Physicochemical properties of carbohydrates and their role in agricultural chemistry

Physical properties.

Monosaccharides are colorless crystalline substances with the following characteristics:

• highly soluble in water, dimethylformamide, and dimethyl sulfoxide;
• sweet-tasting;
• decompose (caramelize) when heated.

Oligosaccharides are solid substances or non-crystallizing syrups. Most of them are soluble in water, lower alcohols, and dimethyl sulfoxide. As molecular weight increases, solubility in water decreases. Reducing oligosaccharides in a solution represent a mixture of tautomers and, like monosaccharides, crystallize as a single anomer. Oligosaccharides can melt, but generally with decomposition.

Polysaccharides have an amorphous structure and do not possess a sweet taste. Polysaccharides are generally highly soluble in water because they are strongly polar compounds due to the presence of a large number of hydroxyl groups. However, some compounds with a stereoregular and linear structure form intermolecular hydrogen bonds, which prevents such substances from dissolving in water.

Chemical properties. The presence of several different functional groups and the capacity for tautomeric transformations make monosaccharides reactive, labile compounds. They participate in chemical reactions via the hemiacetal, i.e., glycosidic hydroxyl, and via alcohol groups. Furthermore, in some reactions, monosaccharides behave as aldehydes or ketones, i.e., they react in their oxycarbonyl form.

As alcohols, they can be converted into ethers and react with acids and their derivatives to form esters. As compounds containing a carbonyl group, monosaccharides can be easily oxidized and reduced, and undergo nucleophilic addition reactions. In some reactions, only chain forms participate, while in others, cyclic forms are involved. In a number of cases, different forms of monosaccharides participate in a reaction simultaneously.

Reduction. All monosaccharides form polyhydric alcohols upon reduction. In plants, the most common are:

• sorbitol, which is formed by the reduction of glucose or fructose;
• mannitol – upon reduction of mannose;
• ribitol – a product of ribose reduction.

Sorbitol is found in mountain ash berries, cherry, plum, apple, and pear juices.

Oxidation. Depending on the nature of the oxidizing agent, monobasic (aldonic) or dibasic (saccharic) hydroxy acids can be formed. Under mild oxidation in an acidic environment, the aldehyde group is oxidized, i.e., the glycosidic hydroxyl, resulting in the formation of aldonic acids. Gluconic acid is formed from glucose as a result of this reaction.

During more vigorous oxidation of aldoses, not only the glycosidic hydroxyl but also the primary alcohol group is oxidized, forming dibasic acids. Consequently, saccharic acid is formed from glucose, and mucic acid from galactose. If only the primary alcohol group of monosaccharides is oxidized, uronic acids are formed.

Acids, the products of monosaccharide oxidation, are often present in plants and serve as intermediate metabolites in the synthesis of various organic compounds. For example, pentoses are formed from glucuronic acid; the latter is a compound necessary for the formation of ascorbic acid in plants; galacturonic acid and its methoxylated derivatives are monomers of pectic substances.

Formation of osazones. When an excess of phenylhydrazine acts on a sugar, phenylosazones are formed – water-insoluble, well-crystallizing substances of a yellowish-orange color. During the formation of osazones, hydrazones are initially produced. Then, the adjacent alcohol group, under the influence of a second molecule of phenylhydrazine, turns into a keto group. This group, interacting with a third molecule of phenylhydrazine, forms an osazone.

Alkylation reaction. The action of alkylating agents on monosaccharides leads to the formation of partial and full ethers. In this process, the hemiacetal hydroxyl interacts most actively with the ether. As a result, an ether—a glycoside—is formed. The bond of the alkyne with the carbon, arising through an oxygen bridge, is called a glycosidic bond. Glycosides are widely distributed in the plant world.

Acylation reaction. Monosaccharides are acylated by anhydrides and acid halides. In this process, along with alcohol hydroxyls, a hemiacetal hydroxyl is formed. As a result of the acylation reaction, full esters of monosaccharides are obtained. Among esters, phosphoric esters are of the greatest importance for the life activities of plants and animals. Carbohydrate metabolism processes in cells occur primarily through phosphate esters.

Pyrolysis of monosaccharides. Heating monosaccharides to temperatures above 100°C leads to their pyrolysis, resulting in the formation of many aroma-forming, dark-colored products that possess a specific odor and taste.

Fermentation of sugars. Fermentation is understood as the breakdown of carbohydrates under the influence of microorganisms or enzymes secreted by them. Sugars with a number of carbon atoms that are a multiple of three are subject to fermentation. This process is carried out in animals, plants, and microorganisms both with the participation of oxygen (aerobic fermentation) and without it (anaerobic fermentation). During fermentation, energy is released in the form of ATP as a result of oxidation-reduction reactions, and cellular metabolites are formed. The following types of fermentation are distinguished: 1) alcoholic; 2) lactic acid; 3) butyric acid; 4) methane.

The process of alcoholic fermentation is characteristic of yeast fungi, especially strains of Saccharomyces cerevisiae. During alcoholic fermentation, they break down with a high yield of ethyl alcohol and carbon dioxide.

Carbohydrates play a key role in feed storage and livestock animal nutrition. Understanding their fermentation processes helps technologists and agronomists effectively manage silage preparation. Various groups of microorganisms use carbohydrates as an energy source, converting them into valuable preserving components or volatile compounds.

The main process during silage preparation and vegetable pickling is lactic acid fermentation, which is carried out by bacteria of the Lactobacteriaceae family. During this process, glucose is converted into lactic acid. The resulting lactic acid lowers the pH of the medium and preserves the succulent feed.

Other types of carbohydrate fermentation proceed via different chemical pathways:

• Butyric acid fermentation. It is carried out by aerobic bacteria belonging to the genus Clostridium. This type of fermentation is used for the industrial production of butyric acid from starch.
• Methane fermentation. Methane-forming bacteria process alcohols and organic acids obtained from the breakdown of carbohydrates into methane. These microorganisms inhabit swamps (where they form marsh gas) and are also part of the essential microflora of the rumen in ruminant livestock animals.

Chemical transformations of complex sugars and their hydrolysis

To assess the nutritional value of feed and the chemical analysis of raw materials, it is important to know the properties of oligosaccharides. Disaccharides are divided into reducing and non-reducing. In reducing disaccharides, the glycosidic bond is formed due to the glycosidic hydroxyl of one monosaccharide residue and the alcoholic hydroxyl of another. In this case, the hemiacetal hydroxyl of the second residue remains free.

The presence of a free hemiacetal hydroxyl provides the ability for ring opening — ring-chain tautomerism. This feature determines the reducing properties of a disaccharide, as well as other reactions characteristic of compounds with a carbonyl group. Like polyhydric alcohols, oligosaccharides are capable of entering into alkylation and acylation reactions.

All oligosaccharides are characterized by the acid hydrolysis reaction, during which glycosidic bonds are broken and monosaccharides are released. Specific chemical transformations are also important for polysaccharides:

• Alkylation. The Haworth method is the most common, taking place in an aqueous solution of alkali using dimethyl sulfate as an alkylating agent. The reaction is used to accurately determine the structure of polysaccharides.
• Acylation. This is carried out by exposing polysaccharides to carboxylic acid anhydrides in the presence of catalysts.
• Hydrolysis of glycosidic bonds. This is carried out in the presence of dilute mineral acids. During the reaction, the polysaccharide first breaks down into oligosaccharide fragments, which, upon further hydrolysis, turn into monosaccharides.