Nomenclature and Chemical Structure Glucose

The compound D-glucose (Greek gleucos, 'sweet wine') or dextrose is 2,3,4,5,6-pentahydroxyhexaldehyde, more conventionally expressed as C6H12O6, with a molecular weight of 180.16 kDa. Glucose is readily soluble in water in a powder form. Below 50 °C, a-D-glucose hydrate is the stable form; at 50 °C the anhydrous form is obtained; and at higher temperatures, a-D-glucose is obtained. Glucose is also present in the diet as part of the disaccharides sucrose (glucose and fructose), lactose (glucose and galactose), and maltose (glucose).

Glucose Oligosaccharides

Oligosaccharides (Greek oligo, 'few') are sugar polymers; the term usually refers to compounds containing 2-9 units but may include polymers containing up to 19 units. The dimer, trimer, and tetramer forms in which glucose molecules are joined by (1-4) linkages are referred to as maltose, malto-triose, and maltotetrose, respectively, since these substances are the products of starch digestion in the malting process. Sucrose, maltose, and lactose are common dietary disaccharides.

Starches

Starches are large-molecular-weight, a-linked polymers of glucose (C6H10O5)«. Most starches show a mixture of a(1-4) and a(1-6) linkages. The a(1-4)-linked polymer forms a linear structure that allows for hydrogen bonding between polymer chains and a more compact starch structure. Introduction of (1-6) linkages results in branch points and a more open structure that allows the (1-4)-linked backbone with the hemiacetal bond in the alpha configuration to coil like a spring into a helical form. Branched starches with the (1-6) linkage are more readily hydrated and digested compared to the (1-4)-linked linear starch. The (1-4)-linked starches are referred to as amylose starch, and (1-6)-linked starches are amylopectin starches.

Resistant Starch

Resistant starches are defined by their resistance to digestion in the human upper gastrointestinal tract. As with the term 'dietary fiber,' the definition is largely physiological. One proposed classification divides resistant starches into three classes: RS1, RS2, and RS3. The first class, RS1, is starch that escapes small intestinal digestion owing to the food form and incomplete enzymatic attack (e.g., large particle size or compact nature of food, or starch entrapment by dietary fiber). The second, RS2, includes the more crystalline starches that resist digestion (e.g., high-amylose starches that resist gelatinization). The RS3 starches are retrograded starches (e.g., high-amylose starches that upon cooling after cooking form a compact, hydrogen-bonded crystalline structure that excludes water).

Cellulose

Like starch, cellulose is a (1-4)-linked glucose polymer (C6H10O5)«, but in this instance the glucose molecules are ^-linked, allowing the development of a linear polymer with strong intrachain hydrogen bonding. Cellulose polymers may consist of as many as 10 000 glucose monomer units. Cellulose is both resistant to small intestinal digestion and insoluble in cold or hot water and most dilute acids and alkali. It is partially degraded by colonic bacteria; the proportion degraded is dependent on the source, with cellulose from vegetables broken down to a greater extent than cellulose from cereals such as wheat.

b-Glucans

In many ways, these predominantly (1-4)-linked glucose polymers are the cellulose equivalent of the starch amylopectin. Here, it is the (1-3) linkages interspersed throughout the polymer that prevent the compact structure achieved with the cellulose polymer where only the (1-4) linkages exist. As a result of the more open molecular structure of the /3-glucan, unlike cellulose, it is readily hydrated and soluble in water, forming a solution of high viscosity. The viscosity, in turn, is dependent on the molecular weight and the presence of the (1-3) linkages. The greater the molecular weight, the greater the viscosity. Thus, reduction of molecular weight by acid or enzymatic hydrolysis, which may also occur during food processing, may greatly reduce viscosity. The common feature shared by cellulose and the ^-glucans is that both are resistant to digestion by small intestinal enzymes. However, whereas cellulose is only partially fermented by the colonic bacteria, ^-glucans are completely fermented.

Hemicellulose

The term 'hemicellulose' should not be taken to imply a class of (1-4)-linked glucose polymers. The similarity with cellulose lies not in the chemical structure but in the fact that hemicellulose is also insoluble in hot or cold water or hot dilute acid. It is, however, soluble in dilute alkali. The polymeric structure is heterosaccharitic with two or more sugars (e.g., arabinoxylans found in cereals), with a relatively small molecular size (50-200 saccharide units).

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