A presentation on carbohydrates and their classification

A presentation on carbohydrates and their classification

1. CARBOHYDRATES – CLASSIFICATION Jaya Upraity B.Voc ( F. P. & P.) 3 rd year

2. INTRODUCTION :- Food Chemistry is a major aspect of food science. Food chemistry deals with composition and properties of food and chemical changes it undergoes during handling, processing and storage. Chemically food consists of carbohydrates, proteins, lipids, vitamins, minerals, preservators , colouring and flavouring reagents of food. In this module we will learn more about them in details. Carbohydrate , class of naturally occurring  compounds  and derivatives formed from them.  In the early part of the 19th century, substances such as  wood ,  starch , and  linen  were found to be composed mainly of molecules containing  atoms  of  carbon  (C),  hydrogen  (H), and  oxygen  (O) and to have the general formula C 6 H 12 O 6 ; other organic molecules with similar formulas were found to have a similar ratio of hydrogen to oxygen. The general formula C x (H 2 O) y  is commonly used to represent many carbohydrates, which means “watered carbon.”  

3. Continue…. Carbohydrates are probably the most abundant and widespread organic substances in nature, and they are essential  constituents  of all living things. Carbohydrates are formed by green  plants  from  carbon dioxide  and  water  during the process of  photosynthesis .  Carbohydrates serve as  energy  sources and as essential structural components in organisms; in addition, part of the structure of  nucleic acids , which contain genetic information, consists of carbohydrate. Carbohydrates are defined as polyhydroxy aldehydes or ketones and their derivatives.

4.   General properties :-   Most of the monosaccharides , disaccharides and lower polysaccharides are sweet starch and cellulose are tasteless in taste, whereas   starch and cellulose are tasteless . They exhibit stereoisomerism. Stereoisomers are compounds whose structural formula is same but arrangement of atoms in 3-Dimensional space is different. E.g., with respect to Carbon no. 5, Glucose has two forms, D-glucose and L-glucose.

5. Continue….   They exhibit optical activity. Optical activity is a phenomenon in which a molecule rotates plane of plain polarised light. If the molecule rotates the plane towards left, it is designated levo rotatory (- sign) and if the plane is rotated towards right, the molecule is designated dextro rotatory (+ sign). D-glucose rotates plane of plain polarised light towards right while L-fructose rotates it towards left.  The sugars in which free aldehyde or ketone groups are present can reduce Fehling’s solution and Benedict’s solution are called reducing sugars. The sugars in which free aldehyde or ketone groups are absent are called as non-reducing sugars. Glucose, fructose, lactose etc., are reducing sugars whereas sucrose is a non-reducing sugar.

6. Functions:- They are a major part of total caloric intake. Starch and glycogen serve as temporary stores of glucose. Water insoluble carbohydrates are major components of cell wall in plant cells and cell membrane in animal cells. Carbohydrates function as lubricants in skeletal joints and help the cells to stick with each other and remain in place.

7. Types of carbohydrate: As per the molecular size, carbohydrate is differentiated into two sections: complex and simple. Simple carbohydrate : Different forms of sugar, like sucrose and glucose, are termed as simple carbohydrates.  The sizes of these molecules are small and can be broken down easily, and absorbed by human body. It helps in increasing blood sugar level quickly, when compared with complex carbohydrate. Some products, which comprise of simple carbohydrate, are dairy products, fruits, maple syrup and honey. It offers a sweet taste in some cakes and candies.

8. Continue…. Complex carbohydrate : Here, the carbohydrates are composed using longer strings of simple carbohydrates . As the sizes of these molecules are larger, they are broken down into simple molecule before absorption. It helps to provide energy to body in a slow manner, and is converted to fatty molecules. These molecules help in increasing blood sugar level slowly. Some examples of complex carbohydrates are fibers and starches, and available in wheat products, root vegetables, beans and other grains .

9. Classification :- Carbohydrates are generally classified into monosaccharides (simple sugars), oligosaccharides (containing few sugar units) and polysaccharides (containing many sugar units). Monosaccharides :- Monosaccharides are sugar molecules containing short chain of carbon atoms, one aldehydic or ketonic group and hydroxyl groups attached to remaining Carbon atoms. If they contain aldehyde group, they are named as aldoses and if they contain ketone group they are named as ketoses. Depending on number of Carbon atoms, monosaccharides are named as trioses (3 C atoms), tetroses (4 C atoms), pentoses (5 C atoms), Hexoses (6 C atoms) and so on.

10. Continue…. Important Monosaccharides :- D-glucose or Dextrose: It is the building block of starch and cellulose, source of energy for the animals. Fructose:  It is found in fruits and honey. Alongwith glucose, it is present in cane sugar (sucrose). Galactose : It is not found in free state in foods but it is component of milk sugar (lactose) and agar-agar. Arabinose and xylose : They are pentoses and found in plant gums.

11. Continue…. Oligosaccharides Oligosaccharides are formed by polymerisation of monosaccharide molecules by elimination of water molecules. Depending on number of monosaccharide units, oligosaccharides are classified as di-, tri- tetra-saccharides etc.The linkage between monosaccharide units in oligosaccharide units is glycosidic linkage.  Important oligosaccharides Sucrose:  It is most abundant in sugarcane and sugarbeet . It consists of one molecule of glucose and one molecule of fructose, joined together by glycosidic linkage between their anomeric –OH groups. Sucrose is thus non-reducing sugar. Maltose:  It is the intermediate product of starch hydrolysis. It consists of two molecules of glucose units with glycosidic linkage between anomeric C of one molecule and –OH molecule on C-4 of the other. Maltose is reducing sugar. Lactose:  It is found only in mammalian milk. It consists of one molecule of glucose and one molecule of galactose .

12. Continue…. Polysaccharides They are high molecular weight substancescomposed of large number of moosaccharide units combined to form one large polymer molecule. They may be straight chain or branched chain polymers. Important Polysaccharides Starch:  It occurs in cereal grains, pulses, tubers, fruits etc. It is the major source of energy in diet of man. Starch is a polymer of glucose. Starch is a mixture of two substances, about 25% amylose (straight-chain polymer) and 75% amylopectin (branched chain polymer). Because of the structural differences between amylose and amylopectin, they exhibit different properties. Amylose contributes to gel-formation while amylopectin is responsible for thickness properties of foods containing starch. Dextrin:  It is the product of partial hydrolysis of starch. Their properties are intermediate between those of sugars and starch. They are produced when starch is subjected to dry heat. E.g. during toasting of bread.

13. Continue…. Cellulose:  It is a straight chain polymer of glucose molecules. The glycosidic linkages are different in cellulose and amylose. Cellulose is insoluble in water. It is not digested in human tract. Modified cellulose is used in food industries  . Carboxymethyl cellulose is used in ice cream. Pectic substances:  They are constituents of plant cell wall. They are mixtures of polysaccharides formed from galactose , arabinose and galacturonic acid. Pectin, a water soluble component is present in fruits and forms gel with sugar and acid. Gums:  They are hydrophilic substances that give a viscous solution or dispersion when treated with hot or cold water. Starches, pectcic substances and derivatives of starch and cellulose are included in this category. They act as thickeners, moisture retainers, emulsion stabilizers, foam stabilizers, clarifying agents, etc. Important gums include Gum arabic (obtained from Acacia plant), Agar (obtained from red-brown algae) and algin (obtained from brown algae).

14. Biological Significance :- The  energy  stores of most animals and plants are both carbohydrate and  lipid  in nature; carbohydrates are generally available as an immediate energy source, whereas lipids act as a long-term energy resource and tend to be utilized at a slower rate.  Glucose, the prevalent uncombined, or free,  sugar  circulating in the blood of higher animals, is essential to  cell  function. The proper regulation of  glucose   metabolism  is of paramount importance to survival. The ability of  ruminants , such as cattle, sheep, and goats, to convert the polysaccharides present in grass and similar feeds into  protein  provides a major source of protein for humans. A number of medically important  antibiotics , such as streptomycin, are carbohydrate derivatives. The  cellulose  in plants is used to manufacture paper, wood for construction, and fabrics .

15. Functional attributes of carbohydrates in the food industry Both slow and fast releasing carbohydrates are used in food industry, for providing functional attributes. It comprises of viscosity, sweetness, coating ability, bulk, consistency, solubility, body, texture and browning capacity. Differences in chemical structure are mostly seen in various carbohydrates and are conferred in different functional uses. Gums, starches and pectins are used in cakes, jams, noodles, cookies and canned food as thickening agents. Alginate is a slow releasing carbohydrate, which provides texture and shape to fascinate food creation. Simple sugar is used for adding sweetness, consistency, texture and browning. In some food items, like ice-cream, corn syrup and sucrose is combined to impart sweetness, smooth texture and a glossy appearance .

16. Role in human nutrition The total caloric, or  energy , requirement for an individual depends on age, occupation, and other factors but generally ranges between 2,000 and 4,000  calories  per 24-hour period (one calorie, as this term is used in  nutrition , is the amount of heat necessary to raise the temperature of 1,000 grams of  water  from 15 to 16 °C [59 to 61 °F]; in other  contexts  this amount of heat is called the kilocalorie). Carbohydrate that can be used by humans produces four calories per  gram  as opposed to nine calories per gram of  fat  and four per gram of  protein . In areas of the world where nutrition is marginal, a high proportion (approximately one to two pounds) of an individual’s daily energy requirement may be supplied by carbohydrate, with most of the remainder coming from a variety of fat sources.

17. Continue…. In East Asia and in areas of Africa, for example, where  rice  or  tubers  such as manioc provide a major  food  source, starch may account for as much as 80 percent of the total carbohydrate intake.  In a typical Western diet, 33 to 50 percent of the caloric intake is in the form of carbohydrate. Approximately half (i.e., 17 to 25 percent) is represented by starch; another third by  table sugar  (sucrose) and  milk sugar  (lactose); and smaller percentages by monosaccharides such as  glucose  and  fructose , which are common in fruits, honey, syrups, and certain vegetables such as  artichokes ,  onions , and  sugar beets .  The small remainder consists of bulk, or indigestible carbohydrate, which  comprises  primarily the cellulosic outer covering of seeds and the stalks and leaves of vegetables. ( See also   nutrition .)

18. Role in energy storage  The principal function of which is the storage of starch; examples of plants with these cells include root vegetables and tubers.   The starch content of plants varies considerably; the highest concentrations are found in seeds and in  cereal  grains, which contain up to 80 percent of their total carbohydrate as starch.  The amylose and amylopectin components of starch occur in variable proportions; most plant species store approximately 25 percent of their starch as amylose and 75 percent as amylopectin. some plants (e.g., the  Jerusalem artichoke  and the leaves of certain grasses, particularly  rye  grass) form storage polysaccharides composed of fructose units rather than glucose. the fructose polysaccharides can be broken down and used to prepare syrups, they cannot be digested by higher animals. Starches are not formed by animals; instead, they form a closely related  polysaccharide ,  glycogen . glycogen is rapidly broken down to glucose, which is subsequently used as an energy source.