Boiling and Microwaving Effects on Hydrophilic Oxygen Radical Absorbance Capacity of Frozen Vegetables

Decreased risks of chronic illnesses, such as cancer, occur with increased consumption of dietary antioxidants. Vegetables are a particularly rich source of dietary antioxidants but these are primarily water soluble compounds. This research determined effects of microwaving or boiling on the antioxidant capacities of commercially frozen vegetables. Hydrophilic components were extracted by Acetone/Deionized water/Acetic Acid (700:295:50, v/v) from commercially frozen broccoli, carrots, sweet corn, and sweet peas before and after microwaving for 5 min or boiling for 10 min. The Oxygen Radical Absorbance Capacity (ORAC) assay was employed to determine the antioxidant capacity. Additionally, color and texture analyses were performed.

ORAC values from uncooked, microwaved or boiled broccoli were 11.33, 8.04 and 5.72 μmol TE/g; ORAC values for peas were 10.2, 5.14 and 2.43 μmol TE/g; ORAC values for corn were 6.32, 8.12 and 4.45 μmol TE/g; and ORAC values for carrots were 2.95, 4.00, and 2.39 μmol TE/g.

No significant ORAC and texture correlations were determined. The only significant color and ORAC correlation was for the a* value of peas (p<0.05). A negative moderate correlation existed; therefore, greener peas had greater ORAC values.

These results demonstrate that boiling vegetables for 10 min results in lower ORAC values and boiled broccoli, peas and corn contained significantly lower values (p<0.05) than uncooked broccoli or peas and microwaved corn. Boiled vegetables have been found to contain significantly lower water soluble nutrients due to loss into the cooking water. ORAC analysis of cooking water from each of the four vegetables verified the loss of antioxidant constituents since the water was found to contain increased antioxidant capacity. The greatest ORAC values were found in the water after boiling broccoli and the lowest ORAC values were found in the water after cooking carrots. Addition of antioxidant capacities of cooked vegetables in nutritional databases would be useful to consumers wanting to increase consumption of antioxidants.