By Lou A. Honary, President of WAVEtek Process Technology, LLC and Emeritus Professor of Innovation, The University of Northern Iowa
Key Takeaways:
Traditional heating methods in food processing, like steam jacketed vessels, are outdated and inefficient, often leading to safety hazards and energy waste, prompting the search for safer alternatives.
Microwave-based vertical cooking vessels have emerged as a promising alternative, using direct heating to products without the need for jacketed vessels, scraping mechanisms, or steam boilers.
This microwave technology offers several advantages, including reduced costs, improved safety, and more efficient energy use, with the potential to revolutionize food processing in both safety and economics.
Food industry leaders maintain vigilance on technological advances that can impact competitiveness and survival. Advances in technology are often unavoidable because of a desire to improve processes, increase safety, and reduce cost. In many cases it becomes impossible to use older technology and remain competitive, because of the superior performance of newer ones.
Background
One area in the food processing industry that is subject to reevaluation is the method of heating for cooking liquid or semi-liquid products. The current heating methods were invented in the late 19th century and were improved during the 20th century. Until the invention of jacketed vessels in the 1940s the majority of food processing heat was via direct flames from wood or petroleum products. The invention of jacketed tanks allowed the use of steam and other heat transfer media to be placed away from the cooking vessel providing a safer cooking environment.
In the case of steam jacketed vessels, the heating medium, in this case steam, is sent through the jackets of the vessel to heat its walls to cook the food inside. The open flame method is still in use in developing countries and in small volume confectioneries using propane tanks and small open top vessels. But since the invention of jacketed vessels, steam and heat transfer oil have been used to heat the vessel walls for cooking food and processing chemicals.
The use of steam or heat transfer oil is in general inefficient and hazardous. Unfortunately, rather than considering the numerous fires in the food processing plants as a reason for looking for alternatives, the blame is often placed on negligence instead of considering the real cause which is an inherently hazardous and old technology. Heating water to generate steam to heat the walls of a vessel to cook products involves several forms of energy conversion each with its own inefficiency. Additionally, hotspots on the walls of jacketed vessels require scrape surface mechanisms with gearbox and drive motors, gear oils etc. An example of a hotspot in home cooking is the bottom of the pan when cooking on a stove, like cooking a cup of milk in a pot. The bottom of the pot that is in touch with the heating element or open flame is much hotter than the milk causing the fat in the milk to burn unless scraped and stirred. This is considered indirect heating where the original heat source heats the vessel not the product. All illustrations of indirect heating show some form of stirring and scraping due to the presence of hotspots of the cooking vessel. This is done intuitively through evolutionary learning.
The same cup of milk in a microwave would heat without the need for stirring or scraping, because microwaves directly the product not the vessel. The cup would remain cool to the touch while the milk is heated to a boil. This direct heating of products has the potential to increase safety and reduce costs significantly. The challenge has been discovering a way of applying microwaves to a metallic vertical tank to cook commercial food volumes.
Photo: The electromagnetic waves directly heat the milk, not the cup. In indirect heating the pot is hotter than the milk.
An alternative heating method for commercial cooking
A 2007 heat transfer oil fire in a biobased lubricant plant in Iowa led to searching for alternative ways to heat products. Of all possible candidates including induction, infrared, etc., the microwave was selected as having the best potential for processing vegetable oils to make soap or grease. The process was later patented and was put into production for heating vegetable oil-based lubricant products. Another heat transfer oil related fire in 2021, at the Chemtool lubricant processing plant in Rockton, Illinois, destroyed the largest grease and lubricant facility in the country.
Photo: A biobased (vegetable-oil-based) grease making vessel.
Microwaves with their cubical cooking chambers have been in use in homes and food service establishments for over seven decades. The idea of applying microwaves to a vertical metallic vessel to directly heat the products appears daunting at first. But in 2010, researchers at the University of Northern Iowa in cooperation with microwave manufacturers showed that microwaves can be applied to non-ferromagnetic metals (like stainless steel, aluminum or copper) without arcing. Since 2010 an Iowa company has been using microwaves to process millions of pounds of vegetable oil-based grease this way.
Microwaves in vertical heating vessels for food processing
Most people are familiar with cooking food inside a cubical chamber of the home microwaves. But microwave-based vertical metal tanks have been in use for heating products for over a decade. What is not known to many is that waves from a microwave generator travel through a small section of a waveguide and enter the microwave oven’s chamber. The opening to the waveguide is covered by a microwave transparent plate seen inside home microwaves. Waveguides can be as long as desired, and the waves can be sent through several feet or several yards away to a cooking chamber.
Knowing that microwaves can be sent through waveguides away from the source; and the fact that the waves can be applied to nonmagnetic metals have led to the development of microwave-based vertical cooking vessels. The images below show that by extending the waveguide from inside a conventional microwave, the waves can be sent to a cylindrical chamber of a cooking pot.
Photo: Waveguide extended from inside a conventional cubical chamber to a cylindrical chamber or cooking pot.
With the knowledge that electromagnetic waves from a microwave can directly excite the molecules of food products in a stainless-steel vessel, one can picture the benefits of this method of heating:
This is a direct heating method and thereby there won’t be a need for heating the tank walls to cook the product. This in turn eliminates the need for jacketed vessels, meaning a less expensive single wall tank can be used for cooking.
Since the tank walls are not hot, there won’t be a need for scraping the wall surfaces, eliminating the need for gear boxes and driving motors on the tank.
The steam boiler and its ancillary components will not be needed, further simplifying the operation. Instead, a pump below the tank circulates the product during wave-application and a temperature sensor reports the product temperature to the microwave controllers.
Photo: Skid mounted microwave-operated side entry vertical cooking vessel.
To illustrate, assume the operator sets the desired temperature to 90°C (194°F), and the power input at 35kW. The PLC applies 35kW of microwaves to the product via the waveguides and the temperature will begin to rise and is shown on the control screen. When the temperature reaches within 1 degree of the set temperature, in this case 89°C (192.2°F), the PLC ramps down the microwave input to near zero. If the temperature drops below 89°C (192.2°F), it ramps up the wave input by a few kW and within a few minutes the temperature stabilizes at 90°C (194°F). This condition will remain with minimal energy requirement until the operator intervenes. If instead of 35kW the operator chooses to apply 70kW, the process would be the same, but the time needed to reach the desired temperature would be cut in half. There is almost a direct linear relationship between the microwave power input and the time to reach the desired temperature.
The current method of heating for processing liquid and semi-fluid food products is old and subject to review and improvement. Microwaves have been in use for decades in homes and restaurants for cooking products. For food industry leaders, wave-based cooking deserves exploration. It offers too many safety and economic advantages over conventional processing to be ignored.
Lou Honary, president of WAVEtek Process Technology, LLC, is a leading advocate for using electromagnetic waves for manipulation of material molecules to achieve a desired process outcome. He holds 12 patents and has published extensively on biobased lubricants, transformer oils, and microwave-based processing. Honary received the 2020 Fellows Award from the National Lubricating Grease Institute.
