Diane M Barrett - Expert in Residence

Microwave Processing – Description

            While in-home microwaves have been common in the U.S. since the late 1960s, larger-scale industrial processing using either batch or continuous microwave systems has been commercialized relatively recently.  Microwaves can preserve fruits and vegetables by heating, pasteurizing and sterilizing foods using electromagnetic radiation in the microwave frequency range.  This energy is transferred volumetrically, similar to high pressure processing (HPP) and pulsed electric fields (PEF), therefore, come-up times are much faster than conventional thermal processing, which heats from the outside in. Detrimental changes in physical, sensory or nutritional properties are minimized due to the short heating treatments.  The U.S. Federal Communications Commission has allocated two frequency bands for industrial, scientific and medical applications.  The 915 MHz band is used for industrial heating, and the 2450 MHz band is used for both domestic and industrial microwave ovens.  Microbial inactivation is thought to occur primarily through the thermal effect, while any effects of non-thermal mechanisms via membrane electroporation or electrical breakdown are still being investigated.  The U.S. Food and Drug Administration accepts that microwaves can be used to heat food for commercial use.  A useful fact sheet on Microwave Heating has been developed by colleagues at Washington State University.

Batch and Continuous Flow Microwave Systems

Both batch and continuous flow microwave systems are in use commercially, for different applications.  Batch systems often utilize multi-component plastic trays, into which various food products may be placed and covered, and this is referred to as in-package processing.  In a recent collaboration between our group at the University of California-Davis and colleagues at Washington State University, critical factors for the thermal pasteurization of ready-to-eat foods and vegetables were highlighted.  These scientists can semi-continuously pasteurize 8-20 oz. pre-packaged chilled foods. Continuous microwave systems require a product to be pumpable, therefore it is used primarily for both high and low acid purees or liquids, which may contain particulates.  For example, colleagues at North Carolina State University successfully processed a multiphase low acid salsa con queso product using a continuous flow microwave system.

Commercial Applications

            Commercial in-package batch microwave systems are currently utilized for precooking bacon (e.g. in Subway restaurants), tempering frozen meats for meat patties, drying pasta and for precooking other food products.  In Europe, some supermarket chains are selling microwave pasteurized and sterilized products, however the use of microwaves to produce stable low acid foods in the U.S. requires FDA approval.  In 2006, FDA acceptance was granted for a sweet potato puree product that was sterilized using continuous flow microwave processing combined with aseptic packaging.  This followed joint efforts by researchers at NC State University, the U.S. Department of Agriculture, Industrial Microwave Systems LLC and a group of sweet potato processors who established the Yamco Company in Snow Hill, NC.  According to their website, they are also producing purees of spinach, pumpkin, butternut squash and carrots.

Domestic institutions currently carrying out microwave research