Starches in Food
Introduction
Starch is a plant polysaccharide stored in roots and seeds of plants, and in the endosperm
of a grain kernel. It provides humans with energy (4 cal/g), and is hydrolyzed into glucose,
supplying the glucose that is necessary for brain and central nervous system functioning.
Starch grains, or granules, contain long-chain glucose polymers that are insoluble in
water. Unlike the small molecules of salt and sugar, the larger starch polymers do not
form a true solution. Instead, starch granules form a temporary suspension when
stirred in water. As uncooked granules, each may swell slightly when it absorbs water.
However, once starch is cooked, the swelling is irreversible and the starch
leaches out. This characteristic of starch granules enables starch to be used as a
thickener.
Overall, the characteristics of a finished starch food product are determined by several
factors:
a) the source of starch, concentration of starch used in a formulation
b) the temperature and time of heating
c) other components used with the starch, such as acid and sugar.
There are many types of starch and modified starches. These thicken, prevent curdling,
and stabilize cooked salad dressings, dips, gravies, desserts, and more. Intermediate,
shorter chain products from starch breakdown, known as dextrins, may be used to
simulate fat in salad dressings and frozen desserts. For example, wheat, potato, and
tapioca maltodextrins may be used as fat replacers. These provide the viscosity and
mouthfeel of fat in a food product, yet, with reduced calories compared to fat.
Starch Sources in the Diet
Starch sources are numerous, with common ones derived from cereal grains such as
wheat, corn, or rice. Wheat yields a cloudy, thick mixture, while cornstarch produces
more clear mixtures such as gravies or sauces. Vegetables, roots and tubers, including
the root of cassava, and potatoes, are frequently used in the preparation of gluten-free
1
,foods, where persons with wheat allergies or intolerances do not use any wheat as a
thickener. Specialty starches are available commercially and some may be available to
the consumer, perhaps purchased through specialty food stores.
Another source of starch is legumes such as soybeans or garbanzo beans. As well, sago
is a powdery starch obtained from the stems and trunks of the sago palm in tropical
Asia. Sago may be used as a food thickener as well as a fabric stiffener. Fruits such as
bananas may also be sources of starch.
Thus it may be seen that starch may come from a variety of sources. Depending on the
source, starches may also have different crystal-line structures.
Starch Structure and Composition
The starch granules from various grains differ in size, ranging in size from 2 to 150 μm.
The shape of starches may also vary—being round or polygonal, as seen in the
photomicrographs of corn, wheat, and waxy maize.
Starch is made up of two molecules, amylose and amylopectin, whose parts are
connected by glycosidic linkages. Amylose molecules typically make up approximately
one quarter of starch. Amylose is a long linear chain composed of thousands of glucose
units with attachment of the carbon 1 and carbon 4 of glucose units, and therefore
contains α-1,4 glycosidic linkages. It forms a three-dimensional network when
molecules associate upon cooling, and is responsible for the gelation of cooked, cooled
starch pastes. While those starches with a high amylose content are able to gel, or hold
their shape when molded, starches without amylose thicken, although do not gel.
Examples of the amylose content of various starch sources include:
Cereal grains—26–28 % amylose
Roots and tubers—17–23 % amylose
Waxy varieties of starch—0 % amylose
Amylopectin molecules (Chap. 3) constitute approximately three-quarters of the
polymers in a starch granule. The glucose chain of amylopectin contains α-1,4 linkages,
similar to amylose, how- ever, with α-1,6 branching at every 15–30 glucose units of the
chain.
2
, There is a linkage between the carbon 1 of the glucose and carbon 6 of the branch in
amylopectin. The chains are highly branched and bushy (however less branched and
less bushy than the animal storage form of carbohydrate, which is glycogen, not
starch). Starches with a high percentage of amylopectin will thicken a mixture, although
cannot form a gel because, unlike amylose, amylopectin molecules do not associate and
form chemical linkages. The greater the amylopectin content, the more viscousthe
starch paste (not a gel), while the greater the amount of amylase, the stronger the gel.
Gelatinization Process in Cooking
Steps in the process of becoming gelatinized will be enumerated in the following text.
Starch in its uncooked stage is insoluble in water. Thus it cannot be referred to as
“going into solution,” or “dissolving.” It forms a temporary suspension of large
granules/particles, which are undissolved in the surrounding medium, and these
particles will settle to the bottom of a container of liquid unless it is continuously stirred
or otherwise agitated. In a suspension, the starch particles may imbibe a small amount
of water; however, generally, a suspension offers minimal change to the starch. Any
uptake of water by the starch granule is reversible if starch is dried while still in the
uncooked state.
Another feature of the uncooked starch molecule is that it exhibits a Maltese cross
formation, or birefringence on the granule when it is viewed under polarized light with
an electron microscope. This is due to the fact that it is a highly ordered crystalline
structure, and light is refracted in two directions. Once cooking has begun when the
starch is heated in surrounding water there occurs imbibition, or the taking of water
into the granule.
This first occurs in less-dense areas, and subsequently in the more crystalline regions of
the starch molecule. At this initial point this is still a reversible step in the gelatinization
process. However, as heating continues, starch granules take up more water irreversibly
and swell; some short chains of amylose come out of the granules. This process, known
as gelatinization, is responsible for the thickening of food systems. The gelatinized
starch mixtures are opaque and fragile, and the ordered crystalline structure of starch is
lost. As starch leaches out of swollen granules in the gelatinization process, the
water–starch mixture becomes a sol. A sol is a colloidal two-phase system containing a
liquid continuous phase and a solid dispersed phase. This solid-in-a-liquid is
pourable and has a low viscosity or resistance to flow.
3
Introduction
Starch is a plant polysaccharide stored in roots and seeds of plants, and in the endosperm
of a grain kernel. It provides humans with energy (4 cal/g), and is hydrolyzed into glucose,
supplying the glucose that is necessary for brain and central nervous system functioning.
Starch grains, or granules, contain long-chain glucose polymers that are insoluble in
water. Unlike the small molecules of salt and sugar, the larger starch polymers do not
form a true solution. Instead, starch granules form a temporary suspension when
stirred in water. As uncooked granules, each may swell slightly when it absorbs water.
However, once starch is cooked, the swelling is irreversible and the starch
leaches out. This characteristic of starch granules enables starch to be used as a
thickener.
Overall, the characteristics of a finished starch food product are determined by several
factors:
a) the source of starch, concentration of starch used in a formulation
b) the temperature and time of heating
c) other components used with the starch, such as acid and sugar.
There are many types of starch and modified starches. These thicken, prevent curdling,
and stabilize cooked salad dressings, dips, gravies, desserts, and more. Intermediate,
shorter chain products from starch breakdown, known as dextrins, may be used to
simulate fat in salad dressings and frozen desserts. For example, wheat, potato, and
tapioca maltodextrins may be used as fat replacers. These provide the viscosity and
mouthfeel of fat in a food product, yet, with reduced calories compared to fat.
Starch Sources in the Diet
Starch sources are numerous, with common ones derived from cereal grains such as
wheat, corn, or rice. Wheat yields a cloudy, thick mixture, while cornstarch produces
more clear mixtures such as gravies or sauces. Vegetables, roots and tubers, including
the root of cassava, and potatoes, are frequently used in the preparation of gluten-free
1
,foods, where persons with wheat allergies or intolerances do not use any wheat as a
thickener. Specialty starches are available commercially and some may be available to
the consumer, perhaps purchased through specialty food stores.
Another source of starch is legumes such as soybeans or garbanzo beans. As well, sago
is a powdery starch obtained from the stems and trunks of the sago palm in tropical
Asia. Sago may be used as a food thickener as well as a fabric stiffener. Fruits such as
bananas may also be sources of starch.
Thus it may be seen that starch may come from a variety of sources. Depending on the
source, starches may also have different crystal-line structures.
Starch Structure and Composition
The starch granules from various grains differ in size, ranging in size from 2 to 150 μm.
The shape of starches may also vary—being round or polygonal, as seen in the
photomicrographs of corn, wheat, and waxy maize.
Starch is made up of two molecules, amylose and amylopectin, whose parts are
connected by glycosidic linkages. Amylose molecules typically make up approximately
one quarter of starch. Amylose is a long linear chain composed of thousands of glucose
units with attachment of the carbon 1 and carbon 4 of glucose units, and therefore
contains α-1,4 glycosidic linkages. It forms a three-dimensional network when
molecules associate upon cooling, and is responsible for the gelation of cooked, cooled
starch pastes. While those starches with a high amylose content are able to gel, or hold
their shape when molded, starches without amylose thicken, although do not gel.
Examples of the amylose content of various starch sources include:
Cereal grains—26–28 % amylose
Roots and tubers—17–23 % amylose
Waxy varieties of starch—0 % amylose
Amylopectin molecules (Chap. 3) constitute approximately three-quarters of the
polymers in a starch granule. The glucose chain of amylopectin contains α-1,4 linkages,
similar to amylose, how- ever, with α-1,6 branching at every 15–30 glucose units of the
chain.
2
, There is a linkage between the carbon 1 of the glucose and carbon 6 of the branch in
amylopectin. The chains are highly branched and bushy (however less branched and
less bushy than the animal storage form of carbohydrate, which is glycogen, not
starch). Starches with a high percentage of amylopectin will thicken a mixture, although
cannot form a gel because, unlike amylose, amylopectin molecules do not associate and
form chemical linkages. The greater the amylopectin content, the more viscousthe
starch paste (not a gel), while the greater the amount of amylase, the stronger the gel.
Gelatinization Process in Cooking
Steps in the process of becoming gelatinized will be enumerated in the following text.
Starch in its uncooked stage is insoluble in water. Thus it cannot be referred to as
“going into solution,” or “dissolving.” It forms a temporary suspension of large
granules/particles, which are undissolved in the surrounding medium, and these
particles will settle to the bottom of a container of liquid unless it is continuously stirred
or otherwise agitated. In a suspension, the starch particles may imbibe a small amount
of water; however, generally, a suspension offers minimal change to the starch. Any
uptake of water by the starch granule is reversible if starch is dried while still in the
uncooked state.
Another feature of the uncooked starch molecule is that it exhibits a Maltese cross
formation, or birefringence on the granule when it is viewed under polarized light with
an electron microscope. This is due to the fact that it is a highly ordered crystalline
structure, and light is refracted in two directions. Once cooking has begun when the
starch is heated in surrounding water there occurs imbibition, or the taking of water
into the granule.
This first occurs in less-dense areas, and subsequently in the more crystalline regions of
the starch molecule. At this initial point this is still a reversible step in the gelatinization
process. However, as heating continues, starch granules take up more water irreversibly
and swell; some short chains of amylose come out of the granules. This process, known
as gelatinization, is responsible for the thickening of food systems. The gelatinized
starch mixtures are opaque and fragile, and the ordered crystalline structure of starch is
lost. As starch leaches out of swollen granules in the gelatinization process, the
water–starch mixture becomes a sol. A sol is a colloidal two-phase system containing a
liquid continuous phase and a solid dispersed phase. This solid-in-a-liquid is
pourable and has a low viscosity or resistance to flow.
3
