Product Humidity in Oven Unit Operations

BACKGROUND OF THE INVENTION

[0001] Baking is a critical unit operation in biscuit manufacturing: the high energy reaction phase maximizing chemical and physical transformations of raw dough into finished products. Traditional industrial baking depends on continuous Direct Gas Fired (DGF) or convection tunnel oven systems or a combination of each mode (sometimes referred to as hybrid or multi-media) all having multiple baking zones (usually between four (4) to eight (8) zones). These "multi-media" ovens permit effective control of the reactions that determine finished product attributes through decoupling of conduction, convection, radiant, and dielectric heat transfer modes. DGF, convection and radiant heat mainly impacts structure, thickness and texture.

BRIEF SUMMARY OF THE INVENTION

[0002] It has been found that product humidity especially in the early stages of baking also plays a key role. This is the focus of the present invention. This approach to the baking processes through multi-media ovens develops understanding of the fundamental characteristics and interactions for baking reactions in terms of materials, process and product of which the role product humidity plays is critical. It permits optimized process and oven designs through specific heat transfer data for scale up from pilot plant to production ovens.

[0003] A method of baking a product may include providing an oven including one or more baking zones; measuring static pressure in one or more of the baking zones; and controlling the static pressure in one or more of the baking zones with exhaust fans to maintain a pre-determined static pressure profile in one or more of the baking zones. The pre-determined static pressure profile may be derived from a pre-determined correlation between static pressure and humidity to provide a desired zone humidity profile for the product. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0004] The foregoing summary, as well as the following detailed description of certain embodiments of the invention will be better understood when read in conjunction with the following exemplary embodiments, the appended drawing and the appendices.

[0005] Figure 1 shows a static pressure arrangement in an oven.

[0006] Figure 2 shows specific humidity as a function of time.

[0007] Figure 3 shows a correlation between humidity and zone pressure.

DETAILED DESCRIPTION OF THE INVENTION

[0008] Methods according to embodiments of the present invention relate to baking a product according to a desired humidity profile.

[0009] Baking Oven Key Operating Modes and Control Parameters

[0010] An oven is defined as an enclosed chamber designed to bake products by applying various energy or heat transfer modes like conduction, convection, radiant, dielectric, microwave or some combination of the aforementioned heat transfer mechanisms. The primary functions of the baking oven are: (1) heat exchange or heat transfer rates and (2) mass exchange based on moisture removal (3) initiating leavening and color change chemical reactions. These processes are interdependent because heat transfer facilitates moisture removal from the dough and provides the activation energy and energy of reaction for the size and color changes typically noted in finished baked products.

[0011] A typical baking process can be defined by the following equations: raw dough + water + heat - final product + water vapor gas (methane) + oxygen - - carbon dioxide + water vapor

[0012] The unit operations of baking are also primarily dependent on the type of dough. For example, hard doughs (i.e. cream crackers, soda crackers) that generally are comprised of relatively low fat content (7-15%) are baked differently than soft doughs (cookies) which comprise slightly higher fat and sugar amounts,

[0013] In order to properly control the baking process, the oven is typically outfitted with zonal temperature sensors, gas flow/energy flow sensors, stack or air velocity sensors, and variable speed controls for product conveying (allows the setting and control of baking time). Each of these systems allows for the development of a baking profile based on the zonal temperatures, gas flows, stack or air velocities, and baking time. These systems are coordinated by programmable logic controller (PLC) or some other appropriate microprocessor-based distributed control system. By coordinating the zonal profiles of each of these parameters, the oven can react to changes in ambient conditions (based upon seasonal or daily weather variability) and variations in the dough products presented to the oven. The control system is typically set to deliver a set of finished product attributes at the exit of the oven. Finished product attributes can be moisture (amount of moisture removal), color, product dimension (thickness and/or diameter), and texture. In-flight testing of the products as they move through the oven also supports the proper profile settings.

[0014] Table 1 - Summary of Typical Oven Control Profiles

[0015] Humidity or water vapor content of the air in each zone above the dough/baking product is also an important control parameter and profile element for industrial baking. Prior to the advent of highly accurate and precise measurement devices in the last 15 years, it was difficult to measure and control humidity on a zone-by-zone basis. Humidity impacts each of the product attributes noted above at varying levels. The burning of gas in a DGF oven also adds to the humidity in a given zone and this additional water vapor (generated by the oxidation of the fuel gas) must be accounted for along with its impact on finished product attributes.

[0016] Humidity or lack of humidity can, for example, accelerate or delay the extraction of moisture from the dough/baking product depending on the balance between baking product moisture and the zone humidity. It can also accelerate or delay chemical reactions responsible for the leavening and color product attributes. Therefore, the balancing of the humidity profile with the other control profiles for the oven can be an important added feature of oven control.

[0017] One major drawback to establishing humidity readings and profiles is the entry cost of the humidity sensors. Typically humidity sensors that can be retrofitted directly to an oven zone are expensive. For example, in 2011 the cost range is from U$D20,000 to U$D35,000 per each. The incremental cost for implementing full zone-by-zone humidity sensors onto the existing oven control platform does not have a typical commercial payback of two to three years versus the benefits provided.

[0018] Static Pressure Measurement and Relationship to Humidity

[0019] Finding a path towards humidity control and profile establishment without the costly sensor expenditures and without having to make substantial modifications to the oven bake chamber is a process control goal. One tactic that can be implemented is by measuring the static pressure in the oven bake chamber. The static pressure is typically defined as the pressure of a fluid in motion, measured when the velocity is undisturbed by the measurement. The fluid velocity or streamline is measured by the time a fluid particle takes to move a known distance. However devices typically cannot measure fluid velocity directly, but can be related to velocity by using Bernoulli's equation. A differential between the zone static pressure and ambient provides a reference point. A probe directly mounted into the oven zone yields a direct process measurement (as opposed to a stack measurement of differential pressure or a measurement of ambient barometric pressure).

[0020] As shown in Figure 1 , the proposed system essentially requires the installation of a single static pressure sensor (readouts and probes obtained from Advanced Manufacturing Technology, Inc. AMT OEM 002 oven exhaust monitor and corresponding probe; model number 79531900002) in each of the bake chambers to measure static pressure, and also installing variable frequency drives (VFD; various suppliers such as Allen-Bradley, Toshiba, Siemens) as part of the overall control system. The overall control system directly controls the oven exhaust fans to provide short response times to vary the baking chamber pressure to maintain the desired pressure profile. Once the static pressure is measured, a direct and proportional correlation with zone humidity can be established. Via significant trials, the humidity component resulting from product moisture equilibrium as well as the humidity component resulting from gas combustion has been determined and quantified.

[0021] The change in static pressure is directly proportional to the change in humidity. By measuring each zone without product and no gas burning, without product and with gas burning, and with product and gas burning, the relative additive effects of humidity can be correlated.

[0022] Previous approaches have attempted to control the final product attributes during baking, as well as increase product throughputs by reducing bake times, by altering the oven temperature profile and adding additional zones to the traditional tunnel oven. Also, alternative energy modes have been investigated and implemented such as microwave technology (US Patent 5,945,022 entitled "Continuous Microwave Assisted Baking"). However, these solutions have proven to be capital intensive and require significant maintenance to keep operational. Adding additional oven zones to the existing oven requires additional space which may not be available due to the infrastructure of the plant facility as well as additional capital investment or cost avoidance per oven.

[0023] By measuring, controlling, and monitoring zone humidity, increased throughput - without significant capital investment in oven infrastructure - can be accomplished by stretching the profile over the existing oven zones without "idling" the end zones due to the onset of the color reactions (see Figure 2).

[0024] Humidity Process Control and Effects on Baking Operations and Baking Products

[0025] Humidity may be important in the baking unit operations because, for example: (1) baking using higher humidity air (increased water vapor content) provides an increased air heat capacity resulting in more efficient heat transfer to the product, thereby enabling a reduction in the bake time (or increase in the production throughput) and (2) higher humidity levels in the zones delay the moisture migration from the dough piece. These result in an "even bake" over a longer portion of the oven, potentially reducing the checking/breakage issues along with reducing case hardening and burned edges (enhancing product texture and appearance).

[0026] Humidity also plays an important role in the early baking zones by controlling the rate of heat or heat transfer to the raw dough. The mechanism is essentially defined as follows. Water vapor is present inside the bake chamber which essentially is derived from two sources: (1) ambient air and derived from the combustion process (as noted above), and (2) evaporation at the boundary layer of previous biscuit pieces adds steam to the oven environment. As water vapor condenses onto the raw dough surface it extends and elevates the sensible heat of the product. This mechanism continues until the dough piece achieves the dew point temperature of the air in each oven zone. The level of humidity should be such that it exceeds the dew point at the dough surface. Humidity then alters the net evaporation rate altering the product's attributes. By controlling the level of humidity, it preserves the extensibility of the dough surface and prevents "crusting" promoting heat transfer into the interior of the dough and baking the product from the inside out to achieve proper stack height. When the optimum humidity level is achieved, and since dough is porous based on its rheological and material properties, the oven chamber moisture will condense onto the dough surface keeping its surface extensible or malleable. This causes the moisture to penetrate into the center of the piece and keeps it flexible and fluid during the initial baking stages to permit the diffusion mechanism to occur and drive out moisture from the center. In summary, reducing the oven exhausting results in increasing relative humidity, oven temperature, thermal conductivity and reducing the rapid evaporation of moisture from the surface of the biscuit.

[0027] If the humidity however is too low in the front end of the oven, the reverse will occur; the dough piece will experience rapid evaporation of moisture from the surface causing the surface to dry out and crust or "case harden". Once the surface of dough has crusted, it is essentially sealed. The crust then results in a reduced heat transfer rate, diffusion and

evaporation rates, which slows the heat penetration rate, causing the dough moisture in the center of the piece to remain high and reducing the ability to achieve proper stack height. The result causes the finished baked piece to have a "doughy" center and limited stack height which is not desired by the consumer. This typically leads to a phenomenon in which the internal moisture gradient is not uniform, leading to differential moisture causing "biscuit checking" which ultimately results in product breakage. It also affects the final piece weight of the product, which affects the weight of the product in the package and fixed length. Lack of humidity may also result in surface blistering of the product which can burn easily, resulting in undesirable dark spots. These blisters may also break and cause shelling on the surface of the product, which again is undesirable for the consumer.

[0028] System Implementation and Practice

[0029] The retrofit of pressure sensors to a wide number of oven control platforms has been completed. This process control approach has been successfully applied commercially in such products as Wheat Crackers, Cheese Crackers, Butter Crackers, Vegetable Chips and some soft dough cookie varieties. In most cases, a throughput increase was observed of at least 7.5-10% (with a potential upside dependent upon extending the control setpoints for humidity).

Additional throughput increases are possible with additional packaging equipment to pack the additional throughput. Product quality has also been improved on all of these products due to the more "even bake" and higher heat transfer coefficient. Leavening and color chemical reactions occur in a more predictable manner with improved product spread while avoiding surface crusting. In addition, there is a general reduction of product variability between production lines in the production network. This technology has also allowed an incremental reduction in energy consumption on an energy per unit pound produced basis.

[0030] Table 2 - Summary of Humidity Profile System Benefits

[0031] It will be appreciated by those skilled in the art that changes could be made to the exemplary embodiments shown and described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the exemplary embodiments shown and described, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the claims. For example, specific features of the exemplary embodiments may or may not be part of the claimed invention and features of the disclosed embodiments may be combined.

[0032] It is to be understood that at least some of the figures and descriptions of the invention have been simplified to focus on elements that are relevant for a clear understanding of the invention, while eliminating, for purposes of clarity, other elements that those of ordinary skill in the art will appreciate may also comprise a portion of the invention.