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Rice Hydrothermal Treatment

The main goal of grain hydrothermal treatment in cereal production is controlled change of the grain baseline processing properties towards their stabilization on the optimal level.

The specific feature of our rice processing technology is the use of the module of water and heat hydrothermal treatment of paddy rice after the grain cleaning department of a groats mills before its hulling.

The hydrothermal treatment module performs the following process operations: rice humidifying, sweating in bins, outer preheating, saturated water steaming and drying.

Each process operation contributes into reaching the final result – manufacturing «steam treated rice».

The process diagram is given in figure.

Phase_1. Humidifying and sweating promote removal with water and accumulation of aqueous solution enriched with vitamins, aromatic substances, macro and microelements between the coat and the kernel.

Phase_2. Steaming enables to activate the process of removing useful substances from the coat and outer kernel layers, initiated at the first stage, due to partial steam condensation on the grain surface, and also to transfer the dissolved useful substances inside the kernel owing to the steaming overpressure. When the condensed moisture penetrates into the kernel, steam and condensate temperature lead to starch gelatinization and protein denaturation, which, in turn, glue inner cracks in rice kernels. This is also encouraged by the fact that the cracks are more actively penetrated by the steam hot condensate.

Phase_3. Grain drying process after steaming, besides its bringing to the required humidity, enables to extend the time of hydrothermal treatment and complete starch gelatinization, which strengthen grains. Thus, the steam-treated cereal allows us to preserve vitamins and minerals, available in the outer grain layers, which are lost under conventional rice cleaning and polishing.

It is vital to know that steam-treated rice:

  • looks yellow, and after cooking becomes absolutely white.
  • Steam treated rice grains do not flu to each other being cooked, rice becomes more airy and fluffy than usual “white” rice polished grains. This rice preserves over 80% vitamins and minerals, which are usually lost as a result of conventional rice cleaning techniques – during the outer shell removal and polishing.

According to the paddy rice process flow, from storage facilities grain is transported by the bucket conveyor p.1.1 to the grain humidifier p.13, where it is humidified to reach 22...28% humidity. The degree of humidifying mainly depends on the sort and cracks of the paddy rice. After the humidifier the grain moves the bins through the separator, p.18, for sweating, pp.14.1 and 14.2. In the bins moisture is redistributed, entailing kernel polymers swelling and crack gluing. Further heat treatment leads to «gluing» cracks owing to the starch gelatinization.

To ensure normal grain discharge in the lower part of the bins there are bin activators pp.15.1 and 15.2. Through rack and pinion gates pp.16.1 and 16.2 paddy rice is transported by the screw conveyor, p.17, is fed to the bucket conveyor, p.1.2, and further to the grain preheating bin p.2. Inside the bin there are feeding and exhaust ducts for the drying agent, and at the end of the tunnel, along its height, there is a discharge chamber to remove light waste. The drying agent is forced to the unit with the fan, p.12.3, exhausting the mixture of air and wet steam from steam dryers, along the shaft height. This mixture is forced for the additional heating to 90...100ºС in steam heaters pp.8.2-8.7. The steam is supplied to the heaters from the central main steam duct, whereas the waste condensate returns to the boiler room through the receiver, p.4.2, and the condensate tank, p.19. The drying agent with these parameters is forced with the fan, p.12.3, into the unit through the feeding ducts, which ensures even distribution in the tunnel and grain heating to 35...45ºС.

Preheated grain is loaded into the steamer, where it is steamed under the pressure of 0,1...0,2 MPa over 3...5 minutes. Grain preheating intensifies steaming process and leads to the accelerated increase of the given steam pressure and decrease of steam consumption for this operation by 12...18 %. Steaming modes are established depending on the grain moisture content. The higher the grain moisture rate, the lower steaming parameter values (exposure and steam pressure). It has been found out that the higher the steam pressure the better not only grain processing properties, but also cereal useful qualities. Steam pressure and exposure to steaming are interchangeable parameters in terms of their efficiency of impact on grain in proportion 1:3. One of the adverse factors is possibility of supplying oversaturated steam from the boiler to the steamer, which leads to excessive humidifying and deterioration of not only processing properties of the grain, interruption of the process flow, but also higher energy consumption for drying. To eliminate this drawback it is suggested to use a special buffer tank before supplying saturated water steam into the steamer for steam cleaning, p.4.1. Passing this buffer tank, steam will be purified from excessive drop moisture.

After grain steaming the waste steam and condensate mixture is discharged from the continuous flow steamer into the sewage, except for the steam emission inside the premises or into the air.

From the steamer the steam-treated grain moves to the over-the-dryer hopper, p.6. In this hopper grain sweats for a while. Sweating enables to complete transformations, started while steaming, but the moisture continuous coming to the kernel, alongside with physical and chemical processes. Since the grain coming out of the steamer is heated and humidified, it should sweat in a bin, having heat-insulated walls and bottom. Otherwise, intensive moisture evaporation from the hot and humidified grain will lead to high moisture condensation on the bin walls, which will prevent grain free flowing out.

Grain drying is a wet stage of hydrothermal treatment. In the steam dryers, pp.7.1 and 7.2, the grain is heated, evaporated moisture is blown off with the drying agent, penetrating the grain. In the dryer grain moves as a solid layer, and drying process is combined conductive and convective: from the grain contact with heating pipes, where steam circulates under the pressure of 0,3...0,4 MPa, and from the drying agent, forced by fans, pp.12.1 and 12.2. The fan, p.12.1, exhausts warm air from the cooling towers, pp.10.1 and 10.2 and forces it in the heaters, pp. 8.6 and 8.7, for additional heating to 90...100˚С. The fan, p.12.2, exhausts hot air from the sections of steam dryers and forces air at 100...120˚С in the upper sections through the heaters in pp.8.2-8.5.

Heat, supplied by convection, is used to moisture evaporation, overheating the generated steam and grain heating. Generated water steam is absorbed by air and exhausted from the drying tunnels. Heated air, thus, operates not only as a heat medium, but also as a dehydrating, or drying agent. The application of well-known dryers, for example VS-10-49М, involves conductive drying, where heat is transferred to the grain from the heated surface of the pressure steam-heating pipes. This not only leads to long-time drying, but also it does not ensure even heat feeding to all the grain volume and is energy consuming. The use of waste heat from dryers may be a solution of these problems.

Drying entails fast shell dehydration, whereas the kernel releases moisture much more slowly. Therefore, while and after drying shells always have lower moisture content than kernels. Under low moisture content shells are very brittle, they are easily cracked and separated from the kernel. Grains that have higher moisture content preserve their plasticity and are less subject to damaging while hulling.

The design feature of the steam dryer is a cooling section in its lower part. Along the height of the dryer tunnel there are two points with air manifolds for the drying agent, which is heated in heaters, where steam is forced from the central main, and the condensate is discharged to the boiler room. As the grain moves along the tunnel to the outlet moisture is removed, and at the outlet of the drying area the grain is cooled down. Cooling is accompanied by further shell drying and some kernel drying, thus facilitating grain hulling. Moisture self-evaporation by 1,2...1,5% is in parallel with grain cooling. Grain moisture content after drying must not exceed 14,5...15%, dried grain cooling is to the temperature that does not exceed air temperature in the production premises by over 6...8˚С. Dried grain is transported for processing by the belt conveyor, p.11.

Grain hydrothermal treatment under the suggested process flow includes the following operations: humidifying, sweating, heating, steaming, drying and cooling. Therefore, it is necessary to consider hydrothermal treatment as a system of grain thermal treatment. Rice hydrothermal treatment increases its food value, since as a result of migration from outer layers into the centre the content of vitamins and other biologically active substances rises.

Specification of rice hydrothermal treatment module, capacity 3 t/hour.

Pp. Name Code Q-ty, pcs Power. per unit., kW
1.1, 1.2 Bucket conveyor N-10 2 1,5
2 Bin (for grain preheating) PB-1200 1 ---
3 Parallel-slide gate valve --- 1 ---
4.1, 4.2 Receiver --- 2 ---
5 Steamer PZ-2 1 0,55
6 Over-the-dryer hopper   1 ---
7.1, 7.2 Dryer VS-14 2 0,75
8.1...8.7 Steam air heater PNP113 7 ---
9.1...9.7 Throttle valve with manual control DK-225 7 ---
10.1, 10.2 Cooling tower ОК 2 ---
11 Belt conveyor TB-30-1,5 1 1,5
12.1 Fan VR-86-77-4В 1 5,5
12.2 Fan VR-86-77-4VK1Zh 1 5,5
12.3 Fan VR-86-77-5К1Zh 1 3
13 Screw grain humidifier   1 1,1
14.1, 14.2 Sweating bin --- 2 ---
15.1, 15.2 Bin activator R6-MVB 2 0,5
16.1, 16.2 Tack and pinion gate ТЗR-300 2 ---
17 Screw conveyor USh2-Ch-200 1 1,1
18 Separator  KDR-7 1 ---
19 Condensate tank №10 1 ---
20.1, 20.2 Cyclone pump BK1/16 2 1,5