Electromagnetic heating improves product value, extends shelf life.

Electromagnetic heating techniques have been successfully applied to treat a variety of vegetable foods, such as cereals, seeds and herbs, in order to optimize their quality, shelf life and value. The techniques involve microwave heating or radio frequency (RF) heating and are applied after product is harvested or before it is frozen and placed in storage.

Microwave energy, like all electromagnetic radiation, travels in a wave pattern. When food is exposed to microwaves, it will absorb that energy and convert it to heat. Food composition--mainly the water content of a product--is a key factor that determines how fast it will heat in a microwave environment. The higher the water content of a food, the faster it will heat.

European scientists have used microwaves to inactivate alpha-amylases in wheat, pre-cook rice, dry yellow beans and pasteurize herbs. They used radio frequencies to inhibit trypsin inhibitors in soybeans and to pasteurize mustard seeds.

In a three-year study, the researchers partially inactivated alpha-amylases in wheat without causing gluten damage. This is possible depending on initial water content. The best results were achieved by using a heat-shocking technique that involved interrupted heating and cooling. The investigators produced par-boiled rice using microwave heating, improving quality and reducing energy consumption and processing times.

The scientists combined microwave heating and conventional heating techniques to germinate yellow peas and improve their nutritional value. Microwave heating also reduced microbial counts up to 3 logs in herbs with little reduction in the volatile value of swelling.

In addition, RF heating of soybeans reduced trypsin inhibitors. RF treatment of mustard seeds may reduce microbial counts to below 4 logs CFU per g. Researchers developed new techniques for measuring the dielectric properties of foods. They built a pilot plant with a capacity of 250 kg per hour and used RF heating (40 kW generator and a power output of 27.12 MHz) to treat mustard seeds and inactivate myrosinase enzyme activity.

Further information. Steven James, University of Bristol, Food Refrigeration and Process Engineering Research Center, Churchill Building, Langford, North Somerset, BS40 5DU Bristol, England, U.K.; phone: [phone]; fax: [phone]; URL: www.bristol.ac.uk.