How to store correctly

One of the problems when storing the harvested crop is the drying process, which often ends up compromising the quality of the grains, especially when very high temperatures are used in the operation.

Drying is an operation to remove moisture from a solid product, which excludes the concentration of a water solution. In a moist grain mass, the water vapor present tends to occupy all intercellular spaces, generating pressure in all directions. For a product to be subjected to the drying process, the vapor pressure on the surface of the product must be greater than the vapor pressure of the water in the drying air.

On the other hand, the water present in the drying air in the form of vapor also exerts a partial pressure, called partial pressure of water vapor in the air. The drying process aims to partially remove water from the grains, through the simultaneous transfer of heat from the air to the grains and mass, through the flow of water vapor from the grains to the air.

Often, grains are harvested with water contents above 18%, making drying essential in order to prevent enzymatic reactions and the development of microorganisms and fungi that produce hydrolytic lipases, which accelerate deterioration and affect grain quality during the storage.

In the drying process, the range of initial and final water content of the product, the temperature of the drying air, the relative humidity of the ambient air, the drying rate and the time of exposure of the product to heated air are taken into account. However, drying grains not only affects the water content of the products, but also changes the physical, chemical and biological properties, such as enzymatic activity. This disadvantage can be minimized by properly carrying out the drying process, which makes it necessary to carry out an in-depth study of the process, equipment and parameters used.

The temperature dosage and drying air flow need to be carefully observed to achieve dryer efficiency and at the same time reduce mechanical damage to the grains. Increasing drying air temperatures alone is often inadequate, both from the point of view of grain quality and the performance of the drying process, with the need to regulate the air flow and increase the volume of air in the drying chamber. , for adequate heat mixing to occur in proportion to the drying air flow rate. The automation of dryers to control air temperature, air flow, water content in and out of the grains and the temperature of the grain mass during drying has been carried out successfully, reducing problems with grain quality and increasing the efficiency of drying equipment.

Drying methods and types of dryers used for corn and soybeans

The drying operation can be carried out naturally, when the product is dried in the plant itself, without human interference, or artificially, when mechanical equipment is used to remove moisture, which allows conditioning of the drying air through temperature control, relative humidity and drying air speed.

Artificial drying causes the grains to be exposed to heated air currents, carrying away the water vapor released by the grains. In artificial drying, the tendency is to improve the process through natural ventilation, forced ventilation and natural convection, using low, high or combined temperatures.

High temperatures cause a higher drying rate, resulting from the greater difference between the water vapor pressure of the drying air and the product, causing water to be removed from the grains more easily. On the other hand, the flow of grains and drying air inside mechanical dryers characterizes the operation of the equipment. Mechanical dryers can work in mixed or cascade systems, when the product and drying air move forming mixed flows within the equipment. They can be crossed, when the product moves downwards in the equipment and the drying air crosses the grain layer; concurrent, when the grain flow and the hot air current move in the same direction within the equipment, normally downward; countercurrent, when the grains flow downwards and the drying air moves upwards, fixed layer or stationary dryers, used mostly for seeds, operation with the product at rest and the drying air moving through of the grain mass.

The way the product is dried can be in continuous or intermittent flow. Continuous drying is characterized by the grains passing through the drying chamber once, while in intermittent drying, the grains pass through the drying chamber and then come to rest, allowing the moisture in the grains to migrate to the periphery. , and then pass through the drying chamber again and remove the remaining moisture. This process can be repeated more than once. The great advantage of the continuous drying process is its speed, however, it works at high temperatures, which when poorly monitored can cause changes that result in interruption in the tegument and embryonic tissue, compromising the seed's permeability or loss of nutritional quality. of grains.

In intermittent drying it is possible to use lower drying air temperatures and gain in grain quality, as water removal is done in a slower process, not as aggressive, as in continuous drying, but it is lost with the drying speed. . Intermittent drying must be carried out with low relative humidity and high air flow. To prevent overdrying from occurring, the grain moisture must be determined every 20 or 30 minutes, establishing the grain moisture at around 12% to 13%.

Some mechanical dryers are designed to work with a drying air reuse system. This system is very important in terms of energy saving, as it makes it possible to use the exhaust air that leaves the dryer that still has drying potential, and directs it back to drying the grains. The most used dryers for soybeans and corn in Brazil are mixed flows and continuous drying, using biomass as fuel, with a direct or indirect fire furnace burning system. Tower-type (vertical) dryers represent the drying of soybeans and corn grains well.

GRAIN QUALITY DEPENDING ON DRYING

In studies carried out with grains, the research group, coordinated by researcher Paulo Carteri Coradi, from the Federal University of Mato Grosso do Sul (UFMS), observed that drying conditions directly influenced the optimization of the process and the quality of the grains. Evaluating the drying of soybean and corn grains at different air temperatures, the authors identified a reduction in drying time with increasing temperature; on the other hand, the acceleration of the water removal process during drying resulted in physical damage to the grains, interfering with quality. The increase in drying air temperature also increased electrical conductivity values ​​and reduced oil content, apparent specific mass, crude protein and grain germination, reducing final quality.

The parameter for evaluating electrical conductivity is related to the amount of ions leached from the cellular tissues that make up the plant structure of the grains, caused by the physical damage caused by the high temperatures used in the drying air. This means that the higher the electrical conductivity values ​​observed in the grains, the greater the physical damage caused by the drying air temperature. The responses to the physical damage caused by drying temperatures were reductions in oil content, germination, protein and specific gravity of the grains. Producers are recommended to dry soybeans and corn grains to storage water levels, preferably below 14% (b.u.), with controlled temperatures in mechanical dryers, below 70°C.

For the drying operation to be carried out safely, quickly and economically, it is essential to monitor the physical phenomena that occur in the process, mainly the temperature of the drying air and the initial and final water contents, establishing limits to avoid possible physical damage. chemical and biological in grains. For researchers and engineers, the use of mathematical models and computational tools, together with the drying system automation process, can minimize the effects on humans in decision making (product x environment), to make a better quality final product. . 

Paulo Carteri Coradi, UFMS

Article published in issue 153 of Cultivar Máquinas.