Carbohydrates: Types and Properties

Sources of Carbohydrates

• Occur in plants, animal tissues, and microorganisms
• Animal sources: main sugar is glucose, storage sugar is glycogen
• Milk: exclusive disaccharide is lactose
• Plant sources: storage sugar is starch, storage polysaccharide is glycogen
• Gums: varied groups of polysaccharides obtained from plants, seaweeds, and microorganisms

Physical Properties of Carbohydrates

• Depend on monosaccharide composition, glycosidic linkage, functional group, molecular size, and branching
• Chemical modification of structure affects physical properties

Solubility of Carbohydrates

• Unmodified mono, oligo, and polysaccharides are generally soluble in aqueous solvents
• Insoluble carbohydrates: crystalline polysaccharides (cellulose), gelling polymers (agarose), and highly branched or cross-linked polymers (starch)
• Polar solvents (formamide, dimethylformamide, dimethyl sulfoxide, and pyridine) can solubilize insoluble unmodified carbohydrates through hydrogen bonding interactions
• Noncovalent derivatives of anionic carbohydrates (pyridinium, alkylphosphonium, and alkylammonium) are readily soluble in polar organic solvents
• Covalent carbohydrate derivatives with esterified or etherified hydroxyl groups are soluble in organic solvents

Viscosity and Crystallinity of Carbohydrates

• Polysaccharides are among the most viscous natural products and are used as thickeners or gelling agents in the food industry
• Viscosity of polysaccharides increases with chain length or molecular weight
• Polysaccharides do not lower surface tension of aqueous solutions, but their affinity for oil-water interfaces makes them good emulsifiers
• Sucrose (disaccharide) and cellulose (polysaccharide) are pure crystalline materials
• Most carbohydrates are isolated and synthesized as amorphous solids, glasses, or syrups
• Difficulty in crystallization of carbohydrates is due to conformational flexibility and multiple structure forms (pyranose, furanose, acyclic, alpha and beta configurations)

Hygroscopicity of Carbohydrates

• Many carbohydrates are hygroscopic, containing 2-10% water even after extensive drying
• Alginic acid (polysaccharide in seaweed) functions as an anti-desiccant, maintaining the viability of seaweed washed ashore on hot beaches

Stability of Carbohydrates

• Polysaccharides have stable secondary structures (helices) in solution
• Secondary structure is disturbed by temperature, pH, and denaturants
• Transition from helix to random coil is often completely reversible denaturation process
• Some polysaccharides form stable high-order structures in solution (triple helix, fibers, and gels)

Optical Properties of Carbohydrates

• Carbohydrates absorb very little UV/visible light
• Multiple chiral centers make carbohydrates optically active
• Many carbohydrates are either enantiomers (D and L glucose) or diastereomers (D-glucose and D-mannose)

Chemical Properties of Carbohydrates

• Chemical properties depend on monosaccharide residues, functional groups, linkage position, and configuration
• Acid-catalyzed sugar reactions: hydrolysis of glycosidic linkages, oxidation to aldonic acids, and dehydration to furfurals
• Base-catalyzed sugar reactions: oxidation to aldonic acids, epimerization, and isomerization

Oxidation of Sugars

• Aldose sugars can be readily oxidized to aldonic acids
• Fehling's test: an alkaline solution of copper(II)sulfate oxidizes aldose to aldonic acid, with cupric ion being reduced to cuprous hydroxide
• Enzyme test kits measure D-glucose content in the presence of other sugars in foods and biological systems
• Glucose oxidase reaction also used for the manufacture of D-gluconic acid and its lactone

Reduction of Carbonyl Group

• Hexitol found widely distributed in the plant world, ranging from algae to higher orders
• D-Mannitol and Xylitol: hydrogenation products of D-mannose and D-xylose, respectively
• D-Mannitol used as a non-sticky coating on candies, and Xylitol used in sugar-free chocolates, pressed mints, cough drops, and hard and soft candies

Reactions with Amines

• Aldoses or ketoses react with amines to produce various compounds, including flavors, aromas, and dark-colored polymeric materials
• The reducing sugar reacts reversibly with the amine to produce a glycosylamine, which undergoes Amadori rearrangement to give a derivative of 1-amino-1-deoxy-D-fructose

Starch Gelatinization

• The ability of starch to thicken mixtures in the presence of heat
• Factors affecting starch gelatinization: ratio of amylose to amylopectin, temperature, agitation, and the presence of other ingredients (sugar, acid)
• Effect of starch gelatinization on food products: browning, chewiness, color, and flavor

Crystallization and Caramelization of Sugars

• Crystallization of sugar: the ability of sugar to dissolve and reform crystals

• Factors affecting crystallization: temperature, acid, agitation, and the presence of other ingredients

• Caramelization of sugar: the decomposition of sugar into glucose and fructose, followed by condensation reactions to form aromatic compounds

• Factors affecting caramelization: temperature, moisture, and the presence of other ingredients### Pectin

• Pectin content is highest in slightly under-ripe fruit

• Over-ripe fruit has enzymes that break down pectin (pectinase and pectinesterase)

• Fruits like apples, currants, gooseberries, and citrus fruits are high in pectin, while softer fruits like strawberries, peaches, and cherries are generally low in pectin

• Low-pectin fruit can still be made to set into a jam or jelly by boosting the pectin content

Fructans

• Fructans are polymers of fructose
• They can begin with or without a single glucose molecule and have up to 1000 fructose units in plants
• Bacterial fructans can consist of up to 100,000 fructose units
• There are several types of fructans in nature, distinguished by their glycosidic linkages
• Inulins, levans, and graminan types are three groups of fructans, with different types of bonds between fructose units
• Inulins are linear fructans with β (2→1) bonds, found in plants and some bacterial fructans
• Levans are linear fructans with β (2→6) bonds, found in monocotyledon plants and bacterial fructans
• Graminan type fructans have both β (2→1) and β (2→6) bonds and are found in grasses

Fructans Applications

• Fructans are commercially obtained from chicory roots
• Inulin can stabilize water into a creamy structure with the same mouthfeel as fat
• Fructo-oligosaccharides (FOS) are used as prebiotics
• Inulin and FOS may enhance calcium and magnesium absorption
• Fructans can also be used as low-calorie sweeteners

Changes in Carbohydrates during Processing

• Carbohydrates can be altered by processing in various ways
• Caramelization is a non-enzymatic browning reaction that occurs when sugars are heated above their melting point
• The process involves the removal of water and breakdown of sugar, resulting in the formation of browning products

Loss through Leaching

• During wet heat treatment, low molecular weight carbohydrates (e.g., mono- and disaccharides) can be lost into the processing water
• The loss of glucose and fructose is higher than that of sucrose
• No leaching of dietary fiber occurs during blanching, boiling, and canning of vegetables and fruits

Alterations of Low Molecular Weight Carbohydrates

• Production of resistant oligosaccharides can occur during processing
• Fructo-oligosaccharides and galacto-oligosaccharides are synthesized from sucrose and lactose, respectively

Starch Heat-Induced Effects

• Gelatinization is the irreversible loss of crystalline regions in starch granules that occurs upon heating in the presence of water
• Gelatinization increases the availability of starch for digestion by amylolytic enzymes
• Retrogradation is the recrystallization of starch molecules upon ageing, which can reduce the digestibility of starch

Starch Texturization

• In pasta products, gluten forms a viscoelastic network that surrounds the starch granules, restricting swelling and leaching during boiling
• Pasta extrusion results in products with slowly digested and absorbed starch
• The slow-release features of starch in pasta relate to the continuous glutenous phase

Dietary Fiber

• Milling and peeling of cereal grains to refined flours remove the outer fiber-rich layers, resulting in a lower content of total dietary fiber
• Heat treatment can break weak bonds between polysaccharide chains, leading to solubilization of dietary fiber
• An increased temperature can lead to a depolymerization of the fiber, resulting in a decreased content of dietary fiber

Hydration Properties

• Grinding of raw materials containing cereal fibers can affect hydration properties
• Heat treatment can also change hydration properties, with boiling increasing the water-binding capacity of wheat bran and apple fiber products
• Roasting has no significant effects on hydration properties

Test your knowledge of carbohydrates, including the main sugar found in animals, the sugar exclusive to milk, and the properties of soluble and insoluble carbohydrates.