The Chemistry of Food Lecture 1 ( Dr. AN Boa )

Carbohydrates – general The Chemistry of Food • Source of energy and sweetness (see later) Lecture 1 Chemistry in Context • Carbohydrates consist of single units known as monosaccharides or multiple units thereof. 06525/06529/06509 • Simple sugars (mono- and disaccharides) • Oligo- and polysaccharides (sugar polymers) Dr AN Boa • Many stereoisomers possible 1 3 Topics in Food Chemistry Polarimetry analyzing Physical and chemical properties of: monochromator l polarizer α • Carbohydrates α – saccharides (mono-, di-, oligo- and poly-) • Proteins c c – amino acids and polypeptides • Lipids sample cell – oils and fats light source polarizer • Colourings • Flavour chemicals Observer rotates analyzing polarizer to view maximum / minimum • Vitamins light and records angle α • Minerals • Water Length of sample cell l(dm) is known, concentration c ( • Preservatives may be known or unknown

Simple Sugars Monosaccharides • Glucose • Monosaccharides mainly exist as cyclic hemiacetals – also known as grape sugar, corn sugar or dextrose • Attributed names on the basis of the ring size, e.g. furanose (5-ring, c.f. furan) and pyranose (6 ring, c.f. pyran) – dextrorotatory (+) – made commercially by hydrolyzing starch • Fructose – also known as fruit sugar or laevulose – laevorotatory (-) (but in the D-sugar series!) – made by hydrolyzing starch and isomerizing the glucose from the corn syrup hemiacetal 5 • α- and β- anomers of the cyclic forms exist 7 Monosaccharides Cyclisation of linear sugars H • General Formula (CH O) O 2 n O O O • Polyhydroxy carbonyl 5 H 5 compounds, e.g. aldoses 1 1 (aldehyde based) and ketoses (ketone based) Carbonyl carbon freely rotates • Carbon content can vary →O can attack either (typically 3-8) for each type H face of C=O OH of saccharide, e.g. pentoses (C5) and hexoses 5 O OH 5 O H (C6) 1 1 FISCHER PROJECTIONS α-anomer β-anomer

The anomeric effect Mutarotation • When compared to the β-anomer, the α-anomer is often much more stable than • Single molecule in solution can interchange expected so that in many cases [α-anomer] > [β-anomer]. between – straight chain and ring α-pyranose β-pyranose – different ring sizes – αand βanomers straight chain • Mutarotation is α-furanose β-furanose – a dynamic equilibrium – all isomers can potentially exist in solution • There is a stereoelectronic preference for conformations in which the best – energy / stability of different forms vary donor lone pair, or bond, is antiperiplanar to the best acceptor bond. 9 11 Sucrose and Lactose: Monosaccharides disaccharides • Fructose • Sucrose – Also known as cane sugar or beet sugar – Obtained commercially from sugar beet or sugar cane – Only one isomer present in solution – Easily crystallized + • Lactose – Also known as milk sugar – Primary sugar in mammalian milk (human > cow) Fischer projection Haworth projection – Also is a by-product of cheese manufacture – Not very sweet (relative intensity ≤0.3 cf sucrose) • Conversion of either anomer into the equilibrium mixture is called mutarotation – Hydrolyzed by enzyme β-galactosidase. Many adults lack this enzyme and are therefore lactose intolerant

Glycosidic linkages Invert sugars • Disaccharides are formed by linking two monosaccharides • Invert sugar is produced by acid hydrolysis of sucrose via a glycosidic link (acetal). • Produces a mixture of glucose and fructose H2O + sucrose →glucose + fructose [α]D +66.5º +52.7 º – 92.4º SUCROSE LACTOSE – Rotation of solution goes from dextrorotatory to laevorotatory overall – Process is called “inversion” of sucrose – producing “invert sugar” 13 15 Tri- and tetrasaccharides Polysaccharides Raffinose • Digestible α-D-galactopyranosyl-(1J6)-α-D-glucopyranosyl-(1J2)- – Starches (glycogen / amylose / amylopectin) β-D-fructofuranose • Indigestible (dietary fibre / roughage) Stachyose – Cellulose α-D-galactopyranosyl-(1J6)- α-D-galactopyranosyl- (1J6)-α-D-glucopyranosyl-(1J2)-β-D-fructofuranose – Pectin – Seaweed polysaccharides (agarose / carageenan) • Both found in legumes (peas, beans, soya) – Xanthan and gellan (gums) • But neither hydrolysed nor digested by humans…… • providing “a feast” for bacteria (e.g. E. coli) in the gut which produce H2 and CO2 causing flatulence.

Glycogen Cellulose • Glycogen is the major carbohydrate storage • Cellulose is a a linear glucan with β-(1,4)-glycosidic molecule in animals. linkages – Humans lack cellulase enzymes, so cellulose is indigestible for us • It is a homopolymer of glucose (a glucan) with α-(1,4)-linkages. • Different stereochemistry of glycosidic bond compared to starches [i.e. β−(1,4)- vs. α-(1,4)-] – results in very different molecular shape in solution • It is also highly branched, with α-(1,6) branch linkages occurring every 8-10 residues. 17 19 Starches: amylose and amylopectin Amylose vs. Cellulose • Starch is the major form of stored carbohydrate in plant cells e.g. grains, potatoes etc. • Unbranched starch is called amylose and branched starch is called amylopectin. – Amylose : linear glucan with α-(1,4)-glycosidic linkages and ~104 subunits – Amylopectin : branched glucan with α−(1,4) and α-(1,6) linkages with ~106 subunits: more compact than amylose for same MW • Structure is identical to glycogen, except for a much lower degree of branching (about every 20-30 Amylose helix Cellulose sheets residues). 18 (Solomons and Fryhle, 2000; © John Wiley & Sons, Inc.)

Pectin • Polysaccharide of D-galacturonic acid (and derivatives) joined by α-(1,4)-linkages – can contain up to 20% neutral sugars – found naturally in fruits, vegetables – charged at higher pH – can form gels – used in the making of jams / preserves 21 Other food polysaccharides Polysaccharide Subunits Linkages Characteristics Alginate D-mannuronic α−(1,4) + Charged, linear + L-guluronic β−(1,4) Agarose D-galactose α−(1,3) + Neutral, linear β−(1,4) Carrageenan sulphonated α−(1,3) + Charged (-SO3-), D-galactose β-(1,4) linear Guar Gum D-mannose & α−(1,6) + Neutral, branched D-galactose β−(1,4)

Bir yanıt yazın

Başa dön tuşu