Chapter 2 :- Carbohydrates

 (A.) Introduction:-

Food is composed of three main constituents, namely carbohydrates, proteins,and fats and their derivatives. 

           Carbohydrates are present in various forms in the food we cook, and processed food which we purchase and form the bulk of our diet. They are available in the market in the natural form, processed form,or modified form as an additive in many different products. 

Definition:- Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones,or substances that yield such compounds on hydrolysis. Carbohydrates are organic compounds made up of carbon, hydrogen, and oxygen. They are called carbohydrates because hydrogen and oxygen are present in the same proportion as found in water.

Classification of Carbohydrates:-

(B.) Effect of cooking (gelatinization and retrogradation):-

Gelatinization and viscosity:-

               

                  When starch granules are mixed with cold water, they do not dissolve, but form a suspension. 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 granule leaches into the cooking water. The temperature at which the granules swell is called the gelatinization temperature and is characteristic for each other. 

               The starch chains in the granules absorb moisture and begin to uncoil from their tightly packed configuration. The size of the granule increases as more and more water enters. The water in the granule gets bonded to amylose and amylopectin. The mixture becomes viscous and translucent after continuous heating. The increase in viscosity is due to the water bonded to starch and increase in size of starch granule as well as reduction in free water in the mixture. Swollen grains find it difficult to move past each other, adding to the viscosity of the mixture. This process of swelling of the starch grains and formation of viscous starch pastes is called gelatinization. 

Retrogradation:-

               Starches rich in amylose from gels more readily but these gels are less stable. 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. 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 gel, causing food defects. 

               A starch gel which has retrograded loses its smooth texture and feels gritty when eaten. The rate and extent of retrogradation are influenced by temperature, size, shape, and concentration of starch. Starch retrogrades rapidly at 0 degree.

               Retrogradation occurs when a starch gel becomes stale or when it is frozen. Bread and starch-thickened puddings stored in the refrigerator develop undesirable textural changes because of retrogradation by formation of crystalline aggregates of amylose. 

                The texture defects caused by retrogradation in foods, which can be heated, are temporarily corrected by warming the food containing starch. Heat energy breaks the hydrogen bonds which hold amylose molecules together forming crystalline areas. Stale bread becomes soft when it is covered and reheated, but as it cools it develops an undesirable texture once again. 

                 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. A number of modified starches are available in the market today. 

(C.) Factors affecting texture of carbohydrates (Stiffness of CHO gel and dextrinization):-

Factors Affecting starch gel formation-

1)         TYPE OF STARCH :-

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 e.g. Waxy starches can form soft gels at high concentration.

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

2)         CONCENTRATION OF STARCH :-

• 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 amts 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

3)         DURATION OF HEATING :-

• 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.
• 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.

4)         STIRRING :-

• Vigorous stirring during heating results in fragmentation of amylose.
• 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.
• Essences and colors should be added to the starch mixture as soon as it is removed from heat and not while mixture is cool.

5)         OTHER INGREDIENTS :-

• Sugar, acids, etc. modify the behavior of starch gel
• The greater the amount of sugar in the product the more delicate the gel is formed, as sugar prevents water from binding to starch
• Acid hydrolyses 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.

6)         AGING OF GEL :-

• 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.
• 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.

Characteristic	Amylose	Amylopectin
Starch structure	Amylose form 20-30% of the starch structure.	Amylopectin forms 70-80% pf starch structure.
Chain structure	Amylose has a linear chain structure.	Amylopectin has a branched chain structure.
Glucose	Amylose has 300-several thousand units of glucose.	Amylopectin has 2000-200,000 units of glucose.
Solubility	Amylose is usually insoluble in water.	Amylopectin is soluble in water
Iodine Test	Amylose gives blue color in iodine test.	Amylopectin gives reddish brown color in iodine test.

Dextrinization:-

When starch is heated without any water, the temperature rises rapidly beyond 100 degrees. Water, which is naturally present in flour, and the high temperature brings about chemical degradation of flour splitting the starch molecule at one or more of the 1,4 a-glucosidic linkages. This reaction is called dextrinization, and the short chain starch molecules of varying lengths formed are called dextrins. Dextrinization is seen when flour in browned while making brown roux for gravies and sauces. Browned flour has lesser thickening ability because of formation of shorter chain dextrins. 

(D.) Uses of carbohydrates in food preparation:-

Unmodified starch:-

1.) Refined flour-- Thickening sauces and soups specially used in the form of a roux.

2.) Rice--- Thickening soups and rice pudding (phirni)

3.) Arrowroot-- For clear sauces, e.g. lemon sauce and as a glaze for fruit flans

4.) Tapioca-- Used for padding 

5.) Potato--- Used to thicken soups which could curdle at high temperature 

6.) High amylose starch -- Edible films for wrapping candies 

7.) Waxy rice flour-- White sauces and starch -thickened pudding which need to be stored frozen and thawed before cooking 

8.) Corn flour--- Thickening soups, sauces, and gravies, anti-caking agent

Modified starch:-

1.) Starch phosphates-- White sauces in cook-freeze operations 

2.) Oxidized starch-- Products which need soft gels

3.) Thin boiling starch-- Gum drops 

4.) Pregelatinized starch--- Instant pudding and soup drops

5.) Cross-linked starch-- Thickeners and stabilizer in salad dressings

Other polysaccharides:-

1.) Cellulose-- Powdered form used to provide bulk in weight-reducing foods 

2.) Cellulose compounds-- Thickening and creaming agents 

3.) Dextrins-- In coffee extracts

4.) Pectin-- Setting agent in jams, jellies, and marmalades

5.) Gums-- Thickening and gelling agents ; stabilization of ice-cream ,cheese, and chocolate milk

6.) Seaweed extracts (algin and agar)-- Prevent ice crystal formation in ice cream, stabilize cream substitutes, modify crystal size in sugar confections, and are used in weight-reduction diets

Sugars:-

1.) Glucose-- Used as a humectant in confectionery 

2.) Caramel-- Used as a colouring and flavouring agent in christmas cake ,soup mixes ,instant puddings, etc.

3.) Invert sugar-- Prevents formation of sugar crystals in preserves and fondant. 

Carbohydrates perform the following functions:

Energy: The principle function of carbohydrates is to serve as a major source of energy for the body. Each gram of carbohydrate yields 4Kcal of energy regardless of its source. In Indian diets 60 – 80 % of energy is derived from carbohydrate.

Glucose: Glucose is indispensable for the maintenance of the functional integrity of the nervous tissue and is the sole source of energy for the proper functioning of the brain. Prolonged lack of glucose may cause irreversible damage to the brain.

Protein Sparing Action: Carbohydrates exert a protein sparing action. If sufficient amounts of carbohydrates are not available in the diet, the body will convert protein to glucose in order to supply energy. Hence to spare proteins for tissue building, carbohydrates must be supplied in optimum amounts in the diet. This is called the protein sparing action of carbohydrates.

Fat Metabolism: Carbohydrates are essential to maintain normal fat metabolism. Insufficient carbohydrates in the diet results in larger amounts of fat being used for energy than the body is equipped to handle. This leads to accumulation of acidic intermediate products called ketone bodies.

Synthesis of Body Substances: Carbohydrates aid in the synthesis of nonessential aminoacids, glycoproteins (which function as antibodies) and glycolipids (which form a part of cell membrane in body tissues especially brain and nervous system). Lactose remains in the intestine longer than other disaccharides and thus encourages growth of beneficial bacteria.

Key Terms:-

• Amylopectin:- Branched form of starch made up of chains of glucose units linked to each other by 1,4 linkages in the chain and 1,6 linkages at point of branching. 
• Amylose:- Form of starch made up of long straight chains of glucose units linked together by 1,4 linkages. 
• Food Rheology:- It is the science of measuring forces, which are needed to deform food materials or to study the flow properties of liquid fluids. It deals with the viscous behaviour of a system. 
• Gelatinization:- Swelling of starch granules on heating with water, resulting in thickening of starch pastes, and leaching out of amylose from the granule into water. 
• Photosynthesis:- The process by which green plants manufacture complex foods from carbon dioxide in the air, and water and minerals from the soil in the presence of sunlight. 
• Relative humidity:- Method of measuring the moisture present in air relative to saturation at the same temperature. 
• Retrogradation:- Gradual increase of crystalline areas in starch gels during storage which affect texture and palatability of puddings. 
• Starch:- Complex polysaccharide made up of amylose and amylopectin. 
• Syneresis:- Separation of liquid from a gel caused by contraction of the solid phase thereby squeezing the liquid out or by cutting the gel.
• Viscosity:- The resistance of a liquid to flow. It is measured by an instrument called a viscometer. This property of a liquid is seen in batters, sauces, syrups, etc.