Green beans (cv. Mustang) obtained from the University of Illinois at Urbana‐Campaign Horticulture plots were assayed within 3 h of harvest. Beans were blanched in covered containers: conventional boiling water (1900 ml, 3 min) (BW), steam (300 ml water, 3 min) (ST), microwave heated in 1 L glass containers (60 ml water, 3 or 5 min, 700 W) (MW‐3, MW‐5), and microwave heated in 1 L Seal‐a‐MealTM bags (45 ml water, 3 min) (MW‐Bag). Baseline data were collected immediately after blanching. Aliquots were frozen at – 18C for 4 weeks. Unblanched beans had 82 units of peroxidase activity/min; activity was zero immediately after all blanching treatments. Reduced ascorbic acid (RAA) content was highest for unblanched, ST and MW‐5 beans and lowest for BW and MW‐Bag beans immediately after blanching. ST beans retained 100% of original RAA. Instron shear force (kg) was highest for unblanched beans and lowest for BW and MW‐Bag beans immediately after blanching. After frozen storage, ST and BW beans had the lowest shear values. Visual color was poorest for MW‐5; other treatments were not different. Microwave blanching green beans for 3 min in a covered container or bag prior to 4 weeks frozen storage resulted in a product that was not different in RAA content, retention or color from BW blanched beans; however, these samples differed in tenderness and crispness from boiling water blanched beans.
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BREWER et al. (1994) studied this question.