One of the earliest automated cracker ovens, which Thomas L Green built in the late 1890s, was the mechanized reel oven. Bakers loaded pans of sheeted crackers into the front of the oven onto a rotating wheel of plates called the reel. As the reel turned at slow speeds, bakers exchanged pans of baked product with pans of newly sheeted dough. This became known as the panning process in the first commercial continuous ovens in the cracker and biscuit industry.
Figure 1: Thomas L. Green “T” Oven
By the early 1930s, mass food production became more common and automated. Machinery manufacturers built brick-and-mortar tunnel ovens in which a conveyor belt transported the crackers through a chamber filled with numerous natural gas line burners. The product entered one end of the oven to be baked and was transported to the other end where finished biscuits and crackers could be unloaded, cooled and packed.
Because intense heat cycles caused masonry tunnel ovens to be frequently rebuilt, ovens built with steel exterior frames and masonry interior liners replaced ovens constructed of bricks and mortar. A series of line burners, evenly spaced in the oven above and below the conveyor belt, provided for a more uniform bake across the width and along the length of the oven. These new steel ovens, commonly referred to as direct gas fired (DGF) tunnel ovens, automated the artisan process of baking further, enabling increases in the mass production of crackers and biscuits.
Figure 2: Thomas L. Green NR Oven
Although today’s DGF baking ovens are similar in concept to the original ones built 100 years ago, the oven design and component technology has substantially improved. Pure DGF ovens have given way to hybrid baking ovens using a combination of DGF and convection zones for mass production of products at higher rates with improved product consistency and quality.
Figure 3: The Transformative Phases of Baking Products
Technology has enabled bakers to better understand the science behind the baking process. Oven-profiling devices, such as the SCORPION® Data Logging Measurement System, have unlocked the mystery within ovens, showing how heat transfer, humidity and air movement can define a product’s attributes. This data show that each product has a single unique baking profile that defines a product’s taste and texture.
With this knowledge, machinery manufacturers now construct ovens to efficiently bake snack products by controlling the type of heat transfer, the humidity and the air movement throughout the baking chamber. The process of baking snack products can generally be divided into three transformative phases, no matter how long the tunnel oven. Each phase plays an important part in establishing the quality of the product.
During the first third of the oven, the dough pieces begin to develop as the product’s temperature slowly increases in the hot, humid zones of the oven. Generally, the ribbon burners in the first third of the oven are set to a high firing rate as the cool dough and the returning mass of the oven belt creates a high heat load. Operators typically set exhaust flows to a minimum to retain humidity, allowing the product to build moisture on the surface. The moist surface shields the product from the high radiant heat of the surrounding oven structure. Lower oven burners beneath the belt are set at higher firing rates than the upper burners to account for heating the combined mass of the oven belt, as well as the product sitting on the belt. The upper ribbon burners heat the passing product, air and oven ceiling.
Figure 4: The Baking Process
The product experiences an expansion as water begins to vaporize and the chemical reactions within the dough entrap forming gases. The moist product surface allows the product to grow and expand without cracking. Surface blisters will begin to develop as water vapor and gases seek paths to escape the product. The internal product temperature increases as heat is continuously applied to the product as it travels in the oven. The time versus temperature profile is a critical parameter in developing the desired flavors and textures within the dough.
The product’s internal temperature profile is usually referred to as an “S” curve, as the temperature slowly increases initially, then rapidly rises as water is removed from the product. As the product’s hydration level reduces from the initial dough level of 30-40% to a final, finished product moisture level of 2-3%, the temperature curve begins to flat-line close to the vaporization temperature of water (100oC or 212oF). As long as the product retains water, the temperature will generally be less than 100oC (212oF).
In the middle third of the oven, the product is now fully developing the internal and external flavors and texture. The upper burners are set to maintain an aggressive heat application to set the product’s surface structure, which prevents the product from collapsing as the gases and water vapor seep outside of it. Lower burners are set to maintain a high belt temperature to ensure the bottom surface of the product is baked. Operators set zone exhaust flows from 50-100%, depending upon the type of product and the amount of water that must be removed. Typically, a full-sheeted product requires the highest exhaust settings in all oven zones, as the amount of water that must be removed from a full-sheeted cracker, versus a round-type cracker, is as much as 30-40% more.
Figure 5: Bread Baking “S” Curve Profile
The outside of the product begins to set and skin. The moisture layer on the product’s surface has evaporated and the product begins to dry from the outside in. Water slowly migrates from the wet interior to the drier exterior, leaving behind a moisture gradient between the insulated interior and the skinning, dry exterior of the product.
The final third of the baking process completes the evaporation of water from the product’s interior and determines the final surface color of the product. The internal temperature of the product, which at this point exceeds 75oC or 170oF, will no longer have any further effect on flavor or texture development. Operators focus on applying heat to achieve the product’s specified final surface color and internal moisture content.
This is the point in the baking process where oven data logging technology collected over the last 20 years reveals the need to move away from the low air flows and high radiant temperatures of DGF oven zones towards a more controllable air heating zone, where both product coloring and drying can be controlled separately and efficiently.
The single-purposed DGF ovens introduced almost a century ago, with slower bake times, free flow exhausts and limited product flexibility, are now more difficult to operate consistently and efficiently in today’s high production-rate culture. Achieving critical product parameters, such as flavor and texture development, stack height, spread, surface color, and final moisture content, at high production rates is challenging with a single type of heat source throughout all oven zones.
The DGF oven zone performs well in the first phases of the oven where flavor and texture development is the primary goal. However, in the final phase of the baking process where controlling product drying is the priority, a zone of multiple high-temperature burners with little airflow is not optimal for operators to achieve uniform product moisture and color.
Over the years, operators have developed ways to compensate for excessive heat in the last zones of a DGF oven. Residual heat in the air, product and belt from the earlier oven zones results in reduced heat requirements in the final phase of baking. Temperatures often exceed controllable set points, thus causing operators to shut off burners, increase exhaust rates and open oven doors.
Figure 6: Heat Transfer within a DGF Oven Zone
To better understand why DGF oven zones work so well in the front of the oven but not so well towards the end of the oven, bakers should understand how the direct gas fired burners add heat to a zone. A DGF oven zone heats the baking chamber with numerous burners, spaced approximately 750-1,000mm (30-40”) apart, above and below the baking band. These burners add three types of heat to the oven. The heated air in the oven bakes the product through convection air currents. The oven belt bakes the product from beneath through conductive contact, and the structure of the oven bakes the product radiantly from all surfaces within viewing range of the product.
The combinations of these three heat forms called convection, conduction and radiation define how a product bakes. In general, the DGF zone provides a good controllable radiant and conductive heat flow, but with very little air movement; the convection heat flow is limited to the exhaust rates. A DGF zone can build much higher levels of humidity than a convection zone because of very low airflow. Having higher humidity ratios in the first phases of baking is important in developing products, but not very beneficial for the drying processes in the last phases of the baking.
Figure 7: Heat Transfer within a Convection Oven Zone
For a drying process, circulating high volumes of air through the baking chamber increases the convection heat. This high heat circulation makes the entire zone’s temperature more uniform. Air adds heat to a product in two ways: Higher air speed applied to the surface of the product drives heat more rapidly and carries away vaporizing water from the product, and higher temperatures in the air applied to the product’s surface increase the coloring rate of the product by drying the surface to very low moisture levels.
To optimize the baking process for crackers and biscuits, a combination of oven zones should be used to provide the most efficient and uniform process that ensures consistent product quality. It is now accepted technology to use DGF zones in the first 33-66% of the oven, with convection zones used for the rest of the oven. An oven with this combination of DGF and convection zones is called a Hybrid Oven.
While hybrid ovens provide for a more flexible baking platform, operators need to be capable of matching existing product attributes established on older DGF ovens. The hybrid oven can deliver the same product flavor and texture profiles at a higher consistency than pure DGF ovens, but operators will need to learn to bake slightly differently in the second and third phases of the baking process.
Achieving uniform product moisture and reduced checking will be the biggest benefits from the convection zones of the hybrid oven. But the control of the product’s surface coloring will require operators to be flexible about where to achieve the highlighting of the product. Normally in a DGF oven, operators add surface color in the last zones of the oven, where a few intensely fired burners, with flame temperatures that can exceed 1,100oC (2,000oF), can quickly color the product’s surface. Temperatures in the baking chamber of convection zones never exceed the circulating air temperature, typically at 175-315oC (350-600oF).
A convection oven applies a consistent lower air temperature across the product’s surface, so the coloring tends to be a more uniform browning process, without the surface highlighting dark and light spots. Therefore, operators need to move the highlighting coloring process to the last DGF zone, before the first convection zone in the hybrid oven.
Figure 8: DGF Cracker Oven Heat Profile
Using the same techniques as a full DGF oven, operators can select a few upper burners in the zone to achieve the surface highlighting color before the product enters the convection zones. Once the product is in the convection zone, operators will have two levers to control the finished product color and moisture.
Setting convection zones at the highest air speeds and lowest temperatures will dry products with less coloring. Setting the lowest air speeds and highest temperatures will color products while lowering drying rates.
If the products are baked on a closed-mesh belt, the differences between a DGF oven and hybrid oven are minimal. If the oven belt is an open mesh, the convection zone offers advantages in product drying, since the bottom of the product will receive both conductive heat from the belt and convective heat from the airflow.
Figure 9: Hybrid Cracker Oven Heat Profile
Figures 8 and 9 compare the radiant and convective baking profiles of two full-sheeted soda crackers from a full DGF oven to that of a hybrid oven, with three zones as DGF and two zones as convection. In the full DGF oven, the convective-to-radiant heat ratios in each zone are similar throughout the oven. In the hybrid oven, the zone temperatures in the middle zones are higher to achieve the surface color earlier in the process. The convective heat transfer is almost 50% higher than that of the DGF final zones.
Every product has a single radiant, convection and conductive heat profile that uniquely define the key flavor and texture attributes of that product. Understanding the basic forms of heat transfer within ovens provides operators a greater ability to make decisions on controlling the process to achieve the highest quality and consistency.
When transferring products between older DGF ovens and new hybrid ovens, the SCORPION® Heat Flux Sensor should be one of the tools used to set up the oven profile. It is more important that the two processes’ heat profiles match than the two ovens’ temperature profiles match.
Figure 10: Thomas L. Green Prism Hybrid Oven
Operators must understand how to shift the highlighting coloring process from the last third phase of the oven to the middle zones of the oven. To fully use the benefits of the convection zones, the circulation rates must be set as high as possible and the temperature must be adjusted to achieve the proper finished moisture levels.
Hybrid ovens are now widely accepted in all snack-baking platforms. For cracker and hard biscuit processes, one half to one third of a DGF oven can be replaced with convection zones, while still matching the existing quality from a full DGF oven. For soft cookies, radiant/convection air re-circulating zones can be combined with full convection zones to create an efficient hybrid oven for cookie processes.
Figure 11: Crackers from a Thomas L. Green Hybrid Oven
What should be the ideal length of tunnel oven for small scale cracker production? Tunnel oven for biscuit snack crackers (output = 50-100kg/hour):