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Radio Frequency Heating

Radio Frequency Heating

Probably  well  over  a  million  Industrial  Radio Frequency  heating  installations  for  various  applications  have  been  sold worldwide in the last seventy years.

Radio Frequency Dielectric Heating was originally introduced into the Food Industry in the UK around the beginning of the 1960's. Early machines were relatively small, air cooled and had simple electrode applicators. They generally suffered from fairly poor reliability and poor performance.

Strayfield Limited was founded by John Holland in 1969 to supply equipment for post baking of biscuits and has been the market leader in RF dryers in the food industry for many years now, having hundreds of dryers installed  in  this  industry  alone  and  several  thousand  other  RF  machines  in  the  Textile,  Fiberglass,  Paper Converting, Plastic Welding, Pre-heating and Woodworking markets.

Over the years, several other companies have attempted to enter the RF food processing market. Virtually all of these have disappeared after a relatively short time.

Industrial Radio Frequency heating at high power is a very specialized field and relies to a large extent on knowledge gained through field experience. Very little, really useful information on this application has been published, apart from the most basic theory. Often,  much  of  the  information  available, including  that  contained  in  patents  has  been  found  to  be  very  simplistic. In many cases, it does not work in practice or is just plain wrong.

RF heaters and microwave ovens offer direct or volumetric heating. The simplest form of RF applicator consists of two metal plates which form an electrical capacitor.

The product becomes a  "lossy"  dielectric  (hence  the  alternative  name  of  "Dielectric  Heating")  and  absorbs energy  from  the  RF  Generator,  which is connected across the two metal plates  ( generally  referred  to  as "electrodes").

For  most  products,  we  are  dealing  with  water  molecules,  which  are  ionic  in  nature,  although  the  technique applies  to  other  ionic  substances  too. The  RF  heating  process  depends  upon  the  ionic  conductivity  of  the material being heated. The effect is analogous to that of two bar magnets. If two like poles are placed together, they repel each other. If two unlike poles are placed together they attract. Similarly, polar molecules have positively and negatively charged ions. If the two electrode plates, between which the material is placed, are charged positive and negative respectively, the molecules will tend to line up all in one direction. If the charge on the plates is then reversed, they will tend to flip around and line up in the opposite direction.

Reversing the charge causes the molecules to rub  against  one  another.  This causes "frictional" heating. The heating rate will increase as the frequency of reversal of the charge on the plates is increased. A rate at which the frictional energy generated within the dielectric material is higher than the rate at which heat is lost to the surroundings by the dielectric decides the rate of temperature rise of the dielectric material. Typically this is caused to occur at high frequencies in the Megahertz range. The RF frequency bands used in dielectric heating are centered on 13.56 MHz, 27.12 MHz and 40.68 MHz. These frequencies are reserved specifically for use by Industrial, Scientific and Medical purposes ( the I.S.M. band) to avoid possible interference with other users of the radio spectrum (i.e. broadcasting, satellites etc). There are very good technical reasons why most companies in the field use the 27.12 MHz band.

Having spent the substantial years in this line of business, I believe that I am probably reasonably well qualified to venture an opinion on the relatively slow adoption this technology in the Food Industry. I would suggest that even today, most food industry technologists either have never heard of RF, or if they have, they consider it as a last resort when all else fails. Why is this?


Radio Frequency Drying


3.1. RF drying – a "Black Box" technology 
Strayfield supplies RF drying equipment to many industries. In the Textile industry for example, dryers are used to dry bobbins of Textile yarn after the coloring (dyeing) process. The RF dryer was introduced into the textile Industry by Strayfield in the late 1970’s. Although this  was  much  later  than  its  original  introduction  into  the  food  industry,  RF  textile drying has  gained universal acceptance and is now the preferred method for drying over conventional hot air dryers.

For  over  25  years  now,  textile  people  have  asked  us  a  very  different  question  from  that  asked  by  the  food industry. They ask not “how does it work?”, but rather, “what capacity RF dryer do I need and how much will it cost?”. There are now well over several thousand RF textile dryers globally, and several of our clients have over 20 dryers on the same site. This is definitely not the case in the food industry. People who come into contact with RF appear to have a resistance to using it. Quite frequently this is due to either fear, ignorance or in some cases, a previous bad experience with a supplier who did not understand the RF technology properly. Purchasing the wrong machine usually results in problems of reliability, efficiency, and poor performance.

3.2. Microwave heating 
Microwave technology probably does not seem to fare quite as badly as RF in this respect. I believe that there are several important reasons for this.

3.2.1. Why do people opt for Microwaves in the first instance, instead of Radio Frequency? 
Today, virtually every house, factory cafeteria, and convenience store has at least one microwave oven.  For many years, small Microwave ovens have been relatively inexpensive. Typically today, a domestic microwave can easily be obtained off the shelf for a very little more than a hundred dollars. Most  manufacturers  have  at  least  one  microwave  oven  in  their  R&D  facilities. Therefore, when food technologists are developing products it is natural to take some of the product, push it in the microwave, switch on and “see what happens”. If this very crude test shows signs of success, then the thought pattern goes  –  “OK  –  now I just need a bigger Microwave for the final process”. It is clearly a natural progression to consider a scale up, rather than to review the alternative, lower frequency option of RF little realizing the ill effects of microwave technology regarding uniformity of drying and reliability of equipment when scaled up to higher powers and sizes.


4.1. A unique attribute of RF – the ability to profile moistures 
The free-running Triode Oscillator circuit can, if correctly designed, be dynamically matched to the load. The power  absorbed  by  the load  is automatically  controlled by itself.  The RF dryer is uniquely suited to moisture control. It is able to profile the moisture content of the product passing through the dryer. If there is high moisture in the product, then more power is drawn automatically.

This  moisture  profiling  effect  is  a  unique feature  specific  to  free-running  triode  tube  oscillators.  Running at fixed power does not accommodate variations in moisture which occurs on all lines due to the modulating of the conventional oven burners.

4.2. A solution for increased production where space is at a premium 
Many bakeries become "landlocked". There is just no room to extend the conventional oven to increase production as the space at the end of the oven is already filled to capacity with coating, sandwiching or packaging equipment. RF can solve this problem.

4.3. A solution for improved product quality control
One big problem especially in the biscuit industry is product checking. This occurs because of thermal stressing during cooling if the distribution of moisture is not even thru the entire mass substrate of the product. RF post baking drying equipment can address this issue, which can result in a dramatic reduction of rejects subsequently.

biscuits cracking

As RF heats the entire mass of the product from within simultaneously, the moisture level from end to end and the center of the product is very close.  This is because the electromagnetic waves cluster together more wherever the moisture content is more as it poses lesser reluctance to their flow.

The exposure to RF energy in these areas increases and they heat faster than the surrounding areas. Hence this becomes an self regulating mechanism whereby at a microscopic level RF energy is distributed based on the moisture level in the product and in  turn creating differential heating levels during the time of exposure to RF energy. Consequently this creates an even moisture profile in the product.

4.4. A solution for improved shelf life 
Generally when product moisture is mentioned it is the average moisture content in the product. This however necessarily does not mean that the moisture content is evenly distributed within the whole mass of the product. Say for example when a biscuit leaves the baking oven and is ready for being packed at 1% residual moisture, more often than not the center of the mass of the biscuit could have moisture levels between 3 to 4% and in some instances beyond 5%. When  such  a  packaged  product  is  put  thru  the  distribution  system  the  product  could  sometimes  wait  for  a month or two before it is consumed. We are all aware that higher residual moistures can lead to rapid growth of microorganisms, which in turn can render the product not consumable.  As discussed above the  excellent impact of moisture profiling of a RF drying system can overcome this problem very easily which in turn helps increase the shelf life over a non RF dried product.

4.5. A solution for cost savings 
A very standard drying attribute of extrinsic heating of any porous product that has moisture distributed throughout its mass is differential heating pattern within the mass. It is very clear that a big part of the energy is required to dry a very small amount of moisture. This is because the energy needs to flow from the outside to the  inside  in  a  laminar  flow  and  in  trying  to  do  so  has  to  encounter  the  thermal  impedance  of  the  product.

Moreover a large part of the energy is also utilized to increase the temperature of the product. Another ill effect of  extrinsic  heating  is  that  the  surface  may  encounter  higher  temperatures  than  the  core  inside. This can sometimes be harmful to the product substrate.


5.1. Post baking of cookies and crackers 
The  most  well  know  application  for  RF  in  the  food  industry  is  in  post baking  of  biscuits  (i.e.  cookies and crackers). In many cases where space is at a premium, and an increase in production throughput of 30% to 40% is required, whilst maintaining product quality, the only realistic option for many bakeries is to install an RF unit.

Uniform shapes can easily be processed in a Monolayer.  It has been determined from experience that more difficult irregular shapes benefit from reduced power density. This is usually achieved by packing more product under the electrode (typically forming a bed), so that although the power used remains the same, the residence time in the RF field is increased, thereby reducing the watts per product piece.

5.2. Drying before slicing 
Many processes exist where bread is sliced following de-panning (removal from the metal baking pan). This is usually  a  discontinuous  process  whereby  the  loaves  are  taken  off-line  after  baking  and  stored  in  huge  conditioning rooms for 24 to 48 hours before they can be sliced. It has been shown  that this labor -intensive activity can be completely eliminated by using RF drying to remove 2-4% of moisture, followed by reduced conditioning on a continuous accumulation conveyor.

The point to make here is that the incorporation of RF into  the  line  allows  the  size  of  the  accumulation  storage  to  become  practically  feasible  and  thus  makes uninterrupted continuous processing possible. Without RF, the size of the accumulation storage required would be very large. This process has worked very well for several customers in producing packaged, toasted bread, and also for customers processing Bagel Chips.

5.3. Pre-heating of yeast-based dough 
Dough  Proofing  on  a  continuous  basis  usually  requires  a  very  long  proofing  oven  in  order  to  raise  the temperature of the dough slowly to prevent the yeast culture being killed due to over -heating. This is because it is easy enough to heat the surface of the pro duct, but the dough is a good insulator and so it takes a long time for the heat energy to penetrate to the centre of the product. RF rapid pre -heating, followed by a reduced length proofing tunnel to hold the temperature at 37°C (98°F) can accelerate this process and allows the length of the proofing tunnel to be reduced by up to 60%. This has been successfully implemented for a variety of products including croissants, cake batters for Italian Pannetoni production, pretzel sticks and bread slabs (for breadcrumbs). In fact for breadcrumbs, good quality Japanese crumb can be manufactured using just RF energy alone.

5.4. Other non-bakery food applications

5.4.1. Dis-infestation of food products 
RF  heating  has  been  demonstrated  to be  a  proven  alternative  to  banned  chemicals  like  Methyl  Bromide  and Phosphine for dis-infestation of post-harvest pests in nuts and similar products as well as processed foods like semolina, spices, pet food, corn, lentils etc. The set up  for  a  3  month  industrial  scale  RF  trial  on  Walnuts  undertaken  jointly  by  WSU,  University  of  California Davis, USDA Fresno and Strayfield at Diamond Walnuts in California. The results were excellent and showed a 100% mortality rate for even the most resistant insect pests. Similar work has been carried out at WSU and elsewhere on almonds and other related products.

5.4.2. In-Tube heating using RF Energy 
Due to the unique ability of RF and Microwave energy to supply direct heat generated within the product, a field of opportunity opens up to be able to heat flow able or pump able products within a tube. This concept has been successfully used to heat rice and also to fix dyestuffs onto textile fibers. There are limitations to this process, and the main one of these is the conductivity of the product. High conductivity limits the penetration depth of the energy and it has been  found  that  products  such  as  meat  for  deli  products  (which  would  be  an  ideal  candidate)  can  only  be processed either in solid form up to 1 inch diameter or in applications where meat in liquid is moved around within the tube.

Radio  Frequency  heating  has  acquired  somewhat  of  a  varied  reputation  over  the  years  due  to  poor  understanding of the process and poor design and implementation. However, with a proper equipment design, properly implemented, RF can be very successful indeed. With suitable products, the correct equipment and an experienced supplier, increased production, reduced costs and improved quality are easily attainable.

A  second  important  point  concerns  how  processes  are  developed:  In  the  past,  many  companies  and  also scientific institutions have purchased small RF  units and carried out research projects alone  with little or  no success. This  happens  because  the  R&D  people  are  primarily  food  scientists  and  they  do  not  properly understand  the  RF  technology  and  how  to  solve  RF  related  problems. Frequently such projects have been abandoned after a short while.  I  would  advocate  from  experience  that  the  only  way  to  properly  exploit  this technology is for academics to work in close partnership with Equipment manufacturers  and the end users.

Strayfield has several such partnerships which have worked to the advantage of both parties. Of particular note here is our partnership with Juming Tang and his team at WSU. The key ingredient for success here is that the arrangement must be a win-win situation for all parties concerned.

Related article:

Heat Transfer for Biscuit Baking


Co-Author : Dilip Wate

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