Rice (Oryza sativa L.) is the second most cultivated and consumed cereal in the world after wheat, being the basis of the diet of more than half of the world's population and also considered the most important, taking into account the extension of cultivated area and the amount of population that depends on its harvest.
Portugal produces 160,000 tons of paddy rice (husk) annually, which represents about half of domestic consumption. We have the highest per capita consumption in Europe, at 16 kg per year, in contrast to the European average of 6.6 kg per capita per year.
The high technical knowledge and specialization of Portuguese organizations and producers have enhanced rice cultivation, especially the Japanese type “Carolino”, to a position of national differentiation, due to its excellent characteristics.
In the rice chain, post-harvest technologies are of vital importance, since it is a living product, which continues its metabolic process even after drying, and whose main objective is the choice of the best and most correct techniques for the preservation of the chemical, biological and physical integrity of the grain.
When receiving paddy rice, laboratory equipment allows the identification of the quality of the samples, greater precision in the control of impurities and greater rigor in the control of humidity. In addition to simulating the work of large scale equipment in terms of functionality and product output quality, they can also be used for direct adjustments to the machines (sieve drilling, size classifiers, etc.).
The purpose of cleaning is to promote the separation of impurities and foreign materials, of dimensions, shapes and densities different from the characteristics of rice grains. Consists-
Tui the first post-harvest operation and prepares the product for the following operations, especially the drying that follows. When effectively performed, cleaning facilitates the movement of grains and air in the equipment, allows for uniform drying and avoids clogs and unnecessary energy costs, as well as reduces the sources of inoculum of microorganisms and pests during storage and subsequent industrial process.
To carry out the cleaning, rotary screen cleaners or flat sieves are used. For impurities that are more difficult to separate, rotary screen cleaners are more effective compared to flat screen cleaners, because of the continuous movement and absence of vibrations, each grain or particle enters the perforation more easily (figure 1).
The cleaners consist of fans, which remove light impurities from the grains by vacuuming and sieves (2 to 4 depending on the cleaning capacity) where larger impurities are removed. The choice of sieve perforations is made according to the type of grain to be cleaned, the humidity level and the properties of the waste to be removed.
Drying is a physico-chemical process that involves the use of thermal energy, leading to the evaporation of water to a vapor state. As the temperature of the medium increases, the vapor pressure at the outer surface of the grain is greater than the water vapor pressure in the ambient air, and the water from the grain immediately passes into the air. Drying ceases when pressures equalize and hygroscopic equilibrium is reached.
Drying not only allows for the necessary moisture reduction, but also lowers the level of physiological activity, and also has an important effect on several other properties of the grain, such as: flavor, color, texture, viability and nutritional retention. These properties, which generally affect the perceptible quality of the final product, are often influenced by temperature, humidity and the time the product stays in the dryer. In practice, the dryer is more complex than a simple machine that only removes moisture.
There are two traditional rice drying systems, recirculation and continuous, which have advantages and disadvantages according to your requirements.
Drying in a recirculation cycle takes place through constant passages of the product inside the dryer, with cooling at the end. The rest zone allows alternating drying and resting periods, ensuring homogeneity and product quality
It is a process that normally lasts 12 hours, where the product comes out with the ideal conservation humidity. There is a longer drying time, and harvest stops may occur at the peak of the campaign. Two dryers are often used in series to increase capacity.
This system is recommended for low flows or when it is necessary to dry different batches of cereal separately and is especially suitable for drying rice that requires longer cycles. Recirculation requires many mechanical movements through the conveyors, so they must be properly adjusted, with large capacity-
of and low rotation, so as not to cause physical damage to the grain.
Continuous cycle drying can be carried out in steps or in a single pass. In this system, the rice, after reception and cleaning, must recover in a silo, in order to achieve the homogenization of moisture, both from the caryopsis itself, by diffusion from the internal to the external extracts, which are the first to evaporate, as well as from the entire wet mass and a regular supply of the dryer.
In this system, a continuous dryer is used with rest compartments in the drying column, in order to alternate the
drying and resting periods, ensuring the homogeneity and quality of the grain. The compartments are placed at the top receiving part for wet grains, in the middle of the drying column and at the bottom for cooling.
Both the upper compartment for receiving the wet grains and the one placed in the middle of the drying column are larger and without ventilation, to allow the water inside the grains to migrate to the periphery by diffusion. In the lower cooling compartment, not only ambient air is used, but also partially heated air from the hot air production column, so that there is no thermal shock and grain ruptures.
The time the grain passes through the various zones in the drying column is determined by an extraction system depending on the humidity, through automatic cyclic openings controlled by computerized commands, until the grain is completely dry. The cooling cycle is only processed according to the drying cycle, so if the grain is not dry it is not released to the cooling zone.
In any of the above-mentioned drying systems, the drying temperature must not exceed 40 °C. Rice grains are extremely sensitive to the action of heat, which, when excessive, seriously compromises some quality and functional attributes, especially the level of cracks, the integrity of the tissues and the germination power when used for seed.
After drying, the paddy rice will be subjected to industrial processing, through the following phases: cleaning, drying
stone separation, peeling, densimetric separation, bleaching, classification according to thickness and size, selection by color and packaging.
The industrial yield of rice after the process is approximately 60 to 70% of white whole rice, 18 to 23% of husk, 7 to 12% of flour, and 5 to 10% of broken grains.
During industrial processing, it is necessary to carry out analyses and controls of rice grains, in order to classify the grain, as well as to help regulate the equipment, thus avoiding qualitative losses, both in mass and in breakage.
Using a single piece of equipment, it is possible to carry out an analysis of the quality indices of rice grain samples. O
rice statistical analyzer S21 analyzes various aspects of rice such as whiteness, breakage index, percentage of chalked grains, percentage of paving area, etc.
Rice quality parameters are obtained using sophisticated digital image analysis software.
A computer attached to a digital image capture system is responsible for sending the grain images to the software, which analyzes them individually. After this process, the program calculates the statistical results and gives the operator various data from the entire sample (figure 5).
In addition to offering faster than traditional methods, the system allows for more comprehensive and accurate analyses, adding greater commercial value to the product. The analyzer has the following advantages:
In the post-harvest stages, it is essential to use equipment and techniques adapted to the grain requirements. Thus, special attention is needed to plan and design projects, adapted to the capacity and needs of each farm and/or industry, with the choice of solutions that guarantee quality and the greatest number of whole grains, in order to add value to the product.
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