Carbohydrates Types & Structure PDF

Carbohydrates Importance & role   Carbohydrates or saccharides come from the Greek word sakkharon meaning sugar)  Important constituent of animal & plant tissues as well as occurs in microorganisms  It is importance ma...

Carbohydrates Importance & role   Carbohydrates or saccharides come from the Greek word sakkharon meaning sugar)  Important constituent of animal & plant tissues as well as occurs in microorganisms  It is importance macronutrient in human body and is major source of energy  It provides fuel for the central nervous system and energy for Introduction working muscles  Carbohydrates also serve as: (1) a short term energy source for all organisms, (2) structural molecules in plants, and (3) storage forms of foods in plants and animals Definition  Chemically, carbohydrates are made up of carbon, hydrogen and oxygen  They are hydrates of carbon with the empirical formula Cm(H2O)n (where m could be different from n)  Structurally, they are more accurately viewed as polyhydroxy aldehydes and or ketones   Simple sugars are generally absorbed and rapidly utilized for energy or converted to glycogen and fat for storage  The more complex carbohydrates range from those that are highly digestible, i.e., starch to those that are non-digestible, i.e., celluloses and chitins  Hemi-cellulose and some oligosaccharides are not digestible but Importance are fermented in the colon resulting in production of beneficial short chain fatty acids  The simple sugars can add sweetness, alter the boiling point and freezing point of foods, and help stabilize proteins in solution  In plants, these molecules (e.g., cellulose) are also components of the supporting tissue (such as the wood in trees)  Carbohydrates can be divided as: Monosaccharides (simplest saccharide) Oligosaccharides (having 2 to 10 units) 1st Polysaccharides (having more than 10 units) Classifications Derivatives (having sugar & non sugar monomers) Also classified as: Simple & 2nd Complex saccharides 1st  Tetroses Pentoses Trioses Hexoses Maltose 2nd This is on the basis of structure and how quickly they are absorbed and digested in body   The simplest sugars or monomeric units are called Monosaccharides monosaccharides  They cannot be hydrolyzed into smaller carbohydrates  Chemically, they are aldehydes or ketones with two or more hydroxyl groups  The general chemical formula of an unmodified monosaccharide is Cn(H2O)n  Are ranging from three to eight carbon atoms which constitute the backbone of the sugar  The carbon backbone is then appended with Hydrogen (H) or Hydroxy (−OH) groups  Classification based on: (1) the placement of the carbonyl group,  If the carbonyl group is an aldehyde then the sugar is an aldose monosaccharides  If the carbonyl group is a ketone the sugar is a ketose Classification of (2) the number of carbon atoms it contains, and  With three carbon atoms, are named trioses, those with four carbons are tetroses, with five carbons pentoses, with six carbons hexoses, and so on.  Monosaccharides of 5 or 6 carbons, however, are the most common (3) chirality  Carbohydrates have a chiral center around which the OH group is either to the left (L) or to the right (D)  Most natural sugars are members of the D series  D and L structures are non-superimposable mirror images, named enantiomers   Tautomers are structural isomers of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydrogen atom within the compound  The sugars are in tautomeric equilibrium in solution results in change in optical rotation  The process will accomplished in several hours or longer until the Tautomerization equilibrium exists and the optical rotation reaches its equilibrium value  Example: the glucose can exist in four tautomeric forms: β- furanoside—0.14%, acyclic aldehyde—0.0026%, β-pyranoside— 62%, and α-pyranoside—38%  Similarly, fructose under the same conditions also exists in four tautomeric forms as follows: α-pyranoside—trace, β-pyrano- side—75%, α-furanoside—4%, and β-furanoside—21% Conformational isomers  Reducing Sugars Non-reducing Sugars  The sugars that can act as a  The sugars do not have free reducing agent due to the aldehyde or ketone presence of free aldehyde functional groups and ketone groups  Don’t form brick-red Sugars Types  Reduce cupric ions of Fehling reagent and Benedict's precipitate reagents with these reagent to cuprous ions and produce brick red  Examples: Sucrose, raffinose, precipitates stachyose and all polysaccharides  Examples: glucose, fructose, maltose, lactose etc Non-reducing sugar   The molecule having saccharides units between 2-10  Oligosaccharides can be homologous or heterologous Oligosaccharides  Sucrose is the most common primary oligosaccharide in plants  These are of two types  Primary: synthesized in vivo from mono- or oligosaccharides by donating glycosyl using glycosyl transferase enzyme e.g. sucrose  Secondary: those formed in vivo or in vitro by hydrolysis of larger oligosaccharides, polysaccharides, glycoproteins, or glycolipids  Examples: raffinose, stachyose, trehalose etc   Two chemically bonded monosaccharides are known as disaccharides  Can be classified into two types: reducing and non-reducing  Disaccharides are formed by a glycosidic link between the reducing groups on one saccharide with the hydroxyl group of Disaccharides another saccharide   Sucrose, a major sweetener, is a primary oligosaccharide (or disaccharide) found widely in plants  The largest commercial sources are from sugarcane or sugar beet  It is non-reducing type of CHO  It is more labile to acid, comparatively Sucrose  When sucrose is heated to 210 °C, partial decomposition takes place and caramel is formed  Sucrose is highly soluble over a wide temperature range, hence is a excellent ingredient for syrups and other sugar containing foods   The sugar in mammalian milk is lactose, which is normally easily digested and converted to energy  It also cause lactose intolerance in human having lactase (β- galactosidase) deficiency  It is less sweet than sucrose and is a reducing sugar Lactose  Major constituent of the dry matter of milk and constitutes about 50% of the total solids  It range from 4.4 to 5.2% in milk  Lactose is a disaccharide of D-galactose and D-glucose and is designated as 4-O-β-D-galactopyranosyl-Dglucopyranose   Are the basic building block of starch and glycogen polysaccharides  Having (4-α-D-glucopyranosyl-β-D-glucopyranose) linkage  The α-1 → 4 linkage is broken by amylases and maltases  Maltose is a reducing disaccharide, shows mutarotation, is Maltose fermentable, and is easily soluble in water   It is a reducing disaccharide resulting from the partial hydrolysis of cellulose  Having (4-β-D-glucopyranosyl-β-D-glucopyranose) linkage  In cellulose and cellobiose the β-1 → 4 linkage is not hydrolyzed by animal and most microbial enzymes Cellobiose  More stable than starch and hence present abundantly in nature  The polymeric structure is important for plant cell wall structure, and fibers such as cotton   They are the polymers of saccharides having more than 10 units attached with glycosidic linkage Polysaccharides  The number of monosaccharides in the polymer is referred to as the degree of polymerization (DP)  There are only a few polysaccharides with DPs less than 100, most are in the 200–300 DP range  Cellulose can have DPs from 7000 to 15,000  It has been estimated that over 90% of the carbohydrate mass in the world is in the form of polysaccharides  These are classified as homoglycans (having same type of monomers) like starch and glycogen  Or heteroglycans, having different type of monomers like agar, gums, pectin etc   The major carbohydrate polysaccharide in plant tubers and seed endosperm is starch  Starch is a homopolymer of D-glucose and is a storage carbohydrate in plants  It occurs as small granules which vary with source  Each granule contains several million amylopectin molecules packed Starch with a much larger number of smaller amylose molecules  corn is the largest single source of commercial starch, other commonly used sources are wheat, rice, potato, and tapioca  The amylose polymer is composed of glucose units in a linear polymer while amylopectin is a highly branched polymer  In amylose these are linked through -(1 → 4) bonds  whereas in amylopectin about one residue in every 20 is also linked -(1 → 6)- forming branchpoints  The amount of both the polymers may differ with source  Amylomaizes contain over 50% amylose whereas ‘waxy’ maize has almost none (~3%)   Most available form of energy  It is used for water binding and as a thickener, an emulsion stabilizer and gelling agent Importance of  Starch is a major component in many foods and ingredients such as wheat flour, where it is 80% of the flour starch  Starch, therefore, inherently delivers function to the final product  Refined starches are also added to foods to provide functionality such as gelation or thickening  Amylose exhibits the important function of acting as a hydrocolloid  Heated starch when cooled, cause retrogradation which is the important deterioration in bread  Amylopectin forms gel when heated in water   Humans, animals, and fungi produce glycogen in muscle as an energy reserve  Glycogen is a highly branched polysaccharide of glucose  Glucoses are linked together linearly by α-(1 → 4) glycosidic bonds, and branches are linked to the chains with α-1 → 6 glycosidic bonds  It affects the glycemic index of the body Glycogen   Starch  Glycogen  Starch is a storage form of  It is storage form of energy in energy in plants animals and fungi  Amylose is un-branched  Highly branched similar to while amylopectin is amylopectin branched  1-4 and 1-6 glycosidic linkage Difference  1-4 glycosidic bond in between the monomers amylose and 1-4 glycosidic and 1-6 glycosidic linkage in  It has branching after around amylopectin every 10 subunits  amylopectin has branching after around every 20 subunits   Cellulose is one of the most widely distributed compounds in nature generally existing as homologus polymers  It provides structural support to most plants and includes the major component in wood.  Cotton cellulose is used for fabric and can be used in some food applications  Cellulose is a polymer of β-D-glucose joined with β-1 → 4 linkages  It is basically resistant to most of the enzymes in crystalline form and Cellulose have very low solubility in water  An example of this is that when foods like carrots are dehydrated, crystallinity increases and the foods become tougher  Cellulose and modified celluloses are used in a wide range of foods to provide important physical characteristics like bulk replacements of digestible carbohydrates in low calorie foods  Chemically modified celluloses provide emulsification, modification of texture, emulsification, foam stabilization, water binding and ice crystal formation   The addition of carboxyl groups increases the hydrophilic nature of the cellulose, where the addition of hydrocarbons such as methyl or ethyl groups makes the cellulose more hydrophobic  Carboxymethyl modification of cellulose is one of the most common modifications used to produce cellulose gum  Higher substitution increases water holding capacity Cellulose   Microcrystalline cellulose is a type of purified and partially depolymerized cellulose Microcrystalline  It is white, odorless, tasteless, and occurs as a crystalline powder made up of porous particles  It can be synthesized by different processes including enzyme mediated reactions and acid hydrolysis etc Cellulose  In the food industry, MCC can be used as an important base in functional foods to:  (1) maintain emulsification and foam stability,  (2) maintain high temperature stability,  (3) improve liquid stability, and  (4) act as a nutritional supplement and thickener Pentosans/Hemicelluloses  They are groups of non –starch and non-cellulosic polysaccharides  Hemicelluloses are water insoluble polysaccharides and pentosans are water soluble non-strachy polysaccharides  Hemicelluloses are structural plant polysaccharides made up of non-glucose containing sugars  Sugars can be xylose, arabinose, mannose or galactose  The hemicelluloses generally contain polymers of two to four different sugar units  Cereals are common sources of hemi-celluloses and pentosans  Arabinose and xylose are the primary pentoses found in pentosans and hemicellulose  cellulose is a highly branched with arabinose on the side chains and an acidic core with seven to eight xylopyranose units and a one D-glucuronic acid attached by a 1 → 2 linkage at the branch point  Glucuronic acid attached with 1-2 linkage Hemicellulose structure   The water-soluble pentosans are highly branched, highly viscous, and gel forming  Because of these properties, it is thought that the pentosans may pentosans contribute to the structure of bread dough Wheat flour pentosan repeating unit of L-arabinofuranosyl linked by β1 → 4 glycosidic bonds to D-xylopyranose backbone   It is synthesized from starch using glycosyltransferase enzyme which contain six, seven, or eight glucose units and form cyclic units  These are known as α-, β-, and γ-cyclodextrins, respectively  The hollow cavity CDs are relatively hydrophobic in nature Cyclodextrins because of hydrogen atoms and internally aligned glycosidic oxygen  While the outer surface is hydrophilic because of exposed polar hydroxyl groups  Therefore, it is form a best complexes with other molecules like vitamins, cholesterol, pigments etc  It is linked in a ring by α-1 → 4 glycosidic bonds   It will provide cementing to the cellulosic network and help control the movement of water  During ripening of fruits, the hydrolysis of pectic substances takes place and soluble type pectins will form  Pectin is primarily composed of repeating units of α-1 → 4- galacturonic acid Pectins  Pectins can also contain α-D-galactouronan formed by repeating units of 1 → 2-α-L-rhamnosyl-(1-L) α-D-galactosyluronic acid  The main blocks in pectin are branched galacturonan chains that are periodically interrupted with rhamnose units which causes bending in the chain  The galacturonic acid is partially esterified with methyl groups, and the degree of esterification varies among plant species  The number of units between branch points ranges from 8 to 20 residues  They are heteroglycans   The methyl esters in pectins are ranged between 9 to 12 % (degree of methylation)  Treatment of pectin with alkali results in hydrolysis of the methyl esters  Complete hydrolysis of the esters results in forming pectic acid which is completely insoluble  In the presence of calcium ions, sugar and acid pectin forms gels Pectins  These gels are three dimensional networks of pectins that bind large amounts of water  Two types of pectins are commonly used in foods are referred to as High methoxy- (HM) and Low methoxy- (LM) based on degree of methylation  HM pectin has 50–80% of the acid sites esterified with methyl esters and the LM has 25–50% esterification  For HM gels to form stable gels they require the pH to be below 3.6 and the sugar content to be at least 55% by weight  Pectin is one of the most versatile stabilizers and gelling agents in food applications Gel formation   They are characterized by its ability to give highly viscous solutions at low concentrations  These polysaccharides are extensively used in the food industry as stabilizers, thickeners, emulsifiers, and gel formers  Gums have hydrophilic molecules, which can combine with water to form viscous solutions or gels  Examples: Gums Gum Arabic: is a dried exudate from acacia trees made up of four sugars, L-arabinose, L-rhamnose, D-galactose, and D-glucuronic acid Guar and Locust Bean gums: are composed of galactomannan polymers and made up of α (1 → 4)-β-linked D-mannan polymer which is appended with single units of (1 → 6)-α-linked galactopyranosyl side chains Agar: is extracted from red algae of the class Rhodophyceae, It is soluble in boiling water but is insoluble in cold water and its structure indicates alternating 1 → 4 linked, 3,6-anhydro-L-galactose units and α1 → 3 linked D-galactose units  Guar gum Agar   It is the extracellular polysaccharide from the plant pathogen Xanthomas campestris high solubility in water  It has a pentasaccharide repeating unit  Having charged trisaccharide sidechains, which solubilize the polymer, occur on alternate glucose residues of a cellulose-like Xanthan backbone, giving rise to a highly branched molecule  Used as thickening agent in foods   Originally, the fiber content of food is known as crude fiber and defined as the residue remaining after acid and alkaline extraction of a defatted sample  Dietary fiber can be defined as a complex group of plant Dietary Fibres substances that are resistant to mammalian digestive enzymes  They includes cellulose, hemicellulose, lignin, cell wall components such as cutin, and soluble polysaccharides such as pectin  They are important dietary constituents because of properties like bulking capacity and water holding capacity  Overall functions of dietary fibers are:  Structural polysaccharides: cellulose, hemicellulose and pectins  Structural non-polysaccharides: mainly lignin  Nonstructural polysaccharides: gums and mucilage Types of fibres  1. Amino Sugars Sugars Derivatives  Usually contain D-glucosamine  These are of high molecular weight compounds such as the chitin of crustaceans and mollusks, insects, as well as in certain mushrooms and in combination with the ovomucin of egg white  Chitin can be hydrolyzed to produce chitosan which has potential as a food ingredient  After cellulose, chitin is the second most important natural polymer in the world  Chitin is a largely inert polymer composed of long chains of acetylglucosamine  Upon hydrolysis the acetates are removed from the amino group leaving a glucosamine unit  Chitin can be hydrolyzed with acid or enzymatically to remove the acetate from the amino sugar leaving a free amino group in chitosan  Chitosan has many applications (food, cosmetics, biomedical, and pharmaceutical applications  2. Glycosides Sugars Derivatives  Glycosides are sugars in which the hydrogen of an anomeric hydroxy group has been replaced by an alkyl or aryl group to form a mixed acetal  They are non reducing in nature because of the formation of full acetal  Hydrolysis of glycosides yields sugar and the aglycone  Example: genistin, is an flavonoid found in edible plants like soybean  Chemically it is the 7-O-beta-D-glucoside form of genistein  Genesteine is the bioactive form that includes antiatherosclerotic, estrogenic and anticancer. 7-O-beta-D-glucoside bond  3. Sugar alcohols  These are compounds obtained when aldo- and keto- groups of a Sugars Derivatives sugar are reduced to hydroxyl groups  Since, sugars are polyhydroxy compounds, their corresponding sugar alcohols merely have one more alcohol group  Sugar alcohols are also referred to as polyols, polyalcohols, or polyhydric alcohols  Some are present, naturally in foods, while some are commercially synthesized  Some are even sweet to the taste and some are used as food sweeteners  Furthermore, sugar alcohols provide fewer calories than sugar and have less of an effect on blood glucose  Examples of sugar alcohols are: glycerol, erythritol, isomalt, lactitol, mannitol, sorbitol, and xylitol