Chemical composition of crops and formation of harvest quality

CHEMICAL AND BIOCHEMICAL COMPOSITION

To solve practical tasks related to obtaining the maximum possible amount of chemical compounds required by the human and animal organism, it is necessary to have knowledge of the chemical composition of plants and the ability to manage it. An agrochemist views the chemical composition of plants not only as an object of knowledge (which is the focus of biochemists and plant physiologists) but also as a subject for influence.

N. M. Gorodniy, 1990

Plants are composed of organic and mineral substances. Organic compounds are primarily represented by proteins, fats, starch, sugars, cellulose, and pectin substances. Their content in plants depends, first and foremost, on the biological properties of the crop and cultivar, age, nutrition conditions, water supply, temperature, and lighting. The quality of the harvest is assessed based on the amount of organic substances in the plant. The harvest quality of cereal crops is determined primarily by the content of proteins and starch, and baking properties – by the amount of gluten in the grain. Grain legumes are evaluated by the level of protein accumulation, and oilseeds – by the content of fats in the seed. The quality of potato tubers is determined by starch content, and root crops of sugar beet – by sucrose. The harvest quality of fiber crops depends on the amount of cellulose: the more it contains in the plants, the greater the fiber yield. At the same time, a high content of cellulose in hay impairs its feed properties. Organic substances such as vitamins and pectin compounds, which are abundant in vegetables, fruits, and berries, also play a huge role in human and animal nutrition. For example, carotene is found in higher amounts in carrots and sweet peppers, while vitamin C (ascorbic acid) is abundant in sweet peppers, leaf parsley, and black currants. Vitamin B9 (folic acid) is rich in spinach, leaf parsley, and soy.

Carbohydrates, fats, and other nitrogen-free organic substances are built from three elements – oxygen, carbon, and hydrogen, while proteins and other nitrogenous compounds also contain nitrogen. Carbon, oxygen, hydrogen, and nitrogen account for 95% of the dry mass of plants. These four elements are called organogenic. When plants are burned, organogenic elements escape in the form of gaseous compounds and water vapor, while ash elements remain in the ash, predominantly in the form of oxides, accounting for about 5% of the dry matter mass.

Inorganic substances included in the plant composition include water, salts, acids, and alkalis.

Water makes up about 75% of the Earth's biomass; however, its content in different types of living organisms and their various tissues and organs fluctuates within a wide range. For instance, tree sap contains 88–99% water, while in plant wood it is significantly lower – 20–45%, and in cereal grain – 12–14%. The younger the organism or organ, the higher its water content. In young plant leaves, it ranges between 85–90%, while in old ones it is 70–80%. Most of the water in plants is intracellular water; less is extracellular water.

In a cell, water is a complex heterogeneous system consisting of: 1) a liquid phase; 2) hydration-bound water; 3) hydrophobically stabilized water (in membranes); 4) spatially stabilized water (in capillary spaces).

Water in a plant exists in the form of constitutional (i.e., chemically bound) water, hydration water, reserve water filling water-collecting cavities in vacuoles and other cellular compartments, and in the form of interstitial (i.e., intercellular) water. In plants, it exists in both a free and bound state (Fig. 4; L. M. Bronshtein, 1986). Free water is defined as water that has retained all or almost all the properties of pure water. Free water moves easily, participates in various biochemical reactions, evaporates during the process of transpiration, and freezes at temperatures below 0°C. Bound water has altered physical properties, mainly due to interaction with non-aqueous components.

Fig. 4. Forms of water in plants

Water that hydrates proteins and other colloidal particles is called colloidally bound, and that which hydrates mineral salts, sugars, and organic acids is called osmotically bound.

Colloidally and osmotically bound water, which forms shells around colloids or ions, is called hydration water. The amount of hydration water depends on the object and its physiological state, accounting for 20–50% of the dry mass.

If an aqueous solution contains ions of any electrolyte, water dipoles orient themselves around them. Water dipoles are attracted to cations by their negatively charged ends, and to anions by their positively charged ones. This binding of water is called electrostatic hydration. High-molecular compounds are hydrated in a similar manner if they contain polar amino groups, or carboxylic, aldehydic, or alcoholic ionogenic groups. In this process, the hydration shell may be continuous or exclusively around the polar groups. The degree of hydration of various ions and molecules depends on the particle size and the magnitude of their charge. The higher the specific charge density, i.e., the larger the charge and the smaller the size, the stronger the degree of hydration. Water molecules are arranged during hydration in three layers: 1) directly around the ion, strictly ordered and oriented by a strong electric field; 2) at some distance from the ion, where the orientation of water molecules is lesser; 3) far from the ion, where water molecules have a standard structure.

Water in a plant exists in different states — from firmly bound at the molecular level to free. The form in which moisture is present directly affects the intensity of cellular respiration and determines the conditions for drying plant materials. At the level of subcellular structures, water is a component of ribosomes, lysosomes, mitochondrial membranes, the endoplasmic reticulum, and the nuclear envelope.

The physicochemical properties of water change depending on its proximity to the protein molecules of cells. The layer of water closest to the protein is the most structured, but some influence of this structure persists even at a greater distance. Water is also part of the tertiary structure of macromolecules.

• Thickness of the structured water layer near a protein — 1–2 nm
• Share of structured water by protein mass — up to 30 %
• Thickness of the external hydration layer — up to 10 nm
• Temperature for intensive drying of plants — 105–130 °C

Based on the energy principle of bonding, there are three forms of water in a plant organism:

• Chemically bound water is part of molecules in a strictly defined ratio. It can be removed only by calcination or chemical treatment, which completely destroys the structure of the substances containing it.
• Physicochemically bound water (adsorption, osmotic, and structural) is held by hydrophilic colloids, primarily proteins. It loses its solvent properties, does not participate in chemical reactions, and does not freeze upon cooling. If biomass contains only such water, all physiological processes within it are inhibited to the maximum extent.
• Mechanically bound water is concentrated in the micro- and macro-capillaries of cell walls. It is easily removed by drying or freezing.

Critical humidity and free water during harvest drying

For practical work with a harvest, it is important to distinguish between hygroscopic and free water. Hygroscopic moisture is removed from plants by intensive drying to a constant mass. This volume includes all free water and almost all physically bound water capable of moving into the extracellular space. Free water constitutes the bulk of the moisture in a plant.

The ratio of moisture forms changes depending on the part of the plant and the cell. The free fraction, which ensures active metabolism, predominates in the protoplasm. In other cytoplasmic structures, these proportions may differ.