Researchers at Wenger Manufacturing Inc.’s technical center in Sabetha, Kan., invested a quarter of a century refining and perfecting an extrusion process for quick-cooking rice. Combining rice flour and water with pressure, heat and mechanical shear to make a gelatinized extrudate that looked like rice could be done from Day One, but duplicating rice’s texture was much more difficult.
Temperature spikes of 100 degrees can occur in seconds in an extruder. The rice process had to account for all the independent variables at play and control their outcome to within a few degrees. After investing hundreds of hours in basic R&D, the researchers succeeded. Because the final product can be hydrated in cold water, it is appropriate for foodservice applications and situations where boiling water is not readily available. Quick-cooking rice also makes an excellent starting material for high-moisture applications such as ready-to-eat and frozen foods. Public health advocates are championing the use of extruded rice in developing countries because it is free of microbes and can be fortified with Vitamin A and other nutrients.
Wenger was founded in 1935 by two brothers creating a machine to mix straw and molasses for animal feed. The company established a technical center in the early 1960s to help refine its pelletizing and extrusion systems and develop new applications for its machinery. When the rice project began, some of the required technology did not exist. Helping guide the project to a successful conclusion were Gordon Huber, Wenger’s director of new concept development; Brian Plattner, process engineer; and Gerry Hertzel, R&D coordinator. Like many of the tech center’s staff, those men have enjoyed long associations with the firm. Hertzel, who joined Wenger as a die designer after graduating from Sabetha High School 29 years ago, helps manage and coordinate technical projects at several universities involved in extrusion research. He recently spoke with Food Engineering about the rice project.
FE: Why did Wenger develop this process?
Hertzel: Up to 40 percent of the grains of rice that are harvested can be broken. Millers allow a small percentage of them to remain in table-grade rice, but that still leaves an abundance of broken grains. In the U.S., there is a large market for those broken grains as the basis for value-added products such as rice cereals, but in other parts of the world, most of these rice kernels end up as animal feed and pet food.
The market price for broken grains typically is $6 to $7 per 100 weight, compared to $13 to $14 for whole-grain rice. The objective was to turn those broken grains into a value-added product. The process we’ve developed adds about $2.20 per 100 weight in cost, which is still substantially below the cost of whole grain rice for a product that actually is superior.
FE: In what way is it superior?
Hertzel: Rice kernels have to be cooked to bring them up to 58 percent moisture. Extruded rice mimics the size, shape and texture of real rice, but it’s already been cooked in the extruder. It is dried to about 12 percent moisture to keep it stable for an indefinite period, then hydrated in hot water in about 10 minutes, half the time of natural rice. It performs very well when hydrated in cold water.
Natural rice is prone to becoming rancid over time, but that’s not a concern with reformed rice. Extrusion stabilizes the rice; the flavor remains the same, even after several years. There also is an opportunity to add essential vitamins and minerals internally, where they won’t be washed off during preparation, an important consideration in developing countries where vitamin deficiency is a serious threat to public health and where rice provides almost 30 percent of caloric intake.
FE: What were the hurdles in developing this process?
Hertzel: When we look at some of our earliest recipes, it’s amazing how close we were to being on target but missed because of some variable we failed to consider. The very first time we had a recipe that would have worked, if we had the understanding we have today of how to evaluate the raw materials, how to properly precondition the raw materials with the right pressure and energy inputs so that they are not overly sensitive to damage. There are things you can build into the process that broaden the processing window, but controlling the mechanical heat that is imparted was critical. An extruder can generate damaging amounts of energy in a very short time. We improved processing control to the point where we can target and maintain proper conditions for product consistency.
FE: What equipment modifications were made?
Hertzel: In 1977, we were working with a single-screw cooker and former; the process now uses a twin-screw extruder. Because it is not an efficiently controlled cooking and conveying device, a single-screw extruder imparts too much mechanical energy into the product.
Metering the rice flour in a controlled manner so that the right amount is delivered for a very precise processing period also is critical. We settled on a gravimetric feeder, rather than a volumetric setup. Raw material sits on load cells that communicate with the feeding device. The feeder delivers a set mass flow rate of raw material to the preconditioner. By slaving all the other inputs to the metering system, we are able to achieve greater control. The industry hasn’t always had the ability to control all the variables of this process as precisely as is necessary.
It’s never been difficult or challenging to make a product that looks like rice. To make a product that eats like rice, on the other hand, is very difficult. Only recently have we been able to do both.
FE: When did you switch to twin-screw extrusion?
Hertzel: That began 12 or 13 years ago. Most of the work has been conducted at the pilot plant at the University of Nebraska. We’ve worked on this process pretty regularly since 1977. A couple of years ago we stumbled across some different techniques for handling and processing the raw materials that have made a great difference. These involve the screw and barrel profile, screw speed, die selection and temperature, moisture and retention time that have significant impact on final product characteristics.
FE: How sensitive is reformed rice to variations in drying conditions?
Hertzel: All rice is susceptible to cracking if it is dried in the wrong manner. As with any high moisture, dense product, the moisture in the center must migrate to the outside, and that requires warm, humid air to prevent case hardening that would prevent outward migration and result in cracks. Drying is not complex, but it needs to be controlled and maintained. We’ve optimized the drying process to make it as cost-effective as possible.
FE: How is reformed rice different from reconstituted rice, which also cooks quickly?
Hertzel: It’s completely different. Reconstituted rice is steam cooked, then dried before the hull is removed. The process is referred to as par boiled. It’s still a natural rice kernel, with broken grains sorted apart and used elsewhere. Extruded rice, on the other hand, starts with broken grains that have been ground into flour, then cooked and formed in an extruder to look like natural rice.
FE: Is this process being commercially applied?
Hertzel: A couple of companies are able to make quick-cooking rice with this process, including one that is about to begin a large-scale project. Because of our respect for our clients’ confidentiality, I can’t say in which geographic area this will be done. This is still a very new product, and a market has yet to be established for it.