Fine tuning the baking and drying parameters to produce reliably uniform, shelf-stable pet foods and treats depends on more than time and temperature. The density and ingredients specific to pet formulas make removing moisture a challenge. Ovens and dryers work in tandem to provide the needed process controls for repeatable results.
The basic thermal process is the same for baking and drying. In baking, the primary purpose is to bring about a chemical or structural change to the product, and moisture reduction is a by-product. In drying, the primary purpose is to remove moisture. “Both processes use temperature and airflow for some given product retention time to get the desired product,” says Tom Barber, vice president of process engineering at Bühler Aeroglide, Cary, North Carolina. “They use convection airflow for the heat transfer and moisture removal. The differences between ovens and dryers relate to operating temperature, process air velocity and distribution, process air humidity, air exhaust volumes, product depth and conveyor speed.”
Baked products, such as dog biscuits, go through three distinct thermal processes. The dough is baked after forming to set the structure, then the baked product must be dried to a shelf stable point and adequately cooled before packaging or storage to prevent moisture condensation. “Ovens normally do not have a high exhaust rate, so it takes higher temperatures and longer retention times to get the final moistures low enough,” Barber explains. “The high discharge temperatures on the product also make it difficult to cool the product, requiring long cooling retention times. The dryer has a much higher exhaust air exchange, allowing drying at much lower temperatures and in a smaller piece of equipment compared to an oven.”
The two major types of products in the pet food industry are formed baked products and extruded products. The equipment used for these processes is similar to the human food industry. “For baked products, the dough structure is set in the oven, and the final baking yields a crisp product with some browning and flavor development,” Barber says. “Sometimes all of this is accomplished in the same oven, but the process may also require a post-bake dryer to more efficiently reduce the moisture content of the product.”
“The challenge is to not case harden the larger products," Pallottini says. "Once the product is case-hardened, it becomes impossible to achieve the desired moisture levels."
Dog biscuits or treats come in a range of sizes. Achieving target moisture levels are important to obtain the required shelf life. Most baked pet treats contain 30% moisture leaving the rotary moulder, 20% leaving the oven and just under 10% leaving the dryer. Sam Pallottini, director of cookie, cracker and pet food sales, Reading Bakery Systems (RBS), Robesonia, Pennsylvania, says, “The challenge is to not case harden the larger products. Once the product is case-hardened, it becomes impossible to achieve the desired moisture levels. Increasing bake time and reducing the airflow in the first part of the oven helps prevent this. Another option is to introduce steam in the beginning, which will enable the product to bake out the moisture in the center of the large biscuits.”
Extruded products most commonly require a dryer. “When a product is made on an extruder there is usually moisture added during this cooking step in the form of steam or water to get the correct product appearance and cook level,” explains Galen Rokey, process technology director, Wenger Manufacturing, Sabetha, Kansas. “This moisture must then be removed for product shelf life. Typical extruded pet foods may contain 20% to 30% moisture prior to drying. Conveyor speeds within the dryer allow the proper bed depth and retention time. Drying temperatures are another operating parameter needed to reach a product moisture level between 6% and 8% as the product leaves the dryer.”
Extruded products for human consumption are typically dried below a moisture content of 5% for bite texture, flavor and structure stability. Pet food extrudates are more dense than human food and are normally dried to a final moisture content of 6% to 9% for shelf stability and food safety.
Oven and dryer manufacturers build in automation and controls designed to optimize the operating parameters. RBS ovens utilize independently controlled blowers for top and bottom airflow. An RBS 3-pass dryer can increase throughput in a smaller footprint by continuously blowing warm air on top of the product as it drops from one tier of the dryer to the next. All sides of the product are exposed to the warm airflow as it gently flips and tumbles to the next level.
“Benchmarking an oven is important,” Pallottini says. “A benchmark defines the bake profile for each product formulation by measuring the four key variables in an oven: temperature, airflow, humidity and heat flux.” The RBS Scorpion Data Logging Measurement System is a hardware and software measurement system, with a 32-input channel, data logger and a group of compatible sensors helping processors better understand each product and process.
Bühler Aeroglide offers two options for precise moisture control: Evaporation Control and Moisture Pro. Evaporation Control measures process air temperatures before and after the process air goes through the product. This differential correlates with the evaporation load at that point in the process, and the operating temperature can be automatically adjusted to compensate for fluctuations in incoming moisture content to bring the drying rate back into specifications early in the process. Moisture Pro provides a microwave-based sensor in the product discharge stream to continuously check expected moisture content versus existing. “When the two options are used together, it provides an opportunity for feedback information to fine tune the process air temperature differential versus evaporation load,” Barber says. “Long-term, sustainable process optimization can be supported by our digital services which provide continuous monitoring and trend analysis.”
Commonly used for extruded products, Wenger dryers offer full automated process management control of conveyor speeds for proper retention time and individual temperature zone control to meet the product needs. The Wenger Series 2 is a 3-pass dryer equipped with woven belts to convey smaller products through the dryer that would normally fall through the openings in other conveyors that use larger-sized mesh.
Other moisture marvels
Dehydrators are similar to dryers. Both use convection airflow for the heat transfer and moisture removal. Marlen International, Riverside, Missouri, has developed a new high-performance industrial dehydrator focused on airflow efficiencies which enables better control of air movement inside the cabinet. Increased air velocity over the surface of the product results in rapid moisture removal, faster drying cycles and more uniform process conditions. Marlen’s design keeps a permanent balance of supply and return air for better performance which is essential for repeatable results. “We monitor relative humidity during the dehydration process,” says Marty Wimmer, thermal product sales manager for Marlen International. “Product uniformity is critical for dehydration. More innovative dehydration technologies ensure efficient management of process air which leads to faster drying times with less energy consumption compared to older, less efficient models.”
Larger, denser and grain-free products hold more water and can be challenging to dry. Radio Frequency, Millis, Massachusetts, offers a Macrowave post-baking dryer that preferentially heats and dries the moist interior of products, eliminating the interior drying requirement from the oven and potentially improving productivity and profitability of an existing line.
“Product moisture uniformity is a critical product characteristic for pet food and pet treats, as it affects product quality, downstream processes and profitability,” Barber says. Finding the right combination of baking and drying technology gives processors the tools needed to deliver consistent, quality pet foods and treats.
Read more about pet food processing equipment on our Equipment page.