Carbohydrates GGF PDF

Food Science MFR1106 Carbohydrates Carbohydrates Also called saccharides, are chemically defined as polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses), i.e., organic compounds with at least three carbons where all carbons have a hydroxyl, except for one, which has the primary carbonyl...

Food Science MFR1106 Carbohydrates Carbohydrates Also called saccharides, are chemically defined as polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses), i.e., organic compounds with at least three carbons where all carbons have a hydroxyl, except for one, which has the primary carbonyl (aldehyde group) or the secondary carbonyl (keto group) Structure Aldose carbonyl at the extremity of the carbon chain Ketosis secondary carbonyl Cellular functions - Energetics – obtaining energy, e.g., glucose, sucrose; - Structural – polymerized or associated with other biomolecules, e.g., cellulose, chitin; - Chemical energy deposit – energy reserve, e.g., starch in vegetables, glycogen in animals and fungi, lactose in milk, fructose in fruits and sucrose in sugarcane; - Carbon donor – for the synthesis of other cellular components, e.g., organic acids. Classification (according to the degree of polymerization): - Monosaccharides - single unit of polyhydroxyaldehyde or ketone, e.g., glucose; - Oligosaccharides - short chains of monosaccharide units, e.g., sucrose; - Polysaccharides - contains more than 20 monosaccharide units, e.g., cellulose. Saccharide, from the Ancient Greek, sakcharon, means sugar Monosaccharides – Are the simplest carbohydrates Ex: glucose (C6H12O6) C1 – aldose 5 hydroxyl groups (C2-C6) 4 chiral centers (C2-C5) Chiral center - carbon atom bonded to four different substituents – They are colorless compounds, crystalline solids, soluble in water and insoluble in nonpolar solvents. Trioses Hexoses Pentoses Representative monosaccharides All monosaccharides, except dihydroacetone, contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms. Glyceraldehy de Isomers (D) OH right (L) OH left Aldoses A molecule with n chiral centers can have 2n stereoisomers Cetoses A molecule with n chiral centers can have 2n stereoisomers Epimers - two sugars that differ only in configuration around a single carbon atom Cyclic structures of monosaccharides For simplification, aldoses and ketoses are represented in linear form. However, in aqueous solutions, aldotetroses and monosaccharides with five or more carbons predominate as cyclic structures (ring) Ring is the result of the general reaction between aldehydes or ketones and alcohols forming derivatives called hemiacetals or hemiketals Formação das duas estruturas cíclicas da D-glicose Free hydroxyl in C5 reacts with the aldehyde group in C1, which becomes a new asymmetric center, producing two new stereoisomers (anomers) (α and β) (α) OH lower plane (β) OH upper plane The interconversion of α and β is called mutarotation. Formation of the cyclic structure of D-fructose Pyranoses and furanoses Six- or five-carbon rings formed Derivatives of hexoses Formation of acetals and ketals Reaction between hemiacetal or hemiketal and alcohols forming derivatives called acetals or ketals O-glycosidic bond Ex: Maltose formation Lactose Disaccharides Sucrose Trehalose Disaccharides Polysaccharides - Frequently classified according to their characteristics of their chemical structure - Composed of glycosidic units arranged in linear or branched form. - Importance: They play an important role in plants and foods. From the nutritional standpoint, starch major polysacharide and the most digestible in the human intestine. Indigestible polysaccharides also have great importance in health (large intestine). Polysaccharides 1) Starch 2) Glycogen 3) Cellulose 4) Hemicellulose 5) Pectins 6) Gums Starch - Found abundantly in plant tissues: tubers and in the endosperms of seeds. - The granules are irregular in shape ranging from 2 – 100 µm. - The shape and size of the granules is characteristic of each species and can be used to identify the origin of any starch or flour. - Formed by 2 glucose polymers: Amylose + Amylopectin - Starches contain 20-25% amylose. Pea starch contains 60% amylose. Starch: Amylose and Amylopectin Amylose: formed by a linear chain of α-D-glucopyranose, joined by 1-4 glycosidic bonds. Amylopectin: branched structure, consisting of 20 to 25 units of α-D-glucose, joined by glycosidic chains. It consists of 10 to 500 thousand units of glucose. Glycogen - Main storage polysaccharide in animal cells (liver (2-8%) and in muscle at low [ ] (0.5-1%) - It has a structure very similar to amylopectin - Presents > molecular weight and degree of branching is much > than amylopectin - Main CHO stored in muscle tissue and animal liver - Consists of glucose units in -1,4 glycosidic bonds and 1,6 bonds in branches. - It is more branched and compact than starch Cellulose - CHO most abundant in nature - Constitutes 1/3 of all plant matter in the world and is the main CHO stored in the cell wall of higher plants. - Formed by linear and unbranched homopolysaccharide - Glucose units joined by -1,4-type bonds. Cellulose The molecular structure of cellulose: the bonds are not broken by the digestive enzymes of animals and the arrangement of glucose units in the molecule allows the formation of hydrogen bonds and the stacking of chains, which makes cellulose extremely resistant Cellulose derivatives Products with very useful properties for the food industry can be obtained by intense chemical modifications of the cellulose molecule, obtained from cellulose treated with NaOH: 1) Carboxy-methyl-cellulose (CMC): Used to increase the viscosity of foods. Dissolved in water, the viscosity of which decreases with increasing temperature. Solutions are stable at pH 5-10. It has wide application in food: it acts as a binder and thickener in puddings, processed cheeses, etc. In ice cream, it slows down the growth of ice crystals. In confectionery, it slows down the growth of sugar crystals. 2) Methyl cellulose (MC) and methyl hydropropyl cellulose (MHPC): Obtained by digesting cellulose with NaOH and methyl chloride. Thickener, in cakes it increases water absorption, in dietary foods it acts as a syneresis inhibitor and provides volume increase, in frozen foods it prevents syneresis, and in whipped cream (emulsifier). Hemicellulose - Complex polymers found in plant cell walls, associated with cellulose and lignin. - Smaller molecules consisting of units of D-xylose, Larabinose, D-mannose and L-rhamnose. - They are dietary fibers not digestible by humans. Pectins - Pectin is a polygalacturonic acid partially esterified with methoxyl groups. - They are macromolecules of galacturonic acid methyl ester, associated with galactans and arabinans; - Units of galacturonic acid that are linked by α-1-4 bonds; - Present linear and crystalline chains - They are used in foods such as fruit-based jellies. The industrial production of pectins developed from food byproducts, e.g., using waste from the fruit juice and beverage industry. - Commercial pectin is obtained mainly from acid extraction of citrus fruit albedo (20-30%) and apple pulp (10-15%). - Main property is the ability of gel formation. Pectins Chemical structure of pectin chain Gums - Long chain polymers with high molecular weight extracted from seaweed, seeds, tree exudates and animal collagen. - Rigid linear polymer due to the glycosidic bonds α-1.4. - They disperse and dissolve in water, increasing viscosity. - Hydrates quickly in cold water. - Viscosity may be little affected by pH and salts, but amounts of sucrose can reduce its viscosity. - They are thickeners. - They may or may not form gels. Gums Gums extracted from tree exudates: 1) Gum Arabic 2) Gum Tragacanth 3) Gum carrageenan Gums obtained from seaweed extracts: 4) Agar-agar 5) Alginate Gums produced by microorganisms: 6) Xanthan gum (Xanthomonas) 7) Gellan gum (Sphinggomonas elodea) 8) Konjac gum Browning reactions - Main problems in the fruit, vegetable and beverage industry, > 50% loss of tropical fruits is due to polyphenoloxydase (PPO) enzyme (dark pigment formation) - Browning reactions can be both oxidative and nonoxidative: a) Oxidative or enzymatic browning: Reactions between O2 and phenolic substance catalyzed by the enzyme PPO and does not involve carbohydrates. b) Non-oxidative or non-enzymatic browning: important in foods, it involves caramelization phenomena and/or interaction of proteins or amines with CHO (Maillard’s reaction). The intensity of the darkening reaction depends on the amount and type of CHO Browning reactions Browning reaction = REDUCING SUGAR + AMINOACID → degradation of sugar = formation of dark pigments They are associated with heating and storage. Two mechanisms for the browning reaction to occur are: a) Caramelization reaction b) Maillard´s reaction Caramelization reaction • Browning occurs due to the heating of CHO, especially sugars and syrups: caramelization • Caramelization involves the breakdown of sugars. • Sugars in solid state are relatively/stable to moderate heating, but if temperatures rises to > 120 oC they are pyrolysed to various high molecular weight and browning degradation products: called caramels. • The low molecular weight fraction present in the caramelized mixture contains, in addition to sugar, pyruvic acid and aldehydes. • Caramel is a brown coloring, flavoring agent prepared by pyrolysis of sugar. • Low-color caramels are more useful as flavoring agents than the same colorings. • Caramels obtained using catalysts need lower temperature (130-200 oC) have high color, used as food coloring. Caramels obtained without catalysts (200-240 oC) have low color intensity and are useful as flavoring agents. • Sucrose is used to produce caramel flavorings and colorings, it is heated with acid solution or ammonium salts (catalysts) to produce various products such as the syrup of “cola” soft drinks. Maillard´s reaction • Louis-Camille Maillard, in 1912, while trying to synthesize peptides under physiological conditions, opened the doors to what would later become the food color and flavor market. • Non-enzymatic browning that occur very quickly in fresh fruits and vegetables when they are cut (apple). • that characterize them, also in cocoa cooked meat, bread, cakes, etc. • Desirable reactions are those that give characteristic flavor, aroma and color to these foods. In some foods such as coffee and cocoa roasting, Maillard’s reaction is responsible for the development of the color and pleasant aromas. • Undesirable reactions for powdered milk causes the development of unpleasant odors in dairy products during storage. Such odors are caused not only by lipid degradation but also by Maillard reactions. In the latter case they derive from the presence of molecules with heterocyclic nucleus, namely pyrazines and pyrroles, causing the loss of nutrients (amino acids). Factors that affect the Maillard´s reaction 1) Temperature: Initial reaction occurs at temperature > 70oC, but at 20oC occurs during processing or storage. At high temperature there is a rapid browning, increasing 2 to 3 times at 10oC. Frozen foods are little affected by Maillard’s reaction. 2) pH: In an acid medium, the amine group predominates, eliminates nucleophilicity and delays reaction. In alkaline medium there is rapid degradation of CHO regardless of the presence of CHO and aminoacids. At pH below 5, darkening occurs caused by the oxidation of ascorbic acid (vitamin C) 3) Types of sugar: The presence of a reducing sugar is important to occur the interaction with the free amine groups. In this way, reactivity occurs with pentoses being more reactive than hexoses and hexoses more reactive than disaccharides. 4) Water activity: Aw > 0.9, the reactions decreases, due to dilution of the reagents, at aw <0.2-0.25 the speed tends to 0 (absence of solvents), > darkening is increased at aw intermediates (0.5 to 0, 8). Inhibition of the Maillard´s reaction • Use of non-reducing sugars such as sucrose (without hydrolyzing); • Reduce to aw; • Removal of reducing sugars by enzymes: treatment with glucose oxidase enzyme; • Addition of SO2, inhibits enzymatic browning, [ ] adequate.