The adequacy of recommended microwave blanch times for carrots and green beans in selected microwave ovens

Abstract

Microwave Blanching instructions for a variety of vegetables have recently been published by several sources. More research is needed, however, to establish microwave blanching times that are sufficient to destroy enzymes which cause nutritional and sensory deterioration in frozen vegetables while maintaining optimal quality.

In the present study the General Electric Company's recommended blanch times for carrots and green beans were investigated using three microwave ovens. Peroxidase was used as an indicator enzyme for measuring blanching adequacy. It was determined that oven variation had no significant effect on percent residual peroxidase activity in either vegetable (p > 0.05). A large degree of standard deviation in the percent residual peroxidase activity was found among the blanched portions of vegetable. This variation was attributed to the inconsistent levels of peroxidase enzyme in the vegetables themselves. Blanch time had the most significant impact on percent residual peroxidase activity (p < 0.01). After a four-minute blanch, carrots retained approximately 21-25% peroxidase activity and green beans retained approximately 37-46% peroxidase activity. These ranges are much higher than the acceptable level of 5% residual peroxidase activity. Six minutes of blanching for green beans and five minutes of blanching for carrots were required to reduce residual peroxidase activity to acceptable levels. Vegetables with such low peroxidase activity show no deleterious enzyme related changes during frozen storage. Microwave blanching has the potential to be an acceptable alternative to hot water blanching, but further research is needed to optimize microwave blanching procedures.