PROCESS EXPO | Expert in Residence
Microwave Thermal Processing
Steve Smith | Purdue University
In 1947 Dr. Percy Spenser, working on a magnetron for microwave energy for Raytheon Corp., reached into his pocket and found a melted candy bar, a revolution had started. In 1965 Amana (owned by Raytheon) sold the first commercially made home microwave for $500. A hefty sum at that time.
By 1975, microwave ovens outsold gas ovens. The advent of microwaves has substantially reduced the amount of time consumers take in food preparation. We use microwaves to defrost, cook, and even pop our foods.
When food is heated conventionally, the heat is transferred to the food surface by temperature gradations by conduction, convection, or radiation. With microwave energy, the energy is delivered directly through molecular interaction with the electromagnetic field (volumetric heating). The microwave energy causes the molecules to vibrate, which causes the food to heat by friction. Thermal conductivity plays less of a role in microwave heating. The result is a more rapid and efficient heating of foods and even particles in the food.
We take advantage of this in the food industry. Microwaves are used to dry, thaw, blanch, pasteurize, and sterilize foods. I’ll focus on pasteurizing and sterilizing foods.
One system out there is made by Industrial Microwave Systems (IMS). We currently have this system at Purdue University and have used it to sterilize a wide variety of food products. It is identical to a commercial unit at Yamco in Snow Hill, North Carolina which processes sweet potato puree aseptically. Our unit generates 100 Kw of microwave energy which is focused directly into the food by an applicator. We can heat food at 3 GPM from 40°F to 280°F in less than 1 second. This has a profound effect on the food itself. For example we processed spinach. As we know, canned spinach isn’t particularly appetizing. It is brown and bland. When we aseptically processed spinach puree using microwaves, we had a product that was bright green in color and tasted like a fresh spinach salad. Clearly there are some food products that would benefit from microwave thermal processing.
Another system commercially operating is at a company called Aseptia or Wright Foods in Troy, North Carolina. This system generates 1.2 MW of microwave energy and can process at 2-80 GPM. The company produces fruit and vegetable purees and now has letters of non-objection to process some low acid products with particulates.
One downside of microwave thermal processing is that heating with electrical energy is potentially more expensive than using natural gas. With microwaves, we use electrical energy to create microwave energy through a magnetron. In traditional thermal processing we use natural gas to produce heat which will then heat the product. However, due to the increased efficiencies of microwave heating, cost may not be higher. Each product should be evaluated individually.