Microwave blanching effects on color, chemical and sensory characteristics of frozen asparagus

Asparagus was blanched for 4 min using: conventional boiling water (BW), steam (ST), microwave (MW), or microwave heated in boilable bags (MWB). Samples were ice-cooled, bagged, heat-sealed and stored at -18C for 4 weeks. Peroxidase activity was reduced from 98-114 units in fresh, unblanched to 1-7 units in blanched asparagus. Reduced ascorbic acid (RAA) content in fresh, unblanched asparagus was similar to 49 mg/100 g; RAA content in fresh, BW-blanched asparagus was similar to 44 mg/100 g. Frozen, unblanched asparagus retained about 40% of original RAA content; BW-blanched asparagus retained 61%, and MWB-blanched asparagus retained 87%. After frozen storage all samples were darker than freshly blanched samples. Blanching increased and freezing decreased greenness of all samples. Blanching increased yellowness of fresh samples; after frozen storage, blanching treatment differences were lost. Unblanched samples had the highest appearance scores; unblanched and MW-blanched samples had the highest color scores. Overall quality of the microwave blanched asparagus was as good as or superior to conventionally blanched asparagus.