starch compositions reactions functional properties common problems

1. STARCH COOKERY
INTRODUCTION
Starch is a nutrient carbohydrate found notably in corn, potatoes, wheat and rice, and is
commonly prepared as a whole tasteless powder. Starch exists in nature as the main component
of cereals and tubers. Wheat flour, cornstarch, and tapioca are starches commonly used as
thickening agents in cooking. Some common starchy foods are bread, cereals, pasta, rice, corn,
wheat, potatoes, beans and chestnuts.
Starch is a solid carbohydrate that is granular in form and naturally present in many plants such
as grains (wheat or rice), pulses (corn), tubers (potatoes), and numerous other plant species.
When cooked, starch can affect foods as it changes the stability, texture and viscosity of the item
being prepared. It is a substance commonly used to alter various aspects of foods being
manufactured as well as being prepared. As an example, sticky rice is a short grain variety of
white rice that sticks together when cooked. This variety is generally used for commercial
product formulations rather than home recipes, converting the rice starch and flour into food
binders for fillings, gravies and sauces. The starch and flour can withstand refrigerated
conditions without separating when frozen or thawed, becoming an excellent option for frozen
food formulations. During food preparation, different varieties of foods cook into a very dense
consistency due to the higher ratio of starch, thus affecting the preparation and results when
finished, similar to the reaction occurring with rice.
USES:
 Pure starch is a white, tasteless and odourless powder that can be used in processed foods
or in paper industry.
 Glucose syrups are liquids which are used in the food industry for their sweetening
power.
 Sweetening power in beverages.

2.  Bulking agent/ texture in dairy products.
 Moistening in bakery products.
 Thickening, binding agent in soups and sauces.
 Browning effect in caramels.
 Cooling effect in chewing-gum.
 It is used as an additive for food processing, food starches are typically used as thickeners
and stabilizers in foods such as puddings, custards, fillings, and salad dressings, and to
make noodles and pastas.
 In the pharmaceutical industry, starch is also used as an excipient and as binder.
 The biggest industrial non-food use of starch is an adhesive in the papermaking process.
 Starch can be applied to some garments before ironing or in laundering, to stiffen cloth.
 Used in food manufacturer in cosmetics, textiles, construction materials and other
industrial uses
Structure & compositionof starch:-
It is present into vegetable cells and contains two types of homopolysaccharides, amylose and
amylopectin.
 Amylose is an unbranched homopolysaccharide formed by about 5-600 glucose units,
linked by a-(1- 4) glycosidic bonds. It has a helix structure with six glucose units for turn,
is soluble in water and places in the core of the starch granules.
 Amylopectin represents about 80% of polysaccharides taken up with diet. It is branched
molecule formed by thousands of glucose units, up to 50000, that in main chain, are
joined by a-(1- 4) glycosidic bonds, Approximately every 25-30 glucose units a branch
point or lateral chain is present, formed by glucose units as well, joined to the main chain
by a-(1-6) glycosidic bonds, glucose units on the lateral chain, from 16 to 25, are linked
again by a-(1- 4) glycosidic bonds. The branches prevents the formation of a helical
structure while allow the creation of spongy reticular structures.

3. CLASSIFICATION OF STARCH:
Starch can be classified into following classes:
 Native starch:
It refers to the starches as originally derived from its plant source. Depending on whether
they are obtained from maize, wheat, potato or pea, native starches present different
characteristics according to the size and structures of the starch granules and depending on
their amylose and amylopectin composition.
Native starch is a powder obtained from plants containing starch. It is used as a thickening
agent and a stabilizer. Good examples of this include custard, desserts, sauces and many
forms of instant foods. The most common native starches are corn, rice, wheat, potato and
waxy maize.
 Modified starch:
Modified starches are those with structures altered by treatment with physical or chemical
agents. Modified starch, also called starch derivatives, are prepared by physically,
enzymatically, or chemically treating native starch to change its properties. Modified starches
are used in practically all starch applications, such as in food products as a thickening agent,
stabilizer or emulsifier; in pharmaceuticals as a disintegrant; as binder in coated paper.
Modified starches are obtained from native starches as a result of physical, enzymatic or
chemical processing methods. Wet and dry chemical processes, drum drying and extrusion
methods are all used. The properties of native starch such as its freeze-thaw stability, acid or
alkali resistance or even its shear stability can be changed by means of these processes.
Depending on the raw materials used starch is used for different applications.
 Slowly DigestedStarch:
Amylose contains 500 to 20,000 molecules of glucose connected together in a straight chain.
The chain twists into a helix and then two chains bond together, forming a structure that
resists the digestive enzymes trying to break the glucose molecules apart. As a result, resists

4. the digestive enzymes trying to break the glucose molecules apart. As a result, amylose is
slowly digested and absorbed, which is why it's called a slowly digestible starch. Amylose
can help keep your blood sugar balanced because it does not cause a large spike in blood
sugar levels. About 20 to 30 percent of the natural starch in plants consists of amylose.
 Rapidly DigestedStarch:
Amylopectin is significantly larger than amylose, with a structure made up of millions of
glucose molecules that branch out and form a crystalline structure. Its glucose units are easily
cleaved during digestion, which makes it a rapidly digestible starch. Amylopectin can boost
your blood sugar temporarily, but it is followed by a hunger-producing drop in blood sugar.
About 70 to 80 percent of the total natural starch in plants is made up of amylopectin.
 Resistant Starch:
Resistant starch (RS) is any starch or starch digestion products that are not digested and
absorbed in the stomach or small intestine and pass on to the large intestine. RS has been
categorized into four types:
 RS1 – Physically inaccessible or undigestible resistant starch, such as that found in
seeds or legumes and unprocessed whole grains.
 RS2 – Resistant starch is inaccessible to enzymes due to starch conformation, as in
high amylose corn starch.
 RS3 – Resistant starch that is formed when starch-containing foods are cooked and
cooled, such as pasta. Occurs due to retrogradation, which refers to the collective
processes of dissolved starch becoming less soluble after being heated and dissolved
in water and then cooled.
 RS4 – Starches that have been chemically modified to resist digestion.
Starch reactions on foods:
Carbohydrates come in various forms, and each form reacts differently when exposed to heat.
The two forms of carbohydrates that are of interest from a basic food science perspective are
sugar and starch.
Caramelization:-
When exposed to heat, sugar will at first melt into a thick syrup. As the temperature continues to
rise, the sugar syrup changes color, from clear to light yellow to a progressively deepening
brown. This browning process is called caramelization. It is a complicated chemical reaction,
and in addition to color change, it also causes the flavor of the sugar to evolve and take on the
rich complexity that we know to be characteristic of caramel. Different types of sugar caramelize
at different temperatures. Granulated white sugar melts at 320°F/160°C and begins to caramelize
at 338°F/170°C.

5. Maillard reaction:-
In foods that are not primarily sugar or starch, a different reaction, known as the Maillard
reaction, is responsible for browning. This reaction involves sugars and amino acids (the
building blocks of protein). When heated, these components react and produce numerous
chemical by-products, resulting in a brown color and intense flavor and aroma. It is this reaction
that gives coffee, chocolate, baked goods, dark beer, and roasted meats and nuts much of their
rich flavor and color.
Gelatinization:-
Starch, a complex carbohydrate, has powerful thickening properties. When starch is combined
with water or another liquid and heated, individual starch granules absorb the liquid and swell.
This process, known as gelatinization, is what causes the liquid to thicken. Gelatinization occurs
at different temperatures for different types of starch. As a general rule of thumb, root-based
starches (potato and arrowroot, for instance) thicken at lower temperatures but break down more
quickly, whereas cereal-based starches (corn and wheat, for example) thicken at higher
temperatures but break down more slowly. High levels of sugar or acid can inhibit gelatinization,
while the presence of salt can promote it.
Gelation:-

6. The process of forming a gel that occurs after the gelatinization process and during cooling is
called Gelation.
Example:- The cooling of Jell-O after preparation of the mixture.
Dextrinization:-
The breakdown of starch molecules to smaller, sweetier tasting molecules (dextrin) by enzymes,
acid or dry heat. Dextrin are partially hydrolyzed starches that are prepared by dry roasting
starch. In home kitchens, dextrinization is achieved by toasting of flour for polvoron, rice flour
for kare-kare sauce and bread slices for breakfast.
Retrogradation:-
When cooled, gelatinize amylase containing starches set into a rigid gel. Retrogradation is a
reaction that takes place when the amylose and amylopectin chains in cooked, gelatinized starch
realign themselves as the cooked starch cools.
When native starch is heated and dissolves in water, the crystalline structure of amylose and
amylopectin molecules is lost and they hydrate to form a viscous solution. If the viscous solution
is cooled or left at lower temperature for a long enough period, the linear molecules, amylose,

7. 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 less digestible.
Chemical modification of starches can reduce or enhance the retrogradation. Waxy, high
amylopectin, starches also have a much lesser tendency to retrogradate. Additives such as fat,
glucose, sodium nitrate and emulsifier can reduce retrogradation of starch.
Syneresis:
It is characterized by the expulsion of moisture from the gel. This reaction occurs in all kinds of
gels:
 puddings
 jellies
 custards
 gelatin
 agar
Retroradation:
It is defined as the reassociation of the linear amylase chains by hydrogen bonding.
The rate of staling is dependent on the product formation, the baking process and the storage
conditions.
Hydrolysis:
Starches undergo hydrolysis during cooking or processing and during storage of food.

8.  Prolonged heating of starches with acid will promote hydrolysis. This can happen when
cooking an acidic food, such as pineapple resulting in reduced viscosity or firmness of
the pie filling.
Functional Properties of Starches
Starch plays various roles in food, a typical multi-tasker.
 Thickeners in gravies, sauces and pudding. It absorbs water and become a gel when
cooked.
 Colloidal stabilizers
 Moisture retainer
 Gel forming agents
 Binders
 Package
 Flavor carriers
Flavor carriers: its ability to trap oils and fats, which absorb flavoring substances more
efficiently.
Starches: are added to processed meats (luncheon meats, hot dogs, sausages, etc.) as a filler,
binder, moisture, retainer, and fat substitute.

9. Properties of Starch:
 Flavor: starch (a white powder) is not sweet in flavor.
 Solubility: starch is insoluble in cold water.
 Hygroscopic: this property relates to how starch absorbs moisture from the air e.g.
biscuits soften if they are not kept air tight.

10.  Hydrolysis: hydrolysis is a chemical breakdown of a molecule by adding water to
produce smaller molecules. Disaccharides become monosaccharaides partly due to
hydrolysis.
 Starch particles do not dissolve in liquid instead they form a suspension.
 Stirring or agitating the liquid keeps the particles suspended.
 If the suspension is not stirred the particles form to the bottom forming lumps.
 When the liquid reaches 60 C the starch grains begin to absorb the liquid.
 At 80 C the particles break open and release starch making the mixture thick and viscose,
this is called gelatinization.
 Gelatinization is complete is complete when liquid reaches 100 C. the thickened liquid
now forms a gel. On cooling the gel solidifies.
Common Problems in Starch Cookery:
1. Thinning of Gel
This problem is usually encountered when using acid or acid
ingredients such as lemon or vinegar.
2. Weak Gel
Weak gel results if there is too much liquid in relation to the starch.
3. Skin Formation
Skin formation is due to loss of water from the starch and protein
molecules near the surface of the mixture. To reduce this problem, cover container of the
starch gel with a waterproof cover.
4. Scorching
This can be avoided by temperature control and constant stirring so the
starch granules do not settle at the bottom of the cooking pan.
5. Raw Starch Flavor
This is due to ungelatinized starch.