and form
hydrogen bridges.
In viscous solutions the viscosity increases to form a gel. At temperatures between and , the aging process is enhanced drastically. Amylose crystallization occurs much faster than crystallization of the amylopectin. The crystal melting temperature of amylose is much higher (about ) than amylopectin (about ). The temperature range between cooking starch and storing in room temperature is optimum for amylose crystallization, and therefore amylose crystallization is responsible for the development of the initial hardness of the starch gel. On the other hand, amylopectin has a narrower temperature range for crystallization as crystallization does not occur at a temperature higher than its melting temperature. Therefore, amylopectin is responsible for development of the long-term crystallinity and gel structure.
Retrogradation can expel water from the polymer network. This process is 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 a type of
resistant starch
Resistant starch (RS) is starch, including its degradation products, that escapes from digestion in the small intestine of healthy individuals. Resistant starch occurs naturally in foods, but it can also be added as part of dried raw foods, or u ...
.
Chemical
modification of starches can reduce or enhance the retrogradation. Waxy, high amylopectin, starches also have much less of a tendency to retrogradate. Additives such as
fat
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers specif ...
,
glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
,
sodium nitrate
Sodium nitrate is the chemical compound with the chemical formula, formula . This alkali metal nitrate salt (chemistry), salt is also known as Chile saltpeter (large deposits of which were historically mined in Chile) to distinguish it from ordi ...
and
emulsifier
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althou ...
can reduce retrogradation of starch.
See also
*
Retrograde (disambiguation)
*
References
External links
*
Starch
Food science
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