Optimize microwave processing, pasteurization.

In fruit juice processing, high-temperature short-time (HTST) methods are important when it comes to minimizing temperature gradients and maximizing product quality. A properly designed microwave-heating unit has the potential to be an effective HTST process that can produce quality products.

The microbial inactivation kinetics involved in microwave energy are similar to those found in conventional thermal processes. Research at Cornell University has also shown that microwave energy can be used to heat apple mash prior to juice extraction in order to increase the amount of phytochemicals in the juice and juice yields.

This multifunctional utilization of microwave heating would make microwaves a cost-effective alternative processing tool. The purpose of the Cornell research was to assess the application of microwave energy toward processing apple cider. Microwaves can improve the quality and extend the shelf life of apple cider. Furthermore, chemical markers can be used to verify the lethality of thermal processes.

Both thermocouples and fiber-optic probes monitored temperatures as part of a continuous microwave pasteurization system. Kinetic parameters for the formation of 5-hydroxymethyl furfural (HMF) were correlated to the total lethality of the process. In addition, scientists optimized the extraction of phenolics by heating the mash before it was pressed.

The investigators monitored total phenolics, color and polyphenol oxidase activity over a six-week storage stability study. The lethality was verified using the chemical kinetics for the formation of HMF.

Apple cider pasteurized for a given lethality was found to contain trace amounts of HMF. These levels were comparable to commercial apple cider. Sensory tests also indicated no off-flavors were produced when compared with microthermic pasteurization. Apple mash heated to 70 C at 1800 W did show an increase in total phenolics. With the optimization of both pasteurization and phenolic extraction at 1800 W, simultaneous heating of the apple mash and the cider would be an efficient process.

Further information. John Roberts, Department of Food Science and Technology, Cornell University, New York State Agricultural Experiment Station, Geneva, NY 14456; phone: [phone]; fax: [phone]; URL: www.cornell.edu.