Fundamentals of proper storage and post-harvest processing of grain crops

Preserving the harvested crop is a task no less complex than growing it. During storage, a significant portion of plant products is lost before reaching the consumer. The main reason lies in the fact that storage under artificial conditions is unnatural for living plant organisms, and it is not always possible to create an ideal environment for them.

Storage losses are caused by three main factors:

• Mismatch between storage conditions and the natural needs of living objects.
• Complex chemical composition of plant products, which complicates their preservation.
• Insufficient knowledge of physiological processes occurring in the harvested crop.

To minimize these losses, specialists need practical skills in post-harvest handling, proper preparation of produce for storage, and its processing.

Why the harvest spoils: composition of the grain mass

Grain mass is a complex living biosystem that forms after threshing. It consists not only of the seeds of the main crop but also of accompanying components. It is important for an agronomist to remember that biological processes are constantly taking place within the bulk, directly affecting the preservation of the harvest.

Any grain mass consists of three main components:

1. Grain of the main crop, as well as seeds of other cultivated plants similar in their usage.
2. Organic and mineral impurities, including weed seeds.
3. Insects and mites — a dangerous living component for which grain serves as a habitat.

Physics of the bulk: flowability and self-sorting

Flowability is a key physical property of grain mass that determines its mobility. It is characterized by the angle of friction (the minimum angle at which grain begins to slide along a surface) and the angle of repose — the angle of the cone formed by freely falling grain. This indicator is influenced by the shape and moisture of the grains, the amount of impurities, and the material of the transport channels. The most flowable are dry, rounded seeds with a smooth surface, such as peas, millet, and lupine.

When loading warehouses and silos, self-sorting inevitably occurs — an uneven distribution of components across the volume of the bulk. Small, shriveled grains, light weeds, dust, and microorganisms accumulate near the storage walls. Large, plump grain and heavy mineral impurities settle in the central part of the bulk.

When unloading grain from silos, the nature of its movement directly depends on the geometry of the container and the location of the discharge opening. There are three main types of mass flow, each of which affects the redistribution of components within the bulk differently:

• Normal — when the vertical column above the opening moves first, gradually drawing the upper side layers along with it.
• Symmetric (in narrow silos) — when the entire grain mass descends simultaneously, but the central part moves faster.
• Asymmetric (in wide silos with asymmetric openings) — when the movement of layers occurs unevenly, increasing separation.

Self-sorting disrupts the homogeneity of the bulk and creates pockets of high humidity and pest activity. If these zones are not detected in time, local heating will quickly escalate into general self-heating. As a result, the farm risks the complete spoilage of the stored batch.

Intergranular space. When placed in warehouses or silos, grain mass does not form a solid body: free gaps filled with air remain between its solid components.

Intergranular space is a state where part of the volume of the grain mass is occupied by grain and other solid particles (impurities, seeds of other cultivated plants, etc.), which characterizes the grain packing density, while the other part of the grain mass volume is filled with air. Grain packing density and intergranular space are expressed as a percentage of the total volume of the grain mass.

The movement of air through the intergranular spaces facilitates heat transfer via convection and moisture movement through the grain mass in the form of vapor. Thanks to these spaces, drying, active ventilation, and grain fumigation are possible.

The intergranular space and packing density of grain mass in storage depend on the following factors:

• shape, elasticity, size, and condition of the solid components of the grain mass;
• quantity and type of impurities within it;
• batch size and grain mass moisture;
• shape and size of the storage facility;
• storage duration.

A grain mass consisting of large, small, thin, and short grains and other particles packs more densely and has lower porosity than a graded grain mass. Moist and damp grain lies less densely and occupies a larger volume in storage than dry grain under otherwise identical conditions. However, an increase in grain moisture during storage is accompanied by an increase in the volume of individual grains due to intergranular spaces and the compaction of the entire mass. In a storage facility with a large cross-section, grain is placed more densely than in one with a small cross-section.

Due to self-sorting, porosity in different parts of the grain mass may be uneven. This leads to an uneven air supply to individual sections of the grain mass.

Thermal stability is the ability of grain to retain its seed, food, and other qualities during the drying process. Under certain thermal regimes, proteins coagulate, which leads to a loss of their ability to swell. As a consequence, the technological properties of grain during milling and dough preparation are sharply impaired, and the germination capacity of seeds is sharply reduced.

The following changes are characteristic of wheat and starch upon heating:

Thermal conductivity is the ability of bodies to conduct heat, characterized by the thermal conductivity coefficient. Thermal diffusivity depends on the rate of temperature change in the grain mass and is characterized by the thermal diffusivity coefficient. Grain mass has low thermal conductivity and thermal diffusivity. This is due to its organic composition and the presence of air in intergranular spaces.

Closely related to the thermophysical properties of grain mass is the phenomenon of thermo-moisture conductivity — the directional movement of moisture in the grain mass caused by a temperature gradient. Moisture moves from the high-temperature zone along with the heat flow to less heated areas, where it condenses. This is observed, for example, when warm grain mass is poured onto an asphalt or concrete floor. Heat capacity is determined by the amount of heat required to increase the temperature of 1 kg of grain by 1°C.

As moisture increases, the heat capacity of the material rises because the heat capacity of water is almost triple that of the dry matter of grain, and a significantly higher energy consumption is required to heat the same grain mass.

Hygroscopicity is the ability of grain mass to absorb (sorption) and release (desorption) water vapor.

Sorption properties are due to the capillary-porous structure of the chemical substances within the grain, which are capable of absorbing and retaining a certain amount of water.

Absorption of water vapor occurs until hygroscopic equilibrium is reached. At this point, the water vapor pressure in the grain and the air equalizes, the exchange between the grain and the air ceases, and the grain moisture stabilizes; such grain moisture is called equilibrium moisture. Grain and seeds reach maximum equilibrium moisture at relative humidity of air 100%. The lower the relative humidity, the drier the air, the more water it can absorb, and the lower the equilibrium moisture of the grain.

Thus, equilibrium moisture is the level of grain moisture that is established at a given relative humidity of air.

Vital activity of grain. Every organism needs a systematic influx of energy to sustain life. In all higher plants and many microorganisms, energy is released as a result of the dissimilation of organic substances, mainly sugars.

During the storage of grain and seeds, two types of dissimilation are observed, the end result of which can be expressed by the following equations, known as respiration equations:

C6H12O6 + 6O2 = 6CO2 + 6H2O + energy (1)

Equation (1) characterizes aerobic respiration (oxidation of sucrose). Equation (2) is anaerobic respiration (the equation of alcoholic fermentation).

With sufficient air access, the process of aerobic respiration prevails in grain and seeds. If the air in the intergranular spaces is not renewed, the carbon dioxide released during respiration accumulates. Grain cells and other organisms capable of anaerobic respiration are forced to switch to this type of respiration.

When storing food and forage grain, the intensity of its respiration plays a key role. The more actively this process proceeds, the more dry matter the bulk loses and the higher the risk of rapid spoilage of the harvest. Respiration leads to the accumulation of hygroscopic moisture in the grain mass and an increase in temperature. At the same time, the intensity of respiration directly depends on the conditions in the storage facility and the physiological state of the batch itself.

Respiration rate is influenced by humidity, temperature, maturity stage, grain size, and integrity. In a damp bulk, the process accelerates sharply not only due to the vital activity of the seed, but also due to the activation of microorganisms. The starchy grain of soft wheat breathes more intensely than the vitreous grain of hard cultivars. Similar patterns are characteristic of weed seeds that may be present in the batch.

How self-heating originates and develops

Grain mass has low thermal conductivity, so the heat released during respiration does not dissipate but accumulates inside the bulk. This process is called self-heating. Because of it, the organoleptic properties of the grain rapidly deteriorate: the natural luster disappears, and the color, taste, and odor change. As a result of chemical changes, the raw material loses its technological, nutritional, and forage qualities.

Self-heating processes are divided into three types: pocket (occurs as a hot spot in any section), layered (the hot spot spreads in a layer), and total. Vertical layers usually form in metal bins due to the heating or cooling of one of the warehouse walls. Horizontal layers are localized depending on the season and the direction of temperature front movement.

Bottom-layer self-heating is considered the most dangerous, as the entire batch of grain can be spoiled. If the process is left to run, the temperature inside the bulk will quickly cross the critical threshold. As a result, the grain darkens, molds, begins to smell of decomposition, and loses its flowability. The final stage of the process is the carbonization of the mass and its transformation into a dense monolith.

• Temperature of deep self-heating — 50 °C or more
• Height of low bulk — 1–1.5 m
• Hot spot of bottom heating from the floor — 20–50 cm