THE CURRENT STATUS OF
RADIO FREQUENCY POST-BAKING DRYING TECHNOLOGY
THE 72ND ANNUAL TECHNICAL CONFERENCE
BISCUIT AND CRACKER MANUFACTURERS' ASSOCIATION
FORT WORTH, TEXAS
OCTOBER 21, 1997
TIMOTHY D. CLARK
ABOUT THE AUTHOR: Mr Timothy D. Clark, V.P. Marketing of Radio Frequency Company's Macrowave™ Division, is a graduate of the University of Arkansas and has 10 years experience in RF processing of foods and post-baking drying systems for the biscuit and cracker industry. His present company, Radio Frequency Company, designs and manufactures RF process heating and drying systems.
Radio Frequency Post-Baking Drying
Radio frequency post-baking dryers were introduced almost thirty years ago. This technology is uniquely suited to seeking out and removing moisture in products where it's most difficult for the conventional oven heat to reach and provides the benefits of increased productivity and improved quality to a broad range of bakery products.
A radio frequency post-baking dryer gives the baker a greater ability to control final moisture content and uniformity throughout the product, which provide the following benefits:
- Up to 40% increase in productivity
- Virtual elimination of checking
- Control over product color
- Longer shelf life
Here's how radio frequency heating or "RF heating," for short, works. As product exiting the baking oven enters the RF post-baking dryer, it passes through an electrode array having an alternating electric field which reverses its polarity at a rate of 40.68 million times per second.
Water is comprised of "polar molecules" which can be likened to each molecule having a north and south pole. When passing through an alternating electric field, polar molecules react similar to the way bar magnets behave in an alternating magnetic field, constantly realigning themselves to face the opposite pole. At a frequency of 40MHz, friction caused by this rapid movement causes the polar molecules of water to quickly heat, wherever moisture is present, throughout the entire thickness of the product. Nonpolar materials such as fat, oil, and dry ingredients do not react and, therefore, are not directly heated by RF energy. In the case of bakery products, this means an RF dryer will preferentially heat and dry the wettest area of a product without heating the areas already dry. Typically, there is greater moisture in the interior of a product as it exits a conventional oven. Therefore, this moist area will preferentially absorb more of the RF energy.
The Complimentary Effect of Conventional and RF Heating
Conventional ovens apply heat to a product most efficiently during the initial stages of baking, when the dough is dense, has a high moisture content, and is a good thermal conductor. This type of heating depends on thermal conduction to transfer heat from the exterior surface to the interior or center of the product.
However, as the product surfaces dry and the crumb structure begins to develop, the product becomes more of a thermal insulator, reducing the oven's heat transfer efficiency and effectiveness in producing a uniform product. Also, since the center of the product was subjected to less heat, it is therefore wetter than the external surfaces. These factors leave the baker with the challenge of trying to bake to specification, a product whose thermal conductivity is rapidly falling at an uneven rate across and along the oven band as the product begins to develop. Often, scrap product that is out of dimensional, moisture, or color specifications is dumped while the oven is "balanced out" by the operator. Even when the correct average moisture content, color, and stack height are achieved, differential shrinkage caused by a nonuniform moisture profile can still occur. This is the primary cause of "checking."
Radio Frequency heating eliminates
checking caused by differential shrinkage
Radio frequency heating overcomes the problem of applying heat specifically to the moist center of the products, eliminating differential shrinkage.
RF post-baking dryers, supplementing the major role played by the conventional oven line make a winning combination.
As we think of the conventional oven baking process, one can divide it into three basic stages: The product development stage, where the product develops the correct loft; the baking stage, where the product's crumb structure is set; and the final stage, where the goal is to achieve correct color and dryness simultaneously. It is at this final stage of finish drying where the application of an RF heating system can provide the greatest benefit to the baking process.
In a typical oven, between 20 and 30% of the
oven length is used for final drying since the
loft and crumb structure has already been set.
A post-baking dryer at the end of an oven line enables the baker to increase the band speed of the conventional oven line to the maximum speed at which the product can be produced with the correct loft, crumb structure and color, without regard to its moisture content upon exiting the oven. Generally, this represents a 20 to 30% increase in band speed and, thereby, productivity too! The RF post-baking dryer will then quickly and efficiently remove the residual moisture trapped within the center of the product.
Adding a Post-Baking Dryer to an Existing Line
The combination of conventional oven heating with RF heating in the final stage of the drying process is an extremely efficient way to produce many types of baked products, and it's easy to do. Radio frequency post-baking dryers are generally supplied as stand-alone turnkey systems, which can be ordered for retrofit to an existing or new oven line.
200kW Macrowave™ Post-Baking Dryer
RF post-baking dryers automatically adjust power to respond to varying moisture loads resulting from oven burner failures, minor dough moisture and sheet thickness variations. A preadjustment assures that the product is still produced within moisture specifications despite varying incoming moisture levels.
Let's quickly review the benefits you can expect with the addition of a radio frequency dryer:
- Increased Throughput - Up to 30% increase for crackers and often up to 40% for cookies.
- Elimination of Checking - By greatly reducing the moisture variation throughout the thickness of the product, checking caused by differential shrinkage is eliminated.
- Excellent Control of Final Moisture Content - Final moisture is controlled to approximately +/-0.25%.
- Independent Color Control - Since moisture is controlled in the RF dryer, color development in the oven can be done without concern for final moisture content.
New Developments in RF Post-Baking Dryer Technology
As we know, radio frequency drying systems have been installed in many bakeries. Historically, there were several challenges unique to this drying technology which we should review.
Probably the most significant problem, particularly with older RF systems, is a phenomenon called "arcing". Arcing is a high voltage discharge from the electrode to the product. Usually an arc is caused by a gross abnormality in the incoming load such as a large lump of wet dough entering the dryer. As I mentioned earlier, RF energy will preferentially heat the wettest areas of a product so if a scrap ball of dough at 40% moisture is conveyed through an RF dryer full of crackers at 5% moisture, it will draw a disproportionate amount of energy to the extent that the energy may arc to the wet dough.
Another cause of arcing is carbonized product. At oven start-up, the leading edge of the product may be overheated and carbonized in the oven. If this conductive, carbonized material is not dumped prior to its entry into the RF dryer, it is likely to cause arcing from the electrodes. Failure to conduct routine cleaning of the RF applicator may also result in conditions which can lead to arcing.
Although instances of arcing events are usually infrequent, they will occur from time-to- time. Because arcs can damage the product, the conveyor belt, and possibly lead to some unforeseen downtime, the RF industry was challenged to find ways to design RF drying systems which are more tolerant of process abnormalities and to suppress the intensity of arcs that do occur to the point where arcing is of minimal concern.
The Macrowave™ Solution
The dielectric heating frequencies reserved for industrial use by the U.S. Federal Communications Commission are as follows:
13.56MHz +/- .05%
27.12MHz +/- .60%
40.68MHz +/- .05%
It is quite important that the frequencies remain within the tolerances allowed so as not to interfere with radio communications.
It is much simpler to design dielectric heating equipment using a frequency with a greater tolerance allowance, so it's easy to see why most RF equipment historically has been produced to operate at a frequency of 27.12MHz, which has a tolerance 12 times wider than the others.
Today, using modern design techniques, Radio Frequency Company produces post-baking dryers trademarked "Macrowave," which operate at 40MHz and remain within the close tolerance required for this frequency. This advancement is particularly significant because a Macrowave™ dryer operating at 40MHz frequency can do the same work as a 27MHz dryer at 20% lower RF voltage, allowing it to deal with most process abnormalities without arcing. This also allows the use of radio frequency drying on a broader range of products. Further, the development of arc detectors and ultra-fast overload devices momentarily turn off power if an arc occurs. This prevents damage from the occurrence and restores the power so quickly that the product drying process is unaffected.
Forty megahertz Macrowave™ Post-Baking Dryers are successfully conditioning snack crackers in bed depths up to six inches deep, after oil spray application, without arcing problems. Additionally, radio frequency systems have benefited from the ongoing technological improvements in all of their electronic components and are operating at better efficiencies with greater reliability than ever before.
Determining If Your Product Is Right for a Macrowave™ Dryer
If you're faced with the dilemma of having a product that is a possible candidate for RF post-baking drying, but need to prove it before seeking approval for a capital expenditure, Radio Frequency Company has provided the answer: The "Macrowave™ OmniTherm™ Simulator." This RF processing system was recently developed for conducting product trials to define the necessary parameters to heat or dry materials under production conditions. It is a fully instrumented system which can apply both 40MHz RF and convection heating to a wide variety of products.
The Macrowave™ OmniTherm™ Simulator
Through the use of this advanced hybrid RF heating system, the scale-up requirements necessary to meet your production goals can be accurately determined. With as little as 50 pounds of product, the RFC testing laboratory can test-dry the product and return it for your evaluation. The Macrowave™ OmniTherm™ Simulator has the following features:
- For each test, the appropriate electrode array (either a flat plate applicator or staggered through-field applicator) can be quickly installed.
- Its conveying system utilizes a flat belt for processing discrete products or a troughed belt for drying snack crackers in a deep bed. Conveyor belts of various materials and constructions are available so the type best suited to the application is utilized.
- A sequence of different RF power levels, heating durations, and relaxation times to simulate the passage of materials through a multizone system can be initiated by the touch screen panel of a programmable logic controller.
- Other programmable features include variable ambient air temperatures up to 300oF and direction of air flow and RF pulsing for each of the simulated heating zones.
It is expected that more of these types of simulators will be available and, perhaps, some of the larger bakeries will acquire them which is expected to accelerate the use of RF post-baking drying technology.
Case History - Nabisco Atlanta
Product: Fat-free Premium® Saltine Crackers
Nabisco Biscuits, Inc. Atlanta Bakery
Atlanta, Georgia USA
Fat-free saltines are particularly challenging products to make. The difficulties in producing this product begin as the dough is machined. Because there's no fat in the dough, it's difficult to consistently process it through the gauge rollers and maintain constant sheet thickness. Differences in sheet thickness and moisture variations from dough to dough both contribute to product moisture variations. Consequently, operators must pay very close attention to product moisture test results and adjust the oven to compensate for these variations while assuring that product color and stack height specifications are simultaneously maintained.
To address these problems, Nabisco's team of bakers and engineers selected a 40MHz, Macrowave™ 100kW Post-Baking Dryer to be installed at the end of their existing oven line. They successfully integrated the dryer into their Allen-Bradley data highway control system providing remote monitoring and control of the system.
Nabisco Fat-Free Premium® Oven Line
This line has made very effective use of the RF post-bake drying system in producing very high quality, fat-free saltines at increased production levels and extremely low scrap rates. They are often achieving in-spec production in spite of burner failures or other anomalies that might have otherwise resulted in scrap.
Case History - Arnott's, Huntingwood NSW Australia
Product: Five multiple-product lines.
Arnott's, a subsidiary of Campbell Soup Company, has been in the biscuit and cracker business since 1865 and is the leading supplier of high quality biscuits and crackers in Australia. Their new facility in Huntingwood, New South Wales, Australia represents one of the most highly automated, state-of-the-art facilities in the world.
From the ingredient handling systems all the way to the finished goods dry storage, the bakery is completely automated and features the very latest equipment designs and control systems.
Arnott's stated that their chief concerns were product consistency and quality to maintain consumer loyalty. They selected a 40MHz Macrowave™ Post-Baking Dryer for each of their four new oven lines. Line versatility is considered a "must" to rapidly respond to changing consumer demands. Therefore, each line is designed to run multiple products. The Macrowave™ dryers provide added resources to achieve Arnott's production and quality goals.
Arnott's Bakery, Huntingwood, NSW
Arnott's bakers discovered that their ovens required practically no hands-on adjustments throughout the shifts as long as the RF dryers were there to handle any variations in moisture of product exiting the ovens. Consumer complaints regarding appearance and checking fell to an all-time low.
Arnott's products include high fiber Vita Wheats®, cheese flavored snack crackers, shortbread, Arrowroot® and several other products. The post-baking dryers on the new lines at Arnott's Huntingwood facility have given them the necessary latitude required to meet their quality and production objectives. The systems have been installed for about a year now with excellent results.
To summarize the current status of RF post-baking drying, we can confidently say that it is now recognized as a bona fide and sound processing technology for many types of cookies, crackers, and snack foods. Today's increasingly competitive environment in the biscuit and cracker industry demands the highest production yields obtainable, without sacrificing quality. The use of state-of-the-art Macrowave™ Post-Baking Dryers provides the modern baker an added tool to achieve these goals... a win-win combination for the baker, his customers, and the bottom line!
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