Food Chemistry - Carbohydrates Overview

Food Chemistry - Carbohydrates

• Carbohydrates are compounds containing carbon, hydrogen, and oxygen.
• General formula: Cn(H₂O)n = CnH₂nOn (e.g., if n=6, C₆H₁₂O₆ (Glucose))
• Common carbohydrates have 6 carbon atoms, like glucose, mannose, and galactose.
• Carbohydrates are a major source of energy for humans.

Topic Outline

• Introduction: General structure, functional roles in foods, sources, general reaction
• Classification: Simple carbohydrates, complex carbohydrates

Simple Carbohydrates

• Sugars: Monosaccharides (single sugars) contain 3-8 carbon atoms.
  • Most common have 5 or 6 carbon atoms (e.g., Glucose, Fructose).
  • General formula: C₆H₁₂O₆.
• Glucose is an aldose sugar - it has an aldehyde group. D- form is more common.
• Two isomers: D-Glucose and L-Glucose.
• The straight chain form exists in equilibrium with ring confirmations (alpha and beta). These forms are fixed when reacting to form other molecules.
  • Alpha and beta forms determine many chemical properties including digestibility.
• D-Fructose is an important ketose sugar.

Simple Carbohydrates: Disaccharides

• Glycosidic bond – formed when the carbonyl group of one monosaccharide reacts with the hydroxyl group of another, with the elimination.
• Examples:
• Maltose (2 glucose units with a-1, 4-glycosidic bond)
• Cellobiose (2glucose units with β-1, 4-glycosidic bond)
• Sucrose (Glucose + Fructose in a-1,2 -glycosidic bonds)

Simple Carbohydrates: Other

• Sugar Alcohols: Carbonyl group reduced to hydroxyl group
  • Examples: Xylitol, Mannitol, Sorbitol
  • Sweet in nature but not as sweet as sucrose.
  • Products containing sugar alcohols are often labeled "sugar-free".
• Reducing Sugars: Sugars that can be oxidized by mild oxidizing agents (e.g., Benedict's reagent or Fehling's solution).
• Non-reducing Sugars: Sugars that cannot be oxidized by these reagents. Sucrose is an example.

Simple Carbohydrates: Nomenclature

• Glycose: Generic name for sugars.
• Number of sugar units:
  • Monosaccharides (1)
  • Disaccharides (2)
  • Oligosaccharides (3-10)
  • Polysaccharides (>10)
• Number of carbon atoms: triose, tetrose, pentose, hexose
• Aldose vs. ketose: glucose (dextrose) vs. fructose (levulose)

Complex Carbohydrates

• Oligosaccharides: Composed of 3-10 monosaccharides, and are joined by the elimination of a water molecule.
• Examples: raffinose (Galactose-Glucose-Fructose)
  • Formed during transition of complex carbohydrates (starch) into simpler di- and monosaccharides.
• Polysaccharides: Large molecules formed by linking many monosaccharides with the elimination of a water molecule at each linkage point.
  • Examples: dextrans, starch, glycogen, cellulose.
    • Non-glycosidic carbohydrates (e.g., pectic substances, gums)

Starch

• Stored in plant cells.
• Intestine enzymes hydrolyze plant starch to glucose.
• Examples of complex carbohydrates (storage carbohydrates): potato, corn, cassava, wheat, and legumes.
• Starch has two fractions: amylose and amylopectin

Glycogen

• Storage form of carbohydrate in animal tissues.
• Similar to amylopectin in structure (long, branched chains of glucose molecules).
• More extensively branched than starch, so it is more compact.
• Released as glucose for energy.

Cellulose

• Key structural component of plant cell walls.
• Linear configuration.
• 1,4-β-glucosidic linkage.
• Insoluble, indigestible.
• Dietary fiber; maintains intestinal mobility.
• Used in the production of low-calorie food products.

Inulin

• Polymer of fructose.
• Built as a linear chain of fructose units by β-1,2- bonds.
• Has one terminal glucose molecule.
• Non-digestible.
• 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 forms gel at low pH, with sugar and/or Ca++. High-ester requires high sugar.
• Stabilizer

Gums

• Found in seeds, plant exudates, and seaweeds.
• Galactose is the dominant monomer.
• Mannose, arabinose, xylose, and rhamnose are other monomers found.
• Emulsifier, binder, foam stabilizer.
• Flavor fixative.
• Retards sugar crystallization, fat separation, and ice formation in ice cream.

Sources of Carbohydrates

• Many foods including fruits and vegetables.
• Commercial source of sucrose: sugarcane and sugar beet.

Explore the essential aspects of carbohydrates in food chemistry, including their structure, classification, and functional roles. This quiz covers simple and complex carbohydrates, highlighting common types such as glucose and their importance as energy sources. Understand how these compounds react and their significance in nutrition.