Food Chemistry – Carbohydrates Lecture 2 PDF

Lecture 5. Food Chemistry - Carbohydrates FDSC 330. FUNDAMENTALS OF FOOD SCIENCE Topic Outline Topics 1 Introduction General structure Functional roles in foods Sources General reaction 2 Classification...

Lecture 5. Food Chemistry - Carbohydrates FDSC 330. FUNDAMENTALS OF FOOD SCIENCE Topic Outline Topics 1 Introduction General structure Functional roles in foods Sources General reaction 2 Classification Simple Carbohydrates Complex Carbohydrates  Introduction ▪ Compounds containing Carbon, Hydrogen and Oxygen ▪ General formula – Cn(H2O)n = CnH2nOn, ▪ if n=6, C6H12O6 (Glucose) ▪ Most common ones are 6-Carbon ones ▪ Examples are: glucose, mannose, galactose CHO CHO CHO HO C H H C OH H C OH HO C H HO C H HO C H H C OH H C OH HO C H H C OH H C OH H C OH CH2OH CH2OH CH2OH D-Mannose D-Glucose D-Galactose Source of carbohydrates ▪ Many foods including fruits and vegetables ▪ Commercial source of sucrose is sugar cane and sugar beets Functional roles in foods ▪ Sweetness: sucrose, fructose, glucose ▪ Colour and flavour by chemical reactions: the Maillard reaction, and caramelization ▪ Structural component of fruits and vegetables: pectin, starch, cellulose ▪ Various functional roles: viscosity, mouth feel, gelation ▪ 75% of biological world ▪ 70-80% of human’s caloric intake ▪ Principal means of energy storage 5 Functional roles in foods ▪ Nutrition ▪ A major energy source: 4 cal/g ▪ Several health beneficial roles ▪ Dietary fibre - coronary heart disease, cancer, diabetes, etc ▪ Sugar alcohol - diabetes, dental caries 6 Classification Nutritional ▪ Available: carbohydrates which are capable of being digested and utilized by the human body, e.g. glucose, sucrose, sorbitol, maltose, fructose, starch. ▪ Unavailable: carbohydrates which are not digested and utilized by the human body; e.g. pectin, cellulose, ▪ Glucogenic: the ability to be converted to D-glucose, e.g. starch, sorbitol etc ▪ Glycaemic: carbohydrate in a food that produces a measurable glycaemic response after ingestion: - Glycemic index (GI) & Glycemic load (GL) Structure-Nomenclature Glycose is generic name for sugars Number of sugar units o Monosaccharides: One sugar ring (1) o Disaccharides: Two (2) sugar rings combined o Oligosaccharides: A few (3-10) sugar rings combined o Polysaccharides: Many (>10) sugar rings combined No. C atoms: triose, tetrose, pentose, hexose Aldose vs. ketose: glucose (dextrose) vs. fructose (levulose) Simple Carbohydrates  Sugars ▪ Monosaccharides – Single sugars – contain 3 to 8 carbon atoms ▪ Most common contain 5 to 6 carbon atoms ▪ Most common 6 – carbon sugars are Glucose & Fructose ▪ Have general formula – C6H12O6 Glucose Fructose Simple Carbohydrates ▪ Glucose is an aldose sugar – it as an aldehyde group ▪ There are two Isomers – D-Glucose and L-Glucose, D form is more common D-Glyceraldehyde L-Glyceraldehyde Reference Carbon Atom Reference Hydroxyl Group  Simple Carbohydrates The straight line confirmation exists as an equilibrium with ring confirmations C1 Anomeric C atom C5 Above the plane for D-sugars; if below its L-sugar Anomeric –OH group is in the Anomeric opposite plane to CH2OH – α- Glucose C atom Anomeric –OH group is in the same plane to CH2OH – β-Glucose ▪ In solution, α and β forms are in equilibrium but when they reacts to form a disaccharide – they are fixed ▪ Knowing whether it is α and β is important since they determine many chemical properties including digestibility ▪ Glucose is the most important aldose sugar – two other aldose sugars are mannose and mannose ▪ D-fructose is the only important Ketose sugar in food  Disaccharide  Glycosidic bond – formed when the carbonyl group of one monosaccharide reacts with the hydroxyl group of another with the elimination of water This free end can form new Glycosidic bond bonds α-1, 4-glycosidic bond Free to rotate – can either take α/β confirmation  Examples – Maltose (2 glucose units with α-1, 4-glycosidic bond)  Cellobiose (2 glucose with β-1, 4-glycosidic bond) Disaccharide ▪ Maltose is the building block for starch – Alpha bonds can be broken by the body, so starch can be digested ▪ Cellobiose has beta linkage – Beta linkage cannot be digested by our enzymes. E.g. Cellulose is known as a dietary fiber ▪ Sucrose = Glucose + Fructose in α-1,2-glycosidic bonds ▪ Sucrose can be hydrolyzed to glucose and fructose by heat, acid, invertase and sucrose ▪ The glucose and fructose obtained like this is called invert sugar – important during production of candies and jellies – invert sugar prevents unwanted excessive crystallization of sugar Properties of Sugar Sugar alcohols  Carbonyl group is reduced to hydroxyl group  Examples are Xylitol, Mannitol, Sorbitol  Sweet in nature but not as sweet as sucrose  Products containing sugar alcohols are labelled ‘Sugar free’  Reducing Sugar ▪ Sugars that can be oxidised by mild oxidising agents are called reducing sugars because the oxidising agent is reduced in the reaction. ▪ A non-reducing sugar is not oxidized by mild oxidizing agents.  The end of a chain with a free anomeric carbon (not involved in a glycosidic bond) is commonly called the reducing end  All common monosaccharides ar e reducing sugars.  The disaccharides maltose and lactose are reducing sugars. Lactose: A reducing Sugar Non-reducing Sugar: Sucrose Differences between reducing and non- reducing sugars are: Reducing Possess a free aldehyde (-CHO) or ketone (-C=0) Group. Can reduce the Cu2+ cupric ions (blue) in Fehling’s or Benedict’s Solution to Cu+ cuprous ions (reddish) that precipitate out as Cu2O (cuprous, lactose, melibiose, gentiobiose, cellobiose, mannotriose, rhamnotriose. Non-reducing ▪ A free aldehyde or ketonic group is lacking. ▪ No such reaction. ▪ Sucrose, trehalose, raffinose, gentiarose, melezitose.  Condensation  Reaction making a disaccharide  chemical reaction linking 2 Losing water! monosaccharides Hydrolysis Adding water! Reaction breaking a disaccharide – water molecule splits – occurs during digestion  Complex Carbohydrates: Oligosaccharides More complex than disaccharides Composed of between 3-10 monosaccharides Joined by the elimination of a molecule of water Less prominent in foods Formed during the transition of complex carbohydrates (starch) into simpler di- and monosaccharides. Raffinose - Galactose-Glucose-Fructose O-a-D-galactopyranosyl-(1->6)-a-D-glucopyranosyl-b-D- fructofuranosid  Complex Carbohydrates Polysaccharides Carbohydrate formed by union of many saccharide units, with the elimination of a molecule of water at each point of linkage – Dextrans – Starch – Glycogen – Cellulose Non-glycosidic Polysaccharides – Pectic substances – Gums Homopolysaccharides – Contain only a single type of monomer Heteropolysaccharides – Contain two or more different kinds of monomers Glycogen  Limited in meat and not found in plants  Long branched chain of Glucose molecules  Not an important dietary source of carbohydrate  all glucose is stored as glycogen  long chains of glycogen allows for hydrolysis and then they release (glucose) as a source of energy Glycogen  Storage form of carbohydrates in animal tissues  Similar to amylopectin in structure  Polymer of (-1,4)-linked subunits of glucose, with (- 1,6)-linked branches  Glycogen is more extensively branched  More compact than starch  Starches ▪ Stored in plant cells ▪ Polymers of glucose ▪ Intestinal enzymes hydrolyzes plant starch to glucose ▪ Complex carbohydrate has two fractions: amylose and amylopectin ▪ Plants (storage carbohydrates): Potato, corn, cassava, wheat and legumes ▪ Have different composition, shape, size and granule crystals Amylose Amylopectin  Amylose  Straight-chain (unbranched)  Slightly soluble starch fraction  Glucose units joined together by 1,4-  -glucosidic linkages formed with the loss of water Non- Reducing Reducing end end  Amylopectin  Very large polysaccharides (molecular weight of a million or more)  Glucose units linked with 1,4--glucosidic linkage interrupted with a 1,6- - glucosidic linkage  Branching occurs at about 24 to 30 residues  Branching results in less solubility  Form transparent highly viscous solution Starch Utilization ▪ Thickening and binding agent ▪ Soups, sauces, puddings ▪ Protecting cover ▪ Dehydrated fruits (Dates & figs) ▪ Decrease oxidation (fries) ▪ Edible wrapping (film packaging) ▪ Stabilizer  Dextrins  Polysaccharides composed entirely of glucose units linked together and distinguishable from starch because of the distinctly shorter chain length.  Produced by Starch hydrolysis - by - amylase or heating with small amount of acidic or basic catalysts  Slightly soluble  Barely sweet  Limited thickening ability  Cellulose  Key structural component (plant cell walls/cotton/wood etc)  Linear configuration, Similar to amylose  1,4--glucosidic linkage  Insoluble  Indigestible  Dietary fiber (maintain intestinal mobility)  Production of low calorie food products  2 Derivatives (methyl- & carboxymethyl-) (-CH2-COOH)  Improve texture & rehydration, decrease oil uptake, retard crystal formation in ice cream  Non-glucosidic Polysaccharides Inulin Pectic substances Gums Inulin (FOS) Polymer of fructose It is built as a linear chain of fructose units by means of  1,2- bonds, having one terminal glucose molecule. Non-digestable Sugar and fat substitute Nutritional source for probiotic bacteria.  Pectic Substances ▪ Group of complex carbohydrates found in fruits. ▪ Polymer of galacturonic acid ▪ Methylated to form methyl esters ▪ Gel formation (MW and esterification) ▪ Low-ester pectin form gel at low pH, sugar and/or Ca++ while High-ester require high sugar ▪ Stabilizer  Pectic Substances ▪ Group of complex carbohydrates found in fruits. ▪ Polymer of galacturonic acid ▪ Methylated to form methyl esters ▪ Gel formation ▪ Low-ester pectin form gel at low pH, sugar and/or Ca++ while High-ester require high sugar ▪ Stabilizer  Gums ▪ Found in seeds, plant exudates, and seaweeds ▪ Galactose is the dominant monomer ▪ Mannose, arabinose, xylose & rhamnose ▪ Gum Arabic & locust bean gum ▪ Emulsifier, binder & foam stabilizer ▪ Flavor fixative ▪ Retard sugar crystallization, fat separation & ice formation in ice creamer Any Questions? Thank you