Preparing the perfect consistency of rice when cooked in small batches on the home stove usually is no problem for household chefs, but processing 10,000 pounds of rice per hour while striving to maintain consistent quality can present a considerable technical challenge for food processors. Conventional high-volume processes used for the cooking and cooling of rice after parboiling - to convert it for use in shelf-stable, ready-to-eat products - can often result in variations in the appearance and texture of the rice, an undesirable physical breakdown of the rice grains, and an adulteration of its flavor.
A more effective method has been sought that preserves the integrity of the parboiled rice throughout the secondary preparation process. The process must deliver a level of consistency comparable to freshly prepared rice for use in high-volume processing of ready-to-eat refrigerated, frozen and canned meals.
For prepared foods, parboiled long grain rice most often is used because of its ease of preparation and durability. Parboiling is an optional processing step prior to rice milling. In parboiling, the rice is subjected to a water-soak and steam-pressure treatment process while still a brown rice. Parboiling gelatinizes the starch within the rice kernel - a process that disrupts the crystalline structure of the rice starch, causing the starch granules to melt together. A crystalline structure does not allow water entry, so heat is applied to cause the crystalline regions to become diffused, allowing water to penetrate. Parboiling increases randomness in the general starch structure and decreases the number and size of crystalline regions. The process also causes nutrients from the outer bran to precipitate into the grain itself, preventing the loss of valuable nutrients during subsequent milling and improving its shelf life. It also mends cracks in the rice, acting as a gluing agent.
After parboiling and drying, the grain structure becomes compact, the content of vitamins and mineral salts is enhanced, and the moisture content is reduced to about 13 percent. The rice then goes through the milling process, which reduces it to about 68 percent of the harvested paddy rice by weight. The result is firmer, more separate rice grains that are more stable, less brittle and less susceptible to overcooking than regular-milled white rice, which simply has the bran milled off.
Parboiled-milled rice, regular-milled white rice and brown rice can be cooked and dried before packaging to provide prepared-food products that require very short consumer preparation times - from as little as 90 sec to 15 min. With such instant, quick-cooking and boil-in-bag products, the rice is blanched in hot water, steamed and rinsed, then placed in dehydration ovens until the moisture content reaches approximately 12 percent. The basic principle involves increasing moisture of the rice by using steam or water to form cracks or holes in the kernels. These products’ fast-cooking properties come from the fact that when the rice is re-cooked, water can penetrate into the cracked grain much more quickly.
Individually quick frozen (IQF) rice goes one step further. The cooked rice grains are individually frozen before packaging to provide a free-flowing ingredient for use in frozen and prepared food products.
High-Volume Rice Cooking and CoolingWhen long-grain rice is used in prepared refrigerated, frozen or canned foods, a number of factors come into play that influence the rice quality as it goes through the cooking and subsequent cooling processes. As mentioned, most rice will already have been parboiled and received by the food processor as such, or as regular-milled white rice and brown rice for instant-rice products.
During cooking or blanching, the rice is introduced into the cooker as small individual kernels. As these kernels are cooked or steamed for long periods of time, at between 195 and 205°F (90.5 to 96°C), if the cooking is not stopped appropriately, the kernels will eventually split open and fall apart. Starch then is released from the kernels and the rice becomes sticky or pasty and may mass into undesirable clumps. The cooking process, and the potential leaching out of the freed starch, will continue until the rice stops cooking, which means the temperature of the rice must be below 100°F (38°C).
Minimizing or eliminating leaching and maintaining firm, compact individual grains of rice throughout the entire cooking cycle are critical conditions for proper texture and appearance in a packaged, ready-to-eat meal or other prepared food product. The cooked quality of the rice should be assayed against rice texture, color, amount of free starch or stickiness of the cooked rice, and kernel integrity.
Cooks talk about rice being al dente, so when they bite into it, the rice is still firm in the middle, and ready to eat. On a stovetop basis, that works. But with a high-volume throughput of 10,000 lb of rice per hour, the process becomes somewhat more complex.
With volume batch processing of rice, the rice is cooked in kettles and a specific level of rice and water are added. At the end of the cooking process, the excess water is drained, cold water is added, and the rice is stirred. The cold water then is drained, and more cold water is added and stirred. No matter how much automation might be employed, this is largely a manual, labor-intensive method, prone to the discrepancies of human intervention in monitoring process times and temperatures. The stirring itself, which is largely a manual process, can cause damage to the rice kernels. Whatever the high-volume quantities that are being processed, the batch method has not been able to achieve consistently firm rice with a high level of kernel integrity.
A more conventional system for high-volume rice cooking is the continuous line using steam cooking. The rice is carried on a belt conveyor through a steaming sequence, and the rice is heated by the steam, which begins the cooking process. Water then is sprinkled onto the grains while they are agitated, causing the grains to absorb it. The rice then is again heated by steam, continuing the cooking process. Some systems utilize conveyor belts running at different levels, independent of each other, to achieve higher volume throughput. To stop the cooking process, the rice is carried by the conveyor while chilled air is force-blown on it. A version of this cooling process uses a water spray within an open cylinder, where the rice is covered with a cold water spray to reduce its temperature after cooking.
These continuous units have the benefit of being controlled with programmable logic controllers (PLCs) that coordinate the belt movements with sensors that determine the rice temperature at various positions throughout the process.
Despite the improvement in automation over the batch method, conveyor cooker/cooler systems are only as good as their ability to stop the cooking process. Some chilling equipment systems have inadequate and non-uniform airflow and cannot stop the cooking process adequately. In addition, conveyor heating and cooling systems require supervision, and the system designs can make cleaning and maintenance difficult. Similar to the batch method, when handling high-volume throughput, conveyor cooker/cooler systems have not demonstrated that they can consistently produce firm rice with a high level of kernel integrity.
Similarly, semi-continuous processing rotary drum cooker/coolers have been employed for some time, but without the ability to handle high-volume throughput with an acceptable level of product consistency.
Continuous processing for cooking and cooling rice, and the integration of PLC controls for automated processing, has opened the door to modern recipe management, enabling rice producers to meet the changing demands of consumers. One high-volume, rice cooking/cooling system, designed by Lyco Manufacturing, Columbus, Wis., provides a continuous process for cooking and cooling rice with high consistency. The system utilizes two enclosed rotary drum cylinders - one for cooking and one for cooling - in a first-in/first-out sequence. The drums have internal augers: a perforated skin sheet is wrapped around the drums and fixed to the augers’ flights. The flights move the rice through the cooker and cooler system while the rice is submerged in water. The rice is carefully agitated as it advances through the cylinders.
Once through the cooker machine, having reached the programmed temperature/time, the rice is deposited in the cooling drum and chilled to its programmed recipe temperature/time factor. According to the manufacturer, the system can process more than 10,000 lb of rice per hour. The cooling process with the auger mixing handles the kernels carefully, even when the rice must be chilled longer to bring the temperature down to 40°F (4°C) for use in frozen meals.
Throughout the production day, the system can be run at different temperatures and retention times to suit the production demands. For example, the equipment can cook rice of different varieties and then vegetables and pasta in the same system without needing to change out processing screens.
Lean manufacturing in rice processing means a reduction in waste. It incorporates shorter runs, faster changeovers and smaller inventories, and that includes flexible equipment.