Carbohydrates.2007

Carbohydrates2007

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Unformatted text preview: CARBOHYDRATES CARBOHYDRATES DEFINITION CONFIGURATION SUGAR CLASSIFICATION CHEMICAL REACTIONS POLYSACCHARIDES GUMS Importance of carbohydrates Importance We use them as our major energy source (4 kcal/g) Humans : starch, sucrose and fructose 80% of our energy intake (average) We use them for their sweet taste We use them to provide structure and texture in food We products products Bread & pudding (starch); Dextrin (soft drinks); Pectin Bread (jellies) (jellies) We use them to lower water activity of food products We and also influence ice crystallization and Intermediate moist foods; Ice cream Importance of carbohydrates Importance We use them as fat substitutes Modifies starches & celluloses, and gums We use them to impart desirable flavors and colors for We certain food products certain Maillard browning We use them as an energy source in fermentation We reactions reactions Yogurt Dietary fiber We use them for their reported health “benefits” Definition of a carbohydrate Definition The word originates from “carbon” and “hydrate” or The “hydrates of carbon” “hydrates Cx(H2O)y The empirical formula showed equal numbers of carbons The and water and X=6 and Y=6 for glucose, galactose and fructose Simple carbs. are polyhydroxy aldehydes (aldoses) & Simple ketones (ketoses) By definition carbs. are aldoses, ketoses and By compounds derived from these via condensation, hydrolysis, reduction, oxidation and substitution hydrolysis, Classification of carbohydrates Classification Monosaccharides The simplest of the CHO forms Building blocks of other higher carbohydrates Two monosaccharide units 2-10 monosaccharide units >10 monosaccharide units Disaccharides Oligosaccharides Polysaccharides Monosaccharide classification 1. The number of carbons (3-9) 1. triose, tetrose, pentose, hexose…. 1 2 3 4 5 6 Fischer projection of monosaccharides Monosaccharide classification Monosaccharide 2. Configuration (simplest of all sugars) Sugars have asymmetric Sugars (chiral) carbons and therefore can exist in two forms (enantiomers) forms D-sugar vs. L-sugar, or + (R) vs. –(S) Based on the location of Based the –OH group of the highest asymmetrical center (right = D; left = L) center Monosaccharide classification Monosaccharide 3. Type of carbonyl group ALDOSE = Aldehyde group Glucose, galactose and mannose most common in foods Fructose most important KETOSE = Ketone group Aldehyde Ketone isomers Sugar ring formation Sugar Most sugar units of carbohydrates in nature (and thus Most foods) have ring structures foods) Formed by a reaction between the aldehyde or ketone Formed aldehyde ketone group and an –OH group of the sugar –OH This results in ring structures called: Hemiacetal (aldoses) Hemiketal (ketoses) These can further react to create di-, oligo- and These polysaccharides (condensation reactions) and react with alcohols Formation of α - and β -anomers of D-glucose A new asymmetric center is created and the carbon at that center is known as the anomeric carbon (labeled *) If the –OH is facing down at C* then we have the αanomer If the –OH is facing up at C* then we have the β -anomer The most common sugar ring forms forms Pyranose Six-member rings More thermodynamically More favorable favorable Most common Five-member rings More kinetically favorable Furanose The more correct representation of the ring form the The pyranose and The furanose rings are not flat not For pyranose rings the For chair and boa...
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