Oats rank sixth, after wheat, corn, rice, barley, and sorghum, in global cereal production, with an annual global production of slightly more than 24 million tons. The majority of the global oat supply (75%) is produced in Canada, Germany, Poland, Russia, and the United States. Canada is a leading producer, with 1.4 million ha dedicated to oat crops. In the United States, cropland dedicated to oat production has decreased over recent years and currently is around 800,000 ha. The United States is now one of the principal importers of oats.
Oats are used mainly as fodder for livestock because of their high carbohydrate content. In addition, oats are not as susceptible to root disease as are wheat and barley, enabling their use as a cover crop in winter rotations. Oats also are commonly used in products for human consumption and less commonly used in industrial and pharmaceutical applications. The focus of this article is oat processing for applications in food products for human consumption.
Oats are processed to create a variety of food products. Oat groats are commonly flaked for use in hot porridge (oatmeal), cold ready-to-eat breakfast cereals, and other food applications, such as snacks, biscuits (cookies), and bars. The sensory (e.g., taste, texture) and nutritional (e.g., protein, soluble fiber) properties of oat flakes are important quality characteristics for consumers that vary depending on oat variety and environmental growing conditions, as well as process conditions used in flaking. The effects of processing conditions, specifically kilning, on flake characteristics are described in this article.
The global oat market is predicted to increase in volume and revenue over the next several years (5), primarily due to the nutritional benefits associated with oat consumption. Oats contain relatively high levels of proteins (11–15%), dietary fibers (e.g., beta-glucan), antioxidants, and micronutrients (e.g., vitamin E, zinc, and iron) and a good balance of amino acids (4). Research has also shown that soluble fibers such as beta-glucan may help lower blood cholesterol levels and act as cancer-fighting agents (4). These properties make oats more attractive from a nutritional standpoint than many other cereal grains. In addition, the gluten level in some oat varieties is <20 ppm, making them suitable for consumption by individuals with coeliac disease.
Oats are harvested and dried before being hulled, cleaned, and stored. These initial steps are followed by a heat treatment known as kilning. Following kilning, groats are flaked using two counter-rotating rollers operating at low or no differential speeds. To produce a specific flake thickness, the gap between the rollers must be precisely controlled. The flaking process has two target outcomes: increase the crunchiness of products such as breakfast cereals and snacks and create a thinner flake with a greater surface area for faster cooking in porridge (oatmeal) applications.
Background on Kilning and Flaking
A kiln is a thermally insulated chamber that is heated to achieve specific conditions for defined applications. It is often compared to an oven. Kilns can be used to transform clay into pottery, dry timber, or, as in this case, dry oat groats for use in food applications.
More than 800 ancient kilns have been excavated in Scotland. These kilns, which have been dated to between the 4th and 13th centuries, were a necessary step in crop production in the region due to the wet conditions commonly experienced during harvesting (3). Kilning systems consisted of a fire that produced heat used to warm a chamber (typically four sided or rounded) made of wood or iron. The heat generated was used to dry harvested grains. Most barns contained a kiln that was used to dry cereal crops prior to storage (2). Oats, along with other grains, were introduced to North America by Scottish settlers in 1602. In 1894, the Kellogg brothers accidentally left wheat kernels soaking overnight. The next day they were able to form and bake the soaked kernels into little flakes. After four years of trials, they applied kilning and flaking to corn. By the end of the 19th century these kilning and flaking processes were applied to a variety of cereals.
Beneficial Effects of Kilning Oats
Oats contain high levels of unsaturated fats and lipase activity—a combination that leads to oxidation, rancidity, and a short shelf life. Enzymes are inactivated by heat-treating oats during kilning, increasing product shelf life. The kilning process also reduces microbiological contaminants on grain and can promote development of aroma via “toasting.” Tempering with steam during kilning enables the creation of a mechanically resistant flake. The use of steam to increase moisture content softens the groat and reduces material breakage and loss during the flaking process.
Standard Kilning Processes
In a standard process, kilning steps are performed in a vertical, moving-bed tower (Fig. 1). The tower is typically filled with raw product at the top, the product moves downward through the process steps, and the treated product is extracted from the base. Steam injection and heaters are used to treat the product. This type of system presents several challenges:
- Voluminous equipment requirements, including feed systems, extraction systems, downstream storage, and dosing before flaking.
- Uneven heat and moisture distribution caused by the static nature of the product inside the kiln.
- Long residence times that are required to compensate for heterogenous behavior.
- Operating temperatures that are limited by steam availability.
- High energy consumption.
Continuous Kilning System
System Description. The Revtech continuous kilning system (Revtech Process Systems) consists of a smooth, continuous stainless-steel tube that is coiled around a central, vibratory support structure (Fig. 2). Heat is generated directly in the walls of the tube using a low-voltage electrical current, and product flows continuously from the bottom to the top of the tube under the influence of two shaker motors. Air or steam can be injected at appropriate locations in the spiral tube. This technology uses electricity as its main energy source and has a number of advantages compared with traditional kilning technologies.
System Design and Testing. System trials have been conducted at MGM Seed & Grain, which recently completed the installation of a new grain processing facility in Saskatoon, SK, Canada, including a continuous kilning machine (flow rate of 5,000 kg/hr) and system. The system is designed to feed product directly into the flaker before transfer of the product into a vibro-fluidized bed cooler, without the need for additional storage or dosing equipment. Flake quality and, by inference, the success of the process were assessed by determining the sensory properties of fresh and aged oat flakes. Analytical methods were used to assess enzyme inactivation and reduction of microbiological loads on groats.
Inactivation of Lipid-Hydrolyzing Enzymes. Samples were taken at different processing steps: raw oat groats, pasteurized whole oat groats, and dried oat flakes. The samples were analyzed using the peroxidase test and following the protocol outlined in AACCI Approved Method 22-80.01 (1).
The results demonstrated that the continuous process is able to inactivate peroxidase under regular processing conditions (Fig. 3). Inactivation of peroxidase significantly delays rancidity and extends product shelf life. Sampling analysis performed on products more than a year old showed the same level of stability.
Pasteurization of Oat Groats. Pasteurization eliminates risk of microbiological contamination by ensuring a reduction in total plate count and elimination of yeasts, molds, and pathogenic bacteria. Process effectiveness was measured by challenge tests to induce inoculation, using a surrogate microorganism displaying the same thermosensitivity as Salmonella muenchen. Enterococcus faecium is typically chosen as the surrogate. The objective was to achieve a minimum 5 log reduction in the inoculated product over a representative period of regular production. All samples tested displayed a >5 log reduction, ensuring conformity with U.S. Food and Drug Administration (FDA) regulations.
Improvement of Sensory Properties. Taste comparisons were made between raw and kilned oat samples using a proprietary method. Internal sensory tests were performed by comparing Revtech-kilned oat samples with commercial samples for different attributes (e.g., bitterness, flavor, crunchiness). The Revtech kilning process promoted aroma and flavor development through Maillard reactions (modifications to starches, sugars, and proteins in the presence of humidity and heat) and ensured the development of a slightly caramelized flavor.
Plasticizing Groat Structure. The Revtech process increased temperature and moisture, enabling groats to reach the glass transition stage. Temperatures around 95°C and relative humidity (RH) levels of approximately 20% are necessary for plasticization of groats to be effective. Plasticizing groats improves flake quality by creating a mechanically resistant groat, which reduces material breakage and loss during flaking and enhances flowability during rolling.
The results of the system test indicate that the new continuous system combines multiple processes in one machine.
Novel Aspects of the System. The plug-flow behavior and precisely controlled operating parameters of the continuous kilning system offer a number of advantages compared with more traditional processes:
- Consistent treatment times, uniform treatment of every grain, and controlled steam input result in optimal quality, as well as controlled caramelization and flavor development (e.g., more roasted or less roasted).
- Processing time is reduced as target temperatures are quickly and evenly achieved via the very high heat transfer coefficients experienced in vibro-fluidized beds of moving product. Final temperatures can be accurately controlled to up to 120°C + 1 degree Celsius. Residence time is around 20 min compared with 1 hr or longer for traditional kilning systems (Table I).
- Pasteurization conditions are optimized. Precise control of temperature and residence time in the presence of steam ensures optimal pasteurization conditions. The absence of cold spots and dead zones results in uniform treatment of groats.
- No hot air is required to compensate for excess steam and heat, as in traditional kilning. This reduces energy and investment costs. Traditional kilns typically require twice the amount of steam compared with the continuous processing system (Table I). Typical steam consumption can be estimated at around 7 kg of steam/100 kg of product and can achieve a final RH of up to 19% prior to flaking.
Other Advantages. The continuous kilning system offers several additional advantages:
- The incorporation of four independently controlled temperature zones allows different conditions to be programmed in to adapt to changes in product feed, as well as to optimize pasteurization, temper groats to improve flaking performance, and promote flavor development.
- An operator-friendly interface and specific formulas facilitate use of the kiln. Once programmed, the kiln requires minimal surveillance during operation.
- The smooth internal surface of the spiral tube is designed to make cleaning easy and fast when using highly efficient pigging (scraping) equipment developed specifically for the system.