agronom.me
Menu

Agrochemistry

The role and distribution of carbohydrates in the tissues of agricultural crops

For students

5 min read

AGROCHEMISTRY A

The role of carbohydrates and their sweetness scale

Carbohydrates make up three-quarters of the dry matter of plants and serve as the main product of photosynthesis. They are used by the plant organism as the primary respiratory material and storage nutrients, which accumulate in roots, tubers, and seeds. Furthermore, carbohydrates form cell walls and plant fibers, and they are also predominant in the composition of fruits and berries. This group of compounds includes starch, cellulose, sugars, pectins, inulin, hemicelluloses, gums, and mucilages.

Sugars serve as a vital source of energy in the diet. At the same time, they differ significantly in their degree of sweetness. If we take the sweetness of glucose as a base indicator, sugars are distributed in decreasing order of sweetness. In this regard, only a limited number of monosaccharides are found in plants.

  • Carbohydrate share in dry matter — 3/4
  • Sweetness of fructose — 220
  • Sweetness of sucrose — 200
  • Sweetness of glucose — 100
  • Furanose forms in fructose — 15%
Carbohydrate Relative sweetness (glucose = 100)
Fructose (fruit sugar) 220
Sucrose (cane or beet sugar) 200
Glucose (grape sugar) 100

Practical properties of basic monosaccharides

Glucose is contained in a free state in all green parts of plants, seeds, various fruits, and berries, especially in grapes. It is a component of starch, cellulose, and many di- and trisaccharides. On an industrial scale, glucose is produced via acid hydrolysis of potato or corn starch. The resulting syrup is widely used in confectionery production and medicine.

Fructose is present in the green mass of plants, flower nectar, fruits, and seeds of grain crops. In the form of beta-D-fructofuranose, it is a component of sucrose, raffinose, stachyose, and polyfructosides. The most well-known polyfructoside — inulin — accumulates as a storage carbohydrate in the tubers of Jerusalem artichoke, chicory, and kok-saghyz. Like glucose, fructose is easily fermented by yeast.

A mixture of almost equal amounts of fructose and glucose forms the basis of natural honey. It is precisely the high fructose content that ensures its pronounced sweetness.

Galactose is a component of raffinose, agar-agar, hemicelluloses, plant gums, and mucilages. In free crystalline form, this monosaccharide is extracted from ivy fruits. The specific rotation of aqueous solutions of galactose after equilibrium is established between alpha- and beta-forms is +80.2°. This sugar is fermented only by specific lactose yeast strains.

Mannose occurs in plants in the form of mannan polysaccharides and glycosides, and is rarely found in a free state. Its derivative, the alcohol mannitol, is more frequently present in plant tissues. For laboratory and industrial purposes, mannose is obtained by the hydrolysis of hemicelluloses from tagua nut shells. The specific rotation of its solutions in an equilibrium state is +14.2°; the monosaccharide is easily fermented by yeast.

Sorbose is contained in rowanberry juice fermented by bacteria and is formed during the oxidation of the hexahydric alcohol D-sorbitol. This monosaccharide is of great importance for the vitamin industry as a raw material for the synthesis of ascorbic acid (vitamin C). The specific rotation of its aqueous solutions after the completion of mutarotation is -43.4°.

Arabinose is a component of mucilages, pectins, gums, and hemicelluloses, and it is usually found in the L-form in plants. For practical needs, it is obtained via acid hydrolysis of cherry gum or beet pulp. The specific rotation of aqueous solutions of arabinose after the completion of mutarotation is +104.5°.

Livestock animals digest arabinose poorly, and yeast is unable to ferment it. This must be taken into account when evaluating the nutritional value of feed based on pulp.

Practical importance of pentoses: from plant residue utilization to enzyme synthesis

Xylose, ribose, and apiose are five-carbon monosaccharides (pentoses) that determine the strength of plant cell walls and participate in vital biochemical processes. For the agronomist and processor, xylose, or "wood sugar," is of greatest practical interest. This carbohydrate is a component of hemicelluloses and pentosan-xylans, which form the rigid framework of stalks and seed coats. Xylose-rich crop residues and processing wastes — straw, bran, sunflower husks, and cotton hulls — serve as valuable raw materials for obtaining pure xylose via the acid hydrolysis method.

  • Specific rotation of aqueous solution — +18.8°
  • Raw material for confectionery xylose — corn cobs
  • Xylose reduction product — xylitol alcohol

Xylose obtained through the hydrolysis of corn cobs has a very sweet taste and is in demand in the confectionery industry. Oxidation of this monosaccharide produces xylonic acid, and reduction produces the pentahydric alcohol xylitol. Xylitol is actively used in the chemical industry for the production of alkyd resins, detergents, varnishes, and glues. Since this alcohol has a sweet taste but is not metabolized by the human body, it is used in the production of products for patients with diabetes mellitus and obesity.

Ordinary yeast is not capable of fermenting xylose. This property of the monosaccharide must be taken into account when planning the targeted processing of plant waste.

Ribose and apiose perform different biological functions in the plant organism. Ribose is an essential component of ribonucleic acids (RNA), vital enzymes, and nucleotides. In its free state, it is rarely found in the plant cell, and upon chemical reduction, it yields the polyhydric alcohol ribitol (or ribonic acid upon oxidation). Apiose belongs to the group of aldopentoses with a branched carbon chain. In crop production, this monosaccharide is known as a component of the flavone glycoside apiin, which accumulates in the tissues of parsley.

Read next