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