Realidad Problemática

Carbohydrates are either polyhydroxyaldehydes (aldoses, oses) or polyhydroxyke-tones (ketoses, uloses); there is an electron gap at their carbonyl carbon atom.

Typically, aldehydes and ketones accept nucleophiles such as water to form hydrates or alcohols to form hemiketals (5.1 and 5.3) and hemiacetals (5.4 and 5.6), respec-tively. In pentoses, pentuloses, hexoses, hexuloses, and higher carbohydrates, one of the hydroxyl groups can play the role of internal nucleophile. Thus, open-chain structure (5.2 and 5.5) cyclizes into internal hemiacetals and ketals, all with either five- (5.1 and 5.3) or six- (5.4 and 5.6) membered cycles.

Since all their carbon atoms are sp³-hybridized, the bond angles in the cycle should be about 109^ο to provide strainless conformations. It implies nonplanar conformations of the cycles (Siemion, 1985).

In some cases, the pyranose ring formation can be either obstructed or blocked, and a five-membered furanose ring dominates for the given sugar. The molecular

Structures 5.1 — 5.6

structure of di- and higher saccharides is additionally controlled by a potential energy benefit resulting from the formation of intramolecular hydrogen bonds, as in cello-biose (5.7a), lactose (5.7b), maltose (5.8 and 5.9), and sucrose (5.10).

Both maltose structures correspond to two energy minima, available thanks to intramolecular hydrogen bonds. Two hydrogen bonds might stabilize the sucrose molecule, provided the fructosyl moiety takes the furanosyl structure. Indeed, such possibility is employed in nature.

In polysaccharides, the structural factors are even more important. A number of different saccharide units in the chain, branching of the chain, and the presence of either more-polar groups (COOH, PO₃H₂, and SO₃H) or less-polar groups (OCH₃

Structure 5.7

Structures 5.8 — 5.10

TABLE 5.1

Essential Natural Saccharides in Food and Their Occurrence and Applications

Saccharide Occurrence Applications

Monosaccharides and Their Natural Derivatives Pentoses

D-Apiose Parsley and celery

L-Arabinose Plant gums, hemicelluloses, saponins, protopectin

Alcoholic fermentation; furan-2-aldehyde production

D-Xylose Accompanies L-arabinose Reduction of xylitol; sucrose substitute; alcoholic fermentation;

production of furan-2-aldehyde Hexoses

L-Fucose Mother milk, algae, plant mucus, and gums

D-Galactose Oligo- and polysaccharides, plant mucus and gums, saponins, glycosides

Diagnostics in liver tests

D-Glucose Plants and animals, honey, inverted sugar, saponins

Alcoholic fermentation; sweetener;

energy pharmacopeial material;

nutrient; food preservative D-Manose Algae, plant mucus, oranges

L-Rhamnose Plant mucus and gums, pectins, saponins, glycosides

Hexuloses

D-Fructose Fruits, honey, inverted sugar Non-cavity-causing sweetener;

sweetener for diabetics; food humidifier and preservative D-Glucosylamine Chitin, chitosan Pharmaceutical aid; antiarthritic

drugs; ion exchanger

L-Sorbose Rowan berries Synthesis of ascorbic acid

Disaccharides (oses)

Lactose Mammalian milk Improves the taste of dairy products;

fermenting component of milk Maltose Starch, sugar beet, honey

Sucrose Sugar beet, sugar cane Alcoholic fermentation; common sweetener; caramel production;

food preservation Polysaccharides

Agar Red algae Microbiological nutrient;

gel-forming agent; emulsifier; bread staling retardant; meat texturizer;

meat substitute

Alginates Brown algae Thickener; gel-forming agent; food

and beer foam stabilizer

and NHCOCH₃) than the OH group are crucial for the overall structure of polysac-charide. Amylose and cellulose are the most regularly built; they form polymer chains of α-D- and β-D-glucose units, respectively. In very random cases, amylose is branched with short chains (Ball et al., 1998). The amylose chain is originally long and randomly coiled (Figure 5.1), but it turns into a more ordered, helical

Carrageenans ι, κ, γ, µ, ν Red seaweed Gel-forming agent; stabilizer;

protein fiber texturizer; milk fat anticoagulant; milk clarifying agent

Cellulose Plants Saccharification to glucose; dietary

fiber; chromatographic sorbent

Dextran Frozen sugar beet Chromatographic sorbent

(Sephadex); blood substitute

Furcellaran Red seaweed Gel-forming agent; filler;

marmalade stabilizer; protein precipitation

Gatti gum Anageissus latifolia tree Emulsifier; stabilizer

Guaran gum Leguminous plants Food; cosmetics; pharmaceutical thickener and stabilizer Gum Arabic Senegal acacia Emulsifier; antistaling stabilizer;

flavor fixative

Gum karaya Sterculiacea tree (India) Foam stabilizer; thickener

Gum locust bean Locust bean Thickener; adhesive

Gum tragacanth Astragalus species (Middle East) Thickener; stabilizer

Glycogen Liver, muscle Glucose reservoir

Hemicelluloses:

Arabinogalactan Larch Emulsifier; stabilizer

Galactan Mannans, xylans

Plants Alcoholic fermentation; reduction to

alcohol

Heparin Liver, tongues Blood anticlotting agent

Hialuronic acid Connective tissues Water absorbent

Inulin Endive, Jerusalem artichoke Prebiotic

Pectins Plants, mainly apples, citrus, sugar beet

Gel-forming agent; beer stabilizer

Protopectin Plants and nonmatured fruits Decomposes to pectins on plant maturation and cooking Starch

Amylose, amylopectin

Tubers, grains, some fruits Food filler; thickener; gel-forming agent, bakery products;

saccharification to syrups Tamarind flour Tamarind tree (India) Thickener; marmalade; jelly; ice

cream; mayonnaise stabilizer Xanthan gum Semiartificial gum Hydrocolloid stabilizer

TABLE 5.1 (CONTINUED)

Essential Natural Saccharides in Food and Their Occurrence and Applications

Saccharide Occurrence Applications

structure. Helical complexes are formed if hydrocarbons, alcohols, lipids, fatty acids, and bar-like anions, such as I₅^–, OCN^–, are present in the amylose environment (Tomasik and Schilling, 1998a, 1998b).

Such compounds and anions, potential guests of the complex, either have hydrophobic fragments or are fully hydrophobic. The possibility of the reduction of the energy of the system by interactions of hydrophobic sides of amylose and the potential guest is the driving force of the formation of a helical complex.

Thus, its cavity is hydrophobic and all hydroxyl groups of the D-glucose units are situated on the external surface of the helix, as well as on the edges of its cavity (the V-type amylose). The number of glucose units in one helix turn depends on the guest molecule present in the helical complex. With KOH as a guest, one turn involves six glucose units. Inclusion of KBr reduces the turn to four glucose units, whereas inclusion of tert-butanol and α-naphthol requires the turns of seven and eight glucose units, respectively (Tomasik and Schilling, 1998a). An additional stabilization of the amylose helix comes from the double-helix formation (Imberty et al., 1991). Depending on the double-helix–double-helix interactions and, in consequence, their mutual arrangement, A- and B-type amylose is formed (Figure 5.2). Recently distinguished C-type amylose appears to be a combination of both A and B patterns.The conformation of β-D-glucose units bound in cel-lulose in the 1→4' manner offers a particularly strong hydrogen bond cross-linked macrostructure of this polysaccharide.

Glycans with 1,2-, 1,3-, and particularly 1,6-linked units have a more irregular, loosely jointed structure. The heterogenicity of the polysaccharide structural units and their volumes introduce further regularities or irregularities in the macrostruc-ture. A decrease in the group polarity, e.g., by methylation, and number results in a more irregular polysaccharide structure.

An opposite effect can be achieved in the presence of more polar groups, or even less polar, but suitably oriented groups. For instance, chitin, a polysaccharide with acetylamino groups, has a very regular, compact structure that provides insol-ubility of chitin in water. This polysaccharide is a common insect carapace-forming FIGURE 5.1 Randomly coiled amylose chain and its helical complex, formed as a result of its interaction with a nonpolar fragment.

x

x

material. Because of this property, chitin has found several technical applications (Goosen, 1997).

Amylopectin (Figure 5.3) presents a special case. It is a highly branched homopolymer of α-D-glucose units. The 1→6-linked terminal branches, which occur at about every 8th glucose unit, contain 15–30 glucose units. These branches can also participate in the formation of helical complexes. Some guest molecules may situate in areas around branching sites in the amylopectin molecule. In spite of an irregular, bulky structure, amylopectin also forms a double helix (Imberty et al., 1991). Because of functional properties, the structure of the polysaccharide matrix, the tertiary structure, is also essential. In solution, polysaccharides such as amylose form separate fibrils that, upon coiling, turn either into micelles at a low-temperature gradient or into gels at a high-temperature gradient. Both amylose and amylopectin participate in the starch granule organization (Gallant et al., 1997). Their seminative mixture has specific functional properties. It strongly depends on the amylose-to-amylopectin ratio; size of granules (Table 5.2); content of residual components of FIGURE 5.2 The crystallographic A- and B-types of amylose depend on the structure of double amylose helices.

FIGURE 5.3 Scheme of amylopectin molecule.

native starch, e.g., lipids, proteins, and mineral salts; and random esterification with phosphoric acid, the latter exclusively in the case of potato amylopectin. The amy-lose-to-amylopectin ratio decides on aqueous solubility of starch and texture of its gels, resulting from a penetration of water into starch granules (swelling) and from pushing the granule interior into a solution where the gel network is formed. The size of granules is essential for smoothness of products prepared from starch (pud-dings, gels). In practice, not all starch granules swell and participate in the gel formation. Larger granules are more susceptible to gelation and chemical modifica-tion (Lii et al., 2001b). The nutrimodifica-tional value of starches usually increases with content of residual components; however, in several cases, when starch is subjected to chemical modifications or is used for specific nonnutritional purposes, such

“contaminants” are nonbeneficial.

Cellulose that is completely insoluble in water forms microfibrils that are com-posed of crystallites and amorphous regions. Such regions may be also distinguished inside of starch granules. Roughly, they form concentric crystalline and amorphous layers surrounding the hilum, the origin of the granule growth (Gallant et al., 1997).

Amorphous regions contain amylopectin (Szymo´nska et al., 2000). The structure of granules is developed on plant vegetation by enzymatic debranching of so-called-plant glycogen (Erlander, 1998). These enzymes reside inside of starch granules and can be activated on starch processing.