UNIT 2 - Carbohydrates

Introduction

Food is composed of three main constituents, namely, carbohydrates, proteins, fats and their derivatives.
In addition these constituents , inorganic mineral elements and diverse organic compounds such as vitamins, pigments, enzymes and acids are also present.
The variation in structure, texture, colour, flavor and nutritive value is because of the varying proportions and arrangement of these constituents.
Knowledge of these constituents, their properties and reactions with other constituents is necessary for a person who processes, severs and stores food.

What are Carbohydrates?

Carbohydrates are an important group of nutrients. It is present in various forms in the foods we cook, processed food which we purchase and form the bulk of our diet.

They are organic compounds made up of C, H &. O
They are called carbohydrates because H & O are present in the same proportion as found in water i.e. 2:1.
They are processed in plants by the process of photosynthesis
Chlorophyll is a green pigment which absorbs energy from sunlight and enables plants to build up carbohydrates from CO2 and H20.

Glucose cannot be stored on a large scale so it is converted to starch with the removal of water and is stored in various parts of the plant. E.g. in cereal grains and potatoes CHO is stored as  starch. In bananas, mango and sugar beets it is stored as sugar.
Tender green peas and maize contain carbohydrate in the form of sugar which is converted into starch as the seed matures. However the reverse is seen in the fruits, immature fruits contain starch which is converted into sugar as the fruit ripens.
The various parts of the plant where CHO is stored form the main source of CHO in diet.

Classification of Carbohydrates

MONOSACCHARIDES:

They are the simplest form of CHO found in nature. These simple sugars are made up of a six carbon chain or ring to which hydrogen groups are attached. The general formula is C6H12O6. They differ from one another because of their arrangement of different atoms around the carbon chain and because of this they have different properties and vary in their degree of sweetness.

Glucose- it is the most important CHO used by the body.
 It is absorbed into the blood stream after CHO is digested in the body. It is also known as dextrose. Available in powder and liquid form. It is found in varying amounts in fruits and vegetables. Found in large amount in fruits like grapes, smaller amount in vegetables like peas.

Fructose-It is sweetest of all sugars, is also known as fruit sugar and levulose because it is found in fruits and honey.
In human body it is converted to glucose and oxidized as a source of energy.

Galactose-It is not present in food as such, but produced when lactose a disaccharide is broken down during digestion.

DISSACHARIDES are double sugars composed of two monosaccharides linked together with the removal of a water molecule. These sugars have a general formula C12H22O11
C6H12O6+C6H12O6………C12H22O11

Sucrose-It is table sugar
It is produced in plants by the condensation of glucose and fructose.
It is found in many fruits and vegetables like sugarcane and sugarbeet contain relatively large quantities.
It is from cane and beet that sugar is extracted commercially.

Lactose-It is milk sugar
Made up of one unit of glucose and one unit of galactose.
It is least sweet of all sugars and easily fermented to lactic acid by lactic acid bacteria while preparing curd and cheese.

Maltose-It is made up of two units of glucose.
During the germination of whole grains starch is broken down into maltose
In the body maltose is formed during digestion of starch.


 Oligosaccharide
Any carbohydrate of from three to six units of simple sugars (monosaccharides). A large number of oligosaccharides have been prepared by partially breaking down more complex carbohydrates (polysaccharides). Most of the few naturally occurring oligosaccharides are found in plants.
Raffinose and stachyose may promote the growth of beneficial intestinal bacteria, but are currently not considered prebiotics.

Raffinose - also called melitose, is composed of 3 sugars: galactose, glucose and fructose. Examples of foods naturally high in raffinose are beans, asparagus, cotton seeds, sugar beet molasses, cabbage, broccoli, Brussel’s sprouts, sweet potatoes and whole grains . Raffinose as a sweetener is extracted from sugar beet molasses.

Stachyose - Stachyose is composed of 4 sugar molecules: 2 galactoses, glucose and fructose. It is found mainly in beans and peas .


POLYSACCHARIDES  - are complex carbohydrates made up of 100-2000 glucose units linked to each other in chain or branched form. The number of glucose units, their arrangement and linkage to one another influence the properties of the polsaccharides.

Dextrins- They are smallest and simplest of all polysaccharides.
They are formed by dry heating or acid hydrolysis of starch.
They are slightly soluble have a mild sweet taste and limited thickening ability.

Starch-It is found in most parts of the plant as a reserve store of carbohydrate.
It is usually present in the seed and root in large amounts.
Starch consists of long chains of glucose units present in two forms amylose and amylopectin.

STARCH
Amylose –
 It is a large molecule made up of 200 or more glucose units .
They are present as linear chain which can bond to each other by hydrogen bonds and form a gel.
Starches from different sources differ in their amylase content. Amylose does not have sweet taste, is slightly soluble, has good thickening ability and is present 20-30% of total starch in most grains.

Amylopectin –
It is also made up of glucose units only and are present in form of large branched polysaccharide.
The molecules of amylopectin are very large and as it is branched structure it is sparingly soluble, not sweet and is predominant form in the starch granule with low gelling
Cereal starches, such as corn, rice, wheat, oats, sago and tapioca are used as thickening and gelling agents.
Genetic research and plant breeding have enabled us to develop starches containing 100 per cent amylopectin, which are called waxy starch and they do not form gels.  Starch with high amylose has also been developed.

STARCH

EFFECT OF COOKING ON STARCH
GELATINIZATION (wet heat)

Ø  When starch granules are mixed with cold water they do not dissolve but form suspension.

Ø  When the water is heated, the granules begin to swell. The heat energy breaks the hydrogen bonds in the starch granules and facilitates the entry of water into the granules. At the same time some amylose from the granules leaches into the cooking water.
Ø  The starch chains in the granules absorb moisture and begin to uncoil from their tightly packed configuration.
Ø  The size of the granules increase as more and more water enters. The water in the granules gets bonded to amylose and amylopectin.
Ø  The mixture becomes viscous and translucent after continuous heating. Swollen granules find it difficult to move past each other, adding to the viscosity of the mixture.
Ø  This process of swelling of the starch granules and formation of viscous starch paste is called
GELATINIZATION
Ø  The temp at which the granules swell is called the GELATINIZATION TEMPERATURE and is characteristic of each starch.

FACTORS AFFECTING THE PROPERTIES OF STARCH AS A THICKENING AGENT

1)     MIXING AND STIRRING
Ø  When starch is used as a thickening agent in soups or custards, it should be dispersed completely to prevent unequal swelling or lumps forming in a starch thickened product. This can be achieved by:
a)     Mixing well with cold H2O
b)     Mixing with melted fat to coat starch particles.
c)     Mixing with another dry ingredient.
Ø  Once starch is dispersed continuous stirring is necessary till gelatinization is complete
Ø  Stirring of hot starch pastes prevent lumps and sticking of gelatinized starch to the sides of the pan.
Ø  Excessive stirring of the starch paste can break the starch granules, releasing amylose & amylopectin into the liquid resulting in a less viscous product.

2)     TEMPERATURE
Ø  Starch paste gradually thicken with increase in temp from 52˚C to 65˚C . The starch granules continue to swell & amylose leaches out of the granule. Shorter amylose molecules have more solubility as the temp approaches 90-100˚C, some granules may burst & fragment.
Ø  Continuous heating decreases the viscosity of starches as granules which reach their maximum volume implode and result in thinning of starch pastes. When cool it may thicken again.

3)     TYPE OF STARCH
Ø  Different starches have different thickening power for eg. Potato starch has the greatest thickening power, followed by waxy starches, tapioca , corn , rice , & wheat which has least thickening  power.
Ø  The texture should ideally be smooth & not stringy & mucilaginous. Root starches such as tapioca are mucilaginous. Root starches such as tapioca & potato, are more mucilaginous than cereal starches. They are more trans lucent when gelatinized. Of  all starches, corn starch is the best thickening agent in terms of texture.

4)     EFFECT OF ADDED INGREDIENTS
 Ø  SUGAR:  When sugar is added to starch thickened paste, because of its hygroscopic nature it competes with starch for H2O needed for gelatinization . Gelatinization temp is higher when sugar is added and the time taken for gel is longer. Sugar reduces the viscosity & strength of the gel. It increases translucency.
Ø  ACID :  When starch paste is heated with acid like lime juice at pH below 4 , starch molecules are hydrolyzed into slightly smaller molecules .Acid hydrolysis results in thinning of the starch paste as smaller molecules move freely in the paste. If acid is added after gelatinization of starch , the paste does not turn thin.
Ø  FATS:  Presence of fat in starch – thickened pastes lowers the gelatinization and thickening temp.
Ø  MILK PROTEINS :  Gelatinization temp is lowered if milk is an added ingredient

GELATION

Ø  Gelatinized starch mixture may exhibit flow properties & remain a sol or may cool and set to from a gel.
Ø  The amylose , which has leached  out  of the swollen starch granule , forms Hydrogen  bonds with other amylose molecules as the starch paste cools & loses energy
Ø  Amylose molecules moves slowly forming bonds & a 3 dimensional continuous network of amylose is formed in which swollen granules are trapped
Ø  This forms a continuous phase of the newly formed starch gel in which water is dispersed. The starch mixture is transformed formed into a gel & no longer exhibits flow properties.

FACTORS AFFECTING GELATION

1)     Type of starch
2)     Concentration of starch.
3)     Duration of heating.
4)     Stirring.
5)     Other ingredients.
6)     Aging of gel.

FACTORS AFFECTING GELATION
TYPE OF STARCH

Ø  The proportion of amylose & amylopectin in the starch determines whether  a gel will form & whether  it will be permanent. The straight chains of amylose form bonds quickly & easily while the branches of amylopectin come in the way & prevent formation of firm gel. Starches rich in amylose can form gel at low concentration while starches lack amylose eg. Waxy starches can form soft gels at high concentration.

Ø  Eg. Wheat & rice flours are good thickening agents but poor gelling agents. Chemically modified starches form stable gel.

CONCENTRATION OF STARCH

Ø  Corn starch form a firm gel at 10% concentration while waxy starches lack amylose can for a soft gel at 30% concentration. Starches containing large amounts of amylose will gel at low concentration.
1Tbsp sp starch in 1 cup liquid – thin sauce
2 Tbsp sp starch in 1 cup liquid -medium consistency
3 Tbsp sp starch in 1 cup liquid -thick sauce

DURATION OF HEATING
Ø  When starch is heated along with water the hydrogen bonds in the starch granule break and amylose fraction of starch leaches into the surrounding water.
Ø  A starch paste should be heated gradually for granules to swell and release sufficient amylose to form a gel. Prolonged heating results in fragmentation of amylose and formation of a weak gel with pasty texture.

STIRRING
Ø  Vigorous stirring during heating results in fragmentation of amylose.
Ø  A firm gel forms when paste is allowed to cool undisturbed. Amylose starts forming bonds as the mixture cools and starts gelling. Stirring disrupts the bonds and results in a weak gel.
Ø  Essences and colors should be added to the starch mixture as soon as it is removed from heat and not while mixture is cool.

OTHER INGREDIENTS
Ø  Sugar, acids, etc. modify the behavior of starch gel
Ø  The greater the amount of sugar in the product the more delicate the gel is formed, as sugar prevents water from binding to starch
Ø  Acid hydrolyse the amylose chain resulting in a more tender gel. this is seen when acids are added before gelatinization of starch. If added after gelatinization of starch, the gel is soft because of extra liquid from lime juice or fruit juice.

AGING OF GEL
Ø  In a starch gel water is trapped as dispersed phase within the gel. Water is also bonded by hydrogen bonding to amylose molecules and starch granules which form the matrix of the gel.
Ø  When the gel stales or the structure is disrupted by cutting the gel, water which is trapped in the gel is released and gel collapses. This weeping or loss of moisture from a gel  is called SYNERESIS

RETROGRADATION

Ø  Amylose starches form gel readily but these gels are less stable as amylose chains have a tendency to recoil and partially recrystallize. Some hydrogen bonds which hold the gel together break and amylose molecules move around forming new bonds.

Ø  As the gel stales amylose molecules rearrange themselves in an orderly manner in crystalline regions. This is accompanied by loss of solubility and release of water from the gels, causing food defect.
Ø  Thus retrogradation occurs when a starch gel stales or when it is frozen. A starch gel which has retrograded loses its smooth texture and feels gritty when eaten.
Ø  The rate and extent of retrogradation are influenced by temp, size, shape and concentration of starch. Starch retrogrades rapidly at 0º C.
Ø  The texture defects caused by retrogradation in foods which can be heated are temporarily corrected by warming the food containing starch.
The problem of retrogradation is of concern in cold starch based gels. This can be corrected by using starches which are stable to freezing and thawing.

DEXTRINIZATION (dry roasting of starch)

Ø  When starch is heated without any water, the temp rises rapidly beyond 100C.
Ø   Water which is naturally present in flour and high temp brings about chemical changes or degradation of flour, splitting the starch molecule at one or more of the α1,4 glucosidic linkages. This reaction is called dextrinization and the short chain starch molecules of varying length formed are called dextrins.
Ø  This process is seen when flour is browned while making brown roux for gravies and sauces. Browned flour has lesser thickening ability because of formation of short chain dextrins.

Uses of Carbohydrates in Food Preparation

Starch from various sources in its natural form is used as a thickening and gelling agent in a wide range of products. It is the primary thickening agent used in soups and sauces.
These sauces are used in vegetable and meat based preparations, salads and pastas. It is also used in custard sauce, puddings, pie fillings and soufflés.
Sugar has a wide range of uses apart from sweetening and energy giving. Sugar cookery involves controlled formation of crystals which has a direct bearing on the texture of crystalline candies such as fondants and fudges.

Some uses of Carbohydrates:

Carbohydrates	Use
Refined flour	Thickening sauces and soups specially used in the form of a roux.
Rice	Thickening soups and rice puddings
Arrowroot	For clear soups
Tapioca	Used for pudding
Potato	Used for soups which could curdle at high temperatures
Waxy rice flour	White sauces and starch thickened pudding which need to be stored frozen and thawed before cooking.
Corn flour	Thickening soups, sauces, gravies and anti-caking agent.
Pectin	Setting agent in jams, jellies and marmalades
Seaweed extracts	Prevent ice crystal formation in ice cream
Glucose	Used as a humectant in confectionery
Caramel	Used as coloring and flavoring agent in Christmas cake, soup mixes, instant pudding etc.
Invert Sugar	Prevents formation of sugar crystals in preserves and fondants

Conclusion

Carbohydrates are one of the most important constituents of food.

They are manufactured by plants through the process of photosynthesis.
The glucose formed in this process is stored in form of starch to be used as food.
On the basis of saccharides or sugar units they are classified into Mono, Di, Oligo and Poly (saccharides)
Starch is present in two forms, Amylose and Amylopectin.

The food industry depends on natural and modified carbohydrates for specific additive functions in many processed foods.

Test your understanding of Carbohydrates