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CARBOHYDRATES (or Glucides, from Greek glucos, sweet) • They are the most abundant and spread organic compounds on earth. • They have a central role in the metabolism of plants and animals. • Their biosynthesis in green plants, i.e. photosynthesis, starting from CO2 and H2O, in presence of light, is the existence base of all the other organisms. • They are the basic constituents of many foods representing a large portion of the total intake of nutrients in human diet (40-75% of the daily energy intake). • Also non digestible carbohydrates are important in a balanced daily diet. • Carbs have other important functions in foods (beyond the energetic): -they act as sweetening; -as gel- and paste- forming; -as thickening; -as stabilizers; -they are forerunners of aromatic substances and colorants which are formed in the food during production and subsequent processes (also cooking). 1 Carbohydrates can be defined as polyhydroxy aldehydes, ketones, alcohols, acids, their simple derivatives and their polymers having acetal type linkages. They may be classified according to their degree of polymerization and may be divided into three principal groups, namely sugars, oligosaccharides and polysaccharides. Cn(H2O)n Carbs chemical formula: This is a simplification, there are other molecules having different formula but reacting as carbs, thus belonging to the same category (e.g. deoxysugars, aminosugars, sugars with carboxyl moiety). CLASSIFICATION Class (DP*) Sub-Group Some components Monosaccharides Glucose, galactose, fructose Disaccharides Sucrose, lactose Polyols Sorbitol, mannitol Malto-oligosaccharides Maltodextrins Other oligosaccharides Raffinose, stachyose, fructo-oligosaccharides Starch Amylose, amylopectin, modified starchs Non-starch polysaccharides Cellulose, Hemicelluloses, Pectins, β Glucans, Fructans, Gums, Mucilages2 Sugars (1-2) Oligosaccharides (2-9) Polysaccharides (>9) DP * = Degree of polymerization • Monosaccharides: polyhydroxy-aldehydes or -ketones with a linear carbon chain (3 to 8 carbon atoms); e.g.: glucose, fructose and galactose; glucose fructose galactose • Oligosaccharides: formally derived from the condensation of monosaccharides, with H2O elimination; disaccharides: sucrose, maltose, lactose; trisaccharides: raffinose; tetrasaccharides: stachyose; raffinose lactose • Polysaccharides: polymers with high MW, having different characteristics with respect to other carbs; often insoluble in H2O, they are not sweet and they are somewhat inert; e.g.: starch, cellulose, pectins. cellulose (portion) 3 Total sugar in various foods Food Total sugar (%) Vaccine milk 4,8 Human milk 7,2 Cheese 0,1-0,9 Yoghurt 7,8 Fruit yoghurt 15,7 Ice cream 22,2 Apples 11,8 Banana 20,9 Grape 15,4 Oranges 8,5 Honey 75 Jam Chocolate 50-70 60 Beer 1,5-2,3 Wine 0-5 Dessert wine 5-15 4 Sugars distribution in fruit and vegetables Glucose Tomatoes Mais Fructose Soy Pineapple Sucrose Orange juice Onions Grapes Maltose Figs Carrots Galactose Cabbages Bananas Stachyose, Raffinose, Verbascose Apples 0% 20% 40% 60% 80% 100% 5 MONOSACCHARIDES NOMENCLATURE The suffix –ose indicates the presence of the carbonyl moiety in an hydroxylated carbon chain. ALDOSES: polyhydroxyaldehydes KETOSES: polyhydroxyketons deriving deriving formally from glyceraldehyde by formally from dihydroxyacetone by the the addition of –CH-OH units (they can addition of –CH-OH units (they can be be triose, tetrose…). triulose, tetrulose,… ); the position of carbonyl is specified by a numeric prefix H (usually is in position 2). O H OH CH2OH CH2OH suffix: -ose O suffix: -ulose CH2OH If two carbonyl moieties are present: -the molecule can be a dialdose (two aldehydes groups); -or osulose (one aldehyde and one ketone group); -or diulose (two ketone groups). When –OH is substituted by –H, the molecule is a deoxysugar, when –OH is substituted by –NH2, the molecule is an aminodeoxysugar. 6 CYCLIZATION All the monosaccharides starting from tetroses and 2-pentuloses cyclize to five and six membered lactols (furanoses from furan, and pyranoses, from pyran) respectively by intramolecular hemiacetal (or hemiketal) formation. With the exception of erythrose, monosaccharides crystallize in cyclic forms; in solution there is an equilibrium between the open chain and the cyclic forms, the second being predominant. 7 CONFIGURATION ALDOSES Glyceraldehyde has a chiral center, thus it exists as a pair of enantiomers, D and L forms. From D-glyceraldehyde it is possible to obtain a mixture of D-erythrose and D-threose, while from L-glyceraldehyde a mixture of L-erythrose and L-threose is obtained. CN CN O CHO H OH HCN H OH HO H H OH H OH CH2OH D-glyceraldehyde 1) H + CH2OH O OH + H H OH OH + OH H OH OH H 2) NaHg/NaBH4 CH2OH D-erythrose D-threose 8 Through the cyanhydrin reaction, starting from D-glyceraldehyde two D-tetroses are obtained and from each of them two D-pentoses, and so forth; thus from Dglyceraldehyde 8 hexoses belonging to the same D-series can be obtained. CHO H OH D-glyceraldehyde (D-glycero-) CH2OH CHO CHO D-erythrose (D-erythro-) H OH HO H OH H OH CH2OH CH2OH CHO H H OH HO H OH H OH HO H OH H OH H CH2OH CHO CHO CHO CHO CHO HO H OH H OH HO H OH H OH H OH H OH HO H OH H OH H OH H OH H D-allose (D-allo-) HO H H HO H OH H CH2OH CHO OH HO H H OH HO H HO H H OH H CH2OH CH2OH H OH CH2OH D-altrose D-glucose D-mannose D-gulose (D-altro-) (D-gluco-) (D-manno-) (D-gulo-) OH D-lyxose (D-lyxo-) CHO OH CH2OH OH D-xylose (D-xylo-) H CH2OH H CHO CH2OH D-arabinose (D-arabino-) CHO H H CH2OH D-ribose (D-ribo-) D-threose (D-threo-) H HO H H CHO H CHO OH HO H OH HO H HO H H HO H HO H OH CH2OH D-idose (D-ido-) H OH CH2OH D-galactose (D-galacto-) H OH CH2OH D-talose (D-talo-)9 An important aldoses degradation reaction takes place via disulfone formation starting from the dithioacetal. CH(SEt)2 CHO EtSH, H Epimers: + CH(SO2Et)2 - OH RCOOOH CHOH CHOH CHOH R R R CH2(SO2Et)2 + HC O R two molecules differing only for the configuration of a chiral center, e.g. D- glucose and D-mannose. Occurence of aldoses 10 KETOSES Name, structure Where is it found? hexulose D-fructose vegetables, honey D-psicose Eptulose, octulose, nonulose D-manno-2-heptulose D-glycero-D-manno-2octulose D-erythro-L-gluco-2nonulose residues of fermented molasses avocado " " L series?? 11 SYSTEMATIC NOMENCLATURE ALDOSE If the number of C atoms is <=6, traditional name can be used, otherwise the molecule portion adjacent to the carbonyl is assigned the maximum possible prefix and the remaining portion of the molecule, (if constituted at least by 2 C atoms), another prefix is assigned and this is named first; then the name is written depending on the total of C atoms. Examples: CHO CHO H OH HO H H OH H OH H HO H H OH H OH OH H OH H OH H OH H OH HO H HO H H CH2OH D-gluco- H HO H OH CH2OH D-glucose or D-gluco-hexose HO L-manno- H OH D-glycero- D-glycero-L-mannoheptose KETOSE If it contains less than 4 chiral carbons, traditional name can be used, otherwise the groups 12 adjacent to the carbonyl must be considered: The longest of the two portions adjacent to carbonyl is named first: HO HO OH H OH HO H HO H L-glycero- H H H HO H H OH H OH D-arabino D-threoD-lyxoD-threo-L-glycero3-hexulose or D-arabino-2-hexulose or D-lixo-3-hexulose When sugar cyclizes to lactol, a new chiral center is formed, thus two diastereomers (anomers) are formed, named α and β anomers. Fructose glucose α-D-glucopyranose β-D-glucopyranose 13 [α]20D= +110° ° ° [α]20D= +19° 64% 36% At equilibrium: ° [α]20D= +52,53° All the monosaccharides can exist in solution in five forms: <1% Cyclic forms are much favoured with respect to open chain forms. Generally, the favourite cyclic form, more stable, is the pyranosic one. 14 PHYSICAL PROPERTIES 1. OPTICAL ROTATION AND MUTAROTATION Non racemic chiral compounds deviate polarized light by an angle α proportional to their concentration in the solution. [α ]λ t 100 ∗ α = l ∗c α = deviation angle at T °C; l = polarimetric tube length (dm); c = grams of optically active substance in 100 ml of solution; [α α]tλ = specific rotation constant; λ = selected wave length (generally sodium D-line light); t = temperature at which the measurement is done (usually 20-25°C). Obviously specific rotation for two anomers is different (and even for furanose and pyranose forms of a same sugar); thus a solution of a pure isomer freshly prepared has a rotation angle varying during time till it reaches a constant value (at equilibrium among the various forms). MUTAROTATION PHENOMENON 15 2. Higroscopicity and solubility • The amount of water kept by the sugars depends on the sugar structure, the isomers present and the sugar purity. • Solubility of mono- and oligosaccharides in water is good. • Anomers can have very different solubility (e.g. α and β lactose). • Monosaccharides have low solubility in ethanol and they are insoluble in organic solvent such as benzene, ethyl ether, chloroform. 16 SENSORY PROPERTIES • Mono-, oligosaccharides and their alcohols are sweet (few exceptions); • Main sweeteners: sucrose, glucose, fructose, invert sugar (glucose and fructose), lactose and alcohols (sorbitol, mannitol, xylitol). • Sugars differ in the quality of sweetness and taste intensity. • As oligosaccharides dimension increases, their sweetness power decreases. • The taste intensity can be quantificated by determining the minimum level of concentration at which the sweet taste is still detected or referring to a reference solution (usually sucrose). Sugar Limit of detection % Fructose Glucose Lactose Maltose Sucrose 0.24 1.17 2.60 1.36 0.36 Sugar Sucrose D-Glucose D-Galactose D-Fructose Invert sugar Maltose Lactose Relative sweetness 100 69 63 114 95 46 39 Sugar D-Mannitol D-Mannose Raffinose D-Ramnose D-Sorbitol Xylitol D-Xylose Relative sweetness 69 59 22 33 51 102 67 17 • The minimum value depends on the affinity between the substance’s structure and the chemoreceptor sites for sweetness. • Further parameters influencing the quality and intensity of sweetness are: pH, temperature, presence of other compounds. Relative sweetness Temperature dependance of relative sweetness of some sugars • There is also a relation between sugar content and volatiles compounds. fructose glucose galactose maltose Temperature (° °C) • Also the color of the solution can influence the organoleptic evaluation. • Composition and concentration of sweetener must be carefully evaluated in each food formulation to give an optimal sensory result. Need of an AH (H donor) B (H acceptor) X (hydrophobic site) system in a substance in order to give sweet taste. D-glucopyranose 18 REACTIVITY 1) REDUCTION to ALCOHOLS • NaBH4 • electrolysis • catalytic hydrogenation Alcohol name: in the sugar name –ulose or –ose is substituted with –itol. Xylitol (pentose), sorbitol (naturally found in many fruits), D-mannitol are used in diet formulations, to decrease water activity, as softeners, etc. They afford 2,4 Kcal/g. 2) a. OXIDATION to ALDONIC ACIDS β-D-glucopyranose NAME: ALDOSE ALDONIC ACID 19 2) b. OXIDATION to ALDARIC ACIDS Stronger conditions (e.g. HNO3) allow oxidation of both the terminal carbons of aldose: It can form mono or dilactones NAME: ALDOSE ALDARIC ACID (dicarboxylic acid) 2) c. OXIDATION to URONIC ACIDS To oxidize saturated terminal carbon only, the carbonyl moiety of the aldose must be protected; then, after deprotection, the uronic acid is obtained. 20 Uronic acids are widespread in nature, forming polysaccharides (e.g. pectines) having industrial applications as gel-forming. 3) REACTION in BASIC and/or ACIDIC MEDIA • Monosaccharides are stable in a pH range of 3-7 (if compounds with amino groups are not present). • At low pH enolization followed by H2O loss, predominates. • At very high pH, enolization followed by chain fragmentation, predominates. 21 3) a. REACTION in STRONGLY ACIDIC MEDIUM • Disaccharides and oligosaccharides are formed (intermolecular glycosidic bond). • When the monosaccharide conformation is proper a glycosidic intramolecular bond can be formed. • Warming in acidic medium, enolization, dehydratation, formation of substituted furans and pyrans take place: H HC H HO HC O HC OH OH H O HO H - H2O HC O O O H O H - H2O H CHO OHC OH H - H2O O O H OH H OH H OH H H OH H OH H OH H CH2OH CH2OH CH2OH Where is the mistake??? OH CH2OH CH2OH CH2OH HMF (Hydroxy Methyl Furfural) 22 3) b. REACIONS in STRONGLY BASIC MEDIUM • In strongly basic medium aldose and ketose enolise quickly, thus fructose, mannose and glucose equilibrate by the formation of the shared 1,2-enediol. • In presence of O2 or other oxidants (Cu2+) the double bond C,C breaks forming carboxylic acids. The method is applied to the quali- quantitative determination of reducing sugars. • Anyway also other transformations can take place leading to formation of several volatile compounds. Some of the volatile compounds formed warming up fructose syrup at pH 8-10 for 3 h. • Acetic acid • Hydroxyacetone • Hydroxybutanone • Furfurylalcohol • 5-Methyl-2-furfurylalcohol • γ-butyrolactone • Various cyclopentenolones Cyclopentenolones are typical compounds with “caramel like” aroma O HO H3C 23 NON ENZYMATIC BROWNING Sugars contribute to the organoleptic characteristics of a food not only by their presence, but also by the products of their degradation. Caramelization (neutral or basic/acidic catalysis, high temperature, sugars) Maillard reaction (neutral or basic/acidic catalysis, high temperature, sugars, amino groups) NR O OH HO glucide RNH2 OH HO Schiff base NHR NR O OH -H2O OH O -NH2R O O HOH2C H CH2 HO HMF enaminol NHR Amadori compound O HO Browning is due to the polymerization of many molecules having low MW. Polymers formed (melanoidins) have structures as the following: N R N R N R X X X=O, NR 24 Aroma compounds are formed: lactones, furanons, pyranons, aldehydes, etc. O CH3 H3C O HO CH3 CH3 Sotolon (typical aroma of brown sugar). OH HO O O O CH3 O Negative aspects: • Milk browning; • Loss of essential aminoacids; • Formation of potentially carcinogenic compounds (heterocyclic amines); • Formation of aroma not always good (acrolein, piruvic aldehyde, glyoxal, etc.). Positive aspects: Formation of desirable aroma (toasting of coffee, cooking of food, etc.); Formation of colour compounds (cooking of bread, of meat); Formation of antioxidant compounds that protect the food against oxidation. 25