With each new grain harvest, Brazil has been breaking production records, even with the challenges encountered by the drought in some regions of the country. This is all the result of serious work carried out by the entire production chain. In addition, in recent years, the disbursed cost for grain production has increased significantly, requiring greater investment by producers, which makes grain production increasingly demanding.
In this sense, monitoring by a technician at all stages of production is important, but it is also essential that the same investment and technical rigor is maintained in the post-harvest and industrialization stages.
The challenges to conserve grain production are great and can be affected by crop factors, such as water deficit or excess, by harvest-related factors, such as the regulatory fermentation stage, grain health and harvester regulation, in addition to of all factors related to the transportation and reception of grains in the stored units.
Aiming at the high quality of grains, the ideal is that all the grain harvested on the day is transported, received, cleaned and dried immediately. The delay in some of these steps can lead to problems, such as an increase in the metabolic rate of the grains, caused by the loss of dry matter and the increase in temperature and the percentage of variable grains in the mass.
The grains are usually harvested with higher humidity than recommended for storage. In this sense, drying becomes a fundamental step to reduce the amount of water present in the grains. In drying, water is first removed from the peripheral region of the grain, originating a water gradient in relation to the central and humid region. This gradient will be responsible for the diffusion of water from the center to the periphery and for the redistribution of water inside the grain. This phenomenon occurs repeatedly until the grain reaches the desired moisture percentage.
To obtain an efficient and safe result, it is necessary to control some parameters that directly influence the drying process. Among the main parameters to be controlled, the initial humidity of the grains, the temperature of the air and the mass of grains, the drying rate, the final humidity, the air pressure and the air and grain flow in the dryer stand out. .
The initial moisture of the grains is an important factor to be considered, as it directly influences the time and cost of the drying process. The wetter the beans are, the more time and energy it takes to dry them. Air temperature and grain mass are other important parameters that must be controlled during grain drying. The ideal temperature varies depending on the grain species and the drying method used. It is important to avoid very high temperatures that could damage the quality of the beans or increase the risk of fire.
Figure 1 shows corn grains that during drying the mass reached temperatures of 30 ºC, 50 ºC, 70 ºC and 90 ºC. Subsequently, these grains were submitted to the tetrazolium test. This test was only used to get an idea of how the cell viability of the grains was after drying under these conditions. When the grains are alive, the tetrazolium turns the corn endosperm red, reducing this coloration as the cell viability decreases, even grains that do not react with the tetrazolium solution, indicating the death of the grain. In this sense, the image showed live grains when the corn mass temperature was up to 30 ºC. At 50 ºC, a reduction of the reddish coloration was observed, indicating a deterioration of the grain mass and at 70 ºC and 90 ºC, the grains showed cell death (Figure 1).
What was represented in the example for maize grains can be taken into account, with some exceptions, for other grain species. Drying at lower temperatures results in greater peace of mind during storage, with the grain's defense metabolism being active to aid conservation. On the other hand, grains that have been subjected to high temperatures light up an alert, which may result in a reduction in quality in storage and in the industry.
Another very common example of using high temperatures during grain drying occurs in rice. In Figure 2, images of the endosperm structure of rice grains subjected to different drying temperatures (40 ºC and 60 ºC) are presented. In this case, the formation of cracks in the rice was evidenced according to the increase in the drying temperature, which generates a loss of quality and added value of the product, mainly due to the reduction of the yield of whole grains in the industry.
Another important parameter to be monitored and controlled during grain drying aimed at high quality is the air pressure inside the dryer. Air pressure within the dryer manufacturer's recommended levels at each indicated point is important to ensure even distribution of drying air throughout the process and throughout the grain column. Measuring and controlling the pressure is important to ensure that ventilation inside the dryer is adequate.
Finally, the final moisture is the residual moisture of the grains after the drying process. The final moisture content varies depending on the species of grain and the storage method that will be used, but generally it should be less than 14% to avoid deterioration of the grains. Therefore, drying and dryer management is essential to ensure that all parameters are in agreement during the entire period and in all dryer chambers.
Following the post-harvest steps presented, the grains are stored when they are not industrialized immediately after drying. In some cases, storage is necessary to guarantee product supply throughout the year, due to the harvest period being limited to only part of the year, or even when storekeepers aim to stock the product waiting for the best time to market. , based on information from the market and its own planning and economic and financial management of the harvest.
Before starting grain storage, silos and warehouses/bulk carriers must be cleaned to remove grain residues from previous harvests, insects and other materials that may hinder the conservation of grains. In addition, all necessary maintenance must be performed on the storage unit. The aeration system must be clean, including fans and air ducts.
Inside the storage cells, it is essential that they have grain mass monitoring systems, capable of measuring the temperature (thermometry systems) and/or the carbon dioxide concentration (CO2 sensors), combined with a properly sized aeration system.
The purpose of aerating stored grains is to reduce and standardize the temperature of the grain mass. For this, a series of factors must be considered, among them the temperature inside the silo, the humidity of the grain and the temperature and relative humidity of the ambient air. For success in this stage, it is important that the storekeeper has mastery over the psychrometric properties of the air and the hygroscopicity of the grains.
Table 1 shows the hygroscopic equilibrium moisture of soybeans as a function of different conditions of temperature and relative humidity of the ambient air. These data help us in making aeration decisions, based on the storekeeper's objectives. Normally, the grains are stored already dry and in this case the objective is that no more moisture is lost through aeration. In this sense, we must seek to aerate in ambient air conditions that result in a hygroscopic equilibrium humidity greater than the humidity of the grains that are stored.
Storage under inappropriate conditions can lead to microbial development, dry matter losses and an increase in damaged grains, in addition to effects on physicochemical and technological properties, such as increased oil acidity, an important parameter for the soybean processing industry, for example. .
The increase in intergranular air temperature during storage results in the growth of microorganisms and can cause fermentation and rancidity of the grains, in addition to the possibility of producing contaminants, such as toxins, which make it difficult to use the product for human and animal consumption. Thus, the main factors that can affect grain storage are grain moisture, temperature, intergranular relative humidity and the presence of insects and microorganisms.
Figure 3 shows the results of a study on the storage of soybean grains with different moisture content (12% and 16%) and at different temperatures (15 ºC and 25 ºC). These results show us that initially the grains with higher moisture had higher weight. However, during storage, even with the highest amount of water, grains with 16% moisture showed a drastic reduction in weight, accentuating the higher the storage temperature.
This occurred due to the higher respiratory rate of grains with high humidity and temperature, resulting mainly in the loss of dry matter in the form of CO2, which translates into loss of product and lower financial profitability due to inadequate storage.
Thus, it is concluded that the post-harvest grain stages must be monitored and controlled to preserve the quality of the product from the crop. In drying, the main control parameters are grain moisture, air and grain mass temperature and air pressure in the dryer. In storage, the main control parameters are grain moisture, temperature, ambient air conditions and pest control.
