Microwave Blanching Superior In Vegetable-Preservation Process

What began as a project by senior food-science students at the University of Illinois has grown into a series of published graduate-level experiments -- green beans (1994), broccoli (1995) and, now, asparagus -- that are setting the standards for efficient microwave blanching.

"In my years of conducting food-preservation classes, I found out that a lot of people are not very interested in using traditional food-preservation methods, because they are time consuming and they heat up the house," said Susan Brewer, a U. of I. food scientist. "People were trying to particularly get away from having to blanch anything. The idea was to simply take something out of the garden, put it in a storage bag and stick it in the freezer."

But that's not a good practice, she said. Vegetables taken straight from the garden to the freezer, without blanching, deteriorate within a month. "Four weeks later, they are going to be of such poor quality that you are not going to want to eat them," she said.

Blanching is very short exposure to high heat -- typically two to five minutes in boiling water or live steam. The heat inactivates enzymes in the vegetables. Preparing for and using traditional steaming or boiling makes for messy kitchens and a lot of unwanted heat. Microwaving has been discouraged because of uncertainties on temperature, time and resulting quality.

"A lot of the recommendations used today are based on USDA guidelines developed in the 1940s and 1950s, and they were used for large quantities," Brewer said. "Our lifestyles have changed a lot. We've got microwave ovens in the kitchen and women in the work force. Women are no longer spending a lot of time in the kitchen doing these things anymore."

The most recent findings -- published in the December issue of the Journal of Food Quality by Brewer and Shahnaz Begum, a graduate student in food science and nutrition -- indicate that microwave-blanched asparagus keeps its nutritional value, taste and texture, as well as and often better than, asparagus blanched using traditional methods.

The success of microwave blanching depends mostly on quantity, timing and wattage. In this case, researchers blanched two to three servings at a time in a covered casserole dish with about two tablespoons of water. They blanched the asparagus, which had been picked within four hours of the process, at 700 watts for four minutes, then sealed it in plastic and put it into the freezer immediately.

In arriving at the optimum time for microwaving, the researchers identified the minimum amount of heating needed to inactivate both peroxidase and ascorbic-acid oxidase activities, which break down a vegetable's quality and vitamin C content, respectively, during storage.

Quality testing was done three months later on the asparagus, cooked in the microwave.